High-energy laser windows: case of fused silica

摘要

Abstract. The engineering of high-energy lasers for applications suchnas the AirBorne Laser u0001ABLu0002 system requires optical windows capable ofnhandling megajoule pulse energies. The selection and/or evaluation of ansuitable window material involves considerations relating to thermal lens-ning, i.e., the beam distortion caused by thermally induced phase aberra-ntions, in addition to issues arising from the thermal stresses generatednby beam-induced temperature gradients. We document analytical meth-nods for evaluating the impact of beam-induced optical distortions andnbeam-induced mechanical stresses, which may enable the designer tonproperly assess the performance of window-material candidates. We il-nlustrate the procedure in the light of an evaluation of the performance ofnthe leading candidate for operation in the near-infrared, i.e., fused silicanu0001SiO2u0002. In terms of allowable peak intensities, and based on availablenmaterial properties, the limiting factor is seen to be related to shearnstresses generated by coating-induced axial compression that may leadnto plastic deformation. The allowable beam fluence is controlled by ther-nmally induced phase distortions rather than planar or axial stresses, thusnreflecting the unusually small u0001E/u0002 ratio of fused SiO2, where u0001E repre-nsents the thermal stress factor, and u0002 designates the optical distortionncoefficient. In conjunction with an evaluation of the required windownthickness as a function of the diameter, our analysis leads to the conclu-nsion that operating the ABL system at the projected power level u00012MWu0002nand pulse duration u00015su0002 requires bulk windows—if made of fusednSiO2—of at least 20 cm in diameter but no more than 7.5 mm innthickness. © 2010 Society of Photo-Optical Instrumentation Engineers.nu0003DOI: 10.1117/1.3484946