Research And Application Of Long Distance φ-OTDR Based On Envelope Detection

Phase sensitive optical time domain reflectometry(φ-OTDR)uses the backward Rayleigh scattering light in optical fiber for vibration sensing,which has the advantages of long sensing distance,multi-point detection at the same time,anti-electromagnetic interference and so on.It has a broad application prospect in the fields of perimeter security,pipeline safety warning,transmission line safety monitoring and so on.Among various technical methods for realizing long-distance φ-OTDR sensing,coherent detection uses local oscillator light to amplify the scattered signal to increase the sensing distance,which has the advantage of high signal-to-noise ratio.But because this method needs to use the digital demodulation method to carry on the high speed signal demodulation,the high performance digital demodulation equipment increases the system cost.In this paper,the envelope detection circuit is adopted to realize the heterodyne coherent demodulation of the intensity information of the-OTDR system,which can effectively reduce the system cost while ensuring the long-distance real-time vibration sensing.The main work of this paper is as follows:(1)After introducing the application basis and sensing principle of φ-OTDR vibration sensing system in the fields of long-distance pipeline safety detection,transmission line health status monitoring,perimeter security and so on,the basic principles of heterodyne detection and direct detection are analyzed theoretically,after an in-depth analysis of the power spectral density of the backscatter signal and its influence on the filter parameters of the envelope detection circuit,And with the help of heterodyne detection method with higher dynamic range and SNR,combined with envelope detection circuit to demodulate the heterodyne signal in real time,theφ-OTDR vibration sensing system can meet the requirements of long distance,low cost and real-time performance.(2)Based on the theoretical analysis of the relationship between the scattered signal bandwidth and the envelope detection low-pass filter parameters,the influence of the envelope detection filter parameters on the envelope output waveform is tested.It is verified that the detection circuit carries the signal from the carrier frequency to the baseband in the frequency domain.The φ-OTDR vibration detection experiment platform was built by using the envelope detection circuit.The contrast experiment between the direct detection and the envelope detection heterodyne detection showed that the envelope detection method proposed in this paper had higher SNR than the direct detection under different pulse widths.The envelope detection heterodyne detection is further applied to the long-distance φ-OTDR vibration sensing system using Raman amplification technology.The experimental results show that the envelope detection method improves the SNR by 10 d B compared with the direct detection method.(3)The envelope detection φ-OTDR vibration sensing system was used to perform intrusion detection experiments,which verified the real-time detection and positioning capability of the sensor system for intrusion events.Three kinds of signals including personal intrusion,knocking and vehicle driving are collected in the experimental site.According to the characteristics of the disturbance signal,the Short-Time Fourier Transform is used to extract the time-frequency characteristics of the vibration signal.The principal component analysis method is used to reduce the dimensionality of the time-frequency feature map..KNN algorithm is used to classify and recognize the three kinds of events,and the average recognition rate can reach up to 90%.