Investigation on the influence of the CO2 laser parameters on the defect healing process of fused silica

During the grinding and polishing processes of hard-brittle fused silica optics, the defects would be inevitably formed on the finished surface. Fused silica has a high absorption coefficient for far-infrared lasers, which makes the CO2 laser processing to be the potential repairing technology for machining-induced defects on fused silica surfaces. In this work, using a low-power CO2 laser, a new repairing method to heal the machining-induced micro-defects on the surface of fused silica is proposed. Then, based on the nonlinear thermodynamic parameters of fused silica material, a thermal transfer model under CO2 laser irradiation and a dynamic defect healing model were established. On basis of that, the influence of CO2 laser parameters on the maximum surface temperature and the temperature distribution inside the silica material was investigated. It is found that, under the low-power and near-continuous CO2 laser irradiation, the maximum melting depth can be obtained under the non-evaporative condition. The defect healing process under various laser powers was explored as well. It is found that the defects would be more difficult to be healed under a laser with higher-power, smaller beam size or shorter pulse width. This work can provide theoretical guidance for the determination of the optimal parameters in the laser healing process and the optical manufacturing strategies of fused silica optics.