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 and some multimode fibers. 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 △ = n 1 − n 2 n 1 ≪ 1 {\displaystyle \triangle \,={\frac {n_{1}-n_{2}}{n_{1}}}\ll \ 1} . The value of n1 is typically between 1.44 and 1.46, and △ {\displaystyle \triangle } 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
pulse dispersion = △ n 1 ℓ c {\displaystyle {\text{pulse dispersion}}={\frac {\triangle \ n_{1}\ \ell }{c}}\,\!}
where
△ {\displaystyle \triangle \,\!} is the fractional index of refraction
n 1 {\displaystyle n_{1}\,\!} is the refractive index of core
ℓ {\displaystyle \ell \,\!} is the length of the optical fiber under observation
c {\displaystyle c} is the speed of light.
See also Graded-index fiber Critical angle Numerical aperture
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).
