Pushing the Limits

By eweek  |  Posted 2001-03-19

Back when 10-gigabit-per-second and 40-Gbps speeds were just a glimmer in an engineers eye, no one worried much about bits bumping into one another on wavelengths of light.

But now that 10-Gbps traffic is the ascendant norm, and 40 Gbps is next years battleground, physicists are pushing the limits of just how fast information can travel across optical networks and still emerge intelligible on the other end.

Optical network companies are teaming physicists with engineers to tap more bandwidth, sharpen fuzzy or dim signals and correct errors on the fly.

Among network equipment companies, customer service and brand-name loyalty may separate the winners from the losers, analysts say. But among components players, cutting-edge technology will still make the biggest difference as carriers attempt to meet the publics apparently insatiable appetite for bandwidth.

Delicate Balance

The first order of business is dealing with chromatic dispersion — the tendency of a light beam to spread as it travels through fiber. In an optical network that uses Dense Wavelength Division Multiplexing (DWDM) to segregate traffic onto dozens of different-colored wavelengths, chromatic dispersion degrades the optical signal, requiring costly signal regeneration.

But theres a delicate balance to be struck — different wavelengths of light travel at different velocities, and without enough dispersion, the signals arent separated and wavelengths will interact with each other. If theres too much dispersion, the signal cant be detected.

Carriers have turned to single-mode fiber from Lucent Technologies and Corning to minimize signal dispersion. But the fiber best at minimizing dispersion cant keep up with the new high speeds. Each time the speed is quadrupled — from, say 2.5 Gbps to 10 Gbps — the dispersion tolerance drops 16-fold.

"Up until 10 gigabits [per second], everything worked pretty well, says Mark Barratt, vice president of business development at LaserComm. "At these speeds, the fact that each wavelength is a bit different has created a problem.

Long-haul and ultra-long-haul systems trying to move traffic at 10 Gbps or faster need extremely precise tools to manage the dispersion. At high speeds, merely a change in the ground temperature where the fiber is buried can increase dispersion. "Its to the point where we think it will take a device that is actively changing the dispersion in concert with the DWDM systems fluctuations, says Mark Stubbe, vice president of marketing at LaserComm. "We refer to it as dynamic variability."

LaserComm, based in Plano, Texas, has developed a single box that solves dispersion problems on long-distance fiber networks. Its 10-inch by 10-inch by 1.5-inch box includes a spool of fiber that reverses the dispersion effect, sharpening the signal of each channel on an 80-channel fiber.

"Without the device, which goes into DWDM subsystems of optical amplifiers, the bands would have to be corrected one at a time," Stubbe says.

LaserComm is testing the Hi-Mode Dispersion Management Device with several DWDM system developers, but hasnt announced any paying customers yet. The company expects to announce a box later this year that can correct dispersion over all channels of a 40-Gbps system.

LaserComms next stop is the longer-wavelength L-band. As bandwidth demand soars, carriers hope to double todays output by tapping 160 channels on the L-band. In February, LaserComm announced a device to manage chromatic dispersion in the L-band.

Avanex offers another way to compensate for chromatic dispersion — a device with two tilted parallel mirrors. Off-the-shelf micropositioners tune the mirrors to bounce light back and forth in a way that slows or speeds the traffic flow through the dozens of wavelengths. Its another way of assuring that the wavelengths arrive simultaneously so the signal isnt lost in the noise.

Avanexs advantage is its wider tunability and smaller size, says Giovanni Barbarossa, senior director of research and development. It doesnt use a special high-mode fiber, or any fiber at all, so "you dont need to spool the fiber or be constrained by the bending radius of fiber.

Avanex says it has shipped its product to all the major manufacturers of equipment for high-speed optical networks.

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 Fiber 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 doesnt 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 todays optical network operate at about half its potential efficiency.

When e-mail 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 fiber itself is used as the gain medium, so the signal weakens much less over a long distance. The launch power of the transmission signal doesnt 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, an Allen, Texas, start-up, is refining Raman amplification so it can awaken the S-band of light waves. "We can increase bandwidth by 50 percent" without adding more fiber 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 isnt alone in developing the technology. Fibermaker Corning and several equipment vendors also are reported to be developing S-band Raman amplification gear.

Short-Haul Solutions

Not all the physics challenges have to do with more wavelengths traveling greater distances. ONI Systems CEO Hugh Martin says his company is more interested in moving traffic down the street, across town and from suburb to suburb.

On the long-haul network, information can travel from Los Angeles to Dallas without being touched by an optical switch. But the metro area is crowded with switches and couplers. It has its own regeneration and reamplification rules, and as speeds begin to zoom, the solutions get more complex.

"One minute, the traffic has to move down the street to the next building; the next, most of the traffic has to get 20 miles away, Martin says. On a 20-mile trek across a metro area, the signal passes through dozens of switches, each robbing the pulse of some of its accuracy.

ONI Systems builds a platform that incorporates devices from companies like Avanex, LaserComm and Xtera with its own Dynamic Power Control technology.

ONIs variable-gain amplifiers monitor the optical power of every wavelength in the metro area and change how much amplification exists in the system at any moment.

If the blue wavelength is on a milk run to the office down the street, the signal wont degrade, even if its not on full power. But if blue suddenly is switched to a crosstown route, signal clarity becomes a factor and ONIs intelligent switch turns up the juice.

Most networks use a fixed system, adjusting, say, the blue wavelength to travel a 20-mile route, never longer than that, never shorter than that. ONIs dynamic power control can tweak the power and correct the dispersion, so that any wavelength can travel any distance and deliver intelligible data, voice or video. "In real time, we adjust the power level of every color, Martin says.

Ciena and Nortel Networks reportedly are working on similar systems, but ONI so far is the only company with announced customers — Qwest Communications International and Williams Communications.

"We either make the signal much brighter at the input, or have amplifiers along the way to juice the wavelength, Martin says. "At 10 gigs the signal-to-noise ratio gets important, and at 40 gigs it gets even worse.

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