2. State Key Laboratory of Safety and Control for Chemicals, SINOPEC Research Institute of Safety Engineering, Qingdao, Shandong 266000, China
Overview: Distributed fiber sensing system can realize long-distance and continuous measurement with a tremendous potential of applications to the fields such as perimeter security, pipeline monitoring and structural health diagnosis for large infrastructure, whose faults or intrusions constantly cause changes of multiple physical parameters, namely vibration, strain and temperature. In addition, the alert and location abilities are also determined by the frequency response range of vibration and the dynamic response ability of strain, which are critical to obtain full information of external events. According to recent research progress in distributed fiber sensing system, wide-frequency vibration measurement based on Rayleigh scattering, dynamic strain measurement based on Brillouin scattering and multi-parameter measurement based on multiple scattering mechanisms are proposed, respectively.
Distributed vibration sensing system based on the combination of Mach-Zehnder interferometer (MZI) and φ-OTDR can realize wide frequency response range and high-precision location. In order to solve the trade-off between the highest frequency response range and signal to noise ratio of location signal, the time-division multiplexing technology based on the merits of MZI and φ-OTDR is proposed. In addition, frequency-division multiplexing technology based on φ-OTDR system can break through the limitation of sensing distance on frequency response range. However, it poses considerable difficulties in realizing the dynamic measurement of vibration-induced strain with large strain range in conventional φ-OTDR system. Through fixing the frequency of probe light at the half height of Brillouin gain spectrum (BGS), slope-assisted technology based on Brillouin optical time domain analysis (BOTDA) system can avoid excessive time of sweeping frequency and improve the dynamic response ability. Compared with slope-assisted technology, the fast BOTDA technology is proposed to improves the dynamic response ability without shortening the dynamic range. Above all, the OTDR system based on Rayleigh scattering and spontaneous Raman scattering has been used to measure vibration and temperature along sensing fiber. Interestingly, the distributed fiber sensing system by integrating φ-OTDR and Brillouin optical time domain reflectometry is proposed for simultaneous multi-parameter detection, including vibration, strain and temperature.