Study On Signal Demodulation Methods Of Weak Fiber Bragg Grating Array

With the rapid development of optical fiber sensor applications,the optical fiber sensing network has gradually developed to the trend of large capacity,long distance,high precision and integration.Quasi-distributed sensing network,using fiber Bragg grating as the sensing element,has the characteristics of multi-parameter measurements and high flexibility,while its multiplexing capability is greatly restricted.Similarly,distributed sensing network,based on optical time domain reflection(OTDR)technique(such as R-OTDR,B-OTDR,etc.),is capable of realizing long-distance and large-scale distributed measurement,while it can only carry out single-parameter measurement and thus,its flexibility is unsatisfactory.This paper integrates time division multiplexing(TDM)technique with OTDR technique,and utilize weak FBG array as a novel optical fiber sensing to realize multi-parameter and long-distance distributed measurements.On one hand,the temperature,strain and other static physical quantities are measured by detecting reflected signal of single weak FBG.On the other hand,highly sensitive distributed vibration measurements can be realized by detecting the low coherence interference signal of adjacent weak FBG.The weak FBG array is written by drawing tower online exposure method,and their reflectivity are controlled within-40 dB and-50 dB to reduce cross-talk so that thousands of weak FBG are multiplexed along a single fiber.The main research results of this paper are:1)The problem of cross-talk and noise in TDM system is analyzed.Two kinds of cross-talk,which are the shadow effect and the multi-reflection,are the most important factors to limit the multiplexing capability of the FBG array.The analysis results show that the effect of these two kinds of cross-talk on the target signal can be reduced by decreasing the reflectivity of FBG.When the reflectivity of FBG is-40 dB,the theoretical multiplexing numbers can reach 5000.2)Based on TDM technique,a quasi-distributed wavelength demodulation system is designed to measure the static parameters,such as temperature,stress,etc.In order to verify the feasibility of the system,the demodulation system is constructed to interrogate 1642 weak FBG with a peak reflectivity of about-40 dB and equal separations of 2.5 m.Experimental results show that the measurement error of the system for peak wavelength of the weak FBG is within 2 pm;The theoretical response frequency of the system is 5 KHz,and the system can work more than 18 days with frequency of 1 kHz continuously,accurately and steadily.The system is able to realize the addressing query of weak FBG array;the system has perfect linear response for temperature and stress.3)By considering two adjacent weak FBG as a sensing unit,a low coherence interferometric sensor network system is designed.The feasibility of the system is verified by 660 weak FBG with the peak reflectivity of about-40 dB and equal separations of 2.5 m.Experimental results show that the system can accurately collect all the interference signals of the weak FBG array;The proposed system has better frequency response,which can reach the sampling rate of 10 kHz while the length of optical fiber is less than 10 km.The proposed system can accurately distinguish weak signal with frequency of less than 1 kHz;The FBG Fabry-Prot(FBG-FP)sensor has higher sensitivity in contrast to the standard of seismic geophone.