Spatial and temporal dispersion in high bandwidth underwater laser communication links

A resurgence is occurring in the area of underwater laser communications. While acoustic systems are currently the more mature technology, they are ultimately band-limited to sub-MHz type data rates due to the frequency dependent absorption of acoustic energies in water. Advances in fiber optic and free space links have shown promise for optical links to provide data rates in excess of a gigabit per second. It is not surprising then, that laser links are being considered for Naval applications involving high bandwidth communications undersea. A major challenge in implementing optical links underwater arises from the spatial dispersion of photons due to scattering. Spatial spreading of the optical beam reduces the photon density at the receiver position. As such, optical links are only expected to be of greatest utility in links <100 m. Nonetheless, it appears that end users may accept limited link range in exchange for the gain in information bandwidth that optical links may provide. Additionally, researchers continue to study how spatial spreading affects the time encoded portion of the transmitted optical signal. Temporal dispersion arising from multiple scattering events may result in inter-symbol interference (ISI), further limiting link range and/or capacity. Researchers at NAVAIR in Patuxent River MD are currently investigating both the spatial and temporal effects of scattering on a laser link in turbid underwater environments. These links utilize an intensity modulated beam to implement coherent digital modulation schemes such as PSK and QAM. Through both modeling and experiment, the underwater channel is characterized both spatially and temporally. Results are providing insight to system requirements of link range, pointing accuracy, photo-receiver requirements, modulation frequency, and optimal modulation format.