Step-index profile

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 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 $\triangle \,={\frac {n_{1}-n_{2}}{n_{1}}}\ll \ 1$. The value of n₁ is typically between 1.44 and 1.46, and $\triangle$ 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

${\text{pulse dispersion}}={\frac {\triangle \ n_{1}\ \ell }{c}}\,\!$

where

$\triangle \,\!$ is the fractional index of refraction

$n_{1}\,\!$ is the refractive index of core

$\ell \,\!$ is the length of the optical fiber under observation

$c$ 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).