Statistical study of UV-laser-induced failure of fused silica

A study was performed to rank the influence of the parameters that can determine the resistance to laser-induced damage of polished synthetic fused silica at 355 nm. These parameters include the type of fused silica, the polishing process, the cleaning method, the sample thickness, and the sample diameter. The study involved both measurements of the statistics of laser-induced damage thresholds (LIDT) and characterization of the damage morphologies. The samples were irradiated at 20 degrees incidence angle with a 6.5-ns-pulse at 355 nm on an automated damage testing (ADT) system. The LIDT was measured on a minimum of 100 sites for each sample to gather sufficient statistical information and the damage morphology was characterized by Nomarski optical microscopy and SEM. The statistical damage curves were very reproducible. The LIDT was most influenced by the polishing process. Surface micro-pits were found on the surface after irradiation with damaging fluences close to the LIDT. Their density varied form less than 0.1 X 10-3 to 26 X 10-3 micrometers -2 for the samples polished with the two different processes. The type of fused silica, the cleaning method, and the sample diameter seemed to have secondary effects on the LIDT. Bulk damage was occasionally noticed for thicker samples in areas with very high surface LIDT. Sample cleaning was found to play a role in reducing the number of low LIDT sites, thereby affecting the statistical behavior of the distribution tail, but did not significantly affect the average damage behavior. Finally, input surface (IS) damage occurred statistically less frequently and at higher fluences than output surface (OS) damage. Further work is discussed to understand if this effect is caused by electric field enhancement on the OS, or fundamental differences in crack propagation behavior between IS and OS.