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.
Single Point Diamond Turning (SPDT) has the potential to cost-effectively manufacture optical materials such as metals and plastic types. However, SPDT generally leaves tool marks on the machined surfaces, which creates problems that can deteriorate the optical performance. Several processes have been studied to eliminate the tool marks caused by SPDT, but it was difficult to carry out without the additional defects like sub-surface damages and other tool marks. To overcome this weakness, we investigated the Magneto-Rheological Finishing (MRF) process to effectively remove the periodic micro structures without surface deterioration for optical performance. The workpiece used in the experiment is a mirror plated with electroless nickel-phosphorus. Through the processing of the SPDT, an initial surface gets periodic tool marks, which have a height of 1.1 μm and a pitch of 20 μm. We studied on the reduction rate of the turning marks by the MRF process with some different conditions of uniform removal. The quantitative analysis of the surface roughness and residual marks was performed using a scanning low-coherence interferometer and through the Power Spectral Density (PSD) respectively. The results showed that reduction rates of tool marks depend on the angles (0, 45, and 90 degs) between the turning direction of the tool marks and the rotation direction of MR wheel. In the case of 45 degs, it indicated the fastest reduction rate.
A magneto-rheological finishing (MRF) process for the post-treatment of diamond turning is presented to remove the periodic micro structures and sub-surface damages with improvement of the original figure and surface roughness. An off-axis aspherical mirror with electro-less nickel-phosphorus plated surface was machined by a Single point diamond turning machine (SPDTM) and MRF polisher. The machined surfaces were examined by a scanning low-coherence interferometer, and the technique of Fast Fourier Transformation (FFT) and Power Spectrum Density (PSD) were introduced to evaluate the residual diamond turning marks on the turned and polished surfaces. The turning marks, which was clearly visible on the diamond turned surface, were absolutely removed after MR process, and the surface roughness of the machined surface was improved from 6 nm(Sa) and 7 nm (Sq) to 2 nm(Sa) and 3 nm (Sq). Consequently, the experimental results indicate that MRF is suitable for removing periodic micro-patterns caused by diamond turning process with the progress of the original figure and surface roughness.
An off-axis optical system can effectively avoid some problems, such as aberrations, shielded area created by the secondary mirror and a narrow field of view (FOV), while an on-axis optical system has the problems. Inspired by the consideration, the off-axis optical system is generally used for hyperspectral sensors and telescopes. However, there are several obstacles limiting the productivity of the off-axis optics in fabrication and measurement processes. In this study, to overcome this weakness, we suggests a new fabrication technique using a customized jig, not separated from the work-piece. A convex aspheric mirror and the off-axis mirror are fabricated by Single Point Diamond Turning Machine (SPDTM) for comparison analysis of surface state. The mirrors are made from aluminum (Al6061-T6) and used for the reflectors of a coastal water remote sensing system. We show fast machining and simple measurement in comparison with traditional off-axis single machining and measurement, provide performance results, such as form accuracy and surface roughness measured by both contact 3D profilometer (UA3P) and non-contact 3D profiler (CCI-Optics). The customized ultra-precision machining process can be effectively used for complex off-axis mirror fabricating.