Fiber testing practices are constantly evolving

The medium has proven itself to be stable and reliable; verifying or certifying that reliability can be a challenge.

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From the October, 2014 Issue of Cabling Installation & Maintenance Magazine

The medium has proven itself to be stable and reliable; verifying or certifying that reliability can be a challenge.

The capabilities of optical fiber are both established and expanding. Whether the medium is singlemode or multimode optical fiber, the ability to transmit enormous amounts of information efficiently via fiber is among the great technological accomplishments of recent history. Yet despite its remarkable information-carrying capacity, cabled optical fiber is a layer-one physical-infrastructure component, installed-proverbially and sometimes literally-"in the trenches" of a not-always-pretty construction project. Likewise, the testing of installed fiber-optic cabling sometimes belies the scientifically graceful manner in which the fiber transmits signal.

Many a teacher or trainer in the cabling industry has countless stories about the frequency with which fiber testing is carried out incorrectly. In many cases, characterizing the performance capabilities of installed fiber involves far more than connecting and pushing the "auto" button on a test instrument. And as fiber-optic systems become more capable as well as sometimes more complicated than they used to be, the opportunities to test incorrectly grow.

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The MPO Switch was created to be used with standard visible fault locators (VFLs) and optical time-domain reflectometers (OTDRs) to test MPO-connected fiber cabling one fiber at a time.

The fundamentals

The Fiber Optic Association (www.thefoa.org) is one training organization that has encountered, and continues to address, the fundamentals as well as the granular specifics of fiber testing that technicians in the field face daily. The FOA has a wide selection of online learning texts and videos. One document on the FOA website is titled "Testing the Installed Fiber Optic Cable Plant." That document explains, "To thoroughly test the cable plant, one needs to test it three times: a continuity test of the fiber before installation, then insertion loss of each segment, and complete end-to-end loss. One should test the cable on the reel for continuity before installing it, to insure no damage was done in shipment from the manufacturer to the jobsite. Since the cost of installation usually is high-often higher than the cost of materials-it only makes sense to insure that one does not install bad cable. It is generally sufficient to just test continuity, since most fiber is installed without connectors and then terminated in place, and connectors are the most likely problem to be uncovered by testing for loss. However, if any damage is visible on cable reels, OTDR [optical time-domain reflectometry] testing may be needed to verify the cable is still good.

"After installation and termination, each segment of the cable plant should be tested individually as it is installed, to insure each connector and cable is good. Finally each end of the run (from equipment placed on the cable plant to equipment) should be tested as a final check. Measured loss should be compared to the calculated loss budget for the cable plant to determine if the measured loss is acceptable."

In a fairly straightforward, field-terminated fiber-optic cabling system, conducting this prescribed testing can be rife with challenges, from properly conditioning out higher-order modes to setting the appropriate zero-reference via a one-, two-, or three-cable reference (previously referred to in standards as Methods A, B, and C). Often it's the zero-reference setting that trips up technicians. And the means of filtering our higher-order modes (e.g. mandrels and the encircled-flux launch condition) continue to evolve within standards-making bodies as well as on jobsites. We have chronicled some of that evolution in this magazine, and will continue to do so. This article, however, will not go into great detail on that issue.

Preterminated cabling

With those common fiber-optic testing issues as a backdrop, adding a variable like preterminated multifiber cabling systems can further challenge the testing and verification of a system. We at Cabling Installation & Maintenance recently hosted a web-delivered seminar, delivered by FOA president Jim Hayes, covering preterminated cabling. Hayes and the FOA refer to the cabling type as "prefabricated" or "prefab" rather than "preterminated" because, as he pointed out during the seminar, these systems are not simply terminated in a factory setting. Rather, they are fully engineered before being delivered to the customer. In that sense, dubbing them "preterminated" tells only part of the story of how these systems have been built.

As these systems can be customized to an extent, the process for testing them must accordingly be customized, or made appropriate, for each particular system. Hayes reminded the webinar audience that the systems should be tested both before and after installation. And as with any fiber-optic system, thorough inspection and cleaning are necessary and should be done as many times as is necessary until the connectors are completely clean.

Commonly, prefab systems include MPO-style fiber-optic connectors on at least one end, which can add a level of complexity to the testing process, including the cleaning process. On the topic of cleaning MPO connectors, a webinar attendee asked Hayes the simple question, "How?" The answer was not quite as simple. Hayes replied that there are cleaners made specifically for MPO connectors, which must be used "because it is hard to get to the fibers between the pins," of MPO connectors. "If you use a connector with retractable pins, you can use a larger cleaning tape or pad, wet or dry, and clean much better around the pins. At least one connector supplier also warns that dirt can build up around the pins and keep the ferrules from contacting. This could be a problem with connectors on test-reference cables. Clean and inspect the test cables often."

