Ultra-long-haul: Ready when you are

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"When the market comes back" is a turn of phrase heard often in telecom circles. None of us knows exactly what that market will look like if and when it appears. But the market segment that has suffered most—long-haul—is the one that may benefit the most from the current slowdown.

While executives and product managers have been struggling for sales, the researchers who first helped fuel the surge in transmission capacity and distance are still at work. For this market to come back, we'll need to see considerably more traffic, of course. We'll also need to see real reductions in operating expenses, especially for 10-Gbit/sec systems. And that's the focus of the research.

Linn Mollenauer, a researcher at Lucent Technologies Bell Labs (Holmdel, NJ), notes that for many scientists, the slowdown has meant a chance to let the science catch up with the performance demands from the carriers. "There really was a point, before the bubble burst, that service providers were beginning to buy the idea of low-cost, efficient service with all-optical technology," he says. "But the technology never really got off the ground before the disaster."

Before and after the disaster, his Lightwave Systems Research group has been working on ways to make the all-optical network appealing to carriers. The group has recently presented results in Optics Letters reflecting some advances in ultra-long-haul technology that have Mollenauer excited.*

The article describes a dispersion-managed soliton technique for 10 Gbits/sec that can extend transmission distances beyond 20,000 km without optical-electrical-optical regeneration. This distance is a considerable advance in scale and, when combined with all-optical switching, would allow carriers to span North America with robust self-correcting networks that lower operating expenses.

Solitons were first observed in the form of solitary waves that traveled along 19th century canals. Mollenauer generated the first fiber-optic solitons experimentally in 1980, but interest was limited until the advent of optical amplification. In 1988, Mollenauer's group extended transmission distance to 4,000 km using Raman amplification in a recirculating loop. While WDM technology was the winner in terms of increasing transmission capacity, WDM used in conjunction with solitons has remained part of product planning strategy at several companies.

A soliton signal can travel great distances over fiber because the pulse spreading caused by chromatic dispersion is offset by the effects of self-phase modulation. More recently, dispersion-managed solitons have been developed to overcome the signal impairments found in classical solitons by splicing together two fibers having different dispersion properties to keep the total dispersion of the two close to zero.Th 130106

In the soliton propagation laboratory at Bell Labs, racks hold spools containing 600 km of optical fiber. To the left of the researcher are the laser sources and pattern generators, and to his right, the receiving equipment. A new periodic-group-delay dispersion-compensating technique developed here extends transmission distances for 10-Gbit/sec signals beyond 20,000 km. Photo courtesy of Bell Labs

Mollenauer's group has now taken another step by installing a periodic group delay dispersion-compensating module. The module is based on etalon technology from Avanex and compensates for only a small part of the transmission span. Other dispersion-compensating fiber is still used, but the result dramatically reduces the distance-limiting effect of jitter—the one significant nonlinear penalty not corrected by previous techniques. Jitter in signal pulses results from the collision between solitons in different wavelength channels.

This modification may seem like a minor tweaking, but as Mollenauer explains, it means that dispersion compensation is independent of distance, economical, and compatible with 10-Gbit/sec DWDM systems. In other words, it's ideal for continent-spanning all-optical networks.

More exotic distance-enhancing techniques such as differential phase-shift keying (DPSK) modulation may be the better dispersion-compensating approach for 40-Gbit/sec systems, says Mollenauer. But with his new dispersion management technique, standard on-off keying works just fine. The result is another enhancement to existing long-haul technology that should serve the industry well "when the market comes back."

*X. Wei, X. Liu, C. Xie, L.F. Mollenauer, Optics Letters, Vol. 28 (June 2003).Th 130115

Conard Holton is chief editor of WDM Solutions and executive editor of Laser Focus World. He can be reached at cholton@pennwell.com.

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