Interference Fading Suppression Based On φ-OTDR

As one of the typical distributed optical fiber sensing technologies,phase-sensitive optical time domain reflectometer(φ-OTDR)has the characteristics of distributed sensing,fast response speed,and high sensing accuracy.It has been widely used in perimeter security,pipeline safety monitoring,geophysical exploration,underwater cable detection and other fields.The monitoring performance of φ-OTDR in different areas will be affected by interference fading effect,resulting in false positives and false positives.Therefore,it is important to study the suppression methods of interference fading in φ-OTDR.Currently,the main solution to suppress interference fading is to overlay multiple unrelated or low-correlation degrees of freedom signals.In this paper,the interference fading suppression method of φ-OTDR is mainly studied from two aspects.The first one is to obtain multi-degree-of-freedom signals without increasing system complexity.The second is to analyze and use three interference fading suppression algorithms to overlay multi-frequency degrees of freedom signals to suppress interference fading,and to analyze the interference fading suppression effect of each algorithm.The specific content of this paper is arranged as follows:(1)The related theory of φ-OTDR is studied,Rayleigh scattering in optical fibers is introduced,the sensing principle and vibration detection principle of φ-OTDR are analyzed,and the generation mechanism and influence of interference fading phenomenon are explained.(2)The coherent detection principle of φ-OTDR system and the orthogonal demodulation method for obtaining the amplitude and phase information along the sensing optical fiber are introduced.The feasibility of extracting the spectrum from the pulse as a multi-frequency degree of freedom is analyzed.After obtaining the beat signal in the experiment,the multi-frequency degree of freedom signal is obtained by extracting the spectrum band from the pulse in the digital domain.(3)Based on the difference of the amplitude distribution of multifrequency degrees of freedom,the rotation-vector-sum(RVS)is used to make full use of the amplitude and phase information of multifrequency degrees of freedom to suppress interference fading.The experimental results show that the ratio of interference fading points to suppression of interference fading is 5.36% and the phase hopping rate is 0.55% by using the RVS.By comparing the beating signals,it is found that the interference fading can be suppressed significantly when RVS is used.(4)Since the effect of phase noise on suppression of interference fading is not taken into account when using the RVS,it is optimized by using the dual rotation-vector-sum(dual RVS).The dual RVS adds time-domain rotation-vector-sum to the RVS to suppress phase noise.The experimental results show that the ratio of interference fading points using the dual RVS is reduced to 5.14% and the phase hopping rate is reduced to 0.07%.(5)In the experimental verification,it is found that the interference fading suppression effect using the dual RVS is unstable,and additional abnormal phase hopping and small phase hopping will occur.Based on this phenomenon,the amplitude-tracking RVS method is proposed to suppress interference fading.Multiple experimental results show that using the amplitude-tracking RVS can reduce the phase hopping rate more effectively than using the dual RVS,and can also effectively suppress the abnormal phase hopping phenomenon and small phase hopping phenomenon introduced by the dual RVS.