June 2001

Building a Better Backbone

Continued from page 4

By Neil Savage

But the name of the game in the backbone remains trade-offs, and speeding up transmission rates causes new complications: putting more bits per second into a fiber requires more power, and at higher powers, the interference between channels increases. Also, at these remarkable rates, tiny flaws in the glass itself start to interfere with the flow of data.

Engineers going for speed must compensate for such effects by increasing the buffer zone of unused spectrum between channels: a 40-gigabit-per-second line speed, for example, may require buffers of 100 gigahertz between channels instead of 50 gigahertz. The math is still favorable: the fibers will deliver half the channels at four times the speed, doubling capacity.

The stakes involved in improving transmission rates in the backbone, however, are so great that for every obstacle, there are teams of engineers working to overcome it. Scientists at NEC America's Public Networks Group are working on a way to squeeze channels together, even at high speeds, by taking advantage of the fact that light is polarized. Imagine moving a jump rope rapidly up and down to make waves, which move up toward the ceiling and down toward the floor. Such waves would be "vertically polarized." Now start moving the jump rope from side to side, so the waves move toward the walls. Your jump rope has become horizontally polarized. The NEC approach divides a light beam into 160 channels, each 50 gigahertz apart, and gives neighboring channels different polarizations. Two channels with the same polarization are thus still 100 gigahertz apart. While channels next to one another are likely to interfere with one another when they have the same polarization, channels with different polarizations are not. Such an approach will boost total capacity per fiber to 6.4 trillion bits (6.4 terabits) per second and is projected to be available in two to three years.

And improvements continue in labs worldwide. In March, researchers from the French company Alcatel, which develops fiber and components for both land-based and undersea optical systems, announced they'd developed a system reaching 10.2 terabits per second. Also in March, researchers at NEC announced an experiment in which they tweaked amplifiers to get access to a wider wavelength band, increasing transmission rates to 10.9 terabits per second.