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 graded index compared to step index is the considerable decrease in modal dispersion.

This type of fiber is normalized by the International Telecommunications Union ITU-T at recommendation G.651.1.[1]

## Pulse dispersion

Pulse dispersion in a graded index optical fiber is given by

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

where

$\delta n\,\!$ 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.