Graded-index fiber

In telecommunication, a graded-index fiber is an optical fiber with a core having a refractive index that decreases with increasing radial distance from the fiber axis.

Note: 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 compensates for multimode distortion.

Source: from Federal Standard 1037C and from MIL-STD-188