Wavelength Converters Go the Distance

A distance record claimed by Kailight Photonics Ltd. shows all-optical wavelength conversion is still alive, even if vendors are still waiting for the market to kick in.

Kailight says it has sent 10-Gbit/s signals 20,000 km using its all-optical wavelength converter to perform 2R regeneration, which involves reshaping and reamplifying a signal (see Kailight Sets 2R Distance Record). The test, run by Bell Labs, involved sending a signal through multiple stages of 2R regeneration, seeing how many times the signal could be repeated before degrading due to jitter and other factors.

"We're showing these things can be cascaded one after another without running into a problem," says Neil Salisbury, vice president of marketing and business development at Kailight. "There wasn't a lot of time to do the test. We think if we'd had a couple of days, we could have gotten better results."

Wavelength conversion was supposed to be a major part of the all-optical network, alongside all-optical switches. The target was the market for wavelength services, where optical channels would be switched without being converted to electrical form, resulting in faster, more advanced communications networks.

Wavelength conversion had been researched in optical physics for more than a decade, and by 2002, startups including Kailight, Kamelian Ltd., and Lightbit Corp. had emerged to tackle the challenge (see Interest Grows in Wavelength Conversion).

Kailight isn't the only distance runner in the pack. Lucent Technologies Inc. (NYSE: LU) hit 1 million km, as described in 2002 research papers, but that was done using 3R regeneration, which adds retiming to quell jitter problems (see Kansans Reset the Clock). Some 2R experiments have gone farther than 20,000 km, but they used optimized conditions, whereas Kailight's test was closer to a real-world case, says Juerg Leuthold, the Bell Labs/Lucent researcher who ran the test.

So, Kailight can lay claim to a 2R distance record, according to Leuthold -- who happens to be the previous record holder and the same guy who achieved the 1 million km mark.

"They had made clever modifications. It's the first time I've seen a really good wavelength converter coming out of a startup," Leuthold says. "Within about half an hour, we had 20,000 km."

One drawback is that Kailight's TASR-4000 module uses return-to-zero (RZ) modulation, as opposed to the less costly non-return-to-zero (NRZ) option. Kailight does have another model that operates with RZ or NRZ, "but we don't get quite the distance on it," Salisbury says. (See Optical Modulation.)

The wavelength converters can also perform 2R regeneration by simply not converting the wavelength -- that is, rebuilding the signal back on its original wavelength rather than shifting it to another channel. That opens up another application, but both wavelength conversion and 2R regeneration are long-haul tricks, tapping a market that's still flat.

"We are making sales on the wavelength converter side [more so than on the 2R side], but it's not into the network. It's into test beds -- which is a good early indicator, but it's a couple of years before the technology will get into the network," says Paul May, CEO of Kamelian. "We are seeing significantly more interest than we were seeing even six months ago, which indicates people are starting to look at what they're going to do in the long haul."

Another sign of interest is Kamelian's round of funding completed last year. Lightbit is getting nibbles as well, having announced some trials in July. One of the then anonymous customers turned out to be Sprint Corp. (NYSE: FON), which presented a paper at NFOEC summarizing its lab trials with Lightbit's 2R regeneration technology. (See Kamelian Scores a Coup and Lightbit Touts Successful Trials.)

Of course, negative signs persist, too. Fellow wavelength-converter startup Optovation Inc. gave up the ghost last year, selling its assets to router firm Laurel Networks Inc., of all people (see Optovation Parts Hit the Market and Ciena Takes Stake in Laurel).

Each startup uses a different approach. Kailight's product lines use a single semiconductor optical amplifier (SOA), exploiting nonlinear effects to regenerate the signal. Kamelian uses a two-SOA setup, while Lightbit's technology is based on a material called partially poled lithium niobate (PPLN). Bell Labs' Leuthold, meanwhile, has researched an optical-filter technique that set the previous 2R distance record of 16,800 km.

Kailight's products are in the prototype stage, with testing samples expected to come in the second half of 2004, Salisbury says.

— Craig Matsumoto, Senior Editor, Light Reading

Dr_Moose 12/5/2012 | 2:24:46 AM
re: Wavelength Converters Go the Distance I heard that Lightbit was sold (recently, in the last few weeks) - any info or rumors on this?
arch_1 12/5/2012 | 2:24:04 AM
re: Wavelength Converters Go the Distance It seems to me that the number of hops is an important metric. This technology can be used to build large "color-space-color" crossbar switches. Such a switch can provide arbitrary corss-connect from any color on any ingress to any color on any egress, but the number of hops inside the switch grows as the log of the number of ingresses.

Of course, a single 5-hop switch in Kansas could probably give each American a personal OC-192 connection, but it's still an interesting question.

I'm guessing that the distance metric is related to the number of hops, but much is left unsaid.
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