The Impact of Link Orientation in Underwater Optical Wireless Communication Systems

Underwater optical wireless has two distinct modes of communication depending on the choice of transmitter technology; short-ranged dispersive LED links (<20m) and long-distance, narrow laser links. In both cases, existing propagation models use a single-value estimation of attenuation but this is not accurate for the latter. Changes in attenuation are caused by variable composition where the natural Gaussian profile of chlorophyll concentration with depth is the biggest contributing factor. With this in mind, this research looks at how varying the orientation of communication affects the average transmission for longer ranged underwater optical links. This is repeated for areas with low, medium and high levels of surface turbidity at 50m depth increments. Highest attenuation in found in links which have the largest portion transmitted through a chlorophyll peak, this occurs between 15-45 degrees for both high and mid turbidity levels which correspond to the peaks below 0m and 50m respectively. This study also calculates how seawater refractive index gradients with depth alter the beam direction for different optical wireless links at angles to the vertical. It is found that angles closer to the vertical undergo higher displacements from their receivers as the gradient over which they are communicating is greater.