Research On Gain Equalization Of Layered-Doped-based Few-Mode Erbium-Doped Fiber Amplifier

With the vigorous development of emerging applications such as cloud computing and the Internet of Things,the data traffic increases explosively and the capacity of the single mode fiber(SMF)is approaching the Shannon limit.The few-mode-fiber-based(FMF)mode-division-multiplexing systems are regarded as an effective instrument to further improve the channel capacity owing to its inherent advantages on bridging the Shannon limitation of SMF,which utilizes different modes as different channels to achieve signal transmission and has drawn more attention.The few-mode erbium-doped fiber amplifier(FM-EDFA)is an indispensable repeater,which guarantees the high-speed long-haul transmission of different signal modes.However,the inherent differential modal gain(DMG),induced by mode competition between different modes,introduces an imbalance of signal power of different signal modes and severely deteriorates the stability of the system.Therefore,aiming to relax the influence of gain imbalance on FMEDFA,this paper optimizes doping profile of the erbium ions utilizing forward design scheme and reverse design scheme.And two schemes,i.e.,layered-doped with ring-core fiber and step-index fiber,are proposed to decrease the DMG and improve the gain equalization performance of FM-EDFA.The main innovations and work are as follows:(1)The mode theories and amplification characteristics of few-mode erbium-doped fiber and the principle of amplifier are analyzed and summarized in this paper.Besides,the optimization schemes of DMG and the impacts of various related factors are also investigated in this paper.Owing to the low complexity on simulation and high stability of the cladding pumped scheme,a theoretical model of the cladding-pumped based fewmode erbium-doped fiber amplifier is established aided by the Runge-Kutta algorithm and the target shooting algorithm.(2)A ring core few-mode erbium-doped fiber with erbium ions layered-doped structure is proposed,which aided by the control-variable-method with a straight forward scheme to optimize the refractive index and doping profile.The ring core fiber is designed to reduce DMG by controlling the mode distribution adjusted by the central recess and outer groove refractive index of the core,combining with the well-designed doping radius and concentration of erbium ions.Results show that about 0.43 d B DMG could be obtained at 1550 nm.In the full C-band,the gain is higher than 22 d B and the DMG is less than 0.45 d B.Besides,a powerful robustness is obtained and results show that the DMG is always beneath 0.84 d B when a 10% deviation of the erbium ions doping radius or concentration is introduced.The straight forward design scheme is beneficial to investigate the impacts of individual factors,which has advantages on flexible operation and low computational complexity.(3)An erbium ions doping profile scheme is proposed utilizing Mento-Carlo(MC)scheme from an inverse aspect.This scheme can achieve automatic design of the erbium ions doping radius and concentrations to minimum DMG when the number of layers in step-index fiber is given.Results show that about 0.016 d B DMG could be obtained at 1550 nm when the number of erbium ions doping layer is three.Besides,about 21 d B gain and 0.444 d B DMG could be obtained in the full C-band.Additionally,the DMG is always beneath 1.09 d B when a 10% deviation of erbium ions concentration or doping radius is introduced.MC scheme is a result-oriented scheme,which could analysis the influence of various parameters simultaneously and flexibly,as well as finding the optimal solutions automatically.Compared with other algorithms,MC is more transparent of the initial state,more insensitive of the factor and stronger to avoid the local optimal solutions,which opens another insight of the design scheme of active fiber.