Study On High-sensitivity Fiber Optic Magnetic Field Sensors

The magnetic field sensing technology has attracted widespread attention and is widely used in the field of geophysical research,oil and mineral exploration,medical diagnosis,military defense,etc.Compared with the traditional electrical magnetic field sensors,fiber optic magnetic field sensors occurred relatively late.In 1980 s,the fiber optic magnetic field sensor developed rapidly since the sensing characteristics of the fiber were found.The fiber optic sensing technology is a multi-disciplinary integration technology and has many advantages.Firstly fiber optic sensors have a strong ability of anti-electromagnetic interference and can work in strong electromagnetic environment.Secondly fiber optic sensors have an ability of corrosion resistance and can work in severe corrosive environment.Thirdly fiber optic sensors have high sensitivity and high resolution.Thus the sensors can be used to detect weak magnetic fields.The fiber optic sensors also possess compact structure,small volume and light weight.In addition,they are easy to be distributed for the multi-point detection and the long-distance measurement.Therefore the fiber optic magnetic field sensing technology has a great development space.The propagating light in the optical fiber is not sensitive to the magnetic field,thus the basic working principle of fiber optic magnetic field sensors can be described as: a fiber optic magnetic field sensor is mainly comprised of an optical fiber and a magnetic field sensing material.The magnetic field sensing material experiences magnetic induced change,leading to the modulation of the light propagating in the optical fiber.Therefore the magnetic filed can be detected by characterizing and analyzing the output light signal.The magnetic sensing materials are the key unit to realize the conversion from magnetic energy to the light energy.The coupling structure between the magnetic sensing materials and the optical fiber mainly depends on the characteristics of those magnetic sensing materials.There are a variety of magnetic sensing materials,such as magnetostrictive materials,materials whose optical polarization will be changed by magnetic fields,and materials whose refractive index will be changed by magnetic fields.Therefore to meet the demand for the detection in different environment fields,various types of fiber optic magnetic field sensors based on different magnetic sensing materials need to be studied.My research mainly studies several types of fiber optic magnetic field sensors.High-sensitivity magnetic field sensors are the purpose of my research.This paper introduces the fiber otptic magnetic field sensors based on two kinds of magnetic sensing materials and they are super rare earth magnetostrictive material and magnetic fluid material respectively.The super rare earth magnetostrictive material can change its physical size by the variation of the magnetic field.The magnetic fluid material can change its refractive index by the variation of the magnetic field.The main work I have completed includes:1.A high-sensitivity ring-shaped magnetic field transducer based on the super rare earth magnetostrictive material is realized.This magnetic field transducer works in the sensing arm of the fiber optic two-beam interferometers,leading to the magnetic field induced changes in the optical path length because of strains transferred to the fiber from the magnetostrictive material.The phase genetation carrier demodulation method is also studied to establish an alternating magnetic field sensing system on fiber optic Michlson interferometer.The sensing system has a high sensitivity and the minimum detectable magnetic field reaches 23 p T/ @ 50 Hz(rms).In addition,the 3×3 coupler based phase demodulation system is established on the configuration of the fiber Mach-Zehnder interferometer.The sensing system for the measurement of the alternating magnetic field has a simple constructure and a high stability.2.The high-finesse fiber optic extrinsic Fabry-Perot interferometer based magnetic field sensor is realized.The Fabry-Perot cavity is formed by aligning the fiber end-face and the magnetostrictive rod end-face,and each end-face is coated by a mirror with a micro-lens.The Fabry-Perot cavity length is changed by the magnetic field induced changes in the magnetostrictive rod length.The fiber optic magnetic field sensor,which has a whole solid state and a compact structure,exhibits a high sensitivity of 41.94 nm/m T with a magnetic resolution of 25 n T.3.A micro-hole induced in-line fiber optic Mach-Zehnder interferometer by using chemical etching for the measurement of weak magntic fields is firstly proposed and realized.The refractive index sensing characteristics of this Mach-Zehnder interferometer is analyzed.The fiber optic magnetic field sensor is formed by coating the magnetic fluid on the in-line fiber optic Mach-Zehnder interferometer.Experimental results show that the sensitivity is as high as 0.2756nm/Oe.4.A tapered fiber optic long period grating based magnetic field sensor is firstly realized.By using the theoretical model of long period fiber optic grating formed by several tapers in the axial direction of the fiber,the refractive index sensing characteristics of this long period grating is analyzed.The magnetic field sensor is formed by combining the long period fiber grating with the magnetic fluid material.The experimental results show that this magnetic field sensor has a high sensitivity,and the magnetic field sensitivity reaches 0.354nm/Oe,which is the highest sensitivity compared with the reported fiber optic magnetic field sensors based on magnetic fluid material.