Lumped Rayleigh reflectors.

Abstract

Distributed acoustic sensing (DAS) via optical fibers makes use of Rayleigh backscattering for the detection of acoustic waves that interact with the fiber along its entire length. The random nature of Rayleigh backscattering leads to nonuniform performance along the fiber and, occasionally, to complete signal fading. In addition, distance-dependent signal-to-noise (SNR) degradation is always present due to propagation loss. In contrast, using arrays of discrete reflectors [such as weak fiber Bragg gratings (FBGs) with equal center wavelengths] offers deterministic performance which can be designed to be uniform along the fiber. Here we describe an approach for implementing Rayleigh-based discrete reflectors that can offer enhanced detection performance in selected regions. It is based on enclosing sections of the fiber in acoustically insulated boxes to create lumped Rayleigh reflectors. Besides diminishing the randomness in detection sensitivity, the method enables increasing the detection SNR far beyond the typical value for Rayleigh-based DAS and obtaining sensitivities comparable with discrete reflectors. The proposed method was successfully tested via both simulation and experiment.