Monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling

In this paper, we present the design, construction, and testing of a monolithic fiber end cap collimator for high-power free-space fiber-fiber coupling applications. The collimator is based on a large-sized fiber end cap and a spherical lens design on the output facet. Values of the spot size and working distance are theoretically analyzed based on Gaussian approximation and ABCD transmission matrix. The free-space fiber-fiber coupling process is also analyzed for different lens curvature radii and coupling distances. In the experiment, a collimated laser beam is obtained with Rayleigh length of about 400 mm. A high-power laser with 1.1 kW output is tested on the end cap collimator, which only heats up by 7°C at the output facet without active cooling. Free-space fiber-fiber coupling between two 20/400 μm fibers is achieved based on these collimators, with measured coupling loss lower than 0.3 dB.