This paper describes the construction of a cryostat and an optical system with a free-space coupling efficiency of 56.5% +/- 3.4% to a superconducting nanowire single-photon detector (SNSPD) for infrared quantum communication and spectrum analysis. A 1K pot decreases the base temperature to T = 1.7 K from the 2.9 K reached by the cold head cooled by a pulse-tube cryocooler. The minimum spot size coupled to the detector chip was 6.6 +/- 0.11 {\mu}m starting from a fiber source at wavelength, {\lambda} = 1.55 {\mu}m. We demonstrated efficient photon counting on a detector with an 8 x 7.3 {\mu}m^2 area. We measured a dark count rate of 95 +/- 3.35 kcps and a system detection efficiency of 1.64% +/- 0.13%. We explain the key steps that are required to further improve the coupling efficiency.
@article{arxiv.1511.05786,
title = {Free space-coupled superconducting nanowire single photon detectors for infrared optical communications},
author = {Francesco Bellei and Alyssa P. Cartwright and Adam N. McCaughan and Andrew E. Dane and Faraz Najafi and Quinyuan Zhao and Karl K. Berggren},
journal= {arXiv preprint arXiv:1511.05786},
year = {2016}
}