| The light intensity of the vortex field is a circular distribution with a dark core in the center,and its phase is a helical distribution exp(ilθ)(where l is the topological charge and θ is the azimuth),and each photon has an Orbital Angular Momentum(OAM)of l?.OAM can be multiplexed/demultiplexed as a new degree of freedom in free space optical communication due to its orthogonality,thus improving the communication capacity.Cylindrical vector beams carry both orbital angular momentum and spatial polarization direction.This unique property can not only greatly improve the communication capacity of free space optical communication,but also reduce the influence of turbulence on beam transmission properties.However,when cylindrical vector beam is transmitted in free space,it is affected by atmospheric turbulence and the cross-talk between channels cannot be ignored.Therefore,how to reduce the influence of atmospheric turbulence on the cross-talk between channels of cylindrical vector beam is very important.Based on this background,the transmission characteristics of cylindrical vector beams in atmospheric turbulence are introduced in detail.The wavefront distortion correction of cylindrical vector beams transmitted in atmosphere is implemented by using adaptive optical correction technology based on Gerchberg-Saxton(GS)phase recovery algorithm.Theoretical and experimental studies are completed.The results show that this method can reduce the influence of atmospheric turbulence on the cross talk between channels,reduce the bit error rate and improve the communication quality.The main work contents are as follows:1.The generation of cylindrical vector beams and their transmission in atmospheric turbulence are studied.Firstly,the theoretical basis of cylindrical vector beam and various experimental generation methods are introduced.Secondly,the theory of atmospheric turbulence is studied and based on the Von Karman power spectrum model of atmospheric turbulence,the atmospheric turbulence is simulated by using the multi-phase plate method.Finally,the propagation of cylindrical vector beams with Laguerre-Gaussian beams as amplitude in atmospheric turbulence are simulated.The results show that when cylindrical vector beams are transmitted in atmospheric turbulence,the intensity and phase of the beams will be distorted due to turbulence disturbance.2.The adaptive correction technique of cylindrical vector beam is studied.The main components and functions of adaptive optics system are introduced.The wavefront distortion of cylindrical vector beam in atmospheric turbulence is simulated and corrected by adaptive optical correction technology based on GS phase recovery algorithm.The theoretical results show that the phase distortion caused by the cylindrical vector beam transmitted in atmospheric turbulence is compensated by the GS algorithm,and the light intensity distribution is recovered well.3.The application of cylindrical vector beam in free space optical communication is studied.Through an improved split-screen experimental method of spatial light modulator,the experimental system is built to generate arbitrary order cylindrical vector beams.Two cylindrical vector beams are combined by a beam splitter to realize coaxial transmission and multiplexing/demultiplexing of multiple cylindrical vector beams.The mode purity and bit error rate of cylindrical vector beams with/without adaptive system correction are measured.The experimental results show that the distortion of cylindrical vector beam transmitted in atmospheric turbulence can be compensated under the adaptive optics system,and the experimental results are consistent with the theoretical results.After compensation,OAM channel mode purity increases and bit error rate decreases.This adaptive correction method can effectively improve the communication quality of cylindrical vector beam. |
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