Numerical simulation of temperature and morphology evolution of fused silica optics under beam shaped CO₂ laser irradiation

CO₂ laser polishing process can significantly improve the surface quality and the laser-induced damage threshold (LIDT) of fused silica optics, which is the most promising ultra-precision processing technology. However, the non-uniformity thermal interaction between laser and material resulted in the high mid-spatial frequency (MSF) error of the polished optic, which seriously reduced the surface quality and even caused light field modulation under high power laser irradiation. In this work, a laser beam shaping method was used to reduce the MSF error of the fused silica optics. The influence of laser beam shaping on thermodynamic temperature and morphology evolution was explored through the numerical simulation method. A multi-physical coupling model including temperature, fluid flow and phase transition was established. The shape types including Gauss and top-hat of the laser beam were given. The temperature and morphology evolution under different beams was simulated and compared through the model established. The difference in the morphology of the molten pool was compared. The surface roughness of fused silica optic irradiated by the top-hat laser was lower than that irradiated by the Gauss laser. Besides that, due to the relatively uniform energy distribution of the top-hat laser, the temperature was higher than the Gauss laser before normalization. The morphology evolution rate decreased under the irradiation of shaped laser, which mean that the MSF error became more controllable. This work can provide theoretical guidance for the control on MSF error of fused silica optics polished by CO₂ lasers.