The optical coupling of superconducting nanowire single-photon detectors （SNSPDs） has always been restricted to a single-mode fiber for a limited detection area. In this study, for enhancing photon coupling, a dual-lens system operating at 2.2 K was used to compress the beam size on the basis of the Gaussian beam theory and geometric approximation. A magnification of approximately 0.3 was obtained, and a focused spot with diameter of approximately 10 ~m was measured from a multimode fiber. Assisted with the compressed beam, a system efficiency of 55 % （1550 nm） was achieved for a SNSPD with a detection area of 10 μm × 10 μm and 62.5 pm multimode fiber coupling. At the same time, a high speed of 106 MHz was measured with the proposed system. The realization of a highly compressed optical beam reduced the optical coupling requirement and helped maintain a high speed for the SNSPD.
Performances of superconducting nanowire single-photon detectors(SNSPDs) based on low TCmaterials strongly depend on the operating temperatures. We have fabricated infrared-sensitive niobium SNSPDs based on doped niobium(Nb*) films and measured them in He-3cryocooler. The critical current approaches to the de-pairing current at 0.3 K. Therefore, with the decrease in temperatures, we have observed a monotonous increase of count rate at the wavelength of 1,521 nm and exponential decrease of dark count rate at all bias currents. The possible origin of dark counts for doped Nb devices is also discussed.
Tao JiaChao WanLimin ZhaoYu ZhouQingyuan ZhaoMin GuXiaoqing JiaLabao ZhangBiaobing JinJian ChenLin Kang
