Combined processing chain for freeform optics based on atmospheric pressure plasma processing and bonnet polishing

Development of major optical projects demands great amounts of freeform optics, which requires mass-production technology. The atmospheric pressure plasma processing (APPP) based on chemical etching shows the advantage of high efficiency in form generation. However, the surface texture gets deteriorated with roughness to 60 nm Ra. Bonnet polishing can achieve ultra-smooth surface texture using flexible inflated tool. In this study, a combined processing chain consisting of APPP and bonnet polishing was proposed, in which APPP plays a role in form generation and bonnet polishing plays a role in rapid surface smoothing. Firstly, the form generation using APPP and the surface finishing using bonnet polishing were analyzed. Then the form-preserving ability of bonnet polishing under various conditions, including polishing parameters and surface form characteristics, was investigated experimentally. Consequently, the influence of bonnet polishing on surface form change was analyzed. Two sinusoidal freeform surfaces with 5 mm and 16 mm spatial period were generated using APPP and then smoothed using bonnet polishing. The experimental results show that the polishing induced form change is smaller than 20 nm PV with roughness improved to 2 nm Ra, which verifies the feasibility of the combined processing chain.