| With the rapid development of information technology,exchange style and range of information are expanding,people are increasingly dependent on the demanding of information transmission capacity,security and confidentiality.The information transmission channel has been stretch to infinite or semi-infinite space(atmosphere or water medium)from space limited optical fibers,information carrier has added orbital angular momentum(OAM)into traditional degrees of freedom,such as amplitude,frequency,polarization and phase.When OAM propagates in turbulent environment,turbulence can cause the crosstalk among the OAM modes,increase the bit error rate of communication system and reduce the performance of communication channel.Therefore,the study on how to effectively reduce the turbulent effects on OAM modes has important significance for improving the performance of OAM-based Free-space optical communication system.In this paper,we investigated the propagation properties of the structured beams(such as,Gaussian beams,Gaussian Schell-model beams,electromagnetic Multi-Gaussian Schell-model beams,Radially polarized Multi-Gaussian Schell-model beams,Bessel-Gaussian beams and Radially polarized vortex beams)in typical turbulent environments,and explored the method to improve the transmission performance of OAM modes in turbulent environment,based on Rytov theory and generalized Huygens-Fresnel principle.The main contents and innovations include the following aspects:1.Analytic expression of the receiver-aperture-averaged scintillation of Gaussian beams in weak non-Kolmogorov turbulent atmosphere is derived based on the Rytov theory.The influences of turbulent parameters and aperture averaging on the scintillation of Gaussian beams and average bit error rate of atmospheric laser communication systems are studied.The difference of maritime and terrestrial atmospheric turbulence on the scintillation of a Gaussian beam is detailed.Based on the modified Rytov theory,the scintillation model in weak turbulence region is extended to that in moderate-to-strong turbulent region,and the scintillation of a Gaussian Schell-model beam in a moderate-to-strong non-Kolmogorov turbulent atmosphere is developed.When ignoring the effects of turbulence inner-and outer-scales,the proposed scintillation model can return to the result in early literature.Meanwhile,the scintillation model of a Gaussian–Schell model beam can also be degraded into those of Gaussian beam,spherical wave and plane wave.2.Irradiance fluctuations in moderate-to-strong turbulent atmosphere are caused by a doubly stochastic process between the effects of large-and small-scale turbulent eddies.Thus,it is necessary to adopt a two independent distributions model to describe the irradiance fluctuations.Classical Gamma-Gamma(GG)distribution assumed that large-scale irradiance fluctuation is modeled by a gamma distribution,but large-scale irradiance fluctuation is widely accepted as a Log-normal(LN)distribution.However,gamma distribution is not a good approximation for LN distribution.Inverse Gaussian Gamma(IGG)distribution is proposed,when Inverse Gaussian distribution is adopted to model the large-scale irradiance fluctuation instead of LN distribution.The IGG distribution can be used as a more accurate model than the GG and LN distributions to describe the irradiance fluctuations in moderate-to-strong turbulent atmosphere.3.Based on the unified theory of coherence and polarization,generalized Huygens-Fresnel principle,models describing electromagnetic and radially polarized Multi-Gaussian Schell-model beams horizontally propagation in non-Kolmogorov anisotropic turbulent atmosphere are established.The effects of anisotropic atmospheric turbulent parameters and source parameters on average intensity,beam spreading and beam wander of electromagnetic and radially polarized multi-Gaussian Schell beams in atmospheric turbulence are detailed.The difference between these two beam wander models are investigated,when adopting free-space beam radius and long-term beam radius in the large-scale filter function,respectively.Employing partially coherent source and radially polarized source both can reduce the turbulence-induced beam spreading and beam wander,but the intensity distributions of radially polarized beams are more stable in the turbulence.4.Take turbulent ocean as an example,the transmission models of orbital angular momentum(OAM)modes carried by fully coherent or partially coherent Laguerre-Gaussian(LG)beams in weak turbulent ocean are developed based on Rytov theory and information optics theory.The influence of oceanic turbulence and source parameters on signal OAM modes detection probability and spiral spectrum are discussed.Results show that partially coherent source can reduce the turbulent effects on intensity modulation system,but it will damage the transmission quality of OAM modes.Besides,the difference between transmission characteristics of the OAM modes carried by Bessel-Gaussian(BG)beams and LG beams in weak turbulent ocean are compared and showed that the detection probability of signal OAM modes carried by BG beam are more higher than those of LG beam,because of its non-diffraction and self-healing ability.Take BG beams as an example,average capacity of OAM-based Free-space optical communication system in a weak turbulent ocean channel is investigated.5.Based on generalized Huygens-Fresnel principle and Rytov theory,transmission models of BG beam and its OAM mode through non-Kolmogorov anisotropic turbulent atmosphere are established.The long-term beam width,effective Rayleigh distance and beam wander of BG beams in anisotropic atmospheric turbulence are numerically simulated.We quantitatively describe the effects of anisotropic atmospheric turbulence parameters and source parameters on the signal OAM mode detection probability and spiral spectrum of partially coherent BG beams.Results show that the performance of OAM carried by BG beams propagation in weak turbulent fluctuation region is better than that of LG beams.As the propagation distance increases,the performance of OAM modes carried by BG beams decreases significantly in strong turbulent fluctuation region.OAM modes carried by BG beams are more suitable for short distance transmission.For different transmission distances,we can improve the performance of BG beam in the turbulence via reasonably setting its axial cone angle.6.Based on generalized Huygens-Fresnel principle,the cross-spectral density matrix and mutual correlation function of radially polarized vortex beams in non-Kolmogorov anisotropic turbulent atmosphere are derived.The effects of anisotropic atmospheric turbulent parameters and source parameters on the average intensity,beam spreading and beam wander of a radially polarized vortex beam are investigated.The difference between the propagation of radial polarized vortex beams and scalar vortex beams in atmospheric turbulence are compared.Results show that radially polarized vortex beams not only can reduce beam spreading and beam wander of scalar vortex beams,but also can slow down the degradation rate of its intensity distribution into circular Gaussian shape induced by turbulence and improve the performance of its OAM modes. |
