Graded-index fiber

(See also: Optical fiber) In Telecommunication, a graded-index or gradient-index fiber is an optical fiber with a core having a refractive index that decreases with increasing radial distance from the fiber axis (the imaginary central axis running lengthways down the fiber).

A graded-index fiber is opposed to a step-index fiber. A step-index fiber consists of a core having a uniform refractive index, with an outer cladding also having a uniform refractive index. On the other hand, parts of the core of a graded-index fiber closer to the fiber axis have a higher refractive index than the parts near the cladding. In effect, this means that light waves follow sine-wave paths down the fiber, instead of following paths composed of straight lines which result due to repeated instances of total internal reflection within the step-index fiber.

An advantage of the graded-index fiber over the step-index fiber is the considerable decrease in modal Dispersion (optics). In a step-index fiber, modal dispersion results in originally distinct light pulses smearing and merging at the exit. This problem is alleviated with the graded-index fiber due to the reduction in modal dispersion.

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

See also: Gradient index optics