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 approximately 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.