Inspect or fail: Why multifiber push-on connector testing is critical in today’s networks

June 8, 2021

By Guillaume Lavallée

With increased service demand driven by an uptick in work-from-home, distance learning, and online socializing trends, reliable network connectivity has never been more important. At the same time, with the advent of 5G mobility, high-speed networking, and inherently more complex and dense networks, service providers are facing new challenges to reliably deliver those complex offerings and data to their customers.

Many are adopting multifiber push-on (MPO) connectors to save space in environments that are dense in fiber, as well as to provide high-speed interconnection and increased redundancy. Central offices are being reconfigured into data centers (CORDs) or headend restructured HERDs, and network operators are deploying MPO cables with 12 or even 24 fibers. MPOs are gaining momentum as the connector of choice to support these dense fiber environments.

However, along with the benefits of deploying MPOs there are challenges. Research has shown that 80% of network owners report having issues linked to bad connectors, which can negatively impact network performance as a whole. For example, CORD operators might have to take critical communications lines out of service for troubleshooting, affecting numerous customers.

Clearly, connectors can have a huge impact on bottom-line operations. In fact, NTT-Advanced Technology Research cites contaminated connectors as the number-one cause of network failure.

So what can service providers do to ensure that small but critical connectors function properly and help reliably meet demand?

Critical connector inspection

Accurate connector validation—including inspection and cleaning—is the critical step to avoid connector issues. It’s important that connectors are inspected at the installation phase. Otherwise, operators will have to shut down live links to fix problems when they arise in the future.

Inspection can detect any imperfections and determine if connectors are clean. Connectors that fail inspection due to contamination can be cleaned. Those with scratches or other physical damage can be rejected and replaced without impacting the network.

Connector validation is key because dirty or damaged connectors have a direct implication on insertion loss and optical return loss levels. That in turn impacts bit error rate (BER), system performance, and network reliability.

There are many sources of contaminants for connectors. Handling connectors can impart skin oils and particles of dust. And even well-protected connectors can be contaminated by their own dust cap because of static electricity.

Dirty connectors can potentially still be useful if cleaned properly and re-inspected. The IEC specifies that several attempts to clean before rejecting a connector may be required. Some organizations suggest performing up to three cleaning attempts before replacement.

It’s important that the right cleaning method and tools be used. For example, if cleaned with improper tools or cleaned with a wet cleaner but not dried properly, the connector may remain dirty.

Implications for MPO connectors

Inspection at the installation phase is even more critical for MPO connectors to ensure faulty connectors are discarded before deployment, and to rectify dirty connectors.

However, attempting to clean a connector when not actually necessary can make the situation worse. This is especially true in the case of highly sensitive MPO connectors where, for example, dirt from the first row could potentially migrate to the second row while cleaning.

It’s also important to inspect both mating connectors, as residue from a dirty connector can transfer to a clean connector once mated.

While it might be tempting to skip inspection at the installation phase—just “plug and pray”—that is not an effective, long-term solution. Often if faulty or dirty connectors are deployed, expensive multifiber cables are replaced rather than trying to address a dirty connector issue in the field to save time and trouble. That can be an expensive workaround indeed.

When high-speed transmission rates are supported, the fiber foundation must be optimized given that any contamination will have a much greater impact at those increased data rates.

Compare, for example, a 1-Gbit Ethernet signal to a 1-Gbit Ethernet signal. With the relatively slow 1-GbE, a few impairments on the physical layer, such as connectors that are not in perfect condition, may not be critical. However, as the data rate increases, 1s and 0s are closer to each other on the signal, leaving an extremely small margin for error. This would be amplified at higher rates, such as 25, 40, 100, and 400 Gbits/sec. The importance of accurate connector validation becomes increasingly apparent as density and rates increase.

Because links in central offices or data centers typically carry high-speed traffic, they are more sensitive to bad connectors. And as noted above, MPOs are increasingly being deployed in these dense fiber environments, making inspection and cleaning crucial. With MPOs, each port represents a potential point of failure and a single dirty or damaged connector can affect as many as 12 or 24 fibers.

