Search

We introduce a different class of thresholdless three-dimensional soliton states that form in higher-order topological insulators based on a two-dimensional Su-Schrieffer-Heeger array of coupled waveguides. The linear spectrum of such structures is characterized by the presence of a topological gap with corner states residing in them. We find that a focusing Kerr nonlinearity allows families of light bullets bifurcating from the linear corner states to exist as stable three-dimensional solitons, which inherit topological protection from their linear corner counterparts and, remarkably, survive even in the presence of considerable disorder. The light bullets exhibit a spatial localization degree that depends strongly on the array dimerization, and may feature large temporal widths in the topological gap near the bifurcation point, thus drastically reducing the otherwise strong instabilities caused by higher-order effects. This research was funded by the Russian Science Foundation under Grant No. 21-12-00096 and partially by Research Project No. FFUU-2021-0003 of the Institute of Spectroscopy of the Russian Academy of Sciences. Support by Agencia Estatal de Investigación Grants No. CEX2019-000910-S and No. PGC2018-097035-B-I00 funded by MCIN/AEI/10.13039/501100011033/FEDER, Departament de Recerca i Universitats de la Generalitat de Catalunya (Grant No. 47Y48R6YK), and Generalitat de Catalunya (CERCA), Fundacio Cellex and Fundacio Mir-Puig, is acknowledged.

This study establishes the extended classical optical solitons for a nonlinear Schrodinger equation describing resonant nonlinear light propagation through isolated flaws in optical wave guides. We use the modified Sardar sub-equation approach to get such innovative results. The innovative optical solitons solutions have been investigated to explain unique physical obstacles, and they entail an extended classical M-truncated derivative, which affects the physical properties of the findings greatly. These advancements have been shown to be beneficial in the transmission of long-wave and high-power communications networks. Furthermore, the figures for the acquired solutions are graphed through the depiction of the 3D and contour plots in order to throw additional light on the peculiarities of the obtained solutions.

This work employs a novel variation of the Sardar sub-equation approach to investigate the optical solitons for the nonlinear Hirota-Schrodinger equation. Diferent soliton solutions, including bright solitons, dark solitons, singular solitons, combined bright-singular solitons, periodic, exponential, and rational solutions are derived along with nonlinear models. The obtained solitons solutions are crucial to mathematics, physics, science, and engineering.

In this paper, exact traveling wave solutions of the conformable differential equations have been examined. By means of the wave transformation and properties of the conformable derivative (CD), conformable nonlinear Schro center dot dinger equation (CNLSE) has been converted into an integer order differential equation. To extract optical solutions, the wave profile has been divided into amplitude and phase components. A new extension of the Ba center dot cklund method has been offered and applied to the CNLSE which has important applications in quantum mechanics. Some novel exact traveling wave solutions to the CNLSE with group velocity dispersion and second order spatiotemporal dispersion coefficients are successfully obtained by means of this method.

We theoretically study the nature of parametrically driven dissipative Kerr soliton (PD-DKS) in a doubly resonant degenerate micro-optical parametric oscillator (DR-DμOPO) with the cooperation of χ (2) and χ (3) nonlinearities. Lifting the assumption of close-to-zero group velocity mismatch (GVM) that requires extensive dispersion engineering, we show that there is a threshold GVM above which single PD-DKS in DR-DμOPO can be generated deterministically. We find that the exact PD-DKS generation dynamics can be divided into two distinctive regimes depending on the phase matching condition. In both regimes, the perturbative effective third-order nonlinearity resulting from the cascaded quadratic process is responsible for the soliton annihilation and the deterministic single PD-DKS generation. We also develop the experimental design guidelines for accessing such deterministic single PD-DKS state. The working principle can be applied to different material platforms as a competitive ultrashort pulse and broadband frequency comb source architecture at the mid-infrared spectral range.

