水下轨道角动量光通信 
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The ocean occupies more than 70% of the earth's surface, which has vast area and rich resources. Research and exploration of the ocean have never ended. Due to the complexity and variability of the underwater environment, the ocean has not yet been fully explored and utilized. Further exploration of the underwater environment plays an important role in climate change, oil and gas detection, disaster early-warning, biological research, and other fields. Underwater wireless communication ensures information transmission and interconnection between unmanned devices in the underwater environment during ocean exploration. As the demand for underwater data transmission increases, high-bandwidth and low-latency underwater communication has become a key technology for exploring and utilizing the ocean at a deeper level.
Commonly used carriers for underwater wireless communications include sound waves, electromagnetic waves (e.g. radio frequencies), and light waves. Each of the three carriers has its own characteristics. Although sound waves, as a traditional underwater communication method, have the advantage of a wide transmission range and have been widely used, the problems of relatively narrow bandwidth and longer delay in the medium limit their applications. Electromagnetic waves are difficult to be widely used in underwater environments as they require complicated equipment and short transmission distances. As a new type of underwater communication technology, underwater wireless optical communication has gained widespread attention due to its advantages such as larger transmission bandwidth, better anti-interference ability, lower latency, and lower costs. Underwater wireless optical communication refers to an underwater communication system that uses light waves as the transmission carrier. In recent years, underwater wireless optical communication has made considerable progress in the transmission capacity through the expansion and utilization of multiple physical dimensions of light waves, such as wavelength, time, amplitude, phase, and polarization. However, there are challenges in further improving the transmission capacity. The exploration of the spatial dimension of light waves has become a feasible way for capacity scaling.
Structured light refers to a special light field that exploits the spatial dimension by tailoring the spatial amplitude, phase, and polarization distribution of light waves to obtain the required characteristics. Especially, structured light with a spiral phase front carrying orbital angular momentum (OAM) has attracted interest in many applications such as optical manipulation, tweezers, sensors, metrology, microscopy, imaging, and quantum science. OAM-carrying structured light appears spatially as an annular intensity distribution due to phase singularity at the beam center. Since OAM-carrying structured light can accommodate multiple orthogonal spatial modes, it has important advantages in expanding the capacity of underwater wireless optical communication. We comprehensively reviewed the advances in underwater OAM optical communications.
We first introduced the development history of three types of underwater wireless communication technology, including underwater acoustic communication, underwater electromagnetic (radio frequency) communication, and underwater optical communication, and summarized their respective advantages and disadvantages. Then, we focused on underwater wireless optical communication using OAM modes, with their basic principle, generation, and measurement methods introduced. The research progress of underwater OAM mode wireless optical communication was comprehensively reviewed, including underwater OAM mode encoding and decoding communication, underwater OAM mode multiplexing communication, and underwater OAM mode broadcasting communication. Moreover, OAM mode optical communications involving air-water interface ("water-air-water" crossing air-water medium, total reflection by "air-water" interface) and fast auto-alignment assisted OAM mode optical communications were presented. In addition to the OAM mode, other underwater structured light (e.g. Bessel beam and Ince-Gaussian beam) communications were also introduced. Additionally, complex medium optical communications using OAM modes assisted by adaptive turbulence compensation and fast auto-alignment were presented.
OAM mode exploits the spatial dimension of light waves, providing a new way for the sustainable capacity expansion of underwater wireless optical communication. The future development trend of underwater wireless optical communication is as follows. From the spatial mode point of view, more flexible and powerful spatial light manipulation, a large number of OAM modes, more general structured light accessing the full spatial dimension (spatial amplitude, spatial phase, and spatial polarization), and full use of multiple dimensions are highly desired. From the underwater communication point of view, complex channel modeling, high capacity, long distance, and high robustness are highly expected. Key devices [lasers, modulators, detectors, converters, and (de)multiplexers] and techniques (high speed, high power, high sensitivity, high efficiency, high scalability, and high integration) are of great importance. Meanwhile, from the perspective of future underwater wireless optical communication, on the one hand, it is expected to be combined with electromagnetic (e.g. RF) communication and acoustic communication. According to different application scenarios and different capacity and distance requirements, one or more suitable communication methods and their combinations can be selected. On the other hand, the integration of underwater wireless optical communication technology and underwater perception technology (i.e. integrated communication and perception) is also an important research direction in the future, which is of great significance for improving the development capacity of marine resources, developing the marine economy, protecting the marine ecological environment, and serving the strategy of becoming a powerful marine country.
