The Fiber Optic Communication Undersea System (FOCUS)
Roy Wilkins
The Fiber Optic Communication Undersea System (FOCUS)
Roy Wilkins
Four years ago the National Science Foundation and the University of Hawai`i funded development of the Fiber Optic Communication Undersea System (FOCUS). FOCUS (Fig. 1) is a real-time deep-sea video system designed from scratch to take advantage of the unique characteristics of fiber optic telemetry. Initial design criteria for FOCUS were somewhat different from those of other observation platforms and Remotely Operated Vehicles (ROV's). Two major drawbacks to existing tethered underwater systems were recognized: 1) Relatively small systems are not capable of operating at great depths, while systems capable of deep work demand dedicated ships, huge cables (due to the bandwidth limitations of copper wire), and large support crews, and 2) Particularly where deep observations are necessary, the cost of the vehicles dictates that they are often not used in a variety of interesting areas due to fears that they will be lost. Interesting questions often go unanswered because submersible and ROV support is expensive and not widely available. It was felt that using the advantages inherent in fiber optic telemetry - small cable size and wide bandwidth - allowed implementation of a different premise. A system has been built that may be operated off any medium or large oceanographic vessel in conjunction with normal marine geological and geophysical programs.
Figure 1.
The heart of FOCUS is the electro-optical tether, a double armored 9.5 mm diameter sea cable. It contains a stranded copper conductor surrounding four single mode fibers in a steel tube. FOCUS uses a 7 km length that weighs approximately 2758 kg (394 kg/km) as well as a 700 m length for work in shallower waters. AC current (800 v) is applied at the top of the sea cable to produce 5 kW of usable power to the vehicle. Each of the single mode fibers is capable of carrying two simultaneous real-time video signals or other data requiring comparable bandwidth. During initial deployment video signals have been sent to the surface on one or two of the fibers (one signal each fiber). A third fiber is reserved for low frequency command and control signals from the surface.
Figure 2.
The cable handling system consists of three pieces; a traction winch to handle the heavy lifting, a storage reel that winds wire under low tension, and a spring-loaded slack accumulator between the two to maintain a consistent pull on the wire during changes in winching speeds and directions. Low tension storage is essential for optical cables because overlays on a winch drum under high tension will crush the stainless steel tube holding the fibers, resulting in permanent loss of optical continuity.
The business end of the sea cable consists of two separate modules - one for initial power conversion and optical encoding/decoding and another that houses lights, camera(s), and eventually other environmental sensors. The power module is attached directly to the end of the electro-optical tether. The camera module hangs below it on a 15 m long kevlar reinforced coaxial umbilical. The camera module currently has one video camera and two 500 W incandescent lights, and a small pressure case to hold communication and power distribution circuits. The power module contains an oil-filled transformer case that supplies 110 v and a pressure case for optical encoding of data.
Testing of the full system was carried out during the Fall of 1994 in offshore O`ahu waters from R/V MOANA WAVE. Examples of volcanic seafloor were recorded in 800 m water depth off the Wai`anae coast (Fig. 2) as well as dredge dump material south of the entrance to Pearl Harbor in 400 - 500 m of water. The system has delivered on its original concept, providing color video images in real-time. In the present configuration, the bottom can be seen from an altitude of approximately 10 m above the seafloor. In practice, the camera module was generally flown at an altitude of 1 and 5 m. Details of bottom morphology were clear, as were images in the dredge spoils areas of numerous man-made objects. Ruggedness of the modules was tested during collisions with coral heads and lava edifices. Several times, due to cable handling problems, the camera module was dragged along the seafloor. The system took a beating but never stopped operating.
While there are still additional environmental sensors (e.g. compass, altimeter, pressure meter) to be integrated into the data stream, FOCUS is now at operational status. Eight days of video surveying have been scheduled for Jan - Feb, 1995, as part of an Army Corps of Engineers sponsored study of dredge spoils sites off Oahu. In the Fall, 1995, FOCUS will work from ATLANTIS II in support of ALVIN operations over the East Pacific Rise during one, and perhaps two, cruises. It may also be used during 1995 to survey sunken wave terraces off Hawaiian Island coasts. Further along, FOCUS will be integrated into the Hawai`i Undersea Geophysical Observatory on L`ihi Volcano, helping to deploy and retrieve instrumentation. It may also play a role in maintenance of the Deep Underwater Muon And Neutrino Detector currently being installed by the U.H. Department of Physics in 4,500 m of water off the island of Hawai`i. Over the next few years a simple capability for seafloor sampling will be developed to augment FOCUS' observational abilities.
It is anticipated that, as confidence in the use of electro-optic sea cables grows, marine experiments and sensors will be developed that will make further use of the information handling capacity of fiber optic communication. Happily, this will happen during a time of great expansion of both scientific and consumer demand for fiber optic technology. Costs of optical encoders and decoders have already dropped by an order of magnitude since the first FOCUS proposal was written and this trend will continue. FOCUS development will continue to take advantage of fiber optic advances - to both increase its usefulness in marine research and lower the costs involved.
reprinted from the Hawaii Center for Volcanology Newsletter, Volume 2, Number 1, December 1994
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