  Dark count rate is one of the key parameters limiting the performance of the
superconducting nanowire single photon detector (SNSPD). We have designed a
multi-layer film bandpass filter that can be integrated onto the SNSPD to
suppress the dark counts contributed by the stray light and blackbody radiation
of the fiber. The bandpass filter is composed of 16 SiO2/Si bilayers deposited
onto the backside of a thermally oxidized Si substrate. The substrate shows an
excellent bandpass filter effect and provides a high transmittance of 88% at
the central wavelength of the pass band, which is the same as that of the bare
substrate. The SNSPDs fabricated on the substrate integrated with the bandpass
filter show conspicuous wavelength-sensitive detection efficiency. The
background dark count rate is reduced by two orders of magnitude to sub-Hz
compared with the conventional SNSPD (a few tens of Hz). The detector exhibits
a system detection efficiency of 56% at DCR of 1 Hz, with the measured minimal
noise equivalent power reaching 2e-19 w/Hz1/2.
