Creation of high-fluence precursors by 351-nm laser exposure on SiO2 substrates

The onset of laser-induced damage on the exit surface of fused silica optics when exposed to UV ns pulses is a limiting factor in the design and operation of most high-energy laser systems. As such, significant effort has been expended in developing laser damage testing protocols and procedures to inform laser system design and operating limits. These tests typically rely on multiple laser exposures for statistical validation. For large aperture systems, testing an area equal to that of the optical components in the system is functionally impossible; instead, sub-scale witness samples are interrogated with elevated fluences. We show that, under certain circumstances, the laser exposure used to test one location on a sample will generate additional, laser-induced damage precursors in regions beyond that exposed to laser light and hence degrade the damage performance observed on subsequent exposures. We hypothesize the precursors result from condensation of vaporized silica, which re-condenses as SiOx particles deposited on the exit surface. In addition, we will outline the conditions under which this phenomenon occurs, as well as methods for mitigating or eliminating the effect.