1 引言
全球气候变化及陆地资源日益枯竭使得对海洋的研究和开发已成为大势所趋，其中围绕水下无线通信（UWC）系统的研究受到了重要关注。水下无线通信指的是通过使用无线载波在水下环境中进行数据的传输，其中无线载波可以是声波、电磁波（如低频射频波）、光波等［1］。声波传播速度较慢、频率较低的特点使得高速、低延迟的数据传输难以实现，电磁波（如低频射频波）在水下的传输需要使用复杂设备，导致灵活性低和传输距离短等问题，难以得到广泛应用［2-4］。水下无线光通信（UWOC）采用光波作为数据信息传输载体，相较于使用声波与射频波进行水下通信，具有更大的带宽、更好的抗干扰能力和更好的保密性能等优点［5］，近年来逐渐成为水下无线通信的研究热点。
水下无线光通信也面临很多问题，携带数据信息的光波会受到水下复杂介质环境（如海洋环境）中湍流、盐度、温度、浊度、海底障碍物等因素的影响，从而导致信号的衰落和传输质量的下降［6-7］。同时，海水中存在着大量的溶解物质和悬浮体，在不同的海域、水深以及季节等情况下的海水特性不同，从而导致光波在传输过程中发生脉冲的衰减和展宽，并产生误码，最终影响传输数据的正确性和传输距离［8］，这种传输介质称为海水信道，其特性与大气、光纤信道不同，对于不同波长光波的衰减也不同。为了在更大衰减长度的海水信道中实现更大容量的通信，光源和调制方式的选择是影响水下无线光通信系统性能的重要因素。除此之外，也可以考虑选择光波的空间结构。目前的水下无线光通信系统主要使用的是高斯光，其具有近似均匀的场分布（束腰位置）。实际上，通过剪裁光波的空间结构可以得到具有非均匀场分布的结构光，如空间变化的幅度、相位、偏振分布等，其相比于高斯光开发了光波空间新维度资源，因此为提升光通信（包括水下光通信）的容量和丰富通信形式提供了新思路。
在多样化结构光中，携带轨道角动量（OAM）的涡旋光是一种光强呈圆环形分布的特殊结构光，其波前呈螺旋状，中心为相位奇点，中心光强为零［9］。涡旋光独特的空间幅度和相位结构使其在光学操控、显微成像、传感测量、量子科学等众多领域获得了广泛应用［10］。特别地，不同于圆偏振光携带的自旋角动量（SAM）仅有两个取值，涡旋光携带的OAM理论上可以为任意整数取值，且相互之间具有正交性。与光波波长维度的多值性和正交性类似，利用光波空间维度的OAM模式也可以携带信息并应用于光通信中［11-12］。目前OAM模式已被应用于自由空间光通信［13］、光纤通信［14］、片上光互连［15］。与此同时，OAM模式也可以应用于水下无线光通信中。例如：在2.96 m的水下信道中，利用螺旋相位板产生和检测涡旋光，在单路激光器直接调制频率为1.5 GHz的情况下，通过两路OAM模式的复用实现了3 Gbit/s的数据传输［16］；利用空间光调制器（SLM）产生和探测涡旋光，在1.2 m水槽中通过4路OAM模式复用和10 GHz的激光器直接调制，实现了40 Gbit/s的数据传输［17］。这些实验验证了使用OAM模式可以在现有水下光通信技术基础上进一步提升通信容量。近年来，在水下无线光通信中使用OAM模式及其拓展的结构光正在受到越来越多的关注。
本文聚焦基于OAM模式的水下无线光通信技术。在简要介绍水下无线通信系统的基本概念和不同种类的水下无线通信技术（水下声波、电磁波和光波通信）后，重点对基于OAM模式的水下无线光通信技术进行了详细阐述，系统回顾了各种水下OAM模式光通信的实验进展，同时也介绍了水下其他结构光通信和复杂介质OAM模式光通信。最后，简要讨论了水下OAM模式光通信的未来发展趋势，并对其应用前景进行了展望。
2 水下无线通信
水下无线通信技术是指不需要借助光纤等传输介质，使用声波、电磁波（如低频射频波）和光波等无线载波在水中进行数据传输。其中，水下声波通信（UAC）被视为几十千米量级长距离通信最实用的方式［18］，是水下长距离高延迟定位中最常用的通信手段，其最早的使用可以追溯到15世纪达·芬奇使用声波来估计船只的距离。电磁波（如低频射频波）通信可以追溯到19世纪无线电在**上用莫斯码通信，电磁波在陆地和水下的传播有较大差异，陆地通信中电磁波传播相对容易控制，水下环境复杂多变且海水中电磁波衰减严重，主要可进行水下短距离通信，其速率通常高于水声通信。不过，水声通信和电磁波通信都存在着有限带宽和数据传输速率的问题，因此，水下无线光通信作为有效的替代方案正在受到广泛关注，其高速和低延时特性可以满足日益增长的水下通信容量和带宽需求。下面将从各自的发展历程和优缺点等方面对这三种水下通信方式进行介绍。
2.1 水下声波通信
由于具有长距离通信的优势，水下声波通信在过去成为了最常用的水下通信方式［19-39］，其发展历程被简单汇总在
2.2 水下电磁波通信
水下声波通信的数据传输速率受限且时延较大，水下电磁波通信［43-59］作为另一种水下通信方案被提出，其发展历程简要汇总在
图 2. 水下电磁波通信的发展历程［2， 43-47］
Fig. 2. Development history of underwater electromagnetic communication[2, 43-47]
2.3 水下光波通信
由于水下声波通信和水下电磁波通信都存在传输能量高、带宽小等问题，可以使用具有大带宽和低延时的光波在有限范围内进行水下无线光通信。用于水下无线光通信的光源主要包括蓝绿光发光二极管（LED）和激光二极管（LD）。水下无线光通信目前已经有很多研究工作［61-71］，
图 3. 水下无线光通信的发展历程［25， 61-68］
Fig. 3. Development history of underwater wireless optical communication[25, 61-68]
除了
表 1. 水下无线光通信近10年的发展情况
Table 1. Development of underwater wireless optical communication in past decade
