Design Of Few-mode Erbium Doped Amplifier System With Low Differential Modal Gain

With the emergence of a series of new services such as mobile Internet and big data,it is increasingly difficult for the capacity of single-mode fiber channel to meet the growing communication demands.Mode division multiplexing is considered as one of the effective ways to further improve the channel capacity,and it has become a research hotspot of optical fiber communication technology.Among them,few-mode erbium-doped fiber amplifier(FM-EDFA),as the key device of long-distance mode division multiplexing system,has attracted much attention.This paper focuses on the low differential mode gain(DMG)in few mode erbium-doped fiber amplifier and its cascaded system design.The main contents and innovations are as follows:1.According to the analytical expression of the mode gain of FM-EDFA,a method to measure the mode absorption coefficient of few-mode erbium-doped fiber is proposed.The single-mode optical devices are used to build an experimental platform to measure the absorption loss coefficient of few-mode erbium-doped fiber(FM-EDF)and to estimate the erbium ion doping concentration of the FM-EDF.The feasibility of this extrapolation method is verified by comparing with the simulation results and the experimental data measured by the transmission method.On this basis,by adjusting the FM-EDF length and pump power,a forward-pumped three-mode EDFA is optimally designed,and the mode gain and DMG are 20d B and 1.8d B,respectively.The simulation process of designing FM-EDFA using VPI software is summarized in detail.2.The fiber-optic optical isolation/wavelength division multiplexer(IWDM)is designed and processed by functionally integrating the wavelength division multiplexer and the optical isolator to construct the all-fiber three-mode EDFA.The amplification experiment of bidirectional signal of LP₀₁ and LP₁₁ is carried out.The pump amplification experiment realizes the tunability of DMG.A simulation method based on the equivalent erbium doping concentration is proposed,and the feasibility of the simulation method is verified by building the same VPI simulation structure as the bidirectional pumping experiment.Compared with the experimental results,the simulation results obtained by the bidirectional pump power weighting way have a maximum difference of 1.3d B.This pump power weighting process can effectively reduce the complexity of the automatic gain control of FM-EDFAs.3.Considering the mode-dependent loss of IWDM and the transmission loss of FMF,the equivalent erbium-doping concentration simulation method is used to optimize the power ratio of the forward and backward pumps,and a two-mode EDFA with a mode gain of about 11 d B and a differential mode gain close to 0 is designed.By building a simulation platform for long-distance amplification system based on optical circulator,the cascade transmission characteristics of the FM-EDFA are studied.When the two mode signals of LP₀₁ and LP₁₁are amplified and transmitted for 550km,the differential mode gain and OSNR of the system output are about 0.56d B and 27d B respectively.Further,by optimizing the design of pump power and length,a three-mode EDFA amplifier with higher gain about 19d B is designed.When the three-mode signals of LP₀₁,LP_11aand LP_11b are amplified and transmitted for 1000km,the differential mode gain and OSNR at the output of the system are about 0.2d B and 33d B respectively,which meets the project requirement of differential mode gain less than 2d B.