The selection of a suitable laser-window material involves considerations relating to "thermal lensing," that is, the process of beam distrotion caused by thermally induced phase-aberrations, in addition to issues relating to the stress field generated by beam-induced temperature gradients. The purpose of this paper is to obtain improved figures of merit (FoM) for ranking high-energy laser-window materials in regard to thermal lensing and thermal stresses. We address this task in the following manner: (a) We provide proper analytical expressions for describing how beam-induced optical distortions and beam-induced hoop stresses control the allowable beam fluence; (b) We re-evaluate the role of axial stresses, which may lead to failure through compressive yielding or thermal shock, and derive appropriate FoMs based on allowable irradiances; (c) We illustrate the procedure through FoM evaluations of six window-material candidates for operation at the chemical oxygen-iodine laser wavelength (1.315 µm). This methodology confirms that low-absorption, impurity-free fused silica is the window material of choice for contemplated high-energy laser systems operating in the near-infrared.
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