The Research On Few-mode Erbium-doped Fiber Amplifier For High-order Mode Gain Compensation

With the advent of the mobile Internet era,the network traffic continues to grow rapidly,while the existing single-mode optical fiber transmission system is close to the capacity limit,and the upgrading and expansion of the communication system are imminent.The mode division multiplexing（MDM）technology based on few-mode fiber（FMF）loads data into various modes for transmission,which can break through the capacity limit of single-mode fiber and has attracted much attention.Few-mode erbium-doped fiber amplifier（FM-EDFA）can realize simultaneous in-line amplification of multiple signal modes,which is an important guarantee for long-distance transmission of MDM.However,in FM-EDFA,the power distribution of higher-order signal mode is more divergent than that of lower-order mode,which faces the dual dilemma of large transmission loss and small modal gain.This will result in a large differential modal gain（DMG）in the MDM system.This difference accumulates continuously under the long-distance transmission,and finally affects the transmission quality of the whole system.Therefore,adaptive gain compensation for high-order modes in FM-EDFA is the key to multi-channel energy equalization.Based on the national research and development plan for high-order optical fiber gain compensation,the specific research topics in this paper are as follows:Firstly,based on the amplification principle of FM-EDFA,the theoretical model of mode gain competition is established,the signal gain characteristics of the core pumping and cladding pumping are studied,and the variation of gain under single signal mode input and multi-signal mode input is analyzed.The results show that the mode gain competition effect is closely related to the signal power density at this position,and the position with higher power density will absorb more pump light.When the azimuthal independent pump mode is used,the gain of different spatial degenerate states in the same the first mock examination group is equal.When using the azimuthal dependent pump mode,the mode gain difference of different spatial degenerate states is up to 7 dB.Secondly,the particle swarm optimization（PSO）algorithm is proposed to design the doping structure of small mode erbium-doped fiber,and high-order mode gain compensation is realized by adjusting the doping radius and concentration of erbium ions.This method is result-oriented and can automatically search the optimal parameter configuration in the set interval.In order to verify the performance of PSO in the design of a few-mode erbium-doped fiber,the design is conducted under cladding-pump and LP₀₁ core-pump respectively,and two kinds of fibers are obtained,which are recorded as EDF-1 and EDF-2 respectively.The two optical fibers can realize the average modal gain greater than 20 dB and compensate for the higher-order modal gain so that the DMG is about 0.12 dB.Compared with the existing methods for the design of few-mode erbium-doped fiber,PSO improves the efficiency of optimization for its simplicity,efficiency,and memory.Finally,the performance of EDF-1 and EDF-2 is analyzed in 1000 km transmission.Due to the large loss of high-order mode LP₀₂,the initial signal power will be different when entering the lower amplifier.The cascade of multi-stage amplifiers causes the accumulation of energy differences.When the transmission is 1000 km,the mode power difference at the output is higher than 6.5 dB.To realize the higher-order mode gain compensation in multistage amplification,based on EDF-1,through adaptive optimization and supplemented by the hybrid pump mode of core pumping and cladding pumping,the mode power difference after 1000 km transmission is 1.239 dB.