Laser pre-exposure to mitigate damage on microparticle-contaminated fused silica surface in high power laser systems

Abstract

Current fused silica surface processing, aimed at reducing known absorbing precursor concentration, has brought laboratory-tested ultraviolet laser-induced damage rates to nearly nil at fluences up to 10 J/cm2 . Yet this damage rate reduction has not been fully realized in large facility operation. A recently discovered source of damage in the facility is from particles ejected from damage of a neighboring optic under laser exposure and deposited onto the substrate surface. This state was observed to provide a means to couple energy from a subsequent laser pulse into the contaminated substrate and cause damage characterized by fracture. In this work, we explore the rate at which particles are removed from the surface and the rate at which particles lead to damage as a function of laser fluence and particle characteristics. This analysis allows for a derivation of an optimal pre-exposure fluence of a contaminated optic which maximizes particle removal probability while minimizing surface damage probability. For fluences up to 9.5 J/cm2 (351 nm, 5 ns square pulse), both particle removal and damage probabilities generally increased with particle size and laser fluence, with damage threshold around 6.5 J/cm2 . Two possible mechanisms that facilitate particle-induced damage on the substrate surface from laser-generated and deposited ejecta will be discussed, namely i) enhanced thermal contact from molten or partially molten ejecta and ii) fracture generated upon impact of solid ejecta with high kinetic energy.