Influence of Local Reflection on Photon Counting OTDR and Its Mitigation

Photon counting optical time-domain reflectometry (PC-OTDR), leveraging single photon detection, offers superior spatial resolution and high dynamic range in the testing of optical fiber links. However, the dead time and afterpulsing of the single photon detector could cause distortions on the PC-OTDR trace, especially for the fiber links with strong local reflections, such as the Fresnel reflection from fiber connection, the influence of which has yet to be investigated. In this work, we study the influences of local reflections with different strengths, then propose and demonstrate a scheme of multiple time-gating with narrow temporal filtering to mitigate these influences. Distortions are observed in a simulation system with a relative intensity of local reflection greater than 25.0 dB. To mitigate these distortions, we employ an optical intensity modulator driven by periodic electronic signals with different relative delays to manipulate the optical signal, and then process the measured traces with narrow temporal filtering. A trace without distortion is obtained in the simulation system, where a local reflection with a relative intensity of 37.5 dB is tolerated using our scheme. Furthermore, a PC-OTDR trace of a 70-m-long fiber link is obtained without distortion, where the relative intensity of the Fresnel reflection reaches 35.2 dB. Our results reveal the influence of local reflection on the PC-OTDR trace, and pave an available avenue for the testing of fiber links with strong local reflections.