In this paper we describe a structurally integrated optical transmitter beacon concept that consists of a side-scattering fiber that can conform to solid surfaces; such as the outer surface of a submersible Remotely Operated Vehicle (ROV); suitable for convenient deployment in underwater applications. By coupling a modulated optical signal from a laser diode into the fiber; an omnidirectional "beacon" is achieved. We demonstrate coarse Wavelength Division Multiplexing (WDM); illustrating that these beacons can transmit optical wireless data through several attenuation lengths in turbid water at aggregate data rates of up to 20 Mb/s.
With increasing demands on underwater information exchange; underwater communication technologies have gained more and more interest in underwater sensor networks; underwater mining; and aquatic rescue; amongst many others. Owing to the large bandwidth; low latency as well as small size; weight; power and cost (SWaP-C); light-emitting diodes (LEDs) / lasers (LDs)-based underwater wireless optical communication (UWOC) (Hanson and Radic; 2008) is becoming a complementary or even a competitive solution to its counterparts; e.g.; acoustic (Song; Stojanovic and Chitre; 2019) and RF-based underwater communication (Palmeiro; Martín; Crowther; and Rhodes; 2011). With great efforts in maturing this technology; the physical layer verifications of UWOC systems are increasingly becoming comprehensive in both achievable transmission distances and speeds (Arvanitakis; Bian; McKendry; Cheng; Xie; He; Yang; Islim; Purwita; Gu; Haas; Dawson; 2020). By exploiting the low attenuation window of the water and single-photon-sensitive detection technologies such as Single-Photon Avalanche Diodes (SPADs); a UWOC system has been demonstrated with a transmission range up to 35.88 attenuation lengths (Hu; Mi; Zhou; and Chen; 2018). On the other side; Tsai; Lu; Wu; Tu; Huang; Xie; Huang; and Tsai (2020) has substantially multiplied the underwater channel capacity; having proved a 500-Gbps UWOC link using a five-wavelength polarization-multiplexing scheme.
Given the development mentioned above; it is an opportune time to consider the system's robustness in real oceanic environments. Previous UWOC links are largely based on a point-to-point configuration; wherein the transmitter maintains continuous alignment with the receiver. However; complex and dynamic marine processes pose great challenges in maintaining the requirements of positioning; acquisition and tracking (PAT). These factors; including oceanic turbulence; turbidity; bubbles; link barriers; etc.; result in severe performance degradation; such as beam scintillation; beam wandering; power fading; and bit transmission errors (Oubei; Shen; Kammoun; Zedini; Park; Sun; Liu; Kang; Ng; Alouini; and Ooi; 2018). Nevertheless; point-to-point links will be constrained with a small end coverage; limiting the mobility and quantity of receivers. Recently; several methods have been proposed to circumvent these issues. One of the solutions is diverging the transmission beam and leveraging the water scattering; forming a diffuse-line-of-sight (diffuse-LOS) link or even a non-line-of-sight (NLOS) link (Sun; Kong; Alkhazragi; Shen; Ooi; Zhang; Buttner; Ng; and Ooi; 2020). Alternatively; enlarging the photodetector's sensitive area could also improve both practicality and connectivity between underwater signal transceivers (Kang; Trichili; Alkhazragi; Zhang; Subedi; Guo; Mitra; Shen; Roqan; Ng; Alouini; Ooi; 2019).