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表 2. 三种水下无线通信技术的比较［25］
Table 2. Comparison of three underwater wireless communication technologies[25]
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3 基于OAM模式的水下无线光通信
在过去几十年里，通过利用光波的波长、幅度、相位、时间、偏振等传统维度，水下无线光通信技术在提高通信容量方面取得了重要进展，不过随着这些传统维度开发殆尽，进一步增加通信容量将面临严峻挑战。探索光波的空间新维度为进一步增加通信容量提供了一种重要解决途径。通过调控光波空间维度和剪裁光波空间结构可以得到结构光，其中包括具有螺旋相位波前携带OAM的涡旋光以及拓展的具有空间变化幅度、相位、偏振分布的广义结构光。相比光波传统维度，OAM模式开发了空间新维度。一方面，OAM模式具有多值性和正交性特点，即OAM可以有很多取值且两两相互正交，可以像其他维度一样用于信息编码和作为载波进行信息复用；另一方面，OAM模式空间维度与光波传统维度相互兼容，即基于OAM模式的光通信技术可以与传统光通信技术有机融合。因此，基于OAM模式的水下无线光通信技术可以在现有水下无线光通信技术基础上进一步进行通信容量扩容。
3.1 OAM模式原理及产生测量方法
OAM模式是一种具有螺旋相位波前的特殊结构光，相比于高斯光（束腰位置为平面波前），其空间相位结构被调控，对应地也产生了被剪裁的空间幅度分布。OAM模式具有
图 4. 具有不同拓扑电荷的OAM模式特征（强度分布、相位分布和波前）
Fig. 4. Characteristics of OAM modes with different topological charges (intensity distribution, phase distribution, and wavefront)
OAM模式的产生关键是要生成具有空间螺旋相位分布的光场，产生方法包括有源与无源两种。有源方法主要利用激光腔直接输出OAM光束，该方法具有良好的光束质量［83-84］；无源方法可以利用衍射光学元件、变换光学方法、螺旋相位板、Q板（Q-plate）、J板（J-plate）、数字微反射镜（DMD）、空间光调制器（SLM）、光纤器件、光子集成器件、超材料、超表面等［85-93］，无源方法的可控性强，实现便捷，方式多样。
产生OAM模式后，可以利用不同的OAM模式进行编码通信，也可以利用不同的OAM模式作为不同的载波携带各自信息进行复用通信，还可以利用不同的OAM模式进行一对多的广播通信。OAM模式通信的接收端通常需要将OAM模式转换为常规具有平面相位波前的类高斯光束，这种转换一方面可以与现有高斯光通信系统相兼容，另一方面也是最直接的解调OAM模式的方法，因为解调后的类高斯光束中心为亮斑，易于从其他OAM模式中滤波出来。目前，使用比较多的方法是利用具有相反拓扑电荷数（
3.2 基于OAM模式的水下无线光通信研究进展
在以往的文献中，OAM技术已被报道并广泛应用于自由空间、光纤、芯片等场景的大容量光通信和互连中［11-15， 94-98］。近年来，基于OAM模式的水下蓝绿光通信也有实验报道［16-17］，OAM技术拓展了空间维度，其为水下无线光通信的发展提供了新思路。
表 3. 近年来基于OAM模式的水下无线光通信研究成果
Table 3. Summary of recent underwater wireless optical communication achievements using OAM modes
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3.2.1 水下OAM模式编译码通信
传统的OOK、BPSK、QPSK、QAM等调制格式信号可以看成是利用光波的幅度和相位维度（复振幅维度）进行编译码，类似地，也可以利用光波的空间维度进行编译码。由于OAM模式的拓扑电荷数理论上可以取值无穷，利用不同OAM模式直接进行数据信息编译码为提高通信容量提供了一种有效解决方案。
利用光波多种OAM模式实现水下OAM模式编译码通信已有研究报道。2017年，Wang等［106］在多进制水下无线光通信系统中采用拉盖尔-高斯光束和轨道角动量移位键控（OAMSK）调制，研究了弱海洋湍流通道，分别推导了基于OAMSK调制的水下无线光通信系统的符号误码率和信道容量的解析表达式，并使用最佳模式间隔详细分析了该多进制水下无线光通信系统在不同弱湍流条件下的符号误码率性能和通道容量。
图 5. 多进制 OAMSK调制的海洋湍流水下无线光通信系统［106］。（a）四进制OAMSK调制的水下无线光通信系统；（b）弱海洋湍流下有效信号能量与传输OAM模式的关系；（c）信道容量与信噪比的关系
Fig. 5. Underwater wireless optical communication system using multi-ary OAMSK modulation over ocean turbulence[106]. (a) Quaternary OAMSK modulation-based underwater wireless optical communication system; (b) effective signal energy versus transmitted OAM mode under weak ocean turbulence; (c) channel capacity versus signal-to-noise ratio
2018年，Cui等［107］研究了一个使用卷积神经网络（CNN）的UOC-OAM-SK解码器，模拟了8种叠加的拉盖尔-高斯（LG）光束作为三进制OAM-SK编码器，并模拟了光波在海洋中的传输。
