LABORATORY FOR SCIENCE
In response to your request, we are sending you the enclosed descriptive literature on the principal members of the Model 200 Series of Ultra-Stable Lasers. As indicated by the price list, there are now additional members of this series, and we expect to mail to you information on these in the coming months. Further, as a result of our extensive research and experience in commercially realizing the very high frequency stability characteristic of the Model 200 Series, we have also developed a number of unique accessories to aid in the attainment of high frequency stability and absolute frequency calibration. Literature on these will also be mailed to you with our next mailing.
It is perhaps worth emphasizing some of the basic features common to all members of the Model 200 Series. They all look the same (cylindrical versions are not available), they have the same self-diagnostic features, the same case temperature regulator, and the same operational controls. Each member employs a triple position output shutter to provide either a single frequency polarized output beam, or provide direct access to the full output beam (as is required for the highest frequency stability). Furthermore the control system in each is such that no frequency dithering is required to stabilize any of this series of lasers.
The servo systems used in the Model 200 Series are of two types: the relatively simple linear analog system of the Models 200 and 210, and the more complex digital phase lock system employed in the Models 220, 230, 240, 260, and 280. Each system allows the corresponding laser to provide some especially useful features.
The Models 200 and 210 feature excellent frequency stability (esp. 210) along with exceptionally low amplitude noise, and a moderate sensitivity to retroreflection. These are features that make these models particularly useful in interferometry and in other applications where amplitude fluctuations caused by mode sweeping are unacceptable. The Model 210 can also be used to generate a known absolute reference frequency.
The phase locking architecture employed in the Model 220, 260, and 280 enable these lasers to provide heretofore commercially unequalled short and long term frequency stability with only a slight trade-off in the ideal characteristic of constant amplitude: they all exhibit some amplitude modulation at a fixed frequency in the range of 100 - 900 kHz. The Model 220 in particular is the most stable and the most versatile of this group. Its frequency can be set over a range of at least 250 MHz either by means of its own reference frequency switches, an external frequency source, or a ramp input signal. On the other hand, the output frequency of the Model 220 can be quite easily and repeatably set to line center despite the small changes in plasma tube parameters that take place with age. (Such settings are made particularly simple with the use of a Model 225 Beat Frequency Register.) The accuracy of such a frequency setting is typically better than I part in 108.
The foregoing and other features of the Model 220 make it particularly useful in high resolution spectroscopy for the active stabilization of Fabry-Perot etalons or other laser cavities, and as an absolute reference frequency standard.
In the enclosed descriptive literature, you will note that considerable emphasis has been placed on the elimination of retroreflection to achieve maximum frequency stability. This emphasis however should not be construed to indicate that the Model 200 Series have an especial sensitivity to retroreflection over and above other types of stabilized laser systems, but rather that the attainment of the high frequency stability obtainable with this series does require some added attention to the problem of retroreflection. With each member of the 200 Series, the headphones and servo-level light will make the user keenly aware of the existence of the problem, and when it has been eliminated.
The problem of retroreflection can be greatly reduced or virtually eliminated even in the most difficult cases, such as occur when the beam is directed by a microscope objective to a spatial filter (precision Fizeau wavemeter applications), or to an optical fiber with its many caprices. Perhaps the least costly device to try in such cases is a Faraday isolator. Such devices typically provide about 30 dB of isolation. At slightly greater cost however, a much higher degree of isolation can be obtained with the use of an acousto-optic modulator or Bragg cell as a frequency shifter. A retroreflected beam then suffers a frequency shift that is twice the acoustic frequency and it thereby falls outside the passband of the laser cavity. A retroreflected beam then has virtually no effect on the servo mechanism of the stabilized laser. We highly recommend the use of this technique along with a pair of our Model 211 Black EtalonsT.M.: one for the total absorption of the residual undeflected beam, and one for the total absorption of specular reflections from the surfaces of the A-O modulator and other components.
The enclosed literature covers just our basic designs: the Model 200, the Model 220, and the Model 260. Information on the other models can be supplied upon request, but briefly these models have the following general features. The Model 210 provides a beam with the very low amplitude noise of the Model 200 along with a significantly improved long term stability: a drift of less than 500 kHz/day. This model also features a 10 turn helipot for precision control of the output frequency and a mode switch to select operation for constant frequency or constant amplitude (accomplished at some expense to frequency stability). The Model 230 is a somewhat simplified version of the Model 220 in that it has no provision for minimizing the Zeeman frequency modulation. It does however provide the excellent frequency stability and tunability of the Model 220. The Model 230 is suitable as a reference laser for the stabilization of Fabry-Perot etalons and other applications where some Zeeman frequency modulation can be easily filtered out without becoming a problem. The Models 240 and 250 are slave lasers that include all the servo electronics to enable them to be phase locked with a frequency offset from a reference laser such as a Model 220. Such lasers then have a frequency stability equal to that of the reference laser, very low amplitude noise, and are relatively insensitive to retroreflection problems.
Superior performance through research and design.