This study investigates the nonlinear Hamiltonian amplitude equation by using two analytical techniques, namely; the extended sinh-Gordon equation expansion method, and the extended rational sine-cosine/sinh-cosh methods. Some important wave solutions are successfully constructed, such as dark, bright, combined dark-bright, singular solitons, periodic and singular periodic wave solutions. The physical features of the acquired solutions are plotted to depict the clear dynamical behaviour of the reported results. All the acquired solutions have satisfied the original equation.

We develop a comprehensive theory for describing the experimental beam profiles from multimode fiber Raman lasers. We take into account the presence of random linear mode coupling, Kerr beam self-cleaning and intra-cavity spatial filtering. All of these factors play a decisive role in shaping the Stokes beam, which has a predominant fundamental mode content. Although the highly multimode pump beam is strongly depleted, it remains almost insensitive to the different physical effects. As a result, the intensity of the output Stokes beam is an order of magnitude higher than the pump intensity at its maximum, in quantitative agreement with the experimental results and in contrast with the simplified balance model.

This article studies dark, bright, trigonometric and rational optical soliton solutions to the perturbed nonlinear Schrödinger–Hirota equation (PNLSHE). Hence, we have examined two cases: first, restrictions have been done to the third-order (TOD) (γ) as constraint relation, and the coupling coefficients (σ) is obtained as well as the velocity of the soliton by adopting the traveling wave hypothesis. Second, the TOD and the coupling coefficients are non-zero value, sending back to the PNLSHE, which has been studied in refs. [4,10,16] recently. By employing two relevant integration technics such as the auxiliary equation and the modified auxiliary equation method, miscellaneous optical solitary wave is obtianed, which is in agreement with the outcomes collected by the previous studies [4,16]. These results help in obtaining nonlinear optical fibers in the future.

In this article, we investigate novel optical solitons solutions for the Lakshmanan–Porsezian–Daniel (LPD) equation, along with group velocity dispersion and spatio-temporal dispersion, via three altered analytical techniques. A variety of bright, singular, dark, periodic singular, and kink solitons solutions are constructed via the Kudryashov method, the generalized tanh method and the Sardar-subequation method. The dynamical behavior of the extracted solutions is demonstrated in graphical form such as 3D plots, 2D plots, and contour plots. The originality of the obtained solutions is recognized by comparison with each other and solutions previously stated in the literature for the LPD model, which displays the efficiency of the methods under consideration.

Purpose: When it comes to third and higher-order dispersion, the Schrödinger–Hirota equation is one of the main models developed outside the classical NLSE management models for optical soliton transmission. The cubic–quartic Fokas–Lenells equation is also one of the recently developed equations, which has importance in the field of telecommunications regarding optical soliton transmission in the absence of chromatic dispersion. In this study, in order to examine the optical solitons, the Schrödinger–Hirota equation in the presence of the chromatic dispersion and the cubic–quartic Fokas–Lenells equation discarding the chromatic dispersion were investigated. For this intent, by obtaining certain soliton types using the unified Riccati equation expansion method (UREEM), optical soliton solutions were obtained for both models and graphical representations and comments were made. Methodology: By developing appropriate computer algorithms and applying UREEM in the following ways, symbolic calculation software was made and analytical optical soliton solutions were obtained. Findings: Through computer algebra software, we plotted the obtained results via 3D, 2D views and we also illustrated the investigation of wave behavior caused by parameter change on 2D graphics. Originality: Different soliton behavior under the parameters effect of the Schrödinger–Hirota equation having chromatic dispersion and the cubic–quartic Fokas-Lenells equation is investigated and the obtained results are reported.

