Research On Single-mode Performance Of Large- Mode-area Photonic Crystal Fiber Based On Mode Separation Method

In the past few decades,high-power fiber lasers have developed very rapidly.This type of fiber laser has many excellent features,including excellent beam quality,good heat dissipation,compact size,and low operating costs.With the rapid development of high-power fiber lasers,the nonlinear effects of optical fibers in fiber lasers have become a key factor limiting the increase in power.In order to suppress optical nonlinear effects and avoid the occurrence of mode instability,optical fibers need to ensure that they can have Excellent single-mode performance,at the same time also to achieve a larger mode area.In this paper,a new large-mode-area photonic crystal fiber(LMA-PCF)is designed based on the mode separation method.A large amount of research is focused on the effective mode area and single-mode properties of photonic crystal fiber(PCF).The main contents are as follows:(1)The research background related to fiber lasers is introduced,and the photonic crystal fibers are classified.The working principle and optical characteristics of photonic crystal fibers in different application scenarios are explained.The physical modeling and simulation analysis methods of photonic crystal fiber are summarized.Based on the existing research,the finite element method is used to study the photonic crystal fiber,and the boundary conditions of the perfectly matched layer(PML)are combined to establish the corresponding physical model.A simulation software called COMSOL Multiphysics is used to assist analysis.(2)The influencing factors of various performance parameters of the photonic crystal fiber are analyzed.Unlike the traditional photonic crystal fiber,this paper focuses on the research of asymmetric photonic crystal fiber with large mode area.After establishing the corresponding physical model for the LMA-PCF with asymmetric structure,various factors that affect the performance of the optical fiber are explored,including the arrangement rule of the air holes and the eccentricity parameters of the air holes to the various modes of the PCF.The effect of leakage losses also includes the effect of bending radius on PCF performance.Then,the method of mode separation is proposed.Based on the LMA-PCF bending filter mode,the eccentricity parameters of the elliptical air hole are adjusted to suppress the fundamental mode loss in the fiber.The bending loss of different modes in the asymmetric fiber responds differently to the eccentricity parameters of the air hole.(3)A novel asymmetric LMA-PCF structure is proposed based on the mode separation method.In this structure,the idea of using the parameter of the eccentricity of the elliptical air hole to control the bending loss of the basic mode and the higher-order mode is further proposed.The LMA-PCF designed in this way can obtain extremely low bending losses while ensuring a large mode field area and effective single-mode operation.Numerical results show that this structure can achieve large mode area and effective single-mode operation with low bend loss for the fundamental mode at the wavelength of 1.064?m.Thanks to the asymmetric design of this type of photonic crystal fiber,according to the actual needs in different scenarios,it is possible to flexibly optimize the parameters of this type of fiber.