图 6. 海洋湍流通道下基于机器学习的自适应OAM移位键控解码器分析［107］。（a） 系统示意图；（b）三种环境下准确率与传输距离的关系；（c） 弱到中等湍流下的结果准确性
Fig. 6. Analysis of adaptive OAM shift keying decoder based on machine learning under oceanic turbulence channels[107]. (a) System schematic diagram; (b) accuracy varying with transmission distance under three different environments; (c) accuracy under weak-to-moderate turbulence
2019年，Cui等［108］通过实验证明了基于CNN的16进制OAMSK解码器在水下无线光通信系统中的性能。该实验在一个充满流动咸水的1 m水箱中验证了OAMSK系统的解码准确率。海洋湍流通过SLM随机相位屏来模拟。
图 7. 基于机器学习的OAM移位键控解码器在水下信道中的实验研究［108］。（a） 实验装置；（b） 浑浊盐水下OAM解码准确率测试结果；（c） 不同海洋湍流下OAM解码准确率测试结果
Fig. 7. Experimental study of machine-learning-based OAM shift keying decoders in underwater channels[108]. (a) Experimental setup; (b) testing results for OAM decoding accuracy in turbid salty water; (c) testing results for OAM decoding accuracy under different ocean turbulences
2023年，Wang等［109］提出了一种使用CNN图像识别器作为解调器的相干解调UWOC-OAM-SK系统。与非相干系统相比，在接收器处从OAMSK信号中解调出OAM模式之前，所提出的系统可以获得具有更高图像对比度和更多模式特征的检测图像，因此具有更高的可靠性。此外，该系统可以识别相互共轭的OAM模式，这可以大大节省OAM通信中的复用通道资源。
图 8. 基于卷积神经网络的相干解调水下无线光通信系统［109］。（a）实验装置示意图； （b）传输60 m条件下的解调准确率；（c）固定水质下的传输解调准确率
Fig. 8. Coherent demodulated underwater wireless optical communication system based on convolutional neural network[109]. (a) Schematic diagram of experimental setup;(b) accuracy of demodulation over 60 m transmission distance; (c) transmission demodulation accuracy in fixed water
3.2.2 水下OAM模式复用通信
为了提高水下光通信的容量，另一个有效的方法是利用空间维度进行复用通信，即空分复用（SDM）技术。基于OAM模式的复用通信可以理解为SDM技术的一种，其利用不同的OAM模式作为不同信道的数据信息载波，类似波长维度的波分复用技术，由于不同OAM模式彼此正交，因此可以将携带不同信道数据信息的多个OAM模式进行复用以有效提高光通信系统的容量。
2016年，Baghdady等［16］使用两个445 nm的LD在2.96 m的距离上演示了一个水下无线光通信链路，并采用OAM模式来实现复用通信，完成了水下3 Gbit/s OOK-NRZ的信息传输。实验装置如
图 9. 水下OAM模式复用光通信链路［16］。（a）实验装置图；（b）发射机和接收机照片；（c）眼图
Fig. 9. Underwater OAM mode multiplexing optical communication link[16]. (a) Experimental setup; (b) photographs of transmitter and receiver; (c) eye diagrams
2016年，Ren等［17］通过复用和传输4个绿光OAM模式，实现水下光通信传输链路容量高达40 Gbit/s。在实验中，系统研究了各种水下环境（例如散射、浊度、水流和热梯度）对光束质量和系统性能的影响，研究发现热梯度引起的光束质量劣化和失真最为严重（模式失真和光束漂移），浊度引起的衰减最大。对于OAM模式复用通信的每个数据信道，实验中给出了两种不同的数据产生方式：一种是先通过红外光（1064 nm）调制产生10 Gbit/s信号，再通过周期极化反转铌酸锂（PPLN）光波导倍频非线性效应得到绿光（532 nm）10 Gbit/s信号；另一种是采用绿光直接调制1 Gbit/s信号。前者的优势是1064 nm红外波长目前有相对成熟的高速光调制器，可以比较容易通过调制产生高速数据信号（如10 Gbit/s），不过需要额外的非线性波长转换得到适合于水下光通信的绿光；后者的优势是比较直接，不过直接调制的速率相对较低（如1 Gbit/s）。实验中产生的绿光高斯光通过专门设计的集成电介质超表面相位板转化为携带数据信息的OAM模式。
图 10. 基于OAM模式复用的高速水下光通信［17］。（a）水下OAM模式复用光通信应用场景；（b） 40 Gbit/s水下OAM模式复用通信实验结果
