Cost drives innovation in OTDRs

During the optical heyday of 1998-2000, the folks at EXFO Electro-Optical Engineering (Quebec City) polled their customers about whether cost or functionality was more important when choosing an optical time-domain reflectometer (OTDR). "At the time, something like 14% said that price was the biggest issue versus something like 55% who said it was all in the specs," recalls Stéphane Chabot, business unit manager, outside plant, at EXFO. Time-to-market was critical then; carriers fought to get their networks up faster than their competitors. Now, say Chabot and other sources, while specs are still important, today's service providers are far more cost-sensitive.

To reduce costs, service providers have begun utilizing the central-office (CO) technician to locate fiber breaks. "Using a little tool like an OTDR with a fiber-break location mode, the CO technician can find the break with a touch of a button

in about 30 sec," contends Peter Schweiger, worldwide sales channel manager for Agilent Technologies' Optical Network Test division (Böblingen, Germany). "The outside plant crew doesn't go away; they just go to fix the problem, rather than find the problem."

This increased reliance on the CO technician—who most likely has a copper background—necessitates that today's OTDRs are easier to use and more dependable than previous devices. They must also be smaller, lighter, and faster. In short, says Schweiger, "[network technicians] want a truck that runs like a Porsche."

Thanks to current market conditions, the installation and maintenance communities have less work to do, which is bad news for OTDR manufacturers. "Service providers may have had an installation crew, but they've been laying people off. Now, some have more OTDRs than they have people," laments Schweiger. "That's the negative."

That said, there are some bright spots in the industry. Today's OTDR vendors have turned their attention to the metro, which they say provides the biggest opportunity in the near-term. Of course, the metro presents a new set of challenges. In the long-haul, a fiber might span 5 km before an event like a point of presence or CO. "But in metro networks, you have a huge number of events very closely spaced together," explains EXFO's Chabot. "You have your patch panel and then another patch panel, then you have your inside plant equipment and then the jumpers and then the outside plant and the manhole and the optical add/drop."

To best test metro networks, OTDRs must feature smaller dead zones. The dead zone occurs in an OTDR trace whenever there is a connector, splice, or other "event" that causes a reflection; the OTDR receiver is temporarily blinded by the aggregate reflection, causing the end user to misread or overlook events. The development of bidirectional testing has helped mitigate this problem, but smaller dead zones are still necessary to handle connection-rich metro environments.

Say you have a 180-km ring with many events closely spaced together, says Chabot, and those are all hidden behind the saturation or reflectance of the detector. "In a case like this, what you have to do is shoot in the other direction of the ring to characterize the other side; you do a bidirectional test." But, he adds, if you have too many events on the other side, you won't be able to see them anyway, because they will be in the dead zone of the first reflective event.

The U.S. cable TV market is also providing opportunities—and challenges—for OTDR vendors. Cable modems are proliferating, reports Schweiger, and old systems are being upgraded to fiber. There's also a big push for video on demand, "which means one fiber pair needs to serve fewer and fewer people in order to deliver the services they want." A few years ago, one fiber pair might have served 2,000 homes; today, it serves just 200. The stakes are higher, notes Schweiger.

The challenge for vendors developing OTDRs for the metro and cable market is not just improving the technology but meeting customers' price points. "Everything is related to cost," reiterates Chabot. "If we want to achieve volume, we must have OTDRs that are very-low-cost."

Despite the price pressure felt in other segments, manufacturers of niche-market OTDRs report they are once again seeing interest—albeit limited—in high-end, half-million-dollar OTDR devices used in long-haul and submarine networks.

Gabriel Odeh, optical applications engineer at Advantest America Measuring Solutions (Edison, NJ), has seen an increased interest in using OTDR technology to map the dispersion characteristics of a fiber—"particularly among long-haul equipment manufacturers, the Cienas, Nortels, and Lucents," he adds. "When they start introducing remote configurable or arbitrary OADMs [optical add/drop multiplexers], dispersion will impact distances and the communication lines between the channels. They will need to know the dispersion characteristics between amplifiers, between routes, and also between fiber cables."

Also generating some interest are coherent OTDRs, designed for long fiber links of up to 10,000 km and typically used in undersea-network applications. "A coherent OTDR allows you to map out the whole fiber," explains Odeh. "It basically tells you within a few hundred meters that you have a break or that one of your amplifiers isn't working."

Daryl Ussery, optical engineer at Anritsu (Richardson, TX), adds that some people are considering the use of coherent OTDRs when forward error correction is implemented. "It's not a common interest," he admits, "but it is something people have been asking about on occasion."

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