Defect mitigation engineering in multilayer dielectric gratings for enhanced nanosecond and picosecond laser-induced damage resistance

Defects and contaminants introduced during the various fabrication stages of multilayer dielectric grating (MLDG) significantly reduces its laser-induced damage resistance. In this study, the damage mechanisms induced by defects in MLDG under 8 ns and 7.6 ps laser irradiation were investigated. The primary risks leading to damage in the ns regime were identified as nodular defects and nanoabsorbing defects, whereas in the ps regime, damage was driven by contamination defects. Several strategies for optimizing the coating, etching, and cleaning processes in the MLDG fabrication were proposed to mitigate the adverse effects of defects. The results suggest that the advanced coating protocol substantially reduced nodular defects, leading to an approximate 23.8% enhancement in the nanosecond-laser-induced damage threshold (ns-LIDT) of MLDG and a more than twofold reduction in damage density. Moreover, the optimal protocol scheme, which simultaneously optimized the multi-stage manufacturing process of MLDG, significantly mitigated the contamination defects on the grating surface and achieved an approximate 64.7% improvement in the ps-LIDT of MLDG compared to that of the prior protocol. Furthermore, under the current LIDT limitations of multilayer dielectric films, the ps-LIDT of the optimal protocol gratings has approached the theoretical value of the ideal case.