Nonlinear waveguides with two distinct domains of anomalous dispersion can support the formation of molecule-like two-color pulse compounds. They consist of two tightly bound subpulses with frequency loci separated by a vast frequency gap. Perturbing such a two-color pulse compound triggers periodic amplitude and width variations, reminiscent of molecular vibrations. With increasing strength of perturbation, the dynamics of the pulse compound changes from harmonic to nonlinear oscillations. The periodic amplitude variations enable coupling of the pulse compound to dispersive waves, resulting in the resonant emission of multi-frequency radiation. We demonstrate that the location of the resonances can be precisely predicted by phase-matching conditions. If the pulse compound consists of a pair of identical subpulses, inherent symmetries lead to degeneracies in the resonance spectrum. Weak perturbations lift existing degeneracies and cause a splitting of the resonance lines into multiple lines. Strong perturbations result in more complex emission spectra, characterized by well separated spectral bands caused by resonant Cherenkov radiation and additional four-wave mixing processes.

We experimentally and numerically study the ignition of helical-shaped plasma filaments in standard optical fibers. Femtosecond pulses with megawatt peak power with proper off-axis and tilted coupling in the fiber core produce plasma skew rays. These last for distances as long as 1000 wavelengths thanks to a combination of linear waveguiding and the self-channeling effect. Peculiar is the case of graded-index multimode fibers; here the spatial self-imaging places constraints on the helix pitch. These results may find applications for fabricating fibers with helical-shaped core micro-structuration as well as for designing laser components and three-dimensional optical memories.

This paper presents the modulation instability (MI) analysis and the different types of optical soliton solutions of the Triki–Biswas model equation. The aforesaid model equation is the generalization of the derivative nonlinear Schrödinger equation which describes the ultrashort pulse propagation with non-Kerr dispersion. The study is carried out by means of a novel efficient integration scheme. During this work, a sequence of optical solitons is produced that may have an important in optical fiber systems. The results show that the studied model hypothetically has incredibly rich optical soliton solutions. The constraint conditions for valid soliton solutions are also reported. The gained results show that the applied method is efficient, powerful and can be applied to various complex models with the help of representative calculations.

Stimulated Brillouin scattering (SBS) lasers based on silicon waveguides with large SBS gain, have been widely used in frequency tunable laser emission, mode-locked pulse laser, low-noise oscillator, optical gyroscope, and other fields. However, current SBS lasers still need long waveguide lengths to realize Brillouin laser output, which increases the waveguide losses and is not conductive to be integrated. In this paper, we propose a silicon ring waveguide, in order to tune the frequency of the phonon field of SBS laser based on the silicon substrate of the ring cavity. The simulation results exhibit that the tailorable forward SBS effect is realized in the silicon-based optical waveguide with a large SBS gain up to 1.90 W-1m-1. Particularly, with the mutual restraint between photoelastic and moving boundary effects, the tunable phonon frequencies emitting from 1 to 15 GHz are realized through the conversion among higher order modes by modifying the widths of the ring cavity. Therefore, this silicon waveguide based on ring cavity will provide a new technical scheme for designing tunable SBS lasers by tuning the ring widths. In addition, this enhanced and broadband acoustic radiation will pave the way for hybrid integration in silicon-based optical waveguide, micro-electromechanical system, and CMOS signal processing technology.

Multimode fibres provide a promising platform for boosting the capacity of fibre links and the output power of fibre lasers. The complex spatiotemporal dynamics of multimode beams may be controlled in spatial and temporal domains via the interplay of nonlinear, dispersive and dissipative effects. Raman nonlinearity induces beam cleanup in long graded-index fibres within a laser cavity, even for CW Stokes beams pumped by highly-multimode laser diodes (LDs). This leads to a breakthrough approach for wavelength-agile high-power lasers. However, current understanding of Raman beam cleanup is restricted to a small-signal gain regime, being not applicable to describing realistic laser operation. We solved this challenge by experimentally and theoretically studying pump-to-Stokes beam conversion in a graded-index fibre cavity. We show that random mode coupling, intracavity filtering and Kerr self-cleaning all play a decisive role for the spatio-spectral control of CW Stokes beams. Whereas the depleted LD pump radiation remains insensitive to them.

