{{Short description|Optical fiber whose core has a varying refractive index}} {{Use American English|date=March 2021}} {{Use mdy dates|date=March 2021}}

A '''graded-index fiber''', or '''gradient-index fiber''', is an optical fiber whose core has a refractive index that decreases ''continuously'' with increasing radial distance from the optical axis of the fiber, as opposed to a step-index fiber, which has a uniform index of refraction in the core, and a lower index in the surrounding cladding.

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 a graded-index or a step-index profile. The advantage of graded-index multi-mode fiber compared to step-index fiber is a considerable decrease in modal dispersion. This means that the trip time of light traversing the fiber is more consistent, allowing shorter and more frequent pulses of light to be discerned by the receiver. Modal dispersion can be further decreased by selecting a smaller core size (less than 10&nbsp;μm) and forming a single-mode step index fiber.<ref>{{citation|title=A Reference Guide to Optical Fibers and Light Guides |url=http://www.pti-nj.com/OldWeb/obb_fiber2.html |archive-url=https://web.archive.org/web/20150722113120/http://www.pti-nj.com/OldWeb/obb_fiber2.html |url-status=dead |archive-date=2015-07-22 |access-date=2015-04-22 }}</ref>

This type of fiber is normalized by the International Telecommunication Union ITU-T in recommendation G.651.1.<ref>{{citation |title=Characteristics of a 50/125 μm multimode graded index optical fibre cable for the optical access network |url=http://www.itu.int/itu-t/recommendations/rec.aspx?id=9181 |access-date=2012-11-23}}</ref>

==Pulse dispersion== Pulse dispersion in a graded-index optical fiber is given by{{Citation needed|reason=This formula seems to assume that a pulse will happily travel along the cladding. In reality, I thought the whole point of GRIN fibre was to make the pulse travelling down the centre of the fibre go slower (due to higher refractive index) than the pulse travelling on a sinusoidal path close to the cladding. If there's a distinction between modal and pulse dispertion that I'm missing, then this should definitely be expounded upon here.|date=March 2013}}

<math display=block>\mathrm{Pulse~dispersion} = \frac{k \delta n\ n_1\ l}{c} \,\!,</math> where

<math display=inline>\delta n\,\!</math> is the difference in refractive indices of core and cladding,

<math display=inline>n_1\,\!</math> is the refractive index of the cladding,

<math display=inline>l\,\!</math> is the length of the fiber taken for observing the pulse dispersion,

<math display=inline>c \approx 3\times 10^8~\mathrm{m/s}\,\!</math> is the speed of light, and

<math display=inline>k\,\!</math> is the constant of graded index profile.

==See also== *Power-law index profile *Gradient index optics

==References== {{Reflist}} *{{FS1037C MS188}}

Category:Optical fiber

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