Vendors target 25-GHz channel spacing

Jan. 1, 2001

In a quest for more capacity from existing fiber, systems manufacturers will likely ship the first DWDM products that support 25-GHz channel spacing this year. Currently, commercial DWDM systems support 100-GHz or 50-GHz channel spacings. The narrower channel spacings will, in effect, allow carriers to double the number of wavelength channels used in their long-haul systems.

Ciena Corp. (Linthicum, MD) announced the ability to support 25-GHz and 12.5-GHz channel spacing using its in-house fiber Bragg grating technology last October. The filter technology is expected to become commercially available in Ciena's Corestream DWDM systems this year. Ciena develops and manufactures all of the fiber Bragg grating technology that is used in its optical transport systems.

The fiber Bragg grating technology that supports 25-GHz and 12.5-GHz channel spacing will allow carriers to increase the number of channels in the C-band without requiring other major component upgrades in the network, such as additional amplifiers, according to Ciena. The same technology could also be used in the L-band and S-band.

"Currently, we are at 96 channels with 50-GHz spacing all on C-band," says Tom Moc, senior director of product management for transport products at Ciena. "So with 25-GHz spacing, one would expect that you could double that, and with the 12.5-GHz spacing that we've announced, we would think that you could probably go to 300 or more channels."

Systems incorporating the new 25-GHz and 12.5-GHz channel-spacing technology have been successfully tested in the lab and shown to customers. At press time, the technology had not been deployed in a network.

"One of the key advantages of this is that if you are looking to go beyond 80 or so channels in a long-haul system, you have a couple of options. You can either space your channels more closely together, or you can use additional amplifier bands-you can start putting channels in the L-band or the S-band," says Moc.

Ciena has used the fiber Bragg grating technology in its long-haul systems since its first systems became available in 1998.

"These basically are the ideal filters for very narrow channel spacings, because they are highly selective filters of a flat passband with very sharp adjacent channel rejection," says Moc. "Our belief is you can't get the same sort of filter selectivity with thin films, al though we do use thin films in other applications. For example, our wider-channel-spacing metro products use the thin-film filters. We usually look at thin-film filters for lower-loss applications that don't require very close channel spacing and high selectivity and then use fiber Bragg grating for applications that require very high selectivity."

Despite its suitability for higher-density applications, the fiber Bragg grating technology presents some unique challenges.

"I think one of the things that you'll find with Bragg gratings is that it is a higher-loss filtering technology," says Moc. "So generally it is easier to use with optical amplifiers, which can make up the loss of that filtering element."

In addition to Ciena, other vendors are also working on developing components that support narrower channel spacings. Lucent Microelectronics, the components arm of Lucent Technologies, expects to offer components that support 25-GHz channel spacing in DWDM systems this year.

"The technology that we have used for the mux and demux-type technology is the arrayed-waveguide [grating] technology," says Ray Nering, director of strategic marketing, optoelectronics, at Lucent Microelectronics. "The arrayed-waveguide technology is certainly one that we have at our disposal that we intend to pursue this market with; the other technology is the thin-film-filter technology."

Toward that end, Lucent acquired Herrmann Technology, a company that specializes in thin-film-filter elements for packaging, in June.

"The big issues are things like crosstalk and passband shape," Nering continues. "What we had done a while ago was to make demuxes that had a certain shape to them, and you might augment that shape and improve the crosstalk with an external filter element. So Herrmann had been making those kinds of filter elements that could be used with an arrayed-waveguide device. Since then, the arrayed-waveguide devices have improved to where these devices don't require that for the 200- and 100-GHz device space systems.

"But when we go down to the 25 GHz, what we may find is that when we take advantage of the fact we have both these technologies in-house to be able to deliver initially the kind of performance that people want and that will get us to market quickly."

Meanwhile, companies such as Wave-Splitter Technologies are pursuing the development of 25-GHz channel demultiplexing using arrayed-waveguide-grating routing technology and interleavers. Interleaving is frequently used in 50-GHz channel spacing. Basically, one set of 100-GHz-spaced channels or wavelengths is going in one direction and another set of wavelengths is going in the opposite direction.

Finally, while carriers and service providers aren't yet asking for DWDM systems with higher channel counts, most want to know how to increase the existing capacity on their networks. "I don't know that there is anyone out there deploying more than 80 channels today," says Ciena's Moc. "But I do know that they all want to know how they get beyond 80 channels, because it won't be very long until they get to that point."