In this paper, the optical solitons with M-truncated derivative and other solutions to the coupled nonlinear Schrodinger equation (NLSE) are obtained. The coupled NLSE describes the pulse propagation through a two-mode optical fiber. Complex amplitude ansatz and polynomial expansion methods are employed to construct such novel solutions. We successfully construct dark, mixed dark-bright, singular optical solitons and other solutions to the considered nonlinear model. Dark soliton describes the solitary waves with lowerintensity than the background, bright soliton describes the solitary waves whose peak intensity is larger thanthe background and the singular soliton solutions is a solitary wave with discontinuous derivatives; examples ofsuch solitary waves include compactions, which have finite (compact) support, and peakons, whose peaks have adiscontinuous first derivative. Numerical simulations of the solutions are used to analyze some of the propagation properties of pulses and their collision dynamics. Fusion along quasi-periodic waves is found to be exhibited. Under nonlinear refractive elements, the graded or phase index waveguide is seen to change the strength of the soliton. In addition, the properties are shown with figures for these solutions. These findings have been usefully extended to the transmission of long-wave and high-power telecommunication systems. In addition, through the figures presented, the effect of the fractional order $$\alpha$$ is reflected.

In this paper, the optical solitons for the complex Ginzburg–Landau equation with Kudryashov’s law of refractive index are established. An improved modified extended tanh–function technique is used to extract numerous solutions. Bright and dark solitons, as well as singular soliton solutions, are achieved. In addition, as the modulus of ellipticity approaches unity or zero, solutions are formulated in terms of Jacobi’s elliptic functions, which provide solitons and periodic wave solutions.

We present a new spatial-spectral mapping technique permitting to measure the beam intensity at the output of a graded-index (GRIN) multimode fiber with sub-nanometric spectral resolution. We apply this method to visualize the fine structure of the beam shape of a sideband generated at 1870 nm by geometric parametric instability (GPI) in a GRIN fiber. After spatial-spectral characterization, we amplify the GPI sideband with a Tm-doped fiber amplifier to obtain a microjoule-scale picosecond pump whose spectrum is finally broadened in a segment of InF3 optical fiber to achieve supercontinuum ranging from 1.7 {\mu}m up to 3.4 {\mu}m, Comment: 4 pages, 6 Figures

Over the last few years, we have experienced a renaissance of nonlinear optics in multimode fibers, driven by both fundamental and applied research. Multimode optical fibers (MMFs) offer a natural environment for investigating multidimensional systems and their complex collective dynamics. Indeed, numerous new nonlinear effects have recently been observed including, for instance, Kerr beam self-cleaning, mode-locking by spatiotemporal filtering, or multimode parametric wave mixing, to name a few [1] . On the other hand, from the application perspective, MMFs are expected to provide interesting novel solutions for high power delivery and increased transmission capacity. Significant work is now in progress to exploit the recently discovered dynamics, and improve the performances of different technologies such as laser endoscopy, nonlinear imaging, and fiber-based laser sources [2] , [3] . In addition, the recent development of advanced measurement techniques, permitting to capture the full spatiotemporal nature of the multimode phenomena has also attracted a strong research interest [4] .

Recent progress in mastering multidimensional nonlinear wave patterns has an interdisciplinary character. It bridges different areas, ranging from nonlinear wave collapse and turbulence phenomena in plasmas, the formation of highly coherent structures, such as solitons in nonlinear optics (so-called "light bullets"), to liquid crystals and Bose-Einstein condensates (BECs) [1] – [3] . Such soliton-like structures could provide unprecedented energy condensation, and lead to a breakthrough in the information capacity of photonic networks and multimode microresonators for optical comb generation [4] – [6] . Here we numerically demonstrate that multidimensional wave pattern stabilization can be substantially enhanced by introducing graded dissipation in a multimode fiber (MMF), which is closely connected with the concept of distributed Kerr-lens mode-locking in an all-fiber lasers [7] .

