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.
Highlights
In the past decades, the development of optical fiber communication technology has promoted the development of the telecom industry and information industry
When a vibration event occurs to the fiber, Rayleigh backscattered trace (RBS) amplitude variation will be modulated at the corresponding location
The principle of the Distributed Acoustic Sensor (DAS) system is described in detail, that is, the amplitude and phase changes of the vibration event modulation are obtained by heterodyne coherence measurements, and the detailed information of the vibration event is further analyzed
Summary
The development of optical fiber communication technology has promoted the development of the telecom industry and information industry. Different from conventional point sensors, the phase sensitive optical time domain reflectometry (Φ-OTDR) is a practical and effective technology for vibration detection applications, which can monitor vibration events occurring along the fiber [1,2,3]. One of the first methods to overcome fading was the frequency hopping technique using a laser source, which changed the laser frequency gradually by changing the temperature and the backscattered signal integration This technique could successfully reduce the amplitude fluctuation, it requires thousands or even tens of thousands of averages, making it unsuitable for real-time applications [7,8]. In the case of seismic waves, it does not require long sensing times, and the monitored disturbances last only a few seconds They pay more attention to the suppression of the fading effect of the amplitude, and have not discussed the phase signal behavior in the fading region [5]. SR and extension suppression of response.of fading, optimize measurement by adjusted SR and extension of response
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