ALBERT

Description: Characteristics of annular beams propagating through atmospheric turbulence along a downlink path and an uplink path are studied in detail by using numerical simulation method. It is found that in downlink the influence of atmospheric turbulence on the characteristics is quite different from that in uplink because of the altitude-dependent index structure constant. It is shown that, when the zenith angle θ is not large enough, it is always \(\sigma_{{I\,{\text{up}}}}^{2} 〉 \sigma_{{I\,{\text{down}}}}^{2}\) on propagation whatever the value of the obscure ratio ε is, where \(\sigma_{{I\,{\text{up}}}}^{2}\) and \(\sigma_{{I\,{\text{down}}}}^{2}\) are the on-axis scintillation index in uplink and downlink, respectively. However, when θ is large enough, \(\sigma_{{I\,{\text{down}}}}^{2}\) is close to \(\sigma_{{I\,{\text{up}}}}^{2}\) as the propagation distance z increases, and \(\sigma_{{I\,{\text{up}}}}^{2}\) and \(\sigma_{{I\,{\text{down}}}}^{2}\) overlap each other as ε increases. Furthermore, as z increases, \(\sigma_{{I\,{\text{up}}}}^{2}\) approaches an asymptotical value when θ is not large enough, and the saturation phenomenon of \(\sigma_{{I\,{\text{up}}}}^{2}\) appears when θ is large enough. But the asymptotical value and the saturation phenomenon of \(\sigma_{{I\,{\text{down}}}}^{2}\) never appear. On the other hand, the energy focusability in downlink is better than that in uplink, and the difference of energy focusability between a downlink and an uplink increases with increasing θ or decreasing ε . In addition, in downlink there may exist sidelobes of intensity distributions when θ is not large enough, but the sidelobes never appear in uplink.