Dirty connectors are major problems in fiber optics. They cause high connector loss and high reflectance, and they contaminate transceivers. Network operators claim the majority of fiber optic network problems can be traced to dirty connectors causing connection problems. My experience with testing problems is similar—dirt is the enemy.
During one of my first visits to a fiber optic network user's facilities back in the early 1980s, a technician explained a fiber was so small that it was very sensitive to dirt on the connector. Every connector needed cleaning, he explained, and he demonstrated how he did it by wiping it on his shirt several times with a circular motion before plugging it into a patch panel.
While his intentions were good, his methods, at least with our current knowledge, was not. The shirt was neither clean or lint free. It probably added more contamination to the connector than it removed. And he had no means to inspect it.
Types of dirt and contamination
Airborne dust particles are of a size comparable to the core diameter of single-mode fiber. When you remove a dust cap from a connector, you create some static electricity, which attracts dust. Of course, the dust cap may itself be a contamination source because they are not kept clean during manufacturing and storage, and they may contain mold from the plastic molding process.
Dropping a connector on the carpet is deadly. Carpet if full of dust. Most cloths and paper wipes will leave lint on a connector. Just leaving the connector ferrule open to the air invites dust settling on it.
One of the worst things you can do is touch the connector end with your finger. The oils on your skin leave the connector contaminated, and you may leave dead skin particles, too.
Here are some microscopic photos of connectors needing to be cleaned.
Many connectors will have both dirt and residue on the ends that need cleaning. The type(s) of contamination and dirt will be known only if the connector is inspected visually.
Transferring dirt when making connections
Brian Teague of Microcare/Sticklers sent me this series of photos showing what happens when you make connections with dirty connectors. It speaks for itself!
Connector ferrule ends should be inspected by microscopes with a magnification of 100X to 400X. Many techs carry a small, portable microscope with a fixture to hold fiber optic connectors and a built-in illuminator. These microscopes are inexpensive and work quite well.
Warning: a microscope will focus any light exiting the fiber into your eye, creating a potential hazard. Be sure to get a microscope with an IR safety filter, and check cables with a fiber optic power meter to ensure no power is present in the fiber before inspecting it visually.
An alternative is to use a video microscope. They are more expensive but much more convenient to use. Video microscopes use small video cameras and microscope lenses to provide a display of the view on a small video screen or the display of a PC or tablet. They offer more flexibility in magnification and image manipulation. Many of these also offer automatic inspection to international standards for cleanliness and produce pass/fail results. Some video microscopes also allow for storing an image of the connector, valuable for documenting the condition of the connector at installation and for reference in the future.
New low power, wide field video microscopes allow looking at the whole ferrule and inside the connector body, searching for contamination. These are very useful tools for inspecting for cleanliness since they allow you to see more of the connector ferrule and body where dust can accumulate and then migrate back to the connector ferrule end where it becomes a problem when mating connectors.
Back in the days of the technician who cleaned a connector on his shirt, the recommended connector cleaning procedure was to use reagent-grade 99-percent isopropyl alcohol (IPA) and a lint-free wipe. IPA was recommended because it was an effective solvent to remove most oily contamination and was not harmful to the epoxies used in termination. However, pure IPA was hard to find, and if bought in bulk, it absorbed water from the air (hygroscopic) and became easily contaminated. But you could buy lint-free pads saturated in IPA and sealed in foil, ideal for techs needing to clean connectors.
In the 1990s, the need for proper cleaning of fiber optic connectors became better known. Manufacturers of cleaning products began to research how to clean connectors properly and created products aimed at this application.
They developed “dry” connector cleaners, using treated lint-free tapes in cassettes, boxes or small hand-held tools (probes) that could reach into a mating adapter and clean a connector at the far end. Some of the probes can even clean inside the mating adapter with a special swab sized to fit inside the adapter perfectly.
Today, “wet/dry” cleaning is generally the preferred method of cleaning connectors. The wet part of the process will loosen dirt and contamination, and the dry process will remove them. Much research has gone into developing cleaning solutions that are better at removing dirt, do not generate static and are safer for use.
The wet/dry process works with a fairly large wipe or the boxes of cleaning wipes shown below. You wet one end of the cleaning wipe with a cleaner solution then wipe the connector from the wet to the dry side of the wipe. This quick process should get the connector clean, but only visual inspection will confirm the connector has been properly cleaned.
You should now have a connector that looks like this: