Research On Propagation Characteristics Of Vortex Beams In Oceanic Turbulent Channels

With the rapid growth of the demand for underwater information transmission,underwater optical communication has become a hot topic of research.At the same time,in recent years,the vortex beams with orbital angular momentum(OAM)have very good application prospects in optical communication.The multiplexing of the vortex beams with different topological charges can greatly improve the capacity and spectral efficiency of the communication system.However,the method of using OAM to enhance the channel capacity of the communication system is easily limited by oceanic turbulence,which seriously affects the performance of underwater optical communication system.The main work and results of this research are as belowFirstly,a brief introduction of partial coherent Hankel-Bessel beam and oceanic turbulence is presented.Based on the Rytov approximation theory,the analytical expressions of ONAM spectra for partial coherent Hankel-Bessel vortex beams under weak horizontal oceanic turbulent channels are derived.And the influences of oceanic turbulence parameters on the OAM spectra of partial coherent Hankel-Bessel vortex beams and the channel capacity and the channel efficiency of partial coherent Hankel-Bessel vortex beams are investigated via numerical calculations.The results show that ocean turbulence leads to mode crosstalk and spiral phase spectrum expansion in the OAM model.The negative effects of Ocean Turbulence on partially coherent r-HB vortex beams increase with the increase of OAM mode number,propagation distance and temperature variance dissipation rate,and decrease with the increase of beam wavelength and turbulence energy dissipation rate.Moreover,partially coherent HB vortex beams are more negatively affected by ocean turbulence dominated by salinity fluctuations.In addition,fully coherent vortex beams provide better performance than partially coherent vortex beams.Secondly,based on the analytical expressions of OAM spectra for partial coherent Hankel-Bessel vortex beams and for partial coherent Laguerre-Gaussian vortex beams,the detection probability and bit error rate are solved numerically and compared under different oceanic turbulence conditions.The results show that under the same turbulence conditions,the system performance of the two kinds of vortex beams is related to the initial waist size,the receiving aperture,the ratio of the transmitting power of the system to the noise power of the receiver,which provides a reference for the selection of the two kinds of vortex beams.Thirdly,a brief introduction of convolutional neural network(CNN)is presented.And Eight types of multiplexed beams are obtained based on sharpness.Multi-step diffraction method is used to simulate numerically the propagation of OAM beams in the oceanic turbulence,and the training and testing database are obtained under different oceanic turbulence.The recognition rate of Multi-OAM beams using convolutional neural network models under different wavelength,transmission distance and oceanic turbulence conditions is studied.The results show that the multi-mode ONAM pattern recognition method based on CNN decreases with the increase of temperature variance dissipation rate and transmission distance,and increases with the decrease of turbulence energy dissipation rate;the multi-mode OAM pattern recognition method based on CNN is greatly affected by salinity fluctuation;compared with the single training set,the model based on mixed training set is more sensitive to salinity fluctuation.Generalization performance is better;with the same recognition accuracy,the size of training set increases with the increase of the number of pixels in the input picture,but with the increase of the size of training set,the training time of the model of the OAM pattern recognition method based on CNN will increase correspondingly;the performance of the multi-mode OAM pattern recognition method based on CNN is better than that of the OAM mode recognition method based on CNN.Traditional conjugate phase mask detection method.Through theoretical derivation and numerical simulation analysis,this paper studies the influences of oceanic turbulence parameters on the vortex beams and the recognition rate of Multi-OAM beams using convolutional neural network models.These works provide some reference value and new ideas for further research on the application of vortex beams in the field of oceanic optical communication.