Study On The Propagation Characteristics And Detection Methods Of LG Beam Based On OAM Multiplexing

With the rapid development of intellectualization and digitization of modern information technology,the transmission rate,information reliability and coverage area of existing communication modes have been unable to meet the ever-increasing demands of users'lives.The channel capacity is further improved by using time division,wavelength division,polarization division and orthogonal frequency division multiplexing in communications.However,due to the limited optical spectrum resources,these methods only alleviate the contradiction between the spectrum efficiency,channel capacity and users'demands.Therefore,a new communication mode needs to be explorred,which can greatly improve the channel capacity and spectrum efficiency for magnitude growth.In recent years,the vortex beams with orbital angular momentum(OAM)have attracted the attention of many scholars.The OAM mode has an infinite eigenstate and different OAM modes are orthogonal with each other which adds a new degree of freedom to the optical communications.Combining OAM with other multiplexing methods,encoding it and using it as carrier carrying information can greatly enhance the channel capacity of the system,which will greatly promote the development of new technologies in the field of optical communications.Therefore,the multiplexing methods,transmitting characteristics and the detecting methods of OAM of the vortex beam in free space have become the research hotspots.In this paper,the propagation characteristics of the rotating beams generated by the superposition of three positive and negative Laguerre-Gaussian(LG)beams in free space are studied.And the propagation characteristics of the double-ring structure of Laguerre-Gaussian beams spatially multiplexed under atmospheric turbulence are simulated and analyzed by Matlab.Finally,the convolutional neural network is used to detect the double-ring structure.The specific research contents are as follows:(1)The propagation characteristics of the rotating beams generated by the superposition of three positive and negative LG beams in free space are studied,then the intensity and phase distribution of the rotating beams are simulated and analyzed.The simulation results are to provide references for the rapid and direct judgment of initial OAM modes and the generation of the rotating beams in the experiment.At the same time,several important factors affecting the angular velocity of the rotating beam are studied and analyzed which are to effectively control the angular velocity.(2)The model of atmospheric turbulence channel is established and the double-ring structure of LG beam with spatial multiplexing is realized.Then through Matlab numerical simulation,we analyze the effects of the characteristics of the beam itself,the intensity of atmospheric turbulence and the combination of different OAM modes on the performance of the system after the beam transmits through atmospheric turbulence.(3)The transmission results of the double-ring structure of LG beam with spatial multiplexing under different intensities of atmospheric turbulence are detected by the convolutional neural network.The network is trained,tested and optimized,respectively.The results show that the pattern recognition method of convolutional neural network has high accuracy and the classification result is more accurate.