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

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    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.


    Elastic coefficients, high-energy laser, interferometric testing, optical distortion, photoelastic coefficients, polycrystalline aggregate, stress birefringence, thermal lensing, window material

    Author Information:

    Klein, CA
    Research Division, Raytheon Company, Waltham, Massachusetts

    Committee/Subcommittee: F01.02

    DOI: 10.1520/STP37003S