Mechanism of beam cleanup by stimulated Brillouin scattering in multimode fibers

The conditions leading to the beam cleanup effect induced by stimulated Brillouin scattering (SBS) have been debated by many researchers. In this paper, we firstly reveal that the key point of the beam cleanup induced by SBS is at the selective excitation of the fundamental mode of the Stokes beam, which is amplified by the counterpropagating high-order-mode pump beam along multimode fibers while preserving the fundamental mode and subsequently realizing the beam cleanup. This mechanism is theoretically modeled and experimentally demonstrated in this work.