Key Technology For Reliability Of Fiber Bragg Grating Strain Sensing System

Due to its distinguished advantages,Fiber Bragg Grating(FBG)strain sensing system has been widely used in various sensing fields.But in practical applications,the sensing system may degrades and fails,which seriously blocks its promotion.The system's reliability issue has caught users' attentions and become an urgent and common problem that needs to be solved.Reliability research involves a wide range of technical difficulties,and it is time-consuming and difficult to make a breakthrough in theory.So,it did not receive enough attention from researchers,making it become the weakest point in the optical fiber sensing research field.In order to achieve a breakthrough in reliability research,the focus must be put on the sensor and the interrogator.Based on basic principles of sensor and signal demodulation,it needs to study FBG sensor's performance change rules in various harsh environments,strain transfer characteristics of sensor package and installation,the robustness of demodulation method,and the method to evaluate the credibility of demodulation results.Solving these problems can not only provide guidance for FBG sensor manufacturers and help for FBG sensor users,but also is very meaningful to promote FBG sensing market,and national economic and military development.Therefore,this thesis studies reliability problems from FBG,strain transfer and demodulation comprehensively.Main research contents are as follows:(1)In order to provide basic theory and method for studying the FBG,formation mechanism of FBG structure is analyzed and the idea of understanding FBG refractive index change from the photochemical reaction is proposed.Based on coupled-mode theory and transfer matrix method,simulation formulas of FBG spectra are deduced.In addition,FBG spectral characterization indexes are proposed for spectral analysis.(2)FBG's performance change rules in mechanical,temperature and irradiation environments are investigated comprehensively.Since existing studies on influences of axial and transversal stresses on FBG spectra are imcomplete,FBG's spectral change rules and reliable sensing conditions under axial and transversal stresses are analyzed based on deduced influencing models respectively.To understand influencing mechanism of bending on the FBG spectrum,a model is established and the bent limiting condition for reliable FBG sensing is given based on this model.Also,degradation mechanism and rule of FBG spectrum under long-term stress are discussed both theoretically and experimentally.To explore FBG's lifetime in normal temperature application range(-40~80?),a thermal lifetime prediction model is established and FBG's lifetime is predicted by combining high-temperature accelerated aging,low-temperature and thermal shock experiments.At last,an irradiation influencing model is established to explain Bragg wavelength drift differences in various irradiation experiments.(3)To disscuss FBG strain sensors' strain transfer characteristics comprehensively,a generalized strain transfer model is established,which unifies current different strain transfer models.Then,formulas of average strain transfer rates for 4 type FBG strain transfer forms are derived based on a simplified FBG strain transfer model.Based on derived formulas,effects of material and geometrical properties of bonding layers on stability and consistency of average strain transfer rate are analyzed.Correspondingly,solutions are proposed to ensure stability and consistency.An influencing model of transversal stresses of bonding layers on FBG spectra is established.Accordingly,influencing rules are studied by simulation and experiment,and the method to avoid FBG spectral distortion is given.In addition,influencing mechanism and rule of degraded bonding layers on FBG spectra are analyzed theoretically and experimentally.(4)To improve the robustness of algorithm for demodulation software,a degradation model of FBG measuring spectrum is established,and this model can fully depict degradations of FBG sensor,interference noise,baseline noise and Gaussian noise caused by the FBG sensing system.Then,influences of spectral noises on some representative demodulation algorithms are analyzed,and a scheme is proposed to improve the robustness of algorithms.By analyzing effects of spectral distortions on demodulation algorithms,a creditability evaluation method using spectral golden side-mode suppression ratio for demodulation results is found and verified by the experiment.Besides,improved correlation and direct peak detection algorithms are found to be relatively reliable,which provides selection reference for users.