Use of Raman for 100-Gbps coherent may make dispersion testing necessary

One of the oft-cited benefits of coherent receivers for 100-Gbps networks is that the receivers are so good at mitigating the effects of dispersion that they render dispersion testing irrelevant. However, test equipment vendor EXFO Inc. (NASDAQ: EXFO) (TSX: EXF) believes there’s at least one coherent 100G application where chromatic dispersion (CD) testing remains useful: brownfield long-haul routes using Raman amplification.

The problem isn’t that coherent receivers don’t work on such routes. It’s that Raman gain varies with fiber type, and specifically with the effective area of the fiber in question. This factor can make achieving the right Raman gain a challenge in brownfield networks whose fiber composition isn’t fully known. In mixed fiber networks, the pump power that provides appropriate gain in one part of the route may overshoot or undershoot the mark elsewhere. Without knowing the effective area of the fiber on each span of the proposed Raman-enabled route, matching the pump power to the span becomes guesswork.

Which is where CD testing comes in, EXFO says in a new whitepaper. A full CD test can identify a fiber’s total chromatic dispersion as a function of wavelength as well as the CD slope. From these parameters, the test instrument measures a “Lambda Zero” – the wavelength on which no dispersion occurs. Add the CD coefficient (CD as a function of span length) to the CD slope and Lambda Zero, and you create a fiber profile that technicians can compare to the specifications of common fibers from any number of manufacturers. Match the profile to a specification set, and you identify the fiber.

Before technicians dig around for their CD mapping records, they should be aware that previous CD measurements may not be adequate to the present task. “Networks have been tested historically with chromatic dispersion analyzers, but it was a different breed of CD analyzers. They wanted to reduce as much as possible the CD value to be within the tolerances of the 10-gig or the 40-gig [system]. So they used either EXFO or other test instruments that were capable of testing through the EDFAs and through all of the active elements,” explains Francis Audet, group manager, system provisioning product line management at EXFO and co-author of the whitepaper with EXFO colleague Tony Lowe. “But in this case here, we don’t want to test the entire span all at once. We want to know the value of the CD span-per-span between the amplifier sites.”

In many networks, this means making new measurements. Audet points to a single-ended CD test instrument that combines CD testing and OTDR-like span measurements as a great tool to perform all of the required measurements simultaneously (and, yes, EXFO happens to make one). However, dual-ended instruments or a CD tester and a separate OTDR can also be used, particularly on spans too long for accurate single-ended testing.

Audet admits that CD testing won’t help technicians identify mystery fibers in spans with multiple fiber types; in such instances, the tests will offer an average of the fiber characteristics. However, even this information can be a useful data point when planning the application of Raman amplification, indicating that this span will require special treatment.

As more network planners consider Raman amplification as a way to improve the performance of coherent 100-Gbps technology on their long-haul routes, Audet expects them to find that CD testing isn’t as obsolete as they might have believed.

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