Effect of residual stress on laser-induced damage characterization of mitigated damage sites in fused silica

• Crack will occur at the inner or the maximum stress location of mitigated site.
• The fluctuation of LIDTs results from the discrepancy of residual stress.
• Annealing treatment can control the fluctuation of LIDT of mitigated site.
• The elimination effect of stress is influenced by the size of CO2 laser beam.

The influences of residual stress on damage morphology and laser-induced damage threshold (LIDT) of mitigated sites in fused silica optical components before and after annealing are investigated. The results indicate that the cracks occur either at the inner location or at the maximum retardation location of mitigated sites once the residual stress (retardation) exceeds 26.49 ± 1.79 MPa (22.25 ± 1.5 nm). Meanwhile, the discrepancy of stress at different locations results in the fluctuation of LIDTs. The results of isochronal and isothermal annealing indicate that annealing temperature is the most important influence factor and the residual stress can be effectively controlled if the residual stress induced by the maximum CO2 laser beam is eliminated. Furthermore, the LIDT of mitigated sites can be enhanced without dramatical fluctuation and the damage growth coefficient that is sharply increasing can be avoided if the retardation of mitigated site is equal to or less than 5 nm. The study demonstrates that annealing significantly improves the resistance of fused silica optics.