Study On The Transmission Characteristics Of Few-Mode Fibers Dependent On The Core Ellipticity

With the rapid development of optical communication networks,space division multiplexing technology has become an important means to improve communication capacity.As a new type of space division multiplexed fiber,the few-mode fiber plays an important role in fiber parametric amplification and fiber orbital angular momentum transmission.In this thesis,the influence of core ellipticity on the transmission characteristics of few-mode fiber is studied.The magjor contents of work and innovations are as follows:1.The mode characteristics of the few-mode fiber are studied by the COMSOL simulation method.The command for obtaining the transmission characteristic parameter information is given,and the influence of the core ellipticity on the distribution of the guided wave field mode and its effective refractive index is analyzed.The overlap integral(or nonlinear coefficient)of the mode field and the four-wave mixing phase mismatch factor between the modes are calculated,which provides a theoretical basis for the optimal design of the fiber parametric amplifier.2.The general form of the Kerr nonlinear coupled mode equation in a few-mode fiber is given.The degenerate four-wave mixing process of three modular groups in an elliptical core few-mode fiber with a step-index distribution is described in detail.The design method of the few mode fiber parametric amplifier.Studies have shown that with the LP21a mode with a pump power of 0.5W,when the core ellipticity of the nonlinear fiber is 48.9%,the LP11a or LP21b mode can obtain a maximum gain of 20.16dB,which is allowed in the range of gain degradation of 2dB.The elliptical core error is 0.6?m.3.The distribution of the exact mode field in the core fiber and the influence of the ellipticity of the few-mode ring fiber on the propagation constant of the fiber's precise mode are studied.The relationship between the exact mode and the orbital angular momentum(OAM)is analyzed.The COMSOL software was used to simulate and plot the effective refractive index difference between the order-precision modes and the odd-order modes of the same order as a function of ellipticity.The research shows that(1)In order to satisfy the effective refractive index difference between the precise modes in OAM transmission ?neff IM should be greater than 10-4,the ellipticity of the core fiber should be controlled within the range of 7.6%;(2)In a toroidal fiber,the smaller the effective refractive index difference ?neff EO between the higher order precision odd and even modes,?neff EO should be less than 10-6 in order to effectively synthesize the OAM mode.In order to ensure the effective synthesis of the OAM mode,and the mode degeneracy does not occur during the transmission process,the ellipticity of the core fiber can be optimized,that is,when the above two conditions are satisfied,the ellipticity of the ring core should be less than 3.80%.