Research On Key Technologies Of Erbium-doped Fiber Amplification In Mode Division Multiplexing System

With the advent of the Internet of Everything,network traffic is growing rapidly at over 35%annually.To meet the future development of information transmission,the mode division multiplexing technology of fiber optic communication has attracted much attention in recent years and has become one of the cutting-edge research hotspots.At present,as one of the key components for developing mode division multiplexing technology,erbium-doped fiber amplifiers still face problems such as noise optimization,bandwidth expansion,mode gain equalization,and improving pump conversion efficiency.In this paper,the basic theory,preparation process,and parameter characterization of the erbium-doped fiber were introduced.On this basis,the key technologies of erbium-doped fiber amplification involved in the mode division multiplexing system are studied and discussed in the current commercial C-band,the future development of the L-band and the extended L-band,respectively,to provide the theoretical basis and technical basis for the development of the amplification technology of mode division multiplexing.This paper presents a method to improve the noise performance of erbium-doped fiber by adjusting the mode field distribution.The depressed refractive index profile was used to flatten the mode field distribution of erbium-doped fiber.The simulation results show that noise figure decreases by 0.53 d B in the C-band compared with the traditional erbium-doped fiber having Gaussian mode field distribution.The effect of the flattening mode field on the population inversion and amplified spontaneous emission（ASE）of erbium-doped fiber was analyzed.The maximum noise figure optimization of 0.63 d B was realized by narrowing the doping radius based on the depressed refractive index profile.Cladding pumping is the most direct and effective way to reduce the differential modal gain（DMG）of few-mode erbium-doped fiber amplifier（FM-EDFA）.However,the low cladding pump absorption of FM-EDF leads to excessive residual pump light,which results in low pump conversion efficiency in C-band.In this paper,the method of co-doping ytterbium in the erbium-doped fiber was employed to enhance the cladding pump absorption.After establishing the theoretical model of cladding-pumped few-mode Er/Yb co-doped fiber amplifier（FM-EYDFA）,a numerical comparison of amplification characteristics between FM-EDFA and FM-EYDFA was presented.The results show that under the same conditions,the cladding-pumped FM-EYDFA achieves modal equilibrium in the C-band,while its gain performance is improved by more than 10 d B compared with that of FM-EDF.According to the simulation results,a double-clad few-mode Er/Yb co-doped fiber was prepared and an all-fiber cladding-pumped test structure was built.The results show that the large signal gain of LP₀₁,LP₁₁,LP_21,and LP₀₂in the C-band was measured up to 28 d B with an average DMG of<1.5 d B.Moreover,the saturated output power of the fiber was more than 35 d Bm with a pump conversion efficiency of>31%,which is over three times higher than the maximum efficiency of cladding-pumped FM-EDFA reported so far.Modal gain equalization is one of the most intractable problems in the core-pumped FM-EDFA,which is generally at the cost of cutting the overlap factor and consequently deteriorates the gain performance.The influence of the structure parameters of uniform doping FM-EDF on the overlap factor was numerically investigated,and results revealed the mutual restriction between DMG and gain performance.In the case of core pumping of LP_11a+LP_11bmode,the increasing core diameter is beneficial to reduce DMG but meanwhile decrease gain.Increasing the NA of FM-EDF helps to increase the mode overlap factor,which can improve the gain performance while maintaining low DMG.According to simulation results,the conventional L-band FM-EDF was prepared,for which the NA was increased to 0.25 by increasing the Al³⁺doping concentration,and the core radius was selected to 8.1μm.An all-fiber LP_11a+LP_11bmode core-pumped configuration was constructed.The experimental results show that the DMG of the FM-EDF was less than 1.8 d B and the large signal gain was up to 19 d B for 5 LP modes in the wavelength range of 1570-1605 nm.In addition,the trench-assisted double-ring doping FM-EDF was proposed to alleviate the instability of amplification performance caused by the change of mode angle,for which the gain equalization between the six mode channels can be achieved in the L-band under the LP₀₁mode pumping configuration.Expanding the bandwidth of the few-mode amplifier in the L-band contributes to improving the capacity of the optical communication system in the dimensions of spatial and wavelength.The principle of the extended L-band erbium-doped fiber amplifier was introduced,and the effects of pump wavelength and co-doped ions on bandwidth expansion were analyzed.The gain and DMG characteristics of core-pumped FM-EYDFA were experimentally studied,and the limitation of the core pumping method in the application of the extended L-band few-mode fiber amplifier was revealed.In this paper,a pseudo-two-stage amplification configuration was proposed to enhance the gain of FM-EYDFA in the extended L-band.The simulation results show that the gain of the pseudo-two-stage structure at 1610 nm was improved by more than 20 d B while maintaining low DMG,compared with that of the traditional cladding-pumped structure under the same conditions.According to the theoretical analysis,the pseudo-two-stage amplification configuration was deployed.The experimental results show that 1μm ASE and C-band ASE were effectively inhibited,and the gain bandwidth of 20 d B was extended to 1620 nm.The average gain of 25 d B with an average DMG of<1 d B for LP₀₁and LP₁₁mode groups in the wavelength range of 1570-1620 nm was obtained.The results indicate that the proposed FM-EYDFA with the pseudo-two-stage configuration owning low DMG and wide bandwidth is expected to improve the capacity of mode division multiplexing systems in the future.