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Graded-index fiber

 
Computer Desktop Encyclopedia: graded-index fiber

A multimode optical fiber in which the refractive index of the core declines from its highest value at the center of the core to a value at the edge of the core that equals the refractive index of the cladding. This design compensates for modal dispersion by allowing light rays in the outer zones of the core to travel faster than those in the center of core. It is typically used for transmitting distances of a couple of kilometers. Standard graded-index fibers typically have a core diameter of 50 or 62.5 µm and a cladding diameter of 125 µm. See step-index fiber and dispersion-shifted fiber.

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Wikipedia: Graded-index fiber
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In fiber optics, a graded-index or gradient-index fiber is an optical fiber whose core has a refractive index that decreases with increasing radial distance from the fiber axis (the imaginary central axis running down the length of the fiber).

Because parts of the core closer to the fiber axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fiber. The advantage of the graded-index fiber compared to multimode step-index fiber is the considerable decrease in modal dispersion.

The most common refractive index profile for a graded-index fiber is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.

This type of fiber is normalized by the International Telecommunications Union ITU-T at recommendation G.651: “Characteristics of a 50/125 μm multimode graded index optical fiber cable”

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