Research On High-speed Demodulation System Based On FPGA And Low-loss Jammed-array Wideband Sawtooth Filter

With the development of optical fiber sensing technology,fiber grating sensing is drawing more and more attention in recent years.Compared with traditional sensors,Fiber Bragg Grating(FBG)sensor has better performances in terms of high sensitivity,corrosion resistance,anti-electromagnetic interference,small size,and so on.Wavelength demodulation is the key technology in FBG sensing system.To some extent,the high-speed wavelength demodulation technology with high precision and low cost can greatly promote the large-scale use of FBG sensing system.The purpose of this paper is to design and validate a high-speed FBG demodulation system with the characteristics of larger dynamic range,higher sensitivity and lower cost.This paper gives a brief introduction to several typical fiber grating demodulation schemes.Furtherly,a high-speed demodulation system is designed and demonstrated,which is on the basis of Field Programmable Gate Array(FPGA)and low-loss jammed-array wideband sawtooth(JAWS)filter.The designed system uses the low-loss JAWS filter as the demodulation method and FPGA as the core hardware which is responsible for data acquisition and transmission,and the collected data is received through the LABVIEW-based upper-computer software.Then,the high-speed fiber grating demodulation has been achieved using the MATLAB-based data analysis.The main work of this paper is displayed as follows:(1)FBG sensor technology has been reviewed briefly,including the principle,development status and trends,and so on.Then,this paper analyzes the performances of several typical FBG demodulation schemes,whose advantages and disadvantages also have been illustrated in detail.Therefore,the wavelength demodulation solution characterized by high-speed,high-precision and low-cost is imperative for FBG sensing in practical applications.(2)A novel JAWS filter is proposed to reduce loss by introducing the advantages and disadvantages of the existing JAWS filter based on virtually-imaged phased array.Using numerical simulation,a comparative analysis has been performed between the low-loss JAWS filter and traditional JAWS filter.At the same time,the characteristic parameters of the low-loss JAWS filter have been optimized to achieve high performance sawtooth waveform,such as linearity,duty cycle and signal-to-noise ratio.The designed filter can reduce about 7dB additional loss.(3)The related theories of hardware circuit have been introduced briefly,including FPGA development,performance characteristics,the typical FPGA chips,USB bus technology,and so on.A high-speed demodulation system based on FPGA has been proposed,which is used to collect the photoelectric conversion signals with the sampling rate of 50MHz.The collected data is stored in DDR3 memory temporarily,and transmitted to PC by the USB2.0 interface.Besides,the upper-computer software based on Labview is proposed and demonstrated to store data information and display data waveform.(4)The experiment of fiber grating stress sensing has been designed,which uses the validated low-loss JAWS filte and data acquisition circuit based on FPGA as wavelength demodulation device.The frequency spectrum and the time-domain waveform of FBG sensing array can be observed respectively by spectrometer and oscilloscope.Using the USB2.0 interface,the collected data is transmitted to the PC software and saved.By analyzing the data carrying vibration information through matlab,the high-speed wavelength demodulation has been accomplished.