Stress-Induced Birefringence, Critical Window Orientation, and Thermal Lensing Experiments

Abstract

A proper assessment of thermal lensing phenomena in high-average-power laser windows involves an evaluation of two optical distortion coefficients: (a) the coefficient X+, which combines the effects of temperature-induced change in refractive index, surface bulging through constrained expansion, and photoelasticity averaged over the principal stress directions, and (b) the coefficient X−, which exists only if there is stress birefringence. This evaluation can be performed for 111 — oriented and for randomly-oriented aggregates of cubic single crystals. It is emphasized that the calculations require correct inputs in terms of elastic and photo-elastic coefficients. Among presently contemplated key laser-window material candidates, only CaF2 can exhibit significant stress-birefringence effects; this is not so in the 111 orientation, which reflects a critical situation in the sense of Joiner, Marburger, and Steier. Windows made of KCl exhibit isotropic distortion patterns because the magnitude of the thermo-optic coefficient is such that the “small-birefringence condition,” X−/X+ << 1, is satisfied. The results of interferometric testing substantiate these considerations.