The three basic elements of a fiber optic cable are the core, the cladding and the coating. • Core: This is the light transmission area of the fiber, either glass or plastic. The larger the core, the more light that will be transmitted into the fiber. • Cladding: The function of the cladding is to provide a lower refractive index at the core interface in order to cause reflection within the core so that light waves are transmitted through the fiber. • Coating: Coatings are usually multi-layers of plastics applied to preserve fiber strength, absorb shock and provide extra fiber protection. These buffer coatings are available from 250 microns to 900 microns.
Fiber can be identified by the type of paths that the light rays, or modes, travel within the fiber core. There are two basic types of fiber: multimode and single-mode. Multimode fiber cores may be either step index or graded index. Step index multimode fiber derives its name from the sharp step like difference in the refractive index of the core and cladding. In the more common graded index multimode fiber the light rays are also guided down the fiber in multiple pathways.
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But unlike step index fiber, a graded index core contains many layers of glass, each with a lower index of refraction as you go outward from the axis. The effect of this grading is that the light rays are speeded up in the outer layers, to match those rays going the shorter pathway directly down the axis. The result is that a graded index fiber equalizes the propagation times of the various modes so that data can be sent over a much longer distance and at higher rates before light pulses start to overlap and become less distinguishable at the receiver end.
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Graded index fibers are commercially available with core diameters of 50, 62.5 and 100 microns. The single mode fiber allows only a single light ray or mode to be transmitted down the core. This virtually eliminates any distortion due to the light pulses overlapping. The core of the single mode fiber is extremely small, approximately five to ten microns.