The optical distortion of a laser beam by a heated window in a typical long-focal-length system can significantly reduce the beam intensity at the target at power levels less than those required to melt or fracture the window. The spatially inhomogeneous incident laser intensity causes a temperature gradient which produces nonuniform changes in the thickness and the index of refraction of the window, causing it to become a lens having aberration and birefringence in addition to a finite focal length. Expressions are derived for the thermally induced optical distortion in terms of measurable parameters such as the index of refraction, the strain-optical coefficients, and the thermal expansion coefficient. Since not all the values of these parameters are known for most materials of interest, alternative expressions are derived for use in obtaining rough estimates of the amount of distortion. 60 pp. Ref. (KB)
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