Figures of Merit for High-Energy Laser-Window Materials: Thermal Lensing and Thermal Stresses

摘要

The selection of a suitable laser-window material involves considerations relating to "thermal lensing," that is, the process of beam distortion 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 hi 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 reevaluate 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 fiva). This methodology confirms that low-aborption, impurity-free fused silica is the window material of choice for contemplated high-energy laser systems operating in the near-infrared.