Performance Enhancement Methods for the Distributed Acoustic Sensors Based on Frequency Division Multiplexing

Abstract

The last years have witnessed the wide application of Distributed Acoustic Sensor (DAS) systems in several fields, such as submarine cable monitoring, seismic wave detection, structural health monitoring, etc. Due to their distributed measurement ability and high sensitivity, DAS systems can be employed as a promising tool for the phase sensitive optical time domain reflectometry (Φ-OTDR). However, it is also well-known that the traditional Φ-OTDR system suffers from Rayleigh backscattering (RBS) fading effects, which induce dead zones in the measurement results. Worse still, in practice it is difficult to achieve the optimum matching between spatial resolution (SR) and signal to noise ratio (SNR). Further, the overall frequency response range (FRR) of the traditional Φ-OTDR is commonly limited by the length of the fiber in order to prevent RBS signals from overlapping with each other. Additionally, it is usually difficult to reconstruct high frequency vibration signals accurately for long range monitoring. Aiming at solving these problems, we introduce frequency division multiplexing (FDM) that makes it easier to improve the system performance with less system structure changes. We propose several novel Φ-OTDR schemes based on Frequency Division Multiplexing (FDM) technology to solve the above problems. Experimental results showed that the distortion induced by fading effects could be suppressed to 1.26%; when the SR of Φ-OTDR is consistent with the length of the vibration region, the SNR of the sensing system is improved by 3 dB compared to the average SNR with different SRs; vibration frequencies up to 440 kHz have been detected along 330 m artificial microstructures. Thus, the proposed sensing system offers a promising solution for the performance enhancement of DAS systems that could achieve high SNR, broadband FRR and dead zone-free measurements at the same time.

Research Progress in Distributed Acoustic Sensing Techniques

“…To ensure the desired outcome without sacrificing spatial resolution, in 2019, Zhang et al proposed a coherent fading suppression method based on frequency division multiplexing (FDM) Φ-OTDR to keep the signal distortion induced by coherent fading in the order of 10 −2 [44] (Figure 8). The beat frequency signal [44].…”

Phase-Sensitive Optical Time Domain Reflectometry With Rayleigh Enhanced Optical Fiber

“…Although, the proposed amplification technique makes the system more complex, expensive, and might impose additional noise to the system. Furthermore, various methods have been proposed and demonstrated, such as using multiple acoustic-optical modulators (AOMs) [9,10], the chirped pulse amplification method [11], frequency division multiplexing [12,13], and multi-frequency laser sources [14,15]. All the above proposed methods require additional expensive hardware components, which makes the system more complex.…”

Study of Intra-Chamber Processes in Solid Rocket Motors by Fiber Optic Sensors

“…The available data suggests the following requirement: an expected total measurement time of less than 10 seconds; typical oscillation frequencies of intra-chamber processes starting from tens of Hz, increasing to hundreds of kHz. In this case, optical time-domain reflectometry (OTDR) systems and devices based on FBG are insufficient due to the low sampling frequency of 30 kHz for phi-OTDR [ 42 , 43 , 44 ] and 10 kHz for FBG in configurations where there are a few sensors in one line [ 45 , 46 , 47 ]. In turn, the small SRM length and the requirement of less sensing points allows for the application of sensors based on the MZI.…”