For anoptical fiber, astep-index profile is arefractive index profile characterized by a uniform refractive index within thecore and a sharp decrease in refractive index at the core-claddinginterface so that the cladding is of a lower refractive index. The step-index profile corresponds to apower-law index profile with the profile parameter approaching infinity. The step-index profile is used in mostsingle-mode fibers^[1] and somemultimode fibers.^[2]

A step-index fiber is characterized by the core and cladding refractive indicesn₁ andn₂ 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${\displaystyle \mathrm{△}=\frac{n_1-n_2}{n_1}\ll 1}$. The value of n₁ is typically between 1.44 and 1.46, and${\displaystyle \mathrm{△}}$ is typically between 0.001 and 0.02.

Step-index optical fiber is generally made bydoping high-purity fusedsilica glass (SiO₂) with different concentrations of materials like titanium, germanium, or boron.

Modal dispersion in a step index optical fiber is given by

${\displaystyle \text{pulse dispersion}=\frac{\mathrm{△}{n}_1\ell }{c}}$

where

${\displaystyle \mathrm{△}}$ is the fractional index of refraction

${\displaystyle n_1}$ is the refractive index of core

${\displaystyle \ell }$ is the length of the optical fiber under observation

${\displaystyle c}$ is the speed of light.

• Graded-index fiber
• Critical angle
• Numerical aperture

 This article incorporatespublic domain material fromFederal Standard 1037C.General Services Administration. Archived fromthe original on 2022-01-22. (in support ofMIL-STD-188).

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