Graded-index fiber

In fiber optics, a graded index 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.

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

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

•  This article incorporates public domain material from Federal Standard 1037C. General Services Administration. Archived from the original on 2022-01-22. (in support of MIL-STD-188).