Enhanced laser ranging with superconducting nanowire single photon detector for low dark count rate

Superconducting nanowire single photon detector (SNSPD) is a competitive candidate in laser ranging at 1064 nm wavelength compared with other single photon detectors such as InGaAs/InP APD for its high sensitivity, high time precision and low dark counts. In this paper, we apply our SNSPD to a laser ranging system measuring target in Qinghai lake area with atmospheric scatter. The echo photons are received by telescope, and transport through the multimode fiber to the SNSPD photon-sensitive area. The SNSPD, integrated in an optical cavity with a resonant wavelength of 1064 nm, is fabricated on a MgF2 substrate. The optical absorption of NbN film goes up to 98% according to FDTD simulation, and the system efficiency is measured to be about 40%. A pulsed laser at 1064 nm, featuring a peak power of 12 MW and a pulse width of 10 ns, is adopted in the laser ranging system. In this experiment, we first measure the system intrinsic noise and the environment noise introduced into the laser ranging system after turning off the laser. After that, we measure the echo rate for the target at 126 km, which increases up to 96% with an attenuator of 10 dB at the receiver side. The maximum distance of the laser ranging system is analyzed based on the experimental results of dark count and echo rate through a theoretical model of laser radar. The analysis indicates that signal-to-noise ratio (SNR) is increased smoothly with the accumulation of time. At the same time, we simulate how the dark counts influence the capability of laser ranging system based on SNSPD, the simulated SNR matches well with the experimental data of target at 126 km. Furthermore, the dark counts, accumulation of time and probability of echo photon affect the SNR according to the simulation results, showing that large dark counts would result in SNR fluctuation and signal annihilation when the probability of echo photon is low. Thus, the maximum distance of laser ranging under the assumption of integration time is estimated through the SNR simulated result, showing that a maximum distance is up to 280 km, 40 km far away from APD detector based system under the same conditions mainly due to the very low dark counts of SNSPD. It should be pointed out that the coupling efficiency between SNSPD and the receiving telescope is low for small view field limited by the 62.5 m fiber of SNSPD. Thus, further work is to fabricate SNSPD with a larger coupling area which is possible to increase the maximum distance with improved coupling settings.