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.

为改进脉冲位置调制方式的性能，结合差分脉冲位置调制和多脉冲位置调制的优势，提出一种新型差分脉冲位置调制方式。在给出其符号结构的基础上，分析了其传输容量、带宽需求和功率需求，推导出Gamma-Gamma湍流信道下自由空间光通信系统的平均误时隙率和误包率模型，仿真分析了湍流强度、传输距离和调制阶数对该系统性能的影响，并将其与开关键控和三种脉冲位置调制方式进行比较。理论分析与仿真结果表明，新型差分脉冲调制方式比单脉冲位置调制、差分脉冲位置调制和多脉冲位置调制具有更高的传输容量，比单脉冲位置调制和差分脉冲位置调制具有更低的带宽需求，比多脉冲位置调制和开关键控具有更高的功率利用率和更好的差错性能，而且不需要符号同步，可以简化系统实现的复杂度。

Free-space optical communication (FSO) can achieve fast, secure, and license-free communication without physical cables, providing a cost-effective, energy-efficient, and flexible solution when fiber connection is unavailable. To achieve FSO on demand, portable FSO devices are essential for flexible and fast deployment, where the key is achieving compact structure and plug-and-play operation. Here, we develop a miniaturized FSO system and realize 9.16 Gbps FSO in a 1 km link, using commercial single-mode-fiber-coupled optical transceiver modules without optical amplification. Fully automatic four-stage acquisition, pointing, and tracking systems are developed, which control the tracking error within 3 μrad, resulting in an average link loss of 13.7 dB. It is the key for removing optical amplification; hence FSO is achieved with direct use of commercial transceiver modules in a bidirectional way. Each FSO device is within an overall size of 45 cm × 40 cm × 35 cm, and 9.5 kg weight, with power consumption of ∼10 W. The optical link up to 4 km is tested with average loss of 18 dB, limited by the foggy test environment. With better weather conditions and optical amplification, longer FSO can be expected. Such a portable and automatic FSO system will produce massive applications of field-deployable high-speed wireless communication in the future.