Improving surface figure and microroughness of IR materials and diamond turned surfaces with magnetorheological finishing (MRF®)

Optics manufactured for infrared (IR) applications are commonly produced using single point diamond turning (SPDT). SPDT can efficiently produce spherical and aspheric surfaces with microroughness and figure error that is often acceptable for use in this region of the spectrum. The tool marks left by the diamond turning process cause high surface microroughness that can degrade performance when used in the visible region of the spectrum. For multispectral and high precision IR applications, surface figure may also need to be improved beyond the capabilities of the SPDT process. Magnetorheological finishing (MRF (R)) is a deterministic, subaperture polishing technology that has proven to be very successful at simultaneously improving both surface microroughness and surface figure on spherical, aspheric, and most recently, freeform surfaces. MRF has been used on many diamond turned IR materials to significantly reduce surface microroughness from tens of nanometers to below 1nm. MRF has also been used to successfully correct figure error on several IR materials that are not diamond turnable. This paper will show that the combination of SPDT and MRF technologies enable the manufacture of high precision surfaces on a variety of materials including calcium fluoride, silicon, and nickel-plated aluminum. Results will be presented for microroughness reduction and surface figure improvement, as well as for smoothing of diamond turning marks on an off-axis part. Figure correction results using MRF will also be presented for several other IR materials including sapphire, germanium, AMTIR, zinc sulfide, and polycrystalline alumina (PCA).