Wireless optical communications underwater is enjoying a renewed interest from researchers due to the wide advances in laser sources and receivers, digital communications, and signal processing. Underwater free space optics (uFSO) fulfills several niche applications for wireless communications in ocean waters. While RF communications have become ubiquitous in our everyday lives above water, the RF portion of the electromagnetic spectrum exhibits high attenuation in seawater. Acoustics on the other hand have long enjoyed success for detection and communication underwater, given their ability to propagate long distances underwater (> km). However, for high-speed data transfer (> Mbps), acoustics are at a disadvantage, as it is well known that acoustic energy exhibits increasing attenuation with increasing frequency. Supported by enormous growth in the telecommunications industry over the past few decades, optical techniques are garnering serious consideration for underwater communications due to the high data rates they may provide. Additionally, as we will learn, the blue/green portion of the visible spectrum exhibits minimal absorption in seawater. Still, scattering of light by organic and inorganic particulates in ocean water can cause significant spatial and temporal dispersion, which may have a measurable impact on link range and available bandwidth.

This chapter serves as both an introduction to the field of light propagation under-water, as well as a survey of current literature pertaining to uFSO. We begin with a simple examination of a link budget equation. Next, we present an introduction of ocean optics in order to gain an appreciation for the challenges involved with implementing free-space optical links underwater.