An SNSPD detector system usually includes: (1) the detector itself, (2) a system for the optical coupling to the detector (based on optical fibers of free-space optics), (3) a system for electrical read-out of the photoresponse of the detector, and (4) a refrigerator. The system detection efficiency can be thought of as the product of the detector detection efficiency (1D, the probability that a photon incident on the detector active area produces a response pulse) and the optical coupling efficiency (1C, the probability that a photon at the input of the optical coupling system is delivered to the detector). The system dark counts are due to the intrinsic dark counts of the detector and the detection of background photons. Since superconducting nanowires are sensitive to a wide range of wavelengths, from the visible to the mid-infrared, the background photon count rate (BCR) is mostly due to the room-temperature blackbody radiation, which has a spectrum spanning mid- and near-infrared wavelengths. Depending on the sensitivity of SNSPDs to longer wavelengths and on the optical coupling system, BCR can, like in this paper, dominate the system dark count rate.
The BCR reduction strategy that has been adopted so far in the SNSPD community is to spectrally filter the broadband black-body photons at cold temperatures. However, adding a cold filter to the detector system has a number of limitations: (1) the filter reduces 1C and a trade-off exists between filter off-band rejection and in-band transmission, (2) the transmission window of the filter shifts with temperature, making the design of the filter around the application wavelength challenging, and (3) the spectral filtering may decrease the timing performance of the system, broadening the optical pulses in the time domain. The interesting result reported in this paper is that BCR was reduced by three orders of magnitude while 1C decreased by ~ 4 dB. Although Dr. Shibata’s team reduced the system detection efficiency, they significantly increased the system sensitivity, which is the highest sensitivity reported to date for a fiber-coupled SNSPD system. The method described in this letter could be used to meet the high-sensitivity requirements of applications that can tolerate low system efficiencies.
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