Research On System Performance Of Free Space Optical Communications Based On Orbital Angular Momentum Multiplexing

The orbital angular momentum(OAM),as the characteristic of the vortex beams,has been widely concerned by people since presented by Allen in 1992.When the vortex beam containing orbital angular momentum transmits in the free space,it may be affected by the atmospheric turbulence and the misalignment,causing dispersion among the orbital angular momentum states,which seriously influences the performance of communication system.In this paper,with the theoretical derivation and MATLAB simulation,the mathematical model of the vortex beam in the presence of atmospheric turbulence and misalignment are established,and a new method of beam alignment is proposed.The main work and achievements of this paper are as follows:Firstly,a summary and analysis of the atmospheric turbulence effect and the main atmospheric turbulence models are carried out.The mathematical models of non-Kolmogorov and modified von Karman atmospheric turbulence channel are established,and the influence of the atmospheric turbulence on the OAM detection probability is analyzed.Secondly,the effect of atmospheric turbulence and the main atmospheric turbulence model are summarized and analyzed.The mathematical model of the atmospheric turbulence is established.The non-Kolmogorov and von Karman OAM turbulence model are analyzed theoretically,and the optimal states of OAM optical communication are selected.Thirdly,the theory of the misalignment of beam is introduced and analyzed,and the model of the vortex beam in a misalignment system is established,and the influences of offset on the transmission of different OAM states are studied.An alignment scheme of a vortex beam with orbital angular momentum is presented,and the alignment process and the driving algorithm are presented in detail.Fourthly,the offset tolerance and the system performance of vortex beams in a misaligned system are investigated by the method of numerical calculation.Through the analysis and research of the above four aspects,this paper studies the performances of the OAM system in the presence of atmospheric turbulence and beam misalignment,and the results of the research provides guidance for further research on the application of OAM technology in optical communication field.