Fig. 10. High-speed underwater optical communications using OAM mode multiplexing[17]. (a) Application scenario for underwater OAM mode multiplexing optical communication;(b) experimental results for 40-Gbit/s underwater OAM mode multiplexing communication
2021年，Zhang等［103］提出并演示了一套高速、低成本、紧凑型、便携式的集成封装OAM模式复用水下无线光通信原型系统。在实验中，使用几何相位Q板器件和偏振分束器件来产生、复用、解复用和检测OAM模式。同时，将信号生成、接收和处理功能集成到现场可编程门阵列（FPGA）中以实现高速、小体积和低功耗。将所有实验元器件封装到两个65 cm×35 cm×40 cm防水箱（带有透明窗口）中作为水下OAM模式复用光通信收发样机。利用该原型系统，实验实现了1.25 Gbit/s水下6 m距离OAM模式复用通信（ℓ=+3和ℓ=-3两个OAM模式信道，每个信道速率为625 Mbit/s），比特误码率性能低于1.5×10-2阈值。
图 11. 基于OAM模式复用的水下无线光通信原型系统［103］。（a） OAM模式复用水下无线光通信概念和原理；（b）几何相位Q板产生OAM模式的概念和原理
Fig. 11. Prototype system of underwater wireless optical communication using OAM mode multiplexing[103]. (a) Concept and principle of underwater wireless optical communication using OAM mode multiplexing;(b) concept and principle of OAM mode generation by geometric phase Q-plate
图 12. 基于OAM模式复用的水下无线光通信原型系统实验装置图和实验结果［103］。（a） 实验装置；（b） 模式信道串扰矩阵及BER性能测试结果
Fig. 12. Experimental setup and results for prototype system of underwater wireless optical communication using OAM mode multiplexing[103]. (a) Experimental setup; (b) measured results of mode channel crosstalk matrix and BER performance
2023年，Hei等［105］采用单光子计数模块来接收光子信号，实验通过建立符合实际系统的理论模型来分析误码率和光子计数统计，并在单光子水平上解调OAM状态，利用FPGA编程实现信号处理。基于这些模块，实验在9 m长的水路上建立了2个OAM模式复用的水下无线光通信链路。通过使用OOK调制和两脉冲位置调制，在数据速率为20 Mbit/s时实现了1.26×10-3的误码率，在数据速率为10 Mbit/s时实现了3.17×10-4的误码率，低于3.8×10-3的前向纠错（FEC）阈值。
图 13. OAM模式复用水下光子计数通信［105］。（a）实验装置图；（b） 光子计数统计；（c） OAM模式复用的误码率性能
Fig. 13. Photon-counting-based underwater wireless optical communication using OAM mode multiplexing[105]. (a) Experimental setup; (b) photon-counting statistics; (c) BER performance of OAM mode multiplexing
3.2.3 水下OAM模式广播通信
在OAM模式复用通信系统中，不同的OAM模式信道携带不同的数据信息，通过多通道复用提高通信容量。值得注意的是，在多样化的光通信系统网络中，也存在一些情况需要对一路信号进行多份复制并分发多个用户，这是一对多的通信，称为广播通信。OAM模式可以有很多取值，如果把信息同时加载到多个OAM模式上也就实现了OAM模式广播通信。在自由空间光通信中，目前已经有基于OAM模式的广播通信报道［110-114］。在水下光通信中，广播通信也具有重要价值，如：在潜艇编队或其他多个水下交通工具之间共享传递数据信息的应用；中心站可以通过广播向多个无人航行器和传感器发送相同的指令，用于海洋环境监测等。水下OAM模式广播通信为这些应用场景提供了有效解决方案。
2017年，Zhao等［101］利用光波空间维度（空间相位结构），提出并演示了一种基于OAM模式的水下无线广播通信。实验实现了水下2 m距离广播传输4束绿光（520 nm）OAM模式（OAM-6、OAM-3、OAM+3、OAM+6），每个OAM模式通道携带1.5-Gbaud 8-QAM正交频分复用（OFDM）信号。基于OAM模式的水下无线广播通信的概念和原理如
图 14. 基于OAM模式的水下无线广播通信［101］。（a）概念及原理示意图；（b） 实验装置图
Fig. 14. Underwater wireless broadcast communication using OAM modes[101]. (a) Concept and principle; (b) experimental setup
图 15. 基于OAM模式的水下无线广播通信实验结果［101］。（a）基于OAM模式1对4广播通信的解调归一化功率分布（OAM谱）； （b）比特误码率性能
