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.

Multimode optical fibers (MMF) recently attracted a renewed attention, because of their
potential for spatial division multiplexing, medical imaging and high-power fiber lasers, thanks
to the discovery of new nonlinear optical effects, such as Kerr beam self-cleaning,
spatiotemporal mode-locking, and geometric parametric instability, to name a few. The main
feature of these effects is that many transverse modes are involved in nonlinear interactions. To
advance our understanding, it is necessary to analyse the modal content of beams at the output

We introduce the concept of third-order Riemann pulses in nonlinear optical fibers. These pulses are generated when properly tailored input pulses propagate through optical fibers in the presence of higher-order dispersion and Kerr nonlinearity. The local propagation speed of these optical wave packets is governed by their local amplitude, according to a rule that remains unchanged during propagation. Analytical and numerical results exhibit a good agreement, showing controllable pulse steepening and subsequent shock wave formation.

A model of coupled modes is proposed, which makes it possible to describe the propagation of a pump wave and a Stokes component in a multimode graded-index fibre. A decrease in the depression in the centre of the profile of the pump wave intensity distribution as a result of the influence of Kerr effects and random linear coupling of modes is demonstrated.

In recent years, nonlinear pulse propagation in multimode fibers (MMFs) is has experienced a dramatic resurgence of interest, because of their potential for opti- cal communications and high-power lasers. However, from the fundamental view- point several aspects still remain to be fully understood. Here we experimentally and theoretically studied the dynamics of high-energy (up to reaching the fiber damage threshold) spatiotemporal solitons in MMFs with a graded-index (GRIN) core profile.

In this work, we reveal that when using femtosecond pump pulses, one has access to a different regime, where the peak power inside the MM fiber may reach and even surpass the critical power Pcr for catastrophic self-focusing (this is about 10 MW in fused- silica). When these peak power levels are reached, the propagating light beam undergoes a so-called filamentation phenomenon, leading to the so-called conical emission (CE)].