Ricerc@Sapienza

We show experimentally that a laser beam scrambled by propagation in a short segment of multimode fiber may be cleaned by the nonlinear propagation in KTP cristal with type-II second-harmonic generation.

Kerr beam cleaning is very sensitive both to the incident angle and beam size. The last one defines the number of excited modes at the entrance of GRIN MMF that should be not too high and not too low. We have measured the output beam spectrum as a function of peak power and analyzed it together with the beam profile variation.

We experimentally and theoretically investigate complex temporal pulse reshaping that accompanies Kerr beam self-cleaning in multimode optical fibers. We also study the output beam shape dependence on initial conditions.

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.

Spatial Kerr beam self-cleaning (KBSC), which transforms an output speckled beam in a quasi-single mode beam in graded index (GRIN) multimode optical fibers (MMFs), has been reported recently [1,2]. GRIN MMFs are also interesting waveguides for supercontinuum (SC) generation in the visible and near infrared regions, as successfully demonstrated by launching femtosecond or subnanosecond pulses in the anomalous (1550 nm) or normal (1064 nm) dispersion regime, respectively [1,3].

Multimode graded index (GRIN) fibers received a renewed interest in recent years, in particular for the development of new laser sources [1]. In many cases, the use of GRIN fibers is limited by multimodal propagation, leading to a spatially modulated intensity distribution (speckles) at the fiber output. Recent studies have found that quasi-single mode propagation can be recovered in GRIN fibers by the so-called Kerr self-cleaning effect [2].