Editing Step-index profile

{{Short description|Refractive index profile in optical fibre}}
[[File:Step-index-profile.svg|thumb|right|Refractive HRS
 index distribution in core and cladding with a step-index profile]]
For an [[optical fiber]], a '''step-index profile''' is a [[refractive index]] profile characterized by a uniform refractive index within the [[Fiber_optics#Principle_of_operation|core]] and a sharp decrease in refractive index at the core-[[Cladding (fiber optics)|cladding]] [[wiktionary:interface|interface]] so that the cladding is of a lower refractive index.  The step-index profile corresponds to a [[power-law index profile]] with the profile parameter approaching infinity. The step-index profile is used in most [[single-mode fiber]]s<ref>{{cite web |url=http://www.tpub.com/neets/tm/107-3.htm |title=SINGLE MODE STEP-INDEX FIBERS |access-date=2012-11-23}}</ref> and some [[multimode fiber]]s.<ref>{{cite web|title=Fiber Optics Overview|url=http://www.wildpackets.com/resources/compendium/fiber_optics/overview|url-status=usurped|archive-url=https://web.archive.org/web/20130117043710/http://www.wildpackets.com/resources/compendium/fiber_optics/overview|archive-date=2013-01-17|access-date=2012-11-23}}</ref>

A step-index fiber is characterized by the core and cladding refractive indices ''n<sub>1</sub>'' and ''n<sub>2</sub>'' and the core and cladding radii a and b.  Examples of standard core and cladding diameters 2a/2b are 8/125, 50/125, 62.5/125, 85/125, or 100/140 (units of μm).  The fractional refractive-index change  <math>\triangle \, = \frac{n_1 - n_2}{n_1} \ll \ 1</math>.  The value of n<sub>1</sub> is typically between 1.44 and 1.46, and <math>\triangle</math> is typically between 0.001 and 0.02.

Step-index optical fiber is generally made by [[Doping (semiconductor)|doping]] high-purity fused [[Fused quartz|silica glass]] (SiO<sub>2</sub>) with different concentrations of materials like titanium, germanium, or boron.


[[Modal dispersion]] in a step index optical fiber is given by

<math>\text{pulse dispersion} = \frac{\triangle\ n_1\ \ell}{c}\,\!</math>

where
:<math>\triangle\,\!</math>  is the fractional index of refraction
:<math>n_1\,\!</math>  is the refractive index of core
:<math>\ell\,\!</math>  is the length of the optical fiber under observation
:<math>c</math> is the speed of light.

==See also==
*[[Graded-index fiber]]
*[[Total internal reflection#Critical angle|Critical angle]]
*[[Numerical aperture]]

==References==
{{reflist}}
{{FS1037C MS188}}

[[Category:Fiber optics]]


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