Understanding The Importance Of Fiber Optic Inspection

Fiber Optic Inspection! Inspection! Inspection!

Written by Ben Hamlitsch, trueCABLE Technical and Product Innovation Manager RCDD, FOI

So much emphasis and focus are put on cleaning. Now don’t get me wrong, dirty connector end faces are 90% of the issues in fiber optic systems. Anywhere from scratches, dirt, grease, or traces of water left from cleaning solutions. It is always extremely important to ensure everything is clean.

However, there is a step in this process that is extremely important: the inspection process. Sometimes the order in which this step is implemented is done incorrectly. What does that mean? Well, for starters every connector or bulkhead should be inspected FIRST. Whether you have a handheld scope or fancy video scope, this important step should always be done first.

Why should inspection be done first? Simply, because the connector may already be clean so there is really no need to clean a connector that is already clean. If the connector or bulkhead or transceiver ports that you are plugging into are already clean, it is ready to make the connection. DONE!

Some recommendations in the past have been, “Just clean it again, it can’t hurt.” Well, it can or could…. Or, you may have heard “Just always clean it!” But the reality is the cleaning process can sometimes introduce new issues from the cleaning process itself. For example, the connector could get some cross contamination from the cleaner tool, or if we are using a cleaning solution it could introduce traces of water or solvent. The more the connector is out in the air and being handled the greater opportunity for static dust to attract to the end face. Additionally, if the connector is not again inspected after cleaning this could be transferred to an already clean bulkhead or transceiver port. And now you have the potential of larger issues not to mention potential costly issues.

Users have no way of knowing if the connector is clean unless it is inspected, and this is done using a fiber inspection tool designed specifically for that purpose. Standard options such as a professional video microscope or a handheld fiber microscope will go a long way in ensuring if it needs cleaning. That is why the rule is to always inspect, and if necessary, clean and inspect again before connecting.

This can get exponentially more important when we talk MPO connectors, because now you must worry about either 8, 12, 16, 24 or 32 fiber arrays. Just one of those fibers being dirty on either the transmit or receive side can cause timely and costly issues no company wants to deal with.

Consider a 12-fiber MPO interface with an array that features a much larger surface area than a single fiber connector. When cleaning these larger surface areas, it is much easier to move contaminants from one fiber to another within the same array. And the larger the array, the higher the risk. With 24-, 48- and 72-fiber MPOs used in high-density fiber interconnects, the greater number of fibers are more difficult to control and not all the fibers always protrude at the same height. Height variances across the fibers in a single multi-fiber connector can increase the risk of not every fiber being properly and equally cleaned.

There are two main types of inspection tools – optical and video.

Inspection microscope for fiber patch cable end faces

Tube-shaped and compact, optical microscopes allow direct inspection of the end-faces. These are popular due to their low cost, and the ability to quickly on the fly inspect your standard fiber connectors like LC, SC, FC, ST and so on. These are not however readily available for MTP/MPO style connectors. Additionally, the optical microscopes don't provide views of end-faces inside equipment or through bulkheads. This is where using a video scope can come in very useful.

Video Inspection for fiber patch cable end faces

Video inspectors consist of a small optical probe connected to a handheld display. The size of the probe makes it excellent for examining ports in hard-to-reach places. Many of these video scopes come with various connector adapters which allow for inspecting various types of connector end faces. MTP/MPO adapters for video inspectors work very well in the inspection process for these types of connectors. These probes are also safer because they show an image and not the actual end-face, thereby reducing the risk of exposing one’s eye to harmful radiation.

Grading and Certification via Standards

A longtime concern of the fiber industry has to do with manually inspecting a fiber connector end face, and how to determine the cleanliness of the fiber end face. This has largely been a subjective and inconsistent process.

What one person may consider clean can vary greatly to another. Other factors such as skill level, years of experience, ambient lighting, and the fiber inspection tool being used can also lead to inconsistencies in determining fiber connector cleanliness.

To establish consistency in fiber inspection and achieve more repeatable results for performance across multiple connectors, the IEC developed the 61300-3-35 "Basic Test and Measurement Procedures Standard for Fiber Optic Interconnecting Devices and Passive Components." This standard contains specific cleanliness grading criteria to assess pass or fail certification for inspection of a fiber connector end face.

The certification criteria in IEC 61300-3-35 varies based on connector type and fiber size, as well as types of events: defects or scratches. Defects include pits, chips, scratches, cracks, particles and embedded and loose dirt and dust. Scratches are identified as permanent linear surface features while defects include all detectable on-linear features that can typically be cleaned. Certification to determine pass or fail is based on the number of scratches and defects found in each measurement region of the fiber connector, including the core, cladding, adhesive layer and contact zones, as well as the quantity and size of the scratches and defects.

For example, multimode fiber with polished connectors can have no scratches greater than 3 μm in width or defects greater than 5 μm in width in the fiber's core. Within the cladding zone, there can be no scratches or defects greater than 5 μm in width, 5 defects ranging between 5 and 10 μm in width and no limit on the number of defects less than 5 μm in width. This number and the size of scratches will vary depending on the type of connector and the different diameters of the core.

MPO Inspection Camera Issues

Compared to a single fiber, MPO connectors have more surface area that can collect contaminants. When the connector is unplugged and re-plugged, particles can move from a spot where they’re not a problem to another area of the connector which then presents a potential performance issue.

Knowing What to Inspect and Clean

The best answer to the question of what to inspect and clean is everything – every connector should be inspected, and every connector that fails IEC 61300-3-35 certification should be cleaned. If upon inspection, the connector passes IEC certification, do not clean it. Cleaning can attract dust due to static electricity.

All connectors, even new and factory-terminated plugs and pigtails should be inspected for cleanliness before mating. That includes both ends of fiber optic test cords, fiber jumpers and pre-terminated trunk cables.

If using an adapter to mate two plugs, the connector on both sides and the sleeve of the adapter itself should be inspected and cleaned before inserting them into the adapter. Interchangeable adapters used with optical power meters also need to be inspected and cleaned on a regular basis.

When testing or troubleshooting any equipment, including the tester itself, all plugs and ports should be inspected and cleaned before mating. This includes test equipment ports, adapters, test cord connectors and any ports into which you will be connecting the test cord.

As previously mentioned, dust caps and mating can be a source of contamination. Therefore, every time a fiber connector is unplugged or removed from a dust cap or port, even when it is brand new, should be inspected. Ports should also always be inspected and cleaned before inserting a connector, even if one was just recently removed. Having a click cleaner in your tool bag can come in handy in these situations.

So, the takeaway from this article is always inspect your fiber optic connectors, ports, and transceiver ports before you do anything else. If they need cleaning, make sure to clean and inspect again. If they pass the first inspection no need to clean, just connect and move on to the next.

Stay Connected!


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