Investigation Of Fiber Grating Fabry-Perot Interferometer Used For Weak Magnetic Field Detection

Based on the research of magnetostrictive principles and sensing properties of a fiber grating Fabry-Perot interferometer (FPI), an optical fiber FPI used for weak magnetic field detection is present in this thesis. Where, the fiber grating FPI works as a sensing element and the magnetostrictive material works as a transducer when applied magnetic field changes. The purpose of magnetic field detention is achieved after the autocorrelation demodulation. Compared with other FPIs, the fiber grating FPI possesses many merits: ease of manufacture, large potential for multiplexing, the fiber grating FPI keeps the advantages of FBG sensors and common FPIs both, 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 magnetic field detection by using the fiber grating FPI, the main work in the thesis is as follows:(1) The magnetostrictive laws are derived through the coherent rotation model, and the formulas between magnetostriction and magnetic field are given. The magetostrictive formulas are deduced in such situations, where magnetostrictive material is under stress and the direction of magnetic field is different from that of measurement. The magnetostrictive curves of Tb-Dy-Fe and Fe-Ga alloy are measured by an experiment set up of Michelson interferometer. These curves agreed with the theoretical result.(2) Starting from the Maxwell-field equations in optical fiber, we derived the coupled-mode equations that describe the interaction between the modes. Then, FBG is analyzed with coupled-mode theory, 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 the fiber grating F-P cavity are deduced through the transfer matrix method and light propagating method. The spectrum of fiber grating F-P cavity is analyzed by numerical simulation, the effects of the grating length and cavity length on the reflective spectrum shape are investigated.(3) Sensing properties to temperature and strain are analyzed for the fiber grating FPI, and related experiment results are given. An autocorrelation demodulation method applied for the fiber grating FPI is introduced. Using the two parameters, A and B, which are got from the linear fit, wavelength demodulation is achieved. This method has the advantages of high precision and a wide measurement range. A temperature experiment is demonstrated to confirm the feasibility of this method. For sensing the magnetic field by proposed fiber grating FPI, the effective magnetostrictive coefficient is calculated. Then, measurement results of a fiber grating FPI are compared with those of a Tesla meter.(4) The frequency division multiplexing technology of a FPI 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 FPIs.