Optical modem technology for seafloor observatories

Cabled ocean observatories will bring broadband Internet to the seafloor around hydrothermal vent sites and other scientifically interesting features. The ideal platform for exploring and monitoring these sites, especially during episodic events, is a remotely-piloted, unmanned, underwater vehicle (UUV) which is capable of sending back high-quality video or other high-rate sensor data. The combined requirement of remote-piloting and high data rates argues for a bidirectional "wireless" communications link capable of sending and receiving data at one to several Mbits per second. Wireless communications at a high data-rate can only be accomplished optically within the ocean. An optical link is practical because of the clear water near the sea floor and recent improvements in diode lasers, LEDs, optical detectors and other subsystem components. In this paper the system approach and several key engineering trades are presented, followed by modeled results deriving maximum range and data rate versus power available at the transmitter.