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The Ultimate Squeeze

Since the demand for bandwidth shows no sign of slowing down, the developers of WDM systems are already thinking about how to pack more wavelengths into the same fiber. At the moment, there are two basic approaches being investigated-and limits to both are apparent.

One approach is to reduce the “space” between wavelengths, by choosing wavelengths that are closer together to carry the multiplicity of signals. Packing wavelengths closer works well up to a point, but it ultimately clashes with basic physics. As bit rates increase, optical pulses get briefer, and-following the dictates of Heisenberg’s Uncertainty Principle-this shortening forces the light signal to spread over a broader range of wavelengths. This spreading can cause interference between closely spaced channels. Lucent’s highest-capacity system handles 10 Gbit/s on wavelength channels separated by 0.8 nanometer but only 2.5 Gbit/s when channel spacing is halved. And few experts think channels can be squeezed much tighter. Among major vendors, only Hitachi Telecom of Norcross, Ga., talks about modulating individual channels at 40 Gbit/s-and admits that those signals could span only limited distances.

Prospects look better for the second option: expanding the range of transmission wavelengths. Pirelli, for example, uses three erbium-fiber amplifiers, optimized for separate bands between 1,525 and 1,605 nanometers, to squeeze 128 wavelength channels at 10 Gbit/s each into a single fiber. Lucent has demonstrated erbium amplifiers covering a similar range in the laboratory, and last year introduced a new optical fiber that opens up a long-neglected block of the spectrum around 1,400 nanometers. Good optical amplifiers are not yet available for other wavelengths.

For WDM to reach its full potential, though, more will be needed than simply packing in additional wavelengths. It will also be necessary to develop better equipment for switching and manipulating the various wavelengths after the signal emerges from the optical “pipe.” Optical switches “are getting close to practical commercial applications,” says analyst Mack of KMI. He adds, however, that “to fully emulate what happens in digital cross-connects, you need to reallocate and reassign wavelengths.” It’s impossible to allocate the same wavelength to one customer throughout an entire system because the huge network has far more customers than it has wavelengths.

The illustration below shows how signals from San Francisco and Cupertino arrive in Palo Alto at the same wavelength, both bound for San Jose. The Palo Alto node must convert one signal to a different wavelength for the final leg of its trip, so that the messages they carry aren’t hopelessly confused. Wavelength conversion now must take the same brute-force approach as regenerators, converting the optical signal to an electronic one that can drive a transmitter at the output wavelength. All-optical conversion approaches, while demonstrated in the lab, have yet to reach commercial practicality.

Even if these technical problems are solved, however, that won’t be enough for the technology to really spread its wings. For that, the price will also have to come down-a trajectory that insiders say has already become apparent. Adel Saleh, head of the broadband access research department at AT&T Labs in Red Bank, N.J., projects that cost per network node will drop by a factor of 10 every five years, starting at $1 million in 1995. Through the next year or two, he says, WDM will be economical only for backbone networks. Once cost drops to $100,000 a node, the technology will make sense for metropolitan and regional networks, starting with service to large businesses. Saleh expects that residential access in large apartment buildings will follow after costs drop to $10,000 a node in about 2005, with WDM reaching individual homes once costs decline to about $1,000 in 2010.

The real strength of WDM lies in how it expands the optical airways so that everyone can inhale more of the oxygen of information. At the dawn of the radio era, each transmitter screamed across the whole radio spectrum, blocking other signals for the duration of its broadcast. Then engineers learned to build circuits that tuned each transmitter to its own frequency, opening the radio spectrum to the many stations we can hear today. In much the same way, WDM replaces a single stream of black-and-white bits with a multitude of different-colored signals.

WDM is creating huge new information pipelines that will bring better service at lower cost. But the real information revolution won’t come until cheap WDM pipelines reach individual residences. Today’s modem connections remain bottlenecks, forcing us to sip the torrent of data through the electronic equivalent of a thin plastic straw. But get ready: As fiber reaches the home, your very own wavelength could deliver a bubbling fountain of bits.

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