Phaethon Embarks on Tunability

Startup Phaethon Communications emerged from stealth mode today, claiming a unique solution to a troubling problem that's emerged in optical networks (see Phaeton 'Unveils Its Vision').

That problem is how to ensure that wavelengths stay strong across fibers that have different physical characteristics. It's a problem that arises when carriers are designing or installing DWDM (dense wavelength-division multiplexing) gear and when wavelengths are switched. When this happens, wavelengths are often driven across fibers of different types and lengths.

At the root of the problem is a technical issue called dispersion -- the degradation of light signals when they're sent over fiber. There are different types of dispersion. "Chromatic dispersion" happens because a light pulse isn't just a single wavelength; it's a bunch of wavelengths representing slightly different colors. The longer wavelengths travel at slightly different speeds than the shorter ones, so over distance the pulse spreads until it bumps into the adjacent one, at which point the signal is lost.

This problem gets worse with speed. At 40 Gbit/s, for instance, the problem is 16 times worse than it is on 10-Gbit/s links.

To compensate for the inevitable effects of chromatic dispersion, Phaethon has designed a tiny device for use by system vendors. It's a fiber Bragg grating (FBG) that has been etched in a special fashion, then microscopically stretched. The end result is a component that can change the compensation values for different bunches of wavelengths in a single fiber. Hence, the term "tunable" compensator.

Today's chromatic dispersion compensators use a different tack. Most are small modules containing dispersion compensating fiber, which are inserted at the receiver on a DWDM system and in the network itself, right before the amplifiers on the fiber -- typically every 80 to 100 kilometers.

These modules, exemplified by the products of Corning Inc. (NYSE: GLW), are preset with so-called "compensation values" that match the type and length of fiber on which they reside. Accommodating different types or lengths of fiber, or switching to different speeds, requires another module.

Phaethon says its solution is useful because one component will be able to automatically adjust to different compensation values.

"Say you had a 100-channel DWDM system with channels organized in groups of ten," says Bruce Barton, VP of marketing at Phaethon. "You can set different compensation values for all of the channel groups, using one device. There's no need to drive out and reinstall the fiber and modules."

Analysts say that if Phaethon can live up to its promise, it's got something good. "I haven't heard anyone offer remote tunability yet," says Tom Hausken, director of optical communication components at Strategies Unlimited, referring to Phaethon's capability to build a component that can make dynamic adjustments.

There are drawbacks. For one thing, Phaethon won't have automatic tunability in its products right off the bat. Instead, for a few months, customers will be offered the ability to tune the compensation values by hand.

Another problem: Phaethon will offer tunable compensation for only small groups of wavelengths at first. It plans to unveil 4- and 8-channel modules at the upcoming NFOEC show in Baltimore.

Phaethon's devices also don't solve all the problems associated with chromatic dispersion. LaserComm Inc., a startup selling dispersion compensating modules based on higher-order fiber, says its devices will take care of "dispersion slope," which refers to the ability to set different degrees of dispersion compensation for different channels. Phaethon's devices can't tackle this problem right now, although the vendor says that capability is on the drawing board.

Phaethon also doesn't yet handle (but plans to handle) polarization mode dispersion (PMD), a problem caused by the fact that light can travel along slightly different paths inside a fiber, depending on how it bounces around as it travels. Vendors such as Yafo Networks have started to tackle PMD (see Can Yafo Lift Speed Limits?).

Some naysayers hold that fiber Bragg gratings aren't the way to handle chromatic dispersion problems, since they tackle a low number of frequencies at once. Not so, says Phaethon, which insists its FBGs are key to the tunability feature and offer low insertion loss and low cost ("less than $10,000 per 4- or 8-channel module," according to Barton).

Phaethon also points to its other assets, which include more than 15 patents from the University of Southern California, thanks to founder Alan Wilner, a professor at USC. The company also has right of first refusal on any new photonic technologies from USC, and it can opt to license them exclusively for a ten-year period.

Still, the complexity of the problems Phaethon hopes to solve ensures it's going to be a while before full success is achieved. Meantime, its competitors, including the likes of LaserComm and Yafo, also are at work on comprehensive solutions for chromatic problems. And there's always a chance they may get there first, given the headstart they've got.

It's also not unthinkable that some as-yet unimagined combinations may result from all these developments. "We're looking at adaptive optical networks, having a way to sense the quality and respond with a standalone system that works with multiple DWDM devices," says Henry Yaffe, founder, chairman, and CTO of Yafo Networks. "A company such as Phaethon could be partners with us. Or, depending on their viewpoint, they could be competitors. Or a bit of both."

- Mary Jander, Senior Editor, Light Reading
http://www.lightreading.com

For more information on NFOEC, please visit the Light Reading NFOEC Site.