Characterizations of UV-laser damage on fused silica surfaces

摘要

Significant improvement in polishing processes of fused silica optical components, has increased optics lifetime at the wavelength of 351 nm. Nonetheless, for large laser operation facilities like the Laser MegaJoule (LMJ), zero defect optics are not yet available. Therefore a damage mitigation technique has been developed to prevent the growth of initiated damage sites: this technique consists in a local melting and evaporation of silica by CO2 laser irradiation on the damage site. Because of the difficulty to produce efficient mitigated sites with large depth, the initial depth of damage to mitigate is a critical issue. An aim of our work was to determine the real extension of the damage site (including fractures) for different laser pulse durations between 3 ns and 16 ns and at different laser fluences. The fractures are non-detectable in conventional microscopy. The depth of the damage can thus be underestimated. Hence confocal microscopy, was used to observe these sub-surface fractures and to measure precisely the depth of damage. Results show that the damage is 2 to 4 times wider than deeper and this ratio is independent of the pulse duration and of the fluence. With this new information, the mitigation process can now be optimized.