OFC/NFOEC: For ROADMs, Less Is More

SAN DIEGO -- OFC/NFOEC 2010 -- Vendors of reconfigurable optical add/drop multiplexer (ROADM) technology have talked about colorless, directionless, and contentionless cases. Next up might be "gridless."

It's all part of making ROADMs more flexible in order to give service providers more options. The gridless ROADM -- vendors seem to favor the phrase "flexible spectrum," actually -- has gained some buzz amidst all the 100-Gbit/s chatter, because the technology could be useful for carrying unusually high-speed signals.

And like a lot of ideas proposed in optical networking, it's not new. "I think this question pops up every five years," says Krishna Bala, executive vice president with Oclaro Inc. (Nasdaq: OCLR).

Being gridless refers to having wavelengths that don't conform to the ITU grid for Dense Wavelength-Division Multiplexing (DWDM). Specifically, it's about letting wavelengths take up irregular numbers of slots -- 1.4 or 2.2 times more spectrum than usual, for instance.

That could be useful for very high-speed signals or experimental modulation schemes, cases where carriers "aren't sure how much passband they'll need," says Craig Iwata, manager of corporate marketing for JDSU (Nasdaq: JDSU; Toronto: JDU).

But Iwata says he was previously hearing those concerns for 100 Gbit/s, and it's already being shown that a serial, long-haul, 100-Gbit/s wavelength can, indeed, fit in a 50GHz ITU grid. And Bala -- who joined Oclaro through its acquisition of Xtellus last month -- notes that the industry is trying to keep to some standards for line-side 100-Gbit/s interfaces, which he thinks would make anything with nonstandard grid spacings unpopular. (See Oclaro Acquires Xtellus.)

Each of them doubts that demand for flexible-spectrum ROADMs will get heavy any time soon.

Others aren't so sure. Ashish Vengsarkar, CEO of ROADM subsystems vendor Nistica , says he's hearing of interest in the technology right now.

And Nokia Networks seems to like the idea of having a gridless ROADM at the ready, as the company experiments with line-side speeds exceeding 100 Gbit/s.

"There's some way to do 200 Gbit/s within a 50GHz grid, and people have done 100 Gbit/s within 25GHz, but these are based on really complex modulation schemes," says Michel Chbat, the company's North American head of solution engagement.

Should a need for gridless ROADMs emerge quickly somehow, Finisar Corp. (Nasdaq: FNSR) might have an edge. The company's wavelength-selective switch (WSS) is software-programmable to arbitrary grid spacings, even irregular ones. The capability appears to make Finisar's the only shipping WSS that could produce a gridless ROADM.

— Craig Matsumoto, West Coast Editor, Light Reading