Propagation Of Vortex Beam And Identification Of Orbital Angular Momentum In Turbulent Environment

Because of its narrow beam width and good directivity,the free-space optical communication technology has strong confidentiality and anti-interference ability.Compared with the traditional beam,the vortex beam carries orbital angular momentum,which can not only increase information transmission,but also greatly improve information security.Therefore,the study on turbulence effect,communication performance and identification of vortex beam in complex environment is helpful to realize the optical communication of vortex beam in free space.Base on the phase screen method and the generalized Huygens-Fresnel principle,the turbulence effects of the Laguerre Gaussian beam and Bessel Gaussian beam in atmospheric and ocean turbulent environment are studied in this paper.In addition,the Bit-Error-Rate of the modified Bessel-Gaussian beam and the Bessel-Gaussian beam in ocean turbulence is studied.The indentification of orbital angular momentum of a vortex beam at the receiving end is investigated by means of machine learning and turbulence effect.The main research content and research achievements include the following aspects: 1.The phase screen of anisotropic atmospheric turbulence is obtained by power spectrum inversion method.According to the generalized Huygens-Fresnel principle,the propagation models of Laguerre Gaussian beam and Bessel Gaussian beam in anisotropic atmospheric turbulence is established respectively,and the turbulence effects are studied,including light intensity evolution,scintillation index,beam width and beam wander.The difference of turbulence effects between Laguerre Gaussian beam and Bessel Gaussian beam in the same environment are compared.The effects of anisotropic and isotropic atmospheric turbulence on the turbulence effects of vortex beams are compared.The results show that the turbulent effects of vortex beam in the anisotropic atmospheric turbulence are weaker than that in the isotropic turbulence.Laguerre Gaussian beam is more dominant in the long-distance transmission than the Bessel Gaussian beam.The non-diffraction and self-healing ability of the Bessel-Gaussian beam keeps the distance limited,and the spread broadens beyond the distance.The Bessel-Gaussian beam is considered to be more suitable for short-range transmissions such as transmissions in ocean turbulence.2.The phase screen of isotropic ocean turbulence is obtained by power spectrum inversion method,and the propagation model of Bessel Gaussian beam in isotropic ocean turbulence was established according to the generalized Huygens-Fresnel principle.Using the generalized Huygens-Fresnel principle and the Rytov theory,the integral formula of the onaxis scintillation index of the modified Bessel Gaussian beam in the ocean turbulent environment is solved.Turbulence effect and Bit-Error-Rate of communication of modified Bessel Gaussian beam and Bessel Gaussian beam are studied.The difference of Bit-ErrorRate between modified Bessel Gaussian beam and plane wave,Bessel Gaussian beam and Laguerre Gaussian beam are compared.The results show that the Bessel Gaussian beam can effectively utilize its non-diffraction and self-healing ability in ocean turbulence environment,and its turbulence effect is closely related to the beam source parameters and the turbulence parameters.The on-axial scintillation index of a modified Bessel Gaussian beam is also closely related to the beam source parameters and turbulence parameters.The Bit-Error-Rate of Bessel Gaussian beam and modified Bessel Gaussian beam in ocean turbulence is better than that of Laguerre Gaussian beam and plane wave.3.According to the research on turbulence effect of vortex beam,the method of turbulence effect combined with machine learning to identify the orbital momentum of vortex beam is proposed.The identification model is established based on support vector machine,genetic algorithm and vortex beam turbulence theory.The communication of Laguerre Gaussian beam in atmospheric turbulence and Bessel Gaussian beam in ocean turbulence are identified by this model.A new method for identifying large quantities of orbital angular momentum is also proposed.The results show that this model not only reduces the number of training samples,reduces the difficulty of learning,but also improves the identification accuracy,so that the model can identify the orbital angular momentum of vortex beam with high accuracy under more realistic conditions.The identification accuracy of this model is much higher than that of spiral spectrum expansion method and most neural network methods.In this paper,a multi-model joint identification method is proposed to detect a large number of orbital angular momentum.