In fibre optics, a graded-index or gradient-index fibre is an optical fibre whose core has a refractive index that decreases with increasing radial distance from the fibre axis (the imaginary central axis running down the length of the fibre).
Because parts of the core closer to the fibre axis have a higher refractive index than the parts near the cladding, light rays follow sinusoidal paths down the fibre. The advantage of the graded-index fibre compared to multimode step-index fibre is the considerable decrease in modal dispersion.
The most common refractive index profile for a graded-index fibre is very nearly parabolic. The parabolic profile results in continual refocusing of the rays in the core, and minimizes modal dispersion.
This type of fibre is normalized by the International Telecommunications Union ITU-T at recommendation G.651.1