Research On Few-Mode Erbium-Doped Fiber Preamplifier For Atmospheric Laser Communication

Atmospheric laser communication is one of the important components of free-space optical communication.Due to the laser transmission through atmospheric channel,the received optical signal will appear light intensity flicker,beam diffusion,drift and other phenomena,seriously affect the quality of laser communication.Compared with the traditional single-mode erbium-doped fiber preamplifier（SM-EDFA）,few-mode erbium-doped fiber preamplifier（FM-EDFA）can effectively suppress the influence of atmospheric turbulence effect and improve the spatial optical coupling efficiency.Therefore,firstly,the performance and characteristics of FM-EDFA are studied by theoretical modeling and simulation,and the parameters of FM-EDFA are designed according to the simulation results.And then a FM-EDFA performance experimental platform is built through three-mode and six-mode Erbium-doped fibers and passive devices,and the gain and noise figure characteristics are studied,according to the results of characteristic research,three-mode erbium-doped fiber preamplifier（3M-EDFA）and six-mode erbium-doped fiber preamplifier（6M-EDFA）prototypes are developed.Finally,the feasibility of atmospheric laser communication based on FM-EDFA is verified by simulation.The communication characteristics of indoor simulated atmospheric laser communication based on 3M/6M-EDFA are studied experimentally,and the coupling efficiency and BER are compared with SM-EDFA to verify the improvement of communication performance by FM-EDFA.The specific research contents can be summarized as follows:（1）Theoretical modeling and simulation of FM-EDFA:On the basis of theoretical research,the theoretical model of FM-EDFA is established.Based on the FM-EDFA theoretical model and the few mode fiber spatial optical coupling theoretical model,software simulations were conducted to study the relationship between the normalized frequency（V value）of the fiber and the spatial optical coupling efficiency under turbulent conditions,as well as the relationship between gain,noise figure and the length of the few mode erbium-doped fiber,pump power.Based on the simulation results,a forward core pumping FM-EDFA is designed.In which the few-mode Erbium-doped fiber has a V value of 5,a core radius of 8.2μm,a mode number of 6 and a length of 1.8m.The spatial light coupling efficiency is higher than 0.83 under the turbulence with atmospheric refractive index constant of 10^-15and 5×10^-15.When the pump power exceeds 100m W,the small signal gain of six spatial mode signals is above 18.5d B.When the pump power exceeds400m W,a small signal gain of more than 28.8d B is obtained,and the noise figure is between 5d B and 5.5d B.（2）Experimental study on FM-EDFA:In order to design and develop a FM-EDFA prototype,3M/6M-EDFA experimental platforms were built,the gain and noise figure under different erbium-doped fiber lengths and different pump powers are studied.By optimizing the length and pump power of erbium-doped fiber,When the input power of the3M/6M-EDFA is-30d Bm and the pump power is 300m W～800m W,the gain is respectively between 22d B～31.8d B and 20.1d B～29.4d B,and the noise figure is respectively less than 6d B and 7d B.The prototype is formed after the parameters are determined.Then,the characteristics of mode gain equalization are studied.It is concluded that the differential modal gain（DMG）between 3M/6M-EDFA modes is within 2d B and 3d B,respectively.（3）Simulation and experimental study of atmospheric laser communication based on FM-EDFA:Firstly,the performance of atmospheric laser communication system based on FM-EDFA is studied by software simulation,and realize six modes of transmission amplification.When the BER is 10^-3,the receiving sensitivity is-37.6d B.Then,based on the 3M/6M-EDFA prototypes,the experimental system of On-Off Keying（OOK）and differential phase shift keying（DPSK）atmospheric laser communication is built.Compared with SM-EDFA,the coupling efficiency and bit error rate（BER）under different logarithmic light intensity jitter varianceσ_l² are studied.Result shows that:1)FM-EDFA is beneficial to improve spatial optical coupling efficiency and reduce received power jitter.Whenσ_l²=0.7204,using 3M/6M-EDFA can improve the coupling efficiency by 4.1d B and 5.8d B respectively,and reduce the received power jitter by 10d B and 17.5d B respectively;2)The advantages of FM-EDFA over SM-EDFA vary with the intensity of turbulence.Whenσ_l²=0.1236 and the transmission power is-10d Bm,OOK/DPSK communication bit error rate based on3M/6M-EDFA is less than one order of magnitude from that of SM-EDFA.However,whenσ_l²=0.7204,the BER of 2.5Gbps OOK communication based on 3M-EDFA and 6M-EDFA decreases by two or three orders of magnitude,and that of 10Gbps DPSK communication decreases by one or two orders of magnitude;3)Whenσ_l²=0.3349 and 0.7204,the communication error rate based on 6M-EDFA is lower than that of SM-EDFA and 3M-EDFA;4)FM-EDFA improves the performance of OOK communication system more than DPSK.In this paper,the gain and noise figure characteristics of FM-EDFA are studied by simulation and experiment,the design process of fiber structure parameters,the length of few-mode erbium-doped fiber and the pump power range is described,and its advantages in atmospheric laser communication are expounded.The research results provide reference for the design of FM-EDFA and the research of atmospheric laser communication based on FM-EDFA.