Research Of All-fiber Optics Sensor Based On Modal Interference And WGM Resonance

Optical fiber sensors based on modal interference have important research value and great potential in the field of optical fiber sensing,and have attracted wide attention because of their unique advantages,such as simple and flexible structure,easy fabrication and low cost.The central principle of modal interferometers is the sensitivity of the interference among different guided modes inside the fiber to the environment change.With the further research on the characteristics of sensors,modal interferometers are currently facing the problems of low detection precision,low demodulating reliability and temperature cross sensitivity and so on.The research work in this dissertation is mainly about the optical fiber sensing technology based on the modal interference and solutions for those problems.Besides,in order to seek the all-fiber sensing method with higher resolution,we did some researches to high-Q resonator in theory and experiment based on whispering gallery mode resonance.The main works of this dissertation are:(1)The background and significance of modal interference sensors are introduced,the basic principle and development of modal interference based on two types of principle,which are the interference among core modes and the interference between core and cladding modes,are described.The structures,principles and applications of the sensors in the presented works are comprehensively summarized from two aspects of mode excitation and introducing special optical fiber.(2)The principles of modal interference sensors by mode theory and evanescent wave theory are given.The relationship between the modal effective index and evanescent wave generated by fiber taper is calculated.Simulation results show shiftd interference spectrum with the external refractive index change,and prove that the evanescent wave can enhance the sensitivity of refractive index sensor demodulated by wavelength.The refractive index sensing characteristics of a typical SMS fiber structure are simulated and experimentally studied,and its structure is improved to realize high-sensitivity and temperature compensated refractive index sensor.(3)In order to further improve the sensing performance in sensitivity and measurement range,structures based on three-core fiber and hollow-core fiber are designed to achieve high sensitivity and wide measuring range of curvature measurement,and high sensitivity of magnetic field and temperature dual-parameter measurement.To raise the measurement resolution,the in-fiber Mach-Zehnder structure is inserted into the erbium-doped fiber ring cavity laser to form a laser sensor with high resolution,which improves the measurement resolution of refractive index,temperature and axial strain by an order of magnitude.(4)The volume change of glass microspheres in the relaxation process are derivated and verified theoretically and experimentally.Systematic experiments showed that the volume of silica microsphere would increase slowly in the relaxation process.The change of volume is concerned with its manufacturing method and the diameter of microsphere.