Undersea-fiber cable sections interlock quickly
DAVE WILSON
A major European establishment has requisitioned a fiber-optic system design that allows acoustic fingerprint measurements of marine vessels that sail through channels and harbors. The flexible fiber-based design permits users to deploy several mid-depth vertical-line-array sensor systems at set distances from the shore. The sensors communicate sensitive data back to stations on the shore. Moreover, the systems can be redeployed at short notice.
Because it is expensive to use large cable-laying ships, and rapid cable deployment is mandatory, the fiber-optic cable is planned to be supplied in 2-km lengths, and the individual lengths connected at sea are restricted to a maximum distance of 6 km. The cable must carry data to and from the sensor array, but it must also serve as the power lines for energizing the sensor arrays. Twisted-pair copper wires carry the electrical signals.
System power is sent down the fiber cable from the shore to the sensor array. There, power is tapped off to activate both the array and the electro-optic converters. The converters change array data into optoelectronic signals that are returned by the fiber-optic cable.
A key challenge faced by the system developers was how to join individual cable lengths together to make a 6-km length. "If you want a pair of connectors for use underwater with a mix of electrical signals, such as optical and power, there are few choices," says David Pinnington, sales and marketing director of Scorpion Oceanics Ltd., in Great Chesterford, Essex, UK. "There is small demand because it`s a specialized underseas application," Pinnington says. "The connectors can be manufactured, but they are expensive in this application."
Modular design is key
According to Peter Miller, technical director for the company, "We approached the problem from a different way and came up with a modular design that easily connects all the components together inside a housing. Within that standard housing, a series of optical connectors, electrical connectors and a large pair of power connectors are used."
Like most submarine cables, the fiber cable incorporates a strength member between an inner sheath and an outer sheath (see Fig. 1). "The strength member is aramid fiber, typically DuPont`s Kevlar," says Pinnington. Within the inner sheath are an earth wire, fiber-optic cables, power cable, and screened twisted-pair copper cable.
The undersea termination provides a water-tight seal on the outer sheath of the cable by employing a poly urethane overmolding (see Fig. 2). A resin-potted strength member supports the cable load. A compression seal covers the inner sheath of the cable, and the termination housing is used to seal a double O-ring assembly. To complete the design, the team developed an outer housing that slides off easily for access to the components inside the termination housing.
To remove the housing, the user loosens several radial bolts around the termination. Then the user can disconnect the individual connectors for the power, fiber and signal conductors. At that time, another cable section can be added to the fiber cable to extend the overall length. "The design advantage is that the termination can transfer any axial loads that might be experienced in use. This means that although the termination is inexpensive, it is also rugged," adds Scorpion`s Miller. "And it means that the user does not have to use a dedicated and expensive underwater connector."
Standard connectors cut costs
Inside the housing, standard quick-fitting connectors reduce costs further. Each fiber is terminated in a typical ST-II coupling; each electrical connector is completed in a push-pull connector.
According to Pinnington, this undersea connector eliminates the need for special tools to separate the connectors. The fiber-optic connections use standard quarter-turn bayonet fittings; the electrical connectors are coupled and uncoupled by a sliding sleeve. Both the fiber-optic and the electrical systems are separated by a printed circuit board to protect the fibers from the electrical connectors.
The termination designers were constrained somewhat by size. The termination housing had to be large enough to accom modate the printed circuit board. At both the shore end and the sensor end, the unit had to interface with a standard Euro board with mounted electro-optic converters. "The termination housings at either end used the same style of housing as the intermediate junctions, but they interfaced through a connector to a converter board rather than to a cable extension," says Pinnington.
A number of stretcher bars (metal rods that run from one end of the housing to other) ensure that the two halves of the housing are aligned and assembled to the correct lengths before the termination housing is brought across the top of the assembly.
"We have balanced the mechanical requirements by developing an undersea fiber-cable design that is small and cost-effective. Other termination designs are complicated, but our system is manageable," says Pinnington. Referring to the rigorous attenuation budget for underwater systems--less than 20 dB throughout the 6-km-long system--he says, "It was a tough problem to meet, but the termination housing design and the connection methodology easily passed the tests. The results show a loss of less than 0.5 dB through an ST-II coupling. The user has approximately 6 to 8 dB to spare, providing more signal than anticipated." q
Dave Wilson writes from London.