Note that a true laser jock will further abbreviate "HeNe laser" to simply "HeNe", pronounced: Hee-nee. Their laser jock colleagues and friends then know this really refers to a laser! :) While other types of lasers are sometimes abbreviated in an analogous manner, it is never to the same extent as the HeNe.
I still consider the HeNe laser to be the quintessential laser: An electrically excited gas between a pair of mirrors. It is also the ideal first laser for the experimenter and hobbyist. OK, well, maybe after you get over the excitement of your first laser pointer! :) HeNe's are simple in principle though complex to manufacture, the beam quality is excellent - better than anything else available at a similar price. When properly powered and reasonable precautions are taken, they are relatively safe if the power output is under 5 mW. And such a laser can be easily used for many applications. With a bare HeNe laser tube, you can even look inside while it is in operation and see what is going on. Well, OK, with just a wee bit of imagination! :) This really isn't possible with diode or solid state lasers.
The red HeNe was the first visible Continuous Wave (CW) laser to have been constructed and tested successfully. I can only imagine the thrill of those who observed a continuous beam of pure monochromatic light for the first time on this planet and we can only imagine what it felt like. But the article "Recollections of the First Continuous Visible Laser" by Alan D. White comes close. It is available from several places, one being Recollections of the First Continuous Visible Laser (Researchgate) uploaded by the Author. However, I rather doubt that the laser in the first photo "is emitting about 80 milliwatts". Probably fuzzy "recollection" or a typo. :)
I remember doing the glasswork for a 3 foot long HeNe laser (probably based on the design from: "The Amateur Scientist - Helium-Neon Laser", Scientific American, September 1964, and reprinted in the collection: "Light and Its Uses" [5]). This included joining side tubes for the electrodes and exhaust port, fusing the electrodes themselves to the glass, preparing the main bore (capillary), and cutting the angled Brewster windows (so that external mirrors could be used) on a diamond saw. I do not know if the person building the laser ever got it to work but suspect that he gave up or went on to other projects (which probably were also never finished). And, HeNe lasers are one of the simplest type of lasers to fabricate which produce a visible continuous beam.
Some die-hards still construct their own HeNe lasers from scratch. Once all the glasswork is complete, the tube must be evacuated, baked to drive off surface impurities, backfilled with a specific mixture of helium to neon (typically around 7:1 to 10:1) at a pressure of between 2 and 5 Torr (normal atmospheric pressure is about 760 Torr - 760 mm of mercury), and sealed. The mirrors must then be painstakingly positioned and aligned. Finally, the great moment arrives and the power is applied. You also constructed your high voltage power supply from scratch, correct? With luck, the laser produces a beam and only final adjustments to the mirrors are then required to optimize beam power and stability. Or, more, likely, you are doing all of this while your vacuum pumps are chugging along and you can still play with the gas fill pressure and composition. What can go wrong? All sorts of things can go wrong! With external mirrors, the losses may be too great resulting in insufficient optical gain in the resonant cavity. The gas mixture may be incorrect or become contaminated. Seals might leak. Your power supply may not start the tube, or it may catch fire or blow up. It just may not be your day! And, the lifetime of the laser is likely to end up being only a few hours in any case unless you have access to an ultra-high vacuum pumping and bakeout facility. While getting such a contraption to work would be an extremely rewarding experience, its utility for any sort of real applications would likely be quite limited and require constant fiddling with the adjustments. Nonetheless, if you really want to be able to say you built a laser from the ground up, this is one approach to take! (However, the CO2 and N2 lasers are likely to be much easier for the first-time laser builder.) See the chapters starting with: Amateur Laser Construction for more of the juicy details.
However, for most of us, 'building' a HeNe laser is like 'building' a PC: An inexpensive HeNe tube and power supply are obtained, mounted, and wired together. Optics are added as needed. Power supplies may be home-built as an interesting project but few have the desire, facilities, patience, and determination to construct the actual HeNe tube itself.
The most common internal mirror HeNe laser tubes are between 4.5" and 14" (125 mm to 350 mm) in overall length and 3/4" to 1-1/2" (19 mm to 37.5 mm) in diameter generating optical power from 0.5 mW to 5 mW. They require no maintenance and no adjustments of any kind during their long lifetime (20,000 hours typical). Both new and surplus tubes of this type - either bare or as part of complete laser heads - are readily available. Slightly smaller tubes (less than 0.5 mW) and much larger tubes (up to approximately 35 mW) are structurally similar (except for size) to these but are not as common. By far the smallest commercial HeNe laser tube in current production is the one found in the Zygo 7705 two-frequency metrology laser. It is approximately 3.5 inches (89 mm) in length and 0.67 inches (17 mm) in diameter. See Zygo 7705 HeNe Laser Tube with AA Cell for Size Comparison. Its output power is typically 0.25 to 0.4 mW in normal operation. However, even smaller HeNes are used in some ring laser gyroscopes with a maximum dimension of under 1 inch. ;-) See the section: Honeywell GG1308 Ring Laser Gyro.
Much larger HeNe tubes with internal or external mirrors or one of each (more than a *meter* in length!) and capable of generating up to 250 mW of optical power have been available and may turn up on the surplus market as well (but most of these are quite dead by now). The most famous of these (as lasers go) is probably the Spectra-Physics model 125A whose laser head is over 6 feet in length. It was only rated 50 mW (633 nm), but new samples under optimal conditions may have produced more than 200 mW. Even more powerful ones have been built as research projects. I've seen photos of a Hughes HeNe laser with a head around 8 feet in length that required a 6 foot rack-mount enclosure for the exciter. See Monster Vintage Hughes HeNe Laser System. (Photo courtesy of Bob Hess.) Its output power is unknown, but probably less than that of the SP-125A. The largest single transverse mode (SM, with a TEM00 beam profile) HeNe lasers in current production by a well known manufacturer like Melles Griot are rated at about 35 mW minimum over an expected lifetime of 20,000 hours or more, though new samples may exceed 50 mW. However, HeNe lasers rated up to at least 70 mW SM and 100 mW MM are available. Manufacturers include: CDHC-Optics (China), Spectral Laser (Italy), and PLASMA, JSC (Russia). However, the lifetime over which these specifications apply is not known and may be much shorter.
Highly specialized configurations, such as a triple XYZ axis triangular cavity HeNe laser in a solid glass block for an optical ring laser gyro, also exist but are much much less common. Most HeNe lasers operate CW (Continuous Wave) producing a steady beam at a fixed output power unless the power is switched on and off or modulated (or someone sticks their finger in the beam and blocks it!). (At least they are supposed to when in good operating condition!) However, there are some mode-locked HeNe lasers that output a series of short pulses at a high repetition rate. And, in principle, it is possible to force a HeNe laser with at least one external mirror to "cavity dump" a high power pulse (perhaps 100 times the CW power) a couple of nanoseconds long by diverting the internal beam path with an ultra high speed acousto-optic deflector. But, for the most part, such systems aren't generally useful for very much outside some esoteric research areas and in any case, you probably won't find any of these at a local flea market or swap meet, though eBay can't be ruled out! :)
Nearly all HeNe lasers output a single wavelength and it is most often red at 632.8 nm. (This color beam actually appears somewhat orange-red especially compared to many laser pointers using diode lasers at wavelengths between 650 and 670 nm). However, green (543.5 nm), yellow (594.1 nm), orange (604.6 and 611.9 nm), and even IR (1,152, 1.523, and 3,921 nm) HeNe lasers are available. There are a few high performance HeNe lasers that are tunable and very expensive. And, occasionally one comes across laser tubes that output two or more wavelengths simultaneously. Although some tubes are designed this way, it is more likely to be a 'defect' resulting from a combination of high gain and insufficiently narrow band optics. Such tubes tend to be unstable with the relative power varying among the multiple wavelengths more or less at random.
Note that the single wavelength described above usually consists of more than one longitudinal mode or lasing line (more on this later). However, some HeNe lasers are designed to produce a highly stable single optical frequency or two closely spaced optical frequencies. These are used in scientific research and metrology (measurement) applications, described in more detail below.
Current major HeNe laser manufacturers include Melles-Griot, JDS Uniphase, and LASOS. This is far fewer than there were only a few years ago. So, you may also find lasers from companies like Aerotech, Hughes, Siemens, and Spectra-Physics that have since gotten out of the HeNe laser business or have been bought out, merged, or changed names. For example, the HeNe laser divisions of Aerotech and Hughes were acquired by Melles Griot; Sieman's HeNe laser product line is now part of LASOS; and Spectra-Physics which was probably the largest producer of HeNe lasers from the very beginning gradually eliminated all HeNe lasers from its product line over the last few years. HeNe tubes, laser heads, and complete lasers from any of these manufacturers are generally of very high quality and reliability. A more complete list can be found in the Photonics Buyers' Guide and in the chapter: Laser and Parts Sources. Information on many specific HeNe lasers can be found in the chapters: Commercial Unstabilized HeNe Lasers and Commercial Stabilized HeNe Lasers.
HeNe lasers have been found in all kinds of equipment including:
Nowadays, many of these applications are likely to use the much more compact lower (drive) power solid state diode laser. (You can tell if you local ACME supermarket uses a HeNe laser in its checkout scanners by the color of the light - the 632.8 nm wavelength beam from a HeNe laser is noticeably more orange than the 660 or 670 nm deep red from a typical diode laser type.)
Melles Griot (now part of IDEX Optics and Photonics Marketplace. Catalogs used to include several pages describing HeNe laser applications. I know this was present in the 1998 catalog but has since disappeared and I don't think it is on their Web site.
Also see the section: Some Applications of a 1 mW Helium-Neon Laser for the sorts of things you can do with even a small HeNe laser.
Since a 5 mW laser pointer complete with batteries can conveniently fit on a keychain and generate the same beam power as an AC line operated HeNe laser almost half a meter long, why bother with a HeNe laser at all? There are several reasons:
However, the market for new HeNe lasers is still in the 100,000 or more units per year. What can you say? If you need a stable, round, astigmatism-free, long lived, visible 1 to 10 mW beam for under $500 (new, remember!), the HeNe laser is still the only choice.
Below are just a few possibilities.
(Portions from: Chris Chagaris (pyro@grolen.com).)
For many more ideas, see the chapters: Laser Experiments and Projects and Laser Instruments and Applications and the many references and links in the chapter: Laser Information Resources.
However, unlike those for laser diodes, HeNe power supplies utilize high voltage (several kV) and some designs may be potentially lethal. This is particularly true of AC line powered units since the power transformer may be capable of much more current than is actually required by the HeNe laser tube - especially if it is home built using the transformer from some other piece of equipment (like an old tube type console TV or that utility pole transformer you found along the curb) which may have a much higher current rating.
The high quality capacitors in a typical power supply will hold enough charge to wake you up - for quite a while even after the supply has been switched off and unplugged. Depending on design, there may be up to 10 to 15 kV or more (but on very small capacitors) if the power supply was operated without a HeNe tube attached or it did not start for some reason. There will likely be a lower voltage - perhaps 1 to 3 kV - on somewhat larger capacitors. Unless significantly oversized, the amount of stored energy isn't likely to be enough to be lethal but it can still be quite a jolt. The HeNe tube itself also acts as a small HV capacitor so even touching it should it become disconnected from the power supply may give you a tingle. This probably won't really hurt you physically but your ego may be bruised if you then drop the tube and it then shatters on the floor!
However, should you be dealing with a much larger HeNe laser, its power supply is going to be correspondingly more dangerous as well. For example, a 35 mW HeNe tube typically requires about 8 mA at 5 to 6 kV. That current may not sound like much but the power supply is likely capable of providing much more if you are the destination instead of the laser head (especially if it is a homemade unit using grossly oversized parts)! It doesn't take much more under the wrong conditions to kill.
After powering off, use a well insulated 1M resistor made from a string of ten 100K, 2 W metal film resistors in a glass or plastic tube to drain the charge - and confirm with a voltmeter before touching anything. (Don't use carbon resistors as I have seen them behave funny around high voltages. And, don't use the old screwdriver trick - shorting the output of the power supply directly to ground - as this may damage it internally.)
And only change electrical connections or plug/unplug connectors with power OFF, being aware of the potential for stored charge. In particular, the aluminum cylinder of some HeNe laser heads is the negative return for the tube current via a spring contact inside the rear end-cap. So, pulling off the rear end-cap while the laser is powered will likely make YOU the negative return instead! You will probably then bounce off the ceiling while the laser bounces off the floor, which can easily ruin your entire day in more ways than one. :( :) This connection scheme is known to be true for most JDS Uniphase and many Melles Griot laser heads, but may apply to others as well.
See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment for detailed information before contemplating the inside or HV terminals of a HeNe power supply!
Now, for some first-hand experience:
(From: Doug (dulmage@skypoint.com).)
Well, here's where I embarrass myself, but hopefully save a life...
I've worked on medium and large frame lasers since about 1980 (Spectra-Physics 168's, 171's, Innova 90's, 100's and 200's - high voltage, high current, no line isolation, multi-kV igniters, etc.). Never in all that time did I ever get hurt other than getting a few retinal burns (that's bad enough, but at least I never fell across a tube or igniter at startup). Anyway, the one laser that almost did kill me was also the smallest that I ever worked on.
I was doing some testing of AO devices along with some small cylindrical HeNe tubes from Siemens. These little coax tubes had clips for attaching the anode and cathode connections. Well, I was going through a few boxes of these things a day doing various tests. Just slap them on the bench, fire them up, discharge the supplies and then disconnect and try another one. They ran off a 9 VDC power supply.
At the end of one long day, I called it quits early and just shut the laser supply off and left the tube in place as I was just going to put on a new tube in the morning. That next morning, I came and incorrectly assumed that the power supply would have discharged on it own overnight. So, with each hand I stupidly grab one clip each on the laser to disconnect it. YeeHaaaaaaaaa!!!!. I felt like I had been hid across my temples with a two by four. It felt like I swallowed my tongue and then I kind of blacked out. One of the guys came and helped me up, but I was weak in the knees, and very disoriented.
I stumbled around for about 15 minutes and then out of nowhere it was just like I got another shock! This cycle of stuff went on for about 3 hours, then stopped once I got to the hospital. I can't even remember what they did to me there. Anyway, how embarrassing to almost get killed by a HeNe laser after all that other high power stuff that I did. I think that's called 'irony'.
A 10 mw HeNe laser certainly presents an eye hazard.
According to American National Standard, ANSI Z136.1-1993, table 4 Simplified Method for Selecting Laser Eye Protection for Intrabeam Viewing, protective eyewear with an attenuation factor of 10 (Optical Density 1) is required for a HeNe with a 10 milliwatt output. This assumes an exposure duration of 0.25 to 10 seconds, the time in which they eye would blink or change viewing direction due the uncomfortable illumination level of the laser. Eyeware with an attenuation factor of 10 is roughly comparable to a good pair of sunglasses (this is NOT intended as a rigorous safety analysis, and I take no responsibility for anyone foolish enough to stare at a laser beam under any circumstances). This calculation also assumes the entire 10 milliwatts are contained in a beam small enough to enter a 7 millimeter aperture (the pupil of the eye). Beyond a few meters the beam has spread out enough so that only a small fraction of the total optical power could possible enter the eye.
The term laser stands for "Light Amplification by Stimulated Emission of Radiation". However, lasers as most of us know them, are actually sources of light - oscillators rather than amplifiers. (Although laser amplifiers do exist in applications as diverse as fiber optic communications repeaters and multi-gigawatt laser arrays for inertial fusion research.) Of course, all oscillators - electronic, mechanical, or optical - are constructed by adding the proper kind of positive feedback to an amplifier.
All materials exhibit what is known as a bright line spectra when excited in some way. In the case of gases, this can be an electric current or (RF) radio frequency field. In the case of solids like ruby, a bright pulse of light from a xenon flash lamp can be used. The spectral lines are the result of spontaneous transitions of electrons in the material's atoms from higher to lower energy levels. A similar set of dark lines result in broad band light that is passed through the material due to the absorption of energy at specific wavelengths. Only a discrete set of energy levels and thus a discrete set of transitions are permitted based on quantum mechanical principles (well beyond the scope of this document, thankfully!). The entire science of spectroscopy is based on fact that every material has a unique spectral signature.
The HeNe laser depends on energy level transitions in the neon gas. In the case of neon, there are dozens if not hundreds of possible wavelength lines of light in this spectrum. Some of the stronger ones are near the 632.8 nm line of the common red HeNe laser - but this is not the strongest:
The strongest red line is 640.2 nm. There is one almost as strong at 633.4 nm. That's right, 633.4 nm and not 632.8 nm. The 632.8 nm one is quite weak in an ordinary neon spectrum, due to the high energy levels in the neon atom used to produce this line. See: Bright Line Spectra of Helium and Neon. (The relative brightnesses of these don't appear to be accurate though at present.) More detailed spectra can be found at the: Laser Stars - Spectra of Gas Discharges Page. And there is a photo of an actual HeNe laser discharge spectra with very detailed annotation of most of the visible lines in: Skywise's Lasers and Optics Reference Section. The comment about the output wavelength not being one of the stronger lines is valid for most lasers as if it were, that energy level would be depleted by spontaneous emission, which isn't what is wanted!
There are also many infra-red lines and some in the orange, yellow, and green regions of the spectrum as well.
The helium does not participate in the lasing (light emitting) process but is used to couple energy from the discharge to the neon through collisions with the neon atoms. This pumps up the neon to a higher energy state resulting in a population inversion meaning that more atoms in the higher energy state than the ground or equilibrium state.
Please refer to Helium-Neon Excitation and Lasing Process for the following description.
It turns out that the upper level of the transition that produces he 632.8 nm line (as well as the other visible HeNe lasing lines) has an nergy level that almost exactly matches the energy level of helium's owest excited state. The vibrational coupling between these two states s highly efficient.
For 632.8 nm, one mirror will be highly reflective at 632.8 nm (typically 99.9 percent or better). This is the "High Reflector" or HR. The other mirror will have a typical reflectivity of 99 percent at 632.8 nm. This is the "Output Coupler" or OC from which the useful beam emerges. In order to suppress lasing at other wavelengths, the mirrors will generally be designed to have lower reflectivity there. (Though given the low gain of all the HeNe lasing lines, especially the "other color" lines, this isn't much of a problem at 632.8 nm.)
The rate at which (4) and (5) can take place ultimately limits the power of a HeNe laser and explains why increasing the excitation (1) actually reduces power above some optimum level.
The gas mixture must be mostly helium (typically 5:1 to 10:1, He:Ne), so that helium atoms can be excited. The excited helium atoms collide with neon atoms, exciting some of them to the state from which they can radiate at 632.8 nm. Without helium, the neon atoms would be excited mostly to lower excited states responsible for non-laser lines. And the gas mixture has to be super pure as any contamination results in excitation of rogue atoms (like H, O, and N) to lower energy states where all that will happen is that they will glow like a poorly made neon sign.
A neon laser with no helium can be constructed but it is much more difficult and the output power will be much lower without this means of energy coupling. Therefore, a HeNe laser that has lost enough of its helium (e.g., due to diffusion through the seals or glass) will most likely not lase at all since the pumping efficiency will be too low.
However, pure neon will lase superradiantly in a narrow tube (e.g., 40 cm long x 1 mm ID) in the orange (611.9 nm) and yellow (594.1 nm) with orange being the strongest. Superradiant means that no mirrors are used although the addition of a Fabry-Perot cavity (e.g., mirrors!) does improve the lateral coherence and output power. This from a paper entitled: "Super-Radiant Yellow and Orange Laser Transitions in Pure Neon" by H. G. Heard and J. Peterson, Proceedings of the IEEE, Oct. 1964, vol. #52, page #1258. The authors used a pulsed high voltage power supply for excitation (they didn't attempt to operate the system in CW mode but speculate that it should be possible).
(From: Steve Roberts.)
"Various IR lines will lase in pure neon, and even the 632.8 nm line will lase, but it takes a different pressure and a much longer tube. 632.8 nm also shows up with neon-argon, neon-oxygen, and other mixtures. Just about everything on the periodic table will lase, given the right excitation. See "The CRC Handbook of Lasers" or one of the many compendiums of lasing lines available in larger libraries. These are usually 4 volume sets of books the size of a big phone book just full of every published journal article on lasing action observed. It's a shame that out of these many thousands and thousands of lasing lines, only 7 different types of lasers are under mainstream use.
There are many possible transitions in neon from the excited state to a lower energy state that can result in laser action. (Only the three found most commonly in commercial HeNe lasers are shown in the diagram, above.) The most important (from our perspective) are listed below:
(1) (2) (3) (4) (5) (6)
Output HeNe Perceived Lasing Typical Maximum
Wavelength Laser Name Beam Color Transition Gain (%/m) Power (mW)
------------------------------------------------------------------------------
543.5 nm Green Green 3s2->2p10 0.52 0.59 2 (5)
594.1 nm Yellow Orange-Yellow 3s2->2p8 0.5 0.67 7 (10)
604.6 nm Orange 3s2->2p7 0.6 1.0 3
611.9 nm Orange Red-Orange 3s2->2p6 1.7 2.0 7
629.4 nm Orange-Red 3s2->2p5 1.9 2.0
632.8 nm Red " " 3s2->2p4 10.0 10.0 75 (200)
635.2 nm " " 3s2->2p3 1.0 1.25
640.1 nm Red 3s2->2p2 4.3 2.0 2
730.5 nm Border Infra-Red 3s2->2p1 1.2 1.25 0.3
886.5 nm " " 2s2->2p10 1.2 1.25 0.3
1,029.8 nm Near-IR Invisible 2s2->2p8 ???
1,062.3 nm " " " " 2s2->2p7 ???
1,079.8 nm " " " " 2s3->2p7 ???
1,084.4 nm " " " " 2s2->2p6 ???
1,140.9 nm " " " " 2s2->2p5 ???
1,152.3 nm " " " " 2s2->2p4 ??? 1.5
1,161.4 nm " " " " 2s3->2p5 ???
1,176.7 nm " " " " 2s2->2p2 ???
1,198.5 nm " " " " 2s3->2p2 ???
1,395.0 nm " " " " 2s2->2p? ??? 0.5
1,523.1 nm " " " " 2s2->2p1 ??? 1.0
3,391.3 nm Mid-IR " " 3s2->3p4 ??? 440.0 24
Notes:
Gain at 1,523 nm may be similar to that of 543.5 nm - about 0.5%/m. Gain at 3,391 nm is by far the highest of any - possibly more than 100%/m. I know of one particular HeNe laser operating at this wavelength that used an OC with a reflectivity of only 60% with a bore less than 0.4 m long. Yet, the output power of the largest 3,391 nm commercial HeNe laser is still only a fraction of that at 632.8 nm.
See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.
The most common and least expensive HeNe laser by far is the one called 'red' at 632.8 nm. However, all the others with named 'colors' are readily available with green probably being second in popularity due to its increased visibility near the peak of the of the human eye's response curve (555 nm). And, with some HeNe lasers with insufficiently narrow-band mirrors, you may see 640 nm red as a weak output along with the normal 632.8 nm red because of its relatively high gain. There are even tunable HeNe lasers capable of outputting any one of up to 5 or more wavelengths by turning a knob. While we normally don't think of a HeNe laser as producing an infra-red (and invisible) beam, the IR spectral lines are quite strong - in some cases more so than the visible lines - and HeNe lasers at all of these wavelengths (and others) are commercially available.
The first gas laser developed in the early 1960s was an HeNe laser operated at 1,152.3 nm. In fact, the IR line at 3,391.3 is so strong that a HeNe laser operating in 'superradiant' mode - without mirrors - can be built for this wavelength and commercial 3,391.3 nm HeNe lasers may use an output mirror with a reflectivity of less than 50 percent. Contrast this to the most common 632.8 nm (red) HeNe laser which requires very high reflectivity mirrors (often over 99 percent) and extreme care to mimize losses or it won't function at all.
When the HeNe gas mixture is excited, all possible transitions occur at a steady rate due to spontaneous emission. However, most of the photons are emitted with a random direction and phase, and only light at one of these wavelengths is usually desired in the laser beam. At this point, we have basically the glow of a neon sign with some helium mixed in!
To turn spontaneous emission into the stimulated emission of a laser, a way of selectively amplifying one of these wavelengths is needed and providing feedback so that a sustained oscillation can be maintained. This may be accomplished by locating the discharge between a pair of mirrors forming what is known as a Fabry-Perot resonator or cavity. One mirror is totally reflective and the other is partially reflective to allow the beam to escape.
One mirror may be perfectly flat (planar) or both may be spherical with a typical Radius of Curvature (RoC = 2 * focal length) slightly longer that the length of the cavity (L) or even longer. Where both mirrors have an RoC equal to L, the configuration is called 'confocal' (the focii of the two mirrors are coincident), but it is marginall stable, so the RoCs will be at least slightly longer than L. A cavity with two planar mirrors is borderline stable and essentially impossible to align or maintain in alignment over time, so it is never used in HeNe lasers (but is in some pulsed solid state and other lasers). Curved mirrors result in an easier to align more stable configuration but are more expensive than planar mirrors to manufacture and are not as efficient since less of the lasing medium volume is used (think of the shape of the beam inside the bore). The confocal arrangement represents a good compromise between a true spherical cavity (r = 1/2 * L) which is easiest to align but least efficient and one with plane parallel mirrors (f = infinity) which is most difficult to align but uses the maximum volume of the lasing medium. (But as noted above, for a practical confocal cavity, RoCs slightly longer than L are used to assure stability.) For more on this topic, see the section: Common Laser Resonator Configurations.
These mirrors are normally made so that the two mirrors together has peak reflectivity at the desired laser wavelength. (For technical reasons, it's sometimes easier to make mirrors like cliffs - high reflectivity that drops to low reflectivity at a given wavelength, in either direction - than to guarantee a particular peak reflectivity.) When a spontaneously emitted photon resulting from the transition corresponding to this peak happens to be emitted in a direction nearly parallel to the long axis of the tube, it stimulates additional transitions in excited atoms. These atoms then emit photons at the same wavelength and with the same direction and phase. The photons bounce back and forth in the resonant cavity stimulating additional photon emission. Each pass through the discharge results in amplification - gain - of the light. If the gain due to stimulated emission exceeds the losses due to imperfect mirrors and other factors, the intensity builds up and a coherent beam of laser light emerges via the partially reflecting mirror at one end. With the proper discharge power, the excitation and emission exactly balance and a maximum strength continuous stable output beam is produced.
Spontaneously emitted photons that are not parallel to the axis of the tube will miss the mirrors entirely or will result in stimulated photons that are reflected only a couple of times before they are lost out the sides of the tube. Those that occur at the wrong wavelength will be reflected poorly if at all by the mirrors and any light at these wavelengths will die out as well.
For most common IR wavelengths, level 4 is the 2s state and level 3 are various 2p states. However, the very strong 3.93 um line originates from the 3s state just like the visible wavelengths - and is the reason it competes with them in long HeNe tubes and must be suppressed to optimize visible output.
The 's' states of neon have about 10 times the lifetime of the 'p' states and thus support the population inversion since a neon atom can hang around in the 2s state long enough for stimulated emission to take place. However, the limiting effect is the decay back to level 1, the ground state, since the 1s state also has a long lifetime. Thus, one wants a narrow bore to facilitate collisions with its walls. But this results in increased losses. Modern HeNe lasers operate at a compromise among several contradictory requirements which is one reason that their maximum output power is relatively low.
While it is commonly believed that the 632.8 nm (for example) transition is a sharp peak, it is actually a Gaussian - bell shaped - curve. (Strictly speaking, it is something called a "Voigt distribution" which is a conbination of Gaussian and Lorentzian - but that's for the advanced course. Gaussian is close enough for this discussion since the discrepency only shows up way out in the tails of the curve.) In order for a linear or (Fabry-Perot) cavity to resonate strongly, a standing wave pattern must exist. This will only occur when an integral number of half wavelengths fit between the two mirrors. This restricts possible axial or longitudinal modes of oscillation to:
L * 2 c * n
W = --------- or F = ---------
n L * 2
Where:
The laser will not operate with just any wavelength - it must satisfy this equation. Therefore, the output will not usually be a single peak at 632.8 nm but a series of peaks around 632.8 nm spaced c/(2*L) Hz apart. Longer cavities result in closer mode spacing and a larger number of modes since the gain won't fall off as rapidly as the modes move away from the peak. For example, a cavity length of 150 mm results in a longitudinal mode spacing of about 1 GHz; L = 300 mm results in about 500 MHz. The strongest spectral lines in the output will be nearest the combined peak of the lasing medium and mirror reflectivity but many others will still be present. This is called multimode operation.
Think of the vibrating string of a violin or piano. Being fixed at both ends, it can only sustain oscillations where an integer number of cycles fits on the string. In the case of a string, n can equal 1 (fundamental) and 2, 3, 4, 5 (harmonics or overtones). Due to the tension and stiffness of the string, only small integer values for n are present with a significant amplitude. For a HeNe laser, the distribution of the selected neon spectral line and shape of the reflectivity function of the mirrors with respect to wavelength determine which values of n are present and the effective gain of each one. And n will be much greater than 1!
For a typical HeNe laser tube, possible values of n will form a series of very large numbers like 948,161, 948,162, 948,163, 948,164,.... rather than 1, 2, 3, 4. :-) A typical gain function showing the emission curve of the excited neon multiplied by the mode structure of the Fabry-Perot resonator and the reflectivity curve of the mirrors may look something like the following:
| 632.8 nm
I| .
| | | |
| | | | | |
| | | | | | | |
_______|______.__|__|__|__|__|__|__|__|__|__._______
n=948,166 -5 -4 -3 -2 -1 +0 +1 +2 +3 +4 +5
Or, see the following for some slightly more esthetically pleasing diagrams of the longitudinal modes of random polarized HeNe lasers. :)
Note 1 to the purists: Due to mode competition, particularly for short lasers like the 1 mW and 3 mW, the envelope of the lasing mode power curves will not be a nice Gaussian. However, factors besides the length and/or power output affect this shape, so these idealized ones are used here. More on this below.
Note 2 to the purists: The actual location of the lasing lines does not quite coincide with the cavity modes except at the very center of the gain curve due to a phenomenon called "mode pulling". But this is for the advanced course :-) and the offset is usually much less than 1 percent of the mode spacing so it would be almost undetectable on the scale of these diagrams.
There is much more on both of these effects below.
The optical frequency of each line is than n * (c/2L) and is thus inversely proportional to the mirror spacing. Very short lasers (e.g., 1 and 2) lase on very few longitudinal modes. In the case of (1), at most 2 modes; for (2), 3 modes are possible if there was one near the center of the neon gain curve. This also means that the total output power varies significantly depending on mode position - as much as 20 percent in a laser like (1).
For longer lasers like (4), over a half dozen longitudinal modes are present at all times and the variation in output power is less than 2 percent. For very short tubes, the output power scale is different because less of the total cavity length is actual gain. For example, a 150 mm tube typically has a bore with a length of around 75 mm (1/2 the cavity length) while a 800 mm tube may have a 700 mm bore. So, there are more modes in a long tube, but also more power per mode.
In the diagrams above, a single arbitrary mode position is shown, but for well behaved lasers, the lasing lines will move smoothly through the gain curve as the laser warms up. This is called by various names including "mode sweep" and "mode cycling". While present with most lasers, the effects are quite striking with low to medium power HeNe lasers due to their relatively narrow neon gain bandwidth (which is only a small multiple of the longitudinal mode spacing in low to medium power HeNe lasers), the rather fortuitous phenomenon that for red (633 nm) HeNe lasers at least, adjacent longidutinal modes tend to be othogonally polarized, and nearly ideal behavior in other respects with the Physics mostly cooperating. (Murphy has seen the LASER DANGER signs and stays away!) Much more on all this below (except perhaps for Murphy).
In the nice diagram above :) of the 8 mW laser, there are 5 longitudinal cavity modes that see gain above the lasing threshold (the right-most just barely). These become lasing modes (red and blue) producing a total output power of somewhat over 8 mW in this specific example. For the 30 mW laser, there are twice as many lasing modes one half the distance apart, and each mode has more power. Interestingly, adjacent modes in a so-called "random polarized" red (632.8 nm) HeNe laser are almost always orthogonally polarized, with the polarization axes fixed relative to the tube. (Here, one of them is arbitrarily referenced as 0 degrees, more on this later). As the distance between the mirrors is increased, the number of oscillating modes increases as well, though the actual power in each mode increases only slightly.
The animated Power Point shows below demonstrate the mode sweep behavior for a variety of random polarized red HeNe lasers. For all of them, the default speed is one increment per second, so the shorter lasers in the animations will take longer to complete a full cycle since the number of "phases" in the slides is larger. (The phases are shown only to be able to identify the specific slides.) This is somewhat similar to real behavior as longer tubes expand faster. The left and right arrow keys can be used to go back and forth at a much faster rate, or to "simulate" the effect of a stabilized laser control circuit keeping a mode or modes in a particular place on the neon gain curve.
These open in a separate window and are known to work in PowerPoint 2003, 2007, and 2010:
However, not all tubes are as well behaved as these. "Flippers" exhibit abrupt changes in polarization at one or more locations on the neon gain curve during mode sweep. HeNe Flipper Mode Sweep: 200 mm (~8 inch) Cavity Length demonstrates this for the same size 2 to 3 mW tube. There is much more on flipper behavior below.
For the 8 mW laser, there is the animation alone in a GIF file if you're using a real computer that doesn't have PowerPoint: :) It cycles at about 1 complete mode sweep cycle per second, which is several times faster than a real HeNe laser even at startup, when it would be fastest. However, some stabilized HeNe lasers will mode sweep this fast initially due to the heater used to control cavity length. See Mode Sweep of 8 mW Random Polarized HeNe Laser.
One complete cycle (red or blue) represents a change in cavity length of one wavelength (at 633 nm) and a change in optical frequency of 2 times the mode spacing of c/2L. The additional factor of 2 arises because the adjacent modes of the red (633 nm) HeNe are orthogonally polarized. This is not true with most other lasers and even HeNe lasers at other wavelengths. Note that while the profile of the mode sweep is affected by the neon gain curve, the period is NOT directly related to it, only c/2L.
However, note that as HeNe lasers get longer, mode competition results in greater and greater instability, so don't expect to see a nice orderly march with a Spectra-Physics 127 (39 inch cavity). In fact while the envelope of the modes will generally follow the gain curve, each mode will be jumping up and down in a quasi-chaotic dance! Instability may appear in the display of a Scanning Fabry-Perot Interferometer (SFPI) when viewing the longitudinal modes of a 633 nm HeNe with tubes rated at 7 to 10 mW. 5 mW lasers are usually quite clean while 35 mW lasers can be a real mess.
For very short HeNe tubes, the width of the gain curve may be similar to or even narrower than the spacing between modes. With those, the output power will become very low or go to zero during portions of the mode sweep. Very few HeNe lasers were produced with cavity lengths where this would be an issue since maximum output power would be very low. The only one I know of was the Spectra-Physics 119 stabilized laser with a 100 mm cavity length (mode spacing of 1.5 GHz). The very short cavity was required to provide special characteristics for this system.
The effects of mode competition where multiple lasing lines are drawing from the same upper state population also become more pronounced with shorter lasers, typically under 3 or 4 mW. For those, the actual appearance may differ substantially from these somewhat idealized plots. For example, the one in the show below can be identified because of its unique shape:
In fact, it's often possible to go so far as to identify a specific manufacturer and even model of a HeNe laser tube based solely on the plots of its polarized mode sweep, providing a sort of "fingerprint" for lasers. :) For example, the type of tube installed in a Zygo or Teletrac/Axsys stabilized laser can be determined without opening the case! This is shown for several tubes in HeNe Laser Mode Sweep Fingerprints. These tubes are all physically similar yet have dramatically different mode sweep plots. And, it's often possible to determine key information about the health of a laser tube by comparing its mode sweep with that of a new one. Over most of its life, the general shape will remain the same, but as the power declines, in addition to the total height of the plot decreasing, the amplitude of the variation (i.e., the AC component) relative to the total will increase. However, near end-of-life when power is way down and fewer modes are oscillating, the distinctions will tend to disappear.
Further, given the physical parameters of the cavity, it is also possible to estimate the actual neon isotope ratio in the gas-fill since that affects the width of the neon gain curve and its shape. A symmetric mode profile indicates a close to equal proportion of Ne20 and Ne22, while a skewed or sloped mode profile indicates that one or the other has been enriched, usually to narrow the gain curve and/or to match an optical frequency specification (mostly stabilized metrology lasers).
For very long tubes like the 30 mW one in the example above where there are many longitudinal modes, the actual appearance of mode sweep may be rather chaotic as power shifts among the modes in a random dance. When I first observed this behavior with a Melles Griot 05-LHP-928 (35 mW) HeNe producing over 40 mW, I thought it might have been defective in some way despite the high power. But two other healthy samples behaved in a similar manner. So, don't expect to see nice well behaved marching modes for these high power lasers. There is often a hint of instability even in shorter tubes though it may be subtle - a few percent variation in the peak amplitudes not attributable to other causes like normal movement under the gain curve or power supply ripple.
The effects of mode sweep are more dramatic with short low pressure carbon dioxide (CO2) lasers because for a given resonator length, the ratio of wavelengths (10,600 nm for CO2 compared to 632.8 nm for HeNe means that the longitudinal mode spacing is 16.7 times larger). In these cases, the laser output will turn on and off as it heats up and the distance between the mirrors increases due to thermal expansion. For this to happen in a 632.8 nm HeNe would require the tube to be less than about 75 mm (3 inches) in length.
A linearly polarized HeNe laser would have the same longitudinal mode spacing, but all the lasing modes would have the same polarization orientation (red or blue) as shown in the diagrams and animations, above. As an example, see Longitudinal Modes of Typical Linearly Polarized 8 mW HeNe Laser. So, someone with red/blue color-blindness (if there is such a thing) would see the diagrams for all them as being linearly polarized!
A label on the polarized laser will indicate the plane or orientation of polarization of the output beam. For a random polarized HeNe laser, a polarizer oriented at 45 degrees with respect to the plane of polarization would produce an output with respect to mode sweep that is similar to that of a linearly polarized laser, except that even with an ideal polarizer, the output power would be cut in half.
Now for some actual numbers: The Doppler-broadened gain curve for neon in a red (632.8 nm) HeNe laser has a Full Width Half Maximum (FWHM, where the gain is at least half the peak value) on the order of 1.5 or 1.6 GHz. So, for a 500 mm long (high gain) tube with its mode spacing of about 300 MHz (similar to what is depicted above), 5 or 6 lines may be active simultaneously and oscillation will always be sustained (though there would be some variation in output power as various modes sweep by and compete for attention). However, for a little 10 cm tube, the mode spacing is about 1,500 MHz. If this laser were to be really unlucky (i.e., the distance between mirrors was exactly wrong) the cavity resonance might not fall in a portion of the gain curve with enough gain to even lase at all! Or, as the tube heats up and expands, the laser would go on and off. There are very few commercial HeNe laser tubes that short. It is possible to widen the gain curve somewhat by using a mixture of neon isotopes (Ne20 and Ne22) rather than a single one since the location of their peak gain differ slightly. This would allow a smaller cavity to lase reliably and/or reduce amplitude variations from mode sweeping in all size HeNe lasers. The actual lasing threshold will also determine the effective width of the neon gain curve over which lasing occurs, so it may be wider than the FWHM.
A high speed silicon photodiode and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with the longitudinal modes of a HeNe laser. The clearest demonstration would be using a short tube where at most two longitudinal modes are active. This will result in a single difference frequency when both modes are lasing. A polarized tube is best as it forces both modes to have the same polarization as a photodiode will not detect the difference frequencies for orthogonally polarized modes. Adjacent longitudinal modes of random polarized tubes are almost always orthogonally polarized (for 633 nm HeNes at least). But, adding a polarizer at 45 degrees to the polarization axes can compensate for this with a slight loss in signal strength. Without a polarizer, the beat frequencies of a random polarized laser will tend to be at multiples of twice the mode spacing since only those modes with the same polarization orientation beat with each-other in the photodiode. (If measured very accurately, it will be seen that these frequencies will not generally be exactly at multiples of the mode spacing based on c/2L and will vary slightly during mode sweep. The is due to mode pulling or pushing effects, reserved for the advanced course!)
Passive stabilization (using a structure made of a combination of materials with a very low or net zero coefficient of thermal expansion or a temperature regulator) or active stabilization (using optical feedback and piezo or magnetic actuators to move the mirrors, or a heating element to control the length of the entire structure) can compensate for these effects. However, the added expense is only justified for high performance lab quality lasers or industrial applications like interferometric based precision measurement systems - you won't find these enhancements on the common cheap HeNe tubes found in barcode scanners. See the section: Stabilized Single Frequency HeNe Lasers.
Thus, a typical HeNe laser is not monochromatic though the effective spectral line width is very narrow compared to common light sources. Additional effort is needed to produce a truly monochromatic source operating in a single longitudinal mode. One way to do this is to introduce another adjustable resonator called an etalon into the beam path inside the cavity. A typical etalon consists of a clear optical plate with parallel surfaces. Partial reflections from its two surfaces make it act as a weak Fabry-Perot resonator with a set of modes of its own. Then, only modes which have the same optical frequency in both resonators will produce enough gain to sustain laser output.
The longitudinal mode structure of an optional intra-cavity etalon might look like the following (not to scale):
| 632.8 nm
I| . . .
| | | |
| | | |
| | | |
_______|______|______________|______________|_______
m=13,542 -1 +0 +1
Notice that since the distance between the two surfaces of the etalon is much less than the distance between the main mirrors, the peaks are much further apart (even more so than shown). (The etalon's index of refraction also gets involved here but that is just a detail.) By adjusting the angle of the etalon, its peaks will shift left or right (since the effective distance between its two surfaces changes) so that one spectral line can be selected to be coincident with a peak in the main gain function. This will result in single mode operation. The side peaks of the etalon (-1, +1 and beyond) will may coincide with weak peaks in the main gain function shown above but their combined amplitude (product) is insufficient to contribute to laser output.
This is shown, again more esthetically in Intracavity Etalon for Line Selection in a Single Mode HeNe Laser. This example is based on the same 30 mW laser as in the diagram in the section: Longitudinal Modes of Operation. Adding an etalon inside the cavity introduces an additional loss function with peaks every GHz or so. (Note that such an etalon would be about 15 cm long, so the plasma tube for this laser needs to be short enough to allow for that much space between it and one of the mirrors, but that's just a detail!) Only where the product of the original net (round trip) gain and the etalon transmission is above one will the laser lase. For this example, there is only place where a cavity mode and etalon mode conincide - just to the left of center of the neon gain curve peak. And, now that there is only a single mode oscillating, it will have an output power of over 15 mW, rather than the ~3 mW or less in each of several multiple modes. There is always some loss in adding an etalon, so the full 30+ mW originally present isn't usually possible, though the ~50 percent reduction in output power shown here may be excessive.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
The standard, small HeNe laser normally lases on only one transition, the well known red line at about 632.8 nm.
The HeNe gain curve is inhomogeneously Doppler-broadened with a gain bandwidth of around 1.5 GHz (at 632.8 nm). (The width of the Doppler-broadened gain curve depends on the lasing wavelength. At 3,391 nm, it is only about 310 MHz.) For a typical laser, say 30 cm long, the axial modes are separated by about 500 MHz. Typically, two or three axial modes are above threshold, in fact as the laser length drifts you typically get two modes (placed symmetrically about line centre) or three modes (one near centre, one either side) cyclically, and a slow periodic power drift results. Shorter lasers, less modes, more power variation unless stabilized. But it needs a huge HeNe laser to get ten modes, and since they are closer of course they still only spread over the 1.5 GHz line width.
Most HeNe lasers which do not contain a Brewster window or internal Brewster plate are randomly polarized; adjacent modes tend to be of alternating orthogonal polarizations. (Note that this is not necessarily true for HeNe lasers operating at wavelengths other than 632.8 nm and/or can be overridden with a transverse magnetic field, see below. See the section: Polarization of Longitudinal Modes in HeNe Lasers. --- Sam.)
Some frequency stabilized HeNe lasers are NOT single mode, but have two, and the stabilization acts to keep them symmetrical about line centre - i.e., both are half a mode spacing off line centre. A polariser will then split off one of them or a polarizing beamsplitter will separate the two.
(From: Sam.)
The party line is that adjacent modes in a HeNe laser will be of orthogonal polarization. However, I've seen samples of small (e.g., 5 or 6 inch) random polarized tubes only supporting 2 active modes where this is not the case - they output a polarized beam that remains stable with warmup and in any case, applying a strong transverse magnetic field will override the natural polarization. So, it's not a strong effect. Only if everything inside the tube is reasonably symmetric, will the modes alternate. Modes may also remain one polarization as they move through part of the gain curve and then abruptly - and repeatably - flip polarization. But the majority of tubes are well behaved in this regard.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes, see An Investigation of the Cavity Modes of the HeNe Laser.
(Portions from: Steve Roberts.)
Flames expected, as I'm ignoring some of the physics and am trying to explain some of this based on what I observe, aligning and adjusting cavities on HeNe and argon ion lasers as part of repairing them. Anyone who only goes by the textbooks has missed out on the fun, obviously having never had to work on an external mirror resonator. It can be quite a education!
