Research On The Detection Of Vortex Beam In Free Space Optical Communication

In recent years,free-space optical communication based on vortex beam has attracted wide attention in the world due to its great communication capacity and extremely high spectral efficiency.In theory,the orbital angular momentum carried by the vortex beam is infinite and orthogonal to each other,which can provide new dimension resources to optical communication.However,when the vortex beam is transmitted in free space,it is inevitably affected by the atmospheric channel,resulting in crosstalk of the orbital angular momentum state of the bearer information,and the performance of the communication system is degraded.In this paper,the research of vortex beam detection technology in free space optical communication is carried out.Firstly,it is analyzed that the transmission characteristics of orbital angular momentum in free space,with focusing on the effects of atmospheric turbulence,atmospheric scattering and alignment error on the orbital angular momentum transmission.Then,according to the detection accuracy caused by these effects,the corresponding vortex light detection technology is proposed,and relevant theoretical analysis and experimental verification are carried out.The main research contents of this paper include the following four aspects:?1?The influencing factors of the free-space vortex orbital momentum transfer and detection are analyzed.Firstly,taking the Laguerre-Gaussian beam as an example,the basic concept of orbital angular momentum is introduced.Then,the vortex optical atmospheric channel transmission is studied.The transmission characteristics of the vortex beam orbital angular momentum state are analyzed under different atmospheric turbulence strength and transmission distance.Combined with the atmospheric turbulence theory and the spiral spectrum concept,the analytic expression of the orbital angular momentum detection probability under the influence of turbulence and alignment error is given.Finally,in order to analyze the interaction of the scattering particle and orbital angular momentum state,the paper uses the finite difference time domain method to give the basic law of the influence of scattering particles on the transfer of vortex beam.?2?Based on the influence analysis of the atmospheric turbulence of the vortex beam orbital angular momentum,a vortex beam detection technique based on convolutional neural network is proposed for the effects of turbulence effect,angular tilt error and lateral offset error.Firstly,a convolutional neural network is used to detect the vortex beam under the influence of turbulence.The detection performance under different turbulence strength and transmission distance is analyzed experimentally.Then,for the angular tilt error,a volume based on angle pooling is proposed.The improved neural network method increases the angle pooling structure between the fully connected layer and the convolutional layer,and realizes the joint training of multi-angle data,which improves the accurate detection of the oblique vortex beam without increasing the network complexity.For models trained using mixed dataset,the detection accuracy is over 98%in the case of weak turbulence and over 88%in moderate turbulence conditions.Finally,for the beam offset error,a vortex beam detection neural network model based on partial convolution generative adversarial network and convolutional neural network is proposed to improve the accuracy of offset vortex beam detection.With cascaded network-based detection technology,the channel capacity can be increased by about 25%at_nC²=7?10^-1414 m^-2/3.?3?The complex-valued neural network is used to detect the vortex beam.On the basis of the original amplitude information detection,the vortex wavefront feature is learned at the same time,and the accuracy of the detection is improved without increasing the space complexity.Aiming at the problem that the neural network has long operation time and it is difficult to meet the real-time needs,a vortex beam detection technology based on all-optical neural network is proposed,which mainly includes three types of pure phase,pure amplitude and amplitude phase hybrid.With the dataset of_nC²=5?10^-1414 m-^2/3,the trained three-layer hybrid network has the best cross-detection performance.With the strong turbulence(_nC²=5?10^-1414 m-^2/3)dataset,the network achieved an accuracy of 88%.?4?Wavefront correction technology can compensate the distortion caused by atmospheric turbulence and reduce the influence of turbulence effect on free space vortex beam optical communication.Firstly,a wavefront reconstruction technique based on generative adversarial network is proposed,which can recover the wavefront distribution of the beam according to the single intensity distribution,and expand the range of recoverable wavefront compared with the existing wavefront restoration method based on generation.Under the condition of strong turbulence,the normalized channel capacity of corrected system is about 3 times that of the pre-correction system.Aiming at the problem that the traditional random parallel gradient descent algorithm has a slow convergence rate,combined with the Adam optimization algorithm in deep learning,an iterative correction algorithm based on adaptive gain factor is proposed.The simulation verifies that the method is more robust to the turbulent environment.The research conclusion have certain theoretical guiding significance in the channel transmission theory,detection technology and wavefront correction technology involved in vortex beam optical communication system.