A simple approach to fiber-based tunable microcavity with high coupling efficiency

We report a simple technique for the realization of fiber-based Fabry-Pérot microcavities with large Q/V values as well as high cavity-to-fiber coupling efficiencies. The open microcavity we demonstrate consists of a flat mirror and a concave mirror on the tip of a single mode optical fiber. Combining hydrofluoric acid chemical wet etching with CO₂ laser reflow, we obtain a fiber-tip atomically smooth concave surface with a diameter of 4.7 μm determined by the fiber core size. The concave surface with a spherical profile is automatically aligned with the fiber core, which enables high cavity-to-fiber power coupling efficiency (higher than 90%) with an optimal fiber-tip mirror's radius of curvature based on numerical simulation results. After distributed-Bragg-reflector coating, we have realized a wavelength-tunable open microcavity with a quality factor Q exceeding 1000 and a mode volume V of 13.6 μm³, and laser emission is demonstrated from this microcavity.