Laser Cavity Density Changes with Kinetics of Energy Release

Abstract

Inherent in any lasing process involving three or more energy levels is the release of heat. The heat is the inefficiency implied in the definition of laser quantum efficiency. In supersonic flow the heat release causes compression waves and a wake, thereby perturbing the gas density. Gas density variations within the laser cavity degrade the beam quality. A comprehensive analysis was made using an extension of the Tsien-Beilock linearized solution for heat addition. A computer program has been developed which incorporates multiple wall reflections, arbitrary beam cross section, and finite kinetics for the transition from the lower laser level to the ground state. Several examples are discussed. Uniform energy release in a rectangular cavity without reflections and without lag for heat addition, with lag and no reflections, and with both lag and reflections has been calculated. A rectangular cavity with a gaussian energy release distribution without lag and with reflections and with both lag and wall reflections has been analyzed. This case is representative of a master-oscillator-power-amplifier arrangement. Finally an energy release distribution in the shape of a crescent was calculated. The crescent was located with convex side upstream and with the peak upstream of the cavity axis. Fractional density perturbation greater than 1% was found for typical conditions in a gas dynamic laser. Larger perturbations would be expected in lasers with a larger output.