  We introduce and experimentally demonstrate the concept of all-optical beam
switching in graded-index multimode optical fibers. Nonlinear coupling between
orthogonally polarized seed and signal beams permits to control the spatial
beam quality at the fiber output. Remarkably, we show that even a weak few-mode
control beam may substantially enhance the quality of an intense, highly
multimode signal beam. We propose a simple geometrical representation of the
beam switching operation, whose validity is quantitatively confirmed by the
experimental mode decomposition of the output beam. All-optical switching of
multimode beams may find important applications in high-power beam delivery and
fiber lasers.
