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Fiber optics is full of jargon but it’s important to understand it. One of the more confusing terms to many is “wavelength.” It sounds very scientific, but it is simply the term used to define what we think of as the color of light.
Light is part of the “electromagnetic spectrum” that also includes x-rays, ultraviolet radiation, microwaves, radio, TV, cell phones and all of the other wireless signals. They are simply electromagnetic radiation of different wavelengths. We refer to the range of wavelengths of electromagnetic radiation as a spectrum.
Wavelength and frequency are related, so some radiation is identified by its wavelength, while other types are referred to by their frequency. For the radiation of shorter wavelengths, light, UV and x-rays, for example, we generally refer to their wavelength to identify them, while we refer to the longer wavelengths such as radio, TV and microwaves by their frequency.
The light we are most familiar with is, of course, the light we can see. Our eyes are sensitive to light whose wavelength is in the range of about 400 nanometers (billionths of a meter) to 700 nanometers, from the blue/violet to the red. If you wonder why this is the range of colors we can see, it’s because it is the same region as the brightest output of the sun. In other words, we developed sight in the spectral range of the output of our local star. This is actually quite a good idea.
For fiber optics, we use light in the infrared region that has wavelengths longer than visible light, typically around 850, 1,300 and 1,550 nm. Why do we use the infrared? We use it because the attenuation of the fiber is much less there. The attenuation of glass optical fiber is caused by two factors—absorption and scattering. Absorption occurs in several specific wavelengths called water bands due to the absorption by minute amounts of water vapor in the glass.
Scattering is caused by light bouncing off atoms or molecules in the glass. It is strongly a function of wavelength, with longer wavelengths having much lower scattering. Have you ever wondered why the sky is blue? It’s because the light from the sun is more strongly scattered in the blue.
Fiber optic transmission wavelengths are determined by two factors: longer wavelengths in the infrared for lower loss in the glass fiber and at wavelengths that are between the absorption bands. Thus the normal wavelengths are 850, 1,300 and 1,550 nm. Fortunately, we are also able to make transmitters (lasers or light-emitting diodes [LEDs]) and receivers (photodetectors) at these particular wavelengths.
If the attenuation of the fiber is less at longer wavelengths, why don’t we use even longer wavelengths? The infrared wavelengths transition between light and heat, like you can see the dull red glow of an electric heating element and feel the heat. At longer wavelengths, ambient temperature becomes background noise, disturbing signals. And there are significant water bands in the infrared.
We often refer to wavelengths in fiber optics. The wavelengths we use for transmission must be the wavelengths we test for losses in our cable plants. Our power meters are calibrated at those wavelengths so we can test the networking equipment we install.
The three prime wavelengths for fiber optics—850, 1,300 and 1,550 nm—drive everything we design or test. The U.S. National Institute of Standards and Technology (NIST) provides power meter calibration at these three wavelengths for fiber optics. Multimode fiber is designed to operate at 850 and 1,300 nm, while single-mode fiber is optimized for 1,310 and 1,550 nm. The difference between 1,300 nm and 1,310 nm is simply a matter of convention, harking back to the days when AT&T dictated most fiber optic jargon. Lasers at 1,310 nm and LEDs at 1,300 nm were used in single-mode and multimode fiber, respectively.
Recent telecom systems use dense wavelength-division multiplexing. In these systems, lasers are chosen with precise wavelengths closely spaced—but not so close they interfere with each other—and transmitted simultaneously on a single fiber. It’s just like the FM radio spectrum.
The final note is on safety. Look closely at the first figure. The visible spectrum is well below the wavelengths used in fiber optics. That means you cannot see the light in fiber systems, so there is no reason to look into the end of a fiber. And as we mentioned last month, some systems do have enough power to be potentially dangerous, so you should never look at the end of a fiber anyway. EC
HAYES is a VDV writer and trainer and the president of the Fiber Optics Association. Find him at www.JimHayes.com.