Microsecond-pulsed CO2 laser cleaning of high damage threshold fused silica

Abstract

Ultraviolet (UV) laser-induced damage from defects and impurities in conventional ground and polished fused silica optics significantly hinders their application in high-power laser systems. Laser cleaning, as a non-contact and polishing fluid-free technology, is likely to be a key technology for improving the damage performance of fused silica optics. However, conventional laser cleaning primarily focuses on surface pollution without removing the material. It overlooks the detrimental effects of multiscale defects and impurities inside the fused silica. In this study, microsecond-pulsed CO2 laser cleaning was proposed to enhance the laser-induced damage threshold (LIDT) by ablating fused silica to nanometer depths. Through multiple-scale simulations, the thermal stress evolution and modulation mechanism of defects and impurities on the damage performance (ranging from the macro- to nanoscale) were revealed. Utilizing multimodal characterization, laser cleaning effectively eliminated and suppressed surface/subsurface defects, absorptive defects, chemical structural defects, and elemental impurities without introducing residual thermal stress and destroying surface roughness. In addition, the evolution of the molecular structures indicated a tendency for intrinsic defects to recombine into shorter-ring structures, which resulted in a densified fused silica surface. The laser-cleaned samples exhibited higher damage thresholds, achieving 47.6 % (0 % probability) and 27.0 % (100 % probability) improvements compared with uncleaned samples. This technology provides a new method for fabricating high damage threshold fused silica optics with superior surface quality for high-power laser applications.