Distributed Acoustic Sensing Based on Pulse-Coding Phase-Sensitive OTDR

Abstract

Phase-sensitive optical time-domain reflectometry (<inline-formula> <tex-math notation= LaTeX >${\\Phi }$ </tex-math></inline-formula>-OTDR), which utilizes the phase information of Rayleigh scattered lightwave inside optical fiber, could turn a fiber cable into a massive sensor array for distributed acoustic sensing (DAS), i.e., an emerging infrastructure for Internet of Things. Given a certain fiber length, there are tradeoffs among the sensing bandwidth, the sensitivity, and the spatial resolution. In this paper, the concept of linearization and Golay pulse-coding for heterodyne <inline-formula> <tex-math notation= LaTeX >${\\Phi }$ </tex-math></inline-formula>-OTDR are proposed and experimentally verified for the first time. First, we gave a full theoretical treatment on how an intensity-coded yet phase-retrieved <inline-formula> <tex-math notation= LaTeX >${\\Phi }$ </tex-math></inline-formula>-OTDR can be built up as a fully linear system, therefore a significant enhancement of signal-to-noise ratio becomes viable and the sensing bandwidth equals the four-times averaging case. Second in the proof-of-concept experiment, submeter gauge length and nano-strain resolution were realized with 10 km sensing range, in other words, more than ten thousand sensitive sensing units were realized along the fiber. This paper makes a significant step toward high-performance DAS with orders-of-magnitude performance enhancement.