Due to the complex number of possible paths down the typical gain medium, you will see lasing as long as the mirrors are reasonably aligned. The cavity spacing is not always that critical and will change anyway as the mirror mounts are adjusted (there will always be some unavoidable translation even if only the angle is supposed to be changed). No, lasers don't really flash on and off in interferometric nulls as you translate the mirrors - they instead change lasing modes. They will find another workable path. You will in some cases see this as a change in intensity but it is more properly observed on a optical spectrum analyzer as a change in mode beating. Eventually you can translate them far apart enough that lasing ceases, but this is a function of your optics not the resonator expansion.
I have seen what you fear in some cases by adding a third mirror to a two mirror cavity with a low gain medium such as HeNe where the third mirror can be positioned in such a way to kill many possible modes. This usually occurs when I use a HeNe laser to align an argon laser's mirrors and the HeNe laser will flicker from back reflections. See the section: External Mirror Laser Cleaning and Alignment Techniques. But unless you have a extremely unstable resonator design, translation will just cause mode hopping, this becomes important on a frequency stabilized or mode locked laser if you have a precision lab application. Otherwise, most commercial lasers are not length stabilized in the least. There are equations and techniques for determining if you have a stable optical design - stable in this case meaning it will support lasing over a broad range of transverse and longitudinal modes. For examples see any text by A. E. Siegman or Koechner. If your library doesn't have any similar texts, find a book on microwave waveguides. It might aid you in visualizing what is going on.
Either an intracavity etalon or active stabilization systems are usually used on single frequency systems anyways, by either translating the mirror on piezos or by pulling on mirror supports with small electromagnets, or in the case of smaller units, heaters to change the cavity length on internal mirror tubes. An etalon is basically a precision flat glass plate in the lasing path between the mirrors, its length is changed by a oven and it acts as a mode filter.
Length stabilization to the 50 or 100 nm you might have expected to be needed would be gross overkill anyhow, and would be impossible to achieve in practice by stabilizing the resonator alone. Depending on the end use of the product, most lasers are simply built with a low expansion resonator of graphite composite or Invar, although in many products a simple aluminum block or L shape is used, a few rare cases use rods made of two different materials designed to compensate by one short high expansion rod moving the mirror mount in opposition to the main expansion. A small fraction of a millimeter is a more reasonable specification.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
The basic idea, that the laser can only work at the frequencies where an integral number of half waves fit in the cavity, is perfectly correct. The separation between adjacent modes is just 1/(2*L) where L is the cavity length in cm. From this we get the separation in 'wavenumbers'. One wavenumber is 30 GHz, so in more usual units it is just 30 GHz/(2*L). Or, to make it easy, in a 50 cm long laser the modes are 300 MHz apart. That is not very far optically.
The laser operates by some molecule, gas, ion in a crystal, etc. making a transition between two levels. But those levels are not perfectly 'sharp'; we say they are 'broadened'. The reason can be many things:
In any case no transition is *perfectly* sharp, the fact that it has a finite lifetime gives it a certain width, but this is not often the real limit, something else is usually more important.
These broadening mechanisms 'blur out' the line - we see optical gain over that *range* of frequencies, the gain bandwidth.
An example is carbon dioxide. The 'natural width' is very small, of order Hz. The Doppler width at 300 °K is about 70 MHz. The collision-broadened width increases about 7 MHz/Torr; so well below 10 Torr the width is Doppler-limited, ~70 MHz; above 10 Torr pressure broadened (e.g. ~700 MHz at 100 Torr).
If I take a typical HeNe laser it might 'blur' out over a GHz or so - **more** than that 300 MHz mode spacing - so there are *always* two or thee modes within the 'gain bandwidth' and it will always lase. For a glass laser there might be *thousands* of modes, because the glass gain is very wide indeed.
But there *are* cases that go the other way. For carbon dioxide, at low pressure, the line is Doppler-broadened and about 70 MHz wide, much **LESS** than that 300 MHz mode spacing. So short carbon dioxide lasers really do turn on and off as the cavity length changes, and you have to 'tune' the cavity length to get a mode inside the gain width. This mainly happens with short, gas lasers in the infrared.
For a *high pressure* CO2 laser at 760 Torr (1 atm), the line width is several GHz, much more than the mode spacing, so the effect disappears.
There are many ways to actually "see" the modes of a laser including the use of an instrument called a Scanning Fabry-Perot Interferometer (see the section: Scanning Fabry-Perot Interferometers). However, for a short tube with only 1 or 2 modes, it's quite straightforward to interpret what's going on from the output power and polarization alone. All that's needed is a photodiode and multimeter (or continuous reading laser power meter), and polarizing filter. (A lens from a pair of polarized Sun glasses or a photographic polarizing filter will do.) The power monitor can be set up in the output beam and the polarizing filter in the waste beam from the HR mirror. Alternatively, a non-polarizing beamsplitter can be used to provide the two beams. Adding a polarizing beamsplitter oriented so that it separates the two polarization orientations in one of the beams can simplify the interpretation of the polarization changes.
Changing the orientation of the polarizer will affect the amplitude of the intensity variations. For most red HeNe lasers, the longitudinal modes will generally remain at two fixed orthogonal orientations, with adjacent modes usually being orthogonal to each other. As the tube heats and the cavity length increases, the modes march along under the gain curve with those at one end disappearing and new ones appearing at the other end as described above. But for well behaved tubes, they don't flip polarization. When the polarizer is oriented at 45 degrees to the polarization axes of the tube, the reading will remain constant. When aligned with the polarization axes of the tube, the reading will fluctuate the most.
As a specific example, consider an HeNe laser tube with a mirror spacing of 120 mm (about 4.75 inches, one of the shortest commercially available laser tubes). This corresponds to a mode spacing of about 1.25 GHz - rather close to the FWHM of 1.5 to 1.6 GHz for the neon gain bandwidth. With this tube, at most 2 modes will be oscillating at any given time. When the output power and polarization is monitored while the tube is warming up, a very distinctive behavior will be observed. One might think that it should be a periodic variation in output power with a simple sinusoidal or similar characteristic. However, there will actually be two peaks for each cycle: A large one corresponding to when there is a single lasing mode at the center of the gain curve, and a smaller one when there are two modes symmetric around the center of the gain curve. For most tubes, the polarization of adjacent modes is orthogonal and will remain fixed with the mode. So, as the modes cycle under the gain curve successive large peaks will have opposite polarization. The small peaks will have equal components of both polarizations. Even though two modes are oscillating, the gain for each one is so much closer to the lasing threshold that their combined power is still lower than for the single mode at the peak of the gain curve. There may also be rather sudden changes in output power as modes on the tails of the gain curve come and go. However, for some tubes which are affectionately called "flippers", the polarization of the modes will tend to suddenly change orientation as they move through the gain curve. This should also be apparent when viewing the beam through a polarizing filter.
For more on these types of experiments along with typical plots, see the section: HeNe Laser Output Power Fluctuation During Warmup.
The photodiode (PD) must have a frequency response that extends beyond at least the longitudinal mode spacing of the laser. A fancy costly one may not be essential, only that the PD is quite small. One with a 1 GHz response is typically around 1 mm square, with the frequency response being roughly inversely proportional to area. Candidate PDs may turn up in all sorts of equipment, even old optical mice. The PD should be back-biased with a few volts to improve frequency response and set up to drive into a 50 ohm load terminating at the scope input. Basing the circuit on something like the Thorlabs DET10A would be perfect. (Search for this on the Thorlabs Web site. The spec sheet will have the circuit diagram.)
The first approach is to view the resulting mode beating on a fast oscilloscope. For a random polarized laser, a linear polarizer will be required in front of the PD oriented at 45 degrees to the principle polarization axes of the laser to force adjacent modes that are usually orthogonal to have the same polarization at the PD. The adjacent longitudinal modes will then produce a beat equal to their difference frequency. There will also be weaker beats from all other combinations of modes. Common HeNe lasers have a fundamental mode spacing of between 1.5 GHz (for a tiny 0.5 mW barcode scanner tube, around 10 cm between mirrors) and 161 MHz (for a 35 mW SP-127, around 95 cm between mirrors). Typical Longitudinal Mode Beat Waveforms of Melles Griot 05-LHP-151 HeNe Laser shows some snapshots during mode sweep. This laser is rated 5 mW with a mode spacing of 438 MHz (around 58 cm between mirrors). The waveforms were taken using a Thorlabs DET210 photodetector and my special edition laser-zapped Tektronix 2467 oscilloscope - formerly resident in the test department of a major laser manufacturer - evident from the 5 unsightly black blobs on the lower part of the screen where the CRT phosphor has been blown away by a high power pulsed laser! :) While the fundamental can usually be seen, information about any higher difference frequencies is hard to interpret. And even this relatively fast scope doesn't have much sensitivity beyond the 438 MHz fundamental. The screen shots are in no particular order in the montage other than to make the sequence somewhat pleasing. :) This is further complicated by higher order effects like mode pulling, which slightly shift the positions of the modes based on their location relative to the center of the neon gain curve. Thus, beyond confirming that the mode spacing is as expected, not much more can be easily determined and switching to the frequency domain will be more fruitful.
The output from the PD may also be applied to an RF spectrum analyzer, there will be significant power detected at the longitudinal mode spacing and its harmonics (hundreds of MHz or more) due to beating between longitudinal modes, as well as under 1 MHz (due to second order beats and mode pulling).
RF Spectra of Melles Griot 05-LHP-151 HeNe Laser During Mode Sweep shows the primary beat signal for the same laser head using a Thorlabs DET210 1 GHz silicon photodetector and an HP 8590L RF spectrum analyzer. (As with the scope, for a random polarized laser, a polarizer would need to be placed in front of the PD oriented at 45 degrees to the polarization axes to detect adjacent beats.) The center frequency is around 437 MHz and the span is 1 MHz. (Each box is 100 kHz.) (The spec'd value for the mode spacing of this laser is 438 MHz but it's possible the spectrum nalyzer is in need of calibration! Otherwise, complain to Melles Griot!) The sequence of screen shots show about half the full mode sweep cycle. Load the Movie Clip of RF Spectra of 5 mW HeNe Laser During Mode Sweep into Windows Media Player in repeat mode and it will look exactly as it does in real life. :) As can be seen, the spectrum goes from being nearly a pure single frequency at the longitudinal mode spacing, to a series of many peaks 10s of kHz apart to 4 peaks almost 200 kHz apart. The displayed width of each peak is much larger than it actually is, an artifact of the spectrum analyzer bandwidth setting.
If there were no mode pulling, the display would always look like the one in the upper left corner (or even narrower) - a single frequency. However, the individual modes move slightly compared to the cavity resonances, so the spectrum spreads out as a function of the position of the modes on the neon gain curve.
Interestingly, the display remains where there is a single narrow peak for longer than could be accounted for based on the normal speed with which the frequencies are changing. In fact, it's impossible to capture a situation where the peak is just slightly wider - it snaps from a FWHM of about 1/5 box (top left in composite photo) to approximately 1 box (top center) and vice-versa. Nothing in between ever apperas. This suggests that there is a self-locking process taking place, as mentioned in the previous section.
When set for a frequency range covering 0 to 200 kHz, peaks are present similar to what appears on the right side of those shown above. But a linear HeNe laser power supply had to be used to avoid seeing the ripple frequency and harmonics of the switchmode brick overlayed on the beats! There are multiple strong beats at around 874 MHz as well, 2 times the mode spacing. They vary a way similar way as the others. This makes sense since there are are 3 longitudinal modes oscillating most of the time, with 4 modes for a brief period during mode sweep. The spectrum analyzer also claims there are weak peaks at around 1,311 MHz and 1,748 MHz during most of the mode sweep cycle, not simply that period where the self mode-locking takes place. However, it's not clear where these originate, or if they are even real. To be direct, the one at 1,748 MHz would require 5 modes to produce a beat at 4 times the mode spacing of 437 MHz. But there are never 5 modes present, let alone for most of the cycle. Perhaps they are the sum of second-order beats. Or, they could simply be an artifact of the analyzer, perhaps leakage from an internal mixer.
Typical Longitudinal Mode Beat Waveforms of JDS Uniphase 1145P HeNe Laser shows scope display for a JDS Uniphase 1145P, with a mode spacing of 438 MHz (around 34 cm between mirrors). The additional complexity is due both to the lower beat frequencies (and thus better response of the scope) as well as the greater number of modes oscillating. An RF spectrum of this laser would have many more peaks closer together, but would look generally similar to that of the 5 mW laser.
Typical Longitudinal Mode Beat Waveforms of Melles Griot 05-LHP-121 HeNe Laser, with a mode spacing of 687 MHz (around 22 cm between mirrors). With only 3 longitudinal modes oscillating (and stressing the bandwidth capabilities of the poor scope), the display is a fairly clean sinewave. The only obvious difference during mode sweep is that the amplitude changes slightly. However, most of the time, it is a relatively clean sinewave and for a higher power healthier tube, the reduction in amplitude is not that great as in this example.
Here is a rough idea of what transverse modes might look like for a rectangular cavity:
O OO OOO Each 'O' represents
O OO O OO OOO a single sub-beam.
TEM00 TEM10 TEM01 TEM11 TEM21
I have only shown the rectangular case because that's the only one I could draw in ASCII!
Other (non-cartesian) patterns of modes will be produced depending on bore configuration, dimensions, and operating conditions. These may have TEMxy coordinates in cylindrical space (radial/angular), or a mixture of rectangular and cylindrical modes, or something else!
To achieve high power from a HeNe laser, the tube may be designed with a wider but shorter bore which results in transverse multimode output. Since these tubes can be smaller for a given output power, they may also be somewhat less expensive than a similar power TEM00 type. As a source of bright light - for laser shows, for example - such a laser may be acceptable. However, the lower beam quality makes them unsuitable for holography or most serious optical experimentation or research. An example of a high power multimode HeNe laser head is the Melles Griot 05-LHR-831 which has a rated output power of 25 mW. Compared to their 05-LHR-827 which is a 25 mW TEM00 laser head, the multimode laser is about 2/3rds of the length and runs on about 3/5ths of the operating voltage at lower current.
(Note that it is easy in principle to convert the output of a TEM00 laser into multimode by using a length of fiber-optic cable with lenses at each end to focus the beam into it and collimate the beam coming out. If the core diameter of the fiber is greater than that needed for the fiber itself to be single mode, then the result will be that multiple modes will propagate inside and the output will be multimode. To assure single mode propagation at 632.8 nm with the index of refraction of a typical glass fiber, a 4 um or smaller core is needed. The actual core diameter of the fiber will determine how many modes are actually generated. A core diameter of 10 um will result in a few modes while one of 125 um will produce dozens of modes. Why this would be desired is another matter.) However, all these modes will be exactly the same wavelength since they originate from a single TEM00 beam.
Sometimes, laser companies don't quite get it right either and a laser tube that is supposed to be TEM00 may actually be multi-transverse mode all the time or whenever it feels like it (e.g., after warmup). I have a 13.5 mW Aerotech tube that is supposed to be TEM00 but produces a beam that has an outer torus (doughnut shape) with a bright spot in the middle. I've also seen an apparently factory-new Uniphase green HeNe laser that produces a similar doughnut beam. Both of these are probably the result of one or both mirrors having a radius of curvature that is too short for the bore diameter. They may have been manufacturing goofups. Everyone can have a bad day, even if it results in a bunch of dud lasers. :) Good for us though. Everyone (well everyone who cares!) has seen a nice TEM00 HeNe laser. How many have one that does three wavelengths with different mode structures! :) (See the section: The Weird Three-Color PMS HeNe Laser Head.)
Note that the mode structure implies nothing about the polarization of the beam. Single mode (TEM00) and multimode lasers can be either linearly polarized or randomly polarized depending on the design and for the multimode case, each sub-mode can have its own polarization characteristics. HeNe (and other) lasers will be linearly polarized if there is a Brewster window or Brewster plate inside the cavity. The majority of HeNe laser tubes produce a TEM00 beam which has random polarization. For internal mirror tubes, linear polarization may be an extra cost option. External mirror HeNe lasers also generally produce a TEM00 beam but are linearly polarized since the ends of the tube are terminated with Brewster windows.
A fast photodiode (PD) and oscilloscope or RF spectrum analyzer can be used to view the frequencies associated with transverse modes. The transverse difference frequencies are very low compared to the longitudinal mode spacing so a really high speed PD isn't needed. A response of a few MHz should be sufficient. Typically less than 2 mm square silicon PD will have an adequate frequency response if back biased. But the modes do have to overlap on the detector so it may be necessary to spread the beam of a multimode HeNe laser using a lens. A polarized tube is best as it forces the modes to have the same polarization (a PD will not detect the difference frequencies for orthogonally polarized modes). But, adding a polarizer can partially compensate for this, though the polarization may drift with a randomly polarized laser.
For a tutorial on both longitudinal (axial) and spatial (transverse) modes, see An Investigation of the Cavity Modes of the HeNe Laser.
All of these are really somewhat equivalent and simply mean that more than one mode fits inside the available active mode volume.
Where there is access to the inside of the cavity (as with a one-Brewster tube), a laser that operates multimode can be forced to operate TEM00 with a stop (aperture) between the external mirror and tube-end. However, there will be a (possibly substantial) reduction is output power. Where both mirrors are external, it may be possible to substitute longer RoC mirrors to force TEM00 mode (again at the expense of some output power).
Note that a speck of dirt or dust on the inside of a mirror or window (if present), or damage to an optical surface, can result in a multi-transverse mode beam even if the bore and mirror parameters are correct for TEM00 operation. Unfortunately, convincing a bit of dust to move out of the way isn't always easy on the inside of an internal mirror HeNe laser tube! Yes, though not common, it can happen. This is one reason not to store tubes vertically. I've heard of people successfully using a Tesla (Oudin) coil to charge up the errant dust particle, causing it to just out of the way via electrostatic repulsion. Your mileage may vary. :)
The following actually applies to all lasers using Fabry-Perot cavities operating with multiple longitudinal modes. It was in response to the question: "Why does the coherence length of a HeNe laser tend to be about the same as the tube length?" The answer is that the coherence period is equal to the tube length but the useful coherence length is generally less (except for the special case above of a single mode). There is also more on coherence in the sections: Determining Coherence Length and Coherence Length and Holography.
(From: Mattias Pierrou.)
In a HeNe laser you typically have only a few (but more than one) longitudinal modes. These cavity modes must fulfill the standing-wave criterion which states that must be an integer number of half wavelengths between the mirrors. In the frequency domain this means that the 'distance' between two modes is delta nu = c/(2L), where L is the length of the laser.
The beat frequency between the modes gives rise to a periodic variation in the temporal coherence with period 2L/c, i.e. full coherence is obtained between two beams with a path-difference of an n*2L (n integer).
If you have only one frequency, the coherence length is infinite (that is, if you neglect the spectral width of this mode which otherwise limit the coherence length). If you have two modes, the coherence varies harmonically (like a sinus curve).
The more modes you have in the laser, the shorter is the regions (path-length differences) of good coherence, but the period is still the same.
You can try this by setting up a Michelson interferometer and start with equal arm-lengths which of course gives good coherence. Then increase the length of one arm until the visibility of the fringes disappear. This should occur for a path-difference slightly less than 2L (remember that the path-difference is twice the arm-length difference!). If there are only two modes is the laser the zero visibility of fringes should occur at exactly 2L. Now continue to increase the path-difference until you reach 4L (arm-length difference of 2L). You should again see the fringes clearly due to the restored coherence between the beams.
Some of these artifacts result from implementation but others are fundamental to the lasing process. The following apply to normal HeNe lasers (not those involving Zeeman splitting or something more exotic):
The closest to a true constant output would probably be a stabilized HeNe operating on a single longitudinal mode, often called "single frequency" with a highly filtered HeNe laser HV power supply. But even that will have some amplitude noise and there is no such thing as absolute single frequency. :)
Although the math can get to be rather hairy, one way of thinking of mode pulling is that the cavity bandwidth has a finite extent which depends primarily on the reflectivity of the mirrors and cavity length. So, if the net gain is greater slightly off to one side due to the position of the gain curve relative to the cavity resonance, the lasing line will be shifted in that direction.
When the laser beam hits a high speed photodetector like a photodiode, which is a non-linear (square law) device, in addition to the DC power term, there are the primary difference frequencies which are close to multiples of c/2L (but not exactly due to mode pulling), but also the differences of the difference frequencies - the second order intermodulation products - which will be at (relatively) low frequencies compared to c/2L. As the cavity length changes and the lasing modes drift across the gain curve, the mode pulling effect on each one varies slightly. But, small differences between large numbers can result in dramatic changes in these second order terms, rapidly rising and falling in frequency, and coming and going as modes drop off one end of the gain curve and appear at the other. The amplitude of the second order beat will be much lower than that of the primary beat but is still detectable with a spectrum analyzer, or in some cases with an audio amplifier.
For a HeNe laser, the range of second order frequencies is typically in the 1 kHz to 100s of kHz range while for a solid state laser it will be in the MHz to 10s or 100s of MHz range. Note that there will generally not be any beat in the range from 0 Hz to some minimum frequency (e.g., 1 kHz or so in the case of the HeNe laser) as would be expected when the modes are almost symmetric on either side of the gain curve where there would be very low second order frequencies. Apparently, a self mode-locking effect occurs to force these to be exactly zero frequency over a small range of mode positions. This behavior can easily be observed in the mode beat RF spectrum of a medium power (e.g., 5 mW) HeNe laser. See the next section.
For these second order beat frequencies to be present, the laser has to be able to oscillate on at least 3 longitudinal modes simultaneously. (With only 2 modes, there will be only a single difference frequency.) The Doppler-broadened gain curve of neon for the HeNe laser is about 1.5 GHz Full Width Half Maximum (FWHM) at 632.8 nm. To get 3 modes requires the modes to be less than about 500 MHz apart implying a c/2L tube length of about 30 cm or more - typical of a 5 mW or more (rated) HeNe laser. It should be polarized to force all modes to be of the same polarization - orthogonal polarizations do not mix in a photodetector. For a randomly polarized laser which typically produces alternating polarizations for adjacent modes, a longer tube length would be required to guarantee enough same-polarized modes and/or a polarizer at 45 degrees to the beam polarizations could be added (but this would cut the power to the photodiode by 50 percent or more).
This effect can be demonstrated using a medium length HeNe laser, high speed photodiode, and audio amplifier. Initially when the laser is turned on and is heating up and expanding the fastest, they may sound like clicks or pops or just non-random noise. As the expansion slows down, more distinct chirps and other interesting sounds will appear. The complexity of the symphony will also depend on the tube length and thus how many modes are oscillating.
A more precise way to look at mode pulling would be to monitor the beat frequencies produced by a high speed photodiode using an RF spectrum analyzer. By expanding the region around c/2L, the changes during mode sweep will be clearly evident. There will be smooth movement as well as sudden shifts corresponding to mode hops. I even did this by beating not a single laser, but two identical stabilized HeNe lasers against each-other. With two modes from each laser, there are then as many as 6 beat frequencies if 45 degree polarizers are placed in front of each laser and they are then combined in a non-polarizing beam-splitter. I'll leave analysis of this behavior as an exercise for the student. It is at first a bit confusing, but with some thought, makes perfect sense. Simply concentrating on the mode pulling of each laser's longitudinal mode where one laser was locked and the other was allowed to mode sweep yielded a shift of about 500 kHz.
(From: Roithner Lasertechnik (office@roithner-laser.com).)
You can "listen" to a single mode HeNe tube: Take an X-rated photodiode and an AC power amplifier - guide a small part of the HeNe laser beam to the photodiode (don't let it saturate!) - and listen to the "chirping oscillations" during warming up with a speaker. Hint: There are no birds inside the tube. ;-) But it sounds similar! Looks like sin(x)/x.
For the following, refer to HeNe Laser Mode Sweep: 200 mm (~8 inch) Cavity Length and click on the last slide to enter the animation. Though slightly shorter, the tube simulated in the animation will have mode sweep similar to that of the tubes in question, with no more than 3 modes present at any time. The red and blue modes denote the two orthogonal polarization axes of the tube called "p" and "s".
ID Make/Model Length Oscillation #1 Oscillation #2 ---------------------------------------------------------------- 1 LASOS LGR-7621 260 mm 270-280 kHz 490-510 kHz 2 " " " " " 293-307 kHz 436-443 kHz 3 Zygo 7701/2 235 mm 496-509 kHz 657-665 kHz 4 " " " " 398-420 kHz 594-624 kHz 5 Zygo Unknown 225 mm 200-360 kHz 200-360 kHz 6 " " " " 280-460 kHz 295-460 kHz 7 " " " " 544-595 kHz 570-612 kHz
Interestingly, both oscillation frequencies for the Zygo Unknown model tubes were similar, and in one case identical within the uncertainty of my measurements. This was also the only sample of those tubes that I had added "wedge" to the HR mirror since they lacked it for some reason. So possibly the slight back-reflections from the outer surface of the mirror substrate do have some minor effect.
Additional frequencies at around 1.95 MHz were seen on some of these tubes but they were at much lower levels, and possibly NOT correlated with the presence of 3 longitudinal modes. Another mystery.
Since the precise behavior depends on whether the p and s polarization is in the center, and polarization axes are locked to tube orientation, the current hypothesis is that these result from (or are at least affected by) mirror birefringence. Mirror birefringence results from mirror coating processes that are not perfectly symmetric due a crystalline structure or whatever. The result in an effective depth of reflection - and thus cavity length - that depends on orientation, with a typical variation of a fraction of 1 nm.
1 nm of cavity length change at 633 nm for a tube with a cavity length of 9 inches (around 225 mm) represents a frequency change of approximately 2 MHz. So, the observed shifts being in the 100s of kHz would be consistent with p and s modes being shifted off of normal c/2L cavity resonances by a fraction of 1 nm, resulting in some sort or low level mode competition. Perhaps the LASOS and Zygo tubes differ in whether 1 or both mirrors have significant birefringence. But exactly how this all works beyond the above statments would be total hand waving, as if it isn't already. ;-)
Not all coating processes exhibit birefringence but it's quite possible those used in most common HeNe laser tubes do since they tend to have fixed polarization axes. And there can also be birefringence resulting from other asymmetries in the tube construction, though they may be smaller. Known exceptions with very low birefringence are REO (based on their own claims and that where fixed polarization axes are required as with most stabilized HeNe lasers, REO tubes must be more complex to get around this "feature") and HP/Agilent Zeeman HeNe tubes (which have very peculiar mode sweep without their Zeeman magnets). Because REO and HP/Agilent tubes are so strange to begin with, it would not be possible to test them for this type of oscillation. Nor is there any easy way of determining if those tubes that don't exhibit the oscillations have lower or no birefringence. But there must be some asymmetries because they all have fixed polarization orientation.
In coming to the conclusion of mirror birefringence or something related, the following have been ruled out:
While I believe the same phenomenon is responsible for these oscillations in LASOS and Zygo tubes, there are differences. Notably with the Zygo tubes, the frequency changes by almost 2:1 compared to only 20 kHz or so for the LASOS tubes. On one sample, ranging from around 200 kHz to 360 kHz. And compared to the LASOS tube where the two frequencies differed by almost 2:1 (around 280 and 500 kHz) they are similar enough for the Zygo tube that they may in fact be the same, but alternately increasing and decreasing depending on whether a p or s mode is in the center.
I'm sticking with mirror birefringence for now. It may be a feature, not a bug. :) Specifically, that a higher birefringence coating on one or both mirrors is used to lock the polarization orientation, required to be able to use these tubes in stabilized HeNe lasers, though I'm not sure LASOS and Zygo are that sophisticated!
The test to reproduce this phenomenon is relatively simple and doesn't require fancy expensive instrumentation. The main components are a polarizer or polarizing beam-splitter, a biased silicon photodiode (almost any type will do using a circuit similar to what's in a Thorlabs DET210 or DET10A (Google will find it), and an oscilloscope (almost any type since the frequencies involved are under 1 MHz). Terminate the output in 2k to 5k ohms and set the scope for a vertical sensitivity of a few mV/division, AC-coupled, and a sweep speed of a few µs/division. The tubes should be red (633 nm), random polarized, and healthy. (I have no idea what happens with other color tubes!) They should be between 8 and 10 inches in overall length. Shorter tubes will have only 2 modes at most. Longer tubes with more modes may work, but that is left as an exercise for the student. :) Since the signal is so low level - around 10 mV even from a tube producing 3 or 4 mW, eliminating other sources of ripple and noise can be a challenge. Specifically, the current ripple from the HeNe laser power supply may overwhelm and bury the low level oscillation signal. A highly filtered linear supply is best though some modern switchmode "bricks" have decently low ripple. And an external ripple reducer can be added to these. (See the section: Reducing the Ripple and Noise in a HeNe Laser Power Supply.) The photodiode will also need to be blocked from room light as the variation in intensity from lamp ballasts will be picked up. Power everything up and look for a clear oscillation in the 100s of kHz range that comes and goes, at a decreasing rate as the tube warms up. Rotate the tube over 45 degrees to align for maximum signal. If you have an SFPI, use a non-polarizing beam-splitter to sample a portion of the tube's output for it to show how the oscillation correlates with the number of modes and their position. As noted above, only selected tubes exhibit this phenomenon and unfortunately, they are not the most commonly available ones. So your mileage - and success - may vary. :)
Mode locking is implemented by mounting one of the mirrors of the laser cavity on a piezo-electric or magnetic driver controlled by a feedback loop which phase locks it with respect to the optically sensed output beam.
Without mode locking, all the modes oscillate independently of one another with random phases. However, with the mode locked laser, all the cavity modes are forced to be in phase at one point within the cavity. The constructive interference at this point produces a short duration, high power pulse. Destructive interference produces a power of almost zero at all other points within the cavity. The mode locked pulse then bounces between the two laser mirrors, and a portion passes through the output coupler on each pass.
As a practical matter, you probably won't run into a mode locked HeNe laser at a garage sale!
The optical switch is the key to making this work. Either it has to actually deflect the beam or change its polarization so some other optical element will then reflect it out of the cavity. There are devices like Kerr cells and Pockels cells that could potentially be fast enough but they require high voltage to operate and may have excessive losses. Other approaches use an Acousto-Optic Modulator (AOM) as a deflector to divert the IC beam just enough to be reflected out of the cavity. However, AOMs don't operate instantaneously since they depend on a high frequency acoustic wave to propagate in their crystal. Even a high performance AOM would require 10s or even 100s of nanoseconds to switch states.
I'm sure a literature search would turn up some moldy papers describing the cavity dumping technique with a HeNe laser, but a "proof of concept" experiment was performed recently by Kevin Zheng, a very talented high school student, while at the Stony Brook Laser Teaching Center. See Kevin's Research Journal and Poster. While the performance wasn't that fabulous (forget any ideas of a hole burning HeNe laser!), just being able to get this work at all with relatively limited resources is impressive.
Note that while the frequency of the power variations in output power of a HeNe laser goes to beyond the GHz range, the following deals with what can be seen by human eyeballs with the aid of only a photodiode and multimeter or chart recorder (or a PC with a data aquisition module).
Here is a plot of the measured output power of mid-size (probably around 5 mW) HeNe laser tube from power-on to 20 minutes: Typical HeNe Laser Output Power Versus Time During Warmup (Plot courtesy of Ryan Haanappel). (Though typically, the output power starts out at a much higher value, often above 75 percent of its value after full warmup. But there are exceptions.) Many more plots can be found later in this section.
Examining the actual plot of output power versus time such as shown in HeNe Laser Output Power Fluctuation During Warmup (or careful observation of laser power meter readings) of a HeNe laser reveals that the curve is not simple but may include several types of behavior:
There is also usually an increase of power due to the heating of the laser tube (independent of thermal expansion effects) as well. But this may be only a fraction of the effects of alignment and is related to the increase in internal temperature and pressure.
In addition, especially with soft-seal tubes, there may be a power increase as the cathode, acting as a weak getter, removes contaminants from circulation that may have accumulated from a period of non-use. Or depending on how far gone they are, the power may go down as various parts outgas from the heat!
Depending on the particular laser, the initial output power can be very low even where the final output power exceeds rated power. Striking examples can be found in a non-negligibale percentage of long JDS Uniphase HeNe lasers like the 1145P. With these, a cold power of 1 or 2 mW for a laser that reaches 24+ mW after 20 minutes isn't unheard of. The dominant cause is a change in mirror alignment.
These effects are collectively called "mode sweep" or "mode cycling". Plotting and analyzing this behavior for various tubes under a variety of conditions can be quite fascinating. More in the next section.
Goofups in design and manufacturering can result in various combinations of these and other effects, though for the most part, HeNe laser companies generally know what they are doing! :) But see the plots below for both normal and abnormal behavior, and a link near the end of the section for a case study of one dramatic example of an "oops". :)
Most of these are from Melles Griot but the behavior of lasers from other manufacturers will be relatively similar, though the detailed shape of the individual polarized modes (more below) can differ significantly. The majority are healthy samples but a few show some rather dramatic peculiarities. There are also plots of a Coherent model 200 and Hewlett Packard model 5517A frequency stabilized HeNe lasers from power-on to locking.
Plots such as these are almost like fingerprints for HeNe lasers. Many of the physical characteristics of the laser can be determined by their appearance, and some features are unique to a particular model or manufacturer.
For most of the plots, my "instrumentation" consisted of a pair of $2 photodiode feeding two of the analog inputs of a DATAQ RS232 Chart Recorder Starter Kit attached to my ancient 486DX-75 Kiwi laptop running Win95. The photodiodes are reverse biased by 30 VDC from a +/-15 VDC power supply with a variable load resistor to set the calibration. The output is taken between the junction of the resistor and the photodiode, and power supply common (0 VDC). One channel is shown below:
R1 PD1
+15 VDC o----/\/\----|<|----+
100 |
/
\<----------+----+---o A/D Input (+/-10 V range)
/ R2 | |
\ 25K | /
R3 | C1 _|_ \ 200K ohms (Zin of A/D module)
-15 VDC o-----/\/\----------+ 1uF --- /
68K | \
| |
0 VDC o-------------------------------+----+----o A/D Ground
The values shown were selected for lasers with a maximum power output of around 1 mW. For higher power lasers, R2+R3 can be decreased or an attenuation filter can be placed in the beam. The later is preferred to avoid shifting the 0 mW reference level, and is what I did for most of the plots.
The capacitor across the input is intended to minimize noise pickup. The resulting filter rolls off at around 0.1 Hz. For reasonably well behaved HeNe lasers, even during the initial warmup period, this bandwidth is more than adequate. The sampling rate for all the plots is at least 10 Hz to allow for averaging since the A/D seems to have an uncertainty of about 2 LSBs.
In most cases, the two photodiodes are positioned at the outputs of a Polarizing Beam-Splitter (PBS) cube and the laser tube is oriented so that they are aligned with its natural polarization axes.
For monitoring power from the waste beam (which is much lower), a dedicated beam sampler assembly was constructed, which along with a photodiode preamp, enabled power levels as low as a few uW to fully utilize the 20 V p-p range of the A/D.
Some of the plots have been acquired with the same photodiodes, but feeding a dual channel preamp and also a summer tp compute the total power without requiring a separate photodiode channel. (Of course, for this to be meaningful, the photodiodes and premaps have to be set up so the two channels have equal gain.) The premap results in lower noise in the plots expecially for low power lasers.
And as of 2011, I've "upgraded" to USB versions of the DATAQ device (DI-158U and DI-145) and laptops that are only 10 years old. :) The original DATAQ RS232 module died and the Kiwi laptop doesn't have USB and is falling apart (though is still usable). The PDs are now each attached to a general purpose trans-impedance amp rather than the simple bias network. [See the section: Sam's Photodiode Preamp 1 (SG-PP1).] And in addition to channels 1 and 2, the outputs also feed a pair of resistors and a pot to adjust balance to channel 3 so their sum (which would usually be total power) is always present.
Although some of these plots aren't as nicely annotated as the one above, zero power is near the bottom of the plot so relative power variations can still be easily seen (who cares about absolute power anyhow!) and the time/division is indicated. The plots are arranged by increasing laser tube length.
For the following, "Total" means all the power in the beam; "Polarized" means a polarizing beam-splitter is used to separate the two orthogonally polarized modes with either one or both plotted. (This is Only done for random polarized lasers.) The scale factor for the "polarized" plot has been adjusted so that the peak amplitude is approximately the same as for the "total" plot for ease of viewing. However, it should be understood, that the sum of the power in the two orthogonal polarizations must add up to the total power. All are red (632.8 nm) HeNe lasers unless otherwise noted.
Note: I have "edited" (doctored?) some of the plots to clean up unsightly randomness and other blemishes, mostly due excessive electrical noise at low optical power levels. However, the important features are unchanged.
Plot of Melles Griot 05-LHR-007 HeNe Laser Tube During Warmup (Polarized). This is the same tube but with a non-polarizing beamsplitter followed by orthogonal polarizing filters inserted in the beam. The orientation of the polarizing filters is adjusted for minimum transmission when its mode is not present since as can be seen, the power actually goes to 0 mW for about half the period of each polarized mode. Alternate similar height peaks on the total power plot correspond to the same mode polarization. A careful examination will confirm that they actually alternate very slightly in amplitude due to minor variations in gain as a function of polarization. (I have adjusted the scale factors to make the plot looks similar.) The reason why the peak spacing on the two plots differs is that the tube was likely not quite at the same temperature when each run was started.
Plot of Melles Griot 05-LHR-640 HeNe Laser Tube During Warmup (Polarized). This is the same tube with the polarized modes separately plotted. Similar comments apply for this tube as for the 05-LHR-007, above.
However, the dramatic variation in mode amplitude over the course of warmup is an artifact of the way that data is being collected for this run and a peculiarity of the tube that doesn't noticeably affect its useful output. Rather than using the output beam, the P and S Modes are taken from the waste beam leaking through the HR mirror at the back of the laser. The Total Power (Waste) is then simply the sum (in an op-amp) of the modes. Compare this to the Total Power (Output) curve, which was measured from the main beam. The cause of the rear beam power variation is interference from multiple internal reflections in the HR mirror glass - between the HR coated inner surface and the uncoated outer surface. The result is a weak Fabry-Perot etalon which varies the effective reflectance of the HR mirror. It doesn't take much: A change from 99.975% to 99.95% would double the waste beam power - from about 15 uW to 30 uW. The 15 uW lost from the main beam power of about 1 mW is almost undetectable on the plot. The HR mirror glass is apparently not wedged on these tubes so the surfaces are very parallel. And indeed there was no ghost beam next to the waste beam as would be the case if wedge was present. The cause was confirmed by putting a dab of 5 minute Epoxy on the outer surface of the mirror. The Epoxy is smooth and clear enough to pass sufficient power for the photodiodes (though it is reduced). But the Epoxy surface is lumpy enough to greatly reduce the power variation. Why? The glass and Epoxy are fairly closely index matched so that the dominant reflection is no longer from the planar glass surface but from the lumpy surface of the Epoxy. There is minimal reflection directly back along the optical axis and thus minimal etalon effect resulting in a reduction of power variation from nearly 100 percent to under 10 percent. Using Norland 65 UV cure optical cement to glue an angled plate to the HR mirror reduced the ripples even more as shown in Plot of Siemens LGR-7641 HeNe Laser Tube With Variable Waste Beam Power During Warmup (Corrected).
More on this phenomenon can be found in the section: Power Variations Due to Lack of Mirror Substrate Wedge which explains the cause in more detail and additional tests that were performed on this specific tube.
Plot of Melles Griot 05-LHR-151 HeNe Laser Head During Warmup (Polarized). This is the same laser head but with the two orthogonal polarizitions separated (as described for the shorter tubes, above) and oriented for maximum variation ("ripple"). They are plotted separately to reduce clutter. Since there are always modes of both polarization present regardless of polarizer orientation, the output power in doesn't go to zero as with the shorter laser but their ripple is almost perfectly complementary. As expected, the size of the fluctuations in each polarization - 5 to 10 percent - is more in line with the total power behavior of a laser with only 2 or 3 modes. Even this amplitude seems remarkable given the almost perfectly smooth behavior of the total (randomly polarized) power. If the plots are examined very carefully, it will be noted that their envelopes are not identical - there is a very subtle slow variation over the course of the warmup period. This may be attributed to a small rotation of the polarization axes as the tube expands. With some samples of these lasers, it can be much more dramatic including polarization flips whenever it feels like it. But such behavior is still considered normal since for a random polarized laser, only the total power really matters, not any peculiar gyrations the modes may go through.
Plot of Melles Griot 05-LYR-173 HeNe Laser Head During Warmup (Polarized). The same laser with a polarizing filter in the beam. The fluctuations are larger as expected both because of the fewer modes in the polarized beam, and the lower gain of the 594.1 nm lasing line.
Since the laser head has optics to separate the modes with orthogonal polarization, the raw beam already varies by more than 2:1 in output power without any additional polarizer. Yes, that is the actual spread - the vertical scale hasn't been stretched! The actual HeNe laser tube inside is a specially selected Melles Griot 05-LHR-120, which by itself would have a normal mode sweep with a small ripple. From a cold start to lock takes about 20 minutes.
Plot of Coherent Model 200 Stabilized HeNe Laser Head Near End of Warmup. This plot zooms in on the last two cycles. Notice that there is a slight distortion on the rising part of the second cycle in the plot. That is probably when the active feedback is switched on. Before then, the heater is simply running at a constant current to bring the tube up to operating temperature. It only takes less than one full additional cycle to achieve lock. The amplitude is then quite stable (uncertainty of less than 0.5 percent on the plot), but the frequency stability which is d(power)*slope(frequency/power), will be under 0.125 percent of the mode spacing of around 750 MHz, so less than 1 MHz.
The actual beat frequency is shown for the last few cycles and after locking in both these plots. This is the actual measured frequency captured along with the F1 and F2 modes, and total output power. (Showing the frequency plot earlier would be a mess.) The beat only appears for a small percentage of the mode cycles with some variation during the time it is present.
The warmup and locking algorithm is partially responsible for the distorted nature of these plots compared to those for "normal" unstabilized HeNe lasers or even other common stabilized HeNe lasers due to the periodic pauses and switching from heating to cooling that may occur. However, the peculiar shape of the mode cycles themselves is due to the fact that these are not linearly polarized modes as with all the previous lasers. Rather, they are Zeeman-split modes distorted by a magnetic field and include (mostly) components differing in frequency by at most a few MHz, rather than the normal longitudinal mode spacing, which is 1.2 GHz for this laser.
Much more on these Zeeman-split HeNe lasers can be found in the section starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers and Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser. For a discussion specifically with respect to characteristics and causes of the strangely shaped plots, see the section: HP/Agilent 5517 Mode Behavior.
However, near the very end of the warmup period (measured in terms of mode cycles, not time) something very interesting occurs: The tube seems to have reverted to being well behaved! This only happens when the tube is approaching thermal equilibrium where each complete mode cycle is taking over 90 seconds. There are perhaps 3 or 4 beyond what is on the plot but the tube temperature is so close to its final value that any disturbance like moving near the laser head will disrupt the sequence. This behavior is consistent from run to run. The cause is unknown, nor is it known whether the tube would continue to behave if stabilization was attempted. But it might since the operating temperature will be somewhat above the natural point of thermal equilibrium.
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head During First Part of Warmup is a closeup of the mode variations when flipping. The shapes are nearly identical from the start of warmup until the transition to normal behavior. Also note that the frequency of the mode cycles for a flipper is double that of a normal tube - each mode would normally be what resulted from tracing the continuous curve and not taking the discontinuities as is evident in Plot of "Flipper" Aerotech OEM1R HeNe Laser Head During First Part of Warmup (Combined). So following red-blue-red, etc., ignoring the green lines. And Plot of "Flipper" Aerotech OEM1R HeNe Laser Head at Transition to Normal Behavior (Combined) is a closeup of the point where flipping ceases. Note that the "envelope" of the mode plot is virtually unchanged at this point but the green transitions have disappeared. At the transition point, the period of a full mode sweep cycle is about 80 seconds. There are then an additional 10 full cycles (only 4 or 5 are shown) requiring about an hour until thermal equilibrium. There is more on flippers in the sectoin: HeNe Mode Flipper Observations.
These have all been high quality HeNe lasers and except as noted, have relatively predictable mode performance. For information and plots for a really ill-mannered beast, see the section: Far East HeNe Laser Tubes.
The primary reason that the output power in any give longitudinal mode doesn't vary in a nice smooth (Gaussian) manner is due to mode competition. If not for mode competition, the gain would not saturate and be the same for all modes. Everyone would thus trace out the envelope of the neon gain curve. However, since the lasing modes are actually competing for a limited resource - the atoms in the upper lasing state - whenever there are more than one mode present, they have to be nice and share. This is most dramatic when only 2 or 3 modes are present since each one has a large fraction of the total output power. With those, the shapes of the envelopes of the polarized output power curves can be decidedly non-Gaussian. And for Zeeman-split lasers, downright weird. But once the various regions are understood - where there are 1, 2, 3, or more modes competing - then the resulting shapes make more sense:
1 mode: The output power will change smoothly during mode sweep roughly following the profile of the Gaussian neon gain curve (minus the lasing threshold). The only way a real laser could be single mode throughout mode sweep would be either for the cavity to be around 10 cm or less (in which case lasing may cease entirely for a part of mode sweep) or for there to be an additional means of forcing SLM operation (such as an etalon inside the cavity). But slightly longer tubes will operate with a Single mode over a portion of mode sweep with 2 modes for the remainder.
Plot of Mode Sweep of Typical 1 mW Random Polarized HeNe Laser Tube shows the appearance for a Melles Griot 05-LHR-007, the shortest modern laser tube I'm aware of. Over approximately 50 percent of the mode sweep cycle, it is pure single mode with power sharing during the remainder.
