Based On Heterodyne Coherent Detection φ-OTDR Research On Vibration Signal Demodulation

Heterodyne coherent detection φ-OTDR is widely used in perimeter security,railway train tracking,oil and gas pipeline anti-digging,oil and gas exploration,and other fields.These applications require real-time monitoring of vibration signals.However,the signal bandwidth of the heterodyne coherent detection φ-OTDR system is relatively high,requiring a high sampling frequency to collect its signals,resulting in large data traffic.Traditional vibration signal demodulation systems usually adopt a FPGA + CPU architecture.The FPGA part is only responsible for collecting data,and the subsequent demodulation algorithm needs to be calculated by the CPU,resulting in a high computational latency and poor real-time performance.These factors limit the engineering application and promotion of the heterodyne coherent detection φ-OTDR system.The main work of this article is as follows:(1)A simple FPGA + ARM architecture and a simple and lightweight gigabit ethernet transmission method are used to complete data demodulation,calculation,and transmission.In the hardware part,this paper designs a high-precision analog-to-digital converter driver module responsible for driving the AD9643 chip with a maximum sampling rate of 250 MSPS and a sampling accuracy of 14 bits to complete the digital conversion of the analog signal output by the heterodyne coherent detection φ-OTDR system.(2)IQ demodulation,differential accumulation,and phase clipping algorithms are implemented on the FPGA side,accelerating data demodulation and calculation.By taking advantage of the efficient parallel computing features of FPGA,the vibration points are located in just 25 ms with a sampling frequency of 4 KHz.At the same time,phase data and differential accumulation curve data enter the ARM-side DDR synchronous dynamic random access memory through the DMA module.The phase data is unwrapped by the Cortex-A9 high-performance microprocessor to restore external vibrations.Differential accumulation curve and phase data communicate with the host computer through the gigabit Ethernet hardware port in real-time,with a communication traffic of 40 Mbps.(3)Developed heterodyne coherent detection φ-OTDR demodulation system upper computer.The PC host computer displays the differential accumulation curve,differential accumulation curve waterfall chart,and external vibration phase change data in real-time.The Web host computer is developed based on the open-source framework Cesium,which can load oblique photography models around the fiber layout position.When a vibration event occurs,the visualization platform developed based on Cesium displays the vibration location in real-time,providing engineers with an intuitive visualization tool.Finally,in the test of the differential coherent detection φ-OTDR demodulation system,the system’s phase amplitude response linearity is up to 0.9942,and the signal-to-noise ratio of the differential accumulation curve positioning of the 5 V voltage-driven PZT piezoelectric ceramic in the 9.7 Km long fiber reaches 13.5 dB.