Spatial self-imaging (SSI) consists of the periodic replication of the transverse profile of an optical beam along its propagation. It can be achieved in guided wave systems when all excited modes interfere in phase [1] . Remarkable is the case of graded-index multimode fibers, where the modes propagation constants are equally spaced, and the interference condition becomes trivial, giving rise to a periodic breathing of the field effective area [2] . In the singlemode propagation regime, SSI is by definition impossible. However, bimodal propagation can be attained in a nominal singlemode fiber (SMF-28), when operating in proximity of its cut-off wavelength. The latter imposes the condition for which only the fundamental mode is guided, so that higher-order modes (HOM) vanish over a certain damping distance (typically of about a few centimeters, depending on the wavelength). Therefore, effects associated to the presence of HOM can be present, over propagation lengths shorter than the damping distance.

The Triki-Biswas equation that is a new model for soliton dynamics of sub pico-second pulses in a monomodal optical fiber with non-Kerr dispersion is considered to get optical solitos of its in this paper. Optical bright and singular solitons are given by trial equation approach. Also, singular periodic solutions are located.

We experimentally report the novel evolutions of dissipative-soliton-resonance (DSR) pulses with respect to the pump power in an all-normal-dispersion fiber laser based on a nonlinear optical loop mirror. By solely increasing the pump power, pulse breaking of the DSR pulses was observed apart from the typical pulse broadening at a fixed pulse peak power. Instead of arbitrarily broadening with the increase in the pump power or directly losing the mode-locking state, the process of pulse breaking was accompanied by the multipulse state and harmonic mode-locking pulses evolving from the original DSR pulse. In addition, to the best of our knowledge, a narrowing of the DSR pulse with the increase in the pump power was observed for the very first time. Further results show that these unusual evolutions of DSR pulses could be attributed to the changes in several laser parameters resulting from the increasing pump power under specific operating conditions.

We present and verify a residual-pump-based temporal overlap estimation method in soliton self-frequency shift-based coherent anti-Stokes Raman scattering system. The residual pump light, output by a highly nonlinear photonic crystal fiber, acts as a crucial link between the pump and Stokes pulses during the temporal overlap estimation. The wavelength-dependent optical delay is estimated to be 0.141 ps/nm when the Stokes wavelength is 900 nm ~1050 nm according to the temporal overlap estimation method. The actual measurement result is 0.138 ps/nm based on the nonresonant signal from a microscope slide, which is very close to the estimated result. In addition, the Raman resonant signals of liquid cyclohexane at 2853 cm-1, 2923 cm-1 and 2938 cm-1 have also been successfully detected at the predicted optical delays 427.27 ps and 428.17 ps.

[EN] We uncover a novel and robust phenomenon that causes the gradual self-replication of spatiotemporal Kerr cavity patterns in cylindrical microresonators. These patterns are inherently synchronized multi-frequency combs. Under proper conditions, the axially localized nature of the patterns leads to a fundamental drift instability that induces transitions among patterns with a different number of rows. Self-replications, thus, result in the stepwise addition or removal of individual combs along the cylinder's axis. Transitions occur in a fully reversible and, consequently, deterministic way. The phenomenon puts forward a novel paradigm for Kerr frequency comb formation and reveals important insights into the physics of multidimensional nonlinear patterns., This work was partially supported by the Government of Spain (Grants No. IJCI-2016-27752, No. MTM2016-75963-P, and No. FIS2015-71559-P; Severo Ochoa CEX2019-000910-S); Generalitat de Catalunya; Centres de Recerca de Catalunya; Fundacio Cellex; and Fundacio Mir-Puig.

We discuss some recent results illustrating the role of input wave-front shaping, propagation dynamics and output beam diagnostics in order to observe spatial beam cleaning in nonlinear multimode fibers and amplifiers.

We review our recent results on fiber lasers with distributed feedback based on π-shifted or random fiber Bragg gratings, and random index structures inscribed by femtosecond pulses in singlemode or multicore/multimode fibers.