2 modes: When a second mode appears, it will start eating into the power of the first mode. Where the modes are balanced on either side of the neon gain curve, their power will be equal. Between these 2 points, they will share power. The total output power may remain relatively constant or increase slightly when equal (usually up to around 20 percent). Tubes with a cavity length of 12 to 16 cm will operate with 1 or 2 modes.
3 modes: When a third mode appears, it will start eating into the power of the other two. The relative and total power will depend on their location on the neon gain curve and is at the very least, not intuitively predictable. :) Tubes with a cavity length of 20 to 25 cm will operate with 2 or 3 modes during mode sweep.
Plot of Mode Sweep of Typical 3 mW Random Polarized HeNe Laser Tube shows the appearance for a Spectra-Physics 088 (same as the Melles Griot 05-LHR-088) used in the SP-117/A/B/C and Melles Griot 05-STP-901 stabilized lasers. It is similar a common barcode scanner tube. At the peaks of the polarized modes (minimum for total power), there are 2 modes. Where the polarized modes cross, there are 3 modes. The overall shape of the mode sweep depends on many factors including the exact length of the cavity which determines where it switches from 2 to 3 modes.
4 or more modes: The same general rules apply, but since the contribution of each mode is smaller, the effects of mode competition are also smaller and more difficult to see and interpret.
What is really being depicted in the gain curve is a combination of a curve derived from what's called the "natural line width of neon" which is homogeneously broadened, and the distribution of atomic velocities of excited neon atoms as they translate into a distribution of Doppler shifts in optical frequency.
Ignoring Special Relativity (which is acceptable for the velocities involved), the Doppler shift in optical frequency is equal to the relative velocity of the excited atom divided by the speed of light multiplied by the optical frequency or:
va
Δf = -f0 * ----
c
Where:
At any temperature above absolute zero, all atoms are in motion and have a probabilistic distribution of velocities (speed and direction), which all contribute to the Doppler broadening. For a Fabry-Perot (linear) cavity, the photons traveling in either direction "experience" the relative speed of the excited atoms. Stimulated emission will only occur when the Doppler-shifted energy of the photon matches a possible lasing transition of an excited atom. The width of the Doppler broadening is directly proportional to optical frequency, but it is also affected by other factors including temperature and pressure, since these impact the distribution of atomic velocities. The shape of the Doppler broadening curve is then the result of the aggregate of the motion of all the atoms available for stimulated emission. And the width of the inhomogeneously broadened neon gain curve is the width due to homogeneous line-width of neon plus the inhomogeneous Doppler broadening. Since they are added like independent noise souces using the square-root of the sum of the squares, the increase in neon gain bandwidth due to the homogeneous line-width is quite small (just over 5 percent even at 3,391 nm). Thus, the change is close to 1/5th even if the homogeneous part is ignored.
Assuming the FWHM value of 1.6 GHz for the entire inhomogeneously Doppler-broadened gain bandwidth of the common red wavelength of 632.8 nm, at the mid-IR wavelength of 3,391 nm it is only 315 MHz. And at the green wavelength of 543.5 nm it is about 1.86 GHz. The optical frequency difference between cavity modes (c/2L) is only dependent on cavity length and the speed of light. Thus, the number of lasing modes possible for a given cavity length decreases as the gain bandwidth becomes narrower at longer wavelengths.
Note that the lasing modes themselves will have a very narrow bandwidth - possibly as small as 5 kHz or even lower for a laser operating with a single mode. At that point, physical vibrations, laser power supply noise, and other external effects are the limiting factors, not the theoretical minimum bandwidth for the HeNe laser which is under 1 Hz! (Schawlow-Townes linewidth). I originally thought that finding values for the bandwidth of commercial HeNe lasers would be straightforward, but it seems to be near impossible. The only specifications I am aware of from a laser manufacturer are in Laboratory for Science brochures. The best is for their model 220 Ultra Stable HeNe laser, which lists 5 kHz over a period of 1 second. But the value for the type of HeNe laser tube that used to be found in barcode scanners may not be all that much greater if it is mounted to minimize vibrations and driven with a well filtered HeNe laser power supply.
One would expect that with the much smaller gain bandwidth at 3,391 nm of 315 MHz, there would be fewer longitudinal modes oscillating compared to 632.8 nm. Or equivalently, a laser tube would need to be much longer for the same number of modes to fit within the FWHM of 315 MHz. But because of the very high gain at 3,391 nm, the lasing threshold will be lower and thus the effective gain bandwidth of the neon gain curve is going to be wider. I do not know by how much, but with a potential gain over 40 times that of the 632.8 nm transition, it could be very significant. There might even be more modes than at 632.8 nm.
Due to the longer wavelength, mode sweep for a laser tube at 3,391 nm will have a complete cycle that is over 5 times as long as one at 632.8 nm. These same numbers would apply to mode competition at 3,391 nm interfering and stealing power from a 632.8 nm laser.
For very low gain wavelengths like green (543.5 nm) and yellow (594.1 nm) - which may have 1/10th the gain or less compared to the common red (632.8 nm) wavelength, the lasing threshold will be far higher on the roughly Gaussian shaped gain curve, where it is narrower. So, while the FWHM of the neon gain curve may be slightly wider at these wavelengths, fewer modes will be oscillating because of the narrowing due to the higher lasing threshold. However, until the lasing threshold approaches the peak of the gain curve, the reduction in number of modes won't be that dramatic. And every effort is made to eliminate losses inside the cavity for these low gain lasers, so in fact, the lasing threshold may not even get that high relative to the peak during the expected life of the laser.
For very high gain wavelengths, the reverse will happen. There's really only one - the mid-IR transition at 3,391 nm which behaves more like a solid state laser with a gain over 40 times that of 632.8 nm. The lasing threshold will be much lower on the gain curve extending the useful region well out into the tails of the distribution. In this situation, many more modes could end up oscillating than would be accounted for by the much narrower FWHM of the neon gain curve of 315 MHz - roughly 1/5th the width compared to 632.8 nm. If calculations based solely on this small gain bandwidth were valid, a 75 cm 3,391 nm laser would have a similar number of longitudinal modes to a 14 cm 632.8 nm HeNe where there are only 1 or 2 active modes at any given time. Since 3,391 nm lasers much shorter than 75 cm are commercially available and don't have dramatic variations in output power with mode sweep, this must not be the case. For example, REO has one with a cavity length of less than 50 cm and maximum power variation of 5 percent, which implies that there are several longitudinal modes always present.
Here are results so far:
With my $2 SFPI modified for 1,5XX nm operation (replaced PD with cut-open germanium transistor photodiode), I have confirmed that the modes of the 05-LIR-151 are also similar in number and appearance to those of a 20 to 25 cm 632.8 nm laser. There are 2 modes most of the time with 3 appearing briefly when a mode is close to the center of the gain curve.
Common internal mirror HeNe laser tubes include a specification called "Mode Cycling Percent" or something similar. This relates to the amount of intensity variation resulting from changes in longitudinal modes due to thermal expansion. Typical values range from 20 percent for a small (e.g., 6 inch, 1 mW) tube to 2 percent or less for a long (e.g., 15 inch, 10 mW) tube. These take place over the course of a few seconds or minutes and are very obvious using any sort of laser power meter or optical sensor. Even the unaided eyeball may detect a 20 percent change. The more modes that can be active simulataneously, the closer those that are active can be to the same output power on the gain curve. Very short tubes or those with low gain (other wavelengths than 632.8 nm or due to age/use or poor design) may vary widely in output intensity or even cycle on and off due to mode cycling. (Note that since the polarization for each mode may be different, reflecting the beam of one of these HeNe lasers from a non-metallic reflective surface (which acts somewhat as a polarizaer) can result in a large variation in brightness as the dominant polarization changes orientation over time.) Trading off between tube size and mode cycling intensity variations is one reason that HeNe tubes with otherwise similar power output and beam characteristics come in various lengths.
There are also stabilized HeNe lasers which use optical feedback to maintain the output intensity with a less than 1 percent variation. (They usually also have a frequency stabilized mode but can't do both at the same time.) An alternative to doing it in the laser is to have an external AO modulator or other type of variable attenuator in a feedback loop monitoring optical output power. See the next section for more info.
Short term changes in intensity may result from power supply ripple and would thus be at the frequency related to the power line or inverter. These can be minimized with careful power supply design.
Intensity variations at 100s of MHz or GHz rates result from beats between the various longitudinal modes that may be simultaneously active in the cavity. For most common applications, these can be ignored since they will be removed by typical sensor systems unless designed specifically to respond to these high beat frequencies.
Also see the section: Amplitude Noise.
If you have, say, $5,000 to spend, you can buy a red (633 nm) HeNe laser that actually produces a single frequency with specifications guaranteed stable for days and that don't change over a wide temperature range. While the operation of such a HeNe laser is basically the same as the one in a barcode scanner (and in fact may use the identical model HeNe laser tube!), several additional enhancements are needed to eliminate mode sweep and select a single output frequency. Simply constructing the laser cavity of low thermal expansion materials isn't enough when dealing with distances on the order of a fraction of a wavelength of light! Active feedback is needed. The most common implementation of these lasers starts with a short red (632.8 nm) tube that can only oscillate on at most 3 longitudinal modes. (For technical reasons, stabilized lasers at the other common visible and IR HeNe wavelengths are more difficult to implement and are much less common. More on this below.) It then adds optical feedback to keep them in a fixed location on the HeNe gain curve by precisely adjusting the distance between the mirrors over a range of about 1/2 the lasing wavelength. This is most often done with a heating coil (inside or outside the tube), but a PieZo Transducer (PZT, an expensive version of the beeper element in a digital watch) may also be used. The PZT reduces the time for the system to stabilize to a few seconds, compared to up to 30 minutes for the heater. But, for a laser that will be left on continuously, this probably doesn't matter. Some lasers use a means of cooling in addition to the heater like a piezo fan, probably to allow the laser to run stably over a wider temperature range. And a few including the Melles Griot 05-STP-909/910/911/912 (originally based on the Aerotech Syncrolase 100) use a miniature RF induction heater surrounding the HR mirror mount to control only its length, not that of the entire tube. With direct heating of such a small mass, the response is quite fast. This also makes for a more compact package than a full tube heater.
Many schemes work well and it's amazing how dirt simple these really are considering their hefty price tags! It's easy to build perfectly usable systems from a common surplus HeNe laser tube and a few common junk parts.
The common ones are listed below:
Type of Stabilization Technique Variation Precision ----------------------------------------------------------------------- Normal (multimode) HeNe laser --- --- Single mode without stabilization 1.5 GHz 3x10-6 Single mode amplitude stabilization 10 MHz 2x10-8 Lamb dip stabilization 5 MHz 1x10-8 Gain peak stabilization 5 MHz 1x10-8 Dual mode polarization stabilization 1 MHz 2x10-9 Second order beat stabilization 200 kHz 4x10-10 Zeeman beat frequency stabilization 100 kHz 2x10-10 External reference (iodine) cell stabilization <5 kHz 1x10-11 External reference (F-P resonator) stabilization <1 Hz 1x10-14
Note that an etalon inside the laser cavity could also be used to select out a single longitudinal mode. For high power lasers which would require long tubes supporting many modes, this would be needed with both the overall mirror spacing and etalon being feedback controlled. But for low power lasers (e.g. 1 to 3 mW), the use of a short tube to limit the number of modes in conjunction with basic feedback control is a much less complex lower cost approach.
Stabilized lasers (or anything that needs to be regulated to some precision) can be classified as two types. The technique is "intrinsic" - basically derived from an internal reference - if what is used to regulate the device is a fundamental property of its construction - the laser physics in this case. It is "extrinsic" if some external reference is used. Most commercial stabilized HeNe lasers are of the first type since they exploit the known and essentially fixed frequency/wavelength and shape of the neon gain curve in the E/M spectrum. Additional techniques may be used to further reduce the uncertainty.
Most common commercial stabilized HeNe lasers are red at 633 nm, partially because of all the available HeNe wavelengths with a single frequency output power of less than 2 mW. Systems like this are both relatively easy to implement and generally useful for a wide range of applications. The approaches usually fall into one of two subclasses:
Some inexpensive (this is relative!) stabilized HeNe lasers only use a single mode for frequency locking. When on the slope, this will be reasonably stable after warmup once the output power has reached equilibrium.
Most commercial stabilized HeNe lasers for general laboratory applications are of type (1) and operate with 2 orthogonal modes for frequency stabilization, though some use 1 mode for intensity stabilization (or can select between them with a switch). (Regardless, only a single longitudinal mode - thus a single optical frequency - may be allowed to exit the laser, the other being blocked with a polarizer.) These include the Coherent 200, Spectra-Physics 117 and 117A (and the identical Melles Griot 05-STP-901), many from Zygo, and various models from REO, Thorlabs, and others. For example, in the Melles Griot 05-STP-901 frequency and intensity stabilized HeNe lasers (no longer in production), the laser cavity permits a pair of orthogonal polarized longitudinal modes to be active and can provide very precise control by straddling these on either side of the gain curve (frequency stabilized mode) or a single longitudinal mode that is also used for the output on one side of the gain curve (intensity stabilized mode). Those from other companies are generally similar.
All the interferometry lasers manufactured by Agilent (formerly Hewlett Packard), Excel, and one model from Zygo (the 7705) are of type (2). While lasers from Teletrac/Axsys, Optodyne, Renishaw, and others are type (1).
And there are hybrid approaches. For example, the Zygo 7701/2/12/14 lasers generate and lock a single frequency via dual mode stabilization, But it is split into two using an Acoutso-Optic Modulator (AOM) rather than the Zeeman effect.
For some photos of the (quite simple) Zeeman split stabilized HeNe tube used in the Hewlett-Packard 5517 laser head, see the Laser Equipment Gallery (Version 1.86 or higher) under "Assorted Helium-Neon Lasers". And for more information on these lasers, see the sections starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers.
It isn't really possible to convert an inexpensive HeNe tube that operates on many longitudinal modes into a single frequency laser. Adding temperature control could reduce the tendency for mode hopping or polarization changes, and the addition of powerful magnets can force a polarized beam. But, selecting out a single longitudinal mode would be difficult without access to the inside of the tube. However, if the HeNe tube is short enough that the mode spacing exceeds about 1/2 the Doppler-broadened gain bandwidth for neon (about 1.5 GHz), it will oscillate on at most 2 longitudinal modes at any given time and these will each be linearly polarized and usually orthogonal to each-other. Then, stabilization is possible using very simple hardware. In fact, even if the mode spacing is a bit smaller - down to 500 or 600 MHz - then only 2 modes will be present most of the time but 3 may pop up if one is close to the center of the gain curve. This, too, is an acceptable situation since the tube can be stabilized with the modes straddling the gain curve and then only 2 modes will oscillate. For intensity stabilization, 4 modes may even be permitted. Note that while the modes of a random polarized and linearly polarized tube are similar (except for polarization), a random polarized tube is desirable to be able to use a tube that supports 2 modes with the benefits they provide, while being able to eliminate the second mode from the output. Also see the section: Inexpensive Home-Built Frequency or Intensity Stabilized HeNe Laser for details.
It may be possible with a combination of what can be done externally, as well as control of discharge current, to force a situation where gain is adequate for only 1 or 2 modes even for a longer tube. Whether this could ever be a reliable long term approach for a HeNe tube that normally oscillates in many longitudinal modes is questionable. What I don't think will have much success are optical approaches such as feeding light back in through the output mirror. Doing this would likely have the exact opposite of the desired effect but may work in special cases (it's called injection locking and is used with great success for other applications).
Coherent, Melles Griot, Spectra-Physics, and others will sell you a small stand-alone stabilized HeNe laser for $5,000 or so and Agilant (HP) and others have interferometers and other similar equipment which includes this type of laser (and are even more expensive!). Other manufacturers includ Zygo, Teletrac, Nikon, Micro-g Solutions, SIOS, NEOARK, and REO. The lab lasers that I've seen all use short HeNe tubes with thermal feedback control of the resonator length and all operate at the red HeNe wavelength (632.8xxxxxx nm to 8 or more significant figures). One typical system is described in the section: Coherent Model 200 Single Frequency HeNe Laser. The Spectra-Physics model 117A/118A laser actually uses a lowly SP088-2 tube similar to those in older grocery store barcode checkout scanners as its heart. A tube like this is visible in the Spectra-Physics Model 117 OEM Stabilized HeNe Laser Assembly. However, some do employ a custom tube with the heater inside to greatly speed up response and reduce heat dissipation to the outside. A stabilized HeNe laser for green or other color visible HeNe wavelength or one of the IR wavelengths is also possible using the same techniques.
As noted above, the actual stabilization mechanism for the general purpose stabilized lasers may be the ratio of amplitudes of two longitudinal modes (which is better for frequency stabilization) or the amplitude of one mode (which is better for intensity stabilization). These are usually stable to within a few parts in 109. However, the frequency drift when intensity stabilized is still not much - probably less than 1 part in 108. Output power variation may be 0.2 percent if intensity stabilized and 1 percent if frequency stabilized. Some allow either method to be selected via a switch, as well as providing for an external tuning input to vary the frequency over several hundred MHz. (However, due to the thermal control most often used, the response time is not exactly fast.)
The Zeeman split interferometer lasers may lock the difference frequency to a crystal clock, though most seem to use the basic polarized modes for stabilization, with the Zeeman beat used only as the reference for the interferometer. See the sections starting with: Hewlett-Packard/Agilent Stabilized HeNe Lasers. A few do lock the Zeeman frequency to a PLL. One of these was the Laboratory for Science Model 220. (Laboratory for Science is now out of business.) See the section Laboratory for Science Stabilized HeNe Lasers. Another example is the NEORK Model 262 Transverse Zeeman Laser.
More sophisticated schemes with even better precision and lower long term drift may lock to the "Lamb Dip" at the center of the neon gain curve or to one of the hyperfine absorption lines of an iodine vapor other type of gas cell, achieving stabilities on the order of 1 part in 1010 or even better. See, the section: Iodine Stabilized HeNe Lasers.
Due to the performance, simplicity, reliability, and relatively low cost of stabilized HeNe lasers, they are still often the preferred frequency reference for many applications. As noted, a typical system might go for $5,000. While this may seem high, it is small compared to many other technologies. The cost is not the result of expensive components or complex manufacturing, but more to the relatively limited number of units produced. If stabilized HeNe lasers were as popular as laser pointers, they would probably cost under $100.
Much additional information and specific details of actual systems may be found in the chapter: Commercial Stabilized HeNe Lasers.
All are basic mode stabilized HeNe lasers. The 5517 is a Zeeman-split laser but the stabilization is mode-based.
The redesign in each case must have cost a fortune. Since none of these lasers had many adjustments in their analog designs, ease of manufacturing is probably not the justification. And there is no need for preventive maintenance as components age - lasers like this will run for years on-end without any adjustments. Cost of components is also not a viable excuse as jelly bean op-amps and other common parts are adequate for any of these lasers. Nor do any require an external computer interface like more complex lasers.
However, one obvious benefit from the company's point of view is serviceability, or lack thereof for anyone not supported by the manufacturer. The new designs are virtually impossible to troubleshoot and repair without detailed service information, and possibly support software. Unless the problem is obvious like a broken wire or blown fuse, attempting to find an electronic fault in these high density surface mount PCBs controlled by firmware programs is just about impossible. And Marketing can promote the "benefits" of digital technology, as bogus as that may be here. If anything, the additional electrical noise from digital signals is a detriment. Digital has to be better, right? :)
An ISHL operating on the common red (633 nm) wavelength typically consists of a HeNe laser tube with one or two Brewster windows, a gas cell containing iodine at low pressure, and at least one external mirror on a PieZo Transducer (PZT) for fine cavity length control. With this arrangement, the iodine cell is installed inside the laser cavity to benefit from the high intra-cavity circulating power as the absorption sensitivity in the vicinity of 633 nm is very low. The sensitivity is further boosted since it is a function of the difference between the gain and cavity losses. However, when operating on the green (543.5 nm) wavelength, the cell can be external despite the lower power generally achievable with green, because the sensitivity is higher. But there may be ways to boost the sensitivity using an internal mirror HeNe laser tube operating single longitudinal mode with 2 mW or more output power.
The basic principles of operation for an ISHL are rather straightforward: The iodine (or actually I2) has a very complex absorption spectra with hundreds of absorption lines. A very small portion of it is shown in: Iodine Absorption Spectrum Near 532 nm. By dithering the laser cavity length via a PZT, a lock-in amplifier (also known as a phase sensitive detector or synchronous demodulator) can maintain the wavelength at the very center of any selected absorption peak (or dip, depending on your point of view!). The challenging part is to be able to reliably select a specific absorption line to lock to. So, although locking to a given line is fairly simple, the overall electronics can get to be quite complex if automatic line selection is desired, though nowadays, an embedded microcomputer does the line selection and most everything else.
Here are some photos of an iodine stabilized laser based on the classic NIST (National Institute of Standards and Technology, formerly the National Bureau of Standards) design originally described in the paper: Howard P. Layer, "A Portable Iodine Stabilized Helium-Neon Laser, "IEEE Trans. on Inst. and Meas, IM-29, pp358-361, 1980. The photos are actually of two different samples of the NIST design. The first one is of a complete laser head while the others are of a physically similar resonator only where it's easier to see the individual components.
Although the laser head does not presently lase, I am hopeful that it will someday. The discharge color of the HeNe laser tube is normal and there is no visible brown crud in the bore indicating that it should be healthy. The iodine cell still has iodine in it based on its response to a green (532 nm) DPSS laser pointer beam. This thing has probably been sitting on a warehouse for years, if not decades (next to the lost Ark), so the non-lasing condition isn't exactly a surprise. However, there seemed to be some type of contamination inside one of the B-windows. So, it may require a replacement 05-LHB-290. The original NIST paper stated that the reflectivity of the OC mirror was only 93 percent, presumably to force single longitudinal mode operation by reducing gain, but this also dramatically reduces output power. However, measurements of the OC mirror in this laser show a reflectivity between 99% and 99.3%, more typical of a normal HeNe laser. Then alignment or some other means could be used to force SLM. It would seem like a better solution to force SLM would be to add a PZT-controlled etalon that tracks cavity length tuning. Then, the output power would be close to the maximum available from the tube - 5 to 10 times higher than this design produces. But I've not seen that anywhere. The paper also states that the laser tube and cavity are 20 and 30 cm long, respectively. On my samples, they are at least 25 and 35 cm. And, their laser tube appears to not be a Melles Griot 05-LHB-290. So perhaps the original prototype was not identical to the versions later reproduced by Frazier (and others), though it's quite clear that Frazier copied nearly every aspect of the laser design down to the controller-in-a-scope and its front panel layout and labeling. ;-)
Putting the iodine cell inside the cavity takes advantage of the high circulating power (approximately 100 times the output power for an OC mirror reflectivity of 99%) and the difference between the gain and lasing threshold based on losses. However, there should be ways to achieve a sufficient boost in sensitivity using an internal mirror HeNe laser tube stabilized to output a single longitudinal mode. With a dual mode polarization stabilized setup, it could be as much as around 2 mW, though the tuning range for the iodine absorption lines may be limited. Two methods that could boost the sensitivity would be to use an external high finesse etalon similar to the cavity of a Scanning Fabry-Perot Interferometer (SFPI) and put the iodine cell inside its cavity. That would similarly boost the circulating power by the inverse of the mirror transmission and iodine cell window losses. The SFPI would need to track the tuning of the laser tube and scan in the vicinity of the lasing line to locate the iodine absorption locations. So, some fancy control would be required. Another simpler approach might be to use a custom long iodine cell or one with a large aperture to allow for multiple passes through it, or a pair of internal mirrors to accomplish this, which would be even better.
More on ISHLs:
And multi-wavelength iodine stabilized HeNe laser have also been built. See: "A Tunable Iodine Stabilized He-Ne Laser at Wavelengths 543 nm, 605 nm, and 612 nm", J. Hu, T. Ahola, K. Riski, and E. Ikonen, Digest of the 1998 Conference on Precision Electromagnetic Measurements, July 6-10, 1998, IEEE Cat. No. 98CH36254. This one used the tube from a PMS/REO LSTP-1010 5 color tunable HeNe laser with a pair of PieZo Transducers (PZTs) behind the rear mirror (tuning prism) and a lock-in amplifier for feedback control. For these wavelengths, the iodine cell can be outside the cavity, but notice that the red wavelength, 633 nm, is not included. Multi-Wavelength Iodine Stabilized HeNe Laser
The only modification to the laser itself was to add a pair of PZT cylinders between the back of the tuning prism and its mount so that the cavity length could be tuned electronically. The iodine cell and laser power detector are external to the cavity.
What I found curious with this (as well as the NIST laser) is that the laser cavity is way too long to restrict the laser to single longitudinal mode operation as would be required for the system to be useful. The authors of the paper don't appear to address this, nor have I found it mentioned elsewhere.
So I performed a quick experiment using a REO tunable HeNe laser. As expected, with the power in each wavelength maximized, there are multiple longitudinal modes oscillating. And also as expected, there would be a range of the mode sweep cycle where the output would be pure SLM if either the Wavelength Selector or Transverse adjustment were set so as to reduce output power below a specific value, differing for each wavelength as follows:
Wavelength Maximum SLM Power
-------------------------------------
632.8 nm 56 µW
611.9 nm 97 µW
604.6 nm 169 µW
594.1 nm 320 µW
543.5 nm 240 µW
These values are very approximate and don't necessarily mean that the laser can be tuned over any significant range and remain SLM as is required to be useful to lock to an I2 line - that would require even lower power. The 543.5 nm SLM power may be somewhat higher than 240 µW but that's as much as my laser wanted to put out at the time. It would appear that 594.1 nm would be a very usable wavelength at higher power, but apparently the authors did not find a suitable I2 absorption transition at that wavelength, or at 632.8 nm either. The latter is rather strange as we know that there are more than a half dozen suitable I2 lines within the normal 632.8 nm gain bandwidth to which the Frazier and NIST lasers can be locked.
The NIST paper describes an ISHL that has an OC reflectance of only 93 percent to raise the lasing threshold and force SLM operation. (Common red HeNe lasers of this size typically have an OC reflectance of 99 percent.) This option is not available for the multi-wavelength ISHL since the authors used a stock PMS/REO tunable laser tube which has a relatively high reflectance (much greater than 99 percent) internal OC. But single mode operation can be forced by misaligning the mirrors or rotating the tube with respect to the iodine cell so the Brewster windows orientations don't line up. And for the yellow and green wavelengths, the effective reflectivity must be higher than something like 99.95% to lase at all.
Assuming this analysis with respect to usable SLM power to be correct, it does explain why direct locked ISHLs typically have very low power. To achieve higher power, some companies offer what is known as an "offset-locked iodine stabilized HeNe laser". With these, a normal SLM HeNe laser with a typical output power of 1 to 2 mW (at 632.8 nm) has its optical frequency phase locked to the lower power ISHL. Implementation is actually easier than it sounds but nonetheless is left as an exercise for the motivated student. ;-)
While it might be assumed that exactly the same approach could be taken for "other color" lasers, this turns out not usually be the case. The principle reason is that the nice behavior that has been counted on to keep the lasers well mannered may not be present. So while the tube will still have a pair of orthogonal axes of polarization, adjacent longitudinal modes will not necessarily be orthogonal and/or even have a consistent relative polarization - they may flip like a banshee.
So, where it is desired to implement a stabilized HeNe laser at other wavelengths (visible or IR), the polarization may be the primary issue, but there are a number of other complications including differences in the neon gain bandwidth and generally much lower power:
However, FWHM or other definition of the gain bandwidth has to be adjusted depending on the actual gain and losses of the tube. For example, the mid-IR 3,391 nm line has a gain over 40 times that of the 633 nm red line, so the lasing threshold will be much lower effectively widening the gain curve. And the gain at 544 nm (green) is roughly 1/20th of that at 633 nm.
Various tricks may be used to stabilize HeNe lasers at other wavelengths but in general, it's often not as easy! Also see the section: A Stabilized HeNe Laser at 1,523 nm.
HeNe lasers are particularly sensitive to back-reflections, though no damage is ever likely to occur. However, the instantaneous polarization state and amplitude of the longitudinal modes will be affected. These effects may not be noticeable for common HeNe lasers without using fancy instruments since they occur at nanosecond time scales. For these lasers, average output power from the laser will not be affected but for random-polarized HeNe lasers, the intensity of any portion of the beam passed through optics that affect polarization may fluctuate dramatically.
Suffice it so say that one should avoid back-reflections to HeNe lasers, but especially for stabilized HeNes. Even the reflection from a piece of fresh transparent tape in the beam may cause the laser to lose lock. What happens is that when a mode swaps polarization, the controller will attempt to relock but that may require several seconds or longer. Some lasers may indicate their unhappiness by flashing the READY or LOCK indicators, or in the case of the Spectra-Physics 117A and Melles Griot 05-STP-901, making clicking noises. :)
The best way avoid such instabilities is to arrange the setup so that there are no back-reflections. ;-) The HP/Agilent interferometer configurations used for metrology applications shown in Most Common Hewlett Packard/Agilent Interferometers are nearly perfect. With their high quality Polarizing Beam-Splitters (PBSs), there are virtually no back-reflections directly to the laser. The next best solution where this is unavoidable is to add an optical isolator at the output of the laser. A Faraday isolator is best but very expensive. For a beam with a single linear polarization, adding a PBS cube and Quarter Wave Plate (QWP) will redirect any reflections downstream that have not had a polarization change off to the side. In many applications, this is sufficient. But in some cases, two Faraday isolators in series are needed to fully tame the HeNe beast. ;-)
But what we want here is the opposite - how the input power is affected by the laser output, which I'll call the "Reverse Incremental Efficiency" or RIE. In other words, compare the input power with the laser operating normally and with the output suppressed, for example, by misaligning a mirror. For a HeNe laser, would there be a detectable change in input power if this were done? With a normal constant current HeNe laser power supply, the result should be a change in tube voltage. If for want of a better term, the "reverse slope efficiency" were 100 percent, then "spoiling" the beam of a 1 mW laser should result in a reduction of 1 mW in power consumed by the tube.
So I did an experiment using a high-mileage JDS Uniphase 1145P laser head with a Melles Griot 05-LPL-915 power supply set at 6.5 mA. The lasing was spoiled using a tube-type Nylon mirror adjuster pushing on the OC mirror mount to kill lasing in a totally reversible manner. Measurements were made while the laser was warming up and outputting 12 mW and then once fully warmed up and outputting 19 mW. The results were rather intriguing:
ΔPo ΔVt ΔPt RIE
------------------------------------
12 mW 4.1 V 26.65 mW 45.0%
19 mW 5.2 V 33.80 mW 56.2%
ΔPo is the output power, ΔVt is the change in tube voltage from 0 mW to ΔPo, and ΔPt is the corresponding change in the tube's power consumption.
At first, my measurements were made with a DMM with only 4 digits of resolution and it appeared as though the the RIE might be exactly 50 percent, which could have had some cosmic significance. :) But it wasn't to be. With the full 5 digits of a Fluke 87, while the RIE isn't far from 50 percent, it isn't 50.00000000%. Too bad. But what this does say is that the incremental efficiency of getting coherent photons out the front of a HeNe laser once it's running at the normal voltage and current and outputting near rated power is order of 50 percent, not a miniscule value like that 0.3 percent! Note that the results depend on whether the laser is running at reduced and full power. If this had been some obscure effect of mechanical stress on the discharge voltage, then the change in tube voltage would be about the same at both output powers. And pushing on the mirror mount beyond where lasing ceases has no effect on tube voltage. At least until it breaks off. :)
To further confirm that this is a true lasing effect, I repeated the experiment with a Melles Griot 05-LHB-570 one-Brewster laser where lasing could be suppressed simply by poking something in the cavity between the tube and OC mirror:
ΔPo ΔVt ΔPt RIE
-------------------------------------
2.55 mW 0.9 V 5.85 mW 43.6%
Even at this much lower output power, the RIE is still fairly high, though uncertainly is greater due to the much lower power and corresponding change in tube voltage.
Then, I did multiple sample points while a like-new 1145P head was warming up:
ΔPo ΔVt ΔPt RIE
------------------------------------
6 mW 4.8 V 32.1 mW 21.0%
10 mW 5.7 V 37.1 mW 27.0%
14 mW 5.8 V 37.7 mW 37.1%
17 mW 6.0 V 39.0 mW 43.6%
19 mW 6.3 V 41.0 mW 46.3%
21 mW 6.3 V 41.0 mW 51.3%
24 mW 6.4 V 41.6 mw 58.0%
Just when I thought this was making some sense, these data appear to show an unexpected very non-linear relationship. Most of the voltage change occurs between 0 mW a few mW, and it is then nearly constant, perhaps due to the gain saturating. There is still significant uncertainty as the measured values for both absolute tube voltage and the voltage difference fluctuate over time.
And finally on the same head when fully warmed up with a stable 24 mW of output power undisturbed, with controlled misalignment of the OC mirror to generate a few intermediate values:
ΔPo ΔVt ΔPt RIE
------------------------------------
1 mW 1.3 V 8.5 mW 12.0%
6 mW 4.4 V 26.8 mW 21.0%
9 mW 5.2 V 34.5 mW 26.1%
24 mW 6.3 V 41.0 mW 58.6%
"" mW 6.5 V 42.3 mW 56.8%
"" mW 6.8 V 44.2 mW 54.3%
These are generally similar to the measurements during warmup. The last two full power entries reflect the variation that may be present even when the laser is in thermal equilibrium. Even so, there can be small changes in the longitudinal mode positions and thus relative efficiencies of the lasing lines or something. :)
A reference to this phenomenon can be found on page 38 of an old NASA report: An Experimental and Theoretical Investigation of Striations in a HeNe Laser. (If this link should decay, simply search for the title.) I'm sure there are many more in depth studies but locating them is left as an exercise for the student. :)
In particular:
See the section: On-Line Introduction to Lasers for the current status and on-line links to these courses, and additional CORD LEOT modules and other courses relevant to the theory, construction, and power supplies for these and other types of lasers.
As of Summer 2012, MEOS has stopped development and support of these kits. (In fact, the company doesn't seem to exist anymore, at least not doing anything remotely related to photonics.) However the creator of the experiments and author of the manuals has been acquired by LD Didactic (Leybold) and is continuing this line, which is represented by Klinger in US. The updated manuals are now available for free download at the Leybold Ld Didactic Web Site.
Several modules would be of particular interest for HeNe lasers. Unfortunately, the on-line manuals (in PDF format) have disappeared from the MEOS Web site. But I have found and archived most of MEOS manuals. (The ones above may be slightly different.)
Bellows Bellows
/\/\/\ Discharge tube with external electrodes /\/\/\
|| \________________________________________________/ ||
|| | | | | | | ||===> Laser
|| ___ __|_|________________|_|______________|_|__ ||===> Beam
|| / || | | | \ ||
\/\/\/ || | o | \/\/\/
Adjustable || +-----------o RF exciter o----------+ Adjustable
totally || partially
reflecting ||<-- to vacuum system reflecting
mirror mirror
Early HeNe lasers were also quite large and unwieldy in comparison to modern devices. A laser such as the one depicted above was over 1 meter in length but could only produce about 1 mW of optical beam power! The associated RF exciter was as large as a microwave oven. With adjustable mirrors and a tendency to lose helium via diffusion under the electrodes, they were a finicky piece of laboratory apparatus with a lifetime measured in hundreds of operating hours.
In comparison, a modern 1 mW internal mirror HeNe laser tube can be less than 150 mm (6 inches) in total length, may be powered by a solid state inverter the size of half a stick of butter, and will last more than 20,000 hours without any maintenance or a noticeable change in its performance characteristics.
Older brochures from several manufacturers of HeNe lasers can be found at Vintage Lasers and Accessories Brochures and Manuals
This fabulous ASCII rendition of a typical small HeNe laser tube should make everything perfectly clear. :-)
____________________________________________
/ _________________ \
Anode |\ Helium+neon, 2-5 Torr Cathode can ^ \ |
.-.---' \.--------------------------------------. '-'---.-. Main
<---| |:::: :======================================: :::::| |===> beam
'-'-+-. /'--------------------------------------' .-.-+-'-'
Totally | |/ Glass capillary ^ _________________/ | | Partially
reflecting | \____________________________________________/ | reflecting
mirror | | mirror
| Rb + - |
+---------/\/\---------o 1.2 to 3 kVDC o-----------+
The main beam may emerge from either end of the tube depending on its design, not necessarily the cathode-end as shown. (For most applications it doesn't matter. However, when mounted in a laser head, it makes sense to put the anode and high voltage at the opposite end from the output aperture both for safety and to minimize the wiring length.) A much lower power beam will likely emerge from the opposite end if it isn't covered - the 'totally reflecting' mirror or 'High Reflector' (HR) doesn't quite have 100 percent reflectivity (though it is close - usually better than 99.9%). Where both mirrors are uncovered, you can tell which end the beam will come from without powering the tube by observing the surfaces of the mirrors - the output-end or 'Output Coupler' (OC) mirror will be Anti-Reflection (AR) coated like a camera or binocular lens. The central portion (at least) of its surface will have a dark coloration (probably blue or violet) and may even appear to vanish unless viewed at an oblique angle.
For a diagram with a little more artistic merit, see: Typical HeNe Laser Tube Structure and Connections. And, for a diagram of a complete laser head: Typical HeNe Laser Head (Courtesy of Melles Griot) and actual structure in X-ray View of Melles Griot 05-LHR-911 HeNe Laser Head. For some photos, see: Typical Small to Medium Size Melles Griot HeNe Laser Tubes. The ratings are guaranteed output power. These tubes may produce much more when new. Another type of construction that is relatively common is shown in the Hughes Style HeNe Laser Tube and a photo in Hughes 3227-HPC HeNe Laser Tube. These are probably disappearing though as Melles Griot bought the Hughes HeNe laser operation and is converting most to their own design but many still show up on the surplus market, including newer ones with the Melles Griot label. Another design that is similar is the NEC Style HeNe Laser Tube. Some specifications for various NEC HeNe lasers can be found at SOC under "Gas Lasers". Most common higher quality HeNe tubes will be basically similar to one of these two designs though details may vary considerably. Most have an outer glass envelope but a few, notably some of those from PMS/REO, may be nearly all metal (probably Kovar but with an aluminum liner which is the actual cathode) with glasswork similar to that of Huches or NEC at the anode-end.
Tubes up to at least 35 mW are similar in design but proportionally larger, require higher voltage and possibly slightly higher current. and of course, will be more expensive.
The discharge at this end produces little heat or damage due to sputtering.
This is a 'cold' cathode - there is no need to heat it (like the ones in the electron guns of a CRT) for proper operation and no warmup period is required before the tube can be started. The discharge is distributed over the entire area of the can thereby spreading the heat and minimizing damage due to sputtering which results from positive ion bombardment. For this reason, although the laser may appear to work (in fact, starting tends to be easier) a HeNe tube should not be run with reverse polarity for any length of time (e.g., more than a minute or so, preferably a lot less) since damage to the anode (now acting as a cathode) and its mirror would likely result. See the section: Damage to Mirror Coatings of Internal Mirror Laser Tubes.
The can-shaped structure is also called a 'hollow cathode' for obvious physical reasons - it is a tube electrode that is large in diameter and hollow like a piece of pipe - and because the plasma discharge flows inside of it. It operates in the abnormal glow current density gas discharge region (should you care). The surface of the cathode can is also not pure aluminum as it appears, but is processed with a very thin layer of oxide which eventually gets depleted, and this is the main determination of tube life. Hollow cathodes are usually used where a tube needs lots of slow moving electrons to excite the gas. They are currently used mainly in HeNe lasers but have been applied to other types of gas lasers having modest current requirements.
Very old HeNe lasers (and some others, old and new, like argon ion) use a heated filament which also acts as the cathode instead of the cold cathode design. This structure can be much smaller than the cold cathode but the added complexities of manufacture, the additional power supply, and the need for a warmup period have delicated it only to those applications where there is no other choice. See the section: Strange High Power HeNe Laser for an example of this technology.
A very few, very tiny HeNe laser tubes, use a small ring-shaped cathode made of either zirconium (expensive) or aluminum. These were likely designed for special applications, presumably requiring very small size or fast turn-on response (due to the reduced capacitance). The examples of these HeNe tubes I've seen are about 5" long by 1/2" in diameter. Life expectancy using the aluminum version (at least) is probably quite limited due to sputtering (since the electrode is very close to the bore, which promotes this due to the increased field gradient).
On some (mostly larger) HeNe tubes, the bore may be ground (but not polished) on the outside, inside, or both:
Note that since the frosting process is done chemically (hydrofluoric acid etch?), the bore will become marginally wider and care must be taken that this doesn't result in multimode (non-TEM00) operation if it goes too far!
One recent paper on such a laser is: "High power HeNe laser with flat discharge tube", Yi-Ming Ling, Journal of Physics D: Applied Physics, Volume 39, Issue 9, pp. 1781-1785, May, 2006.
Abstract:"A high-power HeNe laser with a flat discharge tube has been realized. Its output power can be enhanced by increasing the transverse size of the discharge tube. This high-power flat HeNe laser tube of 1.4 m discharge length can achieve above 180 mW of output power at a wavelength of 632.8 nm. Its optimum discharge parameters and the gain characteristics are investigated experimentally. The experiments indicate that the optimum current increases with decreasing total gas pressure. But the increase in the optimum current is almost independent of the gas mixture ratio. The increase in the gain coefficient at the axis of the discharge tube with discharge current is not obvious. The boost in laser output power is mainly caused by the expansion of the lasing gain region. To achieve the higher output power, four of the laser tubes mentioned above are placed into one laser box. The laser beams are coupled into a quartz optic fibre and the output power from the end of the optic fibre can reach above 480 mW. This high-power HeNe laser has been used in a clinical application, photodynamic therapy (PDT) of cancer, and its effective rate is above 90% in 183 clinical cases. The structure, characteristics and applications of this high-power flat HeNe laser are introduced and discussed in this paper."
Wow! 480 mW at 633 nm (even if it is an ugly beam)! :)
HeNe tubes used in barcode scanners tend to use a simpler (possibly cheaper) design. Some typical examples are the Uniphase 098-1 HeNe Laser Tube and Siemens LGR-7641S HeNe Laser Tube. A typical small barcode scanner tube is shown in Uniphase HeNe Laser Tube with External Lens. That negative lens is used in the barcode application to expand the beam at a faster rate than with the bare tube. A second positive lens about 4 inches away is then used to recollimate the beam. (In many cases, the required curvature is built into the output mirror but not here. The lens was removed by soaking the end of the tube in acetone overnight.)
CAUTION: While most modern HeNe tubes use the mirror mounts for the high voltage connections, there are exceptions and older tubes may have unusual arrangements where the anode is just a wire fused into the glass and/or the cathode has a terminal separate from the mirror mount at that end of the tube. Miswiring can result in tube damage even if the laser appears to work normally. See the section: Identifying Connections to Unmarked HeNe Tube or Laser Head if in doubt.
The getter material is then available to chemically combine with residual oxygen and other unwanted gas molecules that may result from imperfect vacuum pumps and contamination on the tube's glass and metal structures (e.g., from the surface as well as in fine cracks and other nooks and crannies). It will also mop up any intruder molecules that may diffuse or leak through the walls of the tube during its life. Helium and neon are noble gases - they ignore the getter and the getter ignores them. :-)
Should the getter spot (if visible) turn to a milky white or red powdery appearance, it is exhausted and the tube is probably no longer functional.
If you had grown up during the vacuum tube age, the getter would be familiar to you since nearly all radio and TV tubes had very visible silvery getters (and CRTs still do).
The getter electrode can be seen in photos of a Typical Small to Medium Size Melles Griot HeNe Laser Tubes. However, no getter spots are visible. I have found many tubes where there is a getter electrode present but the getter spot is undetectable. Some modern getters use a zirconium based material which is colorless as opposed to old style getters which were barium based with a very visible spot. (Really long life HeNe tubes like those from Hewlett-Packard actually use a zirconium cathode. They are rated for a 100,000 hour life!) It's also possible that the getter was included as insurance and never activated. I suppose that modern vacuum systems and processing methods are so good and hard-seal tubes don't really leak, so there is not as much need for a getter as there used to be.
Note that a high mileage HeNe (or other gas discharge) tube may exhibit metallic deposits (usually) near electrodes which look similar to the getter spot. However, these are due to sputtering and won't change appearance if there is a leak! The tube is usually near death at this point in any case.
The mirrors used in lasers are a bit more sophisticated than your bathroom variety:
However, note that for a sufficiently long HeNe tube (one with high enough gain), it would be possible to use a pair of freshly coated or protected aluminum mirrors though performance would be pretty terrible. And, getting a useful beam out of such a laser would be difficult because aluminized mirrors tend to not be even partially transparent! I've gotten a 10" long HeNe tube with an internal HR and Brewster window at the other end to lase using the aluminized mirror from a barcode scanner - just barely. But the first HeNe laser would not have been possible without dielectric mirrors despite its length since the wide bore resulted in very low gain.
I have also come across HeNe laser output mirrors with a slight *negative* RoC - they are convex rather than concave with respect to inside the cavity. At first I thought these were a mistake, coating the wrong sides of the mirror glass or something like that. But the slightly convex curvature does indeed result in a stable resonator configuration and actually has a slightly lower divergence than a similar concave mirror when tested in my one-Brewster external mirror HeNe laser (though I can't tell if this might also have been more due to the curvature of the outer surface). I have since found a sample of a HeNe laser tube (probably from a barcode scanner) that had such a mirror, though it's certainly not a common configuration.
