Research On Mitigation Technologies Of Atmospheric Turbulence Effects In Free Space Optical Communication

Free Space Optical(FSO)communication is a new type of communication method,which is developing rapidly and has wide applications in satellite communication,ground communication,military communication and so on.The atmospheric turbulence effect caused by the instability of refractive index greatly degrades the communication system's performance,which is an important problem in FSO system.This paper studies two important mitigation technologies of atmospheric turbulence effects in FSO--adaptive optics(AO)and partially coherent beam(PCB)technology.The main research works are as follows:1.The mitigation technologies of atmospheric turbulence effects in FSO are investigated and summarized,including the aperture averaging technology,diversity technology,lens improvement technology,adaptive optics technology,partially coherent beams and so on.2.The transmission of laser beams in turbulence atmosphere is studied,and the numerical simulation of laser beams propagating in the turbulence is investigated based on the Split-Step Fourier Method(SSFM)and FFT method.Based on the Fresnel scale theory,the principle of using the experiment in the laboratory to simulate the actual long distance communication is introduced.3.The experiment using wave-front sensor-less adaptive optics based on SPGD algorithm to mitigate the turbulence effects is performed,and the improvement of fiber coupling efficiency is investigated.The experimental results show that under different turbulence condition,the coupling efficiency can be improved by 2.7 to 9.0 dB and is more concentrated.4.The partial coherent beams technology is studied.The numerical simulation of the pseudo-partially coherent Gaussian-Schell mode beam propagating in turbulence atmosphere is performed.The simulation results show that the partially coherent beam can effectively decrease the scintillation index in the case of long distance or strong turbulence condition,and the partially coherent beam with small coherent length have stronger turbulence mitigation ability.Under the condition of Cn2?1×10-13m-2/3 and transmission distance of 2.5 km,the Gaussian-Schell mode beam can decrease the scintillation index from 2.8 to 2.2 compared with the coherent beam.