Research On Magnetic Field Sensing Based On Special Structure Of Optical Fiber And Magnetic Fluid

Magnetic field sensing methods have been rapidly developed so far, traditional methods of magnetic field sensing have many restrictions, such as, electromagnetic interference, etc. As a new kind magnetic field sensor, fiber optic is insulative. Fiber magnetic field sensing method has many unique advantages, it is not only immune to electromagnetic interference, but also possesses small size, low weight and high sensitivity, and it is easy to realize distributed measurement. Tilted fiber Bragg grating(TFBG), a kind of fiber grating whose grating has a certain angle perpendicular to the fiber axis, not only has characteristics of ordinary fiber Bragg grating(FBG), but also has the external environment refractive index sensitive characteristics of long period fiber grating(LPFG), which can be used for multi-parameter measurement, so TFBG can solve cross sensitive problem effectively. As a new kind of functional material, magnetic fluid(MF) has both the magnetism and the liquidity, especially the refractive index of MF is closely related to the external magnetic field. Based on the two points above, the thesis presents a method of sensing magnetic field based on the TFBG and MF. Considering the absorption and scattering effect of MF, also combined with the property of evanescent wave, we propose a intensity interrogated magnetic field sensor. By utilizing the high sensitivity of the fiber loop ring-down(FLRD) on the measurement of absorption loss, we realize the magnetic field sensing. The main contents are as follows:(1) TFBG is analyzed in theory,demonstrating mode coupling equations. At the same time, theoretical analysis on its refractivity sensing characteristics have been taken.(2) Takes advantage of the external environment refractive index sensitive characteristics of TFBG and the tunable refractive index property of the MF, we proposed and analyzed the magnetic field sensor. The transmission spectra and the relationship between the resonant wavelength of cladding mode shift and the magnetic field intensity of the magnetic field sensor were obtained. The results show that the higher-order cladding mode has a relative large shift under the same magnetic intensity which indicates a more sensitive response to magnetic field. Based on the super paramagnetic characteristic of the ferromagnetic particles, the magnetic field sensor has good repeatability. In addition, combined with the temperature sensing characteristics, we improve the performance of the sensor and realize the temperature compensation.(3)We have analyzed the light transmission theory in optical fiber. Also, we have analyzed and researched the occurrence, effect and property of the evanescent wave when light transmits in the fiber.(4) Combined the theory of evanescent wave sensing with the absorption and scattering effect of MF, we propose an intensity interrogated magnetic field sensor. Due to its high sensitivity, high signal-noise ratio, quasi-real-time response, impervious to the power fluctuation of the light source, and freedom from collimation, we have constructed the FLRD system to realize magnetic field sensing.