Study Of Magnetic Field Sensing Based On Magnetic Fluid And Fiber Bragg Grating

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 has small size, low weight and high sensitivity, and it's easy to perform distributed measurement. Fiber Bragg grating (FBG) sensors have already been used in the strain and temperature sensing and have got significant achievement. As we all know, FBG itself is not sensitive to refractive index, but when the diameter of the fiber cladding is reduced to a certain degree, there will be an interaction between the evanescent wave of the fundamental guided mode and the surrounding materials, the effective refractive index of core guided mode will be changed with the change of surrounding refractive index, this is the basic sensing principle of this thesis. As a kind of new functional material, magnetic fluid (MF) has both the magnetism and the liquidity, especially the refractive index of MF will be changed with the magnetic field surrounding MF. Based on the two points above, the thesis presents a novel method of sensing magnetic field. That is, based on MF and FBG, we select MF as the outer cladding of FBG whose cladding diameter is reduced, through changing the surrounding magnetic field, the MF's refractive index will change, and this change will be sensed by FBG.This paper introduced the principle, and we calculated the sensitivity of surrounding refractive indexes of different FBG sensors with different cladding diameters using MATLAB software. The results show that the thinner the cladding diameter is. the higher the sensitivity is. The results also show that there is still the guided mode in the core of fiber optic when the surrounding refractive index is higher than the cladding's.In this paper, chemical etching method is used to etch the fiber cladding, hydrofluoric acid with a concentration of22%is used, and the etching rate is about0.548μm/min. A plexiglass box is used to fabricate the etched FBG sensor, and protects the etched fiber effectively from being damaged. Refractive index measurement experiments of glucose and sodium chloride are been done, the results show that the sensor is sensitive to surrounding refractive index, experiments'results are in agreement with the calculated results. After the sensor is fabricated, the magnetic field measurement experiments have been done, results show that when the added magnetic field increases, the Bragg wavelength increases correspondingly. The sensitivity of the sensor reaches to about4pm/mT. The experimental results show the feasibility of the magnetic field sensing method in the thesis.