Beam cleanup via stimulated Raman scattering in multimode fibers is modeled by numerically considering the competition between the Stokes modes of graded-index and step-index fibers. The relative gain of each Stokes mode is calculated by considering the overlap the various pump and Stokes modes of the fibers. Mode competition in a graded-index fiber favors the LP01 Stokes mode while mode competition in a step-index fiber does not favor the LP01 Stokes mode. This model explains why beam cleanup has only been reported for graded-index fibers and not for step-index fibers.
Terry, Nathan B.; Alley, Thomas G.; and Russell, Timothy H., "An Explanation of SRS Beam Cleanup in Graded-index Fibers and the Absence of SRS Beam Cleanup in Step-index Fibers" (2007). Faculty Publications. 217.