Extended-range and faster photon-counting Brillouin optical time domain reflectometer

Distributed fiber optic sensors are used to monitor civil infrastructures and detect earthquakes and for energy transport surveillance. Over the past 20 years, various technological and numerical advances have pushed back the limits of these sensors and diversified their applications. However, the maximum range of distributed fiber optic sensors such as Brillouin optical time domain reflectometers (BOTDRs) is currently limited by the signal-to-noise level of the detectors. We present a fast, long-range measurement technique with a high signal-to-noise ratio that overcomes these difficulties. We propose to use a gated single-photon detector triggered by multiple gating pulses delayed by sub-dead time duration. The length of the pulse sequence considerably reduces measurement time without compromising spatial resolution, maximum range, or sensitivity. The proposed approach is demonstrated experimentally by measuring the Brillouin signal up to a distance of 150 km in a standard single-mode fiber. The measurements were performed without need for an optical amplification module remotely placed along a standard single-mode fiber, thus surpassing the state of the art and providing excellent agreement with theory. We experimentally demonstrate a hot-spot measurement at 125 km with a spatial resolution of 20 m. By extrapolating our results, we pave the way for the future 200 km BOTDRs.