Study On Key Issues Improving The Performance Of Fiber-Optic Interferometric Sensors Detecting Weak Magnetic Field

The weak magnetic field sensors can be widely used in military affairs, aeronautics, spaceflight, and etc. With the progress and improvement of narrow linewidth laser sources, single-mode fiber and related fiber-optic devices, not only more types of fiber optic sensors have been developed but also the performances have been improved tremendously. Fiber-optic sensor detecting weak magnetic field is considered to be the most competitive alternative for traditional pure electrical detections because of its high sensitivity in theory. Fiber-optic interferometric sensors based on fiber-optic interferometer and magnetostrictive material is believed to be one of the sensing technologies that can be put into practical applications. Therefore, it is significant to carry out a study on fiber-optic interferometric sensors detecting weak magnetic field.This dissertation presents some key issues in high sensitive and stable fiber-optic Michelson interferometric weak magnetic field sensors, including theory and principle of fiber-optic interferometric weak magnetic field sensor, design and implementation of fiber-optic interferometric transducer, the demodulation schemes, the ability in detecting magnetic field, the stability and the temperature dependence of the fiber-optic interferometric sensor developed, etc.Firstly, the dissertation systematically analyzes and summarizes the fundamentals of fiber-optic interferometric sensors and the operating principles of fiber-optic Michelson interferometric sensors, including the magnetostriction effect and the principle of optical phase modulation in optical fibers, the basic structure and principle of fiber-optic interferometric sensors based on fiber-optic Michelson interferometer, the principle and realization of key techniques of phase detection of fiber-optic Michelson interferometric sensors, the cause of system instability and corresponding solving method, etc.Secondly, the dissertation investigates the fiber-optic interferometric weak magnetic field transducer based on fiber-optic polarization- insensitive Michelson interferometer. Based on the analysis of the dependent factors of the system sensitivity, the designing and implementing schemes of the key components are presented. By comparison the performances of the most popular shapes and structures that are generally adopted in fiber-optic interferometric magnetic field transducers, a cylindrical bakelite transducer framework is chosen and implemented. The performance of fiber-optic Michelson interferometer is an important factor to determine the system sensitivity, therefore the mathematical model of visibility is established for the polarization- insensitive fiber-optic Michelson interferometers so that the limitation factors of visibility can be analyzed. Numerical analysis indicates that the fiber length difference between the interferometric arms is the primary factor that limits the visibility. With the help of the precision reflectometer, the fiber length difference between the interferometric arms can then be precisely measured and maintained to be less than 1 mm to ensure high visibility. Based on the analysis of stress and loading effects, appropriate adhesive and selected and bonding methods are studied. A set of comprehensive techniques on design and implementation of the magnetic field transducer are developed through lots of experimental comparison and experience accumulation.Thirdly, the dissertation studies two kinds of sensing signal detection techniques, the hardware circuit based demodulation technique and software demodulation technique. In hardware circuit based demodulation technique, the traditional lock-in amplification circuit is modified so that a better detection can be ensured. In software demodulation technique, the noise sources of polarization-insensitive fiber-optic Michelson interferometric sensor are studied, including environmental noise, optical source noise, thermal noise, shot noise and electronic noise, etc. Their characteristics as well as their influences on the optical phase of the output signal of fiber-optic interferometers are carefully analyzed. Thermal noise is found to be the major noise to decide the minimum detectable optical phase of fiber-optic interferometer. Accordingly, a sensing signal demodulation scheme based on software demodulation and adaptive line enhancer is presented and realized. By using hardware circuit based demodulation scheme, the dissertation systematically tests and analyzes the system performance of polarization-insensitive fiber-optic Michelson interferometric sensor developed, including the transducer's mechanical resonant frequency and frequency response, system stability, the effect of input optical power and amplitude of AC dither magnetic field on system output, system sensitivity and resolution, and the directivity of magnetic field transducer, and etc. The results show that the fiber-optic Michelson interferometric magnetic field sensor has not only good stability but also excellent ability in detecting weak magnetic field. The minimum detectable magnetic field is about 0.10 nT/Hz1/2 and 0.04 nT/Hz1/2 at 1 Hz and 5 Hz, respectively, without any mumetal shield in the ordinary laboratory environment. This part of the dissertation is collaborated with the members of the research group that the author is engaged.Forthly, the dissertation experimentally investigates the temperature dependence of the performance in fiber-optic interferometric magnetic field sensors. The cause of temperature dependence is briefly described. The theoretic model of the mechanical resonant frequency is evolved from elastic wave theory for ribbon-like magnetic field transducers based on magnetostrictive material and optical fiber. The factors that are responsible for the ambient temperature induced change of the mechanical resonant frequency are discussed. Among these factors, the primary one is analyzed by using modal analysis. The temperature dependences of the transducer's mechanical resonant frequency, the sensor system output, and the sensor system sensitivity are tested, and the corresponding temperature dependence functions are fitted through performing a series of experiments. The experimental results show that the magnetic field transducer's mechanical resonant frequency and the sensor system performance are strongly affected by the ambient temperature. These results also indicate that essential measures must be applied to make the measurement of the fiber-optic interferometric weak magnetic field sensor be independent of ambient temperature. Accordingly two temperature compensation schemes are presented and experiments are conducted to verify the feasibility.Last, a conclusion and expectation are made for the dissertation.