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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-10 billion bits-per second. Today some systems can carry 80 channels in the central band, and are able to push more than a half-trillion 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-art 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 parts 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, doub-ling the capacity of each fiber.

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