The influence of laser beam shaping on surface roughness, surface figure and mid-spatial frequency of fused silica glass in CO2 laser smoothing

CO2 laser with Gaussian and top-hat laser intensity distribution, were utilized to process fused silica with 50 mm diameter in raster scanning path to explore the evolution of surface roughness, waviness and surface figure. The results show that surface roughness can be decreased to < 10 nm with both Gaussian and top-hat CO2 laser smoothing; Top-hat laser will cause higher PSD2 value compared with Gaussian, but Gaussian has led to added mid-spatial frequency at 0.4395 mm(-1) (2.28 mm) with 75 % overlap rate (raster scanning distance is 2.327 mm); The surface profile of fused silica has evolved from the initial convex to concave profile after being irradiated by both CO2 lasers. From experiments, improper overlap rate will cause greater distortion, and higher power density can result in serious damage to the PV value of surface figure.