For optical fibers, a class of graded-index profiles characterized by
where n ( r ) is the nominal refractive index as a function of distance from the fiber axis , n 1 is the nominal refractive index on axis, n 2 is the refractive index of the homogeneous cladding ( n ( r ) = n 2 when r ), is the core radius, and g is a parameter that defines the shape of the profile. Note 1: is often used in place of g . Hence, this is sometimes called an alpha profile. Note 2: For this class of profiles, multimode distortion is smallest when g takes a particular value depending on the material used. For most materials, this optimum value is approximately 2. When g increases without limit, the profile tends to a step-index profile .