Research In Gain Equalization Of Few Mode Erbium Doped Fiber Amplifier

At present,the network traffic communication system exponentially grow rapidly.However,the existing single-mode optical fiber transmission system has been close to capacity limits,unable to meet the growing capacity demand for optical fiber communication system.As a new way of space division multiplexing,mode division multiplexing transmission with the new spatial dimensions(fiber mode),can break through the single-mode transmission capacity limit,which greatly improves the capacity of optical fiber communication.As a repeater of optical transmission system,erbium-doped fiber amplifier(EDFA)plays an important role in long-distance optical communication transmission.Conventional erbium-doped fiber amplifiers generally take single-mode erbium-doped fiber as the gain medium and can only amplify a single mode,unable to meet the mode-division multiplexing systems.In the mode division multiplexing system,if there is a larger different mode gain,it will decrease the system capacity and increase outage probability.Therefore,the research in Fewmode erbium-doped fiber amplifier(FM-EDFA)and realizing gain equalization of different modes have become a hot field of optical communication in recent years.In this paper,in order to achieve the gain equalization of FM-EDFA,we firstly introduce the history and trends of optical communications,analyze and summarize the current research about FM-EDFA.Based on Maxwell's equations of the electromagnetic wave,we use "scalar method" and "vector method" to derive optical scalar mode and vector mode,respectively.We also analysis the cut off condition and well wave guiding condition of the two kinds of mode.Furthermore,based on the theoretical model of FM-EDFA,we introduce the main analytical method,which can solve FM-EDFA theoretical model.By adjusting pump structure,we realize two signal modes gain equalization through regulating the power ratio,rotation angle and loop phase respectively.Using genetic algorithm optimization,we design erbium ion distribution and realize gain equalization of four signal modes.Finally,we preliminary build the experimental setup of FMF transmission and amplification system,to analyze the gain property of EDF.We observe the gain characteristics of different signal modes in FM-EDF with changed pump power.We also measure the gain after filtered ASE.