To increase system capacity of underwater optical communications, we employthe spatial domain to simultaneously transmit multiple orthogonal spatialbeams, each carrying an independent data channel. In this paper, we multiplexand transmit four green orbital angular momentum (OAM) beams through a singleaperture. Moreover, we investigate the degrading effects ofscattering/turbidity, water current, and thermal gradient-induced turbulence,and we find that thermal gradients cause the most distortions and turbiditycauses the most loss. We show systems results using two different datageneration techniques, one at 1064 nm for 10-Gbit/s/beam and one at 520 nm for1-Gbit/s/beam, we use both techniques since present data-modulationtechnologies are faster for infrared (IR) than for green. For the higher-ratelink, data is modulated in the IR, and OAM imprinting is performed in the greenusing a specially-designed metasurface phase mask. For the lower rates, a greenlaser diode is directly modulated. Finally, we show that inter-channelcrosstalk induced by thermal gradients can be mitigated using multi-channelequalisation processing.
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机译：为了增加水下光通信的系统容量，我们采用空间域来同时传输多个正交的空间光束，每个光束都承载一个独立的数据通道。在本文中，我们通过一个单孔多路传输四个绿色轨道角动量（OAM）光束。此外，我们研究了散射/浊度，水流和热梯度引起的湍流的降解作用，发现热梯度引起的变形最大，而浊度造成的损失最大。我们显示了使用两种不同的数据生成技术的系统结果，一种是1064 nm的10 Gbit / s /光束，另一种是520 nm的1 Gbit / s /光束，因为目前的数据调制技术对于红外线（IR ）比绿色。对于更高速率的链接，数据是在IR中进行调制的，而OAM压印是通过使用专门设计的超表面相位掩模以绿色进行的。对于较低的速率，直接调制绿激光二极管。最后，我们表明，使用多通道均衡处理可以缓解由热梯度引起的通道间串扰。
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