Sparse bi-step raster path for suppressing the mid-spatial-frequency error by fluid jet polishing.

The periodic ripple errors (mid-spatial-frequency (MSF) error) produced by computer-controlled sub-aperture polishing severely limit the improvement of high-performance optical systems. At the same time, the fluid jet polishing (FJP) method is non-destructive and non-contact, but it is still hard to widely use it due to the defect of small spot-size and low efficiency. In this paper, we found that FJP has a significant advantage in removing the residual periodic ripples in sub-aperture polishing. The mathematical model developed by complex spectrum optimization verifies the existence of the sparse "bi-step raster path" (BSRP), which can achieve efficient periodic ripple error removal by suppressing the first two-order peaks of the error spectrum. In the experiments, it was observed that the MSF error has been significantly reduced after BSRP processing while the surface form and surface roughness have not been deteriorated, which demonstrates the validity of the proposed method. The proposal of the BSRP provides a new approach for the application of FJP and the suppression of the MSF error.