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The modified uplink and downlink atmospheric turbulence channel models were established and employed to assess the system performance of air–ground orbital angular momentum (OAM) communication. The advantage of the vector vortex beam taking the place of the scalar one in the OAM communication system operated in the atmospheric turbulence was verified, that vector vortex beam can guarantee the more homogeneous energy in the circular hollow beam profile and the less phase distortion on signal OAM in the turbulence, which can reduce OAM crosstalk and improve OAM communication performance, especially small topological charge in strong turbulent regime. With the increase in turbulence strength, the vortex beam with a larger topological charge suffered more OAM mode crosstalk, and the average BER of the OAM communication system increased. Bessel–Gaussian (BG) beams with larger beam shape parameters had the strong capability of turbulence disturbance rejection in short-distance atmospheric applications, conversely, Laguerre–Gaussian (LG) beams with suitable parameter selection were preferred for long-distance atmospheric applications. Additionally, compared to the downlink channel, the transmission of OAM mode and the related communication system in the uplink channel are dramatically deteriorated due to atmospheric turbulent effects. [ABSTRACT FROM AUTHOR]

The match between the orbital angular momentum (OAM) transmission and receipt is a prerequisite for vortex beams as a carrier of the wireless optical communication system in free space. However, the vortex splitting induced by atmospheric turbulence results in an offset in the average OAM measured after propagating over the free space optical communication link. Therefore, how to reduce the vortex splitting effect to improve the stability of orbital angular momentum propagation is studied in this paper. First, a new parameter (m) is introduced to modulate the radial distribution of the polarization state of the autofocusing Airy vortex beams (AAVBs). Second, to control the optical field distribution in focal plane, the wavefront isophase line curvature of the AAVBs is reconstructed by using the Pancharatnam-Berry phase generated in the polarization conversion process. Finally, by comparing the vortex splitting rates of the AAVBs with different wavefrontisophase line curvatures under different turbulence environments, it can be found that the vortex splitting in free space can be effectively mitigated by choosing an appropriate wavefront isophase line curvature. In addition, the mitigation effect is more obvious in the atmosphere with stronger turbulence. This study provides a new feasible method to control the phase structure of vortex beams and promises potential applications in the OAM-based free space optical communication system. [ABSTRACT FROM AUTHOR]

This paper introduces the propagation properties and polarization characteristics of radially polarized partially coherent vortex beams in anisotropic plasma turbulence and discusses in detail the distribution of light intensity in such beams as a function of propagation distance. In addition, we analyze how the parameters of the anisotropic plasma turbulence affect the degree of polarization, polarization orientation angle, and degree of ellipticity of the beam. The results show that the degree of polarization of the radially polarized partially coherent vortex beam satisfies the modification of the central origin zero and the gradual increase with the radius during plasma turbulent propagation. At the same time, smaller turbulence eddies in the anisotropic plasma turbulence render, more obvious the irregular structure perpendicular to the vortex-stretching direction, the smaller the influence on the polarization characteristics of the radially polarized partially coherent vortex beam. It is advantageous to the research on optical communication and the study of anisotropic plasma turbulence. [ABSTRACT FROM AUTHOR]

The perfect optical vortex (POV) beam with the characteristic of the topological charge (TC)- independence radial profile is first considered to improve the spatial-mode-based underwater wireless optical communication (UWOC) performance. The evolution model of POV beams is proposed on the basis of Rytov approximation and the mutual coherence function to explore the impact of wave parameters on the orbital angular momentum (OAM) detection probability spectrum. The instantaneous crosstalk coefficients describing the POV-based UWOC systems are simulated by the multiple phase screen method with the signal coefficient modeled by a mixture Gamma-Gamma and normal distribution. A discrete and memoryless channel and a set of noncollaborative channels are modeled to obtain the capacity performances of the spatial-mode modulating and multiplexing systems, respectively. Results show the transmission quality of POV beams in oceanic turbulence is most related to the beam radius, while it is nearly free from the wavelength, TC, and radius-thickness ratio. Enjoying the advantage of the TC-independence beam radius, POV beams-based UWOC systems could achieve better bit error rate and capacity performances than the widely used Laguerre-Gauss beams when the higher-order TCs are adopted to modulate and multiplex information, which also confirms the superiority of POV beams in spatial-mode-based UWOC systems. [ABSTRACT FROM AUTHOR]

