Optical EngineeringHigh-energy laser windows: case of fused silica
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The engineering of high-energy lasers for applications such as the AirBorne Laser (ABL) system requires optical windows capable of handling megajoule pulse energies. The selection and/or evaluation of a suitable window material involves considerations relating to thermal lensing, i.e., the beam distortion caused by thermally induced phase aberrations, in addition to issues arising from the thermal stresses generated by beam-induced temperature gradients. We document analytical methods for evaluating the impact of beam-induced optical distortions and beam-induced mechanical stresses, which may enable the designer to properly assess the performance of window-material candidates. We illustrate the procedure in the light of an evaluation of the performance of the leading candidate for operation in the near-infrared, i.e., fused silica (SiO2). In terms of allowable peak intensities, and based on available material properties, the limiting factor is seen to be related to shear stresses generated by coating-induced axial compression that may lead to plastic deformation. The allowable beam fluence is controlled by thermally induced phase distortions rather than planar or axial stresses, thus reflecting the unusually small αE/∫χ ratio of fused SiO2, where αE represents the thermal stress factor, and χ designates the optical distortion coefficient. In conjunction with an evaluation of the required window thickness as a function of the diameter, our analysis leads to the conclusion that operating the ABL system at the projected power level (2 MW) and pulse duration (5 s) requires bulk windows-if made of fused SiO2-of at least 20 cm in diameter but no more than 7.5 mm in thickness.