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Spatiotemporal multimode complex optical systems

Periodic Reporting for period 4 - STEMS (Spatiotemporal multimode complex optical systems)

Reporting period: 2021-11-01 to 2023-04-30

Photonics is a key enabling technology for the information and communication society. Optical glass fibers provide a natural means of transporting laser beams over long distances without spreading them in space, in virtually loss-free manner. More than 99% of all fibers in the world are the so-called single-mode fibers. In them, light is guided by total internal reflection in a micrometer sized thin hair of glass known as the fiber core. These fibers do allow for the propagation of bell-shaped light beams, however because of their extremely small cross-section the optical energy that they can deliver is also very small.

For the delivery of powerful laser beams, which is a must in a variety of industrial applications like metal cutting, welding and drilling, the only solution is to use glass fibers with a very big cross-section. Light propagating in these fat pipes of light is however naturally spread in hundreds of individual modes of oscillation, like the music of a symphonic orchestra resonates in a noise-like myriad of independent instruments before the concert begins. As a result, a light beam with initial smooth bell-shaped profile or a clear image is quickly fragmented into noise-like pixels, like a TV out of sync.

Nevertheless, research and industrial interest in the use of multimode optical fibers has emerged in recent years. Multimode fibers permit to address the current bottleneck of capacity of the fiber optics-based internet backbone. Moreover, they permit to scale up by orders of magnitude the power of continuous wave fiber laser sources in high-power industrial applications, and the energy of pulses from mode-locked ultrashort pulse fiber lasers. In addition, multimode fibers are an ideal test-bed to study complex spatio-temporal nonlinear wave propagation phenomena.
Since the invention of fiber optics in 1960, a key question has remained so far unanswered: how to transport light with a multimode optical fiber from a powerful and bell-shaped laser beam, without compromising capacity to transport images and spatial information? Answering to this important question will open the way to using multimode glass fibers for a variety of key photonic technologies, from lensless endoscopy to laser cutting and oil well drilling, and even help solving the internet capacity crunch.

In STEMS, we study a simple and robust way to cancel the natural tendency of multimode optical fibers to scramble the pixels of images that they carry, because each of them travels with its own speed. Quite strikingly, we found that by simply increasing the beam power above a certain level, all image pixels suddenly synchronize and travel in unison, thus keeping intact the initial bell-like shape of the laser beam. We have discovered and baptized this spectacular effect “spatial beam self-cleaning”. In the presence of a sufficiently strong nonlinear coupling among all fiber modes, they get mutually synchronized and pulsate in unison, as when the director signals to the musicians that the concert begins. The spectacular result of this nonlinear synchronization of oscillators, which is analogous to the synchronization of coupled pendula discovered by the dutch scientist Christiaan Huyghens in the 17th century, is that the superposition of hundreds of modes coherently vibrates in the glass pipe to sustain a clean, bell shaped, fat beam of light.

The overall project objective, which has been demonstrated by publishing more than 160 articles and conference papers, is the demonstration of original strategies to control the spatial and temporal content of multimode light beams by exploiting the optical nonlinearities of the fiber material itself, by addressing the three main issues:

(1) Achieving the nonlinear spatiotemporal self-control of beam quality in a multimode fiber.

(2) Design and analyze multimodefiber amplifiers that permit the nonlinear spatiotemporal self-control in active multimode fibers.

(3) Inventing novel pulse multimode fiber laser devices including passive and active multimode fibers.
We studied the polarization dynamics of Kerr beam self-cleaning in multimode fibers. We revealed that spatial beam cleanup is accompanied by nonlinear polarization evolution, and a significant increase of the degree of polarization. Modal attraction towards high-order modes was experimentally observed, also via wavefront shaping, enriching the dynamics of the Kerr self-cleaning effect.

Kerr beam self-cleaning in GRIN MMFs is accompanied by power-dependent temporal pulse reshaping. We also introduced the concept of spatial and spectral control of nonlinear parametric sidebands in MMFs by tailoring their linear refractive index profile. Spatial beam self-cleaning and supercontinuum generation were obtained in a tapered Ytterbium-doped multimode optical fiber with parabolic core refractive index profile. We introduced a spatiotemporal mode-locking mechanism in a fiber laser, based on nonlinear mode-cleaning enhanced by graded dissipation.

We observed spatial self-cleaning of a highly multimode optical beam, thanks to second-harmonic generation in a quadratic nonlinear KTP crystal. Multicolor spiral-shaped beams were generated in silica optical fibers, by using a tilted input laser beam. The output far-field intensity has spiral shape, independently of the input laser power value. With a high-power femtosecond pulse, a visible supercontinuum spiral emission is generated. With appropriate control of the input laser coupling conditions, the colors of the spiral spatially self-organize in a rainbow distribution.

We observed up-conversion luminescence generation, excited by multiphoton absorption of femtosecond infrared pulses in MMFs, based on nonbridging oxygen hole centers and oxygen-deficient center defects.

The process of high-energy soliton fission was investigated in a GRIN MMF. A novel regime was 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. Our studies of spatiotemporal femtosecond solitons revealed that initial pulses naturally and irreversibly evolve into a single mode soliton, carried by the fundamental mode, which acts as a dynamical attractor of the multimode system. We have shown that femtosecond multimode solitons composed by non-degenerate modes have unique properties: when propagating in graded-index fibers, their pulse width and energy do not depend on the input pulse width, but only on input coupling conditions and linear dispersive properties of the fiber, hence on their wavelength.

The self-induced spatiotemporal reshaping of optical beams from multimode fibers allows for improving the performances of nonlinear fluorescence microscopy and endoscopy. We experimentally demonstrated that the beam brightness increase, induced by self-cleaning, enables two and three-photon imaging of biological samples with high spatial resolution.

The project results have been disseminated via press releases, articles on non-specialized Italian magazines, by participating in 2018 at the “international festival of light” in Brescia, and by organizing an international workshop and several summer or winter schools, and by delivering invited and keynote talks at major international conferences, numerous workshops, and at universities or industrial research laboratories around the world.
Spatial beam Kerr self-cleaning in multimode GRIN fiber