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 optical axis 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 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.

Multi-mode optical fiber can be built with either graded index or step index. The advantage of the multi-mode graded index compared to the multi-mode step index is the considerable decrease in modal dispersion. Modal dispersion can be further decreased by selecting a smaller core size (less than 5-10μm) and forming a single mode step index fiber.[1]

This type of fiber is normalized by the International Telecommunications Union ITU-T at recommendation G.651.1.[2]

Pulse dispersion[edit]

Pulse dispersion in a graded index optical fiber is given by

\mathrm{Pulse~dispersion} = \frac{k \delta n\ n_1\ l}{c} \,\!,    [citation needed]


\delta n\,\! is the difference in refractive indices of core and cladding,

n_1\,\! is the refractive index of the cladding,

l\,\! is the length of the fiber taken for observing the pulse dispersion,

c \approx 3\times 10^8~\mathrm{m/s}\,\! is the speed of light, and

k\,\! is the constant of graded index profile.