基于相位突变超构表面的微波波段OAM模式高效激发机理及传播特性研究

In microwave region, devices used to generate orbital angular momentum (OAM) mode are usually bulky and highly complicated, and there is no analytical expression to describe the origin and evolution of the generated OAM mode. In this project, research will be focused on the high-efficiency generation of OAM by metasurfaces with phase discontinuities and the transmission characteristics of the generated OAM mode. First, the transmission coefficient and abrupt phase changes of the cross-polarized component in the transmission wave will be studied in the scenario that a circular-polarized plane wave incidents on the metasurface. Then high-efficiency generation of OAM mode will be discussed based on the abrupt phase changes introduced in the cross-polarized component in the transmission wave. Second, the relationship between sub-wavelength unit cell and dipole source will be constructed, and the origin and evolution of OAM mode will be investigated analytically when a phase singularity appears in the transmission path of incident plane wave. Third, method of controlling the radius and the transmission path of the beam carrying OAM will be proposed based on the superposition of spatial phase function. The relationship between the radius of the beam, aperture of the metasurface and propagating distance will be studied. Fourth, effects of medium in the propagating path on the phase distribution of the OAM mode will be considered, including the atmospheric turbulences and absorption, and then abreaction phase can be calculated at the receiving end based on these effects in order to conquer the perturbation in phase distribution. Fifth, the mutual influences of multi-mode crosstalk on the received signal will be discussed, and OAM mode based on metasurface with phase discontinuities will be considered as a new multiplexing technology, aiming at enhancing spectrum effectiveness and information transfer rate. It is believed that the results in this research will provide theoretical basis for the application of OAM mode in microwave engineering.

针对微波波段轨道角动量模式(OAM)激发器件体积大、复杂度高、且所激发的OAM模式产生过程无表达式准确描述，本项目提出基于相位突变超构表面，实现OAM模式的高效激发，并对其传播特性展开研究。首先，研究超构表面对交叉极化波的激发效率和相位突变，建立OAM模式的高效激发方法；其次，基于超构表面亚波长单元与偶极子源的等效，从解析角度研究相位突变引入的空间相位奇异点所激发的OAM模式在近场区的产生过程及随传播距离的演变过程；第三，研究OAM波束半径及传播轨迹的控制方法，提出传播距离、波束半径与超构表面口径间的函数关系；第四，研究传播媒介对OAM模式的影响，包括大气湍流及大气衰减，建立接收端的校正相位；第五，分析模式间串扰影响接收信号的物理机制，建立基于相位突变超构表面的OAM复用链路，探索OAM模式复用对频谱效率及信息传输速率的改善。预期研究结果将为OAM模式在微波工程中的应用奠定理论基础。

由于轨道角动量模式（OAM）的多样性及模式间的正交性，在微波通信扩容方向体现出了很大的应用潜力。然而利用传统天线或天线阵方式很难实现OAM模式的高效率激发及传播轨迹控制。超构表面能够引入附加的空间相位变化，因此为OAM模式的激发提供了理论方法。本项目基于超构表面所引入的附加空间相位变化，围绕OAM模式的高效率激发、波束半径控制、大气湍流对模式纯度的影响、OAM模式与极化的复用等内容展开研究，研究结果为OAM模式的激发及传播提供了理论参考。首先，提出利用小型化频率选择表面单元结构（MEFSS）在高效率传播的基础上，实现对不同单元特定相位要求，从而构造高效率超构表面进而实现特定OAM模式；其次，分别引入聚焦相位和准贝塞尔波束相位，利用相位叠加的方式实现了对携带OAM模式的涡旋波束半径控制，研究结果表明，附加相位函数的方法能够实现将近50%的波束半径减小；第三，基于Kolmogorov统计学模型，分析了弱湍流、中度湍流和强湍流情况下，涡旋波束的模式纯度变化，研究结果表明，在相同模式数、相同传输距离、弱湍流情况下，主模式数所占能量比接近90%，接收端可以清晰的分辨出传输模式数；第四，利用超构表面对极化通道的操控，实现了四条圆极化通道上附加四个不同的OAM模式，拓展了极化与OAM模式的复用。本研究的理论研究结果，为OAM模式的高效率激发、波束半径控制、实际传播过程中的湍流影响、OAM模式与极化的复用等四方面，提供了理论参考。