Test and measurement (T&M) tools tend to follow deployment curves. First roll out the new technology, then figure out how to troubleshoot it.

But for increasingly high-speed optical networks, the opposite may prevail. It’s a matter of how the spreading of pulses can create a loss of signal and how the margin for error narrows at faster speeds.

"Light traveling on fiber will follow different light paths," said Stephane Chabot, Exfo’s director of optical product management business unit. "At 2.5G (Gbps), it was not really an issue. But at 10G, it became a big issue."

The issue gets even bigger. "The threshold of polarization mode dispersion (PMD) at 10G is 10 picoseconds, but at 40G, it’s 2.5 picoseconds," said Chabot. "For chromatic dispersion (CD), it’s a factor of 16."

Options for managing these constraints are to deploy optical compensators, adopt advanced modulation schemes, or replace the fibers that are displaying too high a PMD.

Possibilities

At last year’s joint Optical Fiber Communications Conference and the National Fiber Optic Engineers Conference (OFC/NFOEC), Exfo engineers and their counterparts at a German network operator shared promising results of a test showing a more efficient way to tackling fiber replacement.

The test involved using a prototype device called a random-scrambling polarization optical time domain reflectometer (RS-POTDR) to measure cumulative PMD along fibers of the fixed network division of Deutsche Telekom group (T-Com).

By showing the "distributed" PMD of the fibers, the tool proved it possible to identify particular fiber spans with especially high dispersion. As a result, the operator could minimize fiber replacement costs, replacing bad and not acceptable spans.

A follow-up paper is slated for the upcoming OFC/NFOEC (March 22-26, San Diego).

Meanwhile, Exfo this month moved ahead with a productized version of that prototype, namely, the FTB-5600 distributed PMD analyzer, which leverages the kinds of field test it conducted with T-Com and other recent development of "single-ended" testers.

"We’ve been working on it for the last three years," said Chabot.

That’s good. The cable industry, its telco competitors and technology suppliers of various stripes pushing optical speeds from 10G to 40G to 100G, and swaths of both dark and lit fiber are of old, undetermined or questionable provenance – including spans strung or laid during the shortages of the early 2000s when fiber came from new and relatively untested sources.

Assuming this tool tests out as claimed, it could see several years of work ahead.

For more on optical test and measurement, click here to read CT’s Optical T&M Tech Guide.

– Jonathan Tombes

Read more news and analysis on Communications Technology‘s Web site at www.cable360.net/ct/news/.

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