You may be able to tell which type you have by looking at a reflection off of the inner surfaces of the mirrors at each end (assuming the one at the non-output end is not painted or covered). Assuming the outer surfaces are flat, a concave mirror will reduce the size of the reflection very slightly compared to a planar mirror. If wedge is present, the reflections from the front and back (interior) surface of the mirror will shift apart as you move further away (though this may be tough to see on the Anti Reflection (AR) coated output mirror since the reflection from the AR coated surface will be very weak). See the section: Ghost Beams From HeNe Laser Tubes.
To further complicate matters, the front (outer) surface of the mirror at the output-end of the tube may be ground to a (slight) convex or concave shape as well resulting in either a positive lens which aids in beam collimation or a negative lens with increases the divergence.
Since the reflection peaks at a single wavelength, this type of mirror actually appears quite transparent to other wavelengths of light. For example, for common HeNe laser tubes, the mirrors transmit blue light quite readily and appear blue when looking down the bore of an UNPOWERED (!!) tube. Blue light from the electrical discharge will also pass out of the mirrors as a diffuse glow when running. No, you don't have a blue HeNe laser!
Also see the section: Mirror Reflectances for Some Typical HeNe Lasers.
There should be no reason for the alignment to have changed unless you whacked the tube - it was set at the factory. But due to the way some tubes are constructed, it can creep with multiple thermal cycles over the years. If you suspect an alignment problem, it is easy to check. Then, you can decide if attempting an adjustment is worth the risks. See the section: Checking and Correcting Mirror alignment of Internal Mirror Laser Tubes.
However, long high power tubes (i.e., 20 mW and up) may require fixtures to maintain mirror alignment even when the mirrors are internal. For example, they may need to be securely mounted in their mating laser head cylinders. Such tubes will not be stable by themselves because thermal expansion will result in enough change in alignment to significantly alter beam power - even to the extent of extinguishing the beam entirely at times! There may even be a 'This Side Up' indication (not related to the orientation for linearly polarized tubes) on the HeNe tube or laser head as gravity affects this as well (the alignment and thus power, not the gas, electrons, ions, or light!) and can significantly affect operation. I do not know if this latter sort of behavior is common or only likely with tubes that are marginal in some way. But, there will always be at least a small change in power with orientation for longer tubes.
There is a slight benefit to having the output coupler mirror at the anode-end of the tube due to the typical long-radius hemispherical cavity configuration. With the bore running almost to the mirror mount, more of the mode volume is inside the bore and thus the gain will be slightly higher. But the difference is only really significant for "other color" HeNe laser tubes which have very low gain and these are more likely to use anode-end output configuration.
However, barcode scanner tubes (typically 5 to 6 inches but even some longer ones) often have anode output. It is not known whether this is simply a preference by the scanner manufacturer out of convenience, or for this reason.
The HRs in all cases showed greater than 99.9 percent reflectivity (T less than 0.001 - virtually undetectable on my fabulous meter).
Due to the behavior of the photodiode at low light levels, the absolute precision of the readings is somewhat questionable. However, the relative reflectivities of these mirrors is probably reasonably accurate. Note, in particular, the high R of 99.4% for the very long external mirror laser compared to the low R of 97.7% (T of 2.3%) for a shorter internal mirror tube. I expect that in addition to the length of the bore, part of this difference is due to the absence of Brewster window losses in the internal mirror tube resulting in a higher gain so that more energy can be extracted via the OC on each pass.
Mirrors for non-red HeNe lasers must be of even higher quality due to the lower gain on the other spectral lines. The OC will also have higher reflectivity for this reason. For green HeNe tubes (which have the lowest gain of all the visible HeNe wavelengths), the transmission is about 1/10th that of a similar length red tube. For example, the reflectivity of a typical green HeNe tube OC is 99.92 to 99.95 percent (.08 to .05 percent transmission) at 543.5 nm.
Notes on making these measurements:
Ion Beam Sputtered (IBS) coatings have a much higher packing density, so they withstand the (i.e., 450 °C) frit sealing temperatures and don't even shift 1 nm. Nowadays, everything is hard sealed, with the exception of the high-end (long precision) Brewster tubes. Hard-sealing a BK-7 window puts a lot of stress on it, and that just isn't acceptable on the high-Q tubes. So, those get fused silica windows optically contacted (lapped and polished surfaces that are vacuum tight.) (In fact, with this type of seal, if there is no adhesive present, the windows can be easily removed from your dead, leaky, or up-to-air tubes by gently heating the Brewster stem and window with a heat gun. The window can then be popped off with your thumb nail!)
As noted elsewhere, the term "random" here really doesn't mean that the polarization is necessarily jumping around to totally arbitrary orientations. In fact, such behavior would be rather unusual, though lasers from some manufacturers do exhibit somewhat erratic mode flipping. It really just means that nothing special is done to control the polarization. The typical HeNe laser will lase on several longitudinal modes (how many will depend on tube length of the resonator). For red (633 nm) HeNe lasers, adjacent modes will generally have orthogonal polarizations. Each of the modes will vary their relative intensities periodically over time as the laser cavity changes length due to thermal expansion.
So "Random polarized" is actually a poor choice of terminology since most random polarized HeNe lasers do NOT exhibit random and/or high speed fluctuations in polarization. Rather there are generally two polarization axes that are orthogonal to each-other and the output power slowly varies between the two axes as the tube cavity length changes due to termperature and the lasing modes drift under the neon gain curve. (In fact, the tube used in a stabilized HeNe laser usually needs to be random polarized!). For most common tubes, the orientation of these polarization axes is determined by slight asymmetries in the tube geometry and/or mirror coatings and is fixed for the life of the tube. Sometimes this is deliberate but most often it is simply as a result of manufacturing tolerances. Lasers from Melles Griot, JDS Uniphase, and Siemens/LASOS generally have well behaved polarization axes. While they are normally not labeled, their orientation can be determined relatively easily.
However, where there is virtually no asymmetry, the polarization could jump around, rotate, or perform some other acrobatics. ;-) It may be virtually impossible to even determine what is going on. The tubes used in Research Electro-Optics random polarized HeNe lasers have somewhat unstable polarization behavior due to (REO claims) their "high quality ion beam sputtered mirror coatings" with virtually no asymmetry. Whether this is true, I can't say. But the effort REO needs to go to in forcing the modes to be well-behaved probably doubles the cost of the tube. A Metrologic metal-ceramic tube was found to have unstable mode behavior as well and its mirrors are probably nothing to write home about. The tubes used in Hewlett Packard (HP)/Agilent/Keysight metrology lasers have very unstable mode behavior when not installed in the Zeeman magnet. However from a test using a Melles Griot plasma tube with two perpendicular windows in place of internal mirrors, the orientation of the external mirrors had no impact on the polarization axes and the modes were well behaved. Only the tube orientation mattered even though the intra-cavity mode volume was nowhere even near the capillary wall. I attribute this to very slight orientation preferences in the windows - when photons make hundreds of passes, even small anisotropies can be significant.
For the special case of a short tube where only two modes fit under the gain curve (typically 5 or 6 inches in length) at the instants when they are equal, the output will appear to be non-polarized (constant intensity as an external polarizer is rotated in the beam) but as the modes shift under the gain curve, one or the other polarization will dominate and for a portion of the entire cycle, the tube will be pure linearly polarized in each of these axes. For longer tubes, there will be much less of an effect because there will be multiple modes with both polarizations at all times.
The main physical effect resulting in a particular polarization direction being favored in a random polarized HeNe tube is a slight preferred axis in the dielectric mirror coatings or in subtle aspects of the geometry of the tube due to manufacturing tolerances. Where these effects are very small or cancel, the resulting polarization axes may indeed not be restricted to a fixed orientation, but this tends to be less common. Most often, the polarization axes are fixed for the life of the tube. It's possible to design a tube with a known orientation for the polarization axes as REO has done for thier stabilized HeNe lasers, but this turns out to be more complex and expensive, so usually it's left up to natural selection. :-)
Most linearly polarized HeNe laser tubes are similar to their randomly polarized cousins but include a Brewster plate or window inside the cavity which results in slightly higher gain for the desired polarization orientation. Such tubes produce a highly polarized beam with a typical ratio of 500:1 or more between the selected and orthogonal polarization. External mirror HeNe lasers almost always use Brewster windows and so are inherently linearly polarized. A strong transverse magnetic field can also be used to force linear polarization and indeed, long before I observed this phenomenon, some commercial HeNe lasers offered a "polarization option" which was a set of magnets to be placed next to the bore. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
Another way to force linear polarization in a HeNe laser (or any other low gain laser) is to add a mirror at 45 degrees reflecting to the actual HR mirror, which is then at 90 degrees to the optic axis (facing sideways). The 45 degree mirror will have a slight polarization preference (or can be designed that way) so its reflectance will be extremely high at the desired polarization and slightly lossy at the unwanted one. Like the Brewster plate, this is enough to force linear polarization in low gain lasers. The undesirable losses from the extra mirror bounce may be less than the losses through a less than perfect Brewster plate or one with a slight orientation error, which is particularly important for "other color" HeNe lasers, especially green, which has the lowest gain. However, this approach is much less common than using a Brewster plate (even for green). I've only seen it in PMS green HeNe laser heads. Based on a test of the mirrors from a broken tube, the reflectance of the 45 degree mirror was about 99.997% for the preferred polarization orientation and 99.9% at the unwanted one. The 90 degree mirror had a reflectance of about 99.997% regardless of polarization. This difference in loss is far less than for a Brewster window but is still more than adequate for the green laser, though probably not for a higher gain red one. And the one PMS polarized yellow HeNe laser head I've had used a Brewster plate. For more info, see: U.S. Patent #6,567,456: Method and Apparatus for Achieving Polarization in a Laser using a Dual-Mirror Mirror Mount.
Linearly polarized HeNe lasers tended to be used in older laser printers (since the external modulator often required a polarized beam) and older LaserDisc players (because the servo and data recovery optics required a polarized beam). Randomly polarized lasers were used in older barcode scanners since polarization doesn't matter there. Note the use of "older". Nowadays, this equipment all use diode lasers which are inherently polarized. I've heard of people retrofitting such equipment to use diode lasers without much difficulty, but your mileage may vary. :)
Helium Neon lasers are offered in either "randomly polarized" or "linearly polarized" configurations.
In randomly (or "random") polarized HeNe lasers, the light contains both S and P polarization vectors - or in simpler terms - polarized components. They are orthogonal with respect to one-another and their orientation remains fixed with respect to the laser structure. But in general, there is no specified or indicated orientation of the two components - their orientation is random. It is important to note that the relative intensity of the two polarized components varies over time and temperature due to mode sweep, with the degree of variation inversely proportional to the length of the laser. This intensity variation between the two polarizations can be smooth (well behaved) or or instantaneously (flippers), depending on certain design parameters of the device, which may be an important consideration for the user.
In linearly polarized HeNe lasers (often referred to as simply "polarized"), the light is emitted almost exclusively in one polarization plane. This polarization plane is established via an internal window at the Brewster angle, which constrains the light to the P-polarization vector, which is marked on the laser housing. Please note, however, that there is a small amount of S-polarized light present in the laser beam, and the ratio of P-polarized light to S-polarized light is often referred to as the polarization ratio, or polarization extinction ratio. Specified values are >500:1, and typical values often exceed 1500:1.
Choosing between a randomly polarized or linearly polarized laser depends on the application, and the types of optics that may be in the beam path. If he beam path contains polarization-sensitive components, alignment of the laser source with respect to those components will be very important. In his case, a linearly polarized model may be the best choice. If the beam path does not contain polarization sensitive components, a random polarized laser may be acceptable and they are generally slightly less expensive.
There are also applications where it is essential to use a random polarized laser so that the S and P polarized components can be directed to separate beam paths. The most common one is probably to be able to stabilize a single longitudinal mode of the laser to a fixed optical frequency using the relative amplitudes of the S and P polarized components for feedback.
However, the output of a laser consists of one or more "lasing lines" which correspond to those optical frequencies which match a cavity resonance ("cavity mode") AND where the round trip net gain within the laser cavity is greater than one. These are the longitudinal (or axial) modes of the laser and each one will have a specific polarization and optical frequency. The cavity modes are spaced at a distance of f=c/2L (called the "Free Spectral Range" or FSR, where f is optical frequency, c is the speed of light, and L is the distance between the mirrors). For the typical HeNe laser, there are between 1 or 2 (for a 15 cm 1 mW tube) and 10 or 12 (for a 1 meter 35 mW tube) present at any given time. Longitudinal Modes of Typical Random Polarized 8 mW HeNe Laser illustrates this for a medium size laser. Much more can be found in the section: Longitudinal Modes of Operation.
For the red (632.8 nm) HeNe laser, unless something specific is done to control the polarization inside the laser tube, adjacent longitudinal modes will usually be orthogonally polarized (the red and blue lines in the diagram, above). The orientation of their two axes will be determined by some very slight asymmetries in the tube's construction or mirror coatings, and will usually remain fixed for the life of the tube. For reasons that are not clear, in Melles Griot tubes at least, one of the two axes often tends to line up approximately with the exhaust tip-off even though nothing special is done to make this happen and there is no obvious structural characteristic of the tube to cause it. The polarization axes can also be forced to be at a particular orientation, though some tubes using this technique may have other quirks. (See the section: The Strange Mischievous REO Tube from a Stabilized HeNe Laser Head.) Melles Griot, JDS Uniphase, and Siemens/LASOS tubes usually (but not always) have well behaved polarization. But symmetry is desirable in some tubes such as those found in HP/Agilent Zeeman-split lasers. These tubes are normally installed in an axial magnetic field and then they are extremely well behaved. But without the magnet, the polarization, while not exactly totally random, does behave rather strangely. And REO claims their mirrors are so good and symmetric that they have problems with polarization and had to implement an more complex scheme to force the polarization axes to be have a fixed orientation for their stabilized HeNe lasers.
Should the temperature of the laser cavity change, the distance between the mirrors increases or decreases resulting in a shift in the position of the cavity modes. For most HeNe lasers, this happens inadvertently as a result of the heating caused by the bore discharge during warmup. But it can also be caused by changes in ambient temperature as well as heating or cooling intentionally applied, usually for the purposes of laser stabilization. For the most common situation, as the tube warms up and the cavity expands, longitudinal modes will drift through the neon gain curve, disappearing at one end (longer wavelength, lower optical frequency) as the gain falls below the lasing threshold, and being replaced at the other end (shorter wavelength, higher optical frequency) as the gain there rises above the lasing threshold. The total output power in each of the two polarization axes will correspond to the sum of the power in its lasing modes. The total output power of the laser is the sum of the output power in both polarizations. In most real HeNe lasers, the variation versus time as the tube warms up - called "mode sweep" or "mode cycling" - is smooth and occurs on a time scale of seconds to hours depending on how close the tube is to thermal equilibrium, being fastest just after the laser is turned on. The modes are not jumping around on a time scale of nanoseconds as has been suggested by at least one major supplier of HeNe lasers! :) However, depending on the size of the laser, there can be high frequency variations in power in each polarization, or in a combination of the two observed with a high speed photodetector and oscilloscope. More on this below.
There are several specific cases depending on the length of the laser cavity. To simplify the explanation, it is assumed that the laser tube has been rotated in its mounts so that the natural polarization axes are at 0 and 90 degrees. In addition, the second order ripple and noise in the output from imperfect power supplies or other external factors are assumed to be small (which is typically the case). Also, fine points like mode pulling (which shift the modes very slightly in position, a small fraction of 1 percent) are ignored. So, the FSR (Free Spectral Range or cavity mode spacing) is equal to the longitudinal (or axial) mode spacing of the lasing lines. And the lasers are assumed to be well behaved and not be "flippers" or "stutterers" or have other pathologic disorders:
And so forth.
Here, the term "non-polarized" means that rotating a polarizer in the beam will result in no variation of optical power passing through it. But the beam in this case actually consists of the two CW longitudinal modes with orthogonal linear polarizion, and equal and constant amplitude. (This is totally unlike a natural non-polarized light source whose output consists of a superposition of a nearly infinite number of independent emitters with arbitrary polarization.)
Note that the axis of polarization is NOT rotating - power is simply shifting back and forth between the two fixed orthogonal polarization axes.
If the output is passed through a polarizer oriented at 0 or 90 degrees, the optical power will be seen to vary smoothly from 0 to to approximately the rated power of the laser in a cycle lasting a few seconds to hours depending on how close the tube is to thermal equilibrium. Aside from this slow variation, the output will be CW with no high frequency oscillation or noise - a pure single optical frequency. However, if a polarizer is oriented at an angle other than 0 or 90 degrees, whenever both modes are present, a high speed photodiode and oscilloscope (or frequency counter or RF spectrum analyzer) would show a beat signal between the two lasing modes at a frequency equal to the longitudinal mode spacing (around 1 GHz for a short tube like this). If the polarizer is at 45 degrees, when both modes are equal in power, the beat would have a peak-to-peak amplitude of double the average power passing through the polarizer.
Longitudinal Modes of Typical Random Polarized 1 mW HeNe Laser is a diagram for this size laser showing the relationship of the neon gain curve, cavity modes, and lasing modes. The Power Point show HeNe Laser Mode Sweep: 140 mm (~5.5 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes. The longitudinal mode spacing and thus the beat frequency (if present) is 1.063 GHz in this example. Specifically note that at no time are more than 2 modes present and they are always orthogonally polarized. (In real life, the motion is continuous, but I didn't have enough patience to generate an infinite sequence of slides!)
Plot of Mode Sweep of Typical 1 mW Random Polarized HeNe Laser Tube shows the appearance of mode sweep using a dual polarization detector for a typical 12 cm random polarized HeNe laser tube. The red and blue plots are the optical power for the two polarization axes. The green plot is the total optical power. Each polarization has exactly 0 power for a approximately 1/3rd of each cycle. (The plot has it slightly raised above 0 so that the green total power curve can be distinguished from the top of the mode it's sitting on, but it really would be almost precisely 0 in real life, limited mainly by the quality of the polarizer in front of each detector.)
During the time while only 2 modes are oscillating, the output through a polarizer oriented at 0 or 90 degrees will be varying slowly with no high frequencies present as with the shorter laser, above.
During the time while 3 modes are oscillating, one of the axes will have 2 modes of the same polarization (but spaced by twice the distance between longitudinal modes) and the other will have only a single mode which is pure CW. For the axis with 2 modes, a polarizer will show a beat at one half the longitudinal modes spacing of the laser. For the axis with a single mode, there will be no beat. If the polarizer is oriented at 45 degrees (or any angle other than 0 or 90 degrees) there will always be a beat at a frequency equal to the longitudinal mode spacing, or one half of it, or both (1.5 GHz and 750 MHz). However, this does NOT mean the polarization is jumping around; only that the power is varying in each of the polarization axes or when combined with the polarizer due to the way the E/M waves add up.
Longitudinal Modes of Typical Random Polarized 3 mW HeNe Laser is a diagram for this size laser showing the relationship of the neon gain curve, cavity modes, and lasing modes. HeNe Laser Mode Sweep: 200 mm (~8 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes. Unlike the 1 mW laser, above, when a longitudinal mode of the 3 mW laser is near the center of the gain curve, there can be modes on both sides of it (3 modes total).
Plot of Mode Sweep of Typical 3 mW Random Polarized HeNe Laser Tube shows the appearance of mode sweep using a dual polarization detector for a typical 22.5 cm random polarized HeNe laser tube. The red and blue plots are the optical power for the two polarization axes. The green plot is the total optical power.
For both of these cases, exactly two modes can be maintained by a feedback circuit with one on either side of the neon gain curve to implement a stabilized HeNe laser. Under these conditions, both of the polarizations are pure single modes with a constant CW output. They are a very pure single optical frequency with ultra-long coherence length when one of them is selected with a polarizer.
Where 4 or more modes are oscillating, the output through a polarizer will show a beat at all times regardless of orientation since there are always at least 2 modes present even at 0 and 90 degrees.
Longitudinal Modes of Typical Random Polarized 8 mW HeNe Laser is a diagram for a laser where 5 modes are present showing the relationship of the neon gain curve, cavity modes, and lasing modes. HeNe Laser Mode Sweep: 400 mm (~16 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes. The longitudinal mode spacing and thus the beat frequency is 373 MHz in this example. Depending on the position with respect to the neon gain curve and orientation of a polarizer, the beat frequency will be at a combination of the longitudinal mode spacing (373 MHz), and 1/2, 1/3, 1/4, and 1/5 of it (186.50 MHz, 124.33 MHz, and 93.25 MHz, and 74.60 MHz, although the last one will not "appear" in the slide show due to the discrete frames skipping over a very small region where 6 modes are present).
Plot of Melles Griot 05-LHR-151 HeNe Laser Head During Warmup (Polarized) shows the appearance of mode sweep using a dual polarization detector for a typical 325 mm random polarized HeNe laser tube. The red and blue plots are the optical power for the two polarization axes. Since multiple longitudinal modes are present at all times, the power variation in each polarization axis is small and the variation in total power is even smaller. As the length of the laser is increased, these power variations become still smaller.
For extremely low power tubes with a cavity length less than 8 or 9 cm, there will never be more than 1 lasing mode present at any time and during a portion of the mode sweep, there may be exactly 0 modes and no beam at all. There will never be any beat frequency detectable in the output. Since two adjacent modes are needed to force orthogonal polarizations and that never occurs, these tubes may lase with the same polarization each time the single mode appears, or the polarization may come up randomly one way or the other (but will remain the same while it's present). So perhaps, such tubes can be truly called random polarized. :) However, they are now almost non-existent.
Finally, for most linearly polarized HeNe lasers, a Brewster plate or Brewster window(s) within the laser cavity provide enough gain asymmetry to force the polarization to be in one plane only. The polarization purity is usually very high - 500:1 or more. Everything above about mode sweep still applies except that all the longitudinal modes have the same polarization. So the diagrams, Power Power shows, and plots will look identical except that all the modes would be the same color. :) A polarizer will not affect the relative amplitude of the modes, only the intensity and angle of the linearly polarized beam. And whenever more than 1 longitudinal mode is present, there will be a beat signal detectable using a fast photodiode which will contain one or more frequencies depending on the possible distances between all the lasing modes.
To complicate this further, some tubes are designed to have very low asymmetry and are thus pathologically poorly behaved. In particular, HP/Agilent/KEysight tubes normally used in Zeeman split HeNe lasers with a magnetic field exhibit very random behavior with no magnet. This is done to assure that the Zeeman characteristics will be stable even at low field strength. But if used without a magnet, their modes may appear chaotic.
So what this all shows is that random is all in the eyes of the polarized beholder. :)
(Portions from: Lynn Strickland (stricks760@earthlink.net).)
Our testing suggested that adjacent modes always have orthogonal polarization - (lets go with S and P designations). BUT, in some two-mode tubes, a given mode doesn't always REMAIN S or P as it changes in frequency (it flips polarization). In "flippers", certain frequencies only support one polarization. If this frequency range is around the center of the gain curve, most power will be of one polarization regardless of temperature (so it appears to be linearly polarized). (However, the extinction ratio varies over time, and is generally poor).
Here's a test setup that shows what's going on if you have access to some nice instrumentation: Send the beam from a two mode, randomly polarized HeNe tube (Example: 05-LHR-006) into a Scanning Fabry-Perot Interferometer. (SFPIs are generally exorbitantly priced, but you can build one if so inclined. See the sections starting with: Scanning Fabry-Perot Interferometers. --- Sam.) Put a polarizer in the beam path, aligned to maximize P polarization (or S polarization, doesn't matter). Normally, the P mode will remain P polarization at all frequencies under the gain curve. So as the frequency changes (due to cavity length changes with temperature), the P mode will trace out a nice pretty sort of bell-shaped curve with a width of about 1.6 GHz FWHM. Bottom line, you can get P-polarized light at every frequency under the gain curve.
In a 'flipper', your curve has missing sections. In other words, there are some frequencies where you cannot get P polarization. When the observed, P mode reaches one of these frequency ranges, it will flip and become S-polarized. When the flip occurs, the other, formerly S mode, turns into a P. If you're just looking at one polarization (as the experiment describes), the observed P mode disappears and pops up again at a frequency delta equal to the longitudinal mode spacing (where the S mode used to be). Some call it mode hop, but it really isn't, because both modes are still there. Both modes still have, and always had, orthogonal polarization - they just swapped. Some tubes flip at one point under the gain curve, some flip many times under the gain curve.
This has to do with gain asymmetry. What brought it to our attention, is that when the polarizations flip, you get high frequency 'noise' if you have polarization sensitive components in your beam path. Solutions are to specify a laser that doesn't flip, go to a three mode (longer) laser, go to non-polarization sensitive optics all the way through the beam delivery/detection train, or put a bandwidth filter on your detector.
A magnetic field will sometimes make a flipper stop, and sometimes make a non-flipper start - but not always. Sans magnetic field, over time (several thousand operating hours) our test population suggested that flippers always flip, non-flippers always behave.
There is more on flippers below.
A "flipper" tube is one where the polarization orientation of adjacent longitudinal modes swap places at one or more locations on the neon gain curve as the modes sweep through it. Most will flip at a single fixed location but some may do it at multiple locations, or locations that are not the same from one cycle to the next but these are less common. The Power Point show HeNe Flipper Mode Sweep: 200 mm (~8 inch) Cavity Length demonstrates the effect of changing cavity length on the lasing modes in a classic 2 to 3 mW flipper tube.
The issue of why some tubes are flippers is apparently one of those grand mysteries of the Universe that even the Ph.D. types at major laser companies have been pondering for eons without resolution, as it's still not always possible to manufacture a tube that is guaranteed to be well behaved. :) Flipper behavior may not be detected where the laser is simply used as a source of photons for the same reason that polarization effects of normal mode sweep tend to be minimal since the total power doesn't vary that much. However, polarization flips will introduce short noise spikes. And if there are any polarization sensitive optical elements (intentional or not), significant sudden power fluctuations will also be evident in the polarized beam(s).
As with random polarized HeNe lasers not being random at all, flipper behavior is also mostly deterministic in that for a given tube, flipping will usually always occur at the same place(s) in the mode sweep, but there are exceptions.
While I haven't seen any discussion of flipper theory, here are some thoughts.
In the absence of external influences like magnetic fields, the mode orientation in a laser will be determined by at least two factors:
Since a transverse magnetic field can also introduce a polarization preference, it is possible to cause a well behaved HeNe laser tube to exhibit flipper behavior by the careful placement of a strong magnet near the tube. I've demonstrated this with a normal Uniphase 098 laser. With no magnet, the mode sweep is perfectly ordinary with no tendency to flipping. By placing a single rare earth magnet next to the tube near the middle, it can be made to turn into a flipper with a mode plot very similar to that of a natural flipper. With too weak a magnetic field, there is no effect or a sort of shortened aborted flipping. With too strong a magnetic field, the polarization becomes locked to the magnetic field and the output ends up being linearly polarized.
For that peculiar tube above which reverts to normal behavior at the very end of the warmup period, a very weak magnetic field will cause it to continue to flip after the point of transition where flipping ceases under normal conditions.
Plot of "Flipper" Aerotech OEM1R HeNe Laser Head with Various Magnetic Fields Applied (Combined) shows the effect of a rare earth magnet at 4 orientations about 4 inches from the center of the laser head compared with no magnetic field. The magnetic field axis was horizontally aligned with one of the polarization axes of the laser. The magnet was rotated 90 degrees approximately every 30 seconds. The first and last orientation shows a mode sweep pattern that is relatively normal. They probably differ slightly because the magnet wasn't in exactly the same position. The tube was allowed to completely warm up with the magnets in the last orientation with no significant change in the plot, even after the transition point where the tube reverts from flipper to normal behavior with no magnetic field A closeup is shown in Plot of "Flipper" Aerotech OEM1R HeNe Laser Head with Magnetic Field Induced Somewhat Normal Behavior (Combined). While very different than the mode plot of the tube after warmup with no magnetic field, the flips are gone (no vertical jumps) and it's relatively well behaved.
Conversely, it should be theoratically possible to suppress flipper behavior with a suitably placed magnet. Getting this to work is more problematic since the magnetic field has to exactly counteract the natural polarization birefringence. But I was able to somewhat do this with my flipper head so that the mode sweep became well behaved. This was more finicky than going the other way. Almost any magnetic field did disrupt the normal flipper behavior. But getting it to be really well behaved was more difficult.
Of course, a magnetic field will also introduce other effects due to Zeeman splitting which may be detrimental depending on the application.
Note that mirror alignment which may affect the resonator orientation preference had no effect on flipper behavior, at least for the one sample I tested. Pressing on the mirror mount of my flipper tube in any direction would reduce the output power significantly due to changing mirror alignment. But the mode flips still occurred, and appeared to be at approximately the same location on the gain curve.
Some observations and questions:
Speculation:
This state can be forced from (2) by a small transverse magnetic field.
This state can be forced from (2) or (3) by a large transverse magnetic field.
This is not necessarily true of "other color" HeNes. My informal tests suggest that in general it is *not*. Long green (543.5 nm), short and long yellow (594.1 nm), and medium length orange (611.9 nm) random polarized HeNe laser heads all exhibited varying degrees of erratic behavior with respect to polarization. Usually, modes when part of the way through the gain curve and then either flipped abruptly or oscillated between polarizations for a short time and then flipped. The long yellow head liked to have pairs of adjacent modes with the same polarization but exhibited the flipper behavior as well. However, adding a modest strength magnet near the long green seemed to force it to behave with adjacent modes having orthogonal polarization. I have no idea if this is significant or the long green HeNe was simply a cooperating sample.
But what is the underlying cause?
(From: A. E. Siegman (siegman@stanford.edu).)
The reason that HeNe lasers can run - more accurately, like to run - in multiple axial modes is associated with inhomogeneous line broadening (See section 3.7, pp. 157-175 of my book) and "hole burning" effects (Section 12.2, pp. 462-465 and in more detail in Chapter 30) in the Doppler-broadened laser transitions commonly found in gas lasers (though not so strongly in CO2) and not in solid-state lasers.
The tendency for alternate modes to run in crossed polarizations is a bit more complex and has to do with the fact that most simple gas laser transitions actually have multiple upper and lower levels which are slightly split by small Zeeman splitting effects. Each transition is thus a superposition of several slightly shifted transitions between upper and lower Zeeman levels, with these individual transitions having different polarization selection rules (Section 3.3, pp. 135-142, including a very simple example in Fig. 3.7). All the modes basically share or compete for gain from all the transitions.
The analytical description of laser action then becomes a bit complex - each axial mode is trying to extract the most gain from all the subtransitions, while doing its best to suppress all the other modes - but the bottom line is that each mode usually comes out best, or suffers the least competition with adjacent modes, if adjacent modes are orthogonally polarized.
There were a lot of complex papers on these phenomena in the early days of gas lasers; the laser systems studied were commonly referred to as "Zeeman lasers". I have a note that says a paper by D. Lenstra in Phys. Reports, 1980, pp. 289-373 provides a lengthy and detailed report on Zeeman lasers. I didn't attempt to cover this in my book because it gets too complex and lengthy and a bit too esoteric for available space and reader interest. The early (and good) book by Sargent, Scully and Lamb has a chapter on the subject. You're probably aware that Hewlett Packard developed an in-house HeNe laser short enough that it oscillated in just two such orthogonally polarized modes, and used (probably still uses) the two frequencies as the base frequencies for their precision metrology interferometer system for machine tools, aligning airliner and ship frames, and stuff like that.
(From: Sam.)
Indeed, HP has several models of two-frequency HeNe lasers but the ones I'm familiar with actually use an external magnet to create Zeeman splitting. Rather than two longitudinal modes, a PZT or heater is used to adjust cavity length so that only a single mode is oscillating, which is split by the Zeeman effect. Then, the difference frequency (in the low MHz range) is used in the measurement system as a reference and possibly for stabilizing the (optical) frequency. See the section: Hewlett-Packard/Agilent Stabilized HeNe Lasers.
The Spectra-Physics model 117A frequency stabilized HeNe laser is designed more like what you are describing - two modes, no magnets. A heater is used to adjust cavity length in a feedback loop using a pair of photodiodes to monitor the two orthogonal polarized modes. However, I would assume that based on its description, the desired operating conditions would be for it to run with a single mode (which it can with carefully controlled cavity length). See the section: Description of the SP-117A Laser. The Coherent and Melles Griot stabilized HeNe lasers are similar.
This setup worked really well. However, the somewhat unexpected result turned out to be that the polarization always remained with the tube regardless of mirror orientation even if the intracavity beam was much smaller than the bore so that any imperfections in its shape should not have had any effect. I attribute this to a very small amount of asymmetry in the transmission through the perpendicular windows. It might be AR coating or stress birefringence, distortion, or even the windows not being mounted quite perfectly perpendicular. With the intracavity photons traversing the windows an average of perhaps 100 times, even a miniscule asymmetry would be amplified into something significant. So on to Plan B, putting everything inside the gas envelope and doing away with the perpendicular windows entirely. Unfortunately, implementation of Plan B is currently not a funded project. :( :)
Like most low current discharge tubes, the HeNe laser is a negative resistance device. As the current *increases* through the tube, the voltage across the tube *decreases*. The typical resistance is -50K ohms. For stability, the net ballast resistance must be greater than +50K ohms. But the magnitude of the tube's negative resistance also increases with descreasing current so for this and other reasone, the required ballast resistance is typically 68K to 100K ohms at the anode, and sometimes an additional smaller ballast such as 12K ohms at the cathode.
In the case of a HeNe tube, the initial breakdown voltage is much greater than the sustaining voltage. The starting voltage may be provided by a separate circuit or be part of the main supply.
Often, you may find a wire or conductive strip running from the anode or ballast resistor down to a loop around the tube in the vicinity of the cathode. (Or there may be a recommendation for this in a tube spec sheet.) This external wire loop is supposed to aid in starting (probably where a pulse type starter is involved). There may even be some statistical evidence suggesting a reduction in starting times. I wouldn't expect there to be much, if any, benefit when using a modern power supply but it might help in marginal cases. But, running the high voltage along the body of the tube requires additional insulation and provides more opportunity for bad things to happen (like short circuits) and may represent an additional electric shock hazard. And, since the strip has some capacitance, operating stability may be impaired. I would probably just leave well enough alone if a starting strip is present and the laser operates without problems but wouldn't install one when constructing a laser head from components.
With every laser I've seen using one of these strips, it has either had virtually or totally no effect on starting OR has caused problems with leakage to the grounded cylinder after awhile. Cutting away the strip in the vicinity of the anode has cured erratic starting problems in the latter case and never resulted in a detectable increase in starting time.
In order for the discharge to be stable, the total of the effective power supply resistance, ballast resistance, and tube (negative) resistance must be greater than 0 ohms at the operating point. If this is not the case, the result will be a relaxation oscillator - a flashing or cycling laser! In practice, the net positive resistance should be 10K ohms or more.
Note: HeNe tube starting voltage is lower and operating voltage is higher when powered with reverse polarity. With some power supply designs, the tube may appear to work equally well or even better (since starting the discharge is easier) when hooked up incorrectly. However, this is damaging to the anode electrode of the tube (and may result in more stress on the power supply as well due to the higher operating voltage) and must be avoided (except possibly for a very short duration during testing).
See the chapter: HeNe Laser Power Supplies for more information and complete circuit diagrams.
A few HeNe lasers - usually larger or research types - have used a radio frequency (RF) generator - essentially a radio transmitter to excite the discharge. This was the case with the original HeNe laser but is quite rare today given the design of internal mirror HeNe tubes and the relative simplicity of the required DC power supply.
Between dropout and nominal, output power will increase, but not in proportion to current and not linearly. The usable output power variation (e.g., for modulation purposes) is usually in the 15 to 25 percent range.
Most healthy tubes will still produce a substantial fraction of their maximum output power even just above the dropout current. However, in rare instances (and probably with a large ballast resistor to push down the stable current as far as possible and/or where the tube low gain due to contamination or end-of-life), lasing will actually cease above the dropout current.
Between nominal and the onset of single frequency noise, output will decrease somewhat, but again not in proportion (or inverse proportion) to current. Attempting to modulate current symmetrically around the nominal current will result in a sort of rectification or absolute value effect on the variation in output power.
Note that the visual effect of increasing current from dropout to cessation of output will just be a smooth increase and then decrease in coherent optical output power. To detect the single frequency or broadband noise will require a sensor and oscilloscope with a bandwidth of at least a few MHz.
I've also seen lasers where single frequency noise occurred close to the dropout current and below the point of maximum output power. However, this was only present with some high mileage tubes in HP-5517 lasers so it's not clear whether this should be listed as a separate regime, or just a special case of a particular tube and power supply combination.
Also of note is that the HeNe laser power supply itself will contribute to optical ripple and noise. A DC input switchmode (inverter) power supply will have ripple at the switching frequency. This is typically in the range of 1 to 5 percent of the operating current and will result in an optical power variation of a few tenths of a percent. An AC input linear power supply will have some ripple at 1X or 2X of the line frequency (with some harmonics) even with a regulator. An AC input switcher (most bricks) will have both types of ripple. Special low noise power supplies are available for critical applications. However, for most common uses, the additional cost is not justified. There are some more comments on this topic in the section: Intensity Stabilized HeNe Laser.
You have probably wondered why the beam from a typical HeNe laser (without additional optics) is so narrow. Is it that making a tube with larger mirrors would be more costly?
No, it's not cost. Even high quality and very expensive lab lasers still have narrow bores. The very first HeNe lasers did use something like a 1 cm bore but their efficiency was even more mediocre than modern ones. A wide bore tube would actually be cheaper to manufacture than one requiring a super straight narrow capillary. However, it wouldn't work too well.
A combination of the current density needed in the bore, optimal gas pressure, gain/unit length in the bore, the bore wall itself aiding in the depopulation of lower energy states, and the desire for a TEM00 (single transverse mode) beam (there are multimode tubes that have slightly wider bores), all interact in the selection of bore diameter.
In fact, there is a mathematical relationship between bore size, gas pressure, and tube current resulting in maximum power output and long life.
The optimal pressure at which stimulated emission occurs in a HeNe laser is inversely proportional to bore diameter. According the one source (Scientific American, in their Amateur Scientist article on the home-built HeNe laser - see the chapter: Home-Built Helium-Neon (HeNe) Laser), the pressure in Torr is equal to 3.6 divided by the ID of the bore. I don't know whether this exact number applies to modern internal mirror tubes but it will likely be similar. Power output decreases on either side of the optimal pressure but a laser with a low loss resonator may still produce some output above twice and below half this value.
Thus, as the bore diameter is increased, the optimal pressure drops. Aside from having fewer atoms to contribute to lasing resulting in a decrease in gain, below a pressure of about .5 to 1 Torr, the electrons can acquire sufficient energy (large mean-free-path?) to cause excessive sputtering at the electrodes. This will bury gas atoms under the sputtered metal (which may also coat the mirrors) leading to a runaway condition of further decreasing pressure, more sputtering, etc. Even with the large gas reservoir of your typical HeNe tube (which IS the main purpose of all that extra volume), there may still be some loss over time. A drop in gas pressure after many hours of operation is one mechanism that results in a reduction in output power and eventual failure of HeNe tubes.
As a result, the maximum bore diameter you will see in a commercial HeNe laser will likely be about 2 mm ID (for those multimode tubes mentioned above where the objective is higher power in a short tube). Most are in the 0.5 to 1.2 mm range. This results in high enough pressure to minimize sputtering, maximize life, provide maximum power output, and optimal efficiency (to the extent that this can be discussed with respect to HeNe lasers! Well, ion lasers are even worse in the efficiency department so one shouldn't complain too much. Since total resonator gain is proportional to bore length and approximately inversely proportional to bore diameter (since the optimal pressure increases resulting in a higher density of lasing atoms), this favors tubes with long narrow bores. But these are difficult to construct and maintain in alignment. Wide bore tubes have lower gain but a higher total number of atoms participating with potentially higher power output at the optimal pressure and current density. Everything is a tradeoff!
However, all this does provide a way of estimating the power output and drive requirements of a HeNe tube or at least comparing tubes based on dimensions. Assuming a tube with a particular bore length (L) is filled to the optimum pressure for its bore diameter (D), power output will be roughly proportional to D * L, discharge voltage will be roughly proportional to L (probably minus a constant to account for the cathode work function), and discharge current will be roughly proportional to D. (Note that D instead of the cross-sectional area is involved because the optimal pressure and thus density of available lasing atoms is inversely proportional to D.)
So, do the numbers work? Well, sort of. Here are specifications for some selected Melles Griot red HeNe tubes rearranged for this comparison:
Total Bore Bore --- Ratio of --- Discharge Discharge Output Lgth Lgth (L) Dia. (D) L D (D * L) Voltage Current Power ------------------------------------------------------------------------------ 135 mm 80 mm .46 mm 1 1 1 900 V 3.3 mA .5 mW 177 mm 115 mm .53 mm 1.4 1.15 1.6 1,130 V 4.5 mA 1.0 mW 255 mm 190 mm .72 mm 2.4 1.57 3.7 1,360 V 6.5 mA 2.0 mW 370 mm 300 mm .80 mm 3.8 1.7 6.4 1,800 V 6.5 mA 5.0 mW 440 mm 365 mm .65 mm 4.6 1.4 6.4 2,150 V 6.5 mA 10 mW 930 mm 855 mm 1.23 mm 11.1 2.7 29.9 4,500 V 8.0 mA 25-35 mW
(Bore length was estimated since the cathode-end of the capillary is not visible without X-raying the tube or by optically determining its position through the mirror!)
The general relationships seem to hold though large tubes seem to produce higher output power than predicted possibly constant losses represent a smaller overhead. As noted elsewhere there is also a wide variation even for tubes with similar physical dimensions. Oh well...
There are more examples in the section:Typical HeNe Tube Specifications. You can do the calculations. And, some large IR HeNe lasers may use a somewhat wider bore. See the section: Spectra-Physics 120, 124, and 125 HeNe Laser Specifications for a comparison of visible and IR HeNe tubes for the same model laser.
Note that there are some multi-mode (non-TEM00) HeNe tubes with wider bores and a different mirror curvature that produce up to perhaps twice the power output for a given tube length. However, with multiple transverse modes, these are not suitable for many applications like interferometry and holography. They are also not very common compared to single-mode TEM00 HeNe tubes.
The most powerful HeNe laser I have ever seen was 160 mW of real power and was the only time I've ever seen a HeNe laser burn anything before with raw beamage. It would slowly burn electrical tape placed in the beam and felt warm on your skin. It was made of two almost 6 foot long Spectra-Physics model 125 tubes hooked electrically to separate power supplies and optically in series in a custom made double-wide sized 125 head. Sadly, it doesn't work anymore and is currently resting piecefully in the NTC laser department's laser graveyard. :-(
(From: Steve Roberts.)
I've seen a normal SP-125 break 160 mW on its own. Two tubes at only 160 mW sounds like it was misaligned, not that I'd like to try to align that one! :)
The current record is for a Chinese researcher using 2 tubes with a flattened elliptical profile in a V fold resonator to get 330+ mW into a fiber. The beam shape and divergence from this are not what you would expect from a typical HeNe laser, even one that runs multi (transverse) mode. Remember that a HeNe laser's power is limited by collisions with the tube wall returning Ne atoms to the ground state, so using a flattened tube means more wall area, hence more power. Optimal gas pressure is a function of bore diameter as well. So you're limited to about a 1 meter tube in most cases by other optics reasons and sputtering. With collisions with the wall increased by a larger wall surface area, what the folks in China did is try tubes with different cross sections. To get enough length they folded the resonator using a 3 optic V-fold. You don't want to see the beam profile. It's nasty! It looks kind of like this: <{[=]}>. And the divergence is high as the optics need to fill that whole lasing volume.
Please note, however, that going to a large rectangular or star shaped tube is not possible due to some quirks in the plasma at the pressure required for HeNe laser operation. Details are in a 1996 issue of Review of Scientific Instruments. A few years ago, Cornell University attempted to sell the rights to the unit in the United States, on behalf of the Chinese Inventor. U.S. patent and marketing were assigned to a group that sadly dropped the ball. At the time, the picture of the unit looked like one of those old foldaway sewing machines like my mom used to have, an ornamental blue box about the size of a PC Tower turned on its side with 4 wooden legs.
In the early days, very long HeNe lasers were constructed in an attempt to obtain higher power. But optimal gas-fill and bore diameter weren't known, and mirrors weren't as good as they are now. Aligning multiple segments with a long narrow bore needed for best gain would have been virtually impossible in any case. Thus, such experimental lasers probably had mediocre performance.
(From: Sam.)
Using a folded resonator, high power HeNe lasers could be constructed in compact packages but the initial machining and/or alignment would be a real treat. I've seen a spec sheet for some with up to 55 mW of output power using a mono-block folded resonator with a volume of 326x280x95 mm (about 13"x10"x4"). I can't imagine this being cost effective though except maybe for space applications where money is no object!
Most laser heads include the ballast resistor since it needs to be close to the HeNe tube anode anyhow (though you may still need additional resistance to match the tube to your power supply). The ballast resistor may be potted into the end cap with the HV cable, a wart attached to the HeNe tube, or a separate assembly. There may be an additional ballast resistor (e.g., 10K) in the cathode circuit as well.
