Dual-sideband-probe BOTDA with distortion-free pump pulse using the distributed Pound–Drever–Hall technique

A dual-sideband probe can effectively enhance the signal-to-noise ratio and the sensing distance of Brillouin optical time domain analysis (BOTDA), but it also causes the risk of pump pulse distortion, which can lead to measurement failure. In this work, we introduce the Pound–Drever–Hall (PDH) technique into the dual-sideband-probe BOTDA to overcome this limitation. With a PDH loop in a distributed form, the Stokes and anti-Stokes frequency offsets (frequency differences between the dual-sideband probe wave and the pump wave) are locked to the Brillouin frequency shift (BFS) throughout the entire length of the sensing fiber, ensuring a flat, distortion-free pump pulse gain during its forward propagation. The demonstration BOTDA system achieves a BFS resolution of 0.84 MHz at the far end of 75 km fiber with 5 m spatial resolution. And the measurement time is accelerated to 3.3 s (4000 averages), benefiting from the frequency scanning free PDH configuration.