Ricerc@Sapienza

Characterization of the complex spatiotemporal dynamics of optical beam propagation in nonlinear multimode fibers requires the development of advanced measurement methods, capable of capturing the real-time evolution of beam images. We present a new space–time mapping technique, permitting the direct detection, with picosecond temporal resolution, of the intensity from repetitive laser pulses over a grid of spatial samples from a magnified image of the output beam.

High energy, ultra-short multimode soliton pulse fission is observed and numerically studied in multimode GRIN fibers, showing complex dynamics bringing to multiple fundamental solitons that do not entirely follow standard single mode soliton perturbation theory predictions.

We study nonlinear self-imaging dynamics in graded-index multimode optical
fibers. Side-scattering of light that accompanies the propagation of intense femtosecond
pulses permits us to directly measure the self-imaging period.

We experimentally demonstrate a previously unforeseen nonlinear effect in optical fibers: up-conversion luminescence generation excited by multiphoton absorption of femtosecond infrared pulses. We directly estimate the average number of photons involved in the up-conversion process, by varying the wavelength of the pump source. We highlight the role of nonbridging oxygen hole centers and oxygen-deficient center defects and directly compare the intensity of side-scattered luminescence with numerical simulations of pulse propagation.

We demonstrate numerically and analytically that the twisting of the 7-core hexagonal fiber leads to an increase in the efficiency of pulse combining and to a reduction of the distance along the fiber to the combining point.

A low intensity light beam emerges from a graded-index, highly multimode optical fibre with a speckled shape, while at higher intensity the Kerr nonlinearity may induce a spontaneous spatial self-cleaning of the beam. Here, we reveal that we can generate two self-cleaned beams with a mutual coherence large enough to produce a clear stable fringe pattern at the output of a nonlinear interferometer. The two beams are pumped by the same input laser, yet are self-cleaned into independent multimode fibres.

Nonlinear mode coupling in multimode optical fibers leads to complex self-organization phenomena. These are associated with the emergence of extreme waves, in the form of stable nonlinear coherent attractors characterized by low-order transverse mode patterns. The periodic or quasi-periodic oscillations of the multimode beam intensity due to mode beating lead to light scattering into a series of spectral sidebands spanning multiple octaves.

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.

We overview recent advances in the nonlinear optics of multimode optical fibers, including ultrabroadband sideband and supercontinuum generation, Kerr and Raman beam cleanup, modal modulation instabilities, four wave mixing, and second harmonic beam cleaning.

We experimentally study the interplay of Kerr and Raman beam cleanup in multimode air-silica microstructure optical fiber. The interplay of modal four-wave mixing and Raman scattering leads to high-brightness multimode supercontinuum