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Source: 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

△

=

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

References

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