Frequency Stabilization by Saturated Absorption

The frequency of a single-mode laser inside the spectral gain profile of its active medium is mainly determined by the eigenfrequency of the active laser cavity mode. Therefore any instability of resonator parameters, such as variation of cavity length, mirror vibrations or thermal drifts of the refractive index will show up as frequency fluctuations and drifts of the laser line. 

The idea of stabilizing the laser frequency 37-39) and making it as far as possible independent of cavity parameters, has been realized by many authors in different ways (see for instance the review article by Basov on optical frequency standards 339). 

One method which employs the saturable absorption of intracavity gaseous absorbers has turned out to be strikingly successful333) As explained in the last section, the absorption profile of a gas interacting with a monochromatic standing wave inside the laser cavity exhibits a sharp minimum at the center of the unsaturated ab- 

Meanwhile hfs splittings and quadiupole moments of many atoms and molecules have been measured with lamp-dip spectroscopy, using fixed frequency lasers 338b—c) j. tunable dye lasers 338) 

The idea of using the same medium as absorber and active material has been proposed and realized by several authors 340-343) Leg and Skolnick 40) used a neon gas discharge at low current and low pressure as saturable absorber inside the cavity of a He-Ne laser oscillating at X = 6328 A. The Lamb-dip halfwidth obtained was 30 Mc/sec compared to 1500 Mc/sec for the doppler line. The disadvantage of this arrangement is that the frequency of the neon transitions depends upon pressure and current 341) in the absorption cell, and this limits the stability and reproducibility of the Lamb dip center frequency. 

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