Research On The Characteristics Of Eccentric-core Long Period Fiber Grating

Because the core deviates from the center axis,the sensing characteristics of the eccentric core fiber are different from the traditional standard single-mode fiber.Compared with the single-mode fiber,the eccentric core fiber core is closer to the external environment,so it has a more powerful evanescent wave,and has a higher sensitivity to changes in the external environment.Long period fiber gratings are widely used in the fields of optical fiber communication and optical fiber sensing due to their advantages such as simple manufacturing process,low integration insertion loss,and no backscattering.Writing a long period grating on an eccentric-core fiber will result in an optical fiber device with both the advantages of an eccentric-core fiber and the advantages of a long period fiber grating.It has a small insertion loss,weak backscatter,high sensitivity,good direction sensitivity,and polarization,and so on.However,the traditional long period fiber grating is written on the standard single-mode fiber.There is less research on writing eccentric-core long period fiber grating.This dissertation systematically studies the theoretical modeling,simulation analysis and grating fabrication of eccentric eccentric-core long period fiber grating.The main research content includes:?1?Based on coupled-mode theory,the theory of long period fiber gratings was studied in detail by analytical and numerical methods.At the same time,avoiding the complex analytical calculations of eccentric-core long period fiber gratings,the coupled-mode theory of long period fiber gratings with eccentric cores was studied by numerical methods.The coupled-mode equations were established and the formulae for calculating the transmittance were deduced.?2?Taking a single-mode long period fiber grating as an example,the single-mode long period fiber gratings were compared and analyzed by analytical and numerical methods,respectively.Among them,the analytical method uses the Matlab programming solution,and the numerical method uses the numerical simulation software COMSOL Multiphysics simulation calculation based on the finite element method.By comparing the simulation results of the two methods,the numerical simulation method is used to verify the correctness of long period fiber gratings.?3?Using COMSOL Multiphysics to model and simulate the fiber structure,determine the effective refractive index and mode field distribution of the fiber mode,as well as the cladding modes involved in the write grating.?4?Comparing the experimental parameters,the numerical simulation method was used to simulate the transmission spectrum of the long period fiber grating.The effects of eccentric core fiber parameters?bias and fiber refractive index difference?and grating parameters?index modulation depth,grating period,and grating length?on eccentric-core long period fiber gratings were simulated and analyzed.At the same time,the polarization characteristics of long period fiber gratings with eccentric cores were analyzed.?5?The long period fiber gratings were fabricated by CO₂ laser.The effects of grating parameters?grating period and grating length?on long period fiber gratings were studied experimentally.?6?Comparing with the simulation results,the theoretical,simulation and experimental conclusions are unified,and a comprehensive systematic study of eccentric-core long period fiber gratings has been formed.