A group of researchers from King Abdullah University of Science & Technology (KAUST) are studying the possibility of using non-line-of-sight (NLOS) optical communication for under-water communications. Through the research study, the team intends to circumvent the data rate limitations of acoustic underwater communication (RF communications being too much attenuated by seawater) and too easily obstructed or misaligned direct line-of-sight (LOS) optical communications by using NLOS optical communications.
In a paper titled "375-nm Ultraviolet-laser Based Non-Line-of-Sight Underwater Optical Communication" published in Optics Express, the researchers note that while an underwater LOS communication link may be easily obstructed NLOS communication can be implemented either through light reflection from the water surface or light scattering from the molecules in the water. In that case, path loss can be adopted as a figure-of-merit, with a lower path loss indicating a higher communication data rate and better received signal-to-noise ratio.
While in previous studies, the wavelengths used for underwater optical communications (UWOC) were mainly in the “transparent window” of 400 to 600 nm for light aquatic-attenuation, the researchers opted for a 375 nm ultraviolet laser, whose short wavelength has enhanced scattering properties in turbid waters, increasing link performance through multiple scattering.
Using a 70 mW ultraviolet diode laser, the researchers placed a transmitter and receiver in a coplanar configuration (hidden from each other), experimenting several azimuth angles, so that the laser beam would enter a small water tank whose turbidity they controlled using different concentrations of micro-particles in suspension, Al (OH) 3 and Mg (OH) 2 to emulate different ocean water types. They found that path loss of such links was favorable for smaller azimuth angles, stronger water turbidity, and shorter transmission wavelength and concluded that a 375 nm laser could potentially be used to establish a NLOS UWOC link over tens of meters, even in turbid water media including harbor waters.