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On Land and in Sea

Long-distance telephone companies were the first to realize that wavelength division multiplexing could cut the cost of bandwidth. Compared with the alternative of adding new fiber, WDM technology provides “a much more effective way to add capacity,” according to Dana Cooperson, optical network analyst for RHK Inc., a market consultancy in South San Francisco. Laying new cable is expensive and time-consuming. And burying new cable along the same route already occupied by an older cable is risky-new excavation invites cable breaks that could put the whole system out of service.

The telecommunications carriers’ desire to save time and money has driven a rapid development in WDM techniques. In the mid-1990s, the carrier companies began using systems transmitting at four wavelengths, and soon upped the count to eight. Developers quickly sliced the spectrum even more finely to squeeze 16 wavelength channels through a single fiber for what has become known as “dense” WDM.

When the carriers saw the need, manufacturers were equally quick to sense the market. Lucent Technologies of Murray Hill, N.J., adapted technology developed at its Bell Labs subsidiary. Ciena, a Linthicum, Md., company founded in 1992, charged ahead faster, delivering its first commercial 16-channel system in 1996-at nearly the same time as the AT&T spinoff. Other telecom giants around the world followed, including Nortel, Alcatel, Pirelli, NEC, Hitachi, Fujitsu and Ericsson. Over the past two to three years, several companies-including Ciena, Lucent and Nortel of Saint-Laurent, Que.-have begun to market systems that slice the erbium-amplifier spectrum into 32 or 40 slivers, each only 0.8 nanometer wide. Last September, Lucent delivered its first 80-channel system to AT&T. Pirelli Cable of Lexington, S.C., followed by promising a 128-channel version, but had not delivered hardware as of mid-January.

Telecommunications carriers don’t need all those channels today-and thanks to WDM’s inherent modularity, they don’t need to buy more channels until they’re ready. A carrier installing a WDM system can start with only the transmitters and receivers needed for the few initial channels. Later, as demand for capacity grows, additional equipment can be plugged in to open up new wavelengths.

Taking full advantage of WDM often requires upgrading older cables by adding components that compensate for a troublesome effect called chromatic dispersion. This refers to the tendency of a short light pulse to stretch out as it travels through a fiber owing to the fact that some wavelengths travel faster than others. Dispersion smears light pulses together and therefore limits transmission speed. Avoiding this phenomenon is especially important in submarine cables, where light signals must travel through several thousand kilometers of fiber from shore to shore. New installations can exploit fibers designed for optimum WDM performance, recently developed both by Lucent and by Corning (Corning, N.Y.).

Last year, the first big submarine cable designed for multiwavelength operation-called Atlantic Crossing 1-began sending 2.5 Gbit/s at four wavelength channels on each of its four fiber pairs. The capacity of this system can be upgraded to 16 wavelengths per fiber at that speed, says Patrick R. Trischitta, director of technical marketing at Tyco Submarine Systems Laboratories in Holmdel, N.J. That promises a total of 160 Gbit/s through the cable, a loop connecting the United States with Britain, the Netherlands and Germany.

Project Oxygen raises the bar. Newer WDM technology will carry 10 Gbit/s at each of 16 wavelengths across the ocean in four fiber pairs, a total capacity of 640 Gbit/s per cable. That’s more than 1,000 times the capacity of the first transatlantic fiber-optic cable, which began service just a decade ago. The whole system will ultimately include 168,000 kilometers of cable-enough to circle the globe four times. Other groups are planning more submarine cable systems, although none is quite so ambitious. It’s no wonder MIT’s Clark predicts, “We’re going to drown in fiber.”

On land, regional telephone companies have just begun to adopt wavelength multiplexing. Last year, Bell Atlantic began testing WDM on a 35-kilometer cable between Brunswick and Freehold, N.J., says Robert A. Gallo, the Bell Atlantic engineer in charge of the trial. Four channels each carried signals at speeds to 2.5 Gbit/s-the top rate between company switching offices-and the Ciena-built system has slots for up to 16 wavelength channels. Bell South tested three of 16 channels in a similar system on a cable spanning 80 kilometers between Grenada and Greenwood, Miss. The economics are clear: “It’s cheaper to add WDM capacity than to add new fiber,” says RHK analyst Cooperson.

Different rules apply to the shorter cables linking switching offices to major business customers. Here, in the so-called “metro” market, “the cost of increasing fiber count is not as big an issue because the runs are so much shorter,” Cooperson explains. Still, WDM improves signal transmission in other important ways. One is by carrying signals in their original digital formats rather than converting them into the digital coding used within the telephone network. Because such conversion requires costly electronics, it can be cheaper to dedicate a wavelength for end-to-end transmission in the original format.

The ability to sort signals by wavelength should streamline the operation of future fiber-optic networks. Traditionally, phone companies organize digital signals in a hierarchy of bit rates, merging many low-bit-rate tributaries into mighty digital rivers carrying gigabits per second. This packs bits efficiently onto transmission lines, but requires unpacking the whole bit stream to extract individual signals. If the signals are organized by wavelength, however, simple optics can tease out the desired wavelength channel without disturbing the others. Engineers speak of adding a new “optical layer” to the telecommunications system. Customers might lease a wavelength in this optical layer instead of leasing the right to transmit at a specific data rate. A television station, for instance, could reserve one wavelength from its studio to its transmitter and another to the local cable company-and transmit both signals in digital video formats not used on the phone network.

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