Concentration And Temperature Simultaneously Sensing Based On A Concatenated Fiber Bragg Grating With Long Period Grating

The research for concentration and temperature sensing plays an important role inthe sensor technique, which is widely used in military, aerospace, medical, chemical,environmental, scientific research and other fields. The method of converting the testedparameters into current or voltage is always taken by traditional concentration andtemperature sensors to demodulate parameters, which results in two weakness oftraditional concentration and temperature sensors. On the one hand, the traditionalsensors are vulnerable to electromagnetic interference because of the use of thetransformed electrical parameters and a lot of electron devices. On the other hand, it isrelatively weak to adapt the harsh surroundings, such as high temperature and powerfulcorrosivity. With the appearance and development of fiber bragg grating, fiber bragggrating shows many advantages, such as corrosion resistance, anti-electromagneticinterference, large measuring range, good insulation and easy to reuse, which makessensing technology based on fiber bragg grating has been concerned widely, therefore,the research on system of concentration and temperature sensing based on fiber bragggrating has theoretical and practical significance.In this paper, based on an in-depth study for the coupling mechanism of FBG andLPG, we present the method to measure concentration and temperature simultaneouslyin concatenated FBG and LPG. The specific work are as follows:1. Analyzing the impact of parameters of the FBG, LPG and spacing fiber toreflection spectrum of concatenated FBG and LPG based on the transfer matrix theory.And under the conditions of appearing two sharp narrow band wave in reflection spectraof concatenated FBG and LPG, optimizing the cascaded structure parameters. Thesimulation result shows that the reflective spectrum for concatenated FBG and LPGwith the simulation method of transfer matrix theory is good consistent with practicalvalue, and under certain restrictions, we can get an ideal waveform in the reflectivespectrum by selecting the optimum parameters.2. Establish experimental system using optimized cascaded structure, and takingthe dual-wavelength method to demodulate sensing signal, and achieving to measure thetemperature and concentration simultaneously. The result of dual-parameter measuringexperiments shows that the cascaded structure can achieve high precision andsimultaneity. 3. Analyzing the spectral characteristics of the noise signal, choosing waveletthresholding method to denoise, processing principle of fusing indicator withexponential fitting algorithm and presenting an inflection point judgment formula todetermine the optimal decomposition scale to get a better performance in de-noising.For sensing signal of concatenated FBG and LPG, the simulation for algorithm ofoptimal decomposition scale shows that the reflectivity rate of central wavelength andthe corresponding light power, with optimal decomposition scale selected in this method,is consistent with the real values. Thus, the proposed method shows better performancein de-noising.