Reamplification redux
Optical networks not only have to manage chromatic dispersion, but reamplify their signals about every 80 kilometers.Traditionally, carriers have used Erbium-Doped Fibre Amplifiers to revive weakened signals. EDFAs excite erbium atoms that release energy to the light beam as it passes through the amplifier. They work well in the C-band where wavelengths are between 1,530 nanometers and 1,563 nm in length. Erbium also can work on the L-band, at 1,570 nm to 1,610 nm.
But erbium reamplification is only good for about 80 km and it doesn't work on the S-band, where wavelengths are shorter, between 1,485 nm and 1,520 nm. The inability to reamplify S-band wavelengths makes today's optical network operate at about half its potential efficiency.
When email was the primary data traffic, the C-band was plenty. But now that consumers want video-on-demand, and bosses want videoconferencing on Fridays, carriers want access to the S-band.
Raman amplification attacks both problems and is at the center of a hot new battleground. In Raman amplification, the fibre itself is used as the gain medium, so the signal weakens much less over a long distance. The launch power of the transmission signal doesn't have to be as high, so fewer amplifiers and less power are needed to carry the information cross-country. Raman amplification is needed about every 100 km.
Xtera Communications is refining Raman amplification so it can awaken the S-band of light waves. "We can increase bandwidth by 50 percent" without adding more fibre or switches, says Chief Executive Jon Bayless, the former chairman at Ciena.
In February, Xtera announced that it had amplified 16 channels in the S-band, using discrete Raman technology to transfer energy from the pump laser to the light traveling through the amplifier.
No one has announced customers for the S-band solution yet, but Bayless suspects Xtera isn't alone in developing the technology. Fibremaker Corning and several equipment vendors also are reported to be developing S-band Raman amplification gear.
