Multi-Wavelength Standard?

A standardization effort announced at Cable-Tec Expo is aiming for improved performance in optical signal transmission.

Led by ARRIS and Cisco, the joint proposal for technical criteria supporting multi-wavelength optical networks leverages the two companies’ understanding of nonlinearities in a particular area of the optical domain.

"The proposal that we put into front of the SCTE is that we actually use wavelengths that are outside the zero dispersion band of the fiber," said John Caezza, president of ARRIS Access, Transport and Supplies. "Effectively, you want to be lower than 1,300 (nm) or higher than 1,324 (nm)." Zero dispersion point(s) The zero dispersion point at which material and waveguide dispersion cancel each other out is commonly regarded to be at a relatively fixed point.

"There lies one of the misconceptions in the industry," Caezza explained. "When we measure zero dispersion of glass, we find that there’s a pretty broad distribution across the range."

Testing 100 spools of fiber-optic cable in the ARRIS labs, for instance, Caezza discovered zero dispersion points ranging from 1,302 nm to 1,321 nm. "And each one of those spools has multiple dispersion points in it," he said.

Outside of a lab environment, there are additional factors such as heat variation and the age of lasers that also contribute to a drifting of the zero dispersion point. The cost of these variations can be significant.

The result of a laser’s center frequency moving into a zero dispersion point (which itself could be moving) can be a degradation of carrier-to-noise ratio of as much as 6 dB, Caezza said.

Apart from avoiding that troublesome range (ARRIS focuses on the 1,291 nm region), the joint proposal also addresses optimal spacing, all the better to avoid other impairments known as stimulated Raman scattering (SRS) and four-wave mixing.

One approach is to put four wavelengths within a single coarse wavelength division multiplexing (CWDM) filter spacing, Caezza said. "It’s wider than DWDM (dense wavelength division multiplexing) but closer than CWDM." All aboard? Given that other vendors have different ways of dealing with both spacing and the zero dispersion point, it’s unclear whether wider consensus will be reached.

Cisco and ARRIS, however, clearly are joining forces. (As are, by extension, the legacy access optical teams of C-COR and Scientific Atlanta.)

 "This collaboration demonstrates the industry coming together to work toward a common goal of standardization," said Mark Palazzo, vice president, general manager, Cisco Access Networks, in a statement at Expo.

"By identifying the limitations and common solutions for multi-wavelength transmission, we are improving cost efficiencies and paving the way for interoperability," he stated.

– Jonathan Tombes

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