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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 4

The easiest way to ensure that the boundary between the inside of the fiber and the outside of the fiber remains constant and unchanging no matter what is to create a permanent boundary of known characteristics. The practical approach is to surround the glass fiber with another layer of glass, while making sure that the speed of light in the outer layer remains faster than the speed of light in the inner fiber. The result is shown here.

A clad optical fiber.

In this figure, the original fiber is now the core of a two-layer construct. The diameter of the core is kept constant, at approximately 50 to 60 µm (micrometers, at one time designated "microns") and its surface is kept as perfectly smooth as possible. The outer layer, known as cladding, is bonded at all points to the surface of the core.

To the outside world, this construction is effectively one solid piece of glass, even though it is constructed of two different types of glass. Thus, it is impervious to water, dirt, and other materials. If the outer surface gets wet, that makes no difference because it still doesn't affect the boundary between the core and the cladding. The whole composite fiber may be covered with rubber or plastic for easier handling and visibility.

This type of optical fiber is known as a multi-mode step-index fiber, because of the fixed and definite boundary, or step, between the core and the cladding, as well as the fact that light traveling through the fiber may assume any of several possible electromagntic "modes." This is the first successful type of optical fiber that was developed. Since then, more advanced types of optical fibers, such as graded-index and single-mode fibers have been produced. We'll get into those a little later, as additional pages in this series are developed.


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