Dual-lens beam compression for optical coupling in superconducting nanowire single-photon detectors

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 mu 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 mu m x 10 mu m and 62.5 mu m 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.