Ricerc@Sapienza

Multimode optical fibers (MMFs) have recently regained interest because of the degrees of freedom associated with their different eigenmodes. In the nonlinear propagation regime in particular, new phenomena have been unveiled in graded-index (GRIN) MMFs such as geometric parametric instabilities and Kerr beam self-cleaning [1, 2]. The speckled pattern observed at the output of the MMF at low powers, is transformed at high powers into a bell-shaped beam close to the fundamental mode. Recent work has also demonstrated that Kerr beam self-cleaning can lead to a low-order spatial mode, different from a bell-shape, by adjusting the laser beam in-coupling conditions [3]. An attractive way to systematically control the spatial excitation conditions at the fiber input is provided by the use of a spatial light modulator (SLM) which permits to profile the beam wavefront entering the MMF. In most cases, experiments involving adaptive optics consider linear propagation through scattering plates or MMFs [4]. So far, few works have dealt with the nonlinear propagation regime[5, 6].