Research On Strain Demodulation System Based On Grating Array

The advantages of fiber grating strain sensor,such as light weight,easy installation,reusability and anti-electromagnetic interference,make it convenient for long-term monitoring of strain field,which plays an important role in the field of structural health monitoring.The distributed high-speed strain measurement system based on the isotactic weak reflection grating array adopts the demodulation method of time division and wavelength division multiplexing.Compared with the strain sensing system realized by ordinary grating technology,it has longer detection distance and larger grating capacity.This thesis takes the strain demodulation technology and system based on the isotactic weak reflection fiber grating array as the research object,and uses the isotactic weak reflection grating with the same central wavelength,bandwidth,and sensitive characteristics to combine time division and wavelength division multiplexing technology to construct a sensor network and analyze it.The composition of the system,the demodulation algorithm and the system acquisition circuit are studied.The main research works of this thesis are as follows:(1)This thesis studies the development status of traditional distributed long-distance strain sensing systems and quasi-distributed systems based on grating arrays.The reflection spectrum characteristics of fiber gratings are analyzed using the mode coupling theory,the strain sensing principle of fiber gratings is analyzed according to the theory,and the demodulation methods of different grating arrays are compared.The total amount of reusable gratings in the system is analyzed theoretically and the calculation is carried out by substituting the actual parameters.(2)The optical path of the strain demodulation system is built,and the placement position and magnification of the EDFA are designed.The FPGA program in the system acquisition module is written,the sampling rate of the acquisition board is 200 MHz,and the data transmission with the PC can reach up to 150Mbit/s.(3)In this thesis,the stability and linearity of the demodulation wavelength of the system under different peak-finding algorithms are compared,and the centroid algorithm is selected as the system peak-finding algorithm according to the computational complexity and experimental performance of each algorithm.An optimization method is proposed to solve the problem of numerical instability when using the centroid algorithm to solve the spectral center wavelength when the light intensity decreases.The wavelength fluctuation range of the tested system within12 hours is within ±3pm.The static and dynamic strain measurement experiments were carried out using the built system,and the experimental results were consistent with the theoretical model,which proved that the system has the ability of static and dynamic strain monitoring.