Laser-induced damage resistance enhancement of fused silica optics by rapid laser micromachining

Abstract

The issue of laser-induced damage on critical components emerges as a bottleneck that limits the high-power or high-energy laser systems, especially for the fused silica optics used in ultraviolet light. Sub-surface defects such as microcracks and impurities on fused silica optics have been discovered as damage precursors and determine the laser-induced damage threshold (LIDT) of the optics. Under the state-of-the-art advanced mitigation processes (AMP) and laser conditioning, only a few destructive damage sites that grow rapidly with successive shots still exist on a large-aperture fused silica optic. Therefore, we propose a method of selectively eliminating the destructive damage sites on fused silica optics by laser micromachining and consequently lead to a significant enhancement of LIDT in this paper. The removal of a damage site is implemented by precisely shaping the destructive damage site into an optically benign cone of special design using a femtosecond laser, with a subsequent CO₂-laser-polishing process to reduce the roughness. Compared with previous methods, the thermal effect on the processed region is dramatically reduced because of the nonthermal ablation by a femtosecond laser. Through optimizing the parameters of laser micromachining, a typical damage site is eliminated and replaced with a designed cone of excellent quality. The manufactured cone typically has a smooth wall with a slope angle of 12°, a diameter of 800 μm, and a negligible raised rim with a height of 14.5 nm (∼ λ/25 @ 355 nm). By employing the raster scan LIDT test procedure, several fused silica optics processed by laser micromachining are investigated and a laser-induced damage threshold (@ 355 nm, 1.6 ns) higher than 14 J/cm² and 10 J/cm² on the input surface and output surface are discovered, respectively. Furthermore, the downstream light intensification is proven to be trivial in the absence of a detrimental high-intensity central spot, owing to the ultra-low raised rim. These results demonstrate that rapid laser micromachining is an effective way to improve laser-induced damage resistance of fused silica optics and eventually enhance the performance of high-power or high-energy laser systems.