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Introduction

Several of the resonances are suitable for looking at the longitudinal modes of a TEM00 red, orange, or yellow laser (650 through 594 nm). For this application the SFPI may also be called a "Laser Spectrum Analyzer". The same configuration may also be used as a tunable etalon or optical frequency filter.

Wavelengths beyond 650 nm and slightly below 594 nm should work as well, but NOT down to 543 or 532 nm. Performance is best with narrow beam lasers like HeNes but the use of an aperture beam reduction optic should allow the modes of fat beam lasers to be displayed as well. The advantage of a confocal SFPI is that alignment with respect to the laser being tested is much less critical than with a planar-planar SFPI and back-reflections can be off-axis so that the laser is less likely to be destabilized.

Here are the available spherical resonances up to N=10 sorted by increasing mirror spacing. Mirror spacings of 1.0 or less are often more useful being able to either reduce the size of an instrument or provide a larger FSR than the confocal spacing. Mirror spacings of more than 1.0 may be useful to provide smaller FSRs. The Planar cavity 1-0 at the top with the same mirror spacing as the confocal cavity (d=RoC=4.2 cm) would have an FSR of 2.0 x CFSR, but is much more difficult to use. (while this instrument could be loaded with planar mirrors and have a continuous range of FSRs, alignment would be really tricky.)

                                        <------ 4.2 cm RoC ----->
      Num         <---- Relative ---->    d     FSR    FWHM  Fin-  FSR Rel to
  N k Rep  d/r    FSR    FWHM  Finesse  (cm)   (GHz)   (MHz) esse   Confocal
----------------------------------------------------------------------------------
  1 0  1  0.000  1.000   1.000  1.000   4.200  3.569   11.9   300  2.00 x CFSR
----------------------------------------------------------------------------------
 10 1 10  0.049  2.043  20.432  0.100   0.206  7.292  243.1   30
  9 1  9  0.060  1.842  16.582  0.111   0.253  6.576  197.3   33
  8 1  8  0.076  1.642  13.137  0.125   0.320  5.861  156.3   38
  7 1  7  0.099  1.443  10.098  0.143   0.416  5.149  120.1   43
  6 1  6  0.134  1.244   7.464  0.167   0.563  4.441   88.8   50
  5 1  5  0.191  1.047   5.236  0.200   0.802  3.738   62.3   60
  9 2  9  0.234  0.475   4.274  0.111   0.983  1.695   50.9   33
  4 1  4  0.293  0.854   3.414  0.250   1.230  3.046   40.6   75
  7 2  7  0.377  0.379   2.656  0.143   1.581  1.354   31.6   43
 10 3 10  0.412  0.243   2.426  0.100   1.731  0.866   28.9   30
  3 1  3  0.500  0.667   2.000  0.333   2.100  2.379   23.8  100
  8 3  8  0.617  0.202   1.620  0.125   2.593  0.723   19.3   38
  5 2  5  0.691  0.289   1.447  0.200   2.902  1.033   17.2   60
  7 3  7  0.777  0.184   1.286  0.143   3.265  0.656   15.3   43
  9 4  9  0.826  0.134   1.210  0.111   3.471  0.480   14.4   33
  2 1  2  1.000  0.500   1.000  0.500   4.200  1.785   11.9  150  Confocal 2-1
  9 5  9  1.174  0.095   0.852  0.111   4.929  0.338   19.1   33
  7 4  7  1.223  0.117   0.818  0.143   5.135  0.417    9.7   43
  5 3  5  1.309  0.153   0.764  0.200   5.498  0.545    9.1   60
  8 5  8  1.383  0.090   0.723  0.125   5.807  0.315    8.6   38
  3 2  3  1.500  0.222   0.667  0.333   6.300  0.793    7.9  100
 10 7 10  1.588  0.063   0.630  0.100   6.669  0.225    7.5   30
  7 5  7  1.623  0.088   0.616  0.143   6.819  0.314    7.3   43
  4 3  4  1.707  0.146   0.586  0.250   7.170  0.523    7.0   75
  9 7  9  1.766  0.063   0.566  0.111   7.417  0.225    6.7   33
  5 4  5  1.809  0.111   0.553  0.200   7.598  0.395    6.6   60
  6 5  6  1.866  0.089   0.536  0.167   7.837  0.319    6.4   50
  7 6  7  1.901  0.075   0.526  0.143   7.984  0.268    6.3   43
  8 7  8  1.924  0.065   0.520  0.125   8.080  0.232    6.2   38
  9 8  9  1.940  0.057   0.516  0.111   8.147  0.204    6.1   33
  1 1  1  2.000  0.500   0.500  1.000   8.400  1.785    5.9  300  Spherical 1-1

Much more info at Mode Degenerate Fabry-Perot Interferometers.

