An explanation of SRS beam cleanup in graded-index fibers and the absence of SRS beam cleanup in step-index fibers.

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 LP(01) Stokes mode while mode competition in a step-index fiber does not favor the LP(01) Stokes mode. This model explains why beam cleanup has only been reported for graded-index fibers and not for step-index fibers.