  We demonstrate up-conversion single-photon detectors based on integrated
periodically poled lithium niobate waveguides, which incorporate two mode
filters and a directional coupler. The two mode filters are optimized for the
fiber-waveguide coupling efficiencies for 1550 nm and 1950 nm respectively
while the directional coupler plays the role of wavelength combiner, making the
overall system portable and low-cost. The two wavelengths pump each other in
our detection system. We achieve detection efficiencies of 28% for 1550 nm and
27% for 1950 nm, respectively. This scheme provides an efficient integrated
single-photon detection method for any two well-separated spectral bands in the
whole low-loss range of lithium niobate waveguides.
