Study On Some Techniques Of Fiber Bragg Grating Sensing System

Fiber Bragg grating (FBG) sensors are one of optical fiber sensors. So they have all theadvantages of optical fiber sensors such as flexible, small size, light weight, dielectric, corrosionresistance, not heating in working, nonradiative, immune to electromagnetic interference, can work incomplicated environment such as inflammable and explosive situation and poisonous gases. Further,they possess flexible multiplexing capability such as wavelength division multiplexing (WDM), timedivision multiplexing (TDM) and space division multiplexing (SDM). Because of their own uniqueadvantages, FBG sensors possess great application prospects in aerospace and civil engineering andthey have becoming a hot and focal point in national and abroad in recent years. However, FBGsensors have some problems in the application of structural health monitoring such as encapsulation,demodulation, spectral reconstruction and development of photonic crystal fiber Bragg grating(PCFBG) with better and more flexible performance. If these problems can well been solved, then theapplication of FBG sensing system will be greatly improved. In this paper, we focus on FBG sensingsystem, mainly research on the encapsulation technology, the demodulation technology, spectralreconstruction technique of FBG and PCFBG technique.The main research contents are described as follows:(1) The theoretical analysis of FBG has been studied. Firstly, we introduce the strain and temperaturesensing model of FBG. Based on the strain and temperature sensing model of FBG, the temperatureand strain sensing properties of FBG sensors are analyzed. Then theoretically and experimentalresearches on the FBG encapsulation technology of strain and temperature are studied, and thepackaging technology are discussed through sensitization and desensitization.(2) In order to demodulate FBG sensing signals, a FBG demodulation system based on a cascadedlong period fiber grating (CLPG) is developed and the improved system is also proposed. A CLPGconsists of two identical long-period fiber gratings separated by a short length of single mode fiber.Reflection light of FBG modulated by CPFG is obtained and studied by use of band-stop filtercharacteristics for CLPG, and wavelength change of FBG is deduced by light intensity achieved byphotodetector. The performance of the demodulation system and the improved system are studiedthrough the FBG temperature, static strain and dynamic strain (vibration) experiments.(3) A FBG parameter reconstruction method based on quantum-behaved particle swarm optimization(QPSO) algorithm is developed, and then the comparison between QPSO and particle swarm optimization (PSO) is presented. In this method, the parameters of the FBG is considered as variables,the reflection spectrum of the FBG is considered as the objective function, the error between theknown reflection spectrum and the reconstructed reflection spectrum is considered as the fitnessfunction.Then the optimal individual with smallest fitness function is finded by parameter search. Theparameter reconstruction of FBG and chirped FBG is described using PSO and QPSO algorithm.(4) The QPSO algorithm and PSO algorithm are further used for the spectrum reconstruction of FBGsensor with non-uniform strain and temperature. The efficiency and reliability of the QPSO algorithmand PSO algorithm are verified through four kinds of non-uniform strain and temperature. Futhher,the comparison between QPSO and PSO is given.(5) The reflection spectrum characteristics of PCFBG are analyzed by OptiFDTD software andcoupled-mode theory. Futhermore, the effect of structural change including radius of air hole, materialrefractive index and collapse of air hole on the resonant wavelength of PCFBG is analyzed.