Multipath distributed acoustic sensing system based on phase-sensitive optical time-domain reflectometry with frequency division multiplexing technique

•Multipath distributed optical fibre sensing network for a modern metropolis.•Different sensing parameters can be applied according to specific applications.•Simultaneously monitoring of different civil structures.•Integrated sensing system with one optical transmitter and receiver. We propose a multipath distributed acoustic sensing (MP-DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) and frequency division multiplexing (FDM) technique. With FDM introduced to Φ-OTDR, simultaneously monitoring of multipath vibration is realized with flexible functionality. In the proposed MP-DAS system, the parameters of each path can be adjusted individually according to the path's specific monitoring requirement. A dual path Φ-OTDR system with 1 km and 22 km measurement distance is demonstrated in the experiment. Through heterodyne detection and FDM technique, both amplitude and phase information are obtained for event locating and vibration sensing in each of the two paths, with a spatial resolution of 5 m and 10 m respectively. With changing optical power and pulse width of one path, its influence on the other path is discussed and analyzed. This proposed MP-DSA system based on FDM technique provides an efficient and cost-effective way for upgrading a conventional single DAS into an integrated DAS network, which shows great potential in many applications such as modern city civil structure monitoring with different demands.