Demodulation Method Of Optical Fiber Sensor Signal Based On Modal Decomposition Technology

With the deep research on fiber mode,the application of fiber mode can open up new channels for fiber-optic communication,and provide direct and effective beam quality feedback for high-power fiber lasers.Therefore,modal demodulation as a new fiber-optic signal demodulation method is setting off a new wave of research in the fiber-related field.At present,the methods of madal decomposition technology for mode field including more than one mode are rich and varied,so it's time to introduce the mode demodulation method based on modal decomposition technology into the fiber sensor signal demodulation,which can further complete the signal demodulation of the fiber sensor.The demodulation method has the advantages of wide application range,low cost,and so on.So far,the research and development of intensity demodulation and wavelength demodulation in the field of optical fiber sensing have been very mature.Many fiber optic devices or structures evolved from spatial optical devices or configuration,such as fiber gratings,fiber interferometers,fiber surface plasmon resonance and so on,are directly performing intensity and wavelength demodulation analysis,which is same as space optic demodulation.Optical fiber sensor demodulation hardly focus on the unique and internal mode characteristics of optical fiber itself.This thesis focuses on the theoretical analysis and implementation of mode demodulation,and has completed several sensing experiments of different sensing parameters.The detailed research content of this thesis is as follows:(1)The development and application range of current intensity and wavelength demodulation have been reviewed.In order to complete the demodulation methods of optical fiber sensing signals,introduced mode demodulation to analyze the signal from optical fiber,and then briefly described the application range of mode demodulation,and finally modal decomposition method based on digital holography was selected for mode demodulation after detailed introduction and analysis of modal decomposition technique.(2)The theories and simulations of optical holography and digital holography technology for acquiring mode field was analysed and applied.Under the weak waveguide condition,the linear polarized fiber mode theory was introduced by simplifing the vector fiber mode theory.Then modal decomposition theory was studied and holographic theory were employed to acquire the fiber output mode field.Finally,the mode field was recorded and reconstructed by the image plane digital holography,and decomposed succussfuly in Matlab simulation.(3)Two holographic mode field acquisition platforms with different light source wavelengths were firstly designed and built,in which the selection of optical laboratory equipments and experimental processes were described and summarized in detail.Then the experimental algorithm was optimized in order to solve some key problems.Combined the platform with algorism,some tests including digital holographic record,reconstruction,modal decomposition,were completed succussfully.(4)Mode demodulation design,experiment and analysis of the signals of microdisplacement,curvature,tensile strain and temperature fiber sensors were carried out.Combined the experimental system with the source wavelength of 1550 nm and the misaligned structure between single-mode fiber and two mode fiber,the micro-displacement mode demodulation experiment was carried out in the misaligned region and the curvature and tensile strain mode demodulation experiments were performed on part of the two mode fiber.A nocore fiber structure with three segments and two offsets,which was covered with temperaturesensitive material,was utilized in mode demodulation experiment associated with experimental system with operational wavelength of 632.8 nm.