For optical fibers, a power-law index profile is an index of refraction profile characterized by where and is the nominal refractive index as a function of distance from the fiber axis, is the nominal refractive index on axis, is the refractive index of the cladding, which is taken to be homogeneous , is the core radius, and is a parameter that defines the shape of the profile. is often used in place of . Hence, this is sometimes called an alpha profile.