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.[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

${\displaystyle \mathrm {Pulse~dispersion} ={\frac {k\delta n\ n_{1}\ l}{c}}\,\!,}$    

where

${\displaystyle \delta n\,\!}$ is the difference in refractive indices of core and cladding,

${\displaystyle n_{1}\,\!}$ is the refractive index of the cladding,

${\displaystyle l\,\!}$ is the length of the fiber taken for observing the pulse dispersion,

${\displaystyle c\approx 3\times 10^{8}~\mathrm {m/s} \,\!}$ is the speed of light, and

${\displaystyle k\,\!}$ is the constant of graded index profile.

References

1. A Reference Guide to Optical Fibers and Light Guides, archived from the original on 2015-07-22, 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)