Many years ago, people complained that the most dangerous part of fiber optic work was the chance you might get your eyeballs burned by laser light in the fiber. They had confused optical fibers to the output of high-powered lasers used in labs. Or perhaps they had been going to the doctor to get warts burned off their skin using lasers with fiber optic probes, or they had maybe seen too many science fiction movies.
In fact, most fiber optic systems do not have sufficient power to cause harm to your eyes. Also, the light coming out of a fiber is expanding, so the farther you are away from the end of the fiber, the lower the exposure. Having said that, consider yourself warned. In more recent times, some fiber optic systems are carrying sufficient power to be dangerous, and some fiber optic inspection techniques increase the chance of harm. But that’s not the biggest danger facing installers. Let’s look at the issues.
The key to understanding the power issue is understanding power levels, wavelength of the light and the nature of light transmission in optical fiber.
Fiber optic medical laser systems used for surgery and laser machining systems certainly have enough power to cause harm to your eyes, as well as burn off warts or machine some types of materials. Those systems use very high-powered lasers, often CO2 lasers, which emit radiation at a wavelength that is really heat, not light, around 10 microns wavelength. This wavelength is readily absorbed by materials and can heat them quickly, cutting those materials easily.
In medical applications, the heat not only cuts tissue, but it cauterizes, stopping bleeding, a major advantage. It also cuts more precisely with a small, focused spot, allowing more exacting work to be done.
Laser machining also takes advantage of the smaller, focused spot of the laser, making it possible to create smaller holes or cut parts more precisely than conventional machining. The smaller spot size also reduces local heating, reducing the need for cooling and preventing thin materials from warping.
Some of these high-powered systems use fiber with focusing lenses on the end for delivery, allowing use of a flexible probe that increases the flexibility of the machine’s use. However, the power is so high that any dust or dirt on the end of the fiber may be vaporized, causing damage to the end of the fiber probe.
Fiber optic communications systems use much less power. First of all, most sources used in fiber optics are optimized for modulation speed, not absolute power. In fact, in most telephone links, the laser has too much power, so much it needs an attenuator at the receiver to prevent overloading. Second, if you put too much power in a fiber, it creates some types of secondary light emission that can add to the noise in the system, causing problems.
Premises cabling with multimode fiber and LED sources has very low power levels, too low to be a hazard. Higher speed premises links use VCSEL lasers, which are still quite low in power levels and generally harmless.
Two types of links have high power, as much as 100 times more than other communications systems. They are CATV or video links at 1,550 nm and telco long-distance links, using dense wavelength division multiplexing (DWDM). The CATV or video links used in fiber to the home (FTTH) may use fiber amplifiers that boost the power to very high levels, potentially dangerous to the eye. Telco DWDM links are used on extremely long distance links. They not only use fiber amplifiers for boosting the power, but they have many different signals operating at different wavelengths carried in one single-mode fiber. Any one single wavelength may not be a problem, but the sum of 16, 32 or 64 individual wavelengths can be very powerful.
The next issue is focusing the light from a fiber into your eye. Light exiting an optical fiber spreads out in a cone, the angle of which is determined by the transmission characteristics of the fiber as defined by the numerical aperture. As your eye is farther from the end of the fiber, the amount of radiation it receives is inversely proportional to the square of the distance—double the distance and you cut the power by one-fourth, 10 times the distance reduces the power to about 1 percent. You do not have to be far away from the fiber for the power to be reduced to low levels.
Because the light is exiting the fiber in a cone-shaped beam, your eye cannot focus it on the retina. This is unlike the typical lab laser or laser pointer that shines a narrow, collimated beam that does not spread out, a beam your eye can easily focus on the retina, causing temporary blindness.
Finally, there is an issue of wavelength. Your eye cannot see many of the wavelengths used in fiber optics because the liquid in your eye—which is mostly water—heavily absorbs light in the infrared (IR). Light from most fiber optic sources will be absorbed by this liquid, so any potential harm is likely to come to the lens or cornea, not the retina.
While the expanding beam of the light exiting the fiber makes it less of an issue for direct viewing, using a fiber inspection microscope can be a problem. I’ve tested this hypothesis myself and can confirm that a microscope will focus virtually all the light back into the eye. Many microscopes used in fiber optics, therefore, have filters to absorb any IR light that could be harmful. Be wary of cheap microscopes, however, which may not have IR-blocking filters.
And to be certain fibers are safe to inspect or work with, always check fibers in an operating network with a fiber optic power meter to ensure no light is present before inspecting any connector with a microscope.
So to sum up, most fiber optic links are harmless to eyes. Some links may be harmful, however; never take a chance. Check the link before inspecting it.
The real danger
The more common problem is getting scraps of fiber in your eye when working with it. While few fiber optic systems have harmful levels of power, every termination and splice produces shards (scraps) of optical fiber, which is potentially very harmful to your eyes and skin.
These shards are tiny, thin and often very sharp where they broke off the fiber. They can easily puncture your skin, burying themselves deep enough to be difficult to pull out, if only you could see them. Being transparent, they practically disappear once imbedded in your skin. In most parts of your body, they merely become a nuisance, perhaps infecting or causing an irritating bump, until they eventually work themselves out.
Around your eye, however, they can be much more difficult to find and remove. The tears that wet your eyes make the transparent glass shards practically impossible to find and remove. The sharp ends of the fiber may cause it to imbed itself in the eye or surrounding tissue, making it even more difficult to remove. Unlike metallic particles, they cannot be removed with magnets.
It is imperative to follow procedures that minimize the dangers to the eye. Always wear protective eyewear with side shields, even if you normally wear glasses, to prevent any flying shards from getting near your eyes. I know from experience not to trust normal eyeglasses, since taking them off for a second allowed a fiber shard to land on my lower eyelash, where, luckily, I found and removed it.
Be extremely careful whenever handling fibers, especially when stripping fiber or scribing and breaking fiber extending out of an adhesive connector. Instead of breaking it, scribe it gently, then slide your fingers up the connector ferrule, grasping the fiber and pulling it off. Then dispose of it carefully.
Most cleavers used for splicing or terminating prepolished/splice connectors hold the fiber after cleaving, so the only problem is disposing of it. I recommend using disposable containers, such as those used for soups at carry-out restaurants. Use the container for all your fiber scraps, and then seal it and dispose of it properly.
You also can set up your workplace to make it easier to avoid problems. Use a black plastic mat for a work surface. The dark background will make it easier to see the fibers you are working with and handle them more carefully. You can easily find any broken fibers that fall on the mat.
Some techs like to place a length of double stick tape or a loop of black electrical tape on the mat and stick fibers to the adhesive surface, then dispose of the tape when finished. I prefer to simply use a disposable container and place every fiber scrap into that container, rather than leave them exposed on the work surface.
This is all very important—important enough to have a few workplace rules for all fiber optic techs that can prevent eye damage:
• Never look directly at the end of a unknown fiber optic cable, especially one that may be attached to a network, without checking the output with a power meter to ensure it has no optical power coming out of it.
• Never use a microscope to inspect a connector without checking for power, and make certain your microscope has a filter to stop IR light from a fiber optic system.
• Always wear protective eyewear with side shields when working.
• Use a black plastic work surface, so you can see the fibers easily. Use techniques that minimize the chances of a loose fiber shard. Dispose of all fiber scraps in a safe container. Dispose of all scraps properly after finishing a job.
HAYES is a VDV writer and educator and the president of The Fiber Optic Association. Find him at www.jimhayes.com.