  We developed superconducting nanowire single-photon detectors with an optical
cavity (OC-SNSPDs) for multichannel systems. For efficient coupling, the
devices were installed in compact fiber-coupled packages after their substrate
thickness was reduced from 400 to 45 $\mu$m. The measured detection efficiency
(DE) measurement at different substrate thicknesses and the estimation of
optical coupling efficiency indicated that ~98% of the input light beam could
be radiated on a 15x15 $\mu m^2$ nanowire area from behind the substrate. The
DEs of a NbN OC-SNSPD system were observed to be 9.5% and 25% at 1550 nm and
1310 nm, respectively (dark-count rate: 100 c/s).
