Advertisement

Advertisement

How Fiber Works (Animated!)

By Jim Hayes | Oct 10, 2024
How Fiber Works - cross-section of a fiber optic cable
Web Exclusive Content

In my time at teaching fiber optics, I learned how hard it was to explain how optical fiber works. I kept looking for ways to make it easier to understand.

Advertisement

Advertisement

Advertisement

Advertisement

In my time at teaching fiber optics, I learned how hard it was to explain how optical fiber works. I kept looking for ways to make it easier to understand.

I started with a large 25 mm acrylic rod and a laser pointer that provides for a simple visible demonstration. That was a good start.

How fiber works

Then I decided to create animated drawings illustrating how the light was carried in the fiber and included them in my presentations. That made explaining it so much easier.

Thanks to the HTML language of the web, I can share these drawings with you in this online article—something that I cannot do on a paper magazine page. So here are five animated illustrations about how fiber really works. 

But first, some background.

Optical fiber comes in three varieties—multimode step index, multimode graded-index and single-mode.

How fiber works - index profiles for multimode step-index fiber, multimode graded-index fiber, and single-mode fiber

 

The light in the fiber travels in the core. The cladding confines the light in the core by “total internal reflection.” 

The index of refraction of glass or any optical material is a measure of the speed of light in the material, and changes in index of refraction are what causes light to bend, as demonstrated by this photo of a plastic rod in a pond.

How Fiber Works - refraction of light

 

Optical fiber uses refraction to “trap” fiber in the core of the fiber. By choosing core and cladding materials with the proper index of refraction, we can cause all the light to be reflected if the angle of the light is below a certain angle. We call that “total internal reflection.” 

How Fiber Works - total internal reflection

There is an angle that, for any given fiber, defines total internal reflection. At higher angles a ray of light will not be refracted enough and is lost in the cladding of the fiber. Below that angle, it will be reflected back into the core of the fiber and transmitted along the core of the fiber. The angle of total internal reflection defines the fiber’s “numerical aperture,” a standard specification. 

Step index multimode fiber

Step index multimode fiber was the first fiber design. The core of step index multimode fiber is made completely of one type of optical material, and the cladding is another type with different optical characteristics. Step index multimode fiber has higher attenuation and inadequate bandwidth for many uses, due to the dispersion caused by the different path lengths of the various modes traveling in the core.

Step index fiber is not widely used—only plastic optical fiber and plastic-clad silica or hard-clad silica. Plastic cladding on a glass core uses a step index design today, and the core of the fiber is large, typically 200 to 1,000 microns (1,000 microns = 1 mm). Step index fiber is mainly used for lighting, consumer audio and video links.

How Fiber Works - step index multimode fiber

Graded-index multimode fiber

Graded-index multimode fiber uses variations in the composition of the glass in the core to compensate for the different path lengths of the modes. It offers hundreds of times more bandwidth than step index fiber—up to about 4 gigahertz/km. Two types are in use, 50/125 and 62.5/125, where the numbers represent the core/cladding diameter in microns. Graded-index multimode fiber is primarily used for premises networks, LANs, fiber to the desk, CCTV and other security systems. While the majority of graded-index fiber is all glass, there are some graded-index POF fibers as well.

Graded-index fiber is made with a range of materials in the core, which are chosen to minimize modal dispersion caused by different path lengths of different modes being transmitted down the fiber. The core index profile is curved—a parabola to be exact—with lower-index glass on the outside of the core. The lower-index glass transmits the higher-angle light rays (called high order modes) faster than the lower-index glass near the center of the core.

How Fiber Works - path of light in the core of a graded-index fiber

 

The index profile of the core of graded-index multimode fiber is not continuous, which is practically impossible to manufacture, but is in steps, from hundreds of steps to thousands, depending on the fiber design and manufacturing process. As a mode of light goes through each step, it is bent slightly by refraction until it is reflected back toward the core of the fiber. 

Index of refraction is related to the speed of light in the fiber: N=C/V, so a higher index of refraction indicates that light travels at a slower speed (V) relative to the speed of light in a vacuum (C). Since the light is going into a material with a lower index of refraction in the outside of the core, it speeds up compared to the speed at the center of the core. By carefully designing and manufacturing the fiber, you can get the average speed of a higher-order mode approximately the same as the modes going straight down the fiber, reducing modal dispersion.

How Fiber Works - graded-index multimode fiber

Single-mode fiber

Single-mode fiber shrinks the core down so small that the light can only travel in one ray or mode, hence the name. Single-mode fiber has a core diameter of 8–10 microns, specified as “mode field diameter,” the effective size of the core, and a cladding diameter of 125 microns.

How Fiber Works - single-mode fiber

Since there is only one mode transmitted in the fiber, there is no problem with modal dispersion. The choice of core material can reduce chromatic dispersion, which increases the bandwidth to almost infinity—although it’s practically limited to about 100,000 gigahertz, which is still a lot. Data rates of terabits per second are easily achieved. 

And that is how fiber works!

About The Author

HAYES is a VDV writer and educator and the president of the Fiber Optic Association. Find him at www.JimHayes.com.

Advertisement

Advertisement

Advertisement

Advertisement

featured Video

;

Advantages of Advertising with ELECTRICAL CONTRACTOR in 2025

Learn about the benefits of advertising with Electrical Contractor Media Group in 2025. 

Advertisement

Related Articles

Advertisement