Propagation Characteristics Of OAM Beams In Atmospheric Turbulence

With the reform of network technology,free-space optical communication(FSOC)had been closely watched by researchers,FSOC is different from radio frequency wireless communication,it uses laser beams instead of high-frequency radio waves for longremoving transmission,in contrast classical communication methods,it possess of the preponderances of higher confidentiality,immunity to stronger electromagnetic fields,and I will reduce the depletion of radio frequency resources,and higher spectrum utilization.Orbital Angular Momentum(OAM)owns all the values of topological charges,and will not affect other optical physical quantities;researchers regard it as a new dimension and propose the way which a FSOC system uses OAM beam as carrier.The OAM multiplexing is achieved by superimposing different OAM modes together,and combined with other multiplexing method such as the WDM,etc.To further more increase the channel band width and the rate of the system,improve the communication capability of the FSOC system.A major challenge facing the FSOC system by using the OAM beam as a carrier is the interference from atmospheric turbulence,which causes the beam expanding and intensity scintillating,which badly influence the capability of the whole system.In this dissertation,the transmission performances of OAM beams among the atmospheric turbulence channels had been studied,the FSOC system based on OAM beams is designed,and the transmission performances of OAM beams among turbulence had been experimentally studied.The main research is as follows:1.The basic theories and concepts of OAM beams are explained,different types of OAM beams are introduced,and different generation methods of OAM beams are analyzed,and the strong and weak of different generation schemes has been fully discussed.Based on the SLM method,the experimental research of OAM beam generation is carried out,and the experimental structure for generating OAM beams is designed.By loading different phase holograms,high-quality OAM beams are generated.The principle of OAM multiplexing is explained.According to the OAM beam characteristics,based on the-SLM method,the multiplexing & demultiplexing of the OAM beam is realized through experiments.2.The basic theory of atmospheric turbulence is elaborated,the physical course of forming atmospheric turbulence had been introduced,the spreading process of the beam carrier among atmospheric turbulence had been theoretically deduced,and the influence of the turbulence effect of atmospheric turbulence on the beam carrier is analyzed.Starting from the atmospheric turbulence simulation method,differences between different atmospheric turbulence simulation methods are analyzed.The structure and principle of the atmospheric turbulence environment simulation equipment are described in detail,and the atmospheric turbulence environment simulation equipment is used to generate atmospheric turbulence environments with different turbulence intensities by adjusting the difference of temperature.3.A FSO transmission system based on OAM beams was designed,and experimental research on the transmission performance of OAM beams in atmospheric turbulence was carried out.A LC-SLM is used to generate the OAM beam,combined with the atmospheric turbulence environment simulation device to constitute the real environment for transmission experiments,and the experimental data such as the power jitter of the OAM beam and the BER are analyzed through the experiment,and the atmospheric transmission performances of the OAM beam had been deduced.4.Conduct the atmospheric turbulence transmission experiment of OAM beam and Gaussian beam.Through experiments,the transmission performance of the two beams was compared,and the ability of the two beams to resist the impaction of atmospheric turbulence had been discussed.In the atmospheric turbulence channel,compared with the Gaussian beam,the OAM beam has a 22.3% reduction in the diameter of the dispersion circle of the pointing error,and the beam center is more concentrated around the center of the receiving surface,and the variance of the power jitter is reduced by 0.13;the transmission system using the OAM beam is the lowest receiving sensitivity is-28.93 d Bm,which is 2.6 d B higher than the Gaussian beam.The study data shows that the transmission system using the OAM beam as the carrier has better resistance to turbulence than the Gaussian beam as the carrier,and the working performance of the system is obvious ameliorated.