Research On Weak Reflection Fbg Demodulation System Based On Frequency Modulated Continuous Wave

FBG(Fiber Bragg Grating,FBG)sensors have been widely used in various fields for its advantages such as small size,high sensitivity,resistance to electromagnetic interference,easy multiplexing and so on.In recent years,the demand for large capacity,high accuracy,low cost has increased in many complex applications.In this paper,we use the identical weak reflection FBG whose center wavelength,reflection rate and other optical characteristics are consistent to build the sensing network,and build the demodulation system based on the frequency modulated continuous wave technology.Based on the research of demodulation system,algorithm and related problems,a large capacity,high precision and low cost identical weak reflection FBG demodulation system is achieved.The main research works of this paper is as follows:(1)Analysis the Fiber Bragg grating temperature sensing model theoretically.Research on the influence of Rayleigh scattering and multiple reflection on multiplexing capacity of the system when using the identical weak reflection FBG to build sensing network,it is proved by simulation and theoretical calculation that we can increase the multiplexing capacity of the system by reducing the reflectivity of the FBG.(2)Building the identical weak reflection FBG array demodulation system based on the frequency modulated continuous wave technology.Using the characteristics of DFB laser that current modulation speed is fast and the wavelength modulation width is wide controlled by the temperature of DFB,cooperating with linear frequency modulation signal,the module outputs light whose linear frequency width is 20 MHz,and wavelength scanning range is 2nm.Theoretically analyzing the influence on light width when high speed modulation signal or random noise drives DFB.Using optical circulator and identical weak FBG array to build a sensing network.The grating reflected signal inputs to the multiplier module after converted to electrical signal by the high speed and gain APD module,after filtering and amplifying,the signal was sent to the ADC controlled by the FPGA for data acquisition and data processing.(3)Analysis the principle of the system from the demodulation of position and wavelength information of each FBG,the demodulation algorithm of the system is proposed.We calibrate the wavelength of the output light via the wavelength-temperature curve of the DFB laser and improve the method by adding a ADC to acquire the real-time temperature of DFB for temperature stability judgment and calibration.We have compared the effect of several common Bragg grating wavelength peak-searching algorithms and select the Gauss fitting algorithm which has the highest precision.(4)Several factors that influence the demodulation speed are discussed,through the experiment,we measure the demodulation speed of the system.We confirm the long distance demodulation ability of system by adding a series of 10 km delay fiber before the FBG array.Through the static temperature experiment,the demodulation ability and the stability of the system are verified.And the system multiplexes 100 FBGs along a single fiber have the same wavelength of 1552.5nm,reflectivity of 0.1%,space interval 5m.The experimental results show that within the temperature range from 0 ? to 80 ?,the measured center wavelength of each FBG remains good linearity of 0.998 higher,and the FBG center wavelength demodulation error is within 13 pm.