Fig. 15. Experimental results for underwater wireless broadcast communication using OAM modes[101]. (a) Demodulated normalized power distribution of OAM mode based 1-to-4 multicasting communication; (b) measured BER performance
3.3 涉空水界面的OAM模式光通信
除了连接不同水下用户间的水下无线光通信，跨空水界面的水下-水上用户之间、水下-水上-水下用户之间以及水下-空水界面-水下用户之间的光通信也有其潜在的重要应用场景，如空中/水下搜救、水下航行器和空中无人机间的通信、石油平台或船舶检查期间的数据传输、海洋环境传感检测数据的传输等。在一些实际应用中，在潮汐起伏的环境下确保准确的接收检测仍然是一个重要挑战，这也是跨空水界面光通信链路的常见情况。此外，某些障碍物如视距通信中的水下生物和固定的岩石也可能会阻挡光路，通过中间中继节点采用多跳传输［115-116］是解决这个问题的一个潜在方法，同时，利用水上中继的水下-水上-水下交互通信也可以绕开障碍物的影响。
2018年，Wang等［117］通过实验证明了一种使用OAM模式的自适应“水下-空气-水下”数据信息传输。在水下链路中采用离散多音频（DMT）调制信号，同时引入反馈机制实现灵活的“水下-空气-水下”光通信，速率达到1.08 Gbit/s。通过比较两个相对水面高度（25 mm和-10 mm）下有反馈和无反馈的系统性能，在相对水面高度为25 mm和-10 mm时，有反馈情况下在7%开销硬判决前向纠错（HD-FEC）阈值（3.8×10-3）下的功率代价分别提高了2.5 dB和1 dB。
图 16. 基于OAM模式的“水下-空气-水下”光通信［117］。（a）概念及原理示意图；（b）实验装置
Fig. 16. "Water-air-water" optical communication using OAM mode[117]. (a) Concept and principle; (b) experimental setup
图 17. 基于OAM模式的“水下-空气-水下”光通信实验结果［117］。（a）输入输出高斯光、OAM模式、解调光斑及有无反馈情况下输出OAM模式的强度分布；（b）误码率性能
Fig. 17. Experimental results for "water-air-water" optical communication using OAM mode[117]. (a) Measured intensity distributions of input/output Gaussian beam, OAM modes, demodulated beams, and output OAM mode with and without feedback; (b) measured BER performance
2018年，Zhao等［118］提出并通过实验证明了利用空气-水界面的全反射非视距水下OAM模式光通信。为了克服空气-水界面起伏引起的光束波动和漂移，开发了自适应反馈控制系统来提供稳定的输出。此外，实验研究了微风效应、盐度（浊度）效应和垂直热梯度引起湍流效应的性能退化影响。结果表明，由微风引起的水波造成了最大的光束漂移，热梯度造成了最大的失真，而盐度则造成了最大的功率损失。利用空气-水界面全反射并采用OAM模式的水下无线光通信的概念和原理如
图 18. 利用空气-水界面全反射基于OAM模式的自适应反馈控制非视距水下无线光通信［118］。（a）概念和原理示意图；（b）实验装置图
Fig. 18. OAM mode based adaptive feedback-control non-line-of-sight underwater wireless optical communication utilizing total reflection at air-water interface[118]. (a) Concept and principle; (b) experimental setup
图 19. 利用空气-水界面全反射的自适应反馈OAM模式水下光通信实验结果［118］。（a）自适应反馈系统OAM模式传输的测量结果；（b）热梯度和盐度影响光束漂移和功率衰减的测量结果
Fig. 19. Experimental results of feedback-enabled adaptive underwater light transmission utilizing all reflection at air-water interface[118]. (a) Measured results for transmitting OAM modes (OAM+5, OAM-5) through adaptive feedback system; (b) measured results for impact of thermal gradient and salinity on beam displacement and power loss
3.4 快速自动对准辅助的OAM模式光通信
2022年，Cai等［104］提出并通过实验报道了基于OAM模式的快速自动对准水下无线光通信系统。利用快速自动对准技术，在4种不同的振动（发射端振动、接收端振动、发射和接收端同时振动、利用空水界面全反射非视距通信时发射和接收端同时振动）条件下，以244 Hz的响应频率演示了传输总容量达4 Gbit/s的双OAM模式复用通信链路。在使用快速自动对准技术之后，各种条件下振动的影响都大大减小，保持了通信链路的稳定。
图 20. 快速自动对准辅助的水下OAM模式复用无线光通信［104］。（a）概念和原理示意图；（b） 实验装置图
