Formation mechanism of surface morphology in the process of CO₂ pulsed laser processing of fused silica optics

In order to comprehensively reveal the surface morphology formation mechanism during CO₂ pulsed laser processing of fused silica optics, the material temperature distribution, the surface morphology evolution law, as well as the relationship between laser parameters and surface quality were analyzed through three-dimensional numerical simulation and experimental research. Results showed that the residual temperature between laser pulses was lower than material's structural transition temperature. The calculated morphologies along laser moving direction and width direction were consistent with the experimental during laser linear scanning. Besides, the superposition of spot energy during multi-pass laser scanning made the processed surface present specific three-dimensional morphology features. Laser processing lines were distributed perpendicular to the scanning direction, and the interval was equal to the track pitch. It was concluded that material's removal depth was proportional to pulse width, and inversely proportional to the track pitch and spot moving speed. The periodic ripple structures on the surface processed by 10 μs pulsed laser were formed by the superposition of materials’ residual height during laser scanning. The conical repaired site with diameter of 2 mm and angle of 12° obtained by CO₂ laser processing could completely eliminate surface damage, and laser-induced periodic surface structures were formed on the repaired surface. These studies could provide guidance for the optimization of CO₂ laser processing technology and the improvement of processed surface quality.