Development of a reflective spiral phase plate based on MRF polishing

In this paper, we propose a fabrication process based on Magneto-Rheological Finishing (MRF) for a reflective Spiral Phase Plate (SPP) with a continuous surface. The front surface of a nickel-plated aluminum disk is machined by diamond turning as a plane mirror, and spiral structure with low topological charge is generated by sub-aperture polishing tools, i.e., MRF. Interferometers are used to analyze the smoothness of the spiral structure, steepness of the center step and surface roughness of the random areas for the fabricated SPP. The results indicate that the direct-polishing approach can be a promising technique to fabricate high-precision SPPs.