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 fiber axis (the imaginary central axis running down the length 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 advantage of the graded-index fiber compared to multimode step-index fiber is the considerable decrease in modal dispersion.

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.

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

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.

See also

• Step-index profile
• Power-law index profile
• Gradient index optics
• Multi-mode optical fiber
• Single-mode optical fiber

References

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