But back to basics. ;-) The general optical layout of a confocal cavity spherical SFPI (which is a special case of the range of mode degenerate spacings) is shown below:

For the confocal configuration, L = RoC of the mirrors, which must be exactly equal. In this kit, the PZT ring has been replaced by the "holey" beeper element which is more sensistive and a lot less expensive. There is no need for an output focusing lens because the beam is already very narrow.

Semi-X-ray drawings of the top and side views of the Selectable FSR SFPI are shown below:

Selectable FSR SFPI using MGN12 Rail (left) and Thorlabs Rail (right)

The rails allow one of the mirrors to be positioned from the mirrors nearly touching to over more than 3.5 inches (8.89 cm), which is the spherical spacing and the largest one that is stable. The carrier may be locked in position so that fine adjustments can be performed using the micropositioner. The kinematic mirror mounts enable the alignment to be fine tuned.

The MGN12 ball bearing rail allows for very smooth movement but is more complex to assemble and locking is not quite as rigid as with the Thorlabs RC1 carrier. Everything is secured with screws except for the joint between the linear slide and MGN15 carriage, which may use 5 minute Epoxy. I was not able to drill a mounting hole in the steel carriage block. :(

Note that since the spacing isn't fixed, a focusing lens is NOT part of the kit since it's focal length depends to some extent on the cavity spacing. However, one can be added externally or attached to the laser if desired. In practice, the benefits of a lens may not be that dramatic for narrow beam lasers. (The Micro and Nano SFPIs don't have focusing lenses and work fine.)

The typical parts are shown below:

The kit includes the following:

• Confocal cavity mirrors - Two high quality dielectric mirrors:
  • S1: 99.4-99.8%@633nm with its RoC around 42 mm. These will be from the same production lot so their RoCs should be the same.
  • S2: AR@633nm.
  • Diameter: 7.75 mm.

  CAUTION: The mirror coating on highly curved surface is extremely delicate. DO NOT TOUCH. These should be clean, requiring at most to have dust blown off with a air bulb. If cleaning is needed, ONLY use laser mirror cleaning techniques! Warranty does not cover damage to mirror surface! I suggest NOT removing the mirrors from the lens tissue until ready to use. They look kind of cool but are just mirrors! :)

  When set up in the confocal configuration where the mirror spacing is equal to the RoC, the Free Spectral Range (FSR) will be approximately 1.75 GHz. The mirror set can be used for other mode degenerate SFPI configurations using different spacings, which is the intent of this kit. ;-) But the common confocal setup is generally most useful as a general purpose laser spectrum analyzer.

• PZT disk with center hole - Requires 5-10 V to move through 1 FSR. A standard electronic function generator (e.g., an old Wavetek) can be used as a driver if its p-p voltage is adequate. Or, construct Scanning Fabry-Perot Interferometer Driver 1. Or, add an amplifier to boost its output. (A dual op-amp with a +/-15 V swing will produce up to 60 V p-p with the PZT floating, which is way more than enough. See the schematic for the driver, above.) The PZT is 27 mm in diameter and the standard hole is 4-6 mm in diameter.

  The attachment of the leads to the PZT disk is rather fragile, especially the one soldered to the metallic coated area which is only loosely bonded to the PZT material itself. So it would be a good idea to coat the area with some 5 minute Epoxy to minimize stress, and then to add a strain relief when installed in the SFPI assembly.

• Silicon photodiode (PD) - This has an active area of around 2x2mm. For mounting, the leads can be put through the holes in a Perf. board and secured with adhesive. Take care soldering as they plastic melts easily.

