Ricerc@Sapienza

In the present study, we reveal the presence of a new type of nonlinear propagation regime in MM fibers, where stable spatiotemporal solitons are created by the fission of the initial pulse. These solitons have different amplitudes and wavelengths, but nearly equal time duration.

Nonlinear optics in crystals with quadratic susceptibility has been largely explored along the last decades, with a particular emphasis on spatial solitons. When in the initial part of the propagation, the nonlinear length is much shorter than the diffraction length, rather than solitons, in these crystals it is possible to observe strong beam reshaping and spectral broadening. This mechanism of nonlinear beam evolution can be induced by combining high laser energies and large input diameters, so to reduce the contribution of diffraction in the initial steps of the propagation.

Multimode fibers received recently a renewed interest because of the ability to control the multimode propagation and to select, at the output, a quasi-single mode supporting the main part of the energy. In this paper we present results on spatial Kerr-beam self-cleaning in multimode optical fiber.
We show how a speckled beam obtained because of multimode propagation can be transformed into a quasi-single-mode emission under the effect of the peak power increase

We start by providing an overview of the emerging field of nonlinear optics in multimode optical fibers [1]. These fibers provide a simple testbed for observing complex wave propagation dynamics, in analogy with other fields of physics ranging from two-dimensional hydrodynamic turbulence and Bose-Einstein condensation. In addition, nonlinear multimode optical fibers enable new methods for achieving the ultrafast, light-activated control of temporal, spatial and spectral degrees of freedom of intense, pulsed light beams, for a range of different technological applications.

The dynamics of high-energy Raman solitons in graded-index multimode fibers is both numerically and experimentally investigated. The propagation of high-power pulses produces nonlinear losses, that quench up to 80% of the fiber transmission. In such a regime, several solitons arising from the fission of ultra-short femtosecond pulses manifest unique features: pulse width, Raman self-frequency shift and soliton order remain nearly constant over a broad range of energies.

The process of high-energy soliton fission is experimentally and numerically investigated in a graded-index multimode fiber. Fission dynamics is analyzed by comparing experimental observations and simulations. A novel nonlinear propagation regime is observed, where solitons produced by the fission have a nearly constant Raman wavelength shift and same pulse width over a wide range of soliton energies.

Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain a new exact solution for the nonlinear evolution of first and second order moments of a laser beam of arbitrary transverse shape carried by a graded-index multimode fiber.

Kerr beam self-cleaning in graded-index multimode fibers is accompanied by power-dependent temporal pulse reshaping. We explore the complex nonlinear dynamics with a single long pulse, where the optical power is continuously varied across its profile

In order to fully exploit laser beam dynamics in multimode fibers for applications to spatiotemporal laser beam mode-locking, it is necessary to accurately determine the conditions for the occurrence of the effect, by optimizing the spatial and temporal properties of the input laser pulses. We will describe a series of recent experiments that permit to unveil the physical mechanism of Kerr-beam self-cleaning, based on a complex cascade of parametric wave mixing processes.