| Distributed dynamic strain fiber sensing is a sensing scheme that utilizes light transmitting in an optical fiber as a carrier to detect the external disturbance.Compared with the existing mechanical and electrical detection solutions,optical fiber sensor technology solution has the advantage of anti-electromagnetic interference,long-distance detection range,and wide frequency bandwidth and so on.At present,the research of distributed dynamic strain detection schemes mainly focus on Rayleigh scattering and Brillouin scattering detection schemes.Phase-OTDR and OFDR,which are both based on the Rayleigh backscattering scheme,have more application potential and research significance in taking account of the fast response,high sensitivity and appropriate spatial resolution.These two kinds of technical solutions are separately studied in depth respectively in this paper: Phase sensitive optical time domain reflectometer(Phase-OTDR)based on Rayleigh scattering and Phase sensitive optical frequency domain reflectometer(Phase-OFDR)based on Rayleigh scattering.The main works in this paper are as follows:1.The detection principle and system architecture of Phase-OTDR sensor system are introduced and illustrated in details.In view of the fact that the specific mechanism and parameter requirements for the effect of the linewidth of the laser on the sensing performance in the current study are not clear enough,a theoretical model is established,which takes account of the coherent detection of Rayleigh scattering and the linewidth of the laser source,to focus on in-depth analysis the relationship between the coherence of the laser and signal to noise ratio of sensing system.The theoretical analysis results show that the linewidth has a great influence on the SNR of the sensor.With the widening of the linewidth,the detection signal-to-noise ratio will be exponentially attenuated.As the linewidth increases from 2 kHz to 1 MHz,the signal to noise ratio of the system detection will decrease by about 4.3 dB.Besides,the relevant linewidth factor experiment that is performed on the Phase-OTDR platform shows that when the linewidth of laser source widens from 2 kHz to 1 MHz,the experimental detection signal-to-noise ratio decreases by 4.1 dB,which the experimental and theoretical analysis results agree with each other.2.A real-time sensor prototype based on Phase-OTDR is designed and set up,which the optical structure and signal data processing flow are introduced as well.After the guidance of the linewidth factor experimental results,a 2 kHz lindwidth laser is used as the light source of the sensing system.Optical coherent detection scheme is used in prototype to achieve the demodulation of vibration's amplitude and phase information.Parallel processing data analysis algorithms is applied to achieve distributed rapid monitoring and real-time analysis of dynamic strain as well.Build a dynamic strain test platform and test the entire sensor system prototype.The experimental results show that: the sensing range is 5 km and spatial resolution is about 5 m,which the signal to noise ratio of the system is 36 dB and the frequency respond range is 1500 Hz.3.A novel Phase-OFDR sensing system based on phase direct demodulation scheme is proposed.This scheme can achieve phase information direct demodulation through 3×3 coupler and the differential and multiplication demodulation algorithm and realize direct detection of external dynamic strain with high spatial resolution.The influence of the instability of the laser source can be avoided as well.Relevant experiments show that: Phase-OFDR scheme can detect and distinguish several millimeter-level breakpoints.It can achieve 10 cm spatial resolution and 100 Hz dynamic strain detection on a 200m-long sensing fiber.It also has the function of static strain monitoring,which the strain sensing sensitivity reaches 1 με. |
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