Fig. 1. Development history of underwater acoustic communication^[19-24]

Fig. 2. Development history of underwater electromagnetic communication^{[2, 43-47]}

Fig. 3. Development history of underwater wireless optical communication^{[25, 61-68]}

Fig. 4. Characteristics of OAM modes with different topological charges (intensity distribution, phase distribution, and wavefront)

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

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

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

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

Fig. 9. Underwater OAM mode multiplexing optical communication link^[16]. (a) Experimental setup; (b) photographs of transmitter and receiver; (c) eye diagrams

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

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

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

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

Fig. 14. Underwater wireless broadcast communication using OAM modes^[101]. (a) Concept and principle; (b) experimental setup

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

Fig. 16. "Water-air-water" optical communication using OAM mode^[117]. (a) Concept and principle; (b) experimental setup

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

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

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

Fig. 20. Fast auto-alignment assisted underwater OAM mode multiplexing wireless optical communication^[104]. (a) Concept and principle; (b) experimental setup

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

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

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

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

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

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

Fig. 27. Adaptive OAM mode optical communication system against turbulence and vibration^[122]. (a) Experimental setup; (b) BER performance

Fig. 28. Future development trend of underwater OAM optical communications

Fig. 29. Future perspective of underwater wireless optical communications