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 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 ${\displaystyle \triangle \,={\frac {n_{1}-n_{2}}{n_{1}}}\ll \ 1}$. The value of n₁ is typically between 1.44 and 1.46, and ${\displaystyle \triangle }$ is typically between 0.001 and 0.02.

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