There are basically two types of multi-mode fibers: graded-index fibers and step-index fibers. In step-index fibers, the refractive index decreases dramatically between the core and the cladding. They are available as quartz/quartz fibers or as so-called HCS fibers (with a quartz core and a plastic cladding). In graded-index fibers, however, the refractive index decreases steadily with increasing radial distance from the fiber axis.
The step index fiber (SI) gets its name from the step-shaped profile of the refractive index between the core glass and the cladding glass