Underwater wireless optical communication (UWOC) has attracted increasing interest in various underwater activities because of its order-of-magnitude higher bandwidth compared to acoustic and radio-frequency technologies. Testbeds and pre-aligned UWOC links were constructed for physical layer evaluation, which verified that UWOC systems can operate at tens of gigabits per second or close to a hundred meters of distance. This holds promise for realizing a globally connected Internet of Underwater Things (IoUT). However, due to the fundamental complexity of the ocean water environment, there are considerable practical challenges in establishing reliable UWOC links. Thus, in addition to providing an exhaustive overview of recent advances in UWOC, this paper addresses various underwater challenges and offers insights into the solutions for these challenges. In particular, oceanic turbulence, which induces scintillation and misalignment in underwater links, is one key factor in degrading UWOC performance. Novel solutions are proposed to ease the requirements on pointing, acquisition, and tracking (PAT) for establishing robustness in UWOC links. The solutions include light-scattering-based non-line-of-sight (NLOS) communication modality as well as PAT-relieving scintillating-fiber-based photoreceiver and large-photovoltaic cells as the optical signal detectors. Naturally, the dual-function photovoltaic-photodetector device readily offers a means of energy harvesting for powering up future IoUT sensors.

Underwater wireless optical communication (UWOC) can offer reliable and secure connectivity for enabling future internet-of-underwater-things (IoUT), owing to its unlicensed spectrum and high transmission speed. However, a critical bottleneck lies in the strict requirement of pointing, acquisition, and tracking (PAT), for effective recovery of modulated optical signals at the receiver end. A large-area, high bandwidth, and wide-angle-of-view photoreceiver is therefore crucial for establishing a high-speed yet reliable communication link under non-directional pointing in a turbulent underwater environment. In this work, we demonstrated a large-area, of up to a few tens of cm², photoreceiver design based on ultraviolet(UV)-to-blue color-converting plastic scintillating fibers, and yet offering high 3-dB bandwidth of up to 86.13 MHz. Tapping on the large modulation bandwidth, we demonstrated a high data rate of 250 Mbps at bit-error ratio (BER) of 2.2 × 10⁻³ using non-return-to-zero on-off keying (NRZ-OOK) pseudorandom binary sequence (PRBS) 2¹⁰-1 data stream, a 375-nm laser-based communication link over the 1.15-m water channel. This proof-of-concept demonstration opens the pathway for revolutionizing the photodetection scheme in UWOC, and for non-line-of-sight (NLOS) free-space optical communication.

For circumventing the alignment requirement of line-of-sight (LOS) underwater wireless optical communication (UWOC), we demonstrated a non-line-of-sight (NLOS) UWOC link adequately enhanced using ultraviolet (UV) 375-nm laser. Path loss was chosen as a figure-of-merit for link performance in this investigation, which considers the effects of geometries, water turbidity, and transmission wavelength. The experiments suggest that path loss decreases with smaller azimuth angles, higher water turbidity, and shorter wavelength due in part to enhanced scattering utilizing 375-nm radiation. We highlighted that it is feasible to extend the current findings for long distance NLOS UWOC link in turbid water, such as harbor water.

The connectivity of undersea sensors and airborne nodes across the water–air interface has been long sought. This study designs a free-space wireless laser communications system that yields a high net data rate of 850 Mbit/s when perfectly aligned. This system can also be used for an extended coverage of 1963 cm² at the receiver while sustaining a net data rate of 9 Mbit/s over 10 m. The utility of this system was verified for direct communications across the water–air interface in a canal of the Red Sea based on a pre-aligned link as well as a diving pool under a mobile signal-searching mode. The canal deployment measured a real-time data rate of 87 Mbit/s when pre-aligned in turbid water over 50 min, which confirms the system robustness in harsh water environments. In the pool deployment, a drone configured with a photodetector flew over the surface of the water and recorded the underwater signals without a structure-assisted alignment. Using a four-quadrature amplitude-modulated orthogonal frequency-division multiplexing (4-QAM-OFDM) modulation scheme provided a net data rate of 44 Mbit/s over a 2.3-m underwater and 3.5-m air link. The results validated the link stability and mitigated problems that arise from misalignment and mobility in harsh environments, which paves the way for future field applications.

In this Letter, we demonstrate a novel distributed-feedback (DFB) InGaN-based laser diode with narrow-linewidth emission at ∼ 480 n m (sky blue) and its application to high-speed visible-light communication (VLC). A significant side-mode suppression ratio (SMSR) of 42.4 dB, an optical power of ∼ 14 m W , and a resolution-limited linewidth of ∼ 34 p m were obtained under continuous-wave operation. A 5-Gbit/s VLC link was realized using non-return-to-zero on–off keying modulation, whereas a high-speed 10.5-Gbit/s VLC data rate was achieved by using a spectral-efficient 16-quadrature-amplitude-modulation orthogonal frequency-division multiplexing scheme. The reported high-performance sky-blue DFB laser is promising in enabling unexplored dense wavelength-division multiplexing schemes in VLC, narrow-line filtered systems, and other applications where single-frequency lasers are essential such as atomic clocks, high-resolution sensors, and spectroscopy. Single-frequency emitters at the sky-blue wavelength range will further benefit applications in the low-path-loss window of underwater media as well as those operating at the H-beta Fraunhofer line at ∼ 486 n m .

Real-time, dynamic, and visual monitoring of underwater scenes would become an important research topic with the development of underwater wireless optical communication (UWOC), autonomous underwater vehicle, and video fusion technologies. To this end, we developed a UWOC-based 2K real-time digital video surveillance prototype named AquaF-seer. Using a light-emitting diode and an avalanche photodetector based diffused line-of-sight UWOC system, real-time video transmission with a high resolution of 1920 × 1080 pixels is achieved over various channels, i.e., 1.5-m free space channel, pure water channel, pure water channel with 1.53-mL/s bubble-induced turbulence, pure water channel with 42.40-mL/s bubble-induced turbulence, simulated pure sea water channel, and coastal ocean water channel. Moreover, 46-m and 5-m video monitoring are implemented in free space and an outdoor diving pool. It indicates the reliability of the prototype, which is the first step to realize underwater visual monitoring in future human-robot interaction applications.