| Optical fiber sensing technology has achieved rapid development in recent years.The use of optical fiber as a sensing and transmission medium can realize the measurement of physical quantities on the entire optical fiber.In this paper,the distributed optical fiber vibration sensing system based on ?-OTDR are studied,which has lower cost,electromagnetic interference resistance,corrosion resistance,high temperature resistant,etc.It has a wide range of applications in resource exploration,oil and gas pipeline safety monitoring,weak acoustic signal detection,classification,perimeter security threat,train real-time positioning,military defense,and other fields.Vibration signal positioning and phase demodulation technology are indispensable for this system,among which the quadrature demodulation method is one of the commonly used phase demodulation methods,which can reduce the hardware cost of the system by using a computer,and the structure is simpler.In this paper,the positioning algorithm and phase demodulation algorithm have been studied in detail.The ?-OTDR system built has a high positioning signal-to-noise ratio and a large signal demodulation range.The highest positioning signal-to-noise ratio can reach 26.3d B,and the lowest it can realize the positioning of 1?Hz vibration signals.At the same time,it can realize the restoration of 5-2500 Hz vibration signal waveform.The vibration signal intensity has a good linear relationship with the phase change amplitude,and the linear fitting coefficient is 0.9974.The main contents of this paper are as follows:1.This article introduces several common distributed optical fiber vibration sensing systems,and focuses on the research progress,application and prospects of distributed optical fiber vibration sensing systems based on ?-OTDR.2.The principle of Rayleigh scattering is introduced in detail.The mechanism of optical fiber vibration sensing is analyzed in detail by formula derivation.After introducing the principle of OTDR,on the basis of its principle,through the establishment of the mathematical model of Rayleigh backscattering light the principle of ?-OTDR system are introduced,and the main parameters of the ?-OTDR system and the corresponding optimization methods are explained.3.A distributed optical fiber vibration sensing system based on ?-OTDR was built,and some key equipment selected for the system is introduced.The digital quadrature demodulation algorithm is used to obtain a pair of orthogonal signals,and then the arc tangent algorithm is used to obtain the phase information.The specific operation of the range expansion of the arc tangent result after orthogonal demodulation is explained,and the process and necessity of two phase unwrapping are theoretically analyzed.4.Introduced the use of Lab VIEW,including the configuration of the acquisition card,the moving average and moving differential positioning algorithm and the implementation process of the subsequent phase demodulation algorithm,and the corresponding experimental results were explained.Use the method of eliminating the initial phase interference to eliminate the phase noise of the laser and the acousto-optic modulator in the system.A relocation method is proposed,which is to use the phase change after the peak of the phase difference curve of the vibration position to restore the vibration waveform.The piezoelectric ceramics driven by the arbitrary waveform generator are loaded on the end of the sensing fiber to simulate external vibration.Through experiments,the positioning and waveform restoration of vibration signals at different frequencies have been realized,and the necessity of two phase unwrapping in the phase extraction process is explained.The linear relationship between the amplitude of the phase change and the voltage applied by the piezoelectric ceramic is verified.The "producer/consumer" design pattern in Lab VIEW is used to realize the real-time positioning of the vibration signal and build the host computer interface. |
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