  The PD can be connected via shielded cable or a twisted pair directly to the vertical input of your scope with a 10K ohm load resistor. However, it is better to connect the PD in series with the load resistor and back-bias it with a few volts (e.g., a 9 V battery). With back bias, the response (across the load resistor) will be linear up to several mW. Something along the lines of:

                              Shielded Cable
         R Protect    PD1         Center
     +-----/\/\-------|<|-----------------------+----o Scope input (direct)
     |    1K ohms            +--------------+   |
     |  (Optional)           |    Shield    |   /
     |                       |              |   \ R Load
     |                       |              |   / 10K ohms
     |                       |              |   \
     |     +| | -            |              |   |
     +------||||-------------+              +---+----o Scope Gnd
         B1 | |
  

  Confirm that the PD is responding to light. Room light will suffice, but a better test source is a dimmable LED flashlight. These use Pulse Width Modulation (PWM) to chop the light output and a pulsed waveform should be clearly visible on the scope.

  For initial setup, leave the entire photodiode uncovered. Once there is a signal, installing an aperture to block all but the central 1 mm or so may improve resolution. For low power lasers, a preamp would be beneficial.

• Mechanical Parts:
  • Ball bearing rail version
    • MR150 rail with MGN12C carriage and 2x 5-32 x 1/2" mounting screws:.

    • Front mirror mount: Thorlabs KMS or KMS/M kinematic mount modified by enlarging the holes to accept the 7.75 mm mirror and adding a setscrew to secure the front mirror. This mounts via an aluminum adapter plate, spacer, and shim(s).

    • Back mirror mount: Thorlabs KMS or KMS/M kinematic mount modified by enlarging the holes to pass the beam.

    • Thorlabs DT12 or DT12M, or Newport MT-X linear stage: This attaches to the mirror mount using a 3/8 inch 8-32 setscrew with plastic washer to provide compliance in tightening. One or both holes may need to be retapped for 8-32 threads.

      This assembly is then glued to the carriage, centered, using 5-minute Epoxy.

    • Carriage Locking Bracket with 4-40 Nylon thumb-screw: This attaches to the front-end of the carriage with two M2.5-??mm screws.

  • Thorlabs rail version
    • Thorlabs RLA0600 rail.

    • Thorlabs RC4 carrier for the front mirror mount: The thumb-screw may be replaced with a short 1/4-20 setscrew since the RC4 should never need to be moved.

    • Front mirror mount: Thorlabs KMS or KMS/M kinematic mount modified by enlarging the holes to accept 7.75 mm mirrors and adding a setscrew to secure the mirror. This mounts via a spacer and washers/shims to the Thorlabs RC4 carrier.

    • Thorlabs RC1 carrier for back mirror mount.

    • Back mirror mount: Thorlabs KMS or KMS/M kinematic mount modified by enlarging the holes to pass the beam. The PZT will be attached using 5-minute Epoxy.

    • Thorlabs DT12 or DT12M, or Newport MT-X linear stage: This attaches to the mirror mount using a 3/8 inch 8-32 setscrew with plastic washer to provide compliance in tightening. One or both holes may need to be retapped for 8-32 threads.

      This assembly is attached to the RC1 using an 8-32 x ?? inch cap-head screw with #8 washer.

    Common to both versions:

    • Baseplate: Aluminum plate 3x6x3/8 inch drilled and tapped for the rail, leveling/alignment screws, and optional cable clamp.

    • Detector PCB: This is for both the single channel photodiode or dual polarization PBSC and photodiodes. This requires 1 or 2 photodiodes, 0 or 1 4 mm PBSC, 4 screw terminal block 1K ohm resistor, and 0.1 µF capacitor.

Dual polarization option:

For simulataneously monitoring the orthogonally polarized longitudinal modes of a random polarized HeNe or other similar laser, this adds an additional photodiode and a small polarizing beam-splitter cube (PBSC). The PBSC splits the beam to the two photodiodes. Both photodiodes can be installed on the detector PCB with one to the side for the S-polarized component and the other over the PBSC for the P-polarized component. The PD outputs feed load resistors or a preamp and are sent to separate channels of your scope. The laser will then need to be oriented so its polarization axes align with the SFPI head.

Initial Adjustments

The confocal cavity SFPI requires that the mirrors be spaced precisely at their RoC, around 42 mm in this case. So, the resonator must have some means of fine adjustment as noted above. Their axes and orientation should be coincident. Slight tilt with respect to each other isn't critical - it just shifts the center point of the spherical cavity. However, an offset may be more detrimental. Once the assembly is complete, it's time to do "first light" with a laser! A single longitudinal mode (single frequency) laser is best for this as it reduces any ambiguity in setting the cavity spacing, but a short normal HeNe (e.g., a JDSU 1508) red alignment laser can be used. A DIODE LASER WILL PROBABLY NOT WORK as most are not even close to single mode.

