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4 Distributed Acoustic Sensing System Based on Phase‐Generated Carrier Demodulation Algorithm

       Tuanwei Xu, Shengwen Feng, Fang Li, Lilong Ma, and Kaiheng Yang

       Key Laboratories of Transducer Technology, Institute of Semiconductors, Chinese Academy of Sciences, Beijing, China; and

       College of Materials Science and Opto-Electronic Technology University of Chinese Academy of Sciences, Beijing, China

      ABSTRACT

      We demonstrate a real‐time distributed acoustic sensing (DAS) system based on phase‐sensitive optical time domain reflectometry (Φ‐OTDR) and phase‐generated carrier (PGC) demodulation algorithm. An unbalanced Michelson interferometer (MI) with specific phase modulation is introduced to overcome phase fading caused by initial phase shift in fiber optic interferometer sensing. Owing to its relatively low data requirement and polarization‐independent structure, PGC‐DAS system exhibits the superiorities of real‐time signal processing and Rayleigh polarization‐induced fading suppression. A proof‐of‐concept system is constructed to demonstrate feasibility and sensing performance. Corresponding to the average phase noise of ~5 × 10^‐4 rad/√Hz, a strain sensitivity of 8.5 pε/√Hz is achieved with a spatial resolution of 10 m, as well as a frequency response range of 2 Hz to 1 kHz over 10 km sensing distance. Further, a field trial of this system is presented to validate it in qualitative seismic monitoring on land.

4.1. INTRODUCTION

      DAS is an advanced technique developed in recent years to accurately measure ground vibration via fiber optic cables. DAS presents a possible new frontier for recording earthquake waves and other seismic signals in a wide range of research and public safety arenas (Juarez et al., 2005; Parker et al., 2014; Tanimola & Hill, 2009). It repurposes standard telecommunication fiber optic cables as a long series of single‐component, in‐line strain, or strain‐rate sensors, which is a completely different way from conventional deployments of nodal devices. DAS can sample passing seismic waves at locations every few meters or closer along paths stretching for tens of kilometers. Therefore, DAS has many advantages, such as passivity, resistance to electromagnetic interference, and cost‐effectiveness.

φ‐OTDR is one of the most widely used schemes to achieve distributed strain or strain‐rate sensing. In the early stage, research focused on detecting the interfering Rayleigh backscattering (RB) amplitude in the sensing fiber. In 1993, Taylor and Lee first monitored intrusion events by detecting RB intensity changes with Φ‐OTDR technology (Taylor & Lee, 1993). However, the nonlinearity between RB amplitude and vibration could not satisfy the need for quantitative seismic measurement in local and regional seismology. Then, researchers began to investigate phase term (Feng et al., 2018; Sha et al., 2017; Yan et al., 2017; Yang et al., 2018; Zinsou et al., 2019), which is almost linear to strain. Currently available DAS systems have characteristics in common that they use pulsed lasers to interrogate optical fibers and process RB phase to provide a nearly continuous estimate of fiber dynamic strain along the fiber. In general, they differ in the method to process RB light and may be separated into coherent detection, dual‐pulse detection, and interferometer detection (Hartog, 2017). Coherent detection represents the fact that the phase is extracted by mixing RB signal and local oscillator (He et al., 2017; Lu et al., 2010; Wang et al., 2016). Dual‐pulse detection uses two separate RBs with different probe frequencies or phases (Alekseev