Recent advances in deterministic, low-cost finishing of sapphire windows

There is an increasing demand for large sapphire windows for a number of defense related pro-rams. Some of these emerging requirements call for windows that are on the order of half a meter in size with tight surface figure and transmitted wavefront requirements. Magnetorheological Finishing (MRF (R)) is a deterministic polishing process capable of rapidly converging to the required surface figure. MRF finishing of sapphire has been demonstrated with surface accuracies better than 0.07 mu m peak-to-valley (0.010 mu m RMS) and surface microroughness less than 1.0 nm RMS on circular and square apertures. As a sub-aperture polishing technique, MRF provides a mechanism for effectively addressing and correcting a variety of optical surface features. This is of particular interest when correcting the transmitted wavefront on windows. The process allows for correction of the optical wavefront when it is aberrated due to inhomogeneity in the material in addition to the errors in the surface. Another benefit is that MRF has been shown to remove subsurface damage left from prior fabrication steps and can improve surface toughness of pre-polished sapphire. We report on a predictable, lower-cost process for fabricating large-scale sapphire windows.