Large-effective-area fiber to address undersea requirements

Large-effective-area fiber to address undersea requirements

By STEPHEN HARDY

The race between Corning Inc. (Corning, NY) and Lucent Technologies` fiber division in Atlanta, GA, to develop a large-effective-area fiber for undersea applications is heading for the wire, with Lucent expected to cross the finish line first. The real winners, however, will be submarine network designers and developers, who will soon be able to construct more powerful and efficient undersea links.

As highlighted when Corning released its LEAF product for terrestrial applications, large-effective-area fiber enables high-power, multichannel transport over long distances while reducing the nonlinear effects that can degrade transmission performance. Manipulation of a fiber`s dispersion can combat some nonlinearities, particularly 4-wave mixing, but other nonlinearities are more significantly affected by the intensity of the photon streams traveling through the fiber. An increase in the effective area of the fiber (the cross-section of the fiber where the light propagates) reduces the intensity of the light, thus enabling an increase in the overall power of the light without reaching the intensity threshold where nonlinearities become a debilitating factor.

Considering the high-power, long-reach requirements of undersea applications, large-effective-area fibers would enable greater distances between expensive amplifiers as well as accommodate higher channel counts. Unfortunately, the positive-dispersion characteristics that work so well in the LEAF fiber`s terrestrial applications represent a disadvantage in undersea applications, according to sources at both Corning and Lucent. In particular, positive dispersion increases the chances of another nonlinear effect, modulation instability, over the long links typical in submarine applications.

The answer would be a negative-dispersion fiber, with a zero-dispersion point of 1585 nm (versus the 1520-nm point of positive-dispersion fiber). However, in the words of Ken Able, Corning`s commercial manager for submarine cable, "One would think that, well, if you can create a positive-dispersion fiber, all you`ve got to do is change the recipe around a little bit and come out with negative dispersion. The negative-dispersion fiber, with the attributes that the submarine community is looking for, turns out to be not such a simple task."

The problem, says Able, is that as you increase the effective area, the zero-dispersion slope also increases. This phenomenon makes it more difficult to balance the power levels in each of the channels transmitted through the fiber. Increased effective area also affects the amount of loss due to both micro- and macro-cable bends.

Both Corning and Lucent have investigated new fiber materials and dopants, in conjunction with adjustments to the refractive index profile of the fiber, to combat these problems. Changing the coating and cladding diameters offers tempting solutions to the bending problems, but would result in undesirable changes to the cabling requirements, says Able. The fact that undersea systems integrators demand extremely high reliability adds to the fiber design challenge. "The specifications in this market are very stringent, very tight. And sometimes they push the edge of even measurement capability, to be able to measure to the requirements that they have," Able explains.

Lucent will be the first company to announce that it has achieved a practical undersea large-effective-area fiber with negative dispersion. The new TrueWave XL fiber, developed with the input of Tyco Submarine Systems (Morristown, NJ) will deliver an effective area of about 75 square microns, according to Dave Peckham, a distinguished member of the technical staff in Lucent`s Preform Technology Group, and Dave Kalish, an optical fiber development and engineering director at the company. TrueWave XL will retain the industry-standard 125-micron diameter to make the fiber easy to cable. Adjustments to the index profile and special doping represent the key design parameters in creating the fiber; however, the Lucent sources declined to provide details of either activity.

As this issue went to press, Lucent expected to announce the availability of the fiber at the end of September. At the same time, the company planned to reveal Tyco Submarine Systems as its first TrueWave XL customer, with the fiber destined for the 9000-km Pan American Crossing network Tyco will build for Global Crossing next year.

Meanwhile, Corning expects to release its undersea large-effective-area fiber, to be called Submarine LEAF fiber, at the end of this month. Able and other Corning sources declined to discuss performance details prior to the fiber`s official debut.

Undersea system integrators are reputed to be fairly conservative when it comes to changing the way they construct their networks, principally because it is difficult and expensive to replace or fix equipment that are so new that all the bugs have yet to be removed. At the same time, the unique requirements of this environment frequently push technology development to previously unattained performance levels. This dichotomy will be readily apparent with large-effective-area fiber.

"Because of that reliability question, there is a tendency not to want to change unless there is a good reason to change," concludes Able. "If there is a good reason to change-- for example, going to 16 channels by 10 Gbits, in order to accomplish that, you have to have a large-effective-area fiber; you can`t go those distances with just the standard non-zero-dispersion-shifted fiber--there`s a great deal of eagerness. And right now in the marketplace, everybody is looking at the introduction of a large-effective-area fiber to facilitate these systems." q