Residual stress modeling of mitigated fused silica damage sites with CO_(2)laser annealing

A numerical model based on measured fictive temperature distributions is explored to evaluate the residual stress fields of CO_(2)laser-annealed mitigated fused silica damage sites.The proposed model extracts the residual strain from the differences in thermoelastic contraction of fused silica with different fictive temperatures from the initial frozen-in temperatures to ambient temperature.The residual stress fields of mitigated damage sites for the CO_(2)laser-annealed case are obtained by a finite element analysis of equilibrium equations and constitutive equations.The simulated results indicate that the proposed model can accurately evaluate the residual stress fields of laser-annealed mitigated damage sites with a complex thermal history.The calculated maximum hoop stress is in good agreement with the reported experimental result.The estimated optical retardance profiles from the calculated radial and hoop stress fields are consistent with the photoelastic measurements.These results provide sufficient evidence to demonstrate the suitability of the proposed model for describing the residual stresses of mitigated fused silica damage sites after CO_(2)laser annealing.

Incident laser modulation of a repaired damage site with a rim in fused silica rear subsurface

Local CO2 laser treatment has proved to be an effective method to prevent the 351-nm laser-induced damage sites in a fused silica surface from exponentially growing, which is responsible for limiting the lifetime of optics in high fluence laser systems. However, the CO2 laser induced ablation crater is often surrounded by a raised rim at the edge, which can also result in the intensification of transmitted ultraviolet light that may damage the downstream optics. In this work, the three-dimensional finite-difference time-domain method is developed to simulate the distribution of electrical field intensity in the vicinity of the CO2 laser mitigated damage site located in the exit subsurface of fused silica. The simulated results show that the repaired damage sites with raised rims cause more notable modulation to the incident laser than those without rims.Speeifically,we present a theoretical model of using dimpled patterning to control the rim structure around the edge of repaired damage sites to avoid damage to downstream optics. The calculated results accord well with previous experimental results and the underlying physical mechanism is analysed in detail

MITIGATING THE DAMAGE OF BROWN BEARS IN MARYO TOWNSHIP

The need for the mitigation of brown bears has become so severe that many nomads have given up repairing their houses in Maryo Township’s Mendang Village(also named the First Administrative Village),a remote place located in the southwestern section of the Changtang National Nature Reserve(CNNR).Only part of the village belongs to Serling Tso Nature Reserve.

ATR-FTIR spectroscopic studies on density changes of fused silica induced by localized CO_2 laser treatment

The surface density changes of the central region of the sites treated by using the CO_2 laser-based non-evaporative damage mitigation for fused silica are investigated by attenuated total reflectance-Fourier transform infrared spectroscopy（ATR-FTIR）.The ATR-FTIR peak shifts of the treated sites of fused silica are monitored to determine the changes of the corresponding density.For the quenching treated sites,the surface density is increased by（0.24±0.01）%compared with the initial density but the laser annealing by the exposure of a power ramp down after damage mitigation effectively suppresses the structural changes of treated sites,which could reduce the increase of the corresponding density to（0.08±0.01）%.The results provide sufficient evidence that the laser annealing by a power ramp down after damage mitigation has a positive effect on the control of the structural change induced by CO_2 laser-based damage mitigation.