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Spatiotemporal multimode complex optical systems

Periodic Reporting for period 2 - STEMS (Spatiotemporal multimode complex optical systems)

Reporting period: 2018-11-01 to 2020-04-30

The STEMS project is about exploiting the new concept that has been recently introduced by the PI and his co-workers, namely the self-control of the spatial coherence of optical beams in multimode nonlinear optical fibers. This concept will enable a breakthrough technology, capable of delivering high-energy optical pulses with high-average powers and much higher beam quality from fiber lasers than what is possible today. High-power fiber lasers are largely limited by transverse mode instabilities, and the loss of spatial coherence in delivery fibers.

Optical fibers provide the backbone of today’s internet communication networks, and enable compact, low cost light sources for a variety of industrial and biomedical applications. In most of these applications, single-mode fibers are used. Replacing single-mode fibers with multimode fibers leads to a dramatic growth of transmission capacity, and a substantial increase of average power and pulse energy from fiber lasers. However, because of spatial dispersion and resulting mode interference, multimode fibers suffer from an inherent randomization of the spatial transverse beam profile, leading to a loss of spatial coherence. Our approach is to exploit the intensity dependent refractive index, or Kerr nonlinearity, of glass fibers to recover the spatial coherence of a multimode wave, and compensate for temporal modal dispersion.
We experimentally studied the polarization dynamics of Kerr beam self-cleaning in a MMF. 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, e.g. the LP11 mode, in a GRIN MMF was experimentally observed, thus enriching the dynamics of the Kerr self-cleaning effect.
We experimentally demonstrated the conservation of the average mode number in the process of Kerr beam self-cleaning, in analogy with wave condensation in hydrodynamic 2D turbulence.
We experimental demonstrated that Kerr beam self-cleaning of many low-order modes in a GRIN MMF can be controlled thanks to optimized wavefront shaping of the coherent excitation beam.
We experimentally demonstrated that spatial beam self-cleaning can be highly efficient when obtained with few-mode excitation in GRIN MMFs. By using optical pulses at 1562 nm, we demonstrated a one-decade reduction of the power threshold for spatial beam self-cleaning, with respect to previous experiments using pulses with laser wavelengths at 1030-1064 nm.
We numerically investigated beam self-cleaning in a GRIN MMF, by using a coupled-mode model. We compared different models of random linear coupling between spatial modes, including coupling between all modes, or only between degenerate ones.

We experimentally studied the interplay of Kerr and Raman beam cleanup in a multimode air-silica microstructure optical fiber. The interplay of modal four-wave mixing and Raman scattering leads to high-brightness multimode supercontinuum.
By controlling pump modes injected in the normal dispersion regime of a few-mode GRIN fiber, we investigated intermodal noise-seeded modulation instability and generation of highly-detuned cascaded intermodal four-wave mixing sidebands.
Kerr beam self-cleaning in GRIN MMFs is accompanied by power-dependent temporal pulse reshaping: we explored the complex nonlinear dynamics with a single long pulse, where the optical power is continuously varied across its profile.
We introduced the concept of spatial and spectral control of nonlinear parametric sidebands in MMFs by tailoring their linear refractive index profile. Our experiments show that introducing a Gaussian dip into the refractive index profile of a GRIN fiber permits us to dramatically change the spatial content of spectral sidebands into higher-order modes.
We demonstrated that spatial Kerr beam self-cleaning can be obtained in a highly multimode multicomponent optical fiber based on lanthanum aluminum silicate oxide glasses, made by using the modified powder in tube technology.

We developed and tested a two-level iterative algorithm for finding stationary solutions of coupled nonlinear Schrödinger equations describing the propagation dynamics of a pulse in multimode and multicore optical fibers.
The process of high-energy soliton fission was experimentally and numerically investigated in a GRIN MMF. Fission dynamics was analyzed by comparing numerical observations and simulations. 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.

We experimentally demonstrated spatial beam self-cleaning and supercontinuum generation in a tapered Ytterbium-doped multimode optical fiber with parabolic core refractive index profile when 1064 nm pulsed beams propagate from the wider into the smaller diameter.
We experimentally demonstrated spatial beam self-cleaning in an Yb-doped GRIN MMF taper with decreasing nonlinearity, both in passive and active configurations. The input laser beam at 1064 nm was injected for propagation from the small to the large core side of the taper, with laser diode pumping in a counter-directional configuration.

We introduced a spatiotemporal mode-locking mechanism in a fiber laser, based on nonlinear mode-cleaning enhanced by graded dissipation.

We experimentally demonstrated the spatial self-cleaning of a highly multimode optical beam, in the process of second-harmonic generation in a quadratic nonlinear potassium titanyl phosphate crystal.
First, we shall develop methods to control fiber nonlinearity, to compensate for temporal and spatial dispersion, thus preventing information spreading in the temporal domain, and coherence loss in the spatial domain.

Second, by adding rare-earth dopants to multimode fibers, we will demonstrate self-control of modal dispersion and beam quality in active multimode fibers.

Third, via the spatio-temporal control of beam propagation, we will introduce a new fast saturable absorber mechanism for the mode-locking of high-power fiber lasers, analogous to Kerr-lens mode-locking with bulk crystals.