Research On Phase Demodulation System Of Real-time Optical Fiber Sensor Network Based On FPGA

Optical fiber sensing technology has gradually become an important technical support for perimeter security,energy exploration and other fields due to its characteristics of high sensitivity and strong anti-interference ability.This project relies on the high-speed railway perimeter security system project to break through the present bottleneck problems such as the increase of modulation complexity and slow signal demodulation speed after the large-scale networking of optical fiber sensing technology,and focuses on the research of real-time phase demodulation system based on off-the-rack programmable logic device(FPGA).Aiming at the time-division multiplexing fiber optic sensor network based on fiber Bragg grating sensor(FBG),this paper designs a binary rectangular pulse phase modulation scheme optimized by time sequence,which can ensure the correct modulation phase for each fiber Bragg grating sensor unit.For fiber optic sensor networks,determining the delay information(or time window)of each fiber optic sensor unit is the prerequisite for the correct demodulation of sensing signals.However,traditional methods can only be obtained by manual calibration,and the implementation process is time-consuming and laborious.This paper proposes an automatic delay information acquisition algorithm based on shape function and cross correlation operation,which greatly reduces the complexity of system implementation.Finally,this paper proposes a hardware phase demodulation module of pipeline structure optimization with low output delay,which is used to calculate the signal of fiber optic sensor in each time window and greatly improve the real-time performance of signal acquisition.Specifically,the work of this paper includes: optical structure,design of a low-cost compact FBG fiber optic sensor network.The binary rectangular pulse phase modulation method is used for phase modulation.By reasonably designing the modulation signal timing sequence,the ?/2 phase-modulated pulse optical signal is generated,which interferes with each other after being reflected by the grating to form interference signal sequences respectively modulated by ?/2,0,-?/2 phase modulation.and then the signal phase value can be obtained.In the aspect of electrical phase modulation system,it mainly realizes the uploading of original optical intensity signal,automatic calculation of delay value,automatic configuration of delay data parameterization,channel switching of multiple sensors optical intensity data and hardware phase unpacking operation with low output delay.When the system works,As soon as the USB communication interface receives the command of the search time window,a certain amount of raw light intensity data and reference pulse signal(16KB)are sent to the upper computer.In the upper computer,the automatic delay information acquisition algorithm proposed in this paper is used to distinguish the signal peak and window peak in the modulation optical signal,determine the delay information required by each time window,and include the information in the command and send it to the FPGA.Analyze the command in FPGA,configure the delay value into the module used to filter the modulation signal light intensity,and verify whether the three-way light intensity is orthogonal according to the Lissajous-Figures drawn by using the filtered light intensity signal,that is,whether the light intensity information is extracted correctly.Finally,the outcome of the delay of hardware phase demodulation module of pipeline structure optimization,which only needs 9 clock cycles,is used to obtain the sensing phase value,and the external intrusion signal can be judged according to the change of the phase value.In this paper,a lot of simulation and real-time debugging are carried out on the FPGA hardware platform for the unpacking process of phase calculation.Debugging results show that the system has a good performance and can quickly and accurately complete a series of tasks,such as obtaining light intensity value,parameter configuration of delay value,filtering light intensity value of modulation position,phase calculation and unpacking.It provides an efficient,real-time and configurable implementation scheme for optical fiber sensor network system.