FPGA Implement Of Demodulation Algorithm For Fizeau Interferometric Fiber Optic Hydrophone

With the development of the distributed optical fiber sensor in the field of underwater acoustic,the requirement of high-speed and real-time demodulation system is becoming higher and higher.For the fiber-optic underwater acoustic 3×3demodulation algorithm based on Fizeau interference,since most of the data in the process of signal acquisition is invalid data that has not yet been peak seeking,the real signal sampling frequency is only the repetition frequency of the acoustic-optic modulator,so the system needs to acquire more than tens of times of the total data amount of the effective signal when demodulating the fixed-length signal.If using the method of collecting data at first and then demodulating the data off-line on the upper computer,the demodulating data processing will be slow and cannot be demodulated in real-time.Therefore,in this paper,the 3×3-arctangent underwater acoustic demodulation algorithm is implemented in FPGA,the demodulation system is designed by hardware description language,and the demodulation algorithm is optimized by three aspects: high-speed signal processing method,cross clock domain signal processing method and special function implementation method.And the phase compensation algorithm is realized by using FSM to correct the inverted signal.The logic delay of the system is reduced while the underwater acoustic signal is correctly demodulated,so that the system can reach a faster clock rate and realize the high-speed real-time demodulating of the underwater acoustic signal.The main research contents of this project are as follows:(1)This paper introduces the development course and current situation of FPGA and FBG hydrophone demodulation technology,analyzes the principle,advantages and disadvantages of three kinds of FBG hydrophone demodulation technology,and determines the research direction of this paper.(2)Based on the structure and working principle of FPGA,this paper introduces three optimization methods of demodulation algorithm based on high-speed signal processing,cross clock domain signal processing and special function realization.Use pipeline to segment complex logic,implement the algorithm of phase compensation using the finite state machine based on the one hot code FSM,and the arctangent function is implemented in FPGA by the IP core addressing method,which improves the system clock frequency and ensures the high-speed demodulation of the system.(3)Introduce the principle of 3×3-arctangent demodulation algorithm,analyzes the requirement of Fizeau interference hydrophone demodulation system based on FPGA.The phase compensation algorithm is designed with finite state machine based on one hot code,the logic module of demodulation system is designed with Verilog language,and the ADC module,peak seeking module,demodulation algorithm module,phase compensation module,FIFO module and PCIe transmission module are introduced.(4)The experimental system of Fizeau interference hydrophone array is introduced and built.The feasibility of the demodulation system based on FPGA is confirmed by comparing the model SIM test with the simulation of Lab VIEW.The experimental research based on vibration liquid column method is carried out and the detection in time and frequency domain of underwater acoustic signal is realized.The results show that the system can not only demodulate the signal correctly,but also achieve a signal-to-noise ratio of up to 14.2 d B,at the same time,it can realize the highspeed real-time of the system,and the system clock can reach up to 106.326 MHz.