Study On The Turbulence Mitigation Scheme In Free-space Optical Communications Using Orbital Angular Momentum State And Orbital Angular Momentum States Multiplexing

Recently, the free space optical communication system based on orbital angular momentumstate and orbital angular momentum states multiplexing have become a hot topic in opticalcommunications and quantum optics, because OAM states are new dimensional informationcarriers，single OAM state has infinite capacity, and different OAM states are orthogonal. However,orbital angular momentum states, as spatial distributions of the wavefront, are suspectable tochannel fading induced by atmospheric turbulence (AT).Firstly，a transmitter diversity scheme using OAM states is proposed to decrease the effect of ATin the FSO communications system. Atmosphere turbulence damages the purity of OAM states andcauses a crosstalk between different OAM states. Therefore，it is difficult to retrieve eachtransmitted OAM state on the receiver. We study the power spread of OAM states with difficultazimuthal mode numbers under AT, and discuss the performance of system with and withoutdiversity. The numerical simulation results show that the communications system with diversityoutperforms the system without diversity, and the OAM state employed in system has a betterperformance with larger azimuthal mode number m.Then, an OAM-multiplexed FSO communications scheme with high efficient sorters ispresented in the thesis. With this OAM sorting method, different orbital angular momentum statescan be obtained simultaneously from the superposition OAM states. That greatly saves the time andthe expensive devices for demodulating the information from the multiplexed OAM states. Thenumerical simulation results show that the bit error rate of the system can be less than10-4when thesignal-to-noise ratio of the system is greater than23dB. Therefore, it is a feasible OAM-multiplexedfree-space optical communications scheme.Lastly，An OAM-multiplexed free-space optical communications scheme based on errorcorrecting code is given to mitigate the impact of AT.(7,4) linear block code is used as the errorcorrection code, and the BER of the OAM-multiplexed communications system with coding andwithout coding is analyzed under different turbulence strengthes. The numerical simulation resultsindicate that (7,4) linear block coding can effectively decrease the effect of AT and reduce the BERperformance of the system. The BER performance can reduce to1~2order when AT is weak andmedium (Cn2<2×10-14m-2/3)。...