The majority of laser heads use a HeNe laser tube with the output beam emerging from the cathode-end of the tube so there is little or no voltage present on the exposed terminals if the output end-cap is removed. However, some laser heads will place the anode and ballast resistors at the output-end. This is particularly true of some "other color" HeNe lasers (e.g., yellow and green) since there are some subtle advantages to this arrangement in terms of output power for a given tube size. But, in some cases, it's just to be able to install a stock tube.
The high voltage cable will likely use an 'Alden' connector which is designed to hold off the high voltages with a pair of keyed recessed heavily insulated pins. This is a universal standard for small to medium size HeNe laser power supplies (the longer fatter pin is negative). Typical cable length is from 6 inches to 6 feet.
Internal wiring may be via fat insulated cables or just bare metal (easily broken) strips. Take care if you need to disassemble one of these laser heads (the round ones in particular) as the space inside may be quite cramped.
CAUTION: The case, if metal, of the laser head may be wired to the cathode of the HeNe tube and thus the negative of the Alden connector and power supply. This is not always the situation but check with an ohmmeter and keep this in mind when designing a power supply or modulation scheme. The case should always be earth grounded for safety if at all possible (or else properly insulated). DO NOT assume that a commercial power supply is designed this way - check it out and take appropriate precautions.
Note: Depending on design, the laser tube itself may be mounted inside the laser head in a variety of ways including RTV Silicone (permanent and alomst impossible to remove), hot-melt glue (permanent but removable), or 3 or 4 set screws at two locations (front and rear) around the outside of the housing. The latter approach permits precise centering of the beam but don't overtighten the screws or you WILL be sorry! (Since RTV silicone has some compliance, very SLIGHT adjustment of alignment may still be possible even if mounted this way - don't force it, however.)
For example, I found that some recent samples of the popular Melles Griot 05-LHR-911 HeNe laser head, rated at 1 mW minimum power output, were all made with neutral density filters to assure that the maximum power output was less than 1.5 mW. With the filters removed, it jumped to between 1.8 and 2.1 mW! Apparently, the filters were individually selected to get the lasers as close as possible to 1.5 mW without exceeding it since their attenuations were not all the same and the weakest laser in the batch (with the filter) actually ended up having the hottest tube.
If you have a laser head that is missing the Alden connector, replacements should be available from the major laser surplus suppliers or salvage one from another (dead) head. I also have many available. Where the end-cap on a cylindrical laser head is also missing, there are no readily available commercial sources - fabricate one from a block of wood and paint it black or find some other creative solution. A suitable ballast resistance must also be installed between the positive power supply output and the HeNe tube anode.
The cylindrical head serves another purpose besides structural support and protection. This is the distribution of heat and equalization of thermal gradients. Thus, removing a long HeNe tube in particular from its laser head may result in somewhat random or periodic cycling of power output due to convection and other non-uniform cooling effects.
Often, particularly inside equipment like barcode scanners, you will see something in between: A HeNe tube wrapped in several layers of thick aluminum foil probably to help distribute and equalize the heating of the tube for the reason cited above. However, I haven't really noticed any obvious difference in stability when this wrap was removed. Spectra-Physics is very fond of this but others may have copied it to sell compatible tubes.
The operating lifetime of a typical HeNe laser tube is greater than 15,000 hours when used within its specified ratings (operating current, proper polarity, and not continuously restarting). Under these conditions, end-of-life occurs when the oxide "pickling" layer of the cathode can gets depleted. Larger diameter (1.5 or 2 inch) tubes last the longest - up to 50,000 hours or more. Small diameter (0.75 or 1 inch) tubes have the shortest lifetime - 10,000 hours or so. Since even 10,000 hours is still very long - over 1 year of continuous operation - HeNe laser lifetime is not a major consideration for most hobbyist applications. Chances are that even a surplus laser will still have thousands of hours of life remaining.
However, the shelf life of the tube depends on types of sealing method used in the attachment of the optics. There are two types of internal mirror HeNe tubes:
The frit is basically powdered low melting point glass mixed with a liquid to permit it to be spread like soft puddy or painted on. The frit can be fired at a low enough temperature that the mirror mount or glass mirror itself is not damaged, there is virtually no distortion introduced by the process, and manufacturing is greatly simplifed compared to using normal (high temperature) glass or ceramic joints. Some tubes use frit seals at other locations in addition to the mirrors (like the end-caps) rather than glass-to-metal seals. The same process is used for other permanently sealed tubes like those in internal mirror argon ion lasers as well as some xenon flashlamps and similar devices.
Note that the electrical connections on those tubes that don't use the mirror mounts will generally be glass-metal seals which do not leak. Mirrors can't use glass-metal seals since they require high temperatures to make which would distort or totally destroy the mirrors. You can tell if a seal is frit or Epoxy by how easily it scratches: Frit is like glass and requires something hard to make a mark while Epoxy can be scratched with a good solid fingernail. Another way to tell is the color: Frit is generally gray or tan while Epoxy is clear or white.
Should you care, the metal parts of the tube are likely made from Kovar, an alloy commonly used with frit seals since there is a very good CTE (Coefficient of Thermal Expansion) match of the Kovar to the frit glass.
CAUTION: The frit seal is thin and relatively fragile, even more so than the fragile optical glass, so avoid placing any stress on the mirrors!
Shelf life of soft-sealed tubes is limited by diffusion of the Helium atoms out and air leakage in, water vapor from Epoxy seals, etc. Helium atoms are slippery little devils and diffuse through almost anything. In the case of HeNe tubes, diffusion takes place mostly through the Epoxy adhesive used to mount the mirrors in non-hard sealed tubes (not common anymore) and through the glass itself but at a much much slower rate. Most of the contamination of air leakage will be cleaned up by the getter (if present) until it is exhausted. However, hydrogen may appear, probably from dissociated wate vapor (the getter will clean up the O2) and hydrogen (1) kills lasing at very low concentrations and (2) appears virtually impossible to remove. The discharge spectrum will reveal much about the gasious health of a HeNe laser tube. See the sections starting with: HeNe Tube Problems and Testing.
. CAUTION: Take care in attempting to clean the Brewster windows or mirror mounts of soft-sealed HeNe or ion laser tubes with alcohol or other solvents as the result may be immediate air leakage and a dead tube. The failure mechanism for this isn't clear - after all, it can take weeks to loosen up these optics by soaking when trying to salvage them for some other use. However, there is anecdotal evidence to suggest that instant tube death may result from such cleaning attempts. So, to be safe, avoid getting the area of the sealing adhesive wet with solvent.
A very few tubes apparently have frit at one end and a soft-seal at the other so check both ends. This probably applies only to some low gain "other color" HeNe lasers with a mirror that would be affected by even the relatively low temperature at which the frit melts.
Note that other parts of most tubes (except for Brewster windows, if present) use glass-to-metal seals but since these must be manufactured at high temperature, they are not an option for delicate optics. The very best tubes with one or two Brewster windows do not use frit because even at the low temperature at which it is fired, there may still be some unavoidable stresses introduced - these tubes continued to be soft-sealed even after frit was common but now use optical contacted seals. With optical contacted seals, the two pieces are ground and polished optically flat and brought together under clean room conditions. The resulting seal is gas-tight. Just a bit of Epoxy is used for mechanical stability but it doesn't do the sealing.
The HeNe gas doesn't 'wear out'. A HeNe tube, when properly connected has a substantial portion of its power dissipated by the bombardment of positive ions at the cathode (the big can electrode) which is made large to spread the effect and keep the temperature down and is "pickled" (coated) to reduce its work function. Hook a tube up backwards and you may damage it in short order and excessive current (operating current as well as initial starting current from some high compliance power supplies) can degrade performance after a while. Electrode material may sputter onto the adjacent mirrors (reducing optical output or preventing lasing entirely) or excessive heat dissipation may damage the electrodes or mirrors directly.
As the tube is used (many thousands of hours or from abuse), operating and starting voltages may be affected as well - generally increasing with the ultimate result being that a stable discharge cannot be initiated or maintained with the original power supply. See the section: How Can I Tell if My Tube is Good?.
(From: Lynn Strickland (stricks760@earthlink.net).)
Typical failure mechanism in a HeNe is cathode sputtering -- seldom gas leakage in the newer (like since 1983) tubes. Shelf life is stated to be about 10 years, but it's not uncommon at all to see HeNe lasers built in the early 1980's that still meet full spec.
Interesting lifetime note - it used to be that you left a HeNe 'on' at all times to prolong life. Since hard-sealing, you should turn it off while not in use. If it's a 20,000 hour tube, and you only turn it on for a few hundred hours a year, it will last a heck of a long time. Not uncommon at all for the HeNe to outlive several power supplies. The larger diameter tubes tend to last longer, but it also depends on fill pressure and operating current (higher fill-pressure tubes last longer). The typical 5 mW red HeNe will commonly live to 40k to 50k operating hours.
As for cathode sputtering, the tube has an aluminum cathode that is 'pickled' during the production process to add a layer of oxidation about 200 microns thick. The oxidation layer prevents aluminum from being bombarded away from the cathode during plasma discharge. As the tube ages, the oxide layer is depleted until aluminum is exposed. Sputtered aluminum can stick to the mirror, causing power decline, or to the inside of the glass envelope, causing the discharge to arc internally. This arcing, if allowed to continue for a period of time, will also cook the power supply. A tube with no oxidation layer on the cathode will die in about 200 hours of use. OR, once the oxidation layer is depleted, the tube will die in about 200 hours. This is why a HeNe life curve is usually pretty flat, then quickly degrading to nothing over about a 200 hour period.
As is typical of Spectra-Physics internal mirror HeNe tubes, these have thick glass walls (at least compared to tubes from most other manufacturers). For the barcode scanner application (at least) there was an outer wrap (removable) of several layers of thick aluminum foil, apparently for thermal stabilization but it would also reduce electrical noise emissions and light spill from the discharge. (The foil wrap also seems to be common with more modern Spectra-Physics HeNe barcode scanner tubes when not installed in cylindrical laser heads.) A 100K ohm ballast resistor stack in heat shrink tubing was attached with a clip and RTV Silicone to the anode end-plate stud, and both ends were capped with rubber covers for protection (of the tube and user).
The SP-084-1 is about 9-1/2" (241 mm) by 1" (25.4 mm) in diameter with a bore length of 5.5" (140 mm). Its output is a TEM00 beam about 0.8 mm in diameter exiting through a hole in the cover on the cathode-end of the tube. Power supply connections are made to a stud on the anode end-plate and the exhaust tube on the cathode end-plate. Their optimal operating point is around a tube current of 5 mA resulting in a total operating voltage (across tube + Rb) of about 1.9 to 2.0 kV using the 100K ballast.
Note from the diagram that unlike modern tubes where the mirrors are on mounts that can be adjusted (by bending) after manufacturer, alignment of the SP-084-1 would appear to be totally fixed. Some possible ways of setting alignment might be:
From appearances, I would guess (2). Since the mirrors are slightly curved (non-planar), their position could be used to adjust alignment slightly - and some were attached very visibly off-center to compensate for end-plates fused to the glass tube at a slight angle.
More recent HeNe laser-based laser pointers became more compact and could typically be powered either from a pair of internal 9 V ("transistor") batteries or a DC wall adapter. The circuitry was often set up so the both batteries could be inserted in either direction and still work correctly. But they never achieved keychain status, unless they were keys for elephants. :) I have HeNe laser pointers badged Kodak, Hitachi, and others. They typically output almost 1 mW. Battery life is, well, short. :) A cutaway view of one such unit is shown in Draper "Linear 1" HeNe Laser Pointer. It is about 6 inches in length with the (probably Siemens) laser tube being just over 5 inches long. It may be an LGR-7656, though the label is hidden. The HeNe laser power supply PCB extends the length of the unit with the pot core inverter transformer at one end and the HV components running to the other end. Note the copper strap "start" electrode surrounding the tube!
It was still possible (in 2010) to buy a HeNe laser in a compact package through Industrial Fiber-Optics (who acquired the Metrologic line of educational lasers). The Metrologic model 811 (red, $399) or 815 (green, $750) is not much over 1" x 2" x 7" and houses a 5 or 6 inch HeNe laser tube with HV power supply built-in. However, these are still tethered to either a DC wall adapter or box with several 9 V batteries. As of 2014, these were being phased out due to lack of demand and parts inventory!
There's not much intrest in these as pointers anymore, though they still are useful as compact lasers for alignment and other optics lab applications. But they are still very cute. :)
Note that the intensity of the light between the mirrors of an HeNe laser may be on the order of 100 times (or more) that of the output beam. Some instruments for making scattering measurements or related applications actually take advantage of this by using this only the 'internal' beam. Such a device could be constructed using an HeNe tube with at least one external mirror with optical sensors to observe only the scattered light from the side. In addition, the amount of attenuation due to the dust will affect the output beam intensity amplified by the gain of the resonator and this behavior can also be used in conjunction with various types of studies. By using these techniques, many of the benefits of a 1 W laser (for example) are available with only a 10 mW tube and at much lower cost. Such a laser is also much safer to use since that 1 W beam is in a sense, virtual - if anything of substantial size intercepts it (like an unprotected eyeball), lasing simply ceases without causing any harm.
Melles Griot and others offer Brewster window HeNe tubes rated up to 30 mW or more of output power and 60 Watts of intra-cavity power! As a rough estimate, a HeNe tube capable of n mW of normal output will be able to do 1000*n mW of circulating power with high quality HRs at both ends. Modern one-Brewster HeNe tubes for partical scattering or particulate monitoring applications may provide as much as 100 Watts of intra-cavity power using super-polished mirror substrates for the two HRs with ion beam sputter coatings and an optically contacted fused silica Brewster window. (The mirrors are about 15 times the cost of those used in common HeNe lasers. Don't ask about the total tube price!) A photo of one such tube is shown in Melles Griot 05-LHB-568 Ultra-High-Q One Brewster HeNe Laser Tube. The "32" was the measured intra-cavity power for this sample.
Specifications for a variety of one and two-Brewster HeNe tubes can be found in the section: Melles Griot Brewster and Zero Degree Window HeNe Tubes.
As noted, the best of these tubes will have optically contacted Brewster windows (rather than frit seals, more on this below). As frit cools, some stresses may build up which can distort the window ever so slightly reducing the tube's performance where hundreds or thousands of paases through the window are involved. Optical contacting uses lapped and polished surfaces to form a glass-to-glass vacuum-tight seal. Adhesive is only really needed for mechanical protection - it doesn't hold the vacuum. Soft-seal windows don't have the distortion problem but do leak over time.
(From: Lynn Strickland (stricks760@earthlink.net).)
"Brewster window terminated HeNe tubes are mostly sold into particle counter applications, where the user pulls an air stream through the cavity. With ultra low-loss ($$$) High Reflecting mirrors on both ends, massively multimode, you can develop 10 to 20 Watts of internal cavity power, we've seen as high as 30 Watts. Selling prices for new tubes is upwards of a thousand bucks in volume quantity (tubes only). The high-end models have an optically contacted Brewster window. There are not too many double-Brewster HeNe laser tubes made anymore, mostly on a special order basis. They're not that hard to align, if you know some tricks."
The tube is a Melles Griot model 05-LHB-570. It has an internal HR mirror and Brewster window at the other end of the tube. The HR is similar to those on other Melles Griot tubes (including the use of a locking collar) though the somewhat more silvery appearance of its surface may indicate that it is coated for broadband reflectivity and/or perhaps for higher reflectivity than ordinary HRs. (The mirror reflectivity of the HR on at least some versions of the 05-LHB-570 is greater than 99.9% from 590 to 680 nm but I don't think this one, which is quite old, has these characteristics.) The total length is about 265 mm (10.5 inches) from the HR mirror to the Brewster window. There is also a power sensor inside the head for (I assume) monitoring what gets through the HR mirror (untested).
CLIMET 9048 One-Brewster HeNe Laser Head shows the aluminum cylinder with its mounting flange at the Brewster window end, ballast resistor, and Alden connector. The other black wire attaches to the solar cell power sensor.
These one-Brewster HeNe tubes are generally used in applications like particle counting which requires high photon flux to detect specks of dust or whatever. Access to the inside of the resonator is ideal since with appropriate highly reflective mirrors at both ends, several WATTs of "virtual" circulating power can be produced inside the cavity of this HeNe laser. Thus, for these applications, they have the benefits of a high power laser without the cost or safety issues. There are even HeNe tubes similar to this that will do up to 45 W using super high quality mirrors and Brewster window. And, of course, they are also super expensive. Of course, you can't siphon off all that power - only be extremely envious and frustrated that it is trapped in there - but also safe from any sneak attacks on an unsuspecting eyeball. :)
A rig similar to the one from which the Climet 9048 was removed is a model 8654, whatever that means. It is shown in Climet Particle Counter Assembly - Front and Climet Particle Counter Assembly - Rear. There really isn't much inside - just some passages for the particle-containing gas which is directed to through the intracavity beam at one focus of a large aspheric lens which directs any scattered light onto a PhotoMultiplier Tube (PMT). The PMT is inside the black box at the lower left with its high voltage power supply above in the front view. The three-screw (sort of) adjustable mount for the external HR mirror is visible in the rear view. What's interesting is that there is really nothing physical to protect either the B-window or mirror from contamination by the flowing gas, except presumably by the flow pattern and pressure. There are separate compartments for the B-window and mirror, but they aren't sealed. However, it appears that during operation, those compartments are provided with a flow of higher pressure gas, filtered by the large canister visible in the photos. But, how they are expected to remain clean when the thing is shut down is a mystery. It is a particle counter after all. Aren't particles basically dust? :) OK, well, part of the secret is that apparently these things are intended to be looking at really clean air without many particles. A typical use would be in a semiconductor Fab Class 10 cleanroom - 10 or fewer particles (2 microns or larger) per cubic foot. This isn't your normal room air, which would be Class 10000 to Class 100000! :) Even so, the recommended service interval printed on the label is only 6 months.
With its wide bore, this tube has an optimal operating point (maximum power) of about 7.5 to 8 mA at about 1 kV (though the recommended current is actually 6.5 mA). This may just be a peculiarity of the sample I tested.
I have constructed a simple mirror mount so that various mirrors could be easily installed and there is easy access to the inside of the cavity. See HeNe Laser Tube with Internal HR and Brewster Window with External OC for a diagram showing this laser assembly. Using various mirrors, both from deceased HeNe lasers as well as from laser printers and barcode scanners, output power reached more than 3 mW and the circulating power inside the resonator peaked at over 1 W (but not with the same mirrors). With optimum high quality mirrors, it should be capable of more power in both areas. Photos of this laser are shown in Sam's External Mirror Laser Using One-Brewster HeNe Laser Head.
See the section: Sam's Instant External Mirror Laser Using a One-Brewster HeNe Tube for details on these experiments and the design of the mirror mount.
I have attempted to get wavelengths other than boring 632.8 nm red out of this and similar 1-B tubes. However, all attempts have failed but one - installing a somewhat larger 05-LHB-670 in place of the dead tube of a PMS/REO tunable HeNe laser. (This 1-B tube did 7.5 mW with the same OC mirror as used above. The 1-B tube in the Climet head probably woudn't have enough gain.) The HR mirror on the tuning prism is broadband coated for 543.5 to 632.8 nm. In this case, I was able to convince just a few 611.9 nm orange photons to cooperate and lase. However, the only way to collect them was from the reflections off the Brewster surfaces of the tube or prism, or from the HR mirror of the 1-B tube. The total orange power was around 225 microwatts - 50 uW from the HR mirror, 65 uW reflected from the Brewster prism, and 110 uW reflected from both surfaces of the tube's Brewster window. When 633 nm was selected, the output from the HR mirrors was about 350 uW (I didn't measure the red power from the Brewster reflections).
H. Weichel and L.S. Pedrotti put out a good summary paper which includes the equations used in the design process of a gas laser. In particular, section V tells you how to calculate mode radius at any point, given mirror curvature, spacing and wavelength. If you know that, the aperture size (the capillary bore usually) and the magic number for the ratio between the two, you can design a TEM00 gas laser. Using a HeNe tube with a Brewster window, you could do some fun stuff with predicting aperture sizes and locations to force TEM00 operation.
The paper was published by the Department of Physics, Air Force Institute of Technology, Wright-Patterson Airforce Base, OH. The title is "A Summary of Useful Laser Equations -- an LIA Report". Don't know where you'd find it, but the Laser Institute of America (LIA) might be a good start.
Needless to say, the parallel plate HeNe laser never took off but it was an interesting approach.
Note: Since the gain of these wavelengths is so low, they also have a shorter life and the chance of finding working surplus green or yellow HeNe lasers is much lower than for red. I would not recommend bidding on an eBay auction for one of these unless guaranteed to be working. The likelihood of the problem for an "unknown condition" green or yellow HeNe laser being just mirror alignment is small to none!
Typical maximum output available from (relatively) small HeNe tubes (400 to 500 mm length) for various colors: red - 10 mW, orange - 3 mW, yellow - 2 mW, green - 1.5 mW, IR - 1 mW. Higher power red HeNe tubes (up to 35 mW or more and over 1 meter long) and 'other-color' HeNe tubes (much lower - under 10 mW) are also available. However, these will be very large and very expensive.
A few tunable HeNe lasers have been produced commercially. These provide wavelength (color) selection with the turn of a knob. However, due to the low gain of most HeNe lasing lines, producing a useful tunable HeNe laser is not an easy task. Everything must be just about perfect to get the "other color" lines to lase at all, and even more so when a laser is to be designed to work at more than one wavelength with a TEM00 beam. The most widely known such laser (as these things go) is manufactured by Research Electro-Optics, Inc. (REO). It produces at least 5 of the visible wavelengths: normal red, two oranges, yellow, and green. A Littrow (or Brewster) prism with micrometer screw adjusters takes the place of the HR mirror in a normal HeNe laser. See the section: Research Electro-Optics Tunable HeNe Lasers.
There used to be a model ML-500 tunable HeNe laser from Spindler and Hoyer that did *14* lines between 611 nm and 1,523 nm. So no 604 nm orange, 594.1 nm yellow, 543.5 nm green, or 3.39 µm IR. The mirror set had to be changed to go between the visible and IR wavelengths. It used a Birefringent Filter (BRF) for wavelength selection instead of the Littrow prism in the REO tunable laser. A BRF has the advantage that there is no loss from a slightly incorrect Brewster angle for all but one wavelength, unavoidable with a Littrow prism. This is because the BRF is always set at exactly the Brewster angle. The birefringent crystal in the BRF filter produces a different optical delay for polarization components oriented in the direction of its slow and fast axes. Only when this difference is a multiple of a full cycle for any given wavelength, will the polarization be unchanged and thus result in minimal loss through the BRF. By rotating the BRF around its optical axis (still maintaining it at the Brewster angle to the laser's optical axis), the wavelength where minimum loss occurs can be selected. In 1987, it was only $5,800 for laser with either wavelength range, an additional $750 for the other mirror set
I don't know why Spindler and Hoyer would have admitted defeat in not including those other wavelengths as they were certainly known at the time. Perhaps, the losses through the two Brewster windows of their laser tube and the Brewster angled plate of the BRF compared to those of the Brewster window and Brewster prism of the PMS/REO tunable laser were just too high. Perhaps, their mirror coating technology was not as good as what PMS/REO had available.
Unfortunately, Spindler and Hoyer no longer makes this laser, only boring normal HeNe lasers and other optical equipment. However, a scan of the original ML-500 product brochure can be found at Vintage Lasers and Accessories Brochures and Manuals. With modern technology, a 17 line tunable HeNe laser should be possible. :) A tube with internal mirrors and a BRF *inside* would reduce the number of Brewster angle reflective surfaces to only 2, compared to the 3 of the PMS/REO design. A magnetic coupling can be used to move the BRF from outside the tube. In addition, the mirrors can be recessed away from the ends of the tube so they don't experience any high temperatures during the sealing process. The tube itself would be hard-sealed with frit or regular glass. Then optical contacting or leaky Epoxy seals can be avoided. Use a Brewster angle window to pass the laser beam out of the tube. One of the mirror mounts would be attached via a metal bellows to allow for alignment.
For reference, here are the approximate red HeNe parameters close enough for Government work: ;-)
Where more decimal points matter, a single mode frequency stabilized HeNe laser will have very nearly a constant single wavelength precise to 9 or more significant figures but it too will be affected by various physical parameters including the exact length of the laser's cavity, gas pressure and He:Ne fill ratio, and temperature - there is no single correct answer!
For example, one typical stabilized HeNe laser from Hewlett-Packard, has a precise vacuum wavelength of 632.991372 nm. Another one from Melles Griot (as noted below) is 632.991058 nm in vacuum or 632.81644 nm in air (divide by the index of refraction of air, n=1.00027593).
(Portions from: Jens Decker (Jens.Decker@chemie.uni-regensburg.de).)
The Melles Griot catalog claims a nominal frequency of 473.61254 THz for their 05-STP series of frequency stabilized lasers. (Elsewhere in the same catalog they are more precise and lists 473.612535 THz for the 632.8 nm line.) Anyhow, with c = 2.997925E8 m/s this gives 632.991058 nm in vacuum or 632.81644 nm in air for n = 1.00027593 (formula from J Phys.E, vol. 18, 1985, pp. 845ff). To find reliable values for all the other HeNe lines is quite difficult. One has to compare a number of books to be sure whether the values are for air or vacuum.
(From: D. A. Van Baak (dvanbaak@calvin.edu).)
Well, here it is depending on the level of precision desired:
The metrologists' answer for a 632.8 nm HeNe laser stabilized to the a-13 component of the R(127) line of the 11-5 transition of the 127-Iodine dimer molecule is:
under certain specified conditions, with uncertainty 2.5x10-11. See: "Metrologia", vol. 30., pp. 523-541, 1993-1994.
The minimum divergence obtainable is affected mostly by beam (exit or waist) diameter (wider is better). Other factors like the ratio of length to bore diameter (narrower is better) may also affect this slightly. The equation for a plane wave source is:
Wavelength * 4
Divergence angle (half of total) in radians = --------------------
pi * Beam Diameter
So, for an ideal HeNe laser with a .5 mm bore at 632.8 nm, the divergence
angle will be about 1.6 mR. Note that although a wider bore should result
in less divergence, this also permits more not quite parallel rays to
participate in the lasing process. This assumes planar mirrors - which few
HeNe lasers use. Where one or both mirrors are curved, the divergence
changes. For example, it is common with HeNe tubes for the Output Coupler
(OC) mirror to be ground slightly concave and for the High Reflector (HR)
mirror to be planar. If the outer surface of the OC glass is not also
curved to compensate for the negative lens that results, the beam will
diverge at a much higher rate than would be expected for the bore diameter.
HeNe laser tubes destined for barcode scanners often have a much higher divergence by design - up to 8 mR or more (where the optimal divergence may be as little as 1.7 mR or less). These tubes either have a negative curvature for the outer surface of the OC mirror glass (concave inward) or even an external negative lens attached with optical cement. See Uniphase HeNe Laser Tube with External Lens. The outer surface of OC in a normal HeNe tube will either be planar or slightly convex depending on whether the OC mirror is planar or slightly concave respectively. In the latter case, the convex surface precisely compensates for the extra divergence produced by the OC mirror curvature and results in a nearly optimally collimated beam. If the outer surface of your HeNe tube's OC is concave, then it will have the high divergence characteristic. Note that the beam is still of very high quality but an additional positive lens approximately one focal length away from the OC will be required to produce a collimated beam.
Also see the section: Improving the Collimation of a HeNe Laser with a Beam Expander.
For a short tube (e.g., 5 or 6 inches), this is easily observed by placing a polarizer in the beam. At certain orientations, the beam brightness will then appear to go through cycles - light, dark, light, etc. However, polarization can be affected by external means. See the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
Most internal mirror HeNe tubes should not have any higher order transverse (non-TEM00) modes. And, for multimode tubes, such modes should show up as part of, or adjacent to the main beam anyhow.
One possible cause for this artifact is that the output-end mirror (Output Coupler or OC) has some 'wedge' (the two surfaces are not quite parallel) built in to move any reflections - unavoidable even from Anti-Reflection (AR) coated optics - off to the side and out of harm's way. Where wedge is present, the small portion of the light that returns from the outer AR coated surface of the OC will bounce back to the mirror itself and out again at a slight angle away from the main beam. In a dark room there may even be additional spots visible but each one will be progressively much much dimmer than its neighbor. Note that if the laser had a proper output aperture (hole), it would probably block the ghost beams and thus you wouldn't even know of their existence!
Without wedge, these ghost beams would be co-linear with the main beam (exit in the same direction) and thus could not easily be removed or blocked. This could result in unpredictable interference effects since the ghost beams have an undetermined (and possibly varying) phase relationship with respect to the main beam. Sort of an unwanted built-in interferometer! The wedge also prevents unwanted reflections from that same AR coated front surface back into the resonator - perfectly aligned with the tube axis - which could result in lasing instability including cyclic variations in output power.
Thus, the ghost beam off to one side is likely a feature, not a problem! The effects of wedge on both the output beam and a beam reflected from a mirror with wedge is illustrated in Effects of Wedge on Ghost Beams and Normal Reflections. Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".
If it isn't obvious from close examination of the output mirror itself that the surfaces are not parallel, shine a reasonably well collimated laser beam (e.g., another HeNe laser or laser pointer) off of it at a slight angle onto a white screen. There will be a pair of reflected beams - a bright one from the inner mirror and a dim one from the outer surface. As above, if the separation of the resulting spots increases as the screen is moved away, wedge is confirmed (there may be higher order reflections as well but they will be VERY weak - see below). Where the mirror is curved, the patterns will be different but the wedge will still result in a line of spots at an angle dependent on the orientation of the tube.
Wedge is often present on the other mirror (High Reflector or HR) as well (in fact, this appears to be more likely than the OC). Wedge at the HR-end won't affect the output beam at all but performing the reflectance test using a collimated laser (as above) at a near-normal angle of incidence may result in the following:
With the exaggerated amount (angle) of wedge in Effects of Wedge on Ghost Beams and Normal Reflections, another effect becomes evident: The weaker spots are spaced further apart. It is left as an exercise for the student to determine what happens when a laser beam is reflected at an angle from such a mirror! Note that his diagrams shows the effect of a beam coming in from the right and reflecting off the mirror. Where the beam is from the tube itself, the main beam corresponds to the one marked "1st Back Surface".
The appearance resembles that of a diffraction grating on such a beam (but for entirely different reasons). The behavior will be similar for an OC with wedge but because the HR mirror isn't AR coated, the higher order spots (from the HR) are much more intense.
It is conceivable that slight misalignment of the mirrors may result in similar ghost beams but this is a less likely cause than the built-in wedge 'feature'. However, if you won't sleep at night until you are sure, try applying the very slightest force (a few ounces) to the mirror mounts (the metal, not the mirrors as they are very fragile) in each while the tube is powered (WARNING: High Voltage - Use a well insulated stick!!!!).
Depending on the type of laser you have, see the sections: Checking and Correcting Mirror Alignment of Internal Mirror Laser Tubes, Quick Course in Large Frame HeNe Laser Mirror Alignment, and External Mirror Laser Cleaning and Alignment Techniques, for more information.
Another much simpler cause of an ugly beam from a HeNe (or other) laser is dirt on the outside of the output mirror since this will decrease the effectiveness of the AR coating. The dirt may also be on other external optics. Some HeNe laser heads have either a debris blocking glass plate glued at an angle to the end-cap or a neutral density filter to adjust output power. Even if AR coated, either of these may also introduce one or more ghost beams and if not perfectly clean, other scatter as well. I'm gotten supposedly bad HeNe lasers where the only problem was dirt on either the output mirror or external plate or filter.
(From: Steve Roberts.)
The mirror is wedged to cut down on the number of ghost beams, however even with a wedged mirror there is almost always one ghost. Nothing is wrong with your coatings on the mirror, it is simply a alignment matter. The mirrors need to be "walked" into the right position relative to the bore. There are many many paths down the bore that will lase, but only a few have the TEM00 beam and the most brightness, this generally corresponds to the one with minimum ghosts.
See the section: Quick Course in Large Frame HeNe Laser Mirror Alignment for more information.
But if you look at the output of a HeNe laser with a spectrometer, there will be dozens of wavelengths present other than one around 632.8 nm (or whatever is appropriate for your laser if not a red one). Close to the output aperture, there will be a very obvious diffuse glow (blue-ish for the red laser) visible surrounding the actual beam. So why isn't the HeNe laser monochromatic as expected?
With one exception, this is just due to the bore light - the spill from the discharge which makes it through the Output Coupler (OC) mirror. As your detector is moved farther from the output aperture, the glow spreads much faster than the actual laser beam and its intensity contribution relative to the actual beam goes down quickly. It is not coherent light but what would be present in any low pressure gas discharge tube filled with helium and neon. However, the presence of these lines can be confusing when they show up on a spectral printout.
The exception is that with a 'hot' (unusually high gain) tube or one with an OC that is not sufficiently narrow-band, one (though probably not more though not impossible) of the neighboring HeNe laser lines (e.g., for other color HeNe lasers) may be lasing though probably much more weakly than the primary line. For example, a red (632.8 nm) laser might also produce a small amount of output at 629.4 or 640.1 nm though this isn't that common. For many applications, a bit of a "rogue" wavelength output is of little consequence and specifications for general purpose HeNe lasers usually don't explicitly include any mention of them. However, rogue output will cause reduced accuracy in metrology applications and since they may not be TEM00, even where the beam is simply used for alignment.
I have a couple of 05-LHP-171 lasers that produce up to 10 percent of their output at 640.1 nm. The first is of unknown pedigree obtained in a lot laser junk from a well known laser surplus dealer. It may have been rejected for other reasons since the output at 632.8 nm is only about 4 mW when it should be well over 7 mW. The 632.8 nm is the normal TEM00 but the 640.1 nm beam may be TEM01 or TEM10 (2 modes) or even TEM11 (4 modes) depending on mirror alignment. With optimal mirror alignment for 632.8 nm, there may be no 640.1 nm at all. The other is a 25-LHP-171-249 system sold to a university lab. It has a manufacturing date of 2000, so this isn't only a problem with old lasers as some people have claimed.
I have one 'defective' yellow (594.1 nm) HeNe tube that also produces a fair amount of orange (604.6 nm), and another that produces in addition some of the other orange line (611.9 nm).
While the probability of a commercial HeNe laser outputting at a rogue wavelength is low, where such a laser is used for measurements assuming pure 632.8 nm, errors could result. For more on this topic, see the paper:
In the course of research for this paper, the first author, Jack Stone, borrowed one of my interesting Melles Griot 633 nm lasers that produced 5 to 10 percent of its output at 640.1 nm! :)
And sometimes HeNe lasers are designed to produce more than one wavelength. PMS/REO used to produce such lasers, usually a visible and IR line, or a pair of IR lines. But one that I've acquired is a combination yellow (594.1 nm) and green (543.5 nm). Unlike many other PMS/REO lasers that produce multiple lines by accident :), this one was either designed that way or their Marketing department decided to convert a bug into a feature, since the laser head has a model number of LHGYR-0300M. Since these wavelengths are so far apart and are low gain, such an occurrence would normally not be likely, but I've also seen one where this was the case - a yellow laser that had a tiny bit of green.
I also have one that does orange (612 nm) and green (543.5 nm) but only under special conditions. It is a high mileage PMS/REO LHOR-0150M so it's presumably supposed to be 612 nm only. But when run at lower than normal current, the green line pops up weakly. Just above the dropout current at 4.5 mA, it produces about 0.5 mW of orange and 0.05 nm of green. At 6.5 mA, it produces around 1 mW of pure 612 nm orange. I rather suspect this is a peculiarity of the tube running near end-of-life with a lower gas pressure. When new, it may not have been so interesting. :)
For more on multiline HeNes, see the sections starting with: The Dual Color Yellow/Orange HeNe Laser Tube. And to make your own (sort of), see the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.
(From: Prof Harvey Rutt (h.rutt@ecs.soton.ac.uk).)
For gas lasers the plasma lines are typically 80 dB or more below the output (measured, of course, within the very small laser mode divergence). This is unlike most semiconductor lasers, which typically have broad 'shoulders' close in to the line, as well as 'lines' due to other modes and instabilities because the initial divergence of the diode is high, and spontaneous emission from the junction high, the broad background tends to be large.
For gas lasers it is usually in the form of narrow lines at remote wavelengths, very easily removed with an interference filter and/or spatial filtering in the *rare* cases where it matters. There is presumably a weak broad background from processes involving free electrons (bound/free and free/free), but I've never seen it even mentioned, let alone observed it. More likely to be significant in the high current density argon laser than the very low current density HeNe.
The only cases I have seen where the plasma lines caused problems were Raman measurements on scattering samples with photon counting detection, and weak fluorescence measurements which are similar.
In most cases scattered light in the monochromator is much more of an issue (hence double monochromators for Raman) and will obscure plasma lines in many cases.
I've gotten most of the well known HeNe lasing lines in this manner including up to 4 mW of red from a 2 mW yellow HeNe laser, both orange lines, various other red lines, and one of the wavelengths that isn't even mentioned in most texts dealing with HeNe lasers. More below. I've only heard of one instance of any yellow being produced from non-yellow tubes, that being a REO 612 nm laser. And I haven't even attempted to obtain green from non-green tubes.
Here's how to get other wavelengths from your HeNe laser. Either a bare tube or complete laser head can be used for these experiments.
The Radius of Curvature (RoC) of the external mirror may need to be consistent with a stable resonator configuration for the overall cavity. I'm not entirely sure this matters that much (and the implication in the next section is that it may not), but I'd still go with a stable configuration given a choice. If you don't want to perform the calculations, a mirror that should work would be one from a dead red HeNe laser at least as long as the tube you are using. Of those I tried that worked at all, minimizing the distance between the ecternal mirror and tube resulted in the best results but this may not always be true. A dielectric mirror is definitely preferred but a good quality aluminized front surface (planar) mirror should work, though it may not be as good.
The quality of the beam from the end of the tube opposite where your external mirror is located will probably be better, especially if the mirror is beyond the HR of the tube (which may have some wedge and is not AR coated). However, the beam from the external mirror end is instructive at least in helping to adjust the alignment.
Using my Melles Griot 05-LYR-170 yellow HeNe tube which for my "broken" sample, actually lases a combination of yellow (594.1 nm) and orange (604.6 nm) from both ends (see the section: The Dual Color Yellow/Orange HeNe Laser Tube), it was quite easy to achieve red output, and all three colors were occasionally present at the same time - an impressive achievement for a HeNe laser. My setup is shown in 05-LYR-170 HeNe Laser Tube Mounted in Test Fixture for Multiline Experiments. The output from the tube's OC was directed at an AOL CD used as a reflective diffraction grating with the first-order beam projected on a white card several feet away. An MSN CD would work just as well :) but a CD-R or CD-RW may not. The lens from a pair of eyeglasses (mildly positive, about 4 diopters or 1/4 meter focal length) narrowed the spots to improve spectral resolution. This rig could easily resolve lines separated by less than 1 nm. The first external "red" mirrors I tried were from an SP-084 HeNe laser tube but due probably to their relatively short RoC, the 05-LYR-170 had to be pushed quite close to the mount to get any red output. Mirrors designed for a longer laser worked better but there wasn't much difference between the behavior using an HR or OC (99 percent).
Then to add to the excitement, with a bit of twiddling, I was able to obtain the other orange line (611.9 nm) as well, and at times, all 4 lines were lasing simultaneously! As expected, this additional line was only present when using an external HR. Depending on the original makeup of the yellow and orange beam (for this tube, their absolute and relative intensities varied with time and were also a very sensitive function of mirror alignment), it was possible to get mostly red or to vary the intensities of the other colors, most easily suppressing yellow in favor of orange and red. The intensity of the red output was never more than 1 mW or so. Its transverse mode structure varied from TEM00 to a star pattern with nothing in the center. Strange. Due to both surfaces of the HeNe tube's HR mirror reflecting some of the intracavity beam resulting in a multiple cavity interference effect, there was a distinct lack of stability. To help compensate for this, a micrometer screw to precisely adjust cavity length without affecting mirror alignment would have been nice.
I also tried this with the external mirror mounted beyond the tube's OC mirror but although there was a definite effect on yellow and orange lasing, it wasn't possible to obtain any red output. (For the 05-LYR-170, the OC already reflects red quite well and the HR doesn't.) Finally, I replaced the red external mirror with a green HR (from a tube of about the same length) mounted beyond the 05-LYR-170's OC (since its HR by appearance looked like it might be a good mirror for green). But, not surprisingly, while this could affect the lasing of the yellow and orange lines, I could detect no coherent green photons. However, I would expect that with a appropriately coated mirrors (or possibly two such mirrors, one beyond each end of the tube), obtaining lasing at the relatively high gain 640.1 nm red line would be easy - the usual "red" mirrors may deliberately kill this line to prevent it from lasing. Although I couldn't detect any evidence of lasing at the other red lines of 629.4 nm and 635.2 nm, these should also be possible with appropriate mirrors as they have higher gain than the yellow and oranges. Another interesting one would be the "Border Infra-Red" line at 730.5 nm. Lasing at the IR lines might also be possible but they are so boring. :)
Next, determined to do something with a more normal HeNe laser tube, I tried a Siemens tube but that refused to do anything interesting. Then, I tried a Melles Griot 05-LHR-150 which typically outputs a 5+ mW red (632.8 nm) beam. Since the OC for this laser is probably around 99% reflective at most, peaking at 632.8 nm, I figured that it would be best to place the external mirror beyond the OC rather than the HR. And, with the same external HR as used above, it was possible to obtain 6 lasing lines, count'm 6: 629.4 nm, 632.8 nm, 635.2 nm, 640.1 nm, a line popping up around 650 nm (all variations on red), ****AND**** 611.9 nm orange! However, since the output is being taken from the HR, none of the colors was more than a fraction of a mW.
Lasing of the 650 nm line was hard to obtain - it only showed up for a few seconds off-and-on every few minutes and increasingly rarely after the tube warmed up. The exact wavelength is very close to 650 nm (649.98 nm) as determined later with an Agilant 86140B Optical Spectrum Analyzer (OSA) which is a lot more expensive than my AOL CD. :) (The wavelength was referenced to the 632.8 line from the same laser resulting in a measurement error bound of +/- 0.02 nm assuming the 632.8 nm line is actually 632.8 nm. But since this could also be slightly shifted, the error may be higher.) Getting anything at 650 nm is really puzzling as there are no HeNe lasing lines between 640.1 nm and 730.5 nm. But I have no doubt it is a true lasing line since it was fluctuating independantly of the others (later confirmed, see below). And all those other lines were quite accurately located corresponding to their handbook wavelengths in the diffracted pattern (and later confirmed with the OSA). So there is little reason to suspect that the funny one isn't as well. When present, it appeared as strong (or weak) as all the expected ones, (except of course, the original 632.8 nm line which was usually, but not always, the strongest). If 650 nm is not a HeNe lasing line - it's certainly not in the sequence of energy level transitions that produce all the other visible HeNe lines - one possible explanation is that there is some trace element present inside the tube and that is what's lasing, not neon. I figured this to be a distinct possibility since the particular tube I am using originally had gas contamination and I revived it by heating the getter. (See the section: Repairing the Northern Lights Tube.) Therefore, the 650 nm wavelength may not be present with another more normal tube. But as it turned out, contamination has nothing to do with it.
I don't think the 730.1 nm line was present but given its low relative perceived brightness, it may not have been visible at all using my AOL Special CD diffraction grating but I couldn't find it with the OSA either. It took awhile to detect the evidence of the 635.2 nm line which only appeared sporatically (but it is the lowest gain of all the known ones above).
A few days later, I tried the same experiment with a couple of my old Spectra-Physics 084-1 HeNe laser tubes which are of soft-seal design so have almost certainly leaked over time (but still work fine). With my "hottest" SP084-1 (about 2.9 mW), I could almost duplicate the results of the 05-LHR-150 including the funny line around 650 nm but minus anything at 635.2 nm. Using a more normal 2.4 mW SP084-1, it was possible to obtain (non 632.8 nm) lines at 629.4 nm and 640.1 nm. For these, an SP084-1 HR worked almost as well for the external mirror as the longer RoC HR I had been using with the 05-LHR-150. I then installed a SP098-1, a common hard-seal barcode scanner tube (this sample puts out about 1.4 mW). With that, the only additional line was at 640.1 nm. Which particular lines appear in each case seem consistent with the length of the tubes (and thus the single pass gain) and the relative gain of the lasing lines.
Some quick calculations predict that the real effect of the external HR mirrors is the obvious one - to increase the circulating power. A 1 percent OC (typical) followed by even a 90 percent external mirror would result in greater than a 99.9 percent effective mirror for a range of wavelengths/modes. An external 99.9 percent HR would result in an even better effective mirror. It looks like the reflectance peak is relatively broad with respect to wavelength (the transmission peak is rather narrow). Specific modes for each of the wavelengths will be enhanced or suppressed. This would also appear to be consistent with the apparent lack of need for the external mirror to result in a stable resonator. All it has to do is form a Fabry-Perot cavity.
These have to be classified right up there in the really fascinating experiments department. Seeing any HeNe laser operating with multiple spectral lines is really neat.
For more examples of these stunts using an already interesting "defective" HeNe laser, see the sections starting with: Melles Griot Yellow Laser Head With Variable Output and in particular, the section: External Mirror Therapy for Variable Power 05-LYR-171 Yellow Laser Head.
