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The use of fiber optics in local area networks (LANs), such as Ethernet, has increased due to the inherent advantages of using fiber. High data rates can be maintained without electromagnetic or radio frequency interference (EMI/RFI). Longer distances can be achieved over that of copper wiring. For the industrial/commercial user, fiber offers high-voltage isolation, intrinsic safety and elimination of ground loops in geographically large installations. Ethernet will function with no difficulty over fiber optics as long as some simple rules are followed.


Optical fiber consists of three basic elements: core, cladding and the coating. The core constructed of either glass or plastic provides the basic means for transmitting the light energy down the cable. The cladding prevents the light from exiting the core and being absorbed by the cable itself. The coating provides protection to the fiber core while providing strength. Final protection is provided by an overall jacket that may consist of other strength and protective elements.

Figure 1 — A single fiber consists of three basic elements.

Fiber Size

Optical fibers are classified by their diameter in microns (1 micron = one-millionth of a meter). Frequently the core, cladding and coating are specified using slashes (/) to separate the values. For example, 50/125/250 means the core is 50 µm, the cladding is 125 µm and the coating is 250 µm. These dimensions all pertain to the concentric diameters of the various elements. A short form way of specifying the fiber is to only list the core and cladding sizes. In the above example, this fiber would be classified as 50/125. Core sizes range from as small as 5 µm to as high as 1000 µm. Depending upon the core size, either one or two modes of light transmission will be experienced. The two modes are called single-mode and multimode.

Figure 2 — Fiber optic cable is available as paired cable (duplex cable)
with an appearance similar to "zip cord."

Single-Mode Operation

With very small diameter fibers in the range of 5 to 10 µm, all light rays have a tendency to propagate along the axis of the fiber. Since there is only one path for the light to take, the light is termed to be experiencing a single-mode of operation. As the core diameter increases, the light rays have the option of traveling at an angle to the core axis while attempting to exit through the cladding. This second effect is called multimode operation.

Multimode Operation

With fiber core sizes of 50, 62.5 and above, multimode operation will be experienced. Not only will the light transfer down the axis of the fiber, but it will also travel away from the axis and toward the cladding. The cladding helps reflect the light rays back toward the fiber axis. The cladding provides this effect because it has a lower index of refraction than the core.

Index of Refraction

The index of refraction of a material (n) is defined as the ratio of the speed of light in a vacuum compared to the speed of light in the material. When light passes from one material to another with a different density, part of the light will be reflected and the remainder refracted. The angle of the refracted ray will be different from the incident wave and will obey Snell’s Law:

n1 sin Θ1 = n2 sin Θ2

where n 1, and n 2 are the corresponding indexes of refraction and the two angles are measured relative to a perpendicular axis to the boundary of the two materials. At some angle called the critical angle, Θ2 becomes 90°. For all values of Θ1, greater than the critical angle, total internal reflection will occur. This is the fundamental principle of fiber optic communications. The light energy is constrained to the inner core. The cladding with its lower index of refraction provides the total internal reflection necessary for proper operation.

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