Fig. 20. Fast auto-alignment assisted underwater OAM mode multiplexing wireless optical communication[104]. (a) Concept and principle; (b) experimental setup
图 21. 快速自动对准辅助的水下OAM模式通信实验结果［104］。（a）不同振动条件下的光束轨迹；（b）不同振动条件下的比特误码率性能
Fig. 21. Experimental results for fast auto-alignment assisted underwater OAM mode communication[104]. (a) Beam's trajectory under different vibration condition; (b) BER performance under different vibration condition
4 水下其他结构光通信
除了上述直接叠加螺旋相位因子的OAM模式外，近年来，其他的一些结构光也被应用于水下光通信中。2017年，Zhao等［100］研究表明，在水下无线光通信中，贝塞尔光束在受到障碍物干扰时表现出更好的性能，这与其无衍射和自恢复特性有关。实验中研究了不同空间模式水下无线光通信时的性能，包括高斯光束、OAM模式和贝塞尔光束，考察了动态气泡和静态障碍物遮挡对三种空间模式传输的影响。
图 22. 气泡和障碍物影响下不同空间模式的水下光通信［100］。（a） 三种空间模式水下无线光通信概念图；（b） 实验装置图
Fig. 22. Underwater optical communications using different spatial modes subjected to bubbles and obstructions[100]. (a) Concept of underwater wireless optical communications employing three different spatial modes; (b) experimental setup
图 23. 气泡和障碍物影响下不同空间模式水下光通信实验结果［100］。（a）有无气泡时不同空间模式的接收光功率；（b）有无障碍物时不同空间模式的输出光强及解调光强分布；（c）有无障碍物时不同空间模式的误码率性能
Fig. 23. Experimental results for underwater optical communications using different spatial modes subjected to bubbles and obstructions[100]. (a) Received optical power of different spatial modes with and without bubbles; (b) output intensity and demodulated intensity profiles of different spatial modes with and without obstruction; (c) measured BER performance for different spatial modes with and without obstruction
2020年，Wang等［119］利用贝塞尔-高斯光束在水下光通信中实现了抗海洋湍流的数据传输，通过使用OOK信号，经过1 m光通信链路，在不同的海洋信道条件下，如水流、温度梯度、散射和气泡，实验演示和分析了水下数据传输性能，发现这些扰动会影响光束波前，进而引起误码。研究表明，相比于高斯光束，贝塞尔-高斯光束可以改善水下数据传输性能，其为抵抗水下光通信复杂环境影响提供了一种潜在的可行性选择。
图 24. 海水信道各种效应下贝塞尔-高斯光束水下数据传输的性能分析［119］。（a） 实验装置图；（b）仿真贝塞尔-高斯光束（左上）、实测贝塞尔-高斯光束（右上）、经过障碍物的贝塞尔-高斯光束（左下）、经过障碍物的高斯光束（右下）的光强分布；（c）水流和热梯度下的误码率性能
Fig. 24. Performance analyses on underwater data transmission using Bessel-Gaussian beams in simulated ocean channel with various effects[119]. (a) Experimental setup; (b) intensity distributions of simulated Bessel-Gaussian beam (upper left), generated Bessel-Gaussian beam (upper right), Bessel-Gaussian beam passing through obstacle (lower left), and Gaussian beam passing through obstacle (lower right); (c) measured BER performance under water current and thermal gradient
2021年，Liu等［120］提出了一种基于CNN的方法用于在海洋湍流信道中识别不同阶贝塞尔-高斯光束。该方法利用了贝塞尔-高斯光束具有的无衍射和自恢复特性以及CNN具有的学习数据多层抽象表示和提取图像固有特征的能力。通过设计一个7层CNN结构，并使用受海洋湍流扰动的贝塞尔-高斯光束的强度分布作为训练数据。通过模拟实验，验证了该CNN在不同海洋环境下对贝塞尔-高斯光束的识别性能，并分析了网络参数、湍流参数、训练数据集类型和编码方法对识别结果的影响。该方法为海洋无线光通信系统中贝塞尔-高斯光束的高效识别提供了一种思路和技术手段。
