Most of us take fiber-optic (FO) safety forgranted. Since fiber-optic cable doesn't carry electricity, we don't worry about electrocution. Similarly, we don't worry about fire because fiber-optic cable isn't a source of heat or combustion. In fact, since its light source is invisible to the naked eye, we aren't even sure when it's transmitting data signals.

Invisible light. Infrared light (Similar to the light used by TV remote controls) is invisible to the naked eye. Visible light has a wavelength between 380nm and 750nm. Light beyond this range is invisible to us. However, even though we cannot see this light, it can cause severe damage to our eyes.

The power levels and wavelengths found in common fiber systems range from 50nW to 10 mW. While this may not sound like a lot of wattage, keep in mind this light can pump through a fiber that's only 9 millionths of a meter (microns) in diameter. Even at these low levels, that's a high level of watts per square centimeter.

Dangerous situations arise when untrained people pick up a live fiber and look directly into it. They see no light. Therefore, they assume there's no danger. However, such unsuspecting people can end up with a burned retina in a very short time.

Do not confuse looking into a live fiber with performing continuity checks with a visual tracer. Essentially, a visual tracer is a visible light you shine down the fiber. Then, you use your eye to trace the fiber through its course to its end. The tracer itself can be a flashlight (difficult to use), a modified flashlight, or even a microscope that holds the fiber in place while you direct light into the fiber. The better tracers use special test sources, which use a bright red LED. The big benefit is the power levels of all visual tracers are too low to cause eye damage.

For single-mode cables, we sometimes use a more powerful tool, called a visual-fault locator (VFL). These testers use red lasers that have enough power to actually show breaks in the fiber through the cable jacket. They are stronger than simple fiber tracers, but still not powerful enough to do bodily damage.

You can purchase a special film card to identify live fibers by eye. This small card, which costs about $10, converts the infrared light to visual light. It tells you quickly whether a fiber is live or not.

The odds of going blind by looking into the broken end of an optical fiber are virtually nil, since the broken surface tends to scatter the light coming through it. However, it is possible for you to suffer injury by mishandling polished optical fibers-but only under certain circumstances, including:

1. The light source must be high-powered. Only the more powerful lasers are strong enough to cause injury. Some Cable TV lasers can do damage.

2. The beam of light exiting the fiber must be narrow and focused. Remember the old trick of starting a fire with sunlight and a magnifying glass? Microscopic glass needles. A more serious hazard of optical fiber work is the fibers themselves. Fibers are pieces of glass. And like all glass, they can cause injury. Because of this, you need to handle fiber with considerable care. First of all, you must be very careful when handling open fibers-that is fibers not contained in a cable. (Modern optical fiber cables are very safe, and pose no danger to you.) If you were to accidentally jab yourself with one of these open fibers, you could easily end up with a painful sliver. What's worse is this sliver may not be visible! Remember: These slivers are made of transparent glass and can be very difficult to see.

You'll be surprised to know that jabbing yourself with a fiber is not, however, the most hazardous situation. The real danger is when fibers are stripped, trimmed, and cut, leaving short, thin, invisible needles. As sharp and thin as these glass shards are, they can easily penetrate your skin. And unlike a wood sliver, these glass slivers will not degrade inside your skin.

These cut pieces of fiber are very dangerous. If they were to end up in your lunch, they could cause internal bleeding and, conceivably, death. To avoid this problem, you should make generous use of masking tape (or any other type of tape) to catch the waste fiber pieces. Some technicians wrap the tape around a few fingers, sticky side out. This catches the fibers as soon as they are cut. You should also frequently blot the entire work area with tape to pick up stray pieces. Once the pickup operation is complete, fold the tape upon itself and carefully dispose of it. This tape should never be left lying around, or wadded up and thrown on the floor. Remember that fibers are insidious, since they are very difficult to see. If you sit on some of these cut pieces, you won't soon forget it.