Graded-index fiber

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

Pulse dispersion in a graded index optical fiber is given by

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

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.

References

1. A Reference Guide to Optical Fibers and Light Guides, retrieved 2015-04-22
2. Characteristics of a 50/125 µm multimode graded index optical fibre cable for the optical access network, retrieved 2012-11-23

•  This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).