As always, depending on mirror reflectivity and other factors, your mileage may vary. But feel free to try variations on these themes. The results from using an HeNe HR beyond the OC of almost any red HeNe laser tube should be easily replicated (except perhaps for the funny 650 nm line). Almost any mirror will do something since even an aluminized mirror will be returning over 90 percent of the otherwise wasted photons to the cavity - enough to boost the gain of all but the weakest lines enough for lasing if everything lines up just right. Aside from getting zapped by the high voltage or dropping the tube on the floor, they are low risk, high reward experiments.
And, can you believe that people get stuff like this published in scholarly journals? I was recently sent an article entitled: "Yellow HeNe going red: A one-minute optics demonstration" by Christopher Hopper and Andrzej Sieradzan, American Journal of Physics, vol. 76, pp. 596-598, June 2008. Geez, they could have saved a lot of time and effort and come here instead. Or, perhaps they did. :)
(From: Bob.)
For neutral neon at low pressure, the lines 640.3 nm, 659.9 nm are listed. For neutral helium, there is one at 667.8 nm. None of the other noble gases have wavelengths listed this short. As far as ionized species go, singly ionized argon has a line at 648.30 nm. Singly ionized krypton has a hand full of lines from 647 nm to 657 nm. Finally, xenon has one at 652 nm.
For atmospheric gases, there is a singly ionized nitrogen line at 648.3 nm. There are no neutral lines of interest for atmospheric gases. The footnotes for the above line were listed as CW lasing in 0.02 torr of krypton. Whats the standard operating pressure of a HeNe laser? Not THAT far out of the ball park I would guess.
(From: Sam.)
The last one sounds promising and would make sense given the history of the particular 05-LHR-150 and the soft-seal design of the SP084-1. Though HeNe lasers operate in the 2 to 3 TORR range - about 100 times higher pressure, the partial pressure of any N2 contamination could very well be down around 0.02 Torr.
However, I now know exactly where the 650 nm line is coming from and it has nothing whatsoever to do with contamination. The exciting writeup from someone who beat me to this by about 15 years follows in the next section preceeded by a condensed version, below.
I've also found a commercial laser that appears to produce a very stable 650 nm line. See the section: The PMS/REO External Resonator Particle Counter HeNe Laser.
(From: Stephen Swartz (sds@world.std.com).)
Lasing of certain HeNe tubes at 650 nm is a known phenomenon and not just a hallucination. The 650 nm line which is never discussed in most standard texts is not due to a "normal" transition of neon. It comes instead from a Raman transition. The 650 nm line is not often observed but when it is it will always be seen simultaneously with operation on a multitude of other lines. A large number of other "unusual" colors have been seen over the years. Higher power tubes with mirrors that are excessively broadband are your best bet for observing them. Often these lines flicker on and off over a few seconds to minutes time scale. A diffraction grating is a good way to look for them.
(From: Someone at a major laser company.)
The 650.0 nm Raman line is a known problem in that it competes for power with the 632.8 nm line intermittently, particularly in long tubes with high circulating power. Polarized tubes are much less susceptible to this effect and using a lower reflectance for the OC mirror helps since it reduces circulating power without affecting output very much (over a reasonable range).
(From: Bruce Tiemann (BruceT@ctilidar.com).)
I have gotten many lines from many different HeNe lasers. In my experience almost every tube is capable of giving at least one other line than 633 nm. (Most wavelengths have been rounded to save bits. So, 632.8 nm becomes 633 nm.) I have never tried doing this with lasers that give other lines than 633 nm, but since that line has the highest gain, it should be no mean feat to at least get that line from lasers that are supposed to not give it. It is also not my experience that calculations to ensure resonator stability, etc., are necessary. Just try it! My best results, in terms of output power, were with a flat grating as the external feedback mirror, and my best results in terms of new lines was obtained with a flat dielectric mirror, formerly used as a facet in a polygonal scanning assembly. Flat mirrors are not stable at any separation for a diverging beam, and HeNe lasers are very rare that give converging beams for their output.
The home stuff had the mirrors on blocks, with the steering accomplished by adjusting the HeNe tube by lifting one or the other end of the tube with sheets of paper, and the azimuth by moving the laser tube back and forth. The lab experiments were done with "real" mirror mounts, supplemented by a single PZT that tilted the feedback mirror a few microns.
(I like PZTs a great deal, and would like to observe that you can get PZT elements from little piezo alarms, from which the useful element can be extracted with some hand-tools and the mind-set of a 9-year-old kid dissecting a bug. :) These are only about $1 each, as opposed to tens to hundreds of bucks for "real" PZTs that you buy from Thor, etc. One of them and a 0 to 50 VDC power supply can precision-wiggle a mirror on the micron scale, which is all that is needed for these experiments.)
(From: Sam.)
I have indeed done something similar using the piezo beeper from a dead digital watch to move a mirror in a HeNe laser based Michelson interferometer. With 0 to 25 V, it went through 4+ fringes which means over 2 full wavelengths at 633 nm. The configuration in these is called a "drum head" piezo element because the movement resembles that of a musical (depending on your point of view!) drum head with the most shift in the center. The piezo material itself doesn't change by very much in thickness but is constructed so it distorts to produce the shape change. With care, the piezo material can be cut to size or drilled to pass light through its center. Much more voltage could have been safely applied if needed.
(From: Bruce.)
Something I also did is cast the spots from a smaller (approximately 3/4 m) spectrometer directly onto the CCD element of a small camera with no lens. I also fabricated a beam block by taping little wires to the side of a block, that would protrude up just in the locations of the very bright lines, like 633, 650, and 612 nm, to block them, but letting light of other colors pass in the ample space between the wires. You could still see when the bright lines were on from light leaking around the wires, but it wouldn't wash out the image when they were.
In this case, when the feedback mirror was tilted, speckle, which was cast everywhere, would kind of shift around all over the place, but the new lines looked like ghostly bullseyes, which would breathe in and out as the mirror was tilted, but remain in the same location unlike the speckle. This was an easy way to see the weakest lines like 624 nm, and it was also how I discovered 668 nm, the CCD being more sensitive than the eye in the deep red. (I searched for but did not find the normal laser line 730 nm even with this very sensitive method.)
That 640 nm line would lase even with a plastic ruler or similar non-mirror mirror and could be established by hand-holding the piece of plastic in the beam, braced against the laser tube.
The gain-bandwidth of the Raman transition is only 60 MHz wide, so the cavity modes for 633 nm must line up with the cavity modes at 650 nm, to within this uncertainty, in order for 650 nm to oscillate. Considering that the Doppler bandwidth is more like 1,500 MHz (1.5 GHz), and the FSR of the laser is ~hundreds of MHz, that is only rarely the case. Hence, 650 nm comes and goes, most of the time being gone. And when it's gone, it's gone. When the laser warms up, however, the cavity expands, and the 633 and 650 nm modes sort of vernier past each other, sometimes bringing them into alignment in difference-frequency space. When they align, 650 nm oscillates. The observed behavior is that 650 nm more rapidly blinks on when the laser is warming up, but only for short periods, and then as the tube comes closer to a steady-state temperature, the periods become less frequent, but 650 nm lasts for a longer duration each time. Eventually, at the steady-state condition, 650 nm will be gone, or more rarely, may persist. However, temperature control of the laser can cause 650 nm to become steady, in the low-tech way of putting a blanket made of paper sheets or something over the laser tube, to stabilize the laser tube temperature to the next-higher value that supports 650 nm oscillation, or in the higher-tech case with a heater tape and thermistor and temperature control unit. When 650 nm goes, it is strong, and one 5 mW tube gives nearly 1.5 mW of output power at 650 nm, when the feedback element was a metal grating, and the output was taken from the first order. It is also perceptibly a deeper red color than 633 or even 640 nm, to me.
I (Sam) have tested two external PMS/REO particle counter assemblies lasing on up to 6 normal HeNe lines (605, 612, 629, 633, 635, 640) and the 650 nm Raman line where the 650 nm line was present 100 percent of the time with little variation in intensity. No stabilization of any kind is involved and the behavior is little changed from power-on to thermal equilibrium. This seems to directly contradict the need to simultaneous resonance mentioned above and in the papers. On one sample, I believe the 650 nm line was actually the strongest one, stronger even than the 633 line. On that laser, there was even the occasional hint of another strange line at around 653 nm. Also see the section: The PMS/REO External Resonator Particle Counter HeNe Laser.
These lines were best observed with the high-R feedback mirror located within about 6 inches of the output face of the laser, closer tending to be a bit better. Except for 589 nm, which required 605 nm to be oscillating, and this only occurred for one exact spacing of feedback mirror about 1.5 cm away from the output coupler, or about 1 mm away from the output flange of the tube, which I didn't remove. (I did, however, find out that you can take a laser tube to the university infirmary, and ask to have it X-rayed, to determine the extent of the internal glass envelope within the aluminum outer casing, and they would only charge you $10 for the cost of the X-ray film and processing, which is not bad for a doctor visit including X-rays.)
To my knowledge, these lines are my discovery.
A brief table shows the relationship between "pump" lines and 4-wave mixing lines, observed on one tube. Upper Sideband is toward shorter wavelengths (higher optical frequencies) from the pump; Lower Sideband is toward longer wavelengths of the pump (all values in nm):
Upper Pump Lower Sideband Wavelength Sideband -------------------------------- 589.7 604.6 ----- 596.6 611.9 627.8 613.3 629.4 646.4 616.5 632.8 (650.0) 618.8 635.2 652.5 623.4 640.1 ----- (632.8) 650.0 668.1
(632.8 and 650.0 nm are parenthesized since they are associated with the genesis of the 4-wave mixing lines.)
All in all this laser produced 17 different lines, many at one time, from a "single line" 633 swap-meet laser. :)
(From: Sam.)
I have augmented the table to include the missing value and those for 594.1 nm which might be present in a multi-line tube but not in Bruce's laser:
Upper Pump Lower Sideband Wavelength Sideband -------------------------------- 579.7 594.1 609.2 589.7 604.6 620.3 596.6 611.9 627.8 613.3 629.4 646.4 616.5 632.8 (650.0) 618.8 635.2 652.5 623.4 640.1 ----- (632.8) 650.0 668.1
References:
The 650 nm discovery paper is:
Abstract: Continuous lasing has been observed at 650 nm with a helium-neon electrical discharge placed in an ultrahigh finesse optical cavity. This new lasing line is attributed to a Stokes-Raman process between the 1s5 and 1s4 electronic states of neon atoms pumped by the 632.8-nm neon lasing line. A gain calculation based on a near-resonant stimulated electronic Raman process predicts a lasing threshold for the 650-nm line near that measured. Lasing output power was measured as a function of discharge current and helium-neon gas pressure for the pump line and for the Stokes line.
Abstract: The 650 nm laser line of a simultaneously operating six-wavelength He-Ne laser was studied experimentally. It is shown that the precise lasing wavelength at 650 nm should be 650.00 + or - 0.05 nm and that this laser line is the result of the stimulated Raman scattering of the 632.8 nm transition between the 1S5 and 1S4 states. The characteristics of the Raman emission are studied and the gain is obtained.
Abstract: Simultaneous CW laser emission has been observed in a He-Ne discharge at 611.8-, 629.3-, 632.8-, 635.1-, 640.1-, and 650.0-nm wavelengths. The output power and the mode spectra have been investigated for various operational conditions. Spontaneous mode locking of the different lines has been observed. The Raman transition (650.0 nm) pumped by the strong intracavity radiation at 632.8 nm has been investigated in detail and its relevance for a secondary multiwavelength standard is discussed.
I have a small Yellow Tube - 05-AYR-006 (as a combo with power supply 05-LPM-496-037). This tube is physically the same size as the 1mW reds, but has a larger bore resulting in multimode output.
I have managed to get the red (632.8nm) line to lase and perhaps orange lines by placing a HR from a 632.8nm HeNe at the HR end of the yellow tube.
Further, I obtained a broadband mirror from an Argon Laser tube, the OC worked best at the OC end of the Yellow tube, have got the laser to output green...
The mirrors hand-held - next to build a small external resonator assembly.
Normally, what comes out in that direction is, well, waste, and is of no consequence. But, there are times where it's convenient to use this low power beam as a reference, expecting its power to track that of the main output beam. Unfortunately, it is sometimes not well behaved in this regard.
In constructing some amplitude stabilized HeNe lasers which depend on the waste beam feeding a photodiode for their feedback loop, an annoying characteristic of the waste beam has become evident with some otherwise perfectly normal and healthy HeNe laser tubes. Namely, that the relative power in the waste beam and the main beam does not remain constant as the tube warms up. In fact, one tube I was using had a variation of almost 2:1 in relative waste beam and output beam power depending on the tube's temperature. This is probably due to one or both of the following:
The coating problem is more likely to result in a strictly increasing, or at least slow change in waste beam power with higher temperature while the etalon would be periodic with temperature going through several cycles, it might be possible to determine which of the two effects is present.
Normally, the waste beam is not used for anything and no one cares. Though there will also be a change in the power of the output beam (inversely relative to the waste beam) from these issues, it will be too small to be detectable without careful measurements, being swamped by the normal mode sweep power variations. But when the waste beam is used as the amplitude reference in a stabilized laser, the supposedly stabilized output will vary based on the relative waste beam power. That 10 uW change would result in the output power changing by 33 percent.
For some plots the mode sweep of normal and naughty tubes, see the section: Plots of HeNe Laser Power Output and Polarized Modes During Warmup. In particular, compare the plots of the Spectra-Physics 088 with those of the tubes that immediately follow it.
Magnets may be incorporated in HeNe lasers for several reasons including the suppression of IR spectral lines to improve efficiency (such as it is!) and to boost power at visible wavelengths, to control its polarization, and to split the optical frequency into two closely spaced components. There are no doubt other uses as well.
The basic mechanism for the interaction of emitted light and magnetic fields is something called the 'Zeeman Effect' or 'Zeeman Splitting'. The following brief description is from the "CRC Handbook of Chemistry and Physics":
"The splitting of a spectrum line into several symmetrically disposed components, which occurs when the source of light is placed in a strong magnetic field. The components are polarized, the directions of polarization and the appearance of the effect depending on the direction from which the source is viewed relative to the lines of force."
Magnetic fields may affect the behavior of HeNe tubes in several ways:
As a result of the Zeeman Effect, if a gas radiates in a magnetic field, most of its spectral lines are split into 2 or sometimes more components. The magnitude of the separation depends on the strength of the magnetic field and as a result, if the field is also non-uniform, the spectral lines are broadened as well because light emitted at different locations will see an unequal magnetic field. These 'fuzzed out' lines cannot participate in stimulated emission as efficiently as nice narrow lines and therefore will not drain the upper energy states for use by the desired lines. The magnitude of the Zeeman splitting effect is also wavelength dependent and therefore can be used to control the gain of selected spectral lines (long ones are apparently affected more than short ones on a percentage basis).
The Doppler-broadened gain bandwidth of neon is inversely related to wavelength. At 632.8 nm (red) it is around 1.5 to 1.6 GHz; at 3,391 nm (the troublesome IR line), it is only around 310 MHz. A magnetic field that varies spatially along the tube will split and move the gain curves at all wavelengths equally by varying amounts depending on position. However, a, say, 100 or 200 MHz split and shift of the gain curve for the 632.8 nm red transition won't have much effect, but it will effectively disrupt lasing for the 3,391 nm IR transition.
Without the use of magnets, the very strong neon IR line at 3.39 µm would compete with (and possibly dominate over) the desired visible line (at 632.8 nm) stealing power from the discharge that would otherwise contribute to simulated emission at 632.8 nm. However, the IR isn't wanted (and therefore will not be amplified since the mirrors are not particularly reflective at IR wavelengths anyhow). Since the 3.39 nm wavelength is more than 5 times longer than the 632.8 nm red line, it is affected to a much greater extent by the magnetic field and overall gain and power output at 632.8 nm may be increased dramatically (25 percent or more). The magnets may be required to obtain any (visible) output beam at all with some HeNe tubes (though this is not common).
The typical higher power Spectra-Physics HeNe laser will have relatively low strength magnets (e.g., like those used to stick notes to your fridge) placed at every available location along the exposed bore along the sides of the L-shaped resonator frame. They will alternate N and S poles pointing toward the bore. Interestingly, on some high mileage tubes, brown crud (which might be material sputtered off the anode) may collect inside the bore - but only at locations of one field polarity (N or S, whichever would tend to deflect a positive ion stream into the wall). The crud itself doesn't really affect anything but is an indication of long use. And on average, tubes with a lot of brown crud may be harder to start, and require a higher voltage to run, and have lower output power.
I do not know how to determine if and when such magnets are needed for long high power HeNe tubes where they are not part of an existing laser head. My guess is that the original or intended positions, orientations, and strengths, of the magnets were determined experimentally by trial and error or from a recipe passed down from generation to generation, and not through the use of some unusually complex convoluted obscure theory. :)
The only thing I can suggest other than contacting the manufacturer (like any manufacturer now cares about and supports HeNe lasers at all!) is to very carefully experiment with placing magnets of various sizes and strengths at strategic locations (or a half dozen such locations) to determine if beam power at the desired wavelength is affected. Just take care to avoid smashing your flesh or the HeNe tube when playing with powerful magnets. Though the magnets used in large-frame HeNe lasers with exposed bores aren't particularly powerful, to produce the same effective field strength at the central bore of an internal mirror HeNe tube may require somewhat stronger ones, though even these needn't be the flesh squashing variety. And, magnets that are very strong may affect other characteristics of the laser including polarization, and starting and running voltage. Enclosing the HeNe tube in a protective rigid sleeve (e.g., PVC or aluminum) would reduce the risk of the latter disaster, at least. :-) If there is going to be any significant improvement, almost any arrangement of 1 or 2 magnets should show some effect.
There may be an immediate effect when adding or moving a magnet. However, to really determine the overall improvement in (visible) output power and any reduction in the variation of output power with mode sweep, the laser should be allowed to go through several mode sweep cycles for 3.39 µm. These will be about 5.4 times the length of the mode sweep for 632.8 nm.
CAUTION: For soft-seal laser tubes in less than excellent health (i.e., which may have gas contamination), changing the magnet configuration near the cathode may result in a slow decline in output power (over several hours) which may or may not recover. I have only observed this behavior with a single REO one-Brewster tube, but there seems to be no other explanation for the slow decline to about half the original power, and then subsequent slow recovery with extended run time after the magnets were removed entirely. Possibly simply leaving the magnets in the new configuration would have eventually resulted in power recovery, but at the time the trend was not encouraging.
(From: Lynn Strickland (stricks760@earthlink.net).)
"They've pretty much nailed the 3.39 micron problem on red HeNes these days so magnets really aren't needed on them. Even the new green tubes don't have much of a problem - especially since the optic suppliers have perfected the mirror coatings. All of the good green mirrors are now done with Ion Beam Sputtering (IBS), as opposed to run-of-the-mill E-Beam stuff.However, you'll probably see a benefit from magnets to suppress the 3.39 µm line on the older HeNe tubes."
Where the capillary of the plasma tube is exposed as with many older lasers, and the magnets can be placed in close proximity to the bore, their strength can be much lower. A few commercial lasers (like the Spectra-Physics model 132) offered a polarization option which adds a magnet assembly alongside the tube. In this case, what is required is a uniform or mostly uniform field of the appropriate orientation rather than one that varies as for IR spectral line suppression though both of these could be probably be combined. However, the polarization purity with this approach never came anywhere close to that using a simple Brewster window or plate, found in all modern polarized HeNe lasers.
Also see the section: Unrandomizing the Polarization of a Randomly Polarized HeNe Tube.
In principle, varying fields from electromagnets could be used for intensity, polarization, and frequency modulation. I do not know whether any commercial HeNe lasers have been implemented in this manner.
But if magnets were not originally present, the only situation where adding some may make sense is for older longer or "other color" HeNes where a series of weak magnets may actually boost output power by 10 to 25 percent or more. On the other hand, most non-Zeeman stabilized HeNes do NOT like magnets at all. Even a relatively weak stray magnetic field from nearby equipment may result in a significant change in behavior. However, unless ferrous metals are used in the laser's construction, any change will likely not be permanent.
Having said all that, it is often possible to convert a short random polarized HeNe tube (even if it is a "flipper") into a well behaved linear polarized laser or a tube suitable for a Zeeman two frequency laser with the appropriate use of magnets. Details are left as an exercise for the student. ;-)
Polarity may alternate with North and South poles facing each other across the tube forming a 'wiggler' so named since such a they will tend to deflect the ionized discharge back and forth though there may be no visible effects in the confines of the capillary:
N S N S N S N
||===================================================||
||======. .=================================. .======||
S ||| N S N S N |_| S
'|' '|'
For some including the Spectra-Physics 120, 124, 125, and 127, the magnets
are actually below and on one side. The objective is usually IR (3.39 µm)
suppression and the magnets are generally relatively weak (refrigerator
note holding strength).
Alternatively, North and South poles may face each other:
N S N S N S N
||===================================================||
||======. .=================================. .======||
N ||| S N S N S |_| N
'|' '|'
With either of these configurations, after long hours of operation, there may be very pronounced brown deposits inside the bore that correlate with the pole positions.
N N N N N N N
||===================================================||
||======. .=================================. .======||
S ||| S S S S S |_| S
'|' '|'
This configuration is found in two very different situations. Strong magnets were used in laser like the Spectra-Physics 132P to polarize the beam. Weaker magnets are used in transverse Zeeman two-frequency HeNe lasers.
+--+ +--+ +--+ +--+
N | | S N | | S N | | S N | | S
+--+ +--+ +--+ +--+
||======================================================||
||====. .========================================. .====||
||| +--+ +--+ +--+ +--+ |_|
'|' N | | S N | | S N | | S N | | S '|'
+--+ +--+ +--+ +--+
Other axial configurations with opposing poles or radially oriented poles may also be used or there may be a single long solenoid type of coil or cylindrical permanent magnet as for a two-frequency laser interferometer.
For the magnet configuration used in a commercial laser, see the section: Description of the SP-124 Laser Head.
Output Tube Voltage Tube Tube Size Power Operate/Start Current Diam/Length ------------ --------------- ------------ ------------- 0.3-0.5 mW 0.8-1.0/6 kV 3.0-4.0 mA 19/135 mm 0.5-1 mW .9-1.0/7 kV 3.2-4.5 mA 25/150 mm 1-2 mW 1.0-1.4/8 kV 4.0-5.0 mA 30/200 mm 2-3 mW 1.1-1.7/8 kV 4.0-6.5 mA 30/260 mm 3-5 mW 1.7-2.4/10 kV 4.5-6.5 mA 37/350 mm 5-10 mW 2.4-3.1/10 kV 6.5-7.0 mA 37/440 mm 10-15 mW 3.0-3.5/10 kV 6.5-7.0 mA 37/460 mm 15-25 mW 3.3-4.0/10 kV 6.5-7.0 mA 37/600 mm 25-35 mW 4.0-5.2/12 kV 7.0-8.0 mA 42/900 mm
Where:
Tubes like this are generally available in both random and linearly polarized versions which are otherwise similar with respect to the above characteristics (for red tubes at least, more below).
At least one other basic specification may be critical to your application: Which end of the tube the beam exits! There is no real preference from a manufacturing point of view for red HeNe lasers. (For low gain "other-color" HeNe laser tubes, it turns out that anode output results is slightly higher gain and thus slightly higher output for the typical hemispherical cavity because it better utilizes the mode volume.) However, this little detail may matter a great deal if you are attempting to retrofit an existing barcode scanner or other piece of equipment where the tube clips into a holder or where wiring is short, tight, or must be in a fixed location. For example, virtually all cylindrical laser heads require that the beam exits from the cathode-end of the tube. It is possible that you will be able to find two versions of many models of HeNe tubes if you go directly to the manufacturer and dig deep enough. However, this sort of information may not be stated where you are buying surplus or from a private individual, so you may need to ask.
The examples above (as well as all of the other specifications in this and the following sections) are catalog ratings, NOT what might appear on the CDRH safety sticker (which is typically much higher). See the section: About Laser Power Ratings for info on listed, measured, and CDRH power ratings.
Note how some of the power levels vary widely with respect to tube dimensions, voltage, and current. Generally, higher power implies a longer tube, higher operating/start voltages, and higher operating current - but there are some exceptions. In addition, you will find that physically similar tubes may actually have quite varied power output. This is particularly evident in the manufacturers' listings. (See the chapter: A HREF="laserhcl.htm#hcltoc">Commercial Unstabilized HeNe Lasers.)
These specifications are generally for minimum power over the guaranteed life of the tube. New tubes and individual sample tubes after thousands of hours may be much higher - 1.5X is common and a "hot" sample may hit 2X or more. My guess is that for tubes with identical specifications in terms of physical size, voltage, and current, the differences in power output are due to sample-to-sample variations. Thus, like computer chips, they are selected after manufacture based on actual performance and the higher power tubes are priced accordingly! This isn't surprising when considering the low efficiency at which these operate - extremely slight variations in mirror reflectivity and trace contaminants in the gas fill can have a dramatic impact on power output.
I have a batch of apparently identical 2 mW Aerotech tubes that vary in power output by a factor of over 1.5 to 1 (2.6 to 1.7 mW printed by hand on the tubes indicating measured power levels at the time of manufacture).
And, power output also changes with use (and mostly in the days of soft-sealed tubes, just with age sitting on the shelf):
(From: Steve Roberts.)
"I have a neat curve from an old Aerotech catalog of HeNe laser power versus life. The tubes are overfilled at first, so power is low. They then peak at a power much higher than rated power, followed by a long period of constant power, and then they SLOWLY die. It's not uncommon for a new HeNe tube to be in excess of 15% greater than rated power."
And the answer to your burning question is: No, you cannot get a 3 mW tube to output 30 mW - even instantaneously - by driving it 10 times as hard!
I have measured the operating voltage and determined the optimum current (by maximizing beam intensity) for the following specific samples - all red (632.8 nm) tubes from various manufacturers. (The starting voltages were estimated.):
Output Tube Voltage Tube Supply Voltage Tube Size
Power Operate/Start Current (75K ballast) Diam/Length
---------- --------------- ------------ ---------------- -------------
.8 mW .9/5 kV 3.2 mA 1.1 kV 19/135 mm
1.0 mW 1.1/7 kV 3.5 mA 1.4 kV 25/150 mm
1.0 mW 1.1/7 kV 3.2 mA 1.4 kV 25/240 mm
2.0 mW 1.2/8 kV 4.0 mA 1.5 kV 30/185 mm
3.0 mW 1.6/8 kV 4.5 mA 1.9 kV 30/235 mm
5.0 mW 1.7/10 kV 6.0 mA 2.2 kV 37/350 mm
12.0 mW 2.5/10 kV 6.0 mA 2.9 kV 37/475 mm
Melles Griot, Uniphase, Siemens, PMS, Aerotech, and other HeNe tubes all show similar values.
The wide variation in physical dimensions also means that when looking at descriptions of HeNe lasers from surplus outfits or the like, the dimensions can only be used to determine an upper (and possibly lower) bound for the possible output power but not to determine the exact output power (even assuming the tube is in like-new condition). Advertisements often include the rating on the CDRH safety sticker (or say 'max' in fine print). This is an upper bound for the laser class (e.g., Class IIIa), not what the particular laser produces or is even capable of producing. It may be much lower. For example, that Class IIIa laser showing 5 mW on the sticker, may actually only be good for 1 mW under any conditions! The power output of a HeNe laser tube is essentially constant and cannot be changed significantly by using a different power supply or by any other means. See the section: Buyer Beware for Laser Purchases.
Also see the section: Locating Laser Specifications.
In addition to power output, power requirements, and physical dimensions, key performance specifications for HeNe lasers also include:
With manufacturers like Aerotech, Melles Griot, and Siemens, a certain amount of information can be determined from the model number. For example, here is how to decipher most of those from Melles Griot (e.g., 05-LHP-121-278):
The vast majority of Melles Griot lasers you are likely to come across will follow this numbering scheme though there are some exceptions, especially for custom assemblies. (Some surplus places drop the leading '05-' when reselling Melles Griot laser tubes or heads so an 05-LHP-120 would become simply an LHP-120.)
For other manufacturers like Spectra-Physics, the model numbers are totally arbitrary! (See the section: Spectra-Physics HeNe Lasers.)
Maximum available power output is also lower - rarely over 2 mW (and even those tubes are quite large (see the tables below). However, since the eye is more sensitive to the green wavelength (543.5 nm) compared to the red (632.8 nm) by more than a factor of 4 (see the section: Relative Visibility of Light at Various Wavelengths), a lower power tube may be more than adequate for many applications. Yellow (594.1 nm) and orange (611.9 nm) HeNe lasers appear more visible by factors of about 3 and 2 respectively compared to red beams of similar power. To get an idea of the actual perceived color at each wavelength, see the section: Color Versus Wavelength.
Infrared-emitting HeNe lasers exist as well. In addition to scientific uses, these were used for testing in the Telecom industry before sufficiently high quality diode lasers became available.Yes, you can have a HeNe tube and it will light up inside (typical neon glow), but if there is no output beam (at least you cannot see one), you could have been sold an infrared HeNe tube. However, by far the most likely explanation for no visible output beam is that the mirrors are misaligned or the tube is defective in some other way. Unfortunately, silicon photodiodes or the silicon sensors in CCD or CMOS cameras do not respond to any of the HeNe IR wavelengths, so the only means of determining if there is an IR beam are to use a GaAs photodiode, IR detector card, or thermal laser power meter. IR HeNe tubes are unusual enough that it is very unlikely you will ever run into one. However, they may turn up on the surplus market especially if the seller doesn't test the tubes and thus realize that these behave differently - they are physically similar to red (or other color) HeNe tubes except for the reflectivity of the mirrors as a function of wavelength. (There may be some other differences needed to optimize each color like the He:Ne ratio, isotope purity, and gas fill pressure, but the design of the mirrors will be the most significant factor and the one that will be most obvious with a bare eyeball, though the color of the discharge may be more pink for green HeNe tubes and more orange and brighter for IR HeNe tubes compared to red ones, more below.) Even if the model number does not identify the tube as green, yellow, orange, red, or infra-red, this difference should be detectable by comparing the appearance of its mirrors (when viewed down the bore of an UNPOWERED tube) with those of a normal (known to be red) HeNe tube. See the section: Determining HeNe Laser Color from the Appearance of the Mirrors. (Of course, your tube could also fail to lase due to misaligned or damaged mirrors or some other reason. See the section: How Can I Tell if My Tube is Good?.)
As noted above, the desired wavelength is selected and the unwanted wavelengths are suppressed mostly by controlling the reflectivity functions of the mirrors. For example, the gains of the green and yellow lines (yellow may be stronger) are both much much lower than red and separated from each other by about 50 nm (543.5 nm versus 594.1 nm). To kill the yellow line in a green laser, the mirrors are designed to reflect green but pass yellow. I have tested the mirrors salvaged from a Melles Griot 05-LGP-170 green HeNe tube (not mine, from "Dr. Destroyer of Lasers"). The HR (High Reflector) mirror has very nearly 100% reflectivity for green but less than 25% for yellow. The OC (Output Coupler) also has a low enough reflectivity for yellow (about 98%) such that it alone would prevent yellow from lasing. The reflectivities for orange, red, and IR, are even lower so they are also suppressed despite their much higher gain, especially for the normal red (632.8 nm) and even stronger mid-IR (3,391 nm) line.
However, to manufacture a tube with optimum and stable output power, it isn't sufficient to just kill lasing for unwanted lines. The resonator must be designed to minimize their contribution to stimulated emission - thus the very low reflectivity of the HR for anything but the desired green wavelength. Otherwise, even though sustained oscillation wouldn't be possible, unwanted color photons would still be bouncing back and forth multiple times stealing power from the desired color. The output would also be erratic as the length of the tube changed during warmup (due to thermal expansion) and this affected the longitudinal mode structure of the competing lines relative to each other. Some larger HeNe lasers have magnets along the length of the tube to further suppress (mostly) the particularly strong mid-IR line at 3,391 nm. (See the section: Magnets in High Power or Precision HeNe Laser Heads.)
In addition, you can't just take a tube designed for a red laser, replace the mirrors, and expect to get something that will work well - if at all - for other wavelenghts. For one thing, the bore size and mirror curvature for maximum power while maintaining TEM00 operation are affected by wavelength.
Furthermore, for these other color HeNe lasers which depend on energy level transitions which have much lower gain than red - especially the yellow and green ones - the gas fill pressure, He:Ne ratio, and isotopic composition and purity of the helium and neon, will be carefully optimized and will be different than for normal red tubes.
Needless to say, the recipes for each type and size laser will be closely guarded trade secrets and only a very few companies have mastered the art of other color HeNe lasers, especially for high power (in a relative sort of way) in yellow and green. I am only aware of four companies that currently manufacture their own tubes: Melles Griot, Research Electro-Optics, Uniphase, and LASOS, with the last two having very few models to choose from. Others (like Coherent) simply resell lasers under their own name.
And, the answer to that other burning question should now be obvious: No, you can't convert an ordinary red internal mirror HeNe tube to generate some other color light as it's (almost) all done with mirrors and they are an integral part of the tube. :) Therefore, your options are severely limited. As in: There are none. (However, going the other way, at least as a fun experiment, may be possible. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.) For a laser with external mirrors, a mirror swap may be possible (though the cavity length may be insufficient to resonate with the reduced gain of other-color spectral lines once all loses taken into consideration). But realistically, this option doesn't even exist where the mirrors are sealed into the tube.
There are also a few HeNe lasers that can output more than one of the possible colors simultaneously (e.g., red+orange, orange+yellow) or selectively by turning knob (which adjusts the angle of a Littrow or other similar dispersion prism) inside the laser cavity using a Brewster window HeNe tube). But such lasers are not common and are definitely very expensive. So, you won't likely see one for sale at your local hamfest - if ever! One manufacturer of such lasers is Research Electro-Optics (REO). See the section: Research Electro-Optics's Tunable HeNe Lasers.
However, occasionally a HeNe tube turns up that is 'defective' due to incorrect mirror reflectivities or excessive gain or magic :) and actually outputs an adjacent color in addition to what it was designed to produce. I have such a tube that generates about 3 mW of yellow (594.1 nm) and a fraction of a mW of orange (611.9 nm) but isn't very stable - power fluctuates greatly as it warms up. Another one even produces the other orange line at 611.9 nm, and it's fairly stable. But, finding magic 'defective' tubes such as these by accident is extremely unlikely though I've heard of the 640.1 nm (deep red) line showing up on some supposedly good normal red (632.8 nm) HeNe tubes.
As a side note: It is strange to see the more or less normal red-orange glow in a green HeNe laser tube but have a green beam emerging. A diffraction grating or prism really shows all the lines that are in the glow discharge. Red through orange, yellow and green, even several blue lines (though they are from the helium and can't lase under any circumstances)!! The IR lines are present as well - you just cannot see them.
See the section: Instant Spectroscope for Viewing Lines in HeNe Discharge for an easy way to see many of the visible ones.
Actually, the color of the discharge may be subtly different for non-red HeNe tubes due to modified gas fill and pressure. For example, the discharge of green HeNe tubes may appear more pink compared to red tubes) which are more orange), mostly due to lower fill pressure. The fill mix and pressure on green HeNes is a tricky compromise among several objectives that conflict to some extent including lifetime, stability (3.39 µm competition), and optical noise. This balancing act and the lower fill pressure are why green HeNes don't last as long as reds. Have I totally confused you, color-wise? :)
The expected life of 'other color' HeNe tubes is generally much shorter than for normal red tubes. This is something that isn't widely advertised for obvious reasons. Whereas red HeNe tubes are overfilled initially (which reduces power output) and they actually improve with use to some extent as gas pressure goes down, this luxury isn't available with the low gain wavelengths - especially green - everything needs to be optimal for decent performance.
The discharge in IR HeNe tubes is usually more orange and brighter due to a higher fill pressure. The discharge in green and yellow HeNe tubes is more pink due to the higher He:Ne ratio and lower pressure. Again, this is due to the need to optimize parameters for the specific wavelength.
I am not sure who first demonstrated the green HeNe laser, but REO first commercialized it in approximately 1984. They launched a 0.2 mW model, using soft-sealed mirrors, because the coatings could not withstand the oven cycle temperatures of hard-sealed mirrors. I was working there at the time, which is how I know this stuff. They were doing their own mirrors in-house. The mirror guru was Ramin Lalezari (now at Five-Nine Optics), the physicist was Robert Knollenburg, Ph.D. (retired) and the production manager was Ken Sample (?). I was the lead engineering technician.
They weren't shipping many units (a few per month) but there was some decent interest in a few application areas in biotech (confocal microscopy, cytometry) and reprographics.
Shortly after REO's launch of the product, Melles Griot began a similar program. MG struggled to get decent mirrors, but did have some reasonable market interest. This was approximately 1985 / early 1986. This activity was led by the Engineering Manager, Ben Cook (retired) and the physicist was William Cobb (I believe he is deceased). MG then recruited me away from REO to turn Engineering demonstration units and prototypes into commercial products. I spent many months qualifying and testing mirrors and mirror suppliers to get the recipe nailed. Also did a lot of work on gas mix and gas fill optimization to get the most power possible. One of the big battles early on was competition from the 3.39 micron line, along with mirror and resonator cavity losses.
By 1987/1988, at least at Melles Griot, we started to get the recipe nailed and shipping regularly. Fairly soon thereafter, MG began to dominate the commercial volume of green HeNe lasers.
Power levels were approaching 1 mW, and we began getting mirrors we could hard-seal. Over the next few years, JDSU (now Lumentum) started making a green HeNe laser, as did Siemens (now LASOS).
The mirrors continued to improve, leading to commercially available yellow (594 nm) and orange (604 nm, 612 nm) products. Yellow did get some market traction in biotech, but the orange lines never saw too much commercial success. The orange lines in particular have very difficult optical coatings, due the sharp cutoff you need to lase at (say 612nm) without lasing at the other orange, yellow or red lines.
Yellow and green HeNe lasers are still produced today, with green power levels over 2 mW.
Take the following with a grain of optical glass - the actual appearance of mirrors can vary widely based on supplier and parameters other than the exact design wavelengths.
Since the mirrors used in all HeNe lasers are dielectric - functioning as a result of interference - they have high reflectivity only around the laser wavelength and actually transmit light quite well as the wavelength moves away from this peak. By transmitted light, the appearance will tend to be a color which is the complement of the laser's output - e.g., cyan or blue-green for a red tube, pink or magenta for a green tube, blue or violet for a yellow tube. Of course, except for the IR variety, if the tube is functional, the difference will be immediately visible when it is powered up!
The actual appearance may also depend on the particular manufacturer and model as well as the length/power output of the laser (which affects the required reflectivity of the OC), as well as the revision number of your eyeballs. :) So, there could be considerable variation in actual perceived color. Except for the blue-green/magenta combination which pretty much guarantees a green output HeNe tube, more subtle differences in color may not indicate anything beyond manufacturing tolerances.
The chart below in conjunction with Typical Appearance of HeNe Laser Mirrors will help to ideentify your unmarked HeNe tube. (For accurate rendition of the graphic, your display should be set up for 24 bit color and your monitor should be adjusted for proper color balance.)
HeNe Laser High Reflector (HR) Output Coupler (OC)
Color Wavelength Reflection Transmission Reflection Transmission
------------------------------------------------------------------------------
Red 632.8 nm Gold/Copper Blue Gold/Yellow Blue/Green
Orange 611.9 nm Whitish-Gold Blue Metallic Green Magenta
Yellow 594.1 nm Whitish-Gold Blue Metallic Green Magenta
Green 543.5 nm Metallic Blue Red/Orange Metallic Green Magenta
Broadband (ROY) Whitish-Gold Blue
IR 1,523 nm Light Green Light Magenta Light Green Light Magenta
IR 3,391 nm Gold (Metal) Coated Neutral Clear
The entry labeled 'Broadband' relates to the HR mirror in some unusual multiple color (combinations of red and/or orange and/or yellow) internal mirror tubes as well as those with an internal HR and Brewster window for external OC optics. And, the yellow and orange tubes may actually use broad band HRs. The OCs would then be selected for the desired wavelength(s) and may also have a broad band coating.
While the appearance of the HR and OC is almost always more or less similar for red (633 nm) tubes, this is usually not true for low gain tubes. It isn't possible to just make a highly selective coating for one wavelength that's narrow enough to have low reflectivity at the nearby lines so they won't lase and/or remain stable over temperature. Or, at least it's not cost effective. So, one mirror will be designed to fall off rapidly on one side of the design wavelength and the other mirror on the other side. That's one reason front and back mirrors on yellow and green tubes in particular generally have very different appearances. And which is which may be swapped.
As noted, depending on laser tube length/output power, manufacturer, and model, the appearance of the mirrors can actually vary quite a bit but this should be a starting point at least. For example, I have a Melles Griot 05-LHR-170 HeNe laser tube that should be 594.1 nm (yellow) but actually outputs some 604.6 nm (orange) as well. It's mirror colors for the HR and OC are almost exactly opposite of those I have shown for the yellow and orange tubes! I don't know whether this was intentional or part of the problem And, while from this limited sample, it looks like the OCs for orange, yellow, and green HeNe lasers appear similar, I doubt that they really are in the area that counts - reflectivity/transmission at the relevant wavelengths.
I do not have any data for the 1,152 nm (IR) HeNe laser wavelength. If you have access to a 1,152 nm or any other non-red HeNe tube and would like to contribute or comment on their mirror colors (or anything else), please contact me via the Sci.Electronics.Repair FAQ Email Links Page.
(From: Steve Roberts.)
You do need a isotope change in the gases for green, and a He:Ne ratio change for the other orange and yellow lines. In addition, the mirrors to go to another line will have a much lower output transmission. The only possible lines you'll get on a large frame HeNe laser will be the 611.9 nm orange and 594.1 nm yellow. The green requires external mirror tubes in excess of a meter and a half long and a Littrow prism to overcome the Brewster losses and suppress the IR.
The original work on green was done by Rigden and Wright. The short tubes have lower losses because they have no Brewsters and thus can concentrate on tuning the coatings to 99.9999% reflectivity and maximum IR transmission. There is one tunable low power unit on the market that does 6 lines or so, but only 1 line at a time, and the $6,000 cost is kind of prohibitive for a few milliwatts of red and fractional milliwatt powers on the other lines. But, it will do green and has the coatings on the back side of the prism to kill the losses.
Also look for papers by Erkins and Lee. They are the fellows who did the green and yellow for Melles Griot and they published one with the energy states as part of a poster session at some conference. Melles Griot used to hand it out, that's how I had a copy, recently thrown away.
Even large HeNe lasers such as the SP-125 (rated at 50 mW of red) will only do about 20 mW of yellow, with a 35 mW SP-127 you're probably only looking at 3 to 5 mW of yellow. And, for much less then the cost of the custom optics to do a conversion, you can get two or three 4 to 5 mW yellow heads from Melles Griot. I know for a fact that a SP-127 only does about 3 mW of 611.9 with a external prism and a remoted cavity mirror, when it does 32 mW of 632.8 nm.
So in the end, unless you have a research use for a special line, it's cheaper to dig up a head already made for the line you seek, unless you have your own optics coating lab that can fabricate state-of-the-art mirrors.
I have some experience in this, as I spent months looking for a source of the optics below $3,000.
(From: Sam.)
I do have a short (265 mm) one-Brewster HeNe tube (Melles Griot 05-LGB-580) with its internal HR optimized for green that operates happily with a matching external green HR mirror (resulting in a nice amount of circulating power) but probably not with anything having much lower reflectivity to get a useful output beam. In fact, I could not get reliable operation even with the HR from a dead green HeNe laser tube as the Brewster window would not remain clean enough for the time required to align the mirror. See the section: A Green One-Brewster HeNe Laser for more info.
I would expect an SP-127 to do more than 3 to 5 mW of yellow, my guess would be 10 to 15 mW with optimized mirrors but no tuning prism. If I can dig up appropriate mirrors, I intend to try modifying an SP-127 to make it tunable and/or do yellow or green. :)
(From: Lynn Strickland (stricks760@earthlink.net).)
You can find 640.1 nm in a lot of red HeNe lasers. I have a paper on it somewhere, and cavity design can influence it to a large extent. If you have a decent quality grating, it's pretty easy to pick up. 629 nm is the one you don't see too much.
I'm no physicist, but the lower gain lines can lase simultaneously with the higher gain lines, no problem, as long as there is sufficient gain available in the plasma. It's really pretty easy to get a HeNe laser to output on all lines at the same time (if you have the right mirrors). The trick is optimizing the bore-to-mode ratio, gas pressure, and isotope mixture to get good TEM00 power. Usually the all-lines HeNe lasers are multi (transverse) mode. I don't know of anyone who makes them commercially though - at least not intentionally.
The 3.39 µm HeNe laser's gain is still, like all other HeNe lines limited by a wall collision to return the excited atoms to the ground state. 3.39 µm HeNe lasers have larger bores then normal HeNe lasers, and the bores are acid etched to fog them and create more surface area, but still the most power I've ever seen published was 40 mW - nothing to write home about. The massive SP-125, the largest commercial HeNe laser, could be ordered with a special tube and special optics for 3.39 µm, and it still only did about 1/3rd the visible power. Superradiance and ultimate power are not tied together.
The reason 3.39 µm got all the writeups it did was that it started on the same upper state as all the other HeNe lines, was easily noticed when it sapped power from the visible line, and was, at the time, a exotic wavelength for which there were few other sources.
