Free-space propagation of autofocusing Airy vortex beams with controllable intensity gradients

Fig. 1 (a) - (d) Intensity distributions of AAVBs with l = 3 for different values of m = 0, 1.6, 3.0, and 5.0 after propagation through a turbulent optical channel. (e) - (h) Phase cross-sections of AAVBs corresponding to (a) - (d), respectively.

Recently, topological charge, which can act as a measure of the angular momentum of vortex beams, has been established as a robust information carrier that is capable of being transmitted over numerous kilometers without any free-space path loss. It suggests that this technology is ready for operational deployment. However, as communication links are further extended to accommodate it, turbulence accumulated during propagation will result in the transmitted orbital angular momentum (OAM) signals becoming severely distorted, even under conditions of relatively weak turbulence. Apart from familiar turbulence effects such as beam wander, beam spreading and scintillation, it will also be important to take into account vortex splitting effects, which affect the precision of OAM signal reception. Vortex splitting refers to the topological charge of vortex beams breaking up where l >1 to give l individual single-charge vortices. These vortices cross the transverse plane quasi-independently. Generally, the larger the vortex splitting ratio, the smaller the average OAM value that can be measured.

The research group led by Prof. Lixin Guo from Xidian University, propose a new method to mitigate vortex splitting in free-space optical communication by using the self-healing effect of auto-focusing Airy vortex beams (AAVB). The value of this work is that two common modes of coupling autofocusing Airy beams with vortices are unified by introducing a parameter that controls the AAVB intensity gradients. Through the action of self-healing effect, the energy of optic field flows inward radially, trapping the individual vortices. For this to be effective, the right intensity gradients need to be selected for different degrees of turbulence, thus improving the accuracy of OAM reception. The results of this work have been published in Chinese Optics Letters, Vol 17, Issue 4, 2019 (Xu Yan, et al., Free-space propagation of autofocusing Airy vortex beams with controllable intensity gradients).

"This is a very valuable study that realizes the simultaneous modulation of beam trajectory and focus intensity without destroying the original AAVB structure, providing an effective new way of reducing vortex splitting", said Prof. Guo.

For now, the research regarding vortex beam propagation is still focused on weak turbulence conditions. The realization of effective transmission and reception of OAM signals under conditions of strong turbulence and scattering media will be the focus of the group's future work.

自聚焦艾里涡旋光束在无线光通信中的应用

图1 在不同光强梯度下，自聚焦艾里光束在湍流大气中的拓扑荷分裂。（a）-（d）光强分布;（e）-（f）相位分布。

目前，拓扑荷（涡旋轨道角动量的量度）作为信息载体在自由空间中已经实现千米级的稳固传输，这为涡旋光束在通信系统中的实用化起到了很好的推动作用。但是，随着通信链路的进一步加长，即使在弱湍流环境下，传输过程中所累积的湍流效应也将导致光信号严重畸变。

除了光束漂移、扩展和抖动等已经被广泛研究的湍流效应外，还有一种被忽略的湍流效应也将影响轨道角动量信息的准确接收，就是拓扑荷分裂。拓扑荷分裂是指拓扑荷数为l的涡旋光束在不均匀介质中传输时会分裂为l个拓扑荷为1的涡旋光束，并且分裂程度越大，发射和接收的拓扑荷信息越不匹配。

由西安电子科技大学郭立新教授领导的研究小组巧妙地利用自聚焦艾里涡旋光束的自愈效应来减弱大气湍流导致的拓扑荷分裂现象。这项工作的价值在于将目前广泛使用的在自聚焦艾里光场中加载涡旋的两种方式进行了统一，同时引进了可以调节背景光强分布的参数。

在自愈效应的作用下，实现了传播过程中光场能量的定向流动，进而将分裂后随机游荡的拓扑荷“圈禁”起来。并且指出根据具体的湍流环境合理地选择背景光强分布和拓扑荷数值可以有效地提高信息的传输精度。该成果发表于Chinese Optics Letters 2019年第17卷第4期（Xu Yan et al., Free-space propagation of autofocusing Airy vortex beams with controllable intensity gradients）。

该团队的郭立新教授认为：“这是一项非常有意义的研究，在不破坏光场原有结构的情况下，实现了光束轨迹和聚焦强度的双重调控，为减弱大气湍流导致的拓扑荷分裂提供了新的方案”。

目前，该课题关于自聚焦艾里涡旋光束的传输特性研究还局限在弱湍流环境当中，希望在接下来的研究中，可以在强湍流环境或强散射介质中实现拓扑荷信号的有效收发。