Spatial beam self-cleaning in multimode fibres

Multimode optical fibres are enjoying renewed attention,
boosted by the urgent need to overcome the current capacity
crunch of single-mode fibre (SMF) systems and by recent
advances in multimode complex nonlinear optics. In this
work, we demonstrate that standard multimode fibres (MMFs)
can be used as ultrafast all-optical tools for the transverse
beammanipulation of high-power laser pulses. Our experimental
data show that the Kerr effect in a graded-index (GRIN) MMF is
the driving mechanism that overcomes speckle distortions, and
leads to a counterintuitive effect that results in a spatially
clean output beam robust against fibre bending. Our observations
demonstrate that nonlinear beam reshaping into the fundamental
mode of aMMF can be achieved even in the absence of
a dissipative process such as stimulated scattering (Raman or
Brillouin).