Research On Transmission Characteristics Of Fiber Grating Written By Femtosecond Laser Point By Point

Fiber Bragg grating is the core passive sensitive filter in fiber sensor which is responsible for information exchange with the outside world.The preparation method has been the focus of research.Femtosecond laser brings many new possibilities for the fabrication of fiber Bragg grating because of its high machining precision,flexibility and excellent thermal stability.When using femtosecond laser point-by-point method to prepare fiber Bragg grating,because of its unique induction mechanism,the structure of the written fiber Bragg grating is quite different from that of the previous preparation methods.As a result,the theory previously used to analyze the transmission characteristics of grating structures is not fully applicable.Aiming at the problem that the relationship between the structure and spectral characteristics of fiber Bragg grating written by point-by-point method is not completely clear.In this article,the finite element simulation method is used to construct a unique grating structure model written by femtosecond laser point-by-point method,and the parameters are adjusted for repeated calculation experiments,and analyze the influence of changes in multiple grating structures on reflection spectra.Finally,the writing experiment is carried out through the constructed writing platform.It is intended to study the FBG transmission characteristics written by femtosecond laser point-by-point method.The main purpose of this study is to provide some reference and help for fiber optic sensor performance improvement and later demodulation in the process of fiber Bragg grating preparation by using the summarized transmission characteristics,which mainly include the following contents:1.First analyze and study the basic principles of fiber grating and femtosecond laser processing mechanism.Start from the optical fiber structure and transmission principle,and summarize the optical waveguide structure and the internal light wave transmission characteristics of the optical fiber guide that causes the total reflection.The theory of fiber mode and Coupled-mode is deduced in detail;Analyze the basic principles of femtosecond laser induced material denaturation.The special structure and refractive index change mechanism of fiber Bragg grating processed by femtosecond laser point-by-point method are summarized.The above analysis and summary of the basic principle of fiber Bragg grating written by femtosecond laser point-by-point method can provide reference for the establishment of geometric models,the selection of boundary conditions and the analysis of calculation results in the subsequent simulation experiments of this study.2.The basic principle and method steps of finite element simulation method.The twodimensional single mode fiber waveguide model is successfully constructed by using the theoretical knowledge summarized.According to the unique modulation structure of femtosecond laser,fiber Bragg grating structure is added to the simulation model;By adjusting the structure and properties of the grating and repeated experiments,the reflectance results corresponding to the above variables are obtained.The changes of structural characteristics corresponding to the reflectance in different modulation regions are analyzed and summarized.It is found that the central wavelength of the reflected light is directly determined by the interval distance of the modulation region;The length of the modulation region directly affects the reflectivity of the light conforming to Bragg reflection condition,nd the reflectance becomes stronger with the increase of structure length;The variation of ellipticity also affects the central wavelength,reflectance and bandwidth of the reflected light;However,too high refractive index change will cause the central wavelength of reflected light to drift,but also the formation of multiple diffraction peaks,resulting in the reflection spectrum tends to disorder.3.An experimental study on fiber Bragg grating written by femtosecond laser point-bypoint method was carried out.First introduce the components and experimental devices of the components of the writing system.The basic process flow of fiber Bragg grating written point by point is introduced in detai;And verify the simulation results by writing and making experiments.The influences of grating length,laser output power and other parameters on the reflector peak height and 3d B bandwidth are analyzed and summarized.The FBG structure was written in different special fibers by using femtosecond point-bypoint method.Display the high flexibility and applicability of femtosecond laser point-bypoint method in writing fiber Bragg grating.