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| How Optical Fibers Work, Part 1 | How Optical Fibers Work, Part 2 | How Optical Fibers Work, Part 3 |
| How Optical Fibers Work, Part 4 | How Optical Fibers Work, Part 5 | How Optical Fibers Work, Part 6 |

How Optical Fibers Work, Part 3

Consider a single glass fiber, such as the one shown in an enlarged view here. The actual fiber is so thin that light entering one end will experience the "mirror effect" described in part 2 of this discussion every time it touches the wall of the fiber. As a result, the light will travel from one end of the fiber to the other, bouncing back and forth between the walls of the fiber.

Light inside an optical fiber.

This is the basic concept of optical fibers, and it correctly describes the fundamental operation of all such fibers. Unfortunately, it is not possible to use fibers of this basic construction for any practical application. The reason for this has to do with the physical realities of the phenomenon of reflection within the fiber, and how the parameters involved will change under different conditions.

The basic fact governing the reflection of light within the fiber has to do with the speed of light inside the fiber, and the speed of light in the medium just outside the fiber. Every possible material through which light can pass has a characteristic called the refractive index, which is a measure of the speed of light through that material as compared to the speed of light in open space. We won't get into the mathematics in this demonstration; it is only necessary for you to understand this concept.

One of the requirements of an optical fiber is that its diameter remain constant throughout its length. Any change in the thickness of the fiber will affect the way light reflects from the inner walls of the fiber. In some cases, this could even mean that the reflected light could exceed the critical angle required for total reflection, and so be lost through the walls of the fiber.

Unfortunately, the same effect will be noticed if the characteristics of the medium outside the fiber should change. For example, if the fiber gets wet (as it would in rain, fog, or some underground situations), the characteristics of the boundary between the inside and the outside of the fiber will change, and hence the effective shape of the fiber will change, and will keep changing as drops of water move along the surface of the fiber.

The question now is, "How can we make the fiber so the boundary layer is permanently fixed and precisely predictable?"


Prev: How Optical Fibers Work, Part 2 Next: How Optical Fibers Work, Part 4

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