You're In My Light

Experienced fiber techs often harp on cleanliness. They know dirt causes many problems with fiber during installation, testing and operation. They understand it is important to emphasize cleanliness when splicing or terminating fiber, but it is even more important to maintain clean connectors when testing cables or connecting systems.

The problem is simple. Fiber optics depends on the transmission of light with low loss through connections. Dirty connections increase loss because dirt blocks the light. Most important, the dirt in a typical building is big—at least compared to the tiny size of the light-carrying core of an optical fiber.

To illustrate the importance of cleaning, at The Fiber Optic Association (FOA), we borrowed a video microscope from AFL (a company that manufactures, engineers and installs fiber optic products and equipment) and fiber optic cleaning kits from Sticklers to run some tests, with the results shown in the photographs below.

First, we cleaned a connector with the Sticklers kit and inspected it in the microscope, shown in Photo 1. The connector shows only two very tiny specs of dirt, easily cleaned with a second try. The microscope also shows that the end of the connector ferrule and the fiber are smoothly finished, which is expected in a good, low-loss connector.

Then, we dropped the connector on the carpet—just like you might do in a typical office—to see what would happen. Again, we used the microscope to inspect the end of the ferrule, and what we saw is shown in Photo 2. The end of the ferrule has a lot of dirt, and several threads from the carpet, both of which are large compared to the fiber’s core. This debris can cause loss by attenuating the light at the connection, and the dirt also can permanently scratch the end of the fiber.

Next, we wiped the connector and inspected it again to ensure it was really clean. Then we touched the end of the connector with our finger and inspected it in the microscope. Photo 3 is what we saw. What looks like drops of oil are exactly that, oil from our skin. That oil is sticky and will attract dirt if open to the air; it would also spread the same mess on another connector if mated. We easily cleaned it using the dry wipes in the Sticklers kit.

As part of our experiments, we also dropped connectors on wood, tile and concrete floors. Sometimes they just got dirty; sometimes they got scratched. We looked at connectors after removing dust caps and discovered dust! Yes, dust caps are not always clean, so using them protects against damage but not against getting dirty.

The photos below are of 50/125-
micron multimode fiber. The core size of single-mode fiber is much smaller, of course, so the effects of dirt are much greater. Most of the specks of dirt in the center photo are larger than the core of a single-mode fiber.

Since keeping connectors clean is so important, having a good microscope to inspect them in your toolbox also is vital. Several types of microscopes are available, including inexpensive ones often included in termination kits, precision optical microscopes designed for rugged field use, and video microscopes.

Any of these microscopes will work just fine—the differences among them mostly involve convenience and ruggedness. Any microscope with a magnification of 100–400 times and proper illumination of the connector end will work well for inspecting connectors. I personally prefer lower magnification, since higher magnifications tend to be harder to focus and sometimes make OK connectors look bad.

If you are buying an optical microscope, it helps to try it first, especially if you wear glasses. Some are easier to use than others. The more expensive ones are the best.

Video microscopes are much easier to use since the magnified view of the connector is shown on a video screen the size of a smartphone or on a laptop PC. The laptop units connect to a USB port and allow saving images for later reference (or, as I did with these images and many more, using them for illustration of the problems of dirt and for training students). Video microscopes are much more expensive but often worth the investment.

Take another look at those photos. Remember that, when working with fiber optics, dirt is a very real problem. Continual inspection is required during the installation of connectors, and during testing and operation, to ensure good connections—both low loss and low reflectance—all very important for single-mode fiber.

Every tech needs a good microscope in their toolkit and needs to know how to use it properly. Make sure each of your installers has one and has been properly trained in its operation.

Photo 1

Photo 2

Photo 3

About the Author

Jim Hayes

Fiber Optics Columnist and Contributing Editor
Jim Hayes is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at .

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