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Fiber Brings 'Live' Response from Ocean Floor

Photonics Spectra
Jan 1997
David Lytle

TUCKERTON, N.J. -- In a scene that would make Jules Verne happy, scientists at Rutgers University and the Woods Hole Oceanographic Institute have established the first permanent fiber optic link that allows live, two-way communication with instruments on the bottom of the ocean.
Previous attempts to gather data from the sea floor have relied on narrow-bandwidth coaxial cable, a limitation that ruled out live control of instruments from the shore. Compared with coaxial cable's 5-MHz bandwidth, the new link to the underwater Long-Term Ecosystem Observatory (LEO-15) supports 600 MHz of video channels and 125 Mb/s of telemetry data. The higher bandwidth allows researchers to control the video cameras and monitor sensor data in real-time. Previous instruments at the research site used on-board computers to store data until it was recovered by divers. But when instruments were lost because of storms or hurricanes, so was the data.
The new communication link consists of three single-mode optical fibers stretching 10 miles from the Rutgers Institute of Marine and Coastal Sciences field station in Tuckerton to two "nodes" that lie 15 m below the ocean surface. The nodes host a variety of advanced, off-the-shelf oceanographic sensors in addition to several remote-controlled video cameras.
Each node also contains eight guest ports for use by future experimenters. The ports supply electricity and a standard computer data interface with a high-bandwidth link to shore via the fiber optic cable.

Home-built connectors
To keep costs down, researchers developed their own low-cost mateable fiber optic connector for underwater use. Although the US Navy had previously developed connectors for such a purpose, their $15,000 unit price was prohibitive. Woods Hole engineer Christopher Von Alt initiated his own tests to determine just how sensitive optical fibers were to contamination in underwater conditions. "We were surprised at just how hard it was to obscure the working tip of a single-mode fiber," he said. "You almost had to scrape the connector through mud to affect it."
The Woods Hole solution was to use a standard ST-style connector (about $25 each). Surprisingly, the 1/2-µm mating tolerance of the connector was unaffected by saltwater particulates.
"If we had been optical engineers, we would have never thought this would work," said Von Alt. The connectors successfully mated on the first attempt about 80 percent of the time, he said, and have proven reliable at depths up to 6000 m.
LEO-15 will study ecosystem processes that could affect commercially important shell fisheries in the Mid-Atlantic Bight region. Designed by the Oceans System Laboratory at Woods Hole, the nodes will also eventually host two torpedo-shaped autonomous research vehicles that will prowl the area.
For more information, contact Rose Petrecca, LEO-15 operations manager, Rutgers University Marine Field Station, 800 Great Bay Blvd., Tuckerton, NJ 08087, (609) 296-5260.

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