Profile control of fiber-based micro-mirrors using adaptive laser shooting with in situ imaging

fiber Fabry-Perot cavities (FFPCs) are used in various studies in cavity quantum electrodynamics (CQED) and quantum technologies due to the cavity's small mode volume and compact integration with optical fibers. We develop a novel CO₂ laser machining method that produces well-controlled surface profiles on the end facets of cleaved optical fibers. Using multiple shots in distinct spatial distribution patterns, our method employs a shooting algorithm that adaptively changes laser ablation parameters during the shooting to suppress deviations from the desired profile. This is made possible by in situ imaging of the machined profile, its inspection, and the usage of the information in the subsequent steps. Underlying this algorithm is a newly found laser ablation parameter, the pause between shots, which controls the accumulation of heat in between successive laser shots, and as a result, determines the area of impact made by an individual ablation sequence. We fabricate fiber-based micro-mirrors with radii of curvature ranging from 250 µm to 700 µm with an effective mirror diameter of 60 µm in either Gaussian or spherical profiles. Due to the self-correcting nature of our adaptive algorithm, we achieve a near 100% success rate in the production of desired profiles with low ellipticity. After furnishing the laser-machined fiber end facets with high reflectivity coating, FFPCs are formed to demonstrate a high finesse up to 150,000 at an optical wavelength of 854 nm.