Externally time-gated photon-counting OTDR in the 1.5 µm telecom band.

Photon-counting optical time-domain reflectometry (PC-OTDR) enables high-sensitivity and high-resolution characterization of optical fiber links. However, its dynamic range is limited by the dead time of the single-photon detector and the resulting maximum count rate. Here, we demonstrate an externally time-gated PC-OTDR system operating at the 1.5 µm telecom band. An optical modulator is employed to gate the back-propagation signal, allowing only photons from a specific fiber segment to be detected by the detector. This configuration effectively suppresses out-of-gate backscattered photons and optimizes the utilization of the finite detector count rate. Benefiting from the scheme, a weak loss event of 0.36 dB is clearly resolved, whereas it is undetectable without gating. With 50-ns external gates, a 70-m PC-OTDR system achieves a dynamic range of 24.1 dB, corresponding to a 10.8-dB improvement over ungated operation. This telecom-band implementation provides a practical route toward high-dynamic-range PC-OTDR for fiber-link monitoring in optical communication and quantum network applications.