Distributed Acoustic Sensing Based on Optical Time‐Domain Reflectometry

Distributed acoustic sensing (DAS) technology has wide application prospects in seismic wave detection, geological exploration, oil well/mine monitoring, etc. Due to the ability to measure time-domain waveform of the dynamic vibration events along the whole sensing fiber, phase optical time-domain reflectometry (ϕ-OTDR) has become one of the most important implementation methods of DAS technology. In this chapter, the basic structure and principle of two typical ϕ-OTDR systems, coherent-detection-based ϕ-OTDR and direct-detection-based ϕ-OTDR, are introduced firstly. The structure, principle, and modulating method of acousto-optic modulator (AOM) and dual parallel Mach–Zehnder modulator (DPMZM) pulse modulators used in system are also discussed in detail. Then, the sensitivity of detecting acoustic signal by optical fiber and two enhance structures are presented. Based on the types of detection pulse used, several DAS systems and experiment results are presented according to two categories: dual-pulse coherent mechanism and linear-frequency-modulation pulse mechanism.