The Design Of Few-mode Fiber Based On Multi-layer Structure And The Brillouin Scattering Characteristics Of All Modes

The rapid development of cloud computing,big data,Internet of Things and other emerging information technologies has put forward higher requirements for the capacity of the existing communication network.Due to the Shannon limit of single-mode fiber,people try to find various ways to improve the capacity of communication system.In recent years,the technology of mode division multiplexing has been widely studied.The increase of the modes will lead to serious dispersion effect among in the fiber.With the advantage of more controllable modes,the few mode fiber is more suitable as the carrier of mode division multiplexing than the multi-mode fiber.In addition,Brillouin scattering effect in optical fiber has important applications in sensor field.The traditional single-mode fiber only supports one mode,which is not conducive to multi-dimensional simultaneous sensing.In the few mode fiber,different modes have different Brillouin scattering characteristics,which has greater potential in multi-dimensional simultaneous monitoring.Therefore,the content of this paper is the design of multi-layer structure few mode fiber and the analysis of Brillouin scattering characteristics of each mode in the few mode fiber.In this paper,we first introduce the application of the few mode fiber in improving the transmission capacity and the sensing field,and then discuss the research of Brillouin scattering characteristics in the few mode fiber at home and abroad.Then we introduce the characteristics of the few mode fiber,and analyze the basic principle of Brillouin scattering,and introduces how to use the finite element method to simulate.Secondly,a multi-layer ring core with different refractive index is designed.Through simulation analysis,the structure supports six transmission modes and has a large effective index difference.The simulation results show that at 1.55 ?m wavelength,the minimum effective index difference between all modes is 1.56 ×10-3,and the mode field area ranges from 132 ?m2 to 298 ?m2.Then,the Brillouin scattering spectrum of the structure and the mode combined Brillouin scattering spectrum are analyzed by finite element method.Through the simulation results,it is found that the fundamental mode has double peak Brillouin scattering spectrum,LP02 and LP12 have three peak Brillouin scattering gain spectrum,and the other three modes LP11,LP21 and LP31 show single peak.Multiple transmission modes of the structure correspond to different Brillouin scattering gain spectra,which is helpful to solve the cross sensitivity problem.At the same time,compared with the traditional single-mode fiber,the structure has larger effective mode field area and higher input power,which is beneficial to improve the signal-to-noise ratio and sensing distance by increasing the signal power in BOTDR system.Finally,a photonic crystal few mode fiber based on multi-layer air holes with different arrangements is designed.By using multi-layer air holes with different geometric arrangements around the core region,the number of transmission modes and the effective mode field area in the fiber are controlled.The results show that at 1.55 ?m wavelength,the fiber supports two modes of stable transmission,the effective refractive index difference is 8.7 ×10-2,the mode field area is 3.44 ?m2 and 5.14 ?m2 respectively,and the corresponding nonlinear coefficients are 35.46 w 1×km-1 and 23.73 w-1×km-1 respectively.Then,the Brillouin scattering spectrum and intermodal spectrum of the two modes are analyzed.It is found that the Brillouin scattering gain spectrum of the two modes are single peak,and the linewidth of the intermodal spectrum is broadened to 455.92mHz due to the large difference of the corresponding Brillouin frequency shift.The smaller effective mode field area of the two modes is beneficial to the stimulated Brillouin scattering between the pump light and the probe light in the BOTDA system.