To enable high-speed underwater wireless optical communication (UWOC) in tap-water and seawater environments over long distances, a 450-nm blue GaN laser diode (LD) directly modulated by preleveled 16-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) data was employed to implement its maximal transmission capacity of up to 10 Gbps. The proposed UWOC in tap water provided a maximal allowable communication bit rate increase from 5.2 to 12.4 Gbps with the corresponding underwater transmission distance significantly reduced from 10.2 to 1.7 m, exhibiting a bit rate/distance decaying slope of −0.847 Gbps/m. When conducting the same type of UWOC in seawater, light scattering induced by impurities attenuated the blue laser power, thereby degrading the transmission with a slightly higher decay ratio of 0.941 Gbps/m. The blue LD based UWOC enables a 16-QAM OFDM bit rate of up to 7.2 Gbps for transmission in seawater more than 6.8 m. Emerging underwater communication technology developed for commercial sea resource exploration, military warship-to-submarine communication, and satellite-to-submarine communication 1 is a compelling research topic. Figure 1 presents three carriers, namely acoustic waves, microwaves, and optical waves, all of which are applicable for underwater communication, but also exhibit specific practical problems. Among these, the underwater acoustic wave (UWAC) system can be applied only in low-noise environments for low-speed content. This is because of its strong attenuation in seawater, exhibiting inverse proportionality to the wavelength, as well as its significant propagation delay and the low signal-to-noise ratio (SNR) of data in the context of background ocean noise. 6 . Moreover, Song et al. demonstrated a UWAC system with 60-kbps 32-QAM data covering a bandwidth of 32 kHz in a seawater environment more than 100 m deep with a distance of over 3-km 7 . However, despite the aforementioned research, the transmission rate of the UWAC system is limited by its narrow modulated bandwidth. To overcome the insufficient data bandwidth of the UWAC system, Moore et al. proposed a preliminary simulation to demonstrate that a microwave with tens of kilowatts power can transmit over tens of kilometers on the surface of seawater . However, the high-frequency (short-wavelength) carrier induced high attenuation in seawater that also limited the transmission capacity and distance of the system 12 . In comparison, at higher frequencies, a microwave carrier enables wider data bandwidth than the acoustic wave does. However, the transmission distance of the UWMC system is notably short because of its extremely high attenuation. To satisfy the requirement of high-capacity underwater machine-to-machine data transmission, a high-speed underwater optical communication (UWOC) system is highly effective compared with currently available carriers such as microwaves and acoustic waves. In contrast to acoustic wave and microwave carriers, light emitting diodes
Recent developments in blue laser diodes enable bit rates up to 7.2 GHz, but in very short distances limited to 6.8 m REF .
Blue Laser Diode Enables Underwater Communication at 12.4 Gbps
Scientific Reports
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Physics Medicine