Borcea Liliana, Garnier Josselin, Sølna Knut
J Opt Soc Am A Opt Image Sci Vis. 2020 May 1;37(5):720-730. doi: 10.1364/JOSAA.384007.
A central question in free-space optical communications is how to improve the transfer of information between a transmitter and a receiver. The capacity of the communication channel can be increased by multiplexing of independent modes using either: (1) the multiple-input-multiple-output (MIMO) approach, where communication is done with modes obtained from the singular value decomposition of the transfer matrix from the transmitter array to the receiver array, or (2) the orbital angular momentum (OAM) approach, which uses vortex beams that carry angular momenta. In both cases, the number of usable modes is limited by the finite aperture of the transmitter and receiver, and the effect of the turbulent atmosphere. The goal of this paper is twofold: first, we show that the MIMO and OAM multiplexing schemes are closely related. Specifically, in the case of circular apertures, the leading singular vectors of the transfer matrix, which are useful for communication, are essentially the same as the commonly used Laguerre-Gauss vortex beams, provided these have a special radius that depends on the wavelength, the distance from the transmitter to the receiver, and the ratio of the radii of their apertures. Second, we characterize the effect of atmospheric turbulence on the communication modes using the phase screen method put in the mathematical framework of beam propagation in random media.
自由空间光通信中的一个核心问题是如何提高发射器和接收器之间的信息传输。通信信道的容量可以通过对独立模式进行复用的方式来增加，具体可采用以下两种方法：（1）多输入多输出（MIMO）方法，即利用从发射器阵列到接收器阵列的传输矩阵的奇异值分解所得到的模式来进行通信；（2）轨道角动量（OAM）方法，该方法使用携带角动量的涡旋光束。在这两种情况下，可用模式的数量都受到发射器和接收器有限孔径以及湍流大气的影响。本文的目标有两个：第一，我们表明MIMO和OAM复用方案密切相关。具体而言，在圆形孔径的情况下，对于通信有用的传输矩阵的主导奇异向量，与常用的拉盖尔 - 高斯涡旋光束基本相同，前提是这些涡旋光束具有一个特殊半径，该半径取决于波长、从发射器到接收器的距离以及它们孔径的半径比。第二，我们使用置于随机介质中光束传播数学框架内的相位屏方法来表征大气湍流对通信模式的影响。
