Using silica fiber coupling to extend superconducting nanowire single-photon detectors into the infrared.

There is growing interest in superconducting nanowire single-photon detectors (SNSPDs) for their high detection efficiency, low noise, and broad wavelength-sensitivity range. Typically, silica fibers are used to deliver light to the detectors inside the cryostat, which works well for wavelengths from visible through 1550 nm. To access longer-wavelength infrared photons, other types of fibers, such as chalcogenide and fluoride fibers, need to be used. Here, we examine the infrared-wavelength transmission of straight and coiled silica optical fibers as candidates to couple infrared light to SNSPDs. We find that the silica fibers offer good transmission up to 2.2 m wavelength. Above this wavelength, the transmission rolls off; the fibers exhibit 3 dB/m loss at 2.5 m. High bend-loss sensitivity of some fibers can be used to adjust the long-wavelength transmission cutoff of the fiber to limit noise photons due to blackbody radiation.