Investigation on control of middle and high-spatial frequency errors of fused silica optics melt polished by CO₂ lasers

Surface and subsurface defects introduced in traditional mechanical polishing seriously influence the laser damage threshold of fused silica. As a non-contact processing technology, CO₂ laser melt polishing could obtain smooth surface by healing surface defects and cracks. However, due to the non-uniformity of the thermal interaction, the mid- and high-spatial frequency errors of the polished optic are pretty high, which seriously reduces the surface quality and even causes light field modulation. In this work, the formation mechanism of the mid- and high-spatial frequency errors of fused silica polished by lasers was explored through temperature and fluid fields multi-physics coupling simulation. Then, the factors influencing the mid-and high-spatial frequency errors were explored. The results showed that the errors of the optic were greatly influenced by the laser scanning speed and track pitch. Then, an innovative polishing strategy was proposed by combining multi-layer high-speed polishing with single-layer low-speed polishing. The mid- and high-spatial frequency error was reduced from 47 nm to 17.7 nm and the high-spatial frequency error was reduced from 8.4 nm to 1.5 nm compared with the previous results. This work revealed the formation mechanism of the mid- and high-spatial frequency errors through multi-physics coupling simulation and effectively controlled the errors by using an innovative polishing strategy. It can offer both theoretical and experimental advice for the ultra-precision machining technique employed on fused silica.