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

TL;DR

This paper introduces a time-of-flight imaging method using a differential-readout SNSPD to measure beam divergence, challenging previous assumptions and enabling smaller, optimized detectors.

Contribution

It demonstrates a novel measurement technique that reveals minimal beam divergence in high-efficiency SNSPDs, contradicting prior beliefs about detector size requirements.

Findings

No beam divergence observed in larger fiber modes.

Beam divergence only present in the smallest fiber mode.

Supports development of smaller, more efficient SNSPDs.

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.