Study Of Fiber Optic Magnetic Field Sensor Based On Magnetic Fluid And Hi-Bi Fiber Loop Mirror

Compared to traditional magnetic field sensor, fiber optic magnetic field sensorshave more advantages in the aspect of magnetic field sensing. In addition to lightweight and small size, they have many advantages, including high resolution, highsensitivity, high precision and immunity to electromagnetic interference, which haveattracted much attention. As a special and attractive interferometer, high-birefringencefiber loop mirror (Hi-Bi FLM) has attracted wide attention and research of manyresearchers, and has been applied in the fields of optical fiber communication andoptical fiber sensing. As the key material in magneto-optical sensors, magnetic fluid(MF) possesses many outstanding properties. We take advantage of the tunablerefractive index property of the MF and the property of the thinned Hi-Bi FLM issensitive to the surrounding. This thesis put forward a novel measurement method ofmagnetic field which composed of magnetic-fluid cladding polarization-maintainingfiber (PMF) as a sensing head. In this novel sensor, the refractive index of MF ischanged by a tunable magnetic field and the resonant dips wavelength produced bythe Hi-Bi FLM drift correspondingly. Therefore, the magnetic field strength isobtained by the wavelength drift of the resonant dip in the Hi-Bi FLM. The sensor inpreference to other sensors has simple structure, great flexibility, anti-electromagneticinterference, high sensitivity and good repeatability. The main job and theachievement of the thesis include five aspects as follows:(1) The development course and status quo of magnetic field sensor areelaborated, the composition and property of the MF are introduced and the sensingapplication of the magnetic field sensor based on MF is presented.(2) The stress and optical transmission modes of the PMF is analyzed by the softnamed as comsol multiphysics, some important graphs are plotted, including thegraphs of stress distribution, refractive index of slow and fast axis, core birefringencedistribution and optical energy distribution.(3) The transmission function of the Hi-Bi FLM is derived with different lengthof the PMFs, and the transmission property of FLM based on the thinned PMF isstudied. The relationship between the wavelength drift of the resonant dip of the Hi-Bi FLM and the refractive index of the ambient environment is obtained.(4) In order to reduce the diameter of the outer cladding, chemical etched isapplied to the PMF to increase the sensitivity to the surrounding. The etching processis monitored in time by the optical spectrum analyzer and the result is discussed too.And then, the experiment of glucose solution refractive index is carried out to verifythe feasibility of the experiment scheme is put forward, and the relationship betweenthe wavelength shift of the resonant dip of the Hi-Bi FLM and the refractive index isobtained. The suitable MF is chose according to the requirement of experiment andthe packaging of sensing head is finished.(5) Whole sensing system is introduced, the design of the experimental system isbuilt up and the sensing experiment of the magnetic field is carried out. The achievedresults show that we proposed the fiber optical magnetic field sensor based onmagnetic fluid and Hi-Bi FLM could be applied to measure the magnetic field andcurrent measurement. To overcome the problems of electromagnetic interference,bulky volume, big temperature-shift, our sensor is easy to built, light, small, has highsensitivity and good reversibility compared to traditional magnetic fields. What’smore, some improved ways are proposed for the inadequacies of the sensor.