Robust Packaging Technique and Characterization of Fiber-Pigtailed Superconducting NbN Nanowire Single Photon Detectors

Efficient packaging and fiber coupling of a superconducting niobium nitride nanowire single photon detector is in itself a real challenge and is often a limiting factor to reach a high system quantum efficiency. Our approach makes use of a controlled expansion alloy, Carpenter Invar 36, that has been adequately heat treated to enhance its characteristics for cryogenic operation. This insures the integrity of the optical coupling done at room temperature while providing a good attenuation for electromagnetic interference due to the high relative permeability of this nickel-iron alloy. The small form factor of this optical fiber pigtailed package makes it versatile and could be easily integrated with a commercial cryogen-free system or simply dipped into a standard helium transport Dewar. We report on our experimental methodology to evaluate the optical coupling quality and present the optoelectronic characterization of two devices packaged in this way.