Only electro-magnetic radiation in the blue/green portion of the spectrum can transfer information at high data rates to useful ranges through seawater. In addition to the fundamental physics, the lack of stealth and the ability to be jammed also impact the viability of both acoustic and low-frequency electro-magnetic systems. This paper will provide an understanding of the fundamental phenomenology and system architecture issues attendant to making a practical underwater blue/green optical communications system (i.e., providing a capability sufficient to justify the financial expenditure). This undersea communications includes both underwater-to-underwater as well as underwater through the air/sea interface to above water platforms. In order to be viable, the latter requires propagation through the dynamically variable multiple scattering/absorption environments of seawater, fog, rain and clouds, all while operating in full sunlight. This may represent the most challenging and most complicated propagation channel in optical communications.
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