Research On Mode Conversion Method Based On Nonlinear Mechanism

Since the transmission capacity of the Wave Division Multiplexing communication system has gradually approached the nonlinear Shannon limit,the combination of mode division multiplexing technology represented by few-mode fiber(FMF)and space division multiplexing technology represented by multi-core fiber(MCF)began to appear,which solved the problem of insufficient transmission capacity.Four-wave mixing(FWM),as a nonlinear effect inside few-mode fibers,was regarded as the main factor limiting the transmission performance of communication systems in early optical communication networks.However,FWM can also be widely used in optical amplification,mode conversion,wavelength conversion,and other research directions.In addition,FWM can also be used in high-speed optical switching,ultrafast signal processing,optical sampling and other emerging fields.In this paper,based on the graded index(GI)and step index(SI)few-mode fibers,a detailed theoretical study and simulation calculation of the inter-mode FWM process are carried out.The nonlinear coupling process is used to realize the conversion between modes.The specific work and results are as follows:(1)Research on mode conversion based on GI FMF Bragg-scattering four-wave mixing(BS-FWM).Firstly,based on wave optics and the nonlinear principle of optical fiber,the equations of pump light and signal light evolution with time and space are derived.Based on the mechanism of intermodal FWM with only two fiber modes in the fiber,the nonlinear theoretical model of BS-FWM for mode conversion is summarized.Secondly,according to the theoretical model,with the help of genetic algorithm,the structural parameters(GI)that meet the phase-matching condition of the FMF BS-FWM are optimized and designed.Next,we will obtain the optimal parameters:the optical fiber core-cladding refractive index difference((35)_G)and core radius(_Ga)parameters are changed by 10%and 5%,respectively,to explore the influence of the change of fiber structure parameters on the phase-mismatch and phase matching wavelength.Numerical simulation results show that the phase matching condition of BS-FWM has relatively low tolerance to the fluctuation of the core radius parameter of the GI fiber,and relatively high tolerance to the fluctuation of the core-cladding refractive index difference.Finally,the classical fourth-order Runge-Kutta algorithm is used to numerically calculate the BS-FWM process coupling equation,and the factors affecting conversion efficiency and bandwidth(fiber length,pump light power,pump light wavelength)are calculated in detail.The results show that the designed and optimized GI fiber,when the fixed pump(P1),signal(S)mode is in the same mode(m),and the pump(P2)is in another mode(l),it passes through the BS-FWM.The linear coupling process can realize the modal conversion from LP₀₁mode to LP₁₁,LP₂₁,LP₀₂ mode,LP₁₁ mode to LP₂₁,LP₀₂ mode,and LP₂₁ to LP₀₂ mode.(2)Based on GI and SI few-mode fiber phase-conjugate four-wave mixing(PC-FWM)mode conversion research,and compare and analyze with BS-FWM.The equations of the time and space evolution of the pump light and signal light of the few-mode fiber PC-FWM are deduced,and the nonlinear theoretical model for PC-FWM mode conversion is summarized.Also with the help of genetic algorithm,optimize the design of GI and SI few-mode fiber structural parameters that meet the PC-FWM phase matching conditions.Then change the obtained optimal parameters of the SI fiber:the core-cladding refractive index difference((35)_S)and the core radius(a_S)parameters are changed by 1%to analyze the influence of the change of the optimal parameters of the SI fiber on the phase mismatch and phase matching wavelength.Numerical simulation results show that the phase matching conditions of PC-FWM have relatively high tolerance to GI fiber parameter fluctuations(core radius,core-cladding refractive index difference),and the tolerance of SI fiber to two parameter fluctuations are relatively low.Finally,the classical fourth-order Runge-Kutta algorithm is used to numerically calculate the PC-FWM process coupling equation,and the factors that affect the conversion efficiency and bandwidth are calculated and analyzed in detail.Through comparative analysis,it is found that in the PC-FWM process of GI and SI fiber,due to the nonlinear contribution of SPM and XPM,the conversion bandwidth will gradually increase with the increase of the pump light power.The results show that the optimized design of GI fiber can satisfy BS-at the same time when the fixed pump(P1)and signal(S)mode are in the same mode(m),and the pump(P2)is in another mode(l).The phase matching process of FWM and PC-FWM realizes the modal conversion from LP₀₁ mode to LP₁₁,LP₂₁,LP₀₂ mode,LP₁₁ mode to LP₂₁,LP₀₂mode,and LP₂₁ to LP₀₂ mode,respectively.But SI fiber can only realize the modal conversion from LP₀₁ mode to LP₀₂ mode.This result provides new design ideas and method support for the development of high-performance mode-related signal processing functional devices.