| Distributed optical fiber sensing technology,with its unique advantages of integrating transmission and sensing,can realize distributed detection in a long distance and a large range,and has the characteristics of anti-electromagnetic interference,intrinsic safety,and high sensitivity.It shows broad application prospects in the fields of oil and gas pipeline leakage,structural health monitoring,and cable condition monitoring.As a typical distributed optical fiber vibration sensing technology,the phase-sensitive optical time domain reflectometer(Φ-OTDR)uses the backscattered Rayleigh light to sense the external vibration signal,which can realize long-distance vibration detection,and has attracted extensive attention and research in the academic community.Coherent detection Φ-OTDR(COTDR)system introduces local light to improve the signal-to-noise ratio and sensitivity,and realizes the quantitative detection of vibration signals by processing the beating signal generated by local signal and backscattered Rayleigh signal.However,due to the extremely long coherence of light source,there is interference fading noise in the system,which drastically deteriorates the phase demodulation result and causes the reconstructed vibration signal to be distorted.False alarms and omissions caused by interference fading noise are the bottlenecks for COTDR system to be applied in practical engineering.Therefore,the suppression of interference fading noise is of great significance for COTDR system towards engineering applications.In this dissertation,through the study of COTDR system,based on the principle of acousto-optic frequency shifting,an interference fading suppression method based on parallel acousto-optic modulators is proposed.In addition,based on the principle of phase modulator,combined with multi-frequency detection technology and auxiliary interferometer feedback structure,an active frequency transformation method with auxiliary interferometer feedback for interference fading suppression is proposed.The research content mainly includes the following three aspects:Firstly,starting from the mechanism of Rayleigh scattering,the principle of COTDR optical fiber vibration sensing system is studied,and the parameters of main optical instruments are introduced.The interference fading effect and its influencing factors in COTDR system are analyzed,the phase demodulation method based on quadrature demodulation is studied,and the influence of interference fading on phase demodulation is expounded.Secondly,based on the principle of acousto-optic frequency shifting,an interference fading suppression method based on parallel acousto-optic modulators is proposed.The simulation analyses of single-frequency and multi-frequency COTDR system show that the intensity distribution of backscattered Rayleigh light with different frequencies is different,and interference fading can be suppressed by signal aggregation processing.A COTDR system based on parallel acousto-optic modulators is then designed.By analyzing the experimental results,it can achieve 98.1% suppression of interference fading points.Thirdly,based on the principle of multi-frequency signal generation by phase modulator,the advantages and limitations of traditional multi-frequency detection to suppress interference fading are analyzed,and an active frequency transformation method with auxiliary interferometer feedback for interference fading suppression is proposed.Whether the signal is distorted is identified through the auxiliary interferometer feedback structure,and the correlation coefficient of vibration signal detected by auxiliary interferometer and COTDR is used as the feedback variable for active frequency transformation.After experimental verification,the scheme can achieve high-fidelity restoration of vibration signals through active frequency transformation on the basis of reducing the probability of interference fading to 1.58%.In addition,the method is still effective for accurate restoration of multi-point vibration signals with the help of blind signal separation technology. |
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