250 km ultra-long distance single-end Φ-OTDR assisted by DRA and RPOA technology

Phase-sensitive optical time-domain reflectometry (Φ-OTDR) can quickly characterize the weak vibration signals along the sensing fiber by demodulating the Rayleigh backscattering (RBS) light. It features a millisecond or sub-millisecond fast response and nanostrain detection sensitivity, and has broad application fields. However, the maximum measurement distance of Φ-OTDR system is limited by the nonlinear threshold and transmission loss of sensing fiber and the ultralow power of RBS light. In this paper, a 250 km ultra-long sensing distance single-end Φ-OTDR system with 20m spatial resolution (SR) is proposed and experimentally demonstrated, assisted by using a semiconductor optical amplifier (SOA) and an acoustic-optic modulator (AOM) cascade to obtain a high extinction ratio (ER) probe pulse, and combining co-pumping distributed Raman amplification (DRA) technology and remotely pumped optical amplification (RPOA) technology. With the elaborate design, the 250 km sensing fiber is divided into three segments, the first 25 km segment sensing fiber is used to delay the injection of the Raman pump, to further achieve the maximum sensing distance without triggering the nonlinear effects; then the co-pumping DRA based on 1480 nm pump laser diode provides gain mainly in the next 100 km segment, then the residual 1480nm pump source provides energy to the erbium-doped fiber to realize the inversion of the particle number, and finally amplifies the RBS light by the process of stimulated radiation in the last 125 km segment. According to our research, this is the maximum sensing distance of Φ-OTDR system achieved without assisted by active repeaters and counter-pumping distributed amplification.