  We investigate the detection of an ultra-bright single-photon source using
highly efficient superconducting nanowire single-photon detectors (SNSPDs) at
telecom wavelengths. Both the single-photon source and the detectors are
characterized in detail. At a pump power of 100 mW (400 mW), the measured
coincidence counts can achieve 400 kcps (1.17 Mcps), which is the highest ever
reported at telecom wavelengths to the best of our knowledge. The multi-pair
contributions at different pump powers are analyzed in detail. We compare the
experimental and theoretical second order coherence functions $g^{(2)}(0)$ and
find that the conventional experimentally measured $g^{(2)}(0)$ values are
smaller than the theoretically expected ones. We also consider the saturation
property of SNSPD and find that SNSPD can be easier to saturate with a thermal
state rather than with a coherent state. The experimental data and theoretical
analysis should be useful for the future experiments to detect ultra-bright
down-conversion sources with high-efficiency detectors.
