Ultra-precision optical surface fabricated by hydrodynamic effect polishing combined with magnetorheological finishing

In the traditional machining method, the surface quality will get worse during the surface figuring process while the surface figure will be destroyed after ultra-smooth polishing process. To obtain an ultra-precision optical surface, these two processes are conducted iteratively which is time-consuming and unstable. A new fabrication method combined with the magnetorheological finishing (MRF) and hydrodynamic effect polishing (HEP) technology was presented. The surface figure error can be quickly depressed by MRF due to its high deterministic figuring ability, while the surface quality will get worse due to the introduction of MRF polishing marks. The material removal of HEP depends on the chemical interaction between the nanoparticles and workpiece in elastic mode, and an ultrasmooth surface with sub-nanometer level can been easily achieved with the surface figure well maintained. The experiment results show that the MRF polishing marks are removed clearly and the surface figure is well maintained after HEP process. Combined with MRF, a quartz glass with initial figure accuracy of 0.415 λ RMS (λ = 632.8 nm), mid-spatial frequency and high spatial frequency roughness of 0.407 nm RMS and 0.525 nm RMS has been improved to figure accuracy of 0.05 λ RMS, mid-spatial frequency and high spatial frequency roughness of 0.268 nm RMS and 0.163 nm RMS. Power spectral density (PSD) analysis indicated that the surface spatial frequency error has all been greatly depressed. It demonstrates HEP is an effective ultra-smooth polishing method with good ability to maintain the initial surface figure.