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Spatiotemporal multimode complex optical systems

Periodic Reporting for period 3 - STEMS (Spatiotemporal multimode complex optical systems)

Reporting period: 2020-05-01 to 2021-10-31

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 brightness and quality 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 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.

The conservation of the average mode number in the process of Kerr beam self-cleaning was demonstrated.
Kerr beam self-cleaning of many low-order modes in a GRIN MMF was controlled, thanks to wavefront shaping of the coherent excitation beam.
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, leading 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 also introduced the concept of spatial and spectral control of nonlinear parametric sidebands in MMFs by tailoring their linear refractive index profile.

Experiments have shown 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.

The process of high-energy soliton fission was investigated in a GRIN MMF. 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.

Spatial beam self-cleaning and supercontinuum generation were obtained in a tapered Ytterbium-doped multimode optical fiber with parabolic core refractive index profile when pulsed beams propagate from either the wider or the smaller diameter.

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.

Multicolour 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 pulses, a visible supercontinuum spiral emission is generated. With appropriate control of the input laser coupling conditions, the colours 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.

A new space–time mapping technique was developed: it permits 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 beams at the MMF output.

Our studies of spatiotemporal femtosecond soliton propagation in GRIN fibers revealed that initial multimode soliton pulses naturally and irreversibly evolve into a singlemode soliton, carried by the fundamental mode, which acts as a dynamical attractor of the multimode system for up to the record value (for multimode fibers) of 5600 chromatic dispersion distances.

We have shown that femtosecond multimode solitons composed by non-degenerate modes have unique properties: when propagating in graded-index fibers, their pulsewidth and energy do not depend on the input pulsewidth, but only on input coupling conditions and linear dispersive properties of the fiber, hence on their wavelength. Because of these properties, spatiotemporal solitons composed by non-degenerate modes with pulsewidths longer than a few hundreds of femtoseconds cannot be generated in graded-index fibers.

The self-induced spatiotemporal reshaping of optical beams from multimode fibers allows for improving the performances of nonlinear fuorescence (NF) 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. Temporal pulse shortening accompanying spatial beam clean-up enhances the output peak power, hence the efciency of nonlinear imaging. We also show that spatiotemporal supercontinuum (SC) generation is well-suited for large-band NF imaging in visible and infrared domains. We substantiated our fndings by multiphoton fuorescence imaging in both microscopy and endoscopy confgurations.
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.
Spatial beam Kerr self-cleaning in multimode GRIN fiber