Research On Distributed Optical Fiber Vibration Sensor Based On Combination Of Interference And Back Rayleigh Scattering Technology

With the continuous improvement of my country’s comprehensive national strength,all walks of life have stricter safety requirements.As an important indicator of safety detection,vibration is used in the fields of structural health monitoring of large buildings,perimeter security,earthquake monitoring,and underwater sound signal detection.The application of vibration sensors effectively protects the safety of people’s lives and property.However,as the detection environment becomes more and more complex,traditional piezoelectric vibration sensors cannot meet the current complex application scenarios.Sensors using optical fiber as the sensing medium and transmission medium have been widely studied and applied because of their small size,corrosion resistance,electromagnetic interference resistance,easy reuse,and high detection sensitivity.In this dissertation,a series of researches on optical fiber vibration sensors based on interference type,back-Rayleigh scattering type and fusion of interference and backRayleigh scattering are carried out.The main contents are as follows:(1)A large dynamic range heterodyne interferometric fiber optic vibration sensor based on undersampling technology is proposed.According to the relationship between the pulse repetition frequency and the heterodyne frequency,the heterodyne interferometric fiber optic vibration sensor is divided into an interferometric fiber optic vibration sensor with a small heterodyne frequency and an interferometric fiber optic vibration sensor with a large heterodyne frequency.The limiting factors of their upper dynamic range are systematically discussed.The application of undersampling technology to the interferometric fiber optic vibration sensor with large heterodyne frequency can not only ensure its large dynamic range,but also reduce the sampling rate of the data acquisition equipment,thereby reducing the system cost and reducing the burden of subsequent digital signal transmission and processing.It is beneficial to realize real-time demodulation of vibration signal.A heterodyne optical fiber vibration sensing system with a pulse repetition frequency of 200 k Hz and a heterodyne frequency of 10 MHz was developed in the laboratory.With the proposed undersampling scheme,the sampling rate of the analog-to-digital converter(ADC)is reduced from 50 MSa/s to 3MSa/s.Compared with normal sampling,the amount of data is reduced to 6% of normal sampling.The experimental results show that the upper limit of the dynamic range of the large heterodyne frequency interferometric fiber optic vibration sensor based on the undersampling technology can reach 71.6 rad at 1 k Hz and 37.7 rad at 2 k Hz,and the maximum uncertainty of demodulation is less than 0.062 %.(2)Aiming at the problems of small measurement bandwidth and low spatial resolution of the current backscattering distributed optical fiber vibration sensor,based on frequency division multiplexing and I/Q receiving technology,a distributed vibration sensing system combining Mach-Zehnder interferometer(MZI)and time-gated digital optical frequency domain reflectometry(TGD-OFDR)is proposed.Among them,the phase of the MZI output signal is demodulated by the homodyne method to detect the frequency and amplitude of the vibration signal;while the TGD-OFDR is used to locate the vibration signal through the heterodyne detection technology.The system has both high frequency response and high spatial resolution multi-point positioning capability.The numerical simulation results show that the upper limit of the measurable vibration frequency of the proposed system can reach the MHz order,and the spatial resolution on the 4 km sensing fiber can reach 0.5 m.(3)A forward transmission distributed optical fiber vibration sensing system based on heterodyne detection is proposed,which can locate broadband vibration signals.Through theoretical analysis and numerical simulation,the positioning ability of the system for broadband vibration signals is verified,and the precise positioning of broadband vibration signals can be realized on the 100 km sensing fiber.