(Spectra for varioue elements and compounds can be easily found by searching the Web. The NIST Atomic Spectra Database has an applet which will generate a table or plot of more spectral lines than you could ever want.)
The shear number of individual spectral lines present in the discharge is quite amazing. You will see the major red, orange, yellow, and green lines as well as some far into the blue and violet portions of the spectrum and toward the IR as well. All of those shown in Bright Line Spectra of Helium and Neon will be present as well as many others not produced by the individual gas discharges. There are numerous IR lines as well but, of course, these will not be visible.
Place a white card in the exit beam and note where the single red output line of the HeNe tube falls relative to the position and intensity of the numerous red lines present in the gas discharge.
As an aside, you may also note a weak blue/green haze surrounding the intense main red beam (not even with the spectroscope). This is due to the blue/green (incoherent) spectral lines in the discharge being able to pass through the output mirror which has been optimized to reflect well (>99 percent) at 632.8 nm and is relatively transparent at wavelengths some distance away from these (shorter and longer but you would need an IR sensor to see the longer ones). Since it is not part of the lasing process, this light diverges rapidly and is therefore only visible close to the tube's output mirror.
(From: George Werner (glwerner@sprynet.com).)
Here is an effect I found many years ago and I don't know if anyone has pursued it further.
We had a recording spectrometer in our lab which we used to examine the incoherent light coming from the laser discharge. This spectrum when lasing was slightly different from the spectrum when not lasing, which one can expect since energy levels are redistributed. As with most detectors, ours used a chopper in the spectrometer light beam and a lock-in amplifier.
Instead of putting the chopper in the path of light going to the spectrometer, I put it in the path of the internal laser beam, so that instead of an open/closed signal going to the amplifier it was a lasing/not-lasing signal. What was recorded then was three kinds of spectrum lines: some deflected positive in the normal way, others deflected negative, and the third group were those that were unaffected by chopping, in which case when we passed over the line we only saw an increase in the noise level. Setting up such a test is easy. The hard part is interpreting the data in a meaningful way.
For HeNe lasers, the primary line (usually 632.8 nm) is extremely narrow and effectively a singularity given any instrumentation you are likely to have at your disposal. Any other lines you detect in the output are almost certainly from two possible sources but neither is actual laser emission:
Close to the output mirror, you may see some of this light seeping through especially at wavelengths in the green, blue, and violet, for which the dielectric mirrors are nearly perfectly transparent. However, such light will be quite divergent and diffuse and won't be visible at all more than a couple of inches from the mirror.
The result will be a weak green beam that can sometimes be observed with a spectroscope in a very dark room room. It isn't really quite as coherent or monochromatic as the beam from a true green HeNe laser and probably has much wider divergence but nonetheless may be present. It may be easier to see this by using your spectroscope to view the bright spot from the laser on a white card rather than by deflecting the beam and trying to locate the green dot off to one side.
Note: I have not been able to detect this effect on the short HeNe tubes I have checked.
Since the brightness of the discharge and superradiance output should be about the same from either mirror, using the non-output end (high reflector) should prove easier (assuming it isn't painted over or otherwise covered) since the red beam exiting from this mirror will be much less intense and won't obscure the weak green beam.
Note that argon and krypton ion lasers are often designed for multiline output where all colors are coherent and within an order of magnitude of being equal to each other in intensity or with a knob to select an individual wavelength. Anything like this is only rarely done with HeNe lasers because it is very difficult (and expensive) due to the low gain of the non-red lines. For more information, see the sections: HeNe Tubes of a Different Color and Research Electro-Optics Tunable HeNe Lasers.
The next best thing is a small HeNe laser laid bare where its sealed (internal mirror) HeNe tube, ballast resistors, wiring, and power supply (with exposed circuit board), are mounted inside a clear Plexiglas case with all parts labeled. This would allow the discharge in the HeNe tube to be clearly visible (and permit the use of the Instant Spectroscope for Viewing Lines in HeNe Discharge). The clear insulating case prevents the curious from coming in contact with the high voltage (and line voltage, if the power supply connects directly to the AC line), which could otherwise result in damage to both the person and fragile glass HeNe tube when a reflex action results in smashing the entire laser to smithereens!
A HeNe laser is far superior to a cheap laser pointer for several reasons:
Important: If this see-through laser is intended for use in a classroom, check with your regulatory authority to confirm that a setup which is not explicitly CDRH approved (but with proper laser class safety stickers) will be acceptable for insurance purposes.
For safety with respect to eyeballs and vision, a low power laser - 1 mW or less - is desirable - and quite adequate for demonstration purposes.
The HeNe laser assembly from a barcode scanner is ideal for this purpose. It is compact, low power, usually runs on low voltage DC (12 V typical), and is easily disassembled to remount in a demonstration case. The only problem is that many of these have fully potted "brick" type power supplies which are pretty boring to look at. However, some have the power supply board coated with a rubbery material which can be removed with a bit of effort (well, OK, a lot of effort!). For example, this HeNe Tube and Power Supply is from a hand-held barcode scanner. A similar unit was separated into its Melles Griot HeNe Tube and HeNe Laser Power Supply IC-I1 (which includes the ballast resistors). These could easily be mounted in a very compact case (as little as 3" x 6" x 1", though spreading things out may improve visibility and reduce make cooling easier) and run from a 12 VDC, 1 A wall adapter. Used barcode scanner lasers can often be found for $20 or less.
An alternative is to purchase a 0.5 to 1 mW HeNe tube and power supply kit. This will be more expensive (figure $5 to $15 for the HeNe tube, $25 to $50 for the power supply) but will guarantee a circuit board with all parts visible.
The HeNe tube, power supply, ballast resistors (if separate from the power supply), and any additional components can be mounted with standoffs and/or cable ties to the plastic base. The tube can be separated from the power supply if desired to allow room for labels and such. However, keep the ballast resistors as near to the tube as practical (say, within a couple of inches, moving them if originally part of the power supply board). The resistors may get quite warm during operation so mount them on standoffs away from the plastic. Use wire with insulation rated for a minimum of 10 kV. Holes or slots should be incorporated in the side panels for ventilation - the entire affair will dissipate 5 to 10 Watts or more depending on the size of the HeNe tube and power supply. (However, if you want to take this thing outdoors, see the section: Weatherproofing a HeNe Laser.
When attaching the HeNe tube, avoid anything that might stress the mirror mounts. While these are quite sturdy and it is unlikely that any reasonable arrangement could result in permanent damage, even a relatively modest force may result in enough mirror misalignment to noticeably reduce output power. And, don't forget that the mirror mounts are also the high voltage connections and need to be well insulated from each other and any human contact! The best option is probably to fasten the tube in place using Nylon cable ties, cable clamps, or something similar around the glass portion without touching the mirror mounts at all (except for the power connections).
Provide clearly marked red and black wires (or binding posts) for the low voltage DC or a line cord for AC (as appropriate for the power supply used), power switch, fuse, and power-on indicator. Label the major components and don't forget the essential CDRH safety sticker (Class II for less than 1 mW or Class IIIa for less than 5 mW).
See: Sam's Demo HeNe Laser as an example (minus the Plexiglas safety cover), contructed from the guts of a surplus Gammex laser (probably part of a patient positioning system for a CT or MRI scanner). The discrete line operated power supply is simple with the HV transformer, rectifier/doubler, filter, multiplier, and ballast resistors easily identified. This would make an ideal teaching aid.
See the suppliers listed in the chapter: Laser and Parts Sources.
Everything needed for such a setup is readily available or easily constructed at low cost but you'll have to read more to find out where or how as each of the components are dealt with in detail elsewhere in Sam's Laser FAQ (but I won't tell you exactly where - these are all the hints you get for this one!).
A system like this could conceivably be turned into an interactive exhibit for your local science museum - assuming they care about anything beyond insects and the Internet these days. :) There are some more details in the next section.
Here are some guidelines for designing an interactive exhibit:
____________________________________________
/ | | | _______ \
Anode |\ | | | \ | Cathode
.-.---' \.-----'-----..-----'-----..-----'------. '-'---.-.
<---| |:::: :===========::===========::============: :::::| |===>
'-'---. /'-----.-----''-----.-----''-----.------' .-.---'-'
|/ | | | _______/ |
\_______|____________|____________|__________/
Or, for a more esthetic rendition, see: Helium-Neon Laser Tube with Segmented Bore.
The third has Brewster angle windows at both ends with an external (fixed) HR mirror and an external screw-adjustable OC mirror. The cathode is also in a side-tube rather than the more typical coaxial can type but is otherwise similar.
Only one of the 3 HeNe tubes of this type that I have works at all and it has a messed up gas fill probably due to age despite its being hard sealed. Its output is perhaps 1 or 2 mW (where it should be around 20 mW). However, to the extent that it works, there doesn't appear to be anything particularly interesting or different about its behavior. Of the other two tubes, one has a broken off mirror (don't ask) but before the mishap, did generate some decent power (perhaps 5 to 10 mW but still nowhere near its 20 mW rating) but erratically. I suspect this was due to a contaminated gas fill resulting in low gain rather than the segmented design since a couple of other similar length tubes of conventional construction behaved in a similar manner. The funky tube with the external mirrors was not hard-sealed at the Brewster windows and leaked over time.
The only obvious effect this sort of structure should have on operation would be to provide gas reservoirs at multiple locations rather than only at the cathode-end of the bore as is the case with most 'normal' HeNe tube designs. I do not know whether this matters at all for a low current HeNe discharge. Therefore, the reason for the unusual design remains a total mystery. It may have been to stabilize the discharge, to suppress unwanted spectral lines, easier to maintain in alignment than a single long capillary, or something else entirely. Then again, perhaps, the person who made the tubes just had a spurt of excessive creativity. :)
I have also acquired a complete laser head with a similar tube, rated 25 mW max with a sticker that says it did 22 mW at one time. It is unremarkable in most respects but does have a large number of IR suppression magnets arranged on 3 sides over most of the length of the tube. Currently, it does not lase because the gas is slightly contaminated but it is also misaligned. The discharge color is along the lines of "Minor - Low Outupt" in Color of HeNe Laser Tube Discharge and Gas Fill so there may be some hope.
Here is the original description (slightly reformatted):
(From: Chris Chagaris (pyro@grolen.com).)
I have recently acquired what I have been told is a 35 mW Helium Neon laser head. However, it is unlike anything I have ever seen before. (See the diagram, below.)
Capillary tube/external starting electrodes
Starting pulse o-------+----------------------+
_|_ _|_
|| //==================================================\\ ||
|| //====. .==================. .=================. .====\\ ||
||| | | |||
Mirror '|' 25K | | 25K '|' Mirror
Anode 1 +---/\/\---o +HV | | +HV o---/\/\---+ Anode 2
.---------------' '--------------.
---|-+ +-|---
| ) Main Spare ( |
---|-+ +-|---
'--------------------------------'
Gas reservoir with heated cathodes and getters
Jodon Laser Head shows the construction in more
detail.
Here is one reply Chris received by email from someone else named Marco. As you will see, this turns out to be a dead end.
(From: Marco.)
"Hi Chris,This seems to be a really old one, or from other location than west Europe, Japan, and the USA. The 'SM' could be an abbreviation for Siemens, they had manufactured lasers from 1966 to 1993; until last year Zeiss/Jena has taken over the production; and since 1997 Lasos has overtaken the production by a kind of management buy-out. You can send them the number, it will be possible that they know it. Contact Dr. Ledig. I will also look around if I can help you further.
HeNe lasers with a heated filament are no longer built. To see if it still runs you can attach a 3.3 V supply to the filament and see if it glows red, not more, to much heat will destroy it. You could use transformers from tube amplifiers for the filament and an old HeNe laser power supply for the anode.
This laser will need around 5,000 V and 10 mA I think. If you could only get a smaller power supply, you may not see any laser beam, but you can see if it will trigger."
(From: Sam.)
Here are my 'guesses' about this device. (I have also had email discussions with Chris.)
I agree with much of what Marco had said.
(From Chris (a few months later).)
Well, tonight while looking through the "Holography Handbook" I spied what looked suspiciously like that elusive laser I have. It said it was made by Jodon Engineering Associates of Ann Arbor, MI. I immediately called them and was fortunate to have the engineer (Bruce) who has built their tubes for the last 18 years answer the phone. I told him of my plight and read off the numbers that were on the plasma tube. Sure enough, it was one of their early lasers. They have been manufacturing HeNe's since 1963. He provided me with many of the details that I had been searching for.
I explained that I planned on trying to re-gas this antique and he offered to help with what ever information I needed. It is truly refreshing to find someone in the industry that is willing to help the amateur without an eye on just making a profit.
I finally located a small supply of HeNe gas, just yesterday. While visiting North Country Scientific to purchase a pair of neon sign electrodes (in Pyrex), I mentioned my need for a small amount of laser gas for my laser refurbishing project. (This was formally Henry Prescott's small company that supplied all the hard to find components for the Scientific American laser projects.) Lo and behold, there on a shelf, covered with dust, were a few of the original (1964?) 1.5 liter glass flasks filled with the 7:1 He/Ne gas mix. He let them go at a very decent price!
(Hopefully, those tiny weeny slippery He atoms have not leaked out! --- sam)
Now, about the magnets:
The magnets are of rectangular shape, one inch long, 3/4 inch in width and 3/8 inch thick. There are a total of 26 magnets placed flat against the top (14) and flat against the bottom (12) of the plasma tube as viewed from the side. All but the ones on the very ends of the plasma tube are attached exactly opposite from one another, top and bottom. (See Jodon Laser Head for placement and field orientation).
They are placed with the long side (1") parallel to the plasma tube with the north and south poles along this axis.
They appear to be of ceramic construction and not very powerful. Sorry, I don't have any means of measuring the actual field strength.
The current status of this project is that the laser needs to be regassed. Chris is equipped to do this and has acquired the needed HeNe gas mixture.
To be continued....
Photos of a similar but much larger Jodon HeNe laser (3.39 µm IR in this case) can be found in the Laser Equipment Gallery (Version 1.41 or higher) under "Jodon Helium-Neon Lasers".
The Brewster windows appear to be glued in place. The OC is a normal 7 or 8 mm diameter curved mirror glued to the inside of the output aperture plate - basically a metal washer. The HR is a square, almost certainly planar mirror, glued to the outside of a 4 screw adjustable mount of sorts. Why is the HR square? Probably because it was cut from a large coated plate, rather than being coated individually. Why 4 screws instead of 3, making mirror adjustment much more of a pain? Another unsolved mystery of the Universe. :) Though it's not obvious from the photo, the Brewster windows aren't quite oriented the same - the angle differs by perhaps 5 degrees - so the gain is already slightly reduced from what's possible. However, I have been assured that this laser did meet specifications when new. The output is still polarized - probably half way in between - but the polarization extinction ratio is certainly lower than it could be. If the laser is still under warranty, it might be worth complaining. ;) As can be seen, this sample still lases after refiring the getter and then letting it run for several hours to allow the cathode to adsorb remaining impurities. The refiring was actually done using a can crusher demonstration apparatus and the remains of the getter coating can be seen as the ugly brown ring encircling the tube just to the left of the anode connection. I don't know whether the getter coating was any the worse for wear after that exciting event as I was not present.
What's a "can crusher"? :) Basically an electromagnetic pulse (EMP) generator: Discharge a really large high voltage capacitor bank into a couple of turns of wire wrapped around the tube (in this case). Since the getter electrode in this tube is conveniently oriented as a ring around the bore and thus acts as the secondary of a transformer, the high current discharge induced enough current to heat the ring to heat it instantly. I wish I could have witnessed that!
The output is only about 2 mW though, when the spec is 4 mW. Spectral line measurements of the discharge in the bore suggest that it's low on helium and low pressure in general. A helium soak may be in its future.
I have a most likely even earlier Aerotech tube which is constructed along the same lines as the LS4P except that:
It doesn't lase and has a very pink discharge - running it now to see if that helps but not much hope by the time it gets that far. The tube originally put out 22 mW according to a hand-written sticker. I had picked it up on eBay in a big blue case and substituted another only slightly newer hard-sealed Aerotech tube which at least lased - 6 mW, wow. :) Its problem appears to be a bad recipe for the gas fill, mirrors, or both.
The cover on one unit bears a sticker from El Don Engineering, 2876 Butternut, Ann Arbor, Michigan 48104, Phone: 1-313-973-0330. The laser was serviced and repaired on 9/28/80 and its output was 2.3 mW, TEM00. Another one had "Tube No. 1170, 2.1 mW TEM00, Jan. 13, 1970". I wonder if they still exist. :)
The AO-3100 appears to be made by Gaertner (whoever they are/were, their model number is not known). Two samples are shown in the Laser Equipment Gallery (Version 2.08 or higher) under "Assorted Helium-Neon Lasers".
The bore is about 2.5 mm in diameter which is extremely wide for a red HeNe laser. I would have expected it to be multi-mode (not TEM00). However, both samples say TEM00 and they must know. The Brewster windows are Epoxy-sealed so needless to say, most of these lasers no longer work (aside from the slight problem that when I received the first tube from one, it was in pieces. While I never expected it to work, being intact would have been nicer.)
AO-3100 HeNe Laser Plasma Tube shows a (dead) tube removed from an AO-3100 laser. Note the wide but thin-walled bore. Cathode in AO-3100 HeNe Laser Plasma Tube is a closeup of the filament and expired getter below it.
Not surprisingly, most of these lasers no longer lase or even light up since the tubes are soft-seal and long past their expiration dates. But if you happen to own a working time machine, it seems that Metrologic was supplying replacement tubes and power supplies for the AO-3100 as late as 1980. And, a bargain at only $225 and $100, respectively. You'll have to pay with old bills though. :)
However, I now have obtained an AO-3100 that does still lase. More below.
Lasing specifications:
HeNe laser tube:
Resonator:
Power Supply:
I have acquired a sample of the AO-3100 that was quite battle weary but the tube did survive cross-county shipping. The case, on the other hand, looks like it lost a fight with one of those Sherman Tanks. :) It was bent and dented in multiple places. How the tube didn't turn to a million bits of glass is amazing.
The better thing about this laser is that the discharge color of the old soft-seal tube looks pretty good and there is still a very distinct getter spot. A measurement of the ratio of the He 587.56 nm and Ne 585.25 nm spectral lines in the discharge show that they are about equal in intensity. This means that the He:Ne fill pressure is still decent, though compared to a barcode scanner HeNe laser tube I tested, about 1/2 the helium intensity. A helium soak might be in its future.
After realigning the mirrors and cleaning the Brewster windows, I now have 0.35 mW of red photons squirting out the front of the laser. Probably only the front mirror was misaligned originally, but since I had to remove them both to get the rubber Brewster covers off, realignment of both were required. Fortunately, getting an alignment laser beam through the wide bore was straightforward. The HR mirror mount was then installed and adjusted to return the alignment beam cleanly through the bore. The OC mirror mount was then installed and that's when it became clear that its alignment was way off. Now I wonder who did that. :) Once the alignment screws were tweaked to center its reflected spot, a bit of fiddling resulted in a weak beam. Some mirror walking and Brewster cleaning helped, but it's not finished.
The discharge color appears to be improving as it is run as well but output power has been decreasing as it is run. I hadn't realized that the spec'd lifetime is only around 100 hours - and I've put on 5 or 10 percent of that just testing it! It might be a power supply problem though since it produces a nice bright beam for an instant when started, but then settles down to perhaps 100 uW on a good day. I do turn it on for a few seconds almost everyday just to keep it happy.
The photos for "Gaertner/American Optical 3100 Helium-Neon Laser 2" in the Laser Equipment Gallery are of this laser in action. The color rendition of my digital camera isn't very good. The color in the main bore and larger sections of tubing actual should look close to that in normal HeNe lasers. But the cathode glow (the bright blob) is actually more yellow, (though not quite the yellow in these photos. :) The double coiled glowing hot filament is clearly visible in Views 03 to 05. A careful examination of Views 03 and 06 reveals the scatter from the Brewster windows at each end of the tube. Note the large difference in scatter size due to the hemispherical resonator. View 07 shows that there is indeed a beam from this laser (if that wasn't obvious from the Brewster windows), though due to its relatively low power, bore light is competing for attention.
I now run this laser for a short time on roughly a weekly basis just to keep it happy. I've never reinstalled the boots, so Brewster cleaning is required every few weeks. The maximum power is now only about 0.2 mW and seemed to be declining with extended run time. Once one realizes that the rated life is only 100 hours or so, it's likely that the few hours I ran it sucked up a substantial percentage of its life. However, the short runs don't seem to be hurting it much. This laser was acquired in July, 2005 and it had been over 2 years now without obvious degradation.
However, as of 2009, it lights up with an seemingly normal discharge color but will not lase despite repeated B-window cleaning. It's possible that the mirrors have become contaminated due to not being sealed, or even degraded since they are soft coated. Eventually, I'll deal with that.
Through screwups in manufacturing (incorrect mirror formula, extra "hot" emission, etc.), an occasional HeNe laser may produce weak lasing at one or more ("rogue") wavelengths other than those for which it was designed. For red tubes, the most likely spurious wavelength is a deeper red at 640 nm since it is also a fairly high gain line. For a low gain yellow laser, orange is most likely since it is a relatively close wavelength and any goofup with the mirror reflectivities may allow it to lase.
I have a tube made by Melles Griot, model number 05-LYR-170, which is about 420 mm long and 37 mm in diameter and can be seen as the middle tube in Three HeNe Tubes of a Different Color Side-by-Side. Its only unusual physical characteristics are that the bore has a frosted exterior appearance (what you see in the photo is not the reflection of a fluorescent lamp but the actual bore). Apparently, larger Melles Griot HeNe tubes are now made with this type of bore - it is centerless ground for precise fit in the bore support. I don't know if the inside is also frosted; that is supposed to reduce ring artifacts. And, of course, the mirrors have a different coating for the non-red wavelengths.
According to the Melles Griot catalog, this is a HeNe laser tube operating at 594.1 nm with a rated output of 2 mW. However, my sample definitely operates at both the yellow (594.1 nm) and orange (604.6 nm) wavelengths (confirmed with a diffraction grating) - to some extent when it feels like it. The output at the OC-end of the tube is weighted more towards yellow and has a power output of up to 4 mW or more (you'll see why I say 'up to' in a minute). The output at the HR-end of the tube has mostly orange and does a maximum of about 1 mW. Gently pressing on the mirrors affects the power output as expected but also varies the relative intensities of yellow and orange in non-obvious ways. They also vary on their own. The mirror alignment is very critical and the point of optimum alignment isn't constant. In short, very little about this tube is well behaved. :)
Why there should be this much leakage through the HR is puzzling. The mirror is definitely not designed for outputting a secondary beam or something like that as there is no AR coating on its outer surface. Thus, that 1 mW is totally wasted. Perhaps, this was an unsuccessful attempt to kill any orange output from the OC. The OC's appearance is similar to that of a broadband coated HeNe HR - light gold in reflection, blue/green in transmission. The HR appears similar to one for a green HeNe laser - light metallic green in reflection, deep magenta in transmission. (However, it's hard to see the transmission color in the intact tube. The OC may be more toward deep blue and the HR may be more toward purple.)
As would be expected where two lines are competing for attention in a low gain laser like this, the output is not very stable. As the tube warms up and expands - or just for no apparent reason - the power output and ratio of yellow to orange will gradually change by a factor of up to 10:1. Very gently pressing on either mirror (using an insulated stick for the anode one!) will generally restore maximum power but the amount and direction of required pressure is for all intents and purposes, a random quantity. If the mirror adjuster/locking collar is tweaked for maximum output at any given time, 5 minutes later, the output may be at a minimum or anywhere in between.
I surmise - as yet unconfirmed - that at any given moment, the yellow and orange output beams will tend to have orthogonal polarizations. But, as the distance between the mirrors changes, mode cycling will result in the somewhat random and unpredictable shifting of relative and total output power as the next higher mode for one color competes with the opposite polarized mode of the other. Is that hand waving or what? :)
A few strong magnets placed along-side the tube reduce this variation somewhat. I'm hoping that adding some thermal control (e.g., installing the tube in an aluminum cylinder or enclosed case) may help as well. I was even contemplating the construction of a servo system that would dither the cathode-end mirror mount to determine the offset direction that increases output and adjusts the average offset to maximize the output. This might have to be tuned for yellow or orange - an exclusive OR, I don't know if maximizing total optical power will also maximize each color individually.
Using an external red HR or OC (99 percent) mirror placed behind the tube's HR mirror, I was able to obtain red at 632.8 nm as well as a weak output at the other orange line (611.9 nm), and at times, all four colors were lasing simultaneously. :) See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes.
(From: Steve Roberts.)
Ah, the Melles Griot defects... These show up from time to time and are highly prized in the light show community for digitizing stations and personal home lumia displays.
The yellow/orange combo is not a goof. I've seen a 7 mW version of that that was absolutely beautiful, but rejected because it was too hot. It's probably slight differences in the length of the tube or bore size. They cut them for a given mode spacing, but fill them all at once with the same gas mixture. A few companies do make dual line tubes, but you can imagine the initial cost is murder.
I used to have a short tube that switched from red (632.8 nm) to orange (611.9 nm) that appeared brighter then the red when it felt like it.
I sometimes wonder if there are a few more HeNe transitions we don't know about. I know they exist in ion lasers. I have seen a 575 nm yellow line in krypton that's not on the manufacture's data and a red in Kr that is between 633 and 647 nm. I had that red in my own laser. 575 nm is preferred for show lasers because it doesn't share transitions with 647 nm like 568 nm does.
When I was interviewing at AVI in Florida they used 4 color 4 scan pair projectors for digitizing - 6 mW of yellow, 5 mW of green, and 8 mW of red, all from HeNe lasers. The blue came up from an ILT ion laser in the basement to each of the four stations via optical fiber. The guy who owned AVI said if you call Melles Griot and ask nicely they will grade some tubes for you for a slight extra cost. Methinks they make all the special colors up and tune them in power somehow, so they can make a price differential, those lines should be consistent by now.
Every two years of so it seems Melles Griot cleans out their scrap pile, and somebody always seems to get there hands on them, grades them and sells em.
(From: Daniel Ames (Dlames2@aol.com).)
The yellow and orange HeNe energy transitions are very similar and possibly competing with each other, especially if the optics are questionable. I have learned that Melles Griot and other HeNe laser manufacturers sometimes suffer from costly mistake on a batch of tubes due to the optics being incorrectly matched to the tube and/or the optics themselves not being correct for the desired output wavelength. One such batch was supposed to be the common red (632.8 nm) but the optics actually caused the gain of the orange to be high enough that the output contained both red and orange (611.9 nm). Then I believe they are rejected and tossed out, only to be saved by professional dumpster divers to show up on eBay or elsewhere. Actually, these misfits such as the yellow/orange tube can be quite fascinating. It would be interesting to shine a 632.8 nm red HeNe laser right through the bore of that tube while powered and see what color the output is. I have been told that if you shine a red HeNe through a green HeNe that it will cause the green wavelength to cease. I have not had this opportunity to try this, so I do not know for sure what really happens, maybe the red just overpowered the green beam. This could be verified with 60 degree prism or diffraction grating on the beam exiting the opposite end of the green tube. Happy beaming. :)
(From: Sam.)
I have tried the experiment of shining a red HeNe laser straight down the bore of a green HeNe laser (my green One-Brewster tube setup). I could detect no significant effect using a low power (1 or 2 mW) laser. This isn't surprising given that the intracavity power of the green laser was probably in the hundreds of mW range so the loss from the red beam would be small in a relative sense. However, wavelength competition effects are quite real as evidenced from experiments with the two color 05-LYR-170 tube.
Its actual total power output after warmup is over 2.50 mW. The 594.1 nm (most intense, LG01/TEM01* doughnut) and 604.6 nm (LG01/TEM01* or TEM10 depending on its mood) are relatively stable but the 611.9 nm (least intense, TEM01) visibly fluctuates. Nonetheless, overall power stability and mode cycling behavior are similar to that of a typical medium power red (632.8 nm) HeNe laser, which contrasts dramatically with the very unstable yellow/orange Melles Griot laser described above. REO does have a couple of dual wavelength HeNe laser heads listed but nothing like this. They are 1,152/3,391 nm and 1,523/632.8 nm.
There is also an additional 2 pin connector on this laser head. The resistance between pins is about 20 ohms and I assume it to be a heater on the OC mirror, though driving it with about 10 V had no detectable effect whatsoever. (This is supposedly used to prevent the formation of "color centers" in the mirror coating. Many older PMS lasers have the heaters and I've never seen any noticeable effect on any of those I've tested either!)
However, I wonder if there is also some screwup in the REO model descriptions as the size of this laser head actually matches that of the REO LHYR-0200M, being almost 17" in length rather than the 13" listed for the LHYR-0100M. I kind of doubt that shorter length can be accounted for by dramatic improvements in HeNe laser technology since my sample was manufactured (1988), though I suppose that's a possibility. But the electrical specifications of the two lasers are supposed to be identical, which doesn't make sense and I don't believe in coincidences. :) And the output power of my sample peaks at 6.5 mA which isn't consistent with the specs for either the LHYR-0100M or LHYR-0200M which are both 5.25 mA.
I've since tested a pair of PMS/REO mode LHOR-0150M laser heads. Both of these produce relatively stable triple wavelengths, though the "color" balance differs:
The ratios change somewhat during mode sweep but not anything sudden or dramatic, and generally not noticeable with an actual measurement.
I've also seen several double-ended LHOR tubes with similar characteristics, in addition to the one with the strange 609.0x nm line, below.
And this one is only supposed to be 611.9 nm orange. However, it's doing stable 604.6 nm (orange toward yellow), 594.1 nm (yellow), AND a wavelength that few if any people have ever seen in a HeNe laser, which appears to be between 608.9 and 609.1 nm (orange). The tube is labeled LTOR-0150ODE, which would normally mean 1.5 mW (rated) 611.9 nm (orange). But we know and love PMS/REO - many of their "other color" HeNes are not what they are spec'd to be. For example, see the section: The Weird Three-Color PMS HeNe Laser Head, above. This is a bare tube which by design (I assume) has about equal output from both ends. (Confirmed because both ends have the strange extra optic glued to the mirror glass, presumably to correct divergence.) Originally, it was misaligned, so the total output power was only about 2 mW consisting of the three common lines - 611.9, 604.6, and 594.1 nm. (Already out of spec but not unusual for REO.) After aligning the OC mirror with a car key (!!), it now produces almost 4 mW total output from both ends. AND a lasing line popped up between 611.9 and 604.6 nm. At first I thought it was simply an artifact of the diffraction grating since it was too unstable to really analyze in detail. But then it came on and stayed on for almost an hour during which photos could be taken of the lasing line spectrum and the wavelength could be measured precisely. The wavelength of the mystery line has now been determined in several ways:
It's difficult to argue with the spectrometer, especially using the known wavelengths for the nearby lines as calibration references.
A search of the NIST database and other sources has shown that there is a transition at 609.5 nm between the 2P4 to 1S4. This is not out of the question, though I do believe my measurements to have an uncertainly of less than 0.2 nm. However, if it is indeed 609.6 nm then there is another mystery: 2S4 is the lower lasing level for 632.8 nm (common red). But there is normally no lasing at 632.8 nm for 2 of these lasers! (Though 632.8 nm can be produced using external mirrors.) So, if that wavelength is accurate and originates there, it may be another Raman transition. The source of the peculiar lasing line is unknown. It has not turned up in a literature search for lasing wavelengths so far. However, in "Gas Lasers", edited by Masamori Endo, Robert F. Walter, pg. 501, there is a diagram with a radiative decay transition at 609.6 nm. This should not be a lasing line though. See link for the graph from "Gas Lasers" Gas Lasers: The Helium-Neon Energy Level Diagram. But a lasing line at around 610 nm using 2P4 to 1S4 does turn up elsewhere.
Could there actually be two lines near 609 nm and the one seen here is previously undiscovered? :-) While this is hardly likely based on the amount of research done in the 1960s on finding every HeNe line that could lase - over 200 of them! - it's not totally out of the question. Perhaps ~609 nm has been seen by many researches who never measured it precisely assuming it was the 609.6 nm line.
Now in fact, those of us who get our excitement from this sort of stuff have never actually seen a laser producing 609.6 nm coherent light (and direct lasing at this wavelength is supposed to be forbidden by Physics). The only time anything appears between 604 and 612 nm is the mystery 609.0 nm line.
Another possibility is that the mystery line is from some gas contamination. Exactly how it would lase in what would almost certainly be very low concentrations isn't clear though. Here are some possible emission lines close to the unknown lasing line:
I thought the most likely would be argon, with its emission line at 609.0785 nm, within +/-0.1 nm of the mystery line. The idea was that that REO used neon intended for NE2 indicator lamps, which apparently may have 0.5% Ar to reduce the starting voltage. Or, perhaps they added Ar for that purpose figuring it would make no difference in lasing - which would be true in most cases, and any additional lines would go unnoticed by 99.9% of users. However, recent tests of the sidelight spectrum of one of these tubes shows absolutely zero indication of any argon lines. Scratch one promising theory.
Stay tuned. The jury is still out on this one. :)
But the remarkable thing about these laser heads revolves around what is inside: A two-Brewster HeNe laser tube! Except for some very early units, the tips of the 2-B tube extend to very nearly touch the mirror plates. On some early ones, the tube is about an inch shorter. (I don't know if this is just a physical difference or whether the newer tubes are actually slightly higher power.) So, these are really external mirror lasers in a nice compact stable package. See View Inside Hughes Model 3184H HeNe Laser Head and Hughes Model 3184H HeNe Laser Head Construction. The end-plates press against aluminum gaskets which allow for mirror adjustment as well as providing a mostly decent environmental seal. The mirror glass is held in place in the end-plate with an aluminum ring press-fit against a rubber cushion. Note the threaded inserts to provide steel-on-steel contact for the adjustment screws. The Brewster window and potting material can be seen within the massive aluminum cylinder - the wall thickness of the sections near each end is at least 5/16ths inch! It's actually made up of 3 pieces (in addition to the end-plates) press-fit together along with a rubber O-ring and an additional rubber ring (maybe just squirted in before completing the press-fit) for sealing. The center section has thinner walls and I found out that clamping it in a vice will crunch the tube. :( But at least the broken heads still make decent hammers. :) The actual tube is the typical Hughes-style but with B-windows at both ends. Although the potting material is soft rubber and not RTV, it appears to mostly fill the inner space, just allowing the Brewster stem at the anode/wiring-end of the tube to poke out and nearly covering the cathode-end, so removing the tube intact would be a challenge. More below.
Several other models may also contain 2-B tubes like this including the 3072H, 3176H, 3178H, 3193H, and 3194H.
Unfortunately, dating from the 1970s, most samples are deader than the standard door nail. They might light up but don't lase. I acquired two of these awhile ago. One, from 1976, appeared to have approximately the correct discharge color (as best as I can determine viewing it from the end) and the tube voltage seemed reasonable. But, no red photons no matter what I've tried. Another, from 1979, did start a couple years ago, though the discharge color and tube voltage characteristics were obviously wrong. But now it only flashes, indicating that it's nearly up to air. However, several of the oldest lasers, dating from the early 1970s, have survived and lase and even produce an output power not much different than what was measured in 1973, the last time they were tested! The beam is TEM00 with low divergence and less scatter than many modern HeNe lasers. I suspect that for those fortunate individuals, the Brewster windows were optically contacted instead of being sealed with Epoxy.
One of the working heads I tested outputs about 3.5 mW at 6.5 mA with an operating voltage to the head of about 1,610 V. The test power in 1973 was 3.4 mW. Based on the 4 in the model number and a CDRH sticker rating of 6.5 mW, I suspect that the rated output power is actually 4 mW. Power continues to increase slightly above 6.5 mA. This may mean that either the optimal current is higher, or more likely, that the tube is low on helium or has some other slight gas fill problem, or it's just high mileage. (Although the power supply that apparently went with these heads is not very well regulated, its behavior suggests that 6.5 mA is correct.) Due to the way the tube is potted inside the metal cylinder, there is no way to easily assess the discharge spectrum to evaluate the gas fill without test instruments.
The mirrors appear to be hard-coated with the HR being flat and the OC having an RoC of about 30 cm. This results in a nearly hemispherical resonator with a mirror spacing just under 30 cm, confirmed by the very small spot visible on the HR mirror when the laser is operating. The OC is AR coated on its outer surface (though it is not as robust as modern AR coatings), and on most of the laser heads, the HR is fine-ground on its outer surface.
Interestingly, the bore of the 3184H appears to be tapered and is wider at the OC-end than at the HR-end. This makes sense to more closely match the mode volume of the hemispherical resonator and thus increase the gain slightly. A tapered bore was apparently an optimization that was popular in the early days of HeNe lasers but went out of fashion due to its higher cost compared to using a uniform size capillary tube for the bore. I've only come across a tapered bore (or at least noticed it) in one modern-style HeNe laser tube, a Melles Griot 05-LHP-170, manufacturing date unknown but it has a serial number of 675P - sounds kind of old! With this asymmetry, the HR and OC cannot simply be swapped without likely seeing a severe penalty in output power. It also would likely not be advantageous to use a confocal or any other symmetric configuration. However, going to a long-radius hemispherical resonator might work even better than the existing arrangement.
With 4 screws holding the end-plates in place against the aluminum gasket, mirror adjustment is somewhat awkward but with persistence, optimal alignment including mirror walking can be performed relatively quickly. However, the aluminum gasket isn't ideal, so for testing, I've replaced it with a rubber O-ring to provide some real compliance. That is, until I decide what to do with the 2-B tube inside! :)
There apparently were some of these for other wavelengths. I've found a (dead) sample of a 3176 that was probably for 1,152 nm as the mirrors are highly transparent at all visible wavelengths but without the greenish tint typical of 1,523 nm mirrors. I suppose it's possible someone replaced the mirrors but they appear to be original.
For a description of several more of these lasers, and a test jig and tests using external mirrors, see the section: Some Semi-Antique Hughes Laser Heads.
Where one is really determined to get the tube out, here is more info on what's involved. But why bother? Aside from esthetics, it's perfectly happy in there and very well protected. The risk of destroying the tube may not be worth the rewards. The press-fit end-sections must be pulled straight out (not twisted) with something along the lines of a gear puller as they are a very tight metal-to-metal press fit with ridges all around. Or, they can be carefully cut off with a metal cutting lathe or band saw. But serious vibrations will likely destroy the tube. Then, the rubber potting material would have to be chipped/gouged/cut/sliced away to actually extract the tube. Then all the remnents of the rubber stuff must be removed from the tube.
Having said that, I was able to get the end-sections off of a dead laser head without serious tools. (I'm not about to risk a good one!) Since the center section has a slightly larger outside diameter than the end-sections, an aluminum HeNe laser head clamp tightened just snug around the end-section provided a way of pressing on the center section to pull the end-sections free. Four clearance holes were drilled in a 1/2" thick piece of aluminum plate and 4-40 screws were then passed through these holes and threaded into the laser head. By carefully tightening these screws in a cyclic manner (e.g., 1,2,3,4,1,2...), the end-section could be pulled out about 1/8". Once this was done, the HeNe head clamp was removed and shorter screws were used to attach the 1/2" plate directly to the head. With the plate clamped in a vice, the entire head could be worked back and forth until it came free. (Alternatively spacer plates and/or shorter screws could be added/substituted to continue the original process until the end-section comes free.) This was not fun, a set of screws survived for only about one end-section, and as noted, this is really only the beginning of the tube extraction process. I have not yet attempted to go any further. But someone else has succeeded in removing one of these tubes intact. Apparently it wasn't much fun.
I've recently come across a 3170H, which is similar in construction to the heads described above - but on steroids. It is 22-3/4" long by 2-1/4" diameter with a thick-walled cylinder for its entire length. The mirror adjustments are equally mediocre with the same aluminum foil seals. The 2-B tube inside is about 22" from Brewster tip to Brewster tip. It had a manufacturing date of 1978. Unfortunately, the HV cable was cut flush with the body of the cylinder, so there was no chance of beaing able to safely apply power, but using an Oudin coil, it does ionize with possibly decent color. It must have been good for 10 or 15 mW.
And I was given a 3178H that is under 9 inches in length with an Alden cable coming out the side instead of the red and green wires, but is otherwise identical to the 3184H. See Hughes 3178H HeNe Laser Head on Original Mount with 3598H Power Supply. It produces over 1 mW at 6.5 mA (a bit under 0.9 mW on 5 mA), which is probably close to the power when new.
PMS has a patent for this setup - U.S. Patent #4,594,715: Laser With Stabilized External Passive Cavity. By linearly oscillating the external mirror at a modest frequency (enough to produce a few cm/sec of movement), the resulting Doppler broadening of the wavelength spectrum will be sufficient to effectively decouple the external cavity from the active cavity. This gets around the stability issues present with open cavity (e.g., Brewster window) particle counter designs. There is a great deal of information in the patent on this and other principles of operation.
Any hapless particles that may pass through the beam in the cavity between the OC of the HeNe laser tube and the external mirror will result in scatter detectable from the side. A large reflector and aspheric lens collects the side-scatter and focuses it on another photodiode (under yellow CAUTION sticker). There is a preamplifier in the box.
It gets better. Viewing the waste beam out the unused HR-end of the tube (far right) with a diffraction grating reveals that the tube is lasing on the normal red line, but also on both of the HeNe orange lines (604.6 nm and 611.9 nm), three other red lines (629.4 nm, 635.2 nm, and 640 nm), *and* on the very rare Raman shifted red line at 650 nm. And there may be others but it's difficult to resolve them since the beam is multimode and the spectra cannot be focused to small spots. Here's a photo of spectrum: Lasing Lines of PMS/REO External Particle Counter HeNe Laser 1. From left to right, the wavelengths are: 604.6 nm, 611.9 nm, 629.4 nm, 632.8 nm, 635.2 nm, 640 nm (very weak), and 650 nm. The 650 nm is actually probably the second strongest after 632.8 nm. How many 7 line HeNe lasers have you seen? :) This is similar but even better than what I've observed in my experiments using external mirrors with normal internal mirror HeNe laser tubes although this one seems particularly stable with little obvious variation in the intensities of the lines, at least over a period of a few minutes. Obtaining the 650 mm line is particularly unusual, especially since it is so stable. See the section: Getting Other Lasing Wavelengths from Internal Mirror HeNe Laser Tubes. These non-632.8 nm lines are probably not an objective of the design but are just an interesting artifact.
In fact, testing much later with a Rees laser spectrum analyzer, a weak line at around 652 nm is also present some of the time. See the section:
I have estimated the reflectivities for the three mirrors which are in this laser. These values are based on measurements of the output power of the HeNe laser tube without the external mirror (about 8 mW after warmup) and the assumption that the internal OC is about 99 percent:
Power with external HR?
Mirror Description Reflectivity No Yes
----------------------------------------------------------------------
HeNe laser tube HR 99.99% 0.9 mW 1.00 mW
HeNe laser tube OC 99% (assumed) 8.00 mW 80.00 mW
External HR 99.9% -- 0.09 mW
The "Power" refers to the optical power passed by the specified mirror depending on whether the external HR mirror is present and aligned. In the case of the HeNe laser tube OC with the external HR, this is the circulating power in the external cavity which is what's available for the particle scatter. Note that the circulating power inside the HeNe laser tube is around 10 WATTS but isn't accessible.
I obtained another particle counter assembly with an internal mirror HeNe laser tube and external resonator. However, there were some differences, most notably an electronics PCB attached to the back of the resonator, and possibly a PZT instead of EO device for cavity length dithering. The tube in this one must be soft-sealed as it arrived with a putrid blue/pink discharge requiring more than a week of run time to clean up until the output power leveled off at about 1.2 mW (50 percent higher than the other laser). It then produced 6 of the same 7 lines through the HR (all but 604 nm). The 650 nm Raman line had been growing steadily during cleanup and is as intense or perhaps even more intense than the 633 nm line. It is also rock stable which is supposed to be impossible. The 640 nm line is very weak, possibly as a result of mode competition with the Raman line but that's just a wild guess. :) There is also a very weak output at around 652 nm - probably another Raman line or something more exotic. But it is only there sporatically. See Lasing Lines of PMS/REO External Particle Counter HeNe Laser 2. Too bad the color rendition of the digital camera is so poor.
And here are some comments on particle counter technology:
(From: Phil Hobbs (pcdh@us.ibm.com).)
There exist particle counters using external resonant cavities, and also intracavity Nd:YAG setups. Intracavity measurements *look* as though they give you amazing sensitivity, but they usually don't. Not only is the circulating power amazingly sensitive to temperature gradients and tiny amounts of schlieren from air currents, but the signal you get is wildly nonlinear and highly position-dependent. Intracavity measurements are a great way to lose sleep and hair. Passive cavities are usually much better, and nonresonant multipass cells are better still.
Raman spectroscopy is used to identify gases by passing a laser beam through the unknown sample. Raman scattering results in a shift toward longer wavelengths depending on the atomic/molecular composition of the gas. By measuring the intensity of the Raman scatter at several longer wavelengths, the gas composition can be determined. For these units, the relevant gases were apparently N2, O2, and N2O based on "linearization constants" printed on a label on the lasers.
To get any sort of sensitivity, the beam must be high power since a very small percentage of photons actually undergo the Raman shift. For the Ohmeda unit, this is achieved by utilizing the intracavity power between 2 super polished HR mirrors and super-polished Brewster window. While I don't know for sure what the intracavity power should be, based on tests of the mirror reflectivities and output power with an external OC mirror with known reflectivity, it is at least several watts and could be over 100 W when using the original exteranl HR mirror!
