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Understanding Wavelengths

No one ever accused fiber optics of being simple technology. Instead, we tend to acknowledge that this powerful technology is difficult to design and complicated in its application. Mastering those complications is often worth it because it enables us to build better networks and do a better job with those networks. In that endeavor, we can tackle a brief tutorial on wavelengths and how they impact fiber optic designs.

What Is Wavelength?

The second word in “fiber optics” already tells us plenty about the nature of this technology. It deals with light — more specifically electromagnetic radiation — as the signaling medium. Wavelength is very simply a measure of the space between two photons in a solid beam of light. Conversely, we have frequency which measures the time between two signals. The two terms are opposite sides of the same coin. If you have a shorter wavelength, it takes less time between signals and a higher frequency.

With this in mind, the wavelength (or frequency) of any light source tells us the physical limitation of how we can use that light in signal processing. We can never send signals that are faster than the frequency of the beam, and we cannot use equipment that is smaller than the wavelength. This is a rough summary, but it paints a good enough picture.

Aside from the basics, the wavelength also tells us how light will interact with other objects. When it comes to designing fiber optics, those interactions are the most important pieces of information hiding within a wavelength.

Absorption

When fiber optics are engineered and tested, there are two issues that can impact their effectiveness. Absorption is one of them. Materials have a natural ability to absorb electromagnetic radiation. For any given substance, only radiation of certain wavelengths can actually interact and be absorbed. When we deal with fiber optic cables, the largest source of absorption actually comes from microscopic water droplets. That drives manufacturers to avoid wavelengths where that absorption is at its worst.

Scattering

Like absorption, scattering happens at different wavelengths for any given material. Also like absorption, the culprits of scattering within a fiber optic cable are small and easy to overlook. Particles of dust and even the air itself can cause scattering problems, so again, the design is to use wavelengths where these problems are smallest.

The term for signal loss related to absorption and scattering is attenuation. Engineers try to make attenuation numbers as small as possible, and when you account for both absorption and scattering at the same time, you find that very specific frequencies work best. The most common wavelengths in use today are 850, 1300, 1310 and 1500 nanometers. You’ll notice large gaps between each of those numbers. Those just happen to be the magic wavelengths where the attenuation values hit minima.

This is only the beginning. We can find that different types of fiber optic signals can further impact which wavelength is best for a function. Multimode and singlemode fiber, for instance, have different naturally-occurring sources of interference. That’s why they tend to operate with noticeably separate wavelength ranges.

Putting it all together, it isn’t necessary to optimize wavelengths when you choose fiber optic systems. That’s baked into the design. Instead, it helps to understand why wavelength is an important identifier and how emerging, more advanced systems might play with wavelength in order to improve the technology.

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