Research On Vortex Beam Wavefront Correction And Mode Detection Under Atmospheric Turbulence

Vortex Beam,also known as Orbital Angular Momentum(OAM)beam,has the characteristics of zero light intensity at the central singularity,phase is spiral,etc.,the orbital angular momentum it carries is a new dimensional resource of light wave in the space domain,carrying different orbital angular momentum mode of vortex beams orthogonal to each other.Therefore,vortex beams are not only used in image processing,particle manipulation,optical computing and other fields,it can also be applied to the field of free space optical communication to improve the capacity of communication systems.When the vortex beam is transmitted in free space,it will be interfered by atmospheric turbulence,resulting in crosstalk between signals,which in turn reduces the reliability and stability of communication,so it is of great theoretical significance and practical value to correct the distortion wavefront of the vortex beam under atmospheric turbulence and effectively detect the orbital angular momentum mode carried by the vortex beam.The main works of this thesis are as follows:(1)The superimposed vortex beam is introduced,and the vortex beams formed by superposition of the same sign and different sign orbital angular momentum modes are simulated by Matlab,and the relationship between the light intensity and phase distribution characteristics and the orbital angular momentum mode value carried by the beam is analyzed.(2)Based on the analysis of the causes of atmospheric turbulence,the atmospheric turbulence refractive index power spectral models of Kolmogorov,Tatarskill,Karman,Hill and Andrews are simulated by Matlab.Based on the power spectrum inversion method,the atmospheric turbulence random phase screen is obtained,and the vortex beam transmission system in atmospheric turbulence is established based on the theory of multilayer atmospheric turbulence random phase screen.(3)The wavefront correction of distorted vortex beams transmitted through atmospheric turbulence is studied by using Gerchberg-Saxton(GS)algorithm,and the conclusion is drawn by analyzing the orbital angular momentum spectra of vortex beams before and after correction: In the case of strong atmospheric turbulence,the GS algorithm is used to correct the vortex beams formed by the superposition of the same sign and different sign orbital angular momentum modes,the purity of the target orbital angular momentum mode in the beam is increased by more than 12% and13%,respectively,The diffusion of the orbital angular momentum spectrum is also effectively suppressed.(4)A convolutional neural network(CNN)model for detecting the angular momentum mode of vortex beam under mixed atmospheric turbulence intensity was designed and experimentally verified.The experimental results show that the detection accuracy of the model can reach 99.5%under the condition of mixed atmospheric turbulence.The model is used to detect the orbital angular momentum mode of the distorted vortex beam at the transmission distance of 1000 m,2000m and 3000 m,respectively,and the accuracy can reach 96.56%.The model is used to detect the orbital angular momentum mode of the distorted vortex beam at the beam wavelengths of850 nm,1310nm and 1550 nm,respectively,and the accuracy can reach 98.81%.The model was used to detect the orbital angular momentum mode of the distorted vortex beam at the radius of the beam beam waist of 0.02 m,0.03 m and 0.04 m,respectively,and the accuracy could reach 98.52%.