A Distributed Acoustic Sensor Based on Dual-Sagnac Interferometer with Counter Loops

We propose a new scheme of a fiber optic distributed acoustic sensor based on two same-length loops of a Sagnac interferometer formed with wavelength division multiplexing, which makes the sensor compact, easy to mount along a perimeter, and both suitable when ring and straight line cable laying application is required. We also describe a dual Sagnac interferometer based sensor operation principle, which allows a disturbance localization by estimation of a time delay, occurring between signals from two loops. An issue of assembling in a single cable an interferometer based sensor systems is studied as an approach of a phase difference occurring between clockwise and counter-clockwise light directions in a Sagnac interferometer loop. A scheme for investigation of the light phase incursion due to disturbance in two directions in the loop is proposed and mathematically substantiated. A potential of implementing the proposed sensor based on Sagnac interferometer with wavelength division multiplexing in a single cable has been proven experimentally.