The relevant patents include:
The first one describes the principles of Raman spectroscopy though the actual drawings do not correspond to the Ohmeda laser assembly. But the other two have diagrams which closely match the specimens I have, though I'm not sure which they are.
A photo of a mostly complete unit is shown in Ohmeda Raman Gas Analyzer Assembly. The metal HeNe laser tube can be seen poking out the left side with the red cap covering its internal HR mirror. The brick power supply is behind it. The tuning prism assembly is at the right, partially hidden by an absolute filter and one of the detector PCBs. That elaborate set of filters and dessicant containers is designed to eliminate *all* particles and condensible vapors from the laser cavity, which must remain perfectly clean. I'm not really sure why the heatsink is clamped to the lsaer tube. It doesn't get *that* hot. :)
The laser tube, Brewster prism, and external mirror are probably made by REO, Inc.. (Other parts of the assembly may be made by REO as well.) The tube looks like a slightly shorter version of the REO/PMS tunable 1-B tubes, but its internal HR mirror is coated so that in conjunction with the HR mirror at the other end of the cavity, the reflectance for 632.8 nm is maximized. Using a 60 cm RoC OC mirror with a reflectance of approximately 98 percent at 632.8 nm, the laser produces about 5.4 mW, multimode. I assume that with an optimal OC mirror, the power would be somewhat higher. (This test was done without the Brewster prism assembly. There would be some loss with the prism present in the cavity.)
At 5 mW - implying 250 mW of intracavity power with the 98 percent OC - the waste beam is about 5 uW and the reflectivity of the internal HR mirror is thus about 99.998 percent. There is very little scatter visible on the B-window under these conditions. (I did have to clean it, but there is a handy access port that can be used for this purpose.) If there were no other losses, putting a similar HR at the other end would result in 125 W of intracavity power! Of course, this is impossible as there ARE other losses, but it is likely to be several watts and perhaps much more. With an SP-084 HR, the output from this mirror was about 0.5 mW and the output from the internal HR was 32 uW corresponding to about 1.5 W of intracavity power. Not too shabby. But with the REO HR (and Brewster prism), the waste beam power for 633 nm was a whopping 122 uW implying about 6 WATTs inside. Not too shabby at all. :) I have cleaned the Brewster prism with no significant change in performance. However, a careful cleaning of all three surfaces would almost certainly improve things some more, especially for this case. Interestingly, with the REO mirrors, the beams exiting the laser appears to close to, if not pure, TEM00.
When used in the normal way, there is a 632.8 nm narrow band filter between the external mirror and a silicon photodiode. So, that is almost certainly used to monitor the power transmitted by that mirror, and by inference, intracavity power.
The 632.8 nm intracavity power would no doubt be greater without the prism but that's where it gets interesting. With the prism in place, the wavelength is tunable with both orange wavelengths being easily selectable for 2 of the lasers. (The 604.6 nm orange line is not present in Laser 3 for unknown reasons, but probably due to mirror reflectivities.)
Here are the stats for three similar laser assemblies with different dates of manufacture:
Laser 1 (Ohmeda PN 6090-2000-513, 15-Jul-04, Tube #IB826-5, S=0.35, T=0.57, Laser Power=3.91. REO tube MN SB/1M/BW, S/N 2856-2204-1063):
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 5 uW 60 cm OC 98.0% 5,400 uW 0.25 W
" " 32 uW SP-084 HR 99.966% 500 uW 1.5 W
" " 122 uW REO HR 99.9984% 186 uW 6.0 W !!
611.9 nm 166 uW " " --- 1,140 uW ---
604.6 nm 14 uW " " --- 0.280 uW ---
Laser 2 (Ohmeda PN 6090-2000-513, 20-Feb-03, Tube #I2348-8, S=1.37, T=0.53, Laser Power=3.45. REO tube MN SB/1M/BW, S/N 1151-0603-911):
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 381 uW REO HR ??? 141 uW ???
611.9 nm 1,120 uW " " --- 93 uW ---
604.6 nm 710 uW " " --- 32 uW ---
Laser 3 (Ohmeda PN 6090-0803-507, 9-Aug-02, Tube #2890-3, S=2.57, T=0.37, Laser Power=2.0. REO tube MN SB/1M/BW(HS), S/N 6093-0501-607):
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 864 uW REO HR ??? 147 uW ???
611.9 nm 2,080 uW " " --- 29 uW ---
604.6 nm 0 uW " " --- 0 uW ---
There were three measured parameters hand-printed on the tube casings of these lasers, but without units: "S", "T", and "Laser Power". Note that S and T have approximately the same ratio as my measured 632.8 nm output power for the internal and external HR mirrors, respectively. While it's not known what these stand for, if the units of these parameters are mW, then this suggests that when new with perfectly clean optics surfaces, the performance at 632.8 nm may be 3 to 4 times what I've measured so far! (There would also be an increase in 611.9 nm output but since significant power is being coupled out of the cavity, the difference won't be nearly as dramatic.) It's also not known what the parameter Laser Power means since nowhere would there be an output where this could be measured.
But these 1-B tubes are considerably shorter than PMS/REO tunable 1-B lasers tubes - 10.25 inches versus 13 inches from internal mirror to B-window. The relative length of the bore discharge differs by a larger relative amount: approximately 8.75 versus 11.5 inches or about 1.3:1. So, their gain will be much lower. And, there is an additional optical surface in the intracavity beam path compared to the tunable laser system since a (2-surface) Brewster prism is used rather than (1-surface) Littrow prism. Thus, even the performance I've measured is rather impressive, especially for Laser 3's orange output (which is really just an accident of the mirror coatings, and not something that was designed in).
Also note that Laser 3 has a different part number than Lasers 1 and 2. I have no idea what differences there may be in the laser part of the system, if any. There is no obvious physical difference.
The orange 611.9 nm beam on Laser 3 when peaked is doughnut mode with a distinct hole in the middle (LG01/TEM01*). There is also an annoying amount of mode-hopping, so adjusting for maximum power is sometimes a challenge as the power jumps around. On Laser 2, the orange beam is TEM00.
I did not test Lasers 2 or 3 with non-REO mirrors, thus the exact reflectivity and intracavity power is not known. Note how the relative mirror reflectivities for these lasers are all different. This may be the reason of a total lack of 604.6 nm orange for Laser 3. Now, Laser 3 was originally sick with a pink discharge and no lasing and had to be run for 100 to 200 hours to recover anything. But since it's total power out of both ends is greater than the others at both 632.8 and 611.9 nm, I doubt low gain to be the cause, though that's still a possibility. Also note that 2+ mW of 611.9 nm orange from a tube of this length with mirrors not optimized for that wavelength is already somewhat impressive. And, the power is actually slightly higher than listed above since that is only the last time all 4 measurements were made. For the complete exciting saga of Laser 3, and up to date measurements, see the section: REO One-Brewster Tube - No Lasing.)
(PMS/REO tubes are soft-sealed since that results in minimal stress on the B-window and higher Q. However, this does mean they should be run periodically. I later found that Laser 2 had a mild case of low poweritis but it's not clear if extended run time will clear it up.)
I do not know what the reflectivity of the internal HR is at 604 nm and 611.9 nm so the intracavity power is not known for these wavelengths either. The purpose of the Brewster prism is no doubt to select only one of the possible wavelengths, which based on the specifications of the filter between the external mirror and photodiode, is no doubt 632.8 nm. The very nice behavior on the orange lines is thus simply an artifact of the mirrors being so highly reflective at 632.8 nm. But note how the power balance between the two mirrors seems to be more or less reversed for Lasers 1 and 2. So, although the internal mirror for both lasers is not AR coated and the external mirror is, the coating formulas appear to have been interchanged.
It would be quite risky to try to run the laser with only the external REO HR but no prism as the mirror glass is glued in place. While the plate that it's glued to could be mounted directly on the adjustable mount, the mirror would be very exposed and susceptible to damage. So, I'm probably not going to attempt that.
Here are how the 8 filters intercepting Raman light from the side of the lasers were labeled and the 633 nm line selection filter in front of the photodiode:
Location Part Number Wavelength
-----------------------------------------------------------
1A BARR #4 4 375-003 7819 1 1993 781.9 nm
1B BARR #1 2 373-030 7777 2 3100 777.7 nm
1C BARR #9 2 373-024 6938 3991 693.8 nm
1D BARR #8 373-027 7421 2 2093 742.1 nm
2A BARR #8 373-026 7364 1 2293 736.4 nm
2B BARR 373-022 6753 4391 675.3 nm
2C BARR #1 373-021 6629 1 2193 662.9 nm
2D BAR #473 CAVITY 7017 1 2293 701.7 nm
Ext HR BARR #1 2 374-002 6328 4102 632.8 nm
I'm deducing the center wavelength based on the part number and observations of visible light transmittance for those in the 600 to 700 nm range. I don't think the exact location of the side mirrors matters except to the extent that it matches up with the appropriate sensor channel.
While these center wavelengths would suggest a rather large wavelength shift, this apparently is the case for gases. But wouldn't there also have to be a 632.8 nm rejection filter in front of the detectors or else that would overwhelm the small Raman signal?
While I had expected the photosensors to be PhotoMultiplier Tubes (PMTs) as in the similar Raman system using an argon ion laser, these are most likely Avalanche PhotoDiodes (APDs). They are in TO18 cans clamped to a ThermoElectric Cooler (TEC, Peltier device) on a large heatsink. Inside the can, there is a little gold colored block perhaps 1.5 mm square, with a 0.5 mm blue dot in the middle, which I presume is the active area. The APD is probably a S9251-05 (or very similar), one of the Hamamatsu S9251 Series Avalanche Photodiodes. There's a fair amount electronics to go with them, though nothing obviously recognizable.
The particle stream passes through the intracavity beam. An elaborate gas flow system maintains positive pressure of clean filtered gas to prevent contamination of the Brewster surface and external HR mirror by the separate gas stream containing the particles being counted. Having been manufactured in 1996, the 1-B design may predate the external resonator design.
The tube is labeled Model: SB/1M, Serial Number: PMS-4638P-2296, and is physically similar to the one described in the section: The Ohmeda Raman Gas Analyzer REO One-Brewster Laser. The glass end of the tube can be seen near the middle of the photo with the Brewster window hidden by a cylindrical dust cover sealed with O-rings that can be pulled back for cleaning. Unlike any of the other PMS/REO lasers (except for the LSTP tunables), this laser also has 3 ceramic magnets glued to the side of the tube, and they do increase the output power by about 5 percent. There are 2 magnets opposite each other near the cathode-end and 1 near the anode-end. The second magnet near the cathode seems superfluous since its effect is minimal but might help a tiny bit. (They may not have put a second magnet near the anode because it would have been dangerously close to the anode connection!)
The power supply is a Voltex brick (which someone had cut all the wires off of, literally 1/4" from the brick. But with wire extensions carefully spliced and insulated, it still works!). The power supply is labeled and set for 5 mA for some reason (perhaps for maximum life), compared to the usual 5.25 mA or 5.5 mA of the other PMS/REO tubes.
With the external HR in place, lasing is mostly on the normal 632.8 nm (red) with a small percentage of several other lines:
Wavelength "Color" Percent
----------------------------------
611.9 nm orange 3%
629.4 nm red 5%
632.8 nm red 80%
635.2 nm red 8%
640.1 nm deep red 4%
For particle counting, only the total intracavity power matters, not the wavelength. Thus, there is no tuning prism in this unit.
The photodetector appears to be identical to the one in the external resonator system (including the safety label), probably using an avalanche photodiode since there is a 200 VDC power supply attached to it. A reflector and big fat focusing lens directs flashes from any particles unlucky enough to pass through the intracavity beam into the photodetector. The only other sensor is a photodiode mounted on the tube's HR mirror, presumably to monitor waste beam power.
As with one of the Ohmeda tubes, this one was also weak at first with an excessively pink slightly dim discharge. But it eventually recovered (though there were a few bumps in the way) with extended run time as the discharge now looks normal (salmon color and bright, possibly near-new and slightly overfilled) and the waste beam power has increased to something very respectable. (See the section: REO One-Brewster Tube - Very Low Output.) So far, the only sick soft-seal tubes that seem to consistently recover to near-new performance with extended run time (as long as there is no contamination from really annoying things like H2 and water vapor) are those from REO. Some other manufacturers' tubes may improve somewhat, but not to this extent, and others simply get worse.
To determine the actual reflectivity of the mirrors and thus the intracavity power, I subsituted a 60 cm RoC, 99%@633nm mirror for the external HR. Rather than attempt to remove the REO mirror itsefl, I simply unscrewed the mounting plate and substituted an instant adjustable mount of my own. :) By measuring the output power from the OC, and knowing its reflectivity, the intracavity power could be calculated. The ratio of the waste beam power from the internal HR to intracavity power represents the transmission (ignoring losses) of the internal HR or Ti. Then, the transmission of the external HR or Te is just the ratio of external to internal waste beam power times Ti. This all went smoothly with the results shown below:
Power from <------- External Mirror -------> Intracavity
Wavelength Internal HR Type Reflectivity Power Power
-----------------------------------------------------------------------------
632.8 nm 2 uW 60 cm OC 99.0% 1,300 uW 0.13 W
" " 86 uW REO HR 99.9959% 246 uW 6.0 W
" " 165 uW " " " " 472 uW 10.7 W
(The last entry is after the full recovery.)
Based on the 60 cm OC's measured reflectivity of 99% and the waste beam power from the internal HR of 2 uW with an intracavity power of 0.13 W, it is allowing only 1 part in 65,000 of the intracavity beam to excape for a reflectivity of around 99.99846%, Wow! If the external HR were that good, the intracavity power would be even higher.
So someone sent me this "thing":
The common autocollimator is an optical instrument for measuring extremely small angular deviations using a point light source, collimating telescope, and beamsplitter to enable the reflection of the light source to be viewed from the side on a graticule. A Web search for "autocollimator" should provide hours of bedtime reading on this subject. :)
The autocollimating alignment laser uses, well, guess what, a laser for the light source and a pair of split photodiodes in place of a human observer. Such instruments can supposedly measure down to arc-seconds.
The Keuffel and Esser 71-2615 is LARGE (over 20 inches long) and MASSIVE (over 10 pounds). And I thought that Metrologic military HeNe laser made a good hammer! :) It is all precision machined and must have cost a fortune new. The thing is also beautiful, with an exterior that is very nicely chrome plated..
The beam out the front is about 1/2" in diameter, only a few hundred uW, rated 1 mW max. The connector on the back has 4 pins that test as diodes.
> I did a brief patent search but didn't find anything relevant. Here is a discussion on the USENET newsgroup sci.optics precipitated by my request for info (loosely based on the description above).
(From: Wade Kelman.)
It's absolutely worthless, and you should send it to me. I'll throw it out for you. :)
Actually, I think you have an alignment telescope that is accurate to a fraction of an arc-second, much better than the visual kind that use reticles for alignment.
I'm surprised that the K&E - Brunson - Cubic Precision Web site doesn't have information on this. Or, you could just call them and ask about it.
(From: Adam Norton.)
What you have is an electronic autocollimator used to measure angle deviation of the reflected beam in the arc-second range. Along with tooling mirrors, penta prisms and such, it is used to do optical alignment, check machine tool way flatness & perpendicularity, surface plate flatness, shaft straightness, etc. In crappy used condition these are worth about $1K (check out ebay). If you had (or could make) the readout, you might get much more. Please do not disassemble as that will ruin the alignment.
(From: Sam.)
I wonder if this was an one of those ideas that never really caught on. There are others out there on eBay and elsewhere, but little (easily located) information.
I did find 5 photodiode outputs on the back that respond to reflected light. I couldn't tell if they were sensitive to slight misalignment though. That would be my next experiment. I wonder what's needed for the readout? Just some op-amps and meters for X and Y?
(From: Adam Norton.)
I replied to the original post before seeing this branch of the thread. This is definitely an idea that has caught on. Check out the Brunson Instruments Web site (which acquired the Cubic Precision/K&E line). Also look at Davidson Optronics and Moeller-Wedel.
To get a signal from the quadrant detector that is proportional to angle and insensitive to reflectance or beam power you need to use the following formulas: Q1, Q2, Q3, Q4 are the signals from the four quadrants:
X = [(Q1+Q2) - (Q3+Q4)]/(Q1+Q2+Q3+Q4) Y = [(Q1+Q4) - (Q3+Q2)]/(Q1+Q2+Q3+Q4)
Older systems used to do this all with analog amplifiers. On-Track technologies among others sell such amplifiers.
(From: Sam.)
This one uses a pair of split detectors so the denominators of the above equations should only have two terms, but would be otherwise similar.
(From: Phil Hobbs.)
In analog, you can do it right down to the shot noise, which typically means something in the hundreds of picoradians rms. Just needs a mildly modified laser noise canceler. See, for example: Ultra-sensitive laser measurements without tears.
Of course, in real life the accuracy will be limited by stray fringes and QE drift in the diodes, but you really can see very very small angular movements this way.
(From: Sam.)
OK, I know you told me not to disassemble the thing. But I may want to do that since the laser tube is very weak - about 30 microwatts out the front and getting weaker with run-time. So, it's end-of-life and is unlikely to get better under any conditions. I assume it should be close to 1 mW when new.
If it were just weak but stable, then the sensitivity would be lower but it will still work so it could be left alone. But it's getting worse. It's clearly an old laser which really needed to be run periodically to maintain its health and was not. (I even found a pic in an auction for one with a notice to this effect.) That would date it to no later than 1980 or so. Soft seal tubes like that went away by 1980. Well, this has probably sat unused for years, if not decades! (However, it seems the tube must have been replaced around 1986, see below.)
It looks like there are 4 setscrews around the perimeter at several locations that do the alignment and lock the laser in place, though not having seen a diagram, there could be others further forward. Originally, I thought the setscrews were covered with hard Epoxy but it turns out that is just a crust over the top, and poking through it with an awl allows these caps to be popped out. Then, there is only some goopy tar-like stuff, for reasons unknown other than to discourage such tampering! :)
If only 2 of the setscrews were removed, the alignment would be maintained, though of course a modern replacement tube - assuming one could be made to fit at all - will also not have the exact alignment of the original. But a jig could be made to adjust it.
Any suggestions other than simply use it or sell it as-is?
It's not a big deal either way. The only reason I have this at all is curiosity! :)
(From: Adam Norton.)
I do not know what this looks like on the inside, but given how stable this thing has to be, I would imagine practically everything would be potted in place inside. If you can replace the laser, trying to align it parallel to the outside housing within a fraction of an arc second might be very tricky. If you can not do that accurately, the gadget still might be useful to measure changes in angle.
(From: Sam.)
That's my feeling. It was useless they way it was with the power declining toward zero the more it was run.
Fortunately, there is no potting anywhere, though some assemblies were locked in place with some globs of Epoxy.
The setscrews seem to adjust the rear of the laser, the front of the laser, and the beam expander position, which makes sense.
I've got the front and rear sections out now.
The front section has the output collimating lens and beamsplitter and photodiode assembly.
The rear section has the laser tube and rear laser mirror. The front laser mirror is still stuck inside. Go figure.
This uses a two-Brewster laser tube with external mirrors. What I haven't figured out yet is hot to get the remaining section with the front laser mirror and expanding lens out. It's about as inaccessible as possible, more than 10 inches in from either end, and doesn't seem to want to move, though I may just need a bigger pry bar. :)
I've also removed the diverging lens and spatial filter assembly.
Unfortunately, so far I have been unable to remove the final remaining piece which holds this as aligns it with the HeNe laser. This also retained the output mirror and mount from the HeNe laser.
Nothing has been damaged so far so it should go back together.
I'll have to replace the laser tube with a modern internal mirror linearly polarized laser tube and arrange to mount it in a similar way. The polarization is needed to optimally separate the outgoing and return beams via a polarizing beamsplitter and Quarter-Wave Plate (QWP).
BTW, the date on the laser tube is 1986. My guess is that it was replaced in 1986 and they used an original design tube, since by then, internal mirror polarized HeNe lasers were widely available and a lot cheaper and less finicky than this contraption. (It was almost recent enough that a red diode laser could have been used but probably not quite.)
It's a custom Hughes two-Brewster HeNe laser tube, a model 3183M. This is short, about 8 inches from tip to tip. Perhaps "M" stands for modified? The mirrors are in massive stainless steel mounts and 1/2" or more in diameter mirrors - unusually large for such a laser. Why? The Radius of Curvatures (RoCs) are 30 cm for the OC and planar for the HR.
I was able to remove the mirror mount deep inside the big cylinder with a hex driver extended with 3/8" copper pipe. :) What was left inside - the mounting plate for the spatial filter/beam expander, electrical connector for the photodiodes, and the OC-end of the HeNe laser - finally yielded to a scrap HeNe cylinder pounded by a 5 pound hammer. :) There appears to be some glue residue that was holding it in place, perhaps the last defense against revealing its secrets. Being able to lay out the parts on the bench will make it a lot easier to realign.
Using a Melles Griot 05-LHP-605 laser head with just the front end-cap removed, it was quite straightforward to install and align the expanding lens and spatial filter to the axis of the main cylinder. The inside diameter of the 05-LHP-605 cylinder is about the same as that of the original laser, so it is a snug fit to the mounting plate at the front. The expanding lens was screwed to the mounting plate snug enough that it would not move on its own, but could be pushed around with the 4 setscrews around the perimeter of the mounting plate. The laser and mounting plate were slid into the main cylinder and then the beam was aligned with its optical axis using the setscrews. After pulling it back out, the spatial filter could be screwed in place and adjusted to cleanly pass the beam. With the 05-LHP-605, the output beam is only about 7 mm in diameter - around half of that with the original laser.
So, I need to find a short polarized HeNe laser tube with a wide beam. A standard cylinder diameter will fit. The trick will be matching the beam diameter so that the expander works correctly and results in a large diameter final beam. I suspect the Hughes has a rather wide beam diameter and possibly a wide divergence as well with its 30 cm RoC OC and planar HR. That is similar to what the gold-cylinder Hughes lasers use. But it may be tough to test since it's so near dead that getting it lasing would be a major issue. Since the axial position of the collimating lens is slightly adjustable, the divergence won't be a big issue. But the laser beam diameter will be proportional to the final beam diameter, and finding a modern tube with sufficiently wide beam may prove challenging.
The Melles Griot 05-LHP-605 I used for testing, about 1 mW, could work. But the divergence and beam diameter result in a final beam that is too narrow for the collimating lens of the autocollimator (about half the original). This would probably be acceptable but not optimal. Matching this may be the hardest part of this retrofit.
A suitable normal tube might be the 05-LHP-410 which has a relatively wide beam (0.85 mm). But I've never seen one of those.
Linearly polarized barcode scanner HeNe laser tubes may also be suitable Possibilities include the 05-LHP-004 and 05-LHP-690 but their beams are closer to 0.5 mm so the final beam diameter wouldn't be much better. But polarized barcode scanner tubes aren't common.
An alternative could be a diode laser. But matching the beam quality of any HeNe would be a challenge.
The chart on that Web page includes the following information:
Cavity Total Tube Working Trigger Working Output Diver-
Model Length Length Diameter Current Voltage Voltage Power gence
-------------------------------------------------------------------------------
150 140 mm* 150 mm* 28 mm 3 mA 4 kV 1.3 kV 0.8 mW
180 180 mm 190 mm 28 mm 4 mA 4 kV 1.4 kV 1.2 mW
200 200 mm 210 mm 30 mm 4 mA 5 kV 1.5 kV 1.4 mW
230 230 mm 240 mm 35 mm 4 mA 5 kV 1.5 kV 1.5 mW 1.25 mR*
250 250 mm 260 mm 36 mm 4 mA 5 kV 1.5 kV 2 mW
280 280 mm 290 mm 36 mm 5 mA 5 kV 1.5 kV 3 mW
300 295 mm* 305 mm* 36 mm 5 mA 5 kV 1.5 kV 5 mW 3 mR*
320 320 mm 320 mm 36 mm 5 mA 5 kV 1.5 kV 5 mW
350 350 mm 360 mm 36 mm 5 mA 5 kV 1.5 kV 7 mW
400 400 mm 410 mm 36 mm 5 mA 5 kV 1.5 kV 8 mW
450 450 mm 460 mm 39 mm 5 mA 5 kV 1.5 kV 10 mW
480 480 mm 490 mm 39 mm 5 mA 5 kV 1.5 kV 12 mW
(The voltage specs for all these tubes are rather suspect since the values don't change much with output power and I haven't measured them even for the tubes I've tested. Values denoted with "*" were measured; all others from their Web site. This doesn't mean they are accurate, just that I haven't measured them.)
Model 150: The first tube I tested was the model 150 (150 mm, ~6 inch tube), similar in performance to a common barcode scanner tube. The construction of this (as well as the others) is, well, strange as shown in Artworldcn 150 mm HeNe Laser Tube. As can be seen, the actual tubes they are shipping bear little resemblence to what's on their Web site. The entire tube is made of glass except for the mini-adjustable mirror mounts, which are similar to those used in Hughes-style tubes except that they have 4 slotted-head screws instead of 3 hex-head screws. Who uses slotted head screws in precision devices anymore? And at least one screw head was already broken! The mirror substrates appear to be attached via a thin layer of glass frit, not the bead that's present on virtually all "normal" tubes. Only the anode uses the mirror mount for the electrical connection; the cathode (which is a normal aluminum cylinder, partially hidden behind the label) has its own terminal via a glass-to-metal feed-through. And to make sure people don't do something stupid, the cathode mirror mount normally has heatshrink over it to prevent its use as the negative electrical connection (removed for the photos). Electrically, the tube behaves normally and should run on a typical HeNe laser power supply for 0.5 to 1 mW lasers. The specs call for 3 mA at 1.3 kV with a 4 kV start.
The output by eye at least is close to TEM00 with a very low M-Squared. So, as a pointer, alignment laser, or barcode scanner, it would be fine. Using a diffraction grating, the only wavelength present appears to be 632.8 nm (at least in the visible). That's the good news, and without actually making measurements, it appears like any other HeNe laser tube of similar size and output power. However, with respect to modes and polarization, this is about the most cantankerous small HeNe laser I've ever seen.
The first thing I noticed after admiring the most artistic (some would say primitive) glass work :-) was that there is no AR coating on the OC mirror, none, not even a puny attempt at an AR coating. I've never heard of any production HeNe laser lacking an AR coating on its OC. Even the 45 year old Spectra-Physics 115 laser had one! Second, as evidenced by the absense of any ghost beams, neither mirror substrate is wedged. So, there will be back-reflections from the outer surfaces of both mirror substrates directly into the laser cavity. I fully expected these back-reflections to make a mess of the tube behavior, and indeed they do. But I was not expecting it to be nearly as strange as reality, or reality to be so strange. ;-)
Using a polarizer with a PC data acquisition system from a cold start to almost 30 minutes, the behavior is extremely bizarre. See: Mode Sweep of Artworldcn 150 mm HeNe Laser Tube During Warmup. This was taken after optimizing alignment (see below) so the total range of the vertical axis is approximately 1 mW. Virtually all common HeNe laser tubes of this length (about 150 mm) go through a predictable mode sweep with two orthogonal polarizations (here called S and P) alternating as the tube expands and the longitudinal modes drift through the neon gain curve. Compare this to plots for a typical tube of similar size and output power in Mode Sweep of Melles Griot 05-LHR-006 HeNe Laser Tube During Warmup. Adjacent modes are orthogonally polarized and the power in each mode goes to exactly zero for a portion of the mode sweep cycle in such a short tube. Even those tubes that are "flippers" generally produce a repeatable pattern, although it might change from flip to non-flip behavior at some point during warmup. But this tube tends to favor one polarization for a few minutes mode sweeping within it alone except for some random burps of the other polarization, and then slowly shifting over to where the other polarization dominats. Within each of these extended temporal regions, one polarization has the most power with occasional dips, while the orthogonal polarization only shows low level twitching and bumps. So it behaves like a poorly polarized tube for awhile (many mode sweep cycles) and then the polarization changes. A few flips can be seen (vertical green lines) but for the most part, the modes change smoothly, so it is not strictly speaking, a flipper. However, even though the total power output doesn't vary that much, it does so in such a way that there is a noticeable discrepancy in the shape of the plots of the P and S polarized modes, especially over a short time period. With normal tubes, they are virtually mirror images of each-other. Yet another very unusual characteristic of this tube.
On a Scanning Fabry-Perot Interferometer (SFPI) the behavior is even more striking. It was necessary to add an ND1 filter to minimize back-reflections from the SFPI before the display settled down, but that's not unusual, and this could also be largely avoided by positioning the SFPI far away from the laser and aligning it slightly off-center. At first, with no polarizer, the display appeared as though it could pass for a normal tube, with the modes happily drifting through the neon gain curve. I wouldn't have given the display on the SFPI a second glance if this were a common tube. But knowing that there was already something very peculiar about this tube, using the polarizer oriented to pass the most power, the display was essentially unchanged from what it looked like with no polarizer - for awhile. Only linearly polarized HeNe lasers behave like that. But then the modes gradually disappeared and it was necessary to reorient the polarizer to get back a similar display. I've have never seen this type of behavior in literally hundreds of HeNe lasers tubes I've tested.
As a test, I put a drop of alcohol first on the HR and then on the OC mirrors. There was little effect with the HR, but the output power in one polarized mode instantly increased dramatically when done on the OC. This was too fast to be a thermal effect, so perhaps an AR coating alone would be enough to make the tube behave. However, while my quick alcohol drop test showed that something changed, it was not clear if behavior was actually significantly improved. And, putting a glass plate at a very slight angle against the OC with some water to index match didn't seem to help, so it's quite possible that other more fundamental modifications would be required.
This sample was also originally annoyingly weak (about 0.4 mW, well below spec) and that of course presented an irresistible challenge. However, it turned out to be rather easy to realign the OC mirror (cathode-end) by adjusting those antique slotted-head screws to boost the power output to over 1 mW. But just when I thought the situation couldn't get any worse, as a result of the optimization, a rogue mode appeared on the SFPI which wasn't there before! (And to confirm that I hadn't simply missed something, misaligning the mirrors makes them disappear, perhaps that's why it was adjusted to be so weak!) The rogue mode can be seen during part of what passes for a mode sweep cycle on this tube as shown in Longitudinal Modes of Artworldcn 150 mm HeNe Laser Tube. The Free Spectral Range (FSR) of this SFPI is 2 GHz. The longitudinal mode spacing of this tube is about 1.034 GHz based on a measured mirror spacing of 14.5 cm. I believe the two tallest peaks on the left photo correspond to the normal (expected) TEM00 modes. Based on the 2 GHz FSR of the SFPI, the tallest and next tallest to the right of it would be just about 1.034 GHz apart. The rogue mode is to the right of one of the main modes, usually but not always the largest one, about 125 MHz higher in frequency than the mode it's hugging (based on the direction of drift of the peaks on the display during mode sweep). At first I thought it was a longitudinal mode. A measurement of the beat frequencies, if any, would prove conclusively that the mode is indeed adjacent, and not aliased as a result of to the 2 GHz FSR of the SFPI (due, for example, to some other lasing line that's not supposed to be there, perhaps IR). And with a Thorlabs DET210 detector (1 GHz bandwidth), there could be no doubt: A beat of around 125 MHz was indeed present for a portion of the time, coming and going as expected. A rogue longitudinal mode would seem to be essentially impossible as there is no way for there to be any reflections inside the cavity at a shorter distance than the mirrors. It would have to be approximately 15.6 mm closer based on the 125 MHz difference. So, could it be a higher order spatial mode? This would seem to be the most likely explanation. And it gets even weirder. Looking closely at the SFPI plots, the rogue mode turns out to actually be a pair of modes, confirmed by their beat frequency to be about 15 MHz apart! At first everything appeared totally perplexing, but assuming these are higher order spatial modes not visible even by careful inspection of the beam profile, it begins to make sense, as they would totally scramble the SFPI display, which generally assumes a TEM00 beam.
As an aside, the cavity geometry of this tube is backwards from nearly all others: The OC is planar and the HR is curved with a RoC measured to be unbelievably long at around 1 meter based on reflecting a parallel beam from outside. With that RoC, Matlab produces a frequency offsets for the first higher order spatial mode that is close to 125 MHz. The fact that it's split could perhaps be due to some asymmetry in alignment.
The main reason that multi-spatial mode operation was the first thing to suspect was the nearly perfect the beam profile. In fact, it may even look better than a more normal short barcode scanner tube. Now in all fairness to Artworldcd, their Web site does say: "Wavelength 632.8nm multi made,long operating time warranty time 1year". OK. so perhaps they need an English translator in addition to some tube redesign. :)
Here's a summary of observations and peculiarities:
Thus, aside from the multitude of unknowns, everything is obvious. ;-)
I've since tested 2 other samples of this same model tube. The serial numbers of two of them are 746 and 1,375, acquired from the company within the last month (April, 2011), so these were likely current production. The third one had no SN label. Even assuming they started at SN1, at most 1,375 had been built to date. Each of the three have unique personalities but generally similar overall behavior. The second tends to remain much more polarized before swapping polarizations while the third produces spikes of the opposite polarization that are fairly regular until it switches polarization and then does the opposite. SNs 746 and 1,375 both had similar higher order spatial modes displayed on the SFPI, while the unmarked tube appeared to be pure TEM00. However that tube was running at slightly lower output power (0.8 versus 1 mW). The alignment screws were too well sealed to attempt to boost it, where higher order spatial modes would be more likely. There are no obvious physical differences but a slightly narrower bore can't be ruled out.
Some aspects of the glass-work are rather crude. For example, the orientation of the tip-off with respect to the cathode terminal differs on all three. And the bore end inside near the cathode-end of the tube has probably been cut by scoring and snapping, not with a diamond saw as there are obvious chunks missing on some places.
So, as noted, using a tube like this for pointing or alignment or anything else that depends solely on the appearance of the beam should be fine. And, I've been told that it isn't too bad for demonstrating the basic principles of a Michelson interferometer. But anyone hoping to build a stabilized HeNe laser or do serious interferometry or holography - or even to explain what longitudinal modes are all about in a classroom - could end up totally frustrated.
But this is a cute little tube! It's possible that only minor modifications would be required to eliminate all these deficiencies, starting with the use of wedged substrates for both mirrors and AR coating of the OC mirror. Using a slightly narrower bore possibly in conjunction with a different RoC for the HR mirror would suppress the higher order spatial modes. (A mirror with that large an RoC is probably the same one they use for their other longer HeNe laser tubes. They then control the reflectivity with the planar OC.) But why not simply copy the relevant parameters from a common 6 inch barcode scanner tube? All of these changes should have only a modest impact on manufacturing cost. Then the tube would not only be cute, but might actually work well and be rather boring like all the others. ;-)
From what I've determined, these tubes are less than half the price of those of similar output power and size from companies like Melles Griot or JDS Uniphase. So there should be some room for well justified added cost while still being much less expensive than the others.
I've since done tests of two other higher power tubes as shown in Several Artworldcn HeNe Laser Tubes The results were totally unremarkable. :) The longer tubes in the photo are rated 1.5 mW and 5 mW.
Model 230: As with the model 150, there is no AR coating on the OC. The output is a nice low divergence beam which appears to be pure TEM00 with a measured output power after a brief warmup of 2.4 mW. Other than some instability when two modes approach equal amplitude, the mode sweep behavior was textbook in nature with no evidence of higher order spatial modes. In the instability region, the modes would bounce up and down, with a possible mode flip. This sort of behavior is not unusual even in some high quality HeNe laser tubes, though it is generally not present with most.
Model 300: Again, no AR coatings. This one is highly multimode (not TEM00) which explains its relatively short length compared to common 5 mW (rated) tubes from other manufacturers. The output power after warmup is over 6.25 mW in an interesting beam. :)
Here are some photos (coming soon):
Here is a rough translation of the general specifications for the OKG-13:
And a similarly rough translation of the description:
"The device OKG-13 is a generator of continuous coherent radiation in the visible part of the spectrum and is designed for use in automatic control systems along the line of for precision optical measurements."
The tube itself appears to be of coaxial construction, with the single filament off to one side, but it is not a side-arm tube. The Brewster windows are attached to rather large bulbs at each end but the bore itself is narrow like that of a modern HeNe laser. The use of a heated filament/cathode went out of fashion :) for most USA HeNes in the late 1960s, though some legacy designs may have persisted into the early 1970s. I have yet to find anything on this laser head assembly that would provide an indication of the manufacturing date. There may be one on the actual glass tube but for reasons that will become clear, I have no current plans to remove it even for inspection or photos. The only label attached to the exterior has the OKG-13 model and a stamped serial number, but no date or date code.
I acquired this laser head on eBay of course, shipped all the way from the Ukraine. :) Here are some photos from the auction (courtesy of Electronics Parts Choice maintained by eBay seller ID: zorolan). These show the laser head in the condition it was received:
When 9 V was applied to the filament leads, the filament immediately lit up nice bright orange indicating that the tube was at least not up to air. Using a variable HeNe laser power supply, it was easy to initiate a discharge but the color was (not unexpectedly) sickly pink/blue. And at first, I was not optimistic about the chances for a miraculous recovery. However, within a few minutes, there was a very obvious improvement. And within less than an hour, the discharge looked relatively normal with a bright salmon complexion. And even though the OC mirror is in rather poor condition with numerous scratches, careful cleaning of the Brewster window at that end of the head resulted in weak lasing. Initially between 25 and 50 µW. (Since these may be soft-coated mirrors, there may be no practical way to clean them.) I did not remove the mirror at the HR-end because it did not appear to have ever been disturbed. The OC mirror had been removed for the auction photos - I would not prop up a mirror like that intentionally!
Of course, a dirty laser would never be happy, so it got a nice scrub and massage! Paint touchup and detailing will come later. ;-)
It was obvious that much more power was possible as the power nearly doubled when run at 6 mA rather than the spec'd 5 mA current.
After several hours, the output power has increased to a peak of over 250 µW at 5 mA, but with a 15 percent mode sweep variation. A maximum power of 340 µW could be reached at around 8 mA.
I'm thinking of building a power supply with a fail-safe circuit preventing the HV from being applied unless there is filament current. The tube would light and lase on a modern power supply without the filament being hot, but that would probably destroy the tube rather quickly from sputtering. In the meantime, it's using my HeNe laser power supply protection widget simply to monitor the filament voltage directly from the tube's filament leads. (See HeNe Laser Power Supply Current Limit Protection.) This will instantly shut off the HeNe laser power supply if the filament voltage either increases significntly or goes away entirely.
Since a proper cleaning of the OC-end Brewster window was never performed, it seemed like a perfect excuse to remove the OC mirror and test the laser with an external mirror.
Using a randomly selected external mirror that was laying around (45 cm, 98.5%), it was possible to get over 0.53 mW easily. Thus, the plasma tube is still in at least decent, if not very healthy condition. Cleaning of the window could now be done while lasing using a generic grocery paper towel :), acetone, and cotton swabs. In fact, the window was already quite clean and the power only increased perhaps 5 percent.
But when the original mirror was replaced, the output power had increased to almost 0.4 mW! I don't think this was due to the Brewster cleaning but simply the luck of the draw on the orientation and position of the mirror. The Russian mirror may simply be too far gone to either achieve optimal output power or consistent results. It is extremely sensitive as to position in the holder. (More so than can simply be accounted for by the associated change in alignment.) The coating quality may also be inferior. If I knew it was hard-coated, proper cleaning might help. But attempting to clean a soft-coated mirror with almost anything will damage or destroy it. Unfortunately, no modern mirror would fit without looking like it had been replaced.
And the HR mirror could also be past its "use by" date, so replacing that might increase power as well.
By fiddling with the mirror and tube alignment via the centering screws, it's now possible to get 0.35-0.4 mW sustained, with it actually peaking at almost 0.45 mW when warming up.
However, running it any more may be counterproductive. It's not known what the life expectancy is of a tube like this. It may be as low as a hundred hours or more likely, several thousand. But not the 10K or 20K hours of a modern tube. The power had declined by a couple percent without doing anything. It's not clear if that's a tube life problem, or simply alignment changing slightly due to thermal cycling. Although the pieces seem to be locked tightly together, torquing the mirror locking ring does affect the power slightly.
The person who sold me the OKG-13 laser (eBay seller ID: zorolan) was kind enough to send me a scan of the operation manual. But unfortunately, it is - no surprise - in Russian. :( :) From what I can deduce looking at the specifications, a photo of the laser head and power supply, and a diagram of the internal construction of the laser head, it's for a slightly newer model as that diagram lacks any reference to connections for the heated filament. See Internal Construction of OKG-13 Laser Head from Operation Manual. (The curvature of the mirrors is greatly exaggerated and some other details do not match my laser head either, but it's better than nothing.) And there may be a date (for the manual at least) in there - 1979 - but that's quite questionable as some of the other date listings don't make sense. That peculiar value of "0,6328 mkilometre" for the wavelength appears to have originated as "0,6328 mkm" in the manual. Perhaps mkm could be interpreted as a "thousand-thousandth" of a meter (1 micron) rather than a thousandth of a kilometer (1 meter). :) Google does find a few papers that reference the OKG-13 (and other OKG) HeNe lasers. One has a publication date of 1975 suggesting that the OKG-13 is likely from much earlier. But the others are much more recent suggesting that the OKG lasers may be or may have been very common. Unfortunately, I am unable to access the full text of these papers (probably also in Russian anyhow!).
If anyone is even moderately fluent in technical Russian and willing to do at last a partial translation of the manual, please contact me via the Sci.Electronics.Repair FAQ Email Links Page.
This tube is intended to be mounted in a three-bar resonator inside an oversize cylinder as shown in Russian HeNe Laser Using Two-Brewster Plasma Tube 1. The resonator assembly appears to simply slip inside secured by screw caps at each end. Nothing else is known about the laser at this time.
And here's another tube acquired separately that appears to be similar or identical: Russian Two-Brewster HeNe Laser Plasma Tube 2. It's a model LG-56 according to the datasheet and that is printed on the tube. This one also requires new gas. ;( :) It came with a Russian Two-Brewster HeNe Laser Plasma Tube 2 Test Sheet which includes the wavelength (6,328 A), current (10 mW), power (2 mW), minimum life (500 hours, wow!), model (LG-55, LG-56), serial number (6708), and date (1978).
(From: Bob Arkin.)
The white rock was a dessicant to remove water vapor from the crappy Epoxy sealed windows. So, it is a sort of very limited getter. (The Optics Technology brand HeNes had carbon chunks instead.)
The power supply uses an bridge rectifier, SCR, and automotive-style induction coil to ionize the gas 120 times per second using two pieces of copper foil wrapped around the tube near the ends. A fluorescent lamp ballast inductor limits current. So it's a pulsed HeNe. :) The connections to the tube are simply pieces of foil. Originally they were copper but my replacements are aluminum. So be it. It is way beyond any hope of lasing but the power supply does work resulting in a blue-white discharge.
The instruction manual (courtesy of Meredith Instruments) may be found at Vintage Lasers and Accessories Brochures and Manuals under "Bausch and Lomb". The only "instructions" are pretty much to plug it in. (There is no power switch.) If it doesn't lase, replace the tube. :-) However, there is a schematic.
More to come.
I later acquired a similar laser head that was more complete so details of the mirror and mounts could be documented. See ENL-911 Two Brewster Hot Filament HeNe Laser Head Mirror Mounts. The HR mirror is small and planar while the OC mirror is much larger and curved. Mirror adjustments are via the 3 screws with a rubber ring providing the compliance force. There doesn't appear to be anything at either end preventing accidental twiddling and total loss of alignment. Although there is no model label on this laser, it is clearly of similar construction with identical wire colors. :) However, it is not identical. For one thing, the cylinder has holes which the ENL911 lacked. And the tube has gas reservoirs at both ends while the ENL911 only has one at the cathode-end. Sorry, no exploded view of this one, some setscrews glued or rusted in place so disassembly would be a major effort. :-) There was no detectable filament glow wen powered at up to 9 V so I'm assuming it is up to air.
(Portions from: Bob Arkin.)
These are from a long defunct company called "Eletro-Nuclear Laboratories, Inc.". There were two models, this uses a single ended tube. There was also a double ended tube with the hot cathode in the middle. The single magnet was indeed for IR suppression.
The power supply had the cathode lead out as two wires from a center tapped filament transformer with a wirewound adjustable Ohmite to set the filament temperature. So the plug had two pins for the cathode, one for the anode, and two used as a jumper to kill the PS circuit if no head was plugged in. And, yes, there was a limiting resistor in the power supply and just a single resistor thrown onto the tube as a ballast.
I've never seen a HeNe tube like this. Although perhaps not obvious from the photos, the machining of the mirror mount stems and glass-to-metal seals is quite rough compared to other tubes. Note the strange mirror mounts with the actual mirrors apparently secured from the inside. The caps appear to have been TIG-welded after the mirrors were installed (though it's not clear how that wouldn't destroy them from the heat). The rest of the stem is a single piece. And the method by which the mirrors are actually sealed to the interior is not clear. Perhaps they simply seat into some soft compliant metal with the caps holding them in place. Or perhaps they are filled with glass frit all around (but then why the caps)? And both mirrors are AR-coated (which for the HR may have been done to minimize back-reflections from the outer surface which could be destabilizing). The electrical connector strips appear to be welded directly to the mirror mount stems. A stub can be seen in the OC View.
As far as operation, it behaves like a normal HeNe. ;-)
The speculation is that it may be from Russia.
