Research of controlling the optical face of CaF2 crystal elements on optical grinding and polishing

As an excellent optical element material, CaF2 crystal has a wide-spectrum transmission range and stable physicochemical properties. Due to the soft and brittle nature of CaF2 crystal, also with high coefficient of thermal expansion and low heat conductivity coefficient, it is of great significance to study the stability and characterization of the removal function during the processing of large-diameter CaF2 optical elements with specific geometric requirements. In this paper, a Phi 200 mm CaF2 crystal plate was taken as the research object. The stable technique of grinding and polishing calcium fluoride is researched and the pressure is treated as the key factor. The specialized tool is designed to control the pressure. The removal model of grinding and polishing is stablished based on the specialized tool. The pressure distribution of grinding and polishing process is simulated and the optimized pressure distribution is obtained. This makes the whole face error converged efficiently and stably, meanwhile the face to face angle can be controlled precisely. As a conclusion, the PV value of experimental element is less than 0.13 lambda within the aperture, and the face to face angle is restrained into 5 " after using new tools.