Multimode Fiber Coupled Superconductor Nanowire Single-Photon Detector

High-performance superconductor nanowire single-photon detectors (SNSPDs) coupled by 50-μm multimode fiber were designed and fabricated for the purpose of space applications. Microlens coupling between multimode fiber and detector chip was proposed, resulting in coupling efficiency of 86% at the chips with detection area of 15 x 15 μm². An optical structure for enhancing the photons absorption and superconductor nanowires with low critical current of 3 μA were fabricated on the chips. A cryogenic amplifier was adopted to increase the signal-to-noise ratio and time jitter of output pulse. The SNSPD exhibited system efficiency (1550 nm) of 56% at a dark count rate of 100 cps, 50% at a dark count rate of 5 cps, and 40% at a dark count rate of 1 cps. The time jitter was 46 ps at full-width at half-maximum. The multimode fiber coupling system with high performance will promote its applications in free space.