Photon-counting optical time-domain reflectometry with centimeter-level spatial resolution over fibers of 100  km

In this work, an experimental system is established to investigate the spatial resolution and signal-to-noise ratio (SNR) of photon-counting optical time-domain reflectometry (PC-OTDR) comprehensively. We analyze the impact of fiber dispersion on the spatial resolution, showing that light pulses at the C-band with a temporal width of several hundred picoseconds can support a spatial resolution of several centimeters over single-mode fibers (SMFs) of 100 km. We analyze the noises in the PC-OTDR measurement and calculate the SNRs along the OTDR traces under different conditions. These results show that the coherent Rayleigh noise (CRN) is the dominate noise, and it can be reduced by using a sampling bin width longer than the light pulse width. The experimental system is optimized accordingly, achieving a PC-OTDR experiment with a spatial resolution of 10 cm and a sensing range of 100 km. This shows that PC-OTDRs have strong capacity for long-distance high-resolution distributed sensing, which has great potential on optical link management and remote sensing over long-distance underground/submarine fiber cables.