Testing connector cleanliness

Achieving an accurate and repeatable pass/fail diagnostic every time for MPO inspection has had its challenges. From a technical standpoint, getting reliable inspection results is more complicated for multifiber than for single-fiber connectors.

A fiber inspection scope is a specialized microscope that takes a picture of the connector endface on top of which an analysis algorithm is applied to provide a diagnostic. Mo matter how good the analysis algorithm is, if the picture lacks quality definition, achieving accurate results remains a challenge.

When selecting an inspection solution, pay attention to the optical assembly. Look for high-definition images with sharp contrast between light and dark areas. That way, when running the analysis on top of the image, the scope will properly distinguish defects and scratches on the surface of the connector from other things, such as reflections, in order to correctly diagnose issues.

Deformable lenses are often used in inspection scopes, but they do not provide the level of detail needed to yield reliable and repeatable results. In optics, the larger the lenses the better the resolution and the image quality. For MPO inspection, a connector inspection scope with larger lenses is more likely to provide better image clarity. This can be tricky, as the tipi at the extremity of the scope must be small enough to fit properly in dense patch panels.

Complementary Metal Oxide Semiconductor (CMOS) detector technology has advanced in recent years, driven by applications such as autonomous vehicles and facial recognition. Next-generation CMOS detectors contribute to obtaining a larger field of view, which is necessary to capture the entire surface of the multifiber connector. While connectors with more than two rows of fiber are not yet common, they are emerging, and a good inspection tool should have a field of view that is large enough to see up to 4 rows of 12 fibers on a single connector.

A uniform level of light should shine across the whole surface of the MPO connector to avoid zones with shadows on the surface and for better visibility. LEDs used for fiber endface inspection are traditionally in the blue spectrum at 435 nm, which is perfectly good for single-fiber connectors.

However, MPO connectors require greater contrast on small details. LEDs in the violet range at 405 nm can identify extremely small defects—as small as 2 µm—which is about one-thirty-fifth the size of a human hair.

What to do when there is no time?

Speed of testing is always an issue, especially when a high volume of tests on multifiber connectors must be performed, such as when an entire patch panel must be certified. The inspection of one MPO connector typically takes about one minute from the moment a connector is inserted, to the analysis and saving of results. Newer solutions have automated every step of the operation and inspection time has been significantly reduced, some completed in less than 10 seconds.

Switching between an MPO and LC adapter (or any other single-fiber connector type) also contributes to complexity and inefficiency of inspecting connectors. Technicians would typically need different adapter tips to fit the varied form factors of the connector types requiring inspection, which is inconvenient when working on top of a tower or when a large volume of testing must be performed.

Pay attention to the tip-changing mechanism when selecting a solution. Ask whether the operation can be done with one hand to avoid parts coming loose in the process. The number of adapter tips needed for a solution is often an afterthought and can increase the cost of a solution. If tests need to be done in dense environments or where there are multiple types of connectors, look for solutions that have a long-reach nozzle by default with as few adapters as possible. This should also help with lowering cost of ownership of the solution.

Tools and solutions available today

Options for inspection and testing are better than ever, enabling technicians to: inspect single and multi-fiber cables using the same tool by simply switching the adapter; take advantage of a long-reach design to easily access recessed connectors and dense panel settings; get an automated analysis of all fibers for multifiber cables; and obtain a clear pass or fail result according to test configuration.

To optimize your inspection process, here are the features to look for when selecting an inspection solution according to the applicable work environment.

CORD/HERD. Look for: speed of test (because of the high volume to be tested); a tip-switching mechanism (for use with multi- and single-fiber connectors in the same location); and reliability (because of the presence of high-speed links).

Webscale/data centers. Look for speed of test, reliability, as well as ease-of-use and automation (because the work may be done by less-experienced technicians).

Fiber-to-the-antenna. Look for a tip-switching mechanism, ease-of-use and automation, as well as a default design (so the technicians needs to carry as few adapters as possible).

Fiber-to-the-home. Look for reliability as well as ease-of-use and automation.

Keeping these options in mind will help minimize manipulations to speed up and facilitate work in difficult environments. It will also increase the number of inspections that are performed in the field, resulting in more robust and reliable networks.

Guillaume Lavallée is product line manager with EXFO.

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