Step-index profile

For an optical fiber, a step-index profile is refractive index profile characterized by a uniform refractive index within the core and a sharp decrease in refractive index at the core-cladding interface.

Note 1: The step-index profile corresponds to a power-law index profile with the profile parameter approaching infinity.

Note 2: The step-index profile is used in most single-mode fibers and some multimode fibers.

Source: from Federal Standard 1037C and from MIL-STD-188