Study On The Transmission Properties Of Beams Carrying Orbital Angular Momentum In Free Space

Compared with the traditional free space optical(FSO)communication,FSO communication based on orbital angular momentum(OAM)can achieve high security,high spectral efficiency,and large capacity transmission,which largely solves the problem of improving channel capacity and spectrum efficiency under the limited spectrum resources.However,in practical applications,atmospheric turbulence obviously attenuate the performance of OAM-based FSO communication systems,such as,reducing channel capacity,crosstalk among channels and inducing the spread of spiral spectrum of OAM modes,etc.Therefore,it is significant to study the effect of atmospheric turbulence on the propagation properties of the beams carrying orbital angular momentum in free space,which helps to optimize the performance of OAM-based free-space optical communication systems.The goal of this paper is to mitigate the effects of atmospheric turbulence.We mainly study the propagation properties of partially coherent Bessel–Gaussian(BG)beams carrying OAM in anisotropic non-Kolmogorov atmospheric turbulence by using numerical simulation methods.As compared with LG beams,partially coherent BG beams are better light sources candidates for mitigating the influences of turbulence due to their self-healing and non-diffraction properties.Besides,we choose the anisotropic non-Kolmogorov spectrum model because it considers the anisotropy and non-Kolmogorov characteristics of the actual atmospheric turbulence.The main research works are as follows:(1)Based on the Rytov approximation theory and anisotropic non-Kolmogorov power spectrum,the analytical expression of detection probability and channel capacity of OAM modes carried by partially coherent BG beams in weak anisotropic non-Kolmogorov atmospheric turbulence are derived.In addition,the spatial coherence length expression for the anisotropic non-Kolmogorov atmospheric turbulence is obtained.(2)Based on the above work,the propagation properties of partially coherent BG beams in anisotropic non-Kolmogorov atmospheric turbulence are simulated and analyzed in detail.We quantitatively study the effects of atmospheric turbulence parameters and beam parameters on the spiral spectrum,detection probability and channel capacity.Besides,the detection probability,crosstalk probability,and channel capacity of Laguerre-Gaussian beams and partially coherent BG beams are compared under the same transmission conditions.The results show that partially coherent BG beams have greater detection probability,lower crosstalk probability and larger channel capacity than LG light,that is,partially coherent BG beams are better light sources carrying OAM modes.According to the simulation results,light source parameters and atmospheric turbulence parameters that can mitigate the effects of atmospheric turbulence on the performance of communication systems are proposed,which provide beneficial theoretic reference for practical links design of FSO communication systems based on OAM.