Orbital-angular-momentum (OAM) multiplexing technology offers a significant dimension to enlarge communication capacity in free-space optical links. The coherent beam combining (CBC) system can simultaneously realize OAM multiplexing and achieve high-power laser output, providing substantial advantages for long-distance communication. Herein, we present an integrated CBC system for free-space optical links based on OAM multiplexing and demultiplexing technologies for the first time, to the best of our knowledge. A method to achieve flexible OAM multiplexing and efficient demultiplexing based on the CBC system is proposed and demonstrated both theoretically and experimentally. The experimental results exhibit a low bit error rate of 0.47% and a high recognition precision of 98.58% throughout the entire data transmission process. By employing such an ingenious strategy, this work holds promising prospects for enriching ultra-long-distance structured light communication in the future.

鉴于目前普遍选用的水下弱湍流模型无法准确拟合实测数据的缺陷，选取经实验测试验证的广义Gamma分布来表征海洋弱湍流，以期合理评估受湍流等因素影响的水下无线光通信系统的误码性能。提出一种包含广义Gamma分布弱湍流、零/非零视轴指向误差以及无衰落信道冲激响应隐路径损耗和多径效应的混合衰落信道模型，据此推导混合衰落串行中继无线光通信系统平均误码率的数学闭型表达；通过数值仿真验证所推导的误码率理论表达式的准确性，并考察不同系统核心参数对误码率的影响。研究结果表明：串行中继节点的引入可有效提高系统的误码性能，以零视轴指向误差、最终传输距离达45 m、误码率需求10^-3为例，引入1至3个串行中继节点后，系统从一开始的无法工作，变为所需的发射功率分别降低至24 dBm、12 dBm和6 dBm；而光源初始发散角增大导致的无衰落信道冲激响应时延扩展，即符号间干扰，则会严重降低此性能改善。

采用功率谱反演法模拟了同轴叠加产生的双拉盖尔-高斯涡旋光束（Double Laguerre-Gaussian Vortex Beam，DLGVB）在海洋湍流中传输时的光强和相位分布，仿真分析了DLGVB光束在不同海洋湍流参数下的闪烁指数以及在基于开关键控调制的水下光通信系统中的通信误码率。结果表明，随着湍流动能耗散率的减小，盐度温度波动平衡参数、温度方差耗散率及传输距离的增加，闪烁指数逐渐增加；随着湍流动能耗散率以及拓扑电荷差值的增加，误码率减小。在海洋湍流环境下，使用DLGVB光束进行传输可以抑制海洋湍流带来的干扰，选择最佳的拓扑电荷差值，可以有效提高传输通信质量及通信系统容量。本文研究结果对涡旋光束及其叠加态在海洋湍流下传输特性研究及水下光通信系统持续扩容的发展需求方面具有重要的参考价值。

This paper has proposed an experimental system for non-orthogonal multiple access (NOMA) wireless optical communication in challenging underwater turbulent environments, employing the gallium nitride (GaN)-based micro-LED array. This design of the GaN-based micro-LED array enables the independent transmission of signals from distinct data streams within the NOMA framework, facilitating direct optical power-domain superposition of NOMA signals. The experimental setup involves emulating oceanic turbulence channels, characterized by varying the level of scintillation intensity, to thoroughly investigate the bit error rate (BER) performance. The outcomes unequivocally demonstrate the superiority of our proposed NOMA scheme, as compared to conventional circuit-driven optical NOMA systems utilizing fixed LED array grouping, particularly in the presence of turbulent underwater channels. The proposed NOMA scheme exhibits consistently superior BER performance and maintains excellent linearity at the lower frequencies while effectively mitigating signal distortion at the higher frequencies.