Frequency division combined machining method to improve polishing efficiency of continuous phase plate by bonnet polishing

Abstract

Continuous phase plate (CPP), as a key diffractive optical element, is difficult to manufacture owing to its random and small features. In this paper, a novel frequency division combined machining (FDCM) method was proposed to improve polishing efficiency of CPP by optimizing the tool influence functions (TIFs) over targeted frequency bands. In addition, the convergence rate of power spectral density (CR-PSD) was proposed to evaluate the correct ability of TIF in different frequency bands, and to determine the division frequency for the combined processing. Through simulation verification, the combined processing with optimized TIFs by FDCM enabled high precision in less total time than that with single TIF processing. The experimental results verified that the method could imprint a 300 × 300 mm CPP with residual root-mean-square 24.7 nm after approximately 6-h bonnet polishing. Comparing the focal spots of designed and fabricated CPPs, the deviation of their energy concentration within 500 microns is only 0.22%. Hence, bonnet polishing using the FDCM is a new technical option for the production of large-aperture CPPs. Furthermore, the FDCM method shows a significant increase in efficiency, and it could be a generic method for CPP processing through other technologies, including magnetorheological and ion beam finishing.