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.

Optical frequency comb sources based on three-wave-mixing in quadratic nonlinear materials allow for reduced pump power threshold and extended spectral coverage. We review recent progress on quadratic optical frequency combs based on second-harmonic generation and optical parametric oscillation.

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

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.

Terahertz (THz) radiation is of great interest for a variety of applications, e.g., particle accelerations, spectroscopy investigations of quantum systems, and high-field study of materials. One of the most common laser-based processes to produce THz pulses is optical rectification, which transduces an infrared pump laser to the THz domain (0.1–20 THz). In this work, we propose and theoretically describe a method to characterize the amplitude and phase of the electric field of the pump laser pulse relying on THz generation and detection.

A great deal of interest over the years has been directed to the optical space solitons for the possibility of realizing 3D waveguides with very low propagation losses. A great limitation in their use for writing complex circuits is represented by the impossibility of making curved structures. In the past, solitons in nematic liquid crystals, called nematicons, were reflected on electrical interfaces, and recently on photorefractive spatial solitons as well.