Ductile machining of single-crystal germanium freeform optics via ultra-precision diamond turning for high-performance infrared imaging systems

This study focuses on the precise fabrication of cubic phase freeform optics on single-crystal germanium (sc-Ge) for infrared (IR) imaging systems, addressing thermal-induced defocus challenges. Achieving nanometric surface finish and high form accuracy in sc-Ge via diamond turning machining (DTM) is challenging due to the material's brittleness. To overcome this, a novel groove generation model is developed to optimize DTM parameters and minimize brittle fractures using various cutting approaches. Additionally, the study demonstrates that a hybrid cutting approach, which combines constant angle and arc methods, achieves ductile-based machining and enhances surface quality significantly. Precise workpiece alignment is vital for ensuring high form accuracy in freeform optics; therefore, a fiducial-based method is developed for characterization using a mechanical profilometer, and a long-wavelength interferometry measurement. Final surface evaluation utilized a computer-generated hologram (CGH) in a Fizeau interferometer, achieving a form error of 0.47 μm and surface roughness (Sa) of 2 nm for cubic freeform optics which is acceptable in the infrared imaging system. The developed optics are integrated into an infrared camera within a wavefront coding setup to counteract thermal defocus. This work provides valuable insights into brittle-ductile transitions and nano-surface generation, enhancing machining techniques for IR freeform optics.