Special Long Period Fiber Grating Device And Its Characteristics Research

The emergence of fiber grating is a landmark revolution to the optical fiber sensing area.Especially, long period fiber grating is a kind of transmission fiber-optic grating, and ithas no reflection. Because its cycle relatively long, the transmission of the same fiber coremode and packet layer modular meet the phase matching condition, which makes long periodfiber grating （LPFG） has temperature, twisted, bending, concentration and refractive indexsensitivities than FBG. With the production methods and theory research developed, somespecial structures of the grating, the emergence of LPFG in sensing and communication of theapplication of the field has attracted more and more attention.This paper is based on the long period of fiber grating mode coupling theory and itssensing properties. Firstly, we analyze the first three layer single-mode optical fiber gratinglong period of the theory of coupling model and all kinds of preparation methods, of whichwe use the structure of the corrugated hot tuned method and mechanical slightly curvedmethod successfully achieve LPFG, and its related characteristics are analyzed.Then we put forward some special structures of the long period fiber grating, includingcapillary long period fiber grating, multi-mode long period fiber grating, LPFG coated withTiO2nano-film.Refer to capillary long period fiber grating, we first analyze the capillary fiberwaveguide, the analysis of the core modes and cladding modes in the fiber, the guidance andthe cut-off characteristics of the fiber; Secondly, based on the established after wall type ofoptical fiber waveguide capillary LPFG model, we solve out the transmission spectrμm andthe different structure parameters for the transmission spectrμm analysis; Finally, thesensitivity of resonance wavelength with the index refraction in air hole by MATLAB.Relate to the structure of SMS long period fiber grating, we use the end coupling theoryof multimode fiber to analyze different core modes and find the energy concentrate on thefirst few modes, the coupling of the core mode LP₀₃ with the largest energy and appropriatecladding mode greatly simplify the coupling calculation process of the multimode LPFG.Based on LPFG coated on thin film, through the establishment of the four layers optical fiber model, we carefully analyze the relationship of the effective index refractive of the coremodes and cladding modes change with the film thickness. As the thickness of the filmincreases to a certain value, some cladding modes will be guided into the coating layer, thestructure eliminated the requirement that for the refractive index of the medium is less thanthe index of refraction of the cladding. Then we analyze the relationship of the resonance andthe film thickness. Finally, we coat about80nm of TiO₂thin film using the liquid phasedeposition method on grating surface, and we do the experiments on the differentconcentration of ethanol.