Research On Digital Demodulation Technology Of Phase Sensitive Optical Time Domain Reflectometer

Distributed optical fiber sensing technology has been widely used in perimeter security,structural health detection,seismic wave detection and ocean monitoring and other fields because of its high sensitivity,anti-electromagnetic interference,and long-distance continuous detection.Among them,the distributed optical fiber sensing system of phase sensitive optical time domain reflectometer(φ-OTDR)is particularly sensitive to the phase information of the vibration signal that cause the system become a research hotspot of the distributed optical fiber sensing system.The phase demodulation plays a key role in the waveform recovery of distributed optical fiber acoustic sensor signals.For most of the existing demodulation methods employ hardware demodulation components to obtain phase change,which increased the cost and complexity of the system.Therefore,this paper mainly investigates the development status of distributed optical fiber sensing systems.Then a new fully digital demodulation scheme is designed and proposed,and the feasibility and accuracy of the scheme are verified through theoretical analysis and experiments.After that,a φ-OTDR system was integrated and a variety of signal tests were carried out by using the system.The specific work is as follows:1)The I/Q quadrature phase demodulation scheme is simulated by Lab VIEW which simulate by using orthogonal transformation,low-pass filtering,arctangent and other methods.Then analyze the existing problem in the analog quadrature demodulation and establish a foundation for proposing a new digital phase demodulation scheme.2)A digital phase demodulation scheme of φ-OTDR system based on the crosscoherence of RBS beat signals is proposed.By utilizing simulated coherence between different back-scattering beat signals and performing Hilbert transform on the collected beat signals.The other two adjacent pulses are simulated to form quadrature signals with a certain phase difference,and the digital cross-coherence method is used to extract the phase.The smoothing and differential methods are used to optimize the phase demodulation process.After that,a φ-OTDR system was built for experimental testing,and the signal waveform was recovered with high quality,reaching a sensing distance of 10 km and a spatial resolution of 10 m.The feasibility of the demodulation scheme and the accuracy of the demodulation information are verified,and it is proved that the scheme can simplify the complexity of the system and reduce the cost.3)A set of distributed optical fiber acoustic sensing system based on φ-OTDR is further integrated.Then the vibration signal was applied by PZT,the performance of the system was tested,and a variety of simulated vibration tests were carried out.The integrated system is used to detect the actual simulated intrusion signal,and the actual intrusion signal can be recovered well after phase demodulation.On this basis,the typical simulated intrusion signal is decomposed by EMD,and the law of the decomposed intrusion signal is analyzed,which lays a foundation for the establishment of pattern recognition.