As for the actual plugging-in-and-testing the MPO end(s) of a preterminated link, several options exist. Multiple test-equipment providers have produced testers that accommodate the direct plugging-in of MPO interfaces.

Another technology recently developed is called the MPO Switch. Manufactured by FiberNext (www.fibernext.com), the product works with standard OTDRs and visible fault locators (VFLs) to test MPO-connected fiber links. When announcing the product in 2013, the company explained, "The challenge of testing high-density MPO/MTP connectivity in data centers and plug-and-play networks is still being perfected. Recently manufacturers have been working to develop new tools to speed the testing of mass connectors as the adoption of MPO connectors skyrockets in data centers, where it's being used in 40- and 100-Gbit connections. FiberNext has responded to these market needs by developing an easy way to use the MPO Switch, which works with existing OTDRs and/or VFLs in a number of test scenarios.

"The device allows users to test through the switch into any of the 12 fibers in the MPO/MTP connector without the hassle and potential damage of unmating and remating fibers during testing, as is typical with fanout cables. Using the switch in conjunction with a VFL, the user can cycle through each of the 12 fibers and note the polarity/continuity visually on the far end of the cable under test. Using the switch in conjunction with an OTDR, the user can cycle through each of the 12 fibers and take discrete OTDR test results without the use of a fanout cable and excessive patching."

FiberNext says the device also can be used for MPO-MPO cable-assembly testing, and for testing bulk cable on a reel by incorporating a mass fusion splicer set at "pause" to enable the orientation of all 12 fibers for a quick validation.

Split decision

MPO-terminated fibers within prefabricated systems can present one type of challenge to technicians conducting field tests. The presence of splitters in systems such as passive optical networks (PONs) can present other challenges. PON technology has become increasingly visible over the past few years as it has made its way into enterprise networks in what are commonly referred to as POLs-passive optical LANs. But the network architecture and technologies have existed for many years, particularly including in fiber-to-the-building networks.

In a webinar titled "Deploying Fiber in MDUs," fiber-optic test-equipment provider EXFO (www.exfo.com) addressed a number of installation and test issues for fiber-to-the-premises systems. The webinar is available for viewing at EXFO's website. In it, product line manager Vincent Racine described the test equipment that can be used in a fiber-to-the-multi-dwelling-unit network. "A power meter and light source is less expensive than other solutions," he explained. "It will deliver accurate insertion-loss measurement, but will not provide testing of optical return loss, or ORL. Many power meters/light sources are manual and require experienced technicians to operate. The OLTS-optical-loss test set-is a more automated tool. But power meters/light sources and OLTS have a drawback: They see that a problem exists, but they do not see where it exists."

For that level of visibility, the OTDR is the tool of choice. Racine said, "The OTDR provides a trace in which you can pinpoint the location of a fault. Another advantage is that it is a single-ended test tool. It can be operated from one location, requiring one technician." Additionally, he explained an OTDR that tests at 1625 nm enables out-of-band and live-network testing. Some OTDRs are PON-optimized, meaning they can deliver a trace even beyond the strong loss event that occurs at a splitter.

The FOA addresses a similar idea in its online documents "Testing Fiber Optic Couplers, Splitters or Other Passive Devices" and "Testing FTTH." In the latter document it explains, "As a result of the complexity of downstream traces, OTDRs are generally used on PONs from the subscriber end toward the CO [central office] to characterize the fiber path. However, the OTDR may also be used from the CO end because … it allows the operator to quickly characterize the length of each fiber link, providing actual fiber length to add to network diagrams for future troubleshooting.

"Special PON OTDRs will test at 1310, 1490 and 1550 nm. Some also test 'out of band' at 1650 nm, which is more sensitive to bending losses and allows in- service testing with a filter to remove signal wavelengths. Since PONs are short, the OTDR needs very high resolution, usually obtained by the shortest test pulse that will give adequate range."

Scratches in a fiber endface can be extremely harmful to a fiber-optic link. This article, over the course of a few pages in this magazine, has only scratched the surface of today's fiber-test issues. Hopefully this scratch has been a useful one.

Patrick McLaughlin is our chief editor.

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