Phase Demodulation ?-OTDR Sensing System Data Acquisition And Processing Key Technology Research

In the past 20 years,china has gradually improved the construction of large-scale infrastructure,such as tunnels,highways,railways,power and oil and gas pipelines.During long-term operation,these large-scale infrastructures are vulnerable to natural disasters,man-made damage,and material aging,and accidents such as tunnel collapse,track damage,damaged cables,broken cables,and pipeline ruptures have seriously affected people's lives and industrial production.Therefore,there is an urgent need for a monitoring technology that can cover these large-scale infrastructures,grasp their structural health,and issue timely and accurate warnings before accidents so as to take countermeasures.The phase-sensitive optical time domain reflectometer(?-OTDR)has the advantages of distributed,long detection distance,high vibration sensitivity and fast response speed.It can use optical fiber as a sensor to monitor the vibration along the line in real time.Information such as frequency determines the type of vibration and plays a role of early warning.As the core component of the ?-OTDR,the data acquisition and processing system directly affects the monitoring performance of the system.In its specific implementation,the following technical difficulties remain to be solved: The phase demodulation type ?-OTDR uses the structure of heterodyne coherent detection to obtain an intermediate frequency comparison.High narrowband signal.When data acquisition,analog down-conversion technology is usually used to move the intermediate frequency signal to the baseband to reduce the requirement on the sampling rate,but it will introduce noise at the analog end and reduce the signal-tonoise ratio of the signal.If Nyquist sampling is directly performed on the intermediate frequency signal,although the deterioration of the signal-to-noise ratio due to analog down-conversion is avoided,the sampling rate is too high at this time,and the amount of data that the system needs to process in real time is extremely large.In addition,traditional ?-OTDR devices mostly use the CPU architecture represented by x86/ARM to complete calculations.When faced with massive amounts of data,the demodulation rate is too slow,resulting in poor real-time performance of the device.In addition,highperformance CPUs need to be matched with the same high-performance motherboards,memory sticks,solid state drives,and computer power supplies,which is not conducive to the miniaturization and low-power design of the equipment.The coherent effect of Rayleigh Backscattering(RBS)in ?-OTDR will cause the problem of fading,causing the RBS intensity of some areas on the fiber to be close to zero,forming a fading dead zone.When the signal is in the dead zone of fading,the result of phase demodulation will be greatly distorted,so relying solely on phase data will cause phase discrimination errors.An effective method to suppress coherent fading is to dynamically select the position with the largest RBS amplitude as a reference for phase discrimination,so as to achieve high-fidelity reconstruction of the vibration signal.This paper designs a special data acquisition and processing system for the phasedemodulation type ?-OTDR.Through the clever band-pass sampling design,not only can the analog-to-digital conversion chip(ADC)completely characterize the amplitude and phase information of the intermediate frequency signal at a sampling rate much lower than the Nyquist limit,but also make the secondary IQ demodulation Image interference is easier to suppress.Use FPGA to accept real-time data from ADC,mix the original signal with IQ digital local oscillator through a high-bit-width complex multiplier,and filter out high-frequency signals while down-sampling through a 3-stage5-fold decimation filter,and pass Cordic algorithm The square root operation and arctangent operation are completed in parallel,and finally the synchronized amplitude and phase demodulation data is output through PCIe.The experimental results show that the data acquisition and processing scheme given in this article can run stably at a sampling rate of 250 MHz,demodulate the amplitude and phase of the original ?-OTDR signal in real time,and the throughput rate reaches 4000 Mbit/s.The overall power consumption of the FPGA is only 6.5 W.The implementation method proposed in this paper can greatly improve the data processing capacity of the ?-OTDR system,and provide a reliable technical approach for the realization of miniaturized,low-power,and high-real-time instruments.It is expected to promote the use of ?-OTDR in large-scale infrastructure structures.Application in the field of health monitoring.