Step-index profile
For an optical fiber, a step-index profile is a refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding 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 fibers<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and some multimode fibers.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
A step-index fiber is characterized by the core and cladding refractive indices n1 and n2 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 n1 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 high-purity fused silica glass (SiO2) 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.