| With the increasing demand for defense in recent years,the increase of optical communication capacity between UAV and ground stations is becoming more and more urgent.The OAM beam containing OAM(Orbital Angular Momentum)is orthogonality when it is propagated coaxially.It provides a new multiplexing dimension and encoding mode on the basis of the original atmosphere window,which can improve the speed of the laser communication in the short distance space of the air.Based on this background,the propagation characteristics of OAM beam in free space and atmosphere are studied,and the Goog Le Net algorithm is used to identify the mode of the beam with mixed OAM state after it propogates through turbulent link.The specific research contents are as follows:Using the scalar diffraction theory and discrete Fourier transform theory to simulate the transmission of a beam with a single OAM state and mixed OAM state in free space and atmospheric turbulence is proposed,which lays a foundation for the analysis of the propagation of OAM beams under different atmospheric turbulence intensity and different zenith angles.Zernike polynomial is used to simulate the atmospheric turbulence phase screen,and the effects of atmospheric turbulence with different intensity and different zenith angles on the transmission with a single OAM state and mixed OAM states in atmospheric turbulence are studied.The OAM spectra of beams transmitted under different atmospheric turbulence and zenith angles are quantitatively analyzed.The transmission characteristics of OAM beams in the atmosphere have been further improved.The OAM beam pattern recognition based on Goog Le Net was proposed.The Goorg Le Net was used to recognition of the turbulent intensity patterns containing mixed OAM states.This method is based on non-coherent detection,which avoids the adverse effect of turbulence on the beam during coherent detection.It can be used to demultiplex beams containing mixed OAM modes. |
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