Spatio-spectral light-by-light moulding in multimode fibre

Controlling complex light waves to achieve desired behaviours or characteristics on demand presents a significant challenge. This task becomes even more complicated when manipulating speckled light beams owing to their inherently fuzzy intensity and phase structures. Here, we demonstrate that a weak speckled second-harmonic signal in a multimode graded-index fibre can be manipulated via its conservative interaction with a high-power co-propagating fundamental pump wave. Specifically, the spatial quality of the signal can be either enhanced or degraded by varying the pump's power or its modal power distribution. The underlying physical mechanism is the optically induced mode conversion, whose phase-matching can be controlled by the mode power distribution of the pump beam. This phenomenon enables new possibilities for manipulating complex light via material nonlinearities in multimode guiding structures. A striking example of this novel light-by-light control is the experimentally observed enhancement or partial suppression of the visible Raman Stokes cascade regulated by the second harmonic beam, while modulated by the mode power distribution of the fundamental beam.