Zhao Yifan, Cai Chengkun, Zhang Jinrun, Cao Xiaoping, Wang Lulu, Li Shuhui, Wang Jian
Opt Express. 2018 Jun 25;26(13):16102-16112. doi: 10.1364/OE.26.016102.
Line-of-sight link is widely used in common free-space optical (FSO) laser communications between two fixed locations. While in practical underwater wireless optical communications (UWOC), the environment is relatively complicated. In some scenarios there exist irremovable obstacles, which block the line-of-sight optical link. Fortunately, the air-water interface can function as a natural mirror to enable non-line-of-sight optical link using the total internal reflection. Very recently, twisted light beams carrying orbital angular momentum (OAM) have attracted researchers' great attention to improve the transmission capacity in UWOC. Here, we propose and experimentally demonstrate a non-line-of-sight underwater twisted light transmission link utilizing the total internal reflection at the air-water interface. To overcome the beam fluctuation and drift caused by the change of interface states, we develop a proof-of-concept adaptive feedback system to provide a stable output. Moreover, we study the degrading effects of the slight wind effect, the salinity (turbidity) effect, and the vertical thermal gradient-induced turbulence effect. The results show that the water wave caused by the slight wind causes the most beam drift, the thermal gradient causes the most distortions, and the salinity causes the most power loss.
视距链路广泛应用于两个固定位置之间的普通自由空间光（FSO）激光通信。而在实际的水下无线光通信（UWOC）中，环境相对复杂。在某些场景中存在不可移除的障碍物，会阻挡视距光链路。幸运的是，空气 - 水界面可作为天然镜子，利用全内反射实现非视距光链路。最近，携带轨道角动量（OAM）的扭曲光束在提高UWOC传输容量方面引起了研究人员的极大关注。在此，我们提出并通过实验证明了一种利用空气 - 水界面全内反射的非视距水下扭曲光传输链路。为克服界面状态变化引起的光束波动和漂移，我们开发了一个概念验证自适应反馈系统以提供稳定输出。此外，我们研究了微风效应、盐度（浊度）效应和垂直热梯度引起的湍流效应的退化影响。结果表明，微风引起的水波导致的光束漂移最大，热梯度导致的畸变最大，盐度导致的功率损耗最大。
