What’s the problem?
Fiber basics
Fiber optic cabling carries pulses of light between transmitters and receivers. These pulses represent the data being sent across the cable. In order for the data to be transmitted successfully, the light must arrive at the far end of the cable with enough power to be measured. Light loss between the ends of a fiber link comes from multiple sources such as the attenuation of the fiber itself, fusion splices, macrobends and loss through adapter couplings where end-faces meet.
In lower data rate networks with shorter lengths, loss budgets may be generous enough to allow for significant attenuation throughout the link and still the link will function properly. However, there is one perpetual trend in structured cabling: the constant push for greater bandwidth. As fiber links are pushed to carry higher data rates, loss budgets get correspondingly smaller, requiring all loss events to be minimized.
Enemy #1 — a dirty face
Among key sources of loss that can bring a fiber network down, dirty and damaged end-faces are the threat most underestimated. In a survey commissioned by Fluke Networks, dirty end-faces were found to be the #1 cause of fiber link failure for both installers and private network owners. Contaminated end-faces were the cause of fiber links failing 85% of the time. It’s astounding and yet easy to prevent. Nevertheless, there continues to be a lack of appreciation for this crucial issue and lots of misinformation about proper techniques.
What to look for and when
Network professionals need to know what to look for when evaluating end-face conditions. There are two types of problems that will cause loss as light leaves one end-face and enters another inside an adapter: contamination and damage.
Contamination
Contamination comes in many forms from dust to oils to buffer gel. Simply touching the ferrule will immediately deposit an unacceptable amount of body oil on the end-face. Dust and small static-charged particles float through the air and can land on any exposed termination. This can be especially true in facilities undergoing construction or renovation. In new installations, buffer gel and pulling lube can easily find its way onto an end-face.
Ironically, protective caps – also called “dust caps” – are one of the most common contributors to contamination. These caps are made in high-speed production processes that use a mold release compound that will contaminate end-faces on contact. Further, as the plastic cap ages the plasticizers deteriorate resulting in an outgas residue. Last, airborne dust itself will find its way into the protective cap and will move to the end-face when the cap is pushed onto a ferrule. It’s a very common mistake to assume that end-faces are clean when patch cords or pre-terminated pigtails are removed from a sealed bag with protective caps in place.
Inspection of the end-face should verify that no contaminants are within the field of view. The most crucial area to clean is the core of the fiber, followed by the cladding. Yet contamination on the ferrule – outside of the end-face – could slide towards to core as the fiber is mated or handled. Therefore, all visible contamination should be removed if possible.
Damage
Deciding to mate every connection first and then inspecting only those that fail is a dangerous approach as the physical contact of mated contaminants can cause permanent damage. This permanent damage would require more costly and time consuming retermination or replacement of pre-terminated links.
Damage will appear as scratches, pits, cracks or chips. These end-face surface defects could be the result of poor termination or mated contamination. Regardless of the cause, damage must be evaluated to determine if action is required as some of it can be ignored or remedied. Up to 5% of the outer edge of fiber cladding generally may be chipped as this is a common result of the polishing process. Any chips on the core are unacceptable. If scratches or excess epoxy bleed is found, repolishing with fine lapping paper can eliminate the problem. If the end-face is cracked or shattered, then the fiber must always be reterminated.
JM Test Systems rents and sells Fluke Networks products for all your fiber optic testing needs. Fiber Optic Cleaning Kit
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