L-Band Amps: The Revolution?

Use of L-band Erbium Doped-Fiber Amplifiers (EDFAs) could become more widespread in the future if U.K. startup Southampton Photonics Inc. (SPI) has its way. The company introduced a new L-band amplifier today (see SPI Announces L-Band Amp).

Up until now, L-band amplifiers, which amplify optical signals in the wavelength range 1565 to 1605 nanometers, have been deployed to increase channel counts in DWDM systems, by opening up new wavelengths alongside the commonly used C-band (1525 to 1565 nm). At the moment, however, no one is building ultra-high channel count systems, except possibly in lab experiments.

But there's another reason for using L-band components, says Southampton's CEO David Parker -- they give better overall system performance than standard C-band ones. Put simply, it is easier to build a high-speed system over long distances using the L-band. The L-band is particularly useful in situations where non-zero dispersion shifted fiber (NZ-DSF) has been installed, because it allows the system to operate away from the zero dispersion point, and thus reduces the crosstalk caused by four-wave mixing (see Nonlinear Effects).

Carriers could start using L-band components in preference to C-band ones, Parker says, and the main thing that's stopping them is the cost of the optical amplifiers.

"We sense from our customers that they see the advantages, but it just costs too much," he contends.

Southampton's new product will overcome the price problem, it claims. "We're not just saying it's a better amplifier," says Parker. "It is massively cheaper than conventional technologies." Although he can't give exact pricing, which depends on the customers' specifications, he does boast that the cost reduction of the new amp will be "fifty percent or better" compared to existing technologies.

Do Southampton's claims stand up to scrutiny? A 50 percent cost reduction would make the cost of L-band amplifiers comparable to C-band amplifiers today, notes David Smith, VP of hardware engineering for Corvis Corp. (Nasdaq: CORV). That means that any decision to use the L-band in preference to the C-band could then be based on performance only.

"The L-band is better in some respects," says Smith. "But I'm not sure that people should be making claims that the L-band is infinitely better."

In terms of performance, the L-band typically has higher dispersion and therefore lower crosstalk. In certain types of fiber with very low dispersion, such as NZ-DSF, crosstalk limits the number of channels. Using the L-band rather than the C makes it possible to cram more channels in this situation, he notes.

The other key parameter is the noise figure, which is lower in the L-band. That means that more amplifiers can be cascaded before the signal has to be electrically regenerated, so ultra-long-haul optical systems could go farther more easily in the L-band.

Smith also points out that most optical components today work in the C-band, and there are relatively few available for the L-band. "The way the industry is looking at it today," he says, "is to use the C-band for as long as possible, then when they exhaust the C-band, move into the L. If, two years from now, components are available in the L-band, and companies are building new networks, maybe they will start in the L-band and move into C later."

Whether Southampton's announcement marks the beginning of a change in the way people build DWDM systems remains to be seen. However, it does mark a turning point in the startup's strategy.

The market Southampton started out in -- making high-density, narrow channel spacing Fiber Bragg Gratings (FBGs) -- isn't large enough to sustain the company in the current downturn. So it is looking to widen its horizons with products that leverage its core expertise in making strange kinds of optical fibers -- including fiber lasers and fiber amplifiers (see Fiber Bragg Gratings on Speed).

The key to Southampton's L-band amplifier is a new kind of fiber, called GT Wave, in which optical power is coupled from pump lasers into the core of the fiber in an extremely efficient manner. Inefficient performance is the fundamental problem with today's L-band amps, says Southampton's Parker.

GT Wave fiber contains multiple cores -- several large ones to carry light from the pump lasers, and a smaller, singlemode core that carries the data signal. As the fibers are drawn on a fiber-pulling tower, the multiple cores are twisted around each other. (Parker tried to demonstrate this with three fingers, but since he was at the other end of a phone line, the finer details of the demonstration were lost.)

Using this technique, Southampton claims it is possible to make amplifiers with outputs of 1W, which is considerably more than is required for telecom applications.

Questions remain about the market opportunity for EDFAs, however. The market for optical amplifiers of any kind is said to be fairly small this year and next, according to recent figures from Communications Industry Researchers Inc. (see Report: Slow Ramp for Optical Amps).

Southampton plans to address a range of applications, not just telecom, with its high-power laser and amplifier products.

Prototypes of the L-band amplifier should be available at the end of the year. General availability is slated for the first quarter of 2003.

— Pauline Rigby, Senior Editor, Light Reading
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