Silica laser damage mechanisms, precursors and their mitigation

Abstract

Controlling laser damage is essential for reliable and cost-effective operation of high energy laser systems. We will review important optical damage precursors in silica up to UV fluences as high as 45J/cm² (3ns) along with studies of the damage mechanisms involved and processes to mitigate damage precursors. We have found that silica surface damage is initiated by nano-scale precursor absorption followed by thermal coupling to the silica lattice and formation of a laser-supported absorption front. Residual polishing compound and defect layers on fracture surfaces are primarily responsible for optic damage below about 10J/cm²; they can be mitigated by an optimized oxide etch processes. At fluences above about 10J/cm², precipitates of trace impurities are responsible for damage; they can be mitigated by eliminating the chances of impurity precipitation following wet chemical processing. Using these approaches, silica damage densities can be reduced by many orders of magnitude allowing large increases in the maximum operating fluences these optics see.