图 25. 基于CNN识别贝塞尔-高斯光束的水下无线光通信系统［120］。（a） 实验装置；（b）不同湍流强度下拉盖尔-高斯光束和贝塞尔-高斯光束的CNN识别准确率；（c）不同湍流强度下CNN识别准确率随传输距离的变化
Fig. 25. Underwater wireless optical communication system based on CNN recognition of Bessel-Gaussian beams[120]. (a) Experimental setup;(b) accuracy of CNN for recognizing Laguerre-Gaussian and Bessel-Gaussian beams under different turbulence intensities; (c) recognition accuracy of CNN versus transmission distance under different turbulence intensities
2023年，Robertson等［121］利用因斯高斯（IG）光束恒定包络调制的方法实现大带宽水下无线光通信。该方法利用二次谐波产生过程生成并调制因斯高斯光束，通过控制二次谐波过程的相位匹配条件，可以控制生成模式的系数，通过将数据调制在光束的相位上，实现恒定包络调制。
图 26. 基于因斯高斯光束恒定包络调制的大带宽水下无线光通信［121］。（a）二次谐波过程产生因斯高斯光束的实验装置图；（b）不同相位匹配条件生成的不同模式系数的因斯高斯光束强度分布；（c）不同调制格式情况下因斯高斯光束的误码率随衰减长度的变化关系
Fig. 26. Constant-envelope modulation of Ince-Gaussian beams for high-bandwidth underwater wireless optical communication[121]. (a) Experimental setup for generating Ince-Gaussian beams through second-harmonic process; (b) intensity distribution of Ince-Gaussian beams with different mode coefficient under different phase matching conditions; (c) BER of Ince-Gaussian beams versus attenuation length for different modulation formats
5 基于OAM模式的复杂介质光通信
除了水下无线光通信，OAM模式也应用于自由空间无线光通信。水下扰动环境和大气扰动环境都属于复杂介质情况。由于大气信道中存在着湍流和散射等影响，OAM模式传输时也不可避免地会失真，同时伴随功率衰减和光束抖动，进而严重劣化通信性能，这需要采取相应的补偿措施。近年来，基于OAM模式的复杂介质光通信也值得特别关注［122-123］。
2021年，Liang等［122］通过引入湍流补偿和快速自动对准装置构成的自适应系统，提出并通过实验实现了一个有效对抗湍流和振动影响的光通信系统。
图 27. 补偿湍流和振动影响的自适应OAM模式光通信系统［122］。（a）实验装置图；（b）误码率性能
Fig. 27. Adaptive OAM mode optical communication system against turbulence and vibration[122]. (a) Experimental setup; (b) BER performance
6 总结及未来发展趋势和展望
OAM开发了光波的空间新维度资源，这为光通信的可持续扩容提供了新途径。在信息一体化趋势下，OAM光通信技术也逐步应用于自由空间、光纤、水下等多样性场景。本文全面总结了近年来水下OAM光通信技术的研究进展。本文对水下无线通信技术的发展历程进行了简要梳理，包括水下声波通信、水下电磁波通信、水下光波通信。针对水下光通信，重点聚焦基于OAM模式的水下无线光通信：一方面介绍了OAM模式的基本原理及主要产生和测量方法；另一方面详细阐述了水下OAM模式无线光通信的研究进展，包括水下OAM模式编译码通信、水下OAM模式复用通信、水下OAM模式广播通信等。在此基础上，进一步介绍了涉空水界面的OAM模式光通信（“水下-空气-水下”跨空水介质、“空气-水”界面全反射）以及快速自动对准辅助的OAM模式光通信。除了介绍OAM模式，同时也拓展介绍了水下其他结构光通信，另外，对基于OAM模式的复杂介质光通信（自适应湍流补偿和快速自动对准）也进行了介绍。
水下OAM模式光通信研究一方面利用OAM模式，另一方面聚焦水下光通信。本文回顾了近年来该领域取得的一系列研究进展，国内外相关研究工作还有很多，本文也不免会有遗漏，未来发展也有更加迅速之势。
图 28. 水下轨道角动量光通信的未来发展趋势
Fig. 28. Future development trend of underwater OAM optical communications
除了上述发展趋势外，
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Article Outline
王健, 王仲阳. 水下轨道角动量光通信[J]. 光学学报, 2024, 44(4): 0400001. Jian Wang, Zhongyang Wang. Underwater Orbital Angular Momentum Optical Communications[J]. Acta Optica Sinica, 2024, 44(4): 0400001.
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