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 microm wavelength. Above this wavelength, the transmission rolls off; the fibers exhibit 3 dB/m loss at 2.5 microm. 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.