Induced thermal distortion effects in active unstable optical resonators with nonuniform gain

Abstract

Thermal distortion effects of mirrors on wave front phase and electric field are evaluated numerically, in a positive branch unstable active resonator. A main source of distortion in high power lasers mirrors is the thermal effects. Thermal distortion is due to absorption of a portion of laser beam energy and consequence heating of the mirror material as well as thermal induced stress within the film by cooling the mirrors. Such distortion can be calculated according to thermoelasticity theory. Using appropriate boundary conditions, diffusion equation, together with thermoelasticity relations was used to estimate such thermal distortions. Based on the Fresnel-Kirchhoff integral, we have calculated the two-dimension phase and intensity in positive branch unstable resonator with rectangular geometry, but the method is applicable for any type of resonator. A finite non-uniform gain was assumed across the resonator. The spatial phase nad intensity in near and far field for active resonator with such finite gain were assessed. Due to absorption of a fraction of laser energy within the mirrors, thermal distortion will occur that in turn affects both phase and intensity. The results show the induced thermal distortion can affect significatly the phase. An asymmetrical wave fron tin near field has also been obtained due to non-uniform gain.