1. The mirror spacing should be set as close to 42 mm as can be done with physical measurements. (I.e., a machinist's scale and Mark II eyeballs.)

2. Connect the PZT to your ramp generator. If driven at a kHz or so at 10 or 20 V p-p, a tone should be audible from the PZT confirming that it's working.

3. Connect the photodiode to your scope's vertical input with a resistor of a few kohms across it. (If a proper photodiode preamp is available, that's even better!)

4. Trigger the scope externally using a sync signal from the ramp generator, or the ramp if none is available. Displaying both the photodiode signal and ramp will confirm that the scope is synced properly.

5. Set up the test laser so it is aimed precisely into the center of the input mirror. (The optional lens should probably not be used at this time as it may make things more confusing.)

6. Drive the PZT with a 10 to 20 V p-p ramp (or triangle) at 50 to 100 Hz.

7. Observe where the intra-cavity beam is located on each mirror and adjust alignment so it is more or less centered and tight. Then check the position of the photodiode and adjust it if necessary so the trasmitted beam is centered on it. The room lights should probably be out for all this.

8. With the scope's vertical sensitivity turned up, watch for any signal from the photodiode that is synchronized with the ramp. If the blips go negative, reverse the PD polarity. If your cavity distance and mirror alignment were perfect, the result scanning through two FSRs for a laser with 3 longitudinal modes would look similar to the photo below.

SFPI Display of Melles Griot 05-LHR-151 5 mW HeNe Laser

More likely, the peaks will be smeared out or composed of multiple small blips as in the sequence of graphics below. Or there may be nothing at all. Adjust the spacing of the mirrors in small increments Slowly and then then let it settle down. With any movement, the display will become quite scrambled, so be patient. If going one way makes it worse, go the other way. :) If the initial cavity spacing was within about 1 mm of being optimal, there should be only one place close by where it resolves into a beautiful display like the one above. ;-)

These simulated "screen shots" depict a display spanning 2 FSRs for an SFPI using 42 mm RoC mirrors with 99.5%R as the cavity length approaches optimum. From left-to-right, top-to-bottom, the error is approximately: 0.3 mm, 0.15 mm, 0.07 mm, 0.03 mm, 0.015 mm, 0 mm.

The entire sequence represents a cavity length change of <1 percent but will also depend on the finesse and the mode order (confocal, half confocal, etc.). The higher the finesse, the more critical it will be. In other words you mileage may vary. :) The amplitude of the peaks would actually increase by a much larger amount than shown. Which side the "crud" is on depends on the relationship of the ramp voltage to cavity length, swap if backwards. :) Some of these diagrams are originallly from the Toptica SFPI 100 manual, I hope they won't mind. :)

First time users do not appreciate how precise the spacing needs to be. But it's less than 1/10th the width of a human hair - a few microns. Once close, the only effective way of fine tuning it is with precision screws that change spacing such as would be found in a linear translation stage, a 3-screw pan/tilt mount (which can be easily constructed from scrap parts), or something equivalent.

It's also essential to avoid back-reflections into the laser, which will likely destabilize it and create chaos in the display. The alignment should be adjusted such the the reflections from the SFPI (mostly the front mirror) do NOT enter the laser's aperture. With the confocal cavity SFPI, a slight offset will not significantly affect resolution. Ideally, an optical isolator could be used but they are really pricey.

Using mirrors identical to the ones in the kit, I've seen a finesse at 633 nm of 500 or more, though this depends on all the stars aligning perfectly. :) And I can't guarantee that all samples are that good. But expect a finesse of several hundred with reasonable care. Performance at other wavelengths may not be as good but it should still be usable to below 594 nm (yellow HeNe) and above 650 nm (may actually be better at longer wavelengths).

While the assembly guidelines here assume the standard confocal cavity configuration, there are smaller and larger mirror spacings that are still mode-degenerate making alignment non-critical. Some of these may be useful by trading off size, FSR, and resolution.

For more on SFPIs, see the section: Scanning Fabry-Perot Interferometers of "Sam's Laser FAQ".