In-Situ Measurement of Beam Divergence in a High Efficiency SNSPD Platform

Abstract

We implement a time-of-flight imaging technique utilizing a differential-readout SNSPD to spatially resolve detection events in a fiber-coupled detector platform. We measure the spatial detection profiles for ultra-high numerical aperture fiber, standard single-mode fiber, and thermally-expanded core fiber (mode-field diameters 4.1{\mu}m, 10.4{\mu}m, 30{\mu}m respectively) in an active area surrounded by an all-dielectric optical stack designed for near-unity detection efficiency. We see no beam divergence in all but the smallest fiber optic modes. This contradicts previously-held beliefs that beam divergence during the detection process necessitates activate areas much larger than coupled optical modes, opening new paths toward smaller and better-optimized detectors.