Temperature Detection Method Based On FBG-FP Cavity

Based on the study of FBG-FP cavity sensor characteristics, it is used for distributed temperature measuring. By using the dependence of the changes of FP cavity on the environment temperature, as well as employing a novel autocorrelation demodulation, this thesis takes FBG-FP cavity as a sensor to achieve the purpose of temperature measurement. Compared with other FPs, the FBG-FP possesses many merits, such as ease of manufacture and large potential for multiplexing. The FBG-FP also keeps both the advantages of normal FBG sensors and common FPs, 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 detection by using the FBG-FP, the main contents in the thesis are as follows:(1) In view of FBG-FP temperature sensing technology is the extension and combination of FBG and fiber FP cavity, their development courses as temperature sensors are introduced respectively, and the main problems existed in detecting physical parameters are analyzed. At last, the main demodulation methods of FBG-FP cavity are listed.(2) Starting from the Maxwell-field equations in optical fiber, and the coupled-mode equations that describe the interaction between the modes are derived.Then,the transmission characteristics of FBG are analyzed with coupled-mode theory, and the relationships between the spectra and grating length as well as index modulation are discussed. Relying on the transmission characters of FBG, the strict expressions of transmissivity and reflectivity for FBG-FP cavity are deduced through the transfer matrix method and light propagating method. The spectrum of FBG-FP cavity is analyzed by numerical simulation, and the effects of the grating length and cavity length on the reflective spectrum shape are investigated.(3) Sensing properties to temperature of the FBG-FP are analyzed. On that basis, an autocorrelation demodulation method applied for the fiber grating FP is introduced. This method has the advantages of high precision and wide measurement range. The frequency division multiplexing technology of a FBG-FP system is discussed and analyzed. By numerically simulating, processes of frequency division multiplexing are explained, and the applied area of autocorrelation demodulation is extended. A multiplexing system based on the wavelength division multiplexing and frequency division multiplexing is suggested to increase the number of multiplexed FP cavity.(4) The whole experiment system is designed, and tnen, the parameters of experimental apparatus and equipment are explained respectively. The thesis completes the temperature detection of the single point successfully and then gives the corresponding experimental results. The feasibility of this demodulation method is confirmed by the temperature measurement experiment.This thesis provides certain theoretical and experimental basis of temperature measure-ment based on FBG-FP. At the same time, thanks to the development of the optical fiber communication, production process of FBG-FP is getting increasingly mature and large-scale production of FBG-FP and on-site inspections have been realized.