Study Of Magnetic Field Sensor Based On Magnetic Fluid And Long Period Fiber Grating

When the modern information-industry is developed rapidly, optical fiber sensing technology which is one of important branches of information technology has been successfully applied in various aspects, such as fire monitoring of petroleum chemical and tunnel, health monitoring of civil engineering, detection of mine gas, online supervision of electrical equipment and so on. In this thesis, a novel current and magnetic field measuring method which is based on magnetic fluid and long period fiber grating is presented. The working medium for current and magnetic field sensor is magnetic fluid, which is intelligent nanometer materials and has an obvious magneto-optical effect; the refractive index of magnetic fluid will be changed by an external magnetic field. In addition, the long period fiber grating which is passive optical component can be used as an ideal fiber optic sensitive element. It is sensitive to the refractive index surrounding its cladding. Combining the magnetic fluid and long period fiber grating, it is very easy to obtain the intensity of magnetic field and current by measuring the shift of resonance wavelength of the long period fiber grating. As compared with the traditional magnetic field sensor based on electromagnetic effect, the new magnetic field sensor that we have designed has many advantages such as anti-electromagnetic interference, high efficiency of conversion and no magnetic saturation phenomenon. In this thesis, the major results of theoretical researches and experiments are as the following:(1) The significance and urgency of magnetic field and current sensing technology are stated, the development history and classification of optical fiber magnetic field sensor are introduced.(2) Coupled-mode equations for long period fiber grating are deduced from coupling theory and the core and cladding effective refractive indices, transmission spectra and relationship between the resonance wavelength shift and the environment’s refractive index are calculated through the method of numerical calculation. The refractive index characteristic for long period fiber grating has been particularly studied.(3) Using krypton fluoride248nm lasers as the UV light source and based on the metal amplitude mask template, we successfully fabricated and annealed many long period fiber gratings. In this section, the conditions and processes of long period fiber gratings’ fabricating are studied too.(4) The theory of method-retro reflection on the end of optical fiber based on the theory of Fresnel reflection is analyzed for measuring magnetic fluid’s refractive index. An index measurement system based on the method-retro reflection on the end of optical fiber is demonstrated. Taking deionized water and air as samples of calibration, we measured the refractive index of water-based Fe3O4magnetic fluid under different magnetic field intensity. The results have shown that the refractive index of water-based Fe3O4magnetic fluid decreases with the increasing of the external magnetic field intensity. The experimental results are in agreement with the theory.(5) The production method and craft of current sensor based on magnetic fluid and long period fiber grating are put forward. A magnetic field sensing experimental system using an equipment generating magnetic field is built. The results of measuring magnetic field intensity experiment show that this novel method based on magnetic fluid and long period fiber grating can be applicable for sensing current and magnetic field intensity.