Free-space optical communications are typically performed in free space, such as air and vacuum, due to the high transmissivity of both mediums in the telecom waveband (1510 to 1600 nm). However, FSO communication can also be performed in non-ideal mediums like water. Categorizing underwater FSO communication as “free-space communication” is possible since no physical link exists between transmitter and receiver. Unlike glass and other transmission mediums, water does not act as a waveguide for light and thus cannot be considered a closed system like a fiber-optic cable.
Establishing underwater communications using FSO eliminates the need to lay expensive fibers across large distances to facilitate communication. Once the transmitters and receivers have been laid on the ocean bed and have been calibrated to face one another, there’s no need to ever fix physically broken links since all communication will be non-physical. FSO communication can be thought of as a lower-maintenance alternative to the fiber backbone currently installed in the ocean.
However, there are some drawbacks to using FSO underwater. The communication links are highly susceptible to beam blocking, such as by sediment deposits or passing undersea life. Underwater currents also cause beam wander and scintillation. The uneven surface of the ocean floor, most of which is still unmapped, makes coordinating the transmitter and receivers difficult since the pair will need to be facing each other to send and receive signals. The most significant effect of underwater communication is the high degree of electromagnetic absorption found in water, ignoring sediments and other scattering/absorbing particles found in the ocean. As seen below, water has a very high absorbance in the low UV and shortwave visible and beyond, leaving a single appropriate communication band from 400 to 500 nm. Thus, telecom would not be able to operate underwater at the ideal 1550, 1310, or 1064 nm that have become industrial standards.