Time-dependent oscillations in a cw chemical laser unstable resonator

Abstract

The amplitude, frequency, and Fresnel number dependence of the time-dependent oscillations which were predicted to occur on lines whose saturated gain does not fill the unstable resonator were measured. The time-dependent oscillations had a period of ~40 nsec independent of flow rates, do not occur for Fresnel numbers less than a demarcation Fresnel number which lies between 1.5 and 3.0, and their amplitudes increased as the fraction of the resonator filled by the saturated gain of the oscillating line decreased. The period of the time-dependent oscillations was determined by the resonator magnification. There was a strong cascade coupling between the oscillating 2 → 1 and 1 → 0 lines. The a priori prediction of these characteristics of the time-dependent oscillations by the MNORO3UR computer model was in agreement with the data. A 7-nsec oscillation, which was probably a mode beat of the laser, was superimposed on top of the 40-nsec oscillation. The time-dependent oscillations occurred in both 2-D and 3-D unstable resonators.