Intensity interferometry distributed acoustic/vibration sensing

Abstract This paper introduces an innovative type of distributed acoustic sensing (DAS) system. Instead of employing an ultra-coherent laser source along with an external modulator, it adopts a broadband laser with direct modulation. The system operates by electronically subtracting Rayleigh backscattered signals, which are collected by probing the sensing fiber with two separately delayed versions of the same pulse. When there is no external disturbance, the system’s output is essentially zero. However, a disturbance at a specific point along the fiber causes a variance between two sequential backscatter signals within a time frame that corresponds to the location of the disturbance. Utilizing the same pulse for two successive interrogations of the sensing fiber permits the use of a broadband laser and direct modulation. Additionally, employing the broadband laser alongside electronic detection of disturbances markedly reduces the bandwidth requirements of the system’s data acquisition, making this DAS configuration significantly simpler and more cost-effective than conventional systems. The primary limitations of the novel method include a significant dependency of its response on frequency and a lack of linearity in relation to the measured perturbations. This paper details the system along with both an experimental and numerical assessment of its performance. The system effectively captures perturbations between 50 and 9500 Hz, achieving a maximum sensitivity of 10 p ϵ / Hz .