Research On Vibration Location Method Of ?-OTDR Based On Amplitude Demodulation

The optical fiber distributed sensing system based on Phase Sensitive Optical Time Domain Reflectometer(?-OTDR)is one of the most commonly used distributed optical fiber sensors(DOFS).Since ?-OTDR is based on coherent Rayleigh backscattering and shows high sensitivity to external vibration,its sensing distance can reach hundreds of kilometers,making it a powerful tool in many important fields.At present,the ?-OTDR system usually adopts a single-path sensing structure,it should be pointed out that the intensity of the backscattered Rayleigh signal fluctuates randomly due to the time-domain intensity fluctuation of the detection pulse,laser phase noise,thermal noise of the detector,or random interference destructive effect,polarization disturbance,etc.,resulting in the change of the vibration signal.This will cause the probability of misjudgment of vibration positioning.Therefore,the experiment in this paper designs a dual-channel/single-channel high-dynamic response vibration detection and recognition system based on ?-OTDR.By optimizing the hardware structure and electrical signal processing algorithm of the ?-OTDR sensing system,the real-time monitoring of dynamic vehicle vibration signals on the highway and the positioning of the static pencil-break vibration in the laboratory are realized respectively.This paper designs a multi-path information fusion algorithm(MIFA)and an optimized amplitude difference accumulation algorithm(DAA)to improve the recognition accuracy of vibration positioning.The specific research content is as follows:(1)Study the basic principles and structural composition of a distributed optical fiber sensing system based on ?-OTDR,and derive the advantages of coherent detection technology and quadrature demodulation in the system.Combined with the mathematical model of the backward Rayleigh scattered light in the sensing fiber,the relationship between the phase and amplitude of the backward Rayleigh scattered light and the vibration of the external environment is analyzed,and the phase or amplitude is extracted to detect the external vibration signal.(2)Analyze the performance of the ?-OTDR system and design sensor systems with different structures,including the selection of system parameters and key components.(3)According to the different propagation speeds of light in different modes(or different channels)in the optical fiber,the electrical signals from the two channels are digitized at slightly different sampling rates,in order to ensure the space between the two sets of discrete data match.Using slightly different sampling rates for the two channels allows us to sample the signals of the two channels at equal spatial intervals.Any two elements with the same index in the two digitized data vectors correspond to the same sensing position.(4)Based on the dual-channel sensing scheme,a MIFA algorithm is proposed to process the vibration signal,and the fusion signal is used to reduce the probability of misjudgment of the interference position.Through these signals,we can determine whether there is vibration on the sensor link.Experimental results show that MIFA can identify the vibration caused by pencil-break and moving vehicles.MIFA has less dependence on the selection of k and N,the signal-to-noise ratio(SNR)is greatly improved.(5)The influence of the vibration frequency on the performance of the amplitude difference accumulation parameters(k,N and SNR)is obtained through simulation,and the optimal value(k,N)of the parameter corresponding to the specific vibration frequency signal is deduced within the detectable range: The best cut-off point is a vibration period corresponding to a specific frequency.For multi-vibration signals of different frequencies,the minimum dominant frequency is selected as the threshold to reduce the probability of misjudgment of multi-vibration recognition.