Pushing the limits
Back when 10-Gigabit-per-second and 40-Gbps speeds were just a glimmer in an engineer's 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 year's 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 public's 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 fibre. 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 there's a delicate balance to be struck - different wavelengths of light travel at different velocities, and without enough dispersion, the signals aren't separated and wavelengths will interact with each other. If there's too much dispersion, the signal can't be detected.
Carriers have turned to single-mode fiber from Lucent Technologies and Corning to minimise signal dispersion. But the fibre best at minimising dispersion can't 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. "It's to the point where we think it will take a device that is actively changing the dispersion in concert with the DWDM system's fluctuations," says Mark Stubbe, vice president of marketing at LaserComm. "We refer to it as dynamic variability."
LaserComm 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 fibre that reverses the dispersion effect, sharpening the signal of each channel on an 80-channel fibre.
"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 hasn't 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.
LaserComm's next stop is the longer-wavelength L-band. As bandwidth demand soars, carriers hope to double today's 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. It's another way of assuring that the wavelengths arrive simultaneously so the signal isn't lost in the noise.
Avanex's advantage is its wider tunability and smaller size, says Giovanni Barbarossa, senior director of research and development. It doesn't use a special high-mode fibre, or any fibre at all, so "you don't need to spool the fibre or be constrained by the bending radius of fibre."
Avanex says it has shipped its product to all the major manufacturers of equipment for high-speed optical networks.
