Uniaxial/Biaxial Stress Paradox in Optical-Materials Hardening

174 LLE Review, Volume 79 Owing to its excellent homogeneity and low-intrinsic absorption properties, fused silica remains the preferred material for high-power laser applications over a wide wavelength range, but especially in the UV. In particular, large-aperture glass and excimer lasers, such as Nova, Beamlet, OMEGA, NIKE, and, in the foreseeable future, the National Ignition Facility (NIF), the Megajoule laser (LMJ, France), and others, owe their existence to readily available, large-diameter fused silica finding use in the form of beam-transport lenses and windows. Often these lenses and windows separate atmospheric pressure from vacuum areas, such as on spatial filters and target tanks, experiencing not only high-fluence irradiation conditions but also pressure-differential–induced stresses. The combination of the two presents an interesting challenge in terms of laser damage, as the formation of pits and cracks during conventional damage may get aggravated by the presence of stress and lead to dramatic device failure by fragmentation and acceleration of the lens or window shards into the evacuated space.