increase is expected this year. The most pressing question is whether, given all the trade-offs, the current rate of improvement can be maintained. "I could give you a macho answer that we're going to continue to improve fiber, but quite frankly, I don't know," says Joseph Antos, technology director for fiber development at Corning. "Every new invention [to increase capacity] gets harder and harder."
More Channels per Fiber Data travels along optical fiber through a series of light pulses from a laser, the offs and ons corresponding to the ones and zeroes of digital coding. Fiber-optic systems use the light spectrum that travels most efficiently through the glass, wavelengths between about 1,300 and 1,600 nanometers. Outside of these wavelengths light tends to be either absorbed and lost or stretched too far to make a usable signal. And of the available spectrum, most transmission takes place in what's called the "central band," between 1,530 and 1,565 nanometers. By breaking the signal into different wavelengths, as a prism separates the colors that make up white light, engineers can send more than one stream of light along a fiber at the same time. Early implementations divided the light into four or eight separate channels, with each fiber carrying about 10 gigabits-l 0 billion bits-per second. Today some systems can carry 80 channels in the central band, and are able to push more than a halftrillion bits per second down a single fiber. But there's a limit to how many channels can be squeezed into the central band. Like closely spaced stations on your car radio, channels that get too close cause interference. On the radio, you might be listening to All Things Considered and suddenly get the Backstreet Boys-or static. The same thing happens with optical signals. To reduce interference, current state-of-the-mt systems require a buffer zone of about 50 gigahertz (a measure of frequency of a billion cycles per second) between channels. As a result of these constraints, the central band is now essentially full, and engineers are looking to add channels by moving out of the central portion of the spectrum and into new territory.
Breaking New Ground In order to make new palts of the spectrum-outside the central band-usable, researchers must develop new versions of devices that help push signals along optical fibers. Take the amplifiers that help boost signals, which lose energy as they bounce back and forth between the walls of the core section of the fiber. To pump them back up, engineers might use devices known as erbium-doped-fiber amplifiers. These are essentially loops of fiber laced with the rare earth element erbium. A laser excites the erbium atoms, which transfer their energy to the optical signal passing through the amplifier, increasing the distance it can travel. Without amplification, high-speed signals wouldn't travel far enough to be useful. Recent developments make it possible for these amplifiers to
work in the longer-wavelength region of 1,570 to 1,625 nanometers, adding a new chunk of spectrum from which to carve additional data channels. Lucent Technologies, for example, has released a system that squeezes 80 channels into the central band and exploits erbium amplifiers to add another 80 channels in the long-wavelength region, doubling the capacity of each fiber. Every time a signal runs through an erbium amplifier, however, it picks up noise-elements that were not a part of the original signal. Over long-distance backbones where a signal needs to be boosted many times, fiber-optic systems must be strung with regenerators, devices that reconstruct signals that have traveled through so many amplifiers that they have degraded. Regenerators take a light signal, convert it to an electrical signal, and then produce a new light beam. A new technique called Raman amplification will allow a signal to be amplified without introducing noise-doing away with the need for regenerators and potentially creating a new way for engineers to increase capacity. Unlike erbium amplifiers, which only work at celtain wavelengths, Raman amplification holds the promise of making even more new channels available. A new company, Xtera, of Allen, Texas, is hoping to take advantage of Raman amplification to enable the long-range transmission of shorter wavelengths of light than current optical networks can support. "It's kind of a new twist on using Raman techniques," says Joe Oravetz, Xtera's product manager, who unveiled the company's first new product at the Optical Fiber Communication Conference and Exhibit in March in Anaheim, California. But using the shorter-wavelength band is a decidedly longterm strategy, since it will require installation of new equipment at every point in the network. "Going into a new band, you have to replace all the components," says Vladimir KozlDv, an analyst at RHK. "You need new sources. You need new amplifiers. It could be very expensive."
Speeding Up Bits An alternative to adding channels is to make the data stream in each channel flow faster. Just as the modems in people's homes have gotten faster, transmitters in the backbone have increased their ability to pump data, from 100 million bits per second a decade ago to a state-of-the-art lObi IIion bits (10 gigabits) per second today. While AT&T issued a press release announcing the first 10gigabit-per-second coast-to-coast Internet protocol backbone in January, it's already old news: 40-gigabit-per-second systems have already been announced by Lucent Technologies, Fujitsu and NEC for sale later this year. The engineering feats involved in advances like these are tremendous: increasing the data rate required engineers to design lasers that can reliably flash on and off 40 billion times per second, and receivers that can pick out one flash from the next, when they're coming at that overwhelming rate. But the name of the game in the backbone remains trade-offs,