Investigation of Control of Residual Stress Induced by CO₂Laser-Based Damage Mitigation of Fused Silica Optics

Abstract

A CO2laser-based annealing technique for the mitigation of damaged sites of fused silica is studied to suppress the residual stress left on the surface. The laser annealing by a linear decrease of the CO2laser power effectively reduces the residual stress. The residual stress of mitigated sites is characterized by polarimetry, the reduction of the maximum retardance around the mitigated sites with the exposure time of laser annealing fits a stretched exponential equation, and the maximum retardance with optimal laser annealing is reduced (36 ± 3)% compared to that without laser annealing. The residual stress regions are destructively characterized by introducing damage. The critical size of damage leading to fracture propagation for the mitigated sites without laser annealing is in the range of 120~230 μm, and the corresponding critical size of damage for the mitigated sites with laser annealing is larger than 600 μm. According to the relationship between maximum damage size and critical stress, the residual stress without laser annealing is in the range of 28–39 MPa and the residual stress with laser annealing is less than 17 MPa. These results indicate that the CO2laser-based annealing technique has a positive effect on the control of residual stress induced by CO2laser-based damage mitigation.

Residual stress modeling of mitigated fused silica damage sites with CO2 laser annealing

“…Thus, the mitigated damage site for the CO 2 laser-annealed case is frozen-in at a lower fictive temperature than for the quenched case, and therefore the residual stress is effectively minimized. [11][12][13] The axial fictive temperature distributions of mitigated damage sites for quenched and CO 2 laser-annealed cases have both been widely characterized. [12,18] According to the reported data for mitigated damage sites quench-cooled at a peak temperature of 2300 K and mitigated damage sites laser-annealed by linear ramp from a maximum of 2300 K to ambient 300 K, [12] depth profiles of fictive temperatures on the axis of mitigated damage sites can be fitted as logistic curves…”

“…[10] CO 2 laser annealing by a linear ramp-down of laser power after damage mitigation has been demonstrated and applied to avoid rapid cooling down, which minimizes the residual stress to an acceptable level, avoiding critical fracture. [11][12][13] Accurate description of residual stress of mitigated fused silica damage sites for quenched and laser-annealed cases is of major interest for optimization of the damage mitigation process. The nondestructive photoelastic method is generally used to characterize stress in transparent materials.…”

Simulations of CO2laser interaction with silica and comparison to experiments

“…Indeed, the residual stresses appear in silica during cooling due to structural relaxation of the glass material after a temperature drop, consequent of the laser heating. These residual stresses are detrimental for the silica components because they can modify the optical properties of the glass (birefringence) and can have an influence of the laser damage resistance [9][10][11] . A method, based on annealing with a second laser heating, allows to reduce significantly the detrimental effect of residual stresses and therefore extends the lifetime of the silica components 12 .…”