Optical fiber falls into three basic classifications: step-index multimode, graded-index multimode, and single mode. A mode is essentially a path that light can follow down the fiber. Step-index fiber has a core with one index of refraction, and a cladding with a second index.
A graded-index fiber has a varying core index of refraction, and a constant cladding index (see Figure 6).
In general, the beam diameters of light sources for optical fibers are larger than the diameter of the fiber itself. Each fiber has a cone of light that it can propagate, known as the cone of acceptance of the fiber. It is driven by the critical angle of the fiber, which in turn varies according to the refractive index of the material. Light outside the cone of acceptance will not undergo total internal reflection and will not travel down the fiber.
Now, if light in the cone of acceptance is entering the fiber at a variety of angles greater than or equal to the critical angle, then it will travel a number of different paths down the fiber. These paths are called modes, and a fiber that can support multiple paths is classified as multimode. Notice that the light hitting at the smallest possible angle travels a longer path than the light at the largest angle, since the light at the largest angle is closest to a straight line. For step-index multimode fiber in which light travels the same speed everywhere, the rays running the longest path will take longer to get to the destination than the light running the shortest path. Thus a sharp pulse, or packet of light, will be spread out into a broad packet as it travels through the fiber. This is known as modal dispersion and can be a disadvantage in many applications. This type of fiber is used for in-house phone lines and data links.
Graded-index fiber offers one method for minimizing dispersion. The index of refraction of the core of graded index fiber increases toward the center. Remember, the refractive index of a material controls the speed of light traveling through it. Light propagating in the
center of the fiber thus goes more slowly than light on the edges. This reduces the pulse spread caused by differing path lengths. While not a perfect transmission, the transmitted pulse is dramatically improved over the step-index multimode fiber output. Graded-index fiber requires very specialized fabrication and is more expensive than step-index multimode. It is commonly used for mid-length communications (see Figure 6).
The best way to avoid modal dispersion, however, is to restrict transmission to only one mode. Single mode fiber is very narrow, with core diameters typically 8 microns, allowing light to propagate in only one mode (see Figure 7). The cone of acceptance is dramatically decreased, however, which makes light injection difficult. Splicing fiber together is more challenging, as well. Single-mode fiber is more costly than step-index multimode but less so than graded-index multimode. Single-mode fiber is used for long distance communication such as transoceanic telephone lines.
Plastic fiber is available in all three types. It is less expensive and lightweight but experiences more signal attenuation. It is practical for very short distance applications such as in automobiles.
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