Study of the gain-dependent refractive index of a CO2 laser

Results of research directed toward the achievement of a better understanding of the gain-dependent index of refraction of an active CO2 laser medium are presented. Experimentally discovered frequency-jumping and gain-discontinuity effects in a 3-mirror laser have been studied using a newly-developed mathematical model in which it was assumed that there was present a gain-dependent portion of the refractive index of the laser medium. The analytical results are related to the experimental results obtained from a long, 3-mirror, frequency-modulated CO2 laser cavity. It was found that the simple model predicts, qualitatively, the experimental results. Specific results indicate that frequency pushing is the dominant mode interaction process in a reflective-type coupled-cavity laser, and that this process is more dependent upon the direction in which the laser frequency is swept than it is upon the location of the pertinent mode frequencies relative to the linecenter frequency. In a 3-mirror coupled-cavity structure, the effects of the gain-dependent portion of the refractive index of a CO2 laser is more than two order of magnitude larger than the effects associated with the anomalous dispersion. As little as 10-7 W feedback to the laser cavity caused frequency-jumping and gain-discontinuity effects in the CO2 ring laser whose gain was approximately 3 dB per pass. Similar results were obtained from an experimental arrangement which contained a straight, single-frequency CO2 laser. (U.S.)