Research On Measurement Of Temperature And Liquid Concentraion Based On PBG-FP Cavity

Based on the study of characteristics of Fiber Bragg Grating and Fabry-Perot cavity, a fiber optic sensor for temperature and liquid concentration measurement using FBG-FP cavity was described. The determination of temperature was by evaluating the the Bragg wavelength shift and the measurement of liquid concentration was achieved by using the Fresnel reflection from the cleaved fiber end. Ulilizing the ratio between the amplitudes of the two peaks in Fourier domain allows intensity-independent accurate measuments for liquid concentration. At the same time, a demodulation algorithm using Fast Fourier Transform and correlation analysis was presented to extract the information of the measurand from the optical signal received.FBG-FP cavity has many attributes compared to traditional FP cavity, such as ease of manufacture and large potential for multiplexing. The FBG-FP also keeps both the advantages of normal FBG sensors and common FP cavity, which means that it can provide high precise measurements and absolute measurements at the same time. As a result, the measurement range is enlarged. Encircling the research on temperature and liquid concentration detection by using the FBG-FP cavity, the main contents in the thesis are as follows:(1) Since the FBG-FP cavity combines the FBG and the FP cavity, their development courses as sensors were introduced respectively, especially as temperature and liquid concentration sensors. The main problems existing in detecting measurands are analyzed. Then the sensinig applications of FBG-FP cavity are studied to providing theoretical basis of chosing FBG-FP cavity parameters.(2) The transmission characteristics of FBG were analyzed with the coupled-mode theory, and the relationships between the spectra and grating length as well as index modulation were discussed. Relying on the transmission characters of FBG, the strict expressions of transmissivity and reflectivity for FBG-FP cavity are deduced. The spectrum of FBG-FP cavity is analyzed ultilizing Matlab and the effects of the grating length and cavity length on the reflective spectrum shape are investigated.(3) Sensing properties to temperature and liquid concentration of the FBG-FP cavity are analyzed, on which basis a demodulation method using FFT applied for the FBG-FP cavity was introduced. This method has the advantages of high precision and wide measurement range.(4) The experimental setup is designed, and the parameters of experimental apparatus and equipment are explained respectively. After detecting the temperature and liquid concentration, the detailed analysis of the experimental results were shown. It also proved that the proposed demodulation procedure provides high accuracy and absolute measurement.This article provides certain theoretical and experimental basis of temperature and liquid concentration measurement based on FBG-FP.