Residual stress and damage-induced critical fracture on CO 2 laser treated fused silica

Abstract

Localized damage repair and polishing of silica-based optics using mid- and far-IR CO 2 lasers has been shown to be an effective method for increasing optical damage threshold in the UV. However, it is known that CO 2 laser heating of silicate surfaces can lead to a level of residual stress capable of causing critical fracture either during or after laser treatment. Sufficient control of the surface temperature as a function of time and position is therefore required to limit this residual stress to an acceptable level to avoid critical fracture. In this work we present the results of 351 nm, 3ns Gaussian damage growth experiments within regions of varying residual stress caused by prior CO 2 laser exposures. Thermally stressed regions were non-destructively characterized using polarimetry and confocal Raman microscopy to measure the stress induced birefringence and fictive temperature respectively. For 1~40s square pulse CO 2 laser exposures created over 0.5-1.25kW/cm 2 with a 1-3mm 1/e 2 diameter beam (T max ~1500-3000K), the critical damage site size leading to fracture increases weakly with peak temperature, but shows a stronger dependence on cooling rate, as predicted by finite element hydrodynamics simulations. Confocal micro-Raman was used to probe structural changes to the glass over different thermal histories and indicated a maximum fictive temperature of 1900K for T max ≥2000K. The effect of cooling rate on fictive temperature caused by CO 2 laser heating are consistent with finite element calculations based on a Tool-Narayanaswamy relaxation model.