  We demonstrate a compact all-fiber polarization-independent up-conversion
single-photon detector based on integrated reverse proton exchanged
periodically poled lithium niobate waveguides. The horizontally and vertically
polarized components of randomly polarized signals are separated with a
fiber-coupled polarization beam splitter, launched into two orthogonally
polarized polarization maintaining fibers and fetched into two adjacent
independent waveguides on the same device. The up-converted outputs from both
waveguide channels are then combined with a multi-mode fiber combiner and
detected by a silicon detector. With this configuration, the
polarization-independent single-photon counting at 1.55 um is achieved with a
system detection efficiency of 29.3%, a dark count rate of 1600 counts per
second, and a polarization dependent loss of 0.1dB. This compact all-fiber
system is robust and has great application potential in practical quantum key
distribution systems.
