Fabrication of microlens arrays on single-crystal CaF2 by ultraprecision diamond turning

Single-crystal calcium fluoride (CaF2) is considered a superior optical material with good transmittance in a wide transmitted wavelength region. However, the production of high surface quality freeform surfaces on CaF2 is difficult owing to its soft, brittle, and anisotropic properties. This study attempted to fabricate spherical and hexagonal microlens arrays on CaF2 using diamond turning with a slow tool servo. In addition, the machining characteristics of CaF2 were systematically investigated. It was found that microcracks tended to be generated in the cut-in and cut-out regions of the lenses. When cutting along the [-211] and [11 -2] directions, the machined surfaces were very prone to cracking. When reducing the maximum undeformed chip thickness to less than approximately 60 nm, continuous ribbon-like chips were formed throughout the cutting process, and all lenses were machined in ductile mode without surface cracking. In addition, a spherical microlens array was successfully fabricated in ductile mode with a surface roughness of approximately 2 nm Sa. Consequently, a hexagonal microlens array with sharp edges and crack-free surfaces was achieved using the proposed depth-offsetting segment cutting method. The surface roughness was approximately 4.1 nm Sa with a form error of approximately 147 nm P-V (peak to valley). Tool wear was insignificant until a machining distance of approx-imately 50 m; however, with further increase in the machining distance, crater wear and microchipping were observed. This study demonstrated the feasibility of fabricating high-quality freeform microstructured surfaces on CaF2 and other soft-brittle materials via diamond turning.