Analysis And Study On Mode Crosstalk Of Vortex Beams For Wireless Communication In Turbulent Environment

Vortex beams with different orbital angular momentum modes are orthogonal.Theoretically,the value of orbital angular momentum can be infinite.Therefore,the capacity of vortex beams wireless communication based on orbital angular momentum multiplexing obviously increases.However,the vortex beam is affected by turbulence in propagation,leading to beam expansion,orbital angular momentum dispersion and so on.Moreover,the turbulence causes crosstalk of orbital angular momentum modes contained in the beams and reduces the performance of wireless optical communication systems.Therefore,it is important to study the crosstalk between orbital angular momentum modes for improving the turbulence resistance of vortex beams and to reduce the interference of turbulence on communication systems.In this thesis,considering the types and parameters of beams and turbulence,we study the influences of beams and turbulence to wireless optical communications in orbital angular momentum spectrum by theoretical models and numerical analysis,and the performance of wireless optical communication is improved.The main innovative works of this dissertation are as following:1.Based on Laguerre-Gaussian beam,the influence of Kolmogorov atmospheric turbulence on the orbital angular momentum spectrum is studied.We derive the general expressions for the probability density of orbital angular momentum modes of the Laguerre-Gaussian beam,and a theoretical model for propagation in the Kolmogorov atmospheric turbulence is established.By analyzing the influence of beam waist widths,wavelengths,topological charges and radial indices on the received probabilities of orbital angular momentum modes of Elegant and Standard Laguerre Gaussian beams in Kolmogorov turbulent atmosphere,we propose the best selection scheme of beam and its parameter which can improve the ability of anti-turbulence interference.The results show that Elegant Laguerre Gaussian beam with smaller beam waist width and radial index,larger wavelength and topological charge is less disturbed by turbulenceand is more suitable for propagation in atmosphere.2.In order to mitigate the disturbance of atmospheric turbulence on the vortex beam,the model of orbital angular momentum spectrum of the perfect vortex beam in non-Kolmogorov atmospheric turbulence is established,and the influence of turbulence parameters and beam parameters on orbital angular momentum spectrum of perfect vortex beam is discussed.We find the turbulence with large inner scale and power spectrum index has little influence on perfect vortex beam,and the larger wavelength is beneficial to improve the turbulence resistance of the beam.Furthermore,we study the effects of propagation distance and the radius of received aperture on the signal probability of the orbital angular momentum mode.The study suggests that there are the optimal initial beam waist width and the radius of received aperture to make the signal probability get maximum for different propagation distances.3.Based on the spatial coherence length of spherical waves in ocean turbulence,we establish the model of the received probability for orbital angular momentum modes of Lommel beam,and study the effect of ocean turbulence on underwater wireless optical communication.The influence of the characteristic parameters of ocean turbulence and beam on the modes crosstalk of Lommel beam is studied by the signal and crosstalk probabilities of orbital angular momentum modes.The results show that the Lommel beam is suitable for propagation in ocean turbulence with small temperature-to-salinity ratio,dissipation rate of mean-squared temperature and dissipation rate of kinetic energy and large inner scale.The Lommel beam with asymmetric parameter with smaller absolute value and larger wavelength possess stronger robustness,it is beneficial to reduce the interference of ocean turbulence to wireless optical communication.