| Along with the advancement of the era of information,the Internet industry has been developed rapidly,and the explosive growth of traffic has put forward higher requirements for communication.In the field of space optical communication,traditional modulation and multiplexing technology is difficult to meet the demand of high channel capacity and high frequency spectral efficiency communication system.The vortex beam with orbital angular momentum provides a completely new dimension for channel multiplexing and provides the possibility for the realization of communication systems with higher channel capacity and spectral efficiency.However,all the beams used in space optical communication are inevitably disturbed by atmospheric turbulence,as is the vortex beam.The most representative effect is the mode crosstalk of vortex beam.In this paper,by studying the theoretical model of vortex beam transmission in atmospheric turbulence,the analytical expression used to characterize the mode crosstalk generated by the interaction between vortex beam and atmospheric turbulence has been obtained.Then,the atmospheric transmission characteristics of vortex beam have been characterized.By studying the mode crosstalk produced by different vortex beams under different intensity atmospheric turbulence conditions,the quantitative relationship between the purity of each mode in the vortex beam and the parameters such as turbulence intensity,transmission distance,beam waist radius and so on have been studied.The vortex beam orbital angular momentum scattering model has been established.By defining the scattering variance and scattering width,the mode scattering law of the beam during transmission has been analyzed,and the selection method of the best beam parameters has been proposed,which provides a theoretical basis for reducing the mode crosstalk of the communication system based on orbital angular momentum multiplexing.The research works main include:1.By studying the theoretical model of vortex beam transmission in atmospheric turbulence,the topological charge detection probability to characterize the vortex beam transmission has been determined.In this paper,based on Rayleigh diffraction theory,by studying the variation law of the rotational coherence function of vortex beam during atmospheric turbulence,the crosstalk between the orbital angular momentum modes of vortex beam transmitted in atmospheric turbulence has been summarized.The crosstalk law has been described by using the topological charge detection probability,and the analytic expression of the topological charge number detection probability has been derived.2.Based on the analytic expression of the probability of topological charge detection,the distribution of angular momentum patterns of vortex beams passing through turbulence has been studied.The correlation between atmospheric turbulence spectra and orbital angular momentum modes of vortex beams has been studied and characterized by defined coherence parameters.The atmospheric turbulence parameters and beam parameters have been further studied for the purity variation of the original mode and the crosstalk between different modes.3.A numerical simulation platform for vortex beam transmission in atmospheric turbulence has been established by using the theory of atmospheric random phase screen and MATLAB software.The influence of different intensity of atmospheric turbulence on the intensity and phase of vortex beam has been studied.By using the theory of rotational coherence function,the distribution of different orbital angular momentum modes has been calculated by the intensity and phase of the received beam,and the crosstalk between the vortex beams under different transmission conditions has been studied.4.In this paper,the experimental platform for generating vortex beam has been established,the vortex beam has been generated by using liquid crystal space light modulator and spiral phase plate respectively and the transmission experiment has been carried out.By combining the atmospheric turbulence simulation device,the vortex beam wavefront detection device,and adjusting the relevant parameters(transmission distance,beam waist size,generation mode,turbulence intensity,etc.),the distribution of the phase plane of the vortex beam has been collected.Thus,the distribution of angular momentum modes has been obtained by calculation,and the degradation of orbital angular momentum during transmission has been further recorded.5.The theoretical derivation results,simulation results and experimental data have been systematically compared and analyzed.It has been found that the theoretical analytic formula can well describe the degradation and crosstalk of the angular momentum modes of vortex beams during atmospheric turbulence.The proposed scattering width based on the theoretical expression can intuitively characterize the crosstalk of atmospheric turbulence with the vortex beam,which provides a theoretical guide for spatial optical communication system based on orbital angular momentum multiplexing to select suitable vortex beam mode intervals and optimal beam parameters. |
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