Evaluation of subsurface damage caused by ultra-precision turning in fabrication of CaF₂ optical micro resonator

The optical micro-resonator, which stores light at a certain spot, is essential in next-generation optical signal processing. Single-crystal calcium fluoride (CaF₂) is the most suitable material for this element. Ultra-precision turning is a feasible fabrication process for CaF₂ optical micro-resonators. In this study, the influence of subsurface damage on the resonator's Q factor is investigated. TEM observation shows that the subsurface layer of up to several tens of nanometers thickness changed from single-crystal to polycrystalline morphology due to ultra-precision turning. A diamond tool with 0° rake angle results in lower damage than one with negative rake angle, which enhances the resonator's performance.