“Dust Cap” may not be the best name for the small piece of plastic that covers the end of a fiber optic connector, since the cap’s purpose isn’t actually to keep dust out. A dust cap’s actual job is not so much to keep the fiber clean, but to shield the polished endface of the connector from being damaged, something which can render a connector useless. A dust cap is designed to generate a static charge on a connector, pulling dust to the cap when it is removed in order to keep your fiber clean.
Plugging a dirty connector into active fiber optic gear can contaminate your equipment, spreading the dirt to the fiber you’re connecting to. This makes it very important to always clean a fiber optic connector before plugging it into anything. Fortunately, there are a variety of methods for keeping your connector’s endfaces clean.
The most common way of cleaning a connector, and the easiest, is by using a fiber optic cleaning tool, such as a Kletop, to scour the head of the connector. These products are designed to abrade away dirt, but are gentle enough that they never risk damaging the endface itself. In a pinch, if the cleaner isn’t working, try rubbing the head of the connector against a clean cotton material (such as a shirt) in order to dislodge obstinate contaminants. Some people will wipe the fiber on their hand before using this method – don’t be one of those people! Finger oils can contaminate the fiber worse than the dirt already obscuring it, making your job that much more difficult.
You can also wet clean a fiber, using Isopropyl Alcohol and lint-free wipes or swabs for a more thorough cleaning of the fiber. By gently wiping at the endface with a slightly alcohol-dampened wipe, you can dislodge more difficult contaminants.
If you’re having trouble getting a good reading on an OTDR, there’s a good chance your problem is a dirty connector endface. Many people, unfortunately, think that a dust cap on the end of an optical fiber connector means it is proof against contaminants, and thus does not require cleaning.
The most important rule of fiber optic testing, though, is that you always clean everything, and if it doesn’t work you clean it all again. Make sure you clean a connector’s endface immediately before mating it to the next connector, or you risk the contaminants from the first being applied to the second connector, despite the protection of a dust cap.