Highlights • Propagation model of radially polarized vortex beams in atmospheric turbulence is established. • A radially polarized vortex beam can reduce beam spreading and wander effects in the atmosphere. • Polarization modulation can be an approach to mitigate turbulent effects on OAM-based systems. Abstract Adopting a vortex beam with poor coherence can reduce turbulence-induced beam spreading and wander effects in the atmosphere, but it intensifies the degradation rate of its beam profile from a doughnut shape into a Gaussian shape. Transmission quality of Orbital–angular–momentum (OAM) mode carried by a vortex beam is closely related to its evolution of intensity distribution. After appropriate polarization modulation, a radially polarized vortex beam has a better performance on the reduction of beam spreading and wander effects than its scalar counterpart. A radially polarized vortex beam can also lower the degradation rate of its beam profile and keep "OAM information" a longer distance. Therefore, appropriate polarization modulation can be an effective approach to mitigate the adverse turbulent effects on OAM-based systems operated in the atmosphere. [ABSTRACT FROM AUTHOR]

Analytical expressions of average intensity and long-term beam width for a radially polarized multi-Gaussian Schell-model (MGSM) beam in anisotropic atmospheric turbulence are developed based on the extended Huygens-Fresnel principle. The influences of anisotropic non-Kolmogorov turbulence and source parameters on the average intensity, polarization and long-term beam spreading of radially polarized MGSM beam are examined in detail. The source correlations, free-space diffraction and turbulent effect jointly affect both the intensity and polarization profiles of the beam propagated in the atmosphere. As a radially polarized MGSM beam propagates in the atmosphere, increasing the associated beam order can lower the evolution rate from its flat-topped beam profile into a Gaussian beam profile because of the turbulent effect. Variations in the average intensity and long-term beam width are closely related to the propagation distance, power spectrum index, inner-scale, initial spatial coherence, and beam width. The atmospheric turbulence-induced beam spreading is generally suppressed with the enhanced anisotropy coefficient. [ABSTRACT FROM AUTHOR]

In this paper we studied the evolution of an optical vortex and an edge dislocation in atmospheric turbulence, It is shown that when mixed screw-edge dislocations beams propagate through atmospheric turbulence, the optical vortex always exists, and the edge dislocation evolves into a pair of optical vortices; when the transmission distance becomes large enough, the pair of optical vortices annihilates. The bigger the refraction index structure constant and off-axis distance of the edge dislocation, the smaller the annihilation distance of the pair of optical vortices. Especially, mixed screw-edge dislocations evolve into two optical vortices with same topological charge when the off-axis distance of the edge dislocation is zero. [ABSTRACT FROM AUTHOR]

The average bit error rate (ABER) and outage performances of decode-and-forward (DF) based multi-hop parallel free-space optical (FSO) communication system with the combined effects of path loss, pointing errors (i.e., misalignment fading), and atmospheric turbulence-induced fading modeled by M distribution have been investigated in detail. Particularly, the end-to-end probability density function (PDF) and cumulative distribution function (CDF) over the aggregated fading channel are derived for the first time. Based on the binary phase-shift keying (BPSK) subcarrier intensity modulation scheme, the analytical expressions for the end-to-end ABER and outage probability are obtained, respectively. The ABER and outage performances of the present FSO system are then analyzed systematically with the effects of turbulence strengths, weather conditions, pointing errors, and structure parameters ( M and N ) taken into account. This study shows that the turbulent atmosphere, weather conditions and pointing errors can be mitigated by increasing the number of cooperative path ( N ) over M fading channels. For the fixed hop length, the FSO system performance will be degraded with the increasing hop numbers ( M ) . But the performance will be improved with the increasing hop numbers ( M ) when the total distance from the source to destination is fixed. Monte Carlo simulation is also provided to verify the correctness of the proposed ABER expression. [ABSTRACT FROM AUTHOR]

Turbulence effects can seriously affect the propagation of structured light, especially for orbital angular momentum (OAM) modes, causing spread and crosstalk. We derive an analytical solution to the mode spectrum of a Laguerre-Gaussian beam (LGB) propagating in weak compressible turbulence. The influence of the radial number and turbulence strength in the mode spectrum on the probability of LGB propagation is mainly considered. The results show that the weak compressible turbulence interferes strongly with the LGB, and this effect is mainly reflected in the sharp drop of the signal mode energy at short transmission distance. We find that the pure radial mode has a higher signal mode energy than the pure OAM mode, which means that the pure radial mode is more resilient to compressible turbulence than the pure OAM mode. In addition, the increase of radial number can improve the performance of the OAM dimension, but on the other hand, it reduces the probability of the radial dimension. Finally, we discussed the propagation properties of higher-order modes. Our research clarifies the interaction of OAM mode and radial mode in turbulence, and provides a theoretical basis for applications based on radial modes in the field of free-space communication. • The analytical solution of the mode spectrum of Laguerre-Gauss beam (LGB) carrying radial indices in weak compressible turbulence is obtained. • The impact of compressible turbulence on the propagation characteristics of LGB mode was clarified, and the correlation between radial number and OAM number was established. [ABSTRACT FROM AUTHOR]

Abstract The symbol error rate (SER) performance of decode-and-forward (DF) based multiple-input multiple-output (MIMO) multi-hop free-space optical (FSO) systems is investigated with space–time block coding (STBC) over the exponentiated Weibull (EW) fading channel. The moment generating function (MGF) and Gauss–Laguerre quadrature rule are adopted to derive the end-to-end (EE) analytical SER expressions for this considered FSO communication system. The SER performance is further studied under various hops, STBC schemes, and turbulence conditions detailly. In addition, the asymptotic EE SER expressions are obtained to evaluate the diversity orders. Results show that the SER performances of this FSO system under EW atmospheric turbulence distribution can be improved considerably by employing the STBC scheme, and the diversity order depends not only on the numbers of transmitting and receiving antennas, but also on the channel fading parameters. The SERs of EW distribution are compared with Lognormal (LN) and gamma–gamma (G–G) distributions in weak and strong turbulence atmosphere, respectively. We also verify these analytical results through Monte Carlo simulations. This work is beneficial to the design of FSO system. Highlights • SERs of multi-hop MIMO FSO system with STBC over EW channels are studied. • The diversity order of multi-hop MIMO FSO system with STBC is studied. • Monte Carlo simulation is provided to verify the correctness of the proposed SER expressions. [ABSTRACT FROM AUTHOR]

The impact of pointing errors on the symbol error rate (SER) performance of multiple pulse-position modulation (MPPM) free space optical (FSO) system with optimized decision threshold (ODT) and dynamic decision threshold (DDT) is investigated over exponentiated Weibull atmospheric turbulence channels. The probability density function (PDF) of the aggregated channel model is derived on the basis of Meijer’s G-function and the closed-form SER expression is then obtained by Gauss–Laguerre quadrature function. The variations of SER performance with different jitter variances, beam width, aperture sizes, and turbulence strengths for these two decision threshold schemes are analyzed in detail. The study shows that the impact of jitter variances is more apparent for ODT than that for DDT scheme and the performance difference between ODT and DDT schemes decreases with the increase of beam width. Monte Carlo simulation is also provided to verify the correctness of the proposed SER models. This work is beneficial to the design of FSO system. [ABSTRACT FROM AUTHOR]

• Single scattering model of single Mie scattering model of the raindrop particles under incident Bessel beams is established. • Energy attenuation characteristics of high-order Bessel beams transmission in rainfall atmospheric medium are simulated. • Penetration ability of high order Bessel vortex beam in rainfall environment is significantly stronger than that of non-vortex beam. Vortex beam propagates in the atmospheric environment, it interacts with the floating cloud, fog, rain and other micro particles, the absorption and scattering effects will lead to the energy attenuation of vortex beam, and result in the degradation of the transmission quality of vortex beam signal. Single scattering between Bessel vortex beam and raindrop particles is simulated based on generalized Lorentz Mie theory. Combined with the raindrop particle size distribution model, the transmission and attenuation characteristics of Bessel vortex beam in rainfall environment are analyzed. Bessel vortex beam with orbital angular momentum (OAM) has multiring intensity structure and spiral phase distribution, and the extinction efficiency factor of Bessel vortex beam caused by raindrop absorption and scattering in rainfall environment is smaller than that of Gaussian beam. Bessel vortex beam has the weaker energy attenuation in rainfall, and has stronger penetration ability of rain and fog. With the increase of topological charge, the extinction efficiency factor and energy attenuation rate caused by raindrop absorption and scattering further decrease. Bessel vortex laser with larger topological charge and bigger axial cone angle has stronger penetrating ability in rainfall environment. [ABSTRACT FROM AUTHOR]

Rytov theory is employed to analyze the spiral spectrum and orbital angular momentum (OAM) crosstalk effects of partially coherent Bessel Gaussian localized wave (BGLW) in the completely weak to strong anisotropic atmospheric turbulence channel. Propagation quality of OAM mode carried by BGLW is superior to that of the traditional Bessel Gaussian (BG) beam in the turbulence, which provide a good choice for OAM based optical communication. Atmospheric turbulence grows up with the increase of Reynolds number, the effects of the atmospheric turbulence parameters (anisotropy parameters, power spectrum index and refractive index structure constant) on the OAM modal detection probability also enhance. The influence of atmospheric turbulence can be reduced by increasing the half pulse width of the input pulse and wavelength of BGLW. The research results provide a theoretical basis for the performance evaluations and parameters optimization of practical OAM based optical communication technology. [ABSTRACT FROM AUTHOR]