Fig. 1. Typical application scenarios of UWOC.

Fig. 2. Intensity distribution of a laser beam after transmitting through (a) 30 m and (b) 60 m in clean sea water.

Fig. 3. Experimental setup of the proposed RGB LD-based WDM UWOC system. Inset: (a) the transmitter module, (b) the receiver module, and (c) the water tank^[18].

Fig. 4. Schematic diagram of the working principle of a DM-DPSSL^[39].

Fig. 5. Possible application scenario of the proposed underwater Fi-Wi system^[40].

Fig. 6. Leaky POF-based distributed UWOC system^[41]. Inset: a “ZJU” symbol generated by a leaky POF originally used for decorative applications.

Fig. 7. (a) Experiment setup of the 46 m UWOC system using an MPPC receiver^[52]. (b) The 46 m PVC tube filled with tap water to simulate a 46 m underwater channel.

Fig. 8. Transmitting optical power for different L-PPM signals^[52]; stimulated/spontaneous: laser worked under stimulated/spontaneous emission state.

Fig. 9. Histogram of incident photon number in each pulse slot for different L-PPMs^[53].

Fig. 10. (a) Waveform and (b) spectrum of the captured 32-QAM OFDM signal with an ROP of −19.9 dBm^[55].

Fig. 11. Constellations after 2 m underwater transmission: (a) 256-QAM with bit loading, (b) 16-QAM with bit loading, (c) 256-QAM without bit loading^[58].

Fig. 12. Experimental setup for the proposed MIMO-OFDM-based UWOC system. The inset shows the schematic arrangement of transmitters (TXs) and receivers (RXs)^[42].

Fig. 13. Experimental setup for verifying information leakage using an MPPC placed aside the light beam^[76].

Fig. 14. Experimental setup of the air–water laser communication scheme^[82]. Inset: (a) the transmitter module, (b) the receiver module, and (c) the water tank.

Fig. 15. (a) Wave/current basin (70 m in length, 40 m in width, and 1.5 m in depth). (b) The research vessel named Zijingang (29.8 m in length with a gross tonnage of 100 tons).