Sabeus Challenges Thin Film

Sabeus Photonics Inc. says it has a solution that can increase the reliability of Erbium Doped-Fiber Amplifiers (EDFAs), which are used in long-haul and ultra-long-haul DWDM applications (see What's Inside Sabeus).

Sabeus, an optical components startup that recently announced a $16 million round of funding, will be displaying its long-period gain flattening filter at the Optical Fiber Communication Conference and Exhibit (OFC) in Anaheim, Calif., next month.

Gain flatteners are used to equalize Optical Amplification over a range of wavelengths, and they're key parts used in all EDFAs. Thin-film filter technology, which has been around for several years, is the most commonly used technique for gain flattening in EDFAs.

But Dmitry Starodubov, a founder of Sabeus and its CTO, says thin-film technology has an inherent flaw that can cause unreliability. Specifically, the erbium used in EDFAs creates a green light inside the fiber that may cause degradation of the optical properties in the thin-film solution used to coat the filters. He adds that this degradation may not happen immediately and often takes weeks or months to show up. He claims the root of the problem lies in the way thin-film filters are manufactured.

“In order to put the thin film in the path of light you have to break the fiber,” he says. “And when that happens, reliability problems occur. Also, there can be problems from the epoxy that is used to put together the thin film and the fiber.”

Starodubov speculates this may have been part of the problem that Marconi PLC (Nasdaq/London: MONI) experienced with the JDS Uniphase Inc. (Nasdaq: JDSU; Toronto: JDU) EDFAs that were recalled last year. Neither Marconi nor JDSU have publicly offered specifics on the problem (see JDSU in EDFA Recall).

Starodubov contends that Sabeus’s approach, which does not use thin-film filter technology, is much more reliable. Sabeus uses long-period grating technology, whereby light is directed to specific locations by gratings within the fiber. As a result, fiber is never broken and light never leaves the fiber. Also, there is no epoxy attaching the filter to the fiber, which Starodubov says could further degrade performance.

Sabeus is not the only company working on fiber grating technology as an alternative to thin-film filters. Over the past two years, a number of startups -- such as Bragg Photonics Inc., Redfern Photonics Pty Ltd., Indigo Photonics Ltd., TeraXion, and Southampton Photonics Inc. -- along with established players like JDSU, Lucent Technologies Inc. (NYSE: LU), and Alcatel SA (NYSE: ALA; Paris: CGEP:PA) have been working on so-called fiber Bragg grating filters (see TeraXion Trots Out FBGs).

Like Sabeus’s long-period grating technology, Fiber Bragg Gratings (FBGs) work within the fiber. Light passes through the core of the fiber, and unwanted wavelengths are reflected back in the opposite direction. The problem with Bragg gratings is that this reflection causes interference with the wavelengths that are supposed to get through. As a result, additional devices called isolators are needed to prevent this. The addition of these isolators adds cost and complexity to the component.

Sabeus has taken a different approach to gratings. Instead of reflecting the unwanted wavelengths in the opposite direction, the Sabeus product directs it off to the side. This eliminates the reflection problem, the startup says, obviating the need for isolators.

Sabeus's new tack may be another bid for an alternative to thin-film filters, but not everyone agrees that thin-film technology is outmoded.

Karen Liu, director of optical components for market research firm RHK Inc. agrees that the technology has its drawbacks, though she says she has never heard of the green-light factor described by Starodubov.

According to Liu, what makes thin-film technology good also makes it potentially bad. For instance, it is batch oriented, which means hundreds of filters are made at once on a single sheet of substrate. Then the filters are broken down into tiny wafers and attached to fibers.

The batch method is good for high-volume production, but it’s difficult to make customizable filters, since an entire batch must be made at once.

What’s more, every filter that comes from the same batch is essentially identical. Liu says that residual errors, which are inherent to any filter, are compounded when the filters are deployed in a series.

“Thin-film filters have some limitations,” she says. “But [the technique] works pretty well, and that is why a lot of companies still use it. The batch objections are really second order. The residual error is residual, after all.”

Jeff Montgomery, chairman and founder of ElectroniCast Corp. is critical of gratings because he says it is difficult to manufacture them in volume, since each grating device is separately made.

Montgomery also is not convinced that thin-film filter technology is unreliable. He says it's working just fine in most applications.

“I haven’t heard that there is a widespread problem with thin film filters,” he says. “And I think if there was a significant problem I would have heard about it.”

Starodubov of Sabeus agrees that traditional manufacturing techniques for gratings is labor intensive, but he says Sabeus has fixed that problem, too. The company has developed a completely computerized manufacturing technique, which inputs a particular customer’s specifications and then replicates those specifications for each grating.

Sabeus claims that it is shipping its gratings to about 40 customers for trials with EDFA applications. Revenue shipments are expected later this year. Although the company wouldn’t comment on specific customers, Agere Systems (NYSE: AGR) could be a good guess, since the company is also an investor.

— Marguerite Reardon, Senior Editor, Light Reading
http://www.lightreading.com