Densification and residual stress induced by CO 2 laser-based mitigation of SiO 2 surfaces

Abstract

Knowing the ultimate surface morphology resulting from CO 2 laser mitigation of induced laser damage is important both for determining adequate treatment protocols, and for preventing deleterious intensification upon subsequent illumination of downstream optics. Physical effects such as evaporation, viscous flow and densification can strongly affect the final morphology of the treated site. Evaporation is a strong function of temperature and will play a leading role in determining pit shapes when the evaporation rate is large, both because of material loss and redeposition. Viscous motion of the hot molten material during heating and cooling can redistribute material due to surface tension gradients (Marangoni effect) and vapor recoil pressure effects. Less well known, perhaps, is that silica can densify as a result of structural relaxation, to a degree depending on the local thermal history. The specific volume shrinkage due to structural relaxation can be mistaken for material loss due to evaporation. Unlike evaporation, however, local density change can be reversed by post annealing. All of these effects must be taken into account to adequately describe the final morphology and optical properties of single and multiple-pass mitigation protocols. We have investigated, experimentally and theoretically, the significance of such densification on residual stress and under what circumstances it can compete with evaporation in determining the ultimate post treatment surface shape. In general, understanding final surface configurations requires taking all these factors including local structural relaxation densification, and therefore the thermal history, into account. We find that surface depressions due to densification can dominate surface morphology in the non-evaporative regime when peak temperatures are below 2100K.

Laser polishing and laser shape correction of optical glass

“…The resulting change of the density leads to a densification of glass material and thus to a quasiablation. 29 This effect occurs already before reaching the evaporation temperature on the surface. The second mechanism is the evaporation of material.…”

Evaluation of the fused silica thermal conductivity by comparing infrared thermometry measurements with two-dimensional simulations

“…11 Since our simulation does not take matter ejection into account, its results are not expected to be valid for T > 2100 K. Analogous fused silica behavior has been recently reported at similar temperatures. 15 Since several different thermal conductivities have been published, we show a comparison with our resulting thermal conductivity in Fig. 3(a).…”

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

“…If the cooling rate of the materials in the irradiated region after damage mitigation is effectively slowed, the structures of these materials that they reached high temperatures may gradually change to the states corresponding to low thermodynamic temperatures during the decrease of the temperature. Feit et al reported [16] a linear ramp down of the CO 2 laser power following the damage mitigation can effectively suppress the plastic deformation induced by the quenching, and the more gradual the cooling, the smaller the plastic deformation. A linear decrease of CO 2 laser power from 19.7 watt to 8.7 watt with different exposure times was performed following the damage mitigation to minimize the residual stress.…”

“…The melt zone of mitigated site can not completely relax the strained material over the time that is shorter compared to structural relaxation time, and a structure corresponding to high temperature is frozen at higher density, so the melt zone is plastically deformed. After collective cooling of the fused silica sample, the thermal expansion of the irradiated region vanishes and the plastic deformation of the melt zone with higher density remains, and the central melt zone undergoes tensile stress [14][15][16].…”