We theoretically introduce a new type of topological dipole solitons propagating in a Floquet topological insulator based on a kagome array of helical waveguides. Such solitons bifurcate from two edge states belonging to different topological gaps and have bright envelopes of different symmetries: fundamental for one component, and dipole for the other. The formation of dipole solitons is enabled by unique spectral features of the kagome array which allow the simultaneous coexistence of two topological edge states from different gaps at the same boundary. Notably, these states have equal and nearly vanishing group velocities as well as the same sign of the effective dispersion coefficients. We derive envelope equations describing components of dipole solitons and demonstrate in full continuous simulations that such states indeed can survive over hundreds of helix periods without any noticeable radiation into the bulk., Comment: 13 pages, 6 figures, to appear in Physical Review A

We analyzed the nonlinear dynamics of pulsed beam self-cleaning in nonlinear tapered Ytterbium doped and Erbium-Ytterbium codoped graded-index multimode optical fibers, with quasi-uniform doping distribution in the core cross-section. By increasing the net gain when operating in active configuration we observed that the output spatial intensity distribution changed from a speckled into a high-quality and bell-shaped beam. By launching pulses in the normal dispersion regime of the taper, from the wider into the smaller core diameter, we generated a supercontinuum emission between 520 nm and 2600 nm. When the laser pulses were launched into the small core side of the tapered fiber or in the Erbium-Ytterbium fiber, self-cleaning was obtained without any self-phase modulation-induced spectral broadening or frequency conversion.

We propose a new semi-analytical model, describing the bandwidth evolution of pulses propagating in dispersion managed (DM) transmission systems using multimodal graded-index fibers (GRIN) with parabolic index. The model also applies to monomodal fiber DM systems, representing the limit case where beam self-imaging vanishes. The model is successfully compared with the direct integration of the (1+1)D nonlinear Schr\"odinger equation for parabolic GRIN fibers, and to experimental results performed by using the transmission of femtosecond pulses over a 5 m span of GRIN fiber. At the high pulse powers that are possible in multimodal fibers, the pulse bandwidth variations produced by the interplay of cumulated dispersion and self-phase modulation can become the most detrimental effect, if not properly managed. The analytical model, numerical and experimental results all point to the existence of an optimal amount of chromatic dispersion, that must be provided to the input pulse, for obtaining a periodic evolution of its bandwidth. Results are promising for the generation of spatio-temporal DM solitons in parabolic GRIN fibers, where the stable, periodic time-bandwidth behaviour that was already observed in monomodal systems is added to the characteristic spatial beam self-imaging.

We observed the generation of a spiral-shaped intensity beams from optical fibers under suitable coupling conditions. In nonlinear regime, we observed rainbow-spiral emission thanks to the generation of supercontinuum in the fiber.

A theory describing experimental beam profiles of multimode fiber Raman lasers is developed: random mode coupling, Kerr self-cleaning and spatial filtering play a decisive role in shaping Stokes beams, whereas depleted pump beams remain insensitive.

Optimization of self-starting in passively mode-locked optical soliton fiber laser resonators results from a complex transient dynamic involving soliton molecules, soliton-based complexes and supramolecular structures, with particle-like properties. By means of time-stretch dispersive Fourier transform based real-time spectroscopy, we identify a new route to the passive mode-locking of fiber laser solitons. A raised relaxation oscillation stage is followed by the generation of shaking-soliton molecular triplets. The relative phase between satellite and main pulses of shaking soliton triplets evolves chaotically, while the two satellite pulses are orthogonal to each other in their state of polarization. These results provide new perspectives into the soliton formation and the internal dynamics of soliton molecules with higher degrees of freedom.

In this paper, a model of an optically active isotropic dielectric, quartz, is determined. Applying the Maxwell equation and the Dirac delta function, we obtain that in a microdisperse medium  0 , the spatial resonance at is added to the time atomic field resonance at  s and the frequency of the spatial resonance s corresponds to the wavelength  , It is found that for the breaking index, the N2  0 frequencyчастота is equal 0 to., and in the interval between 0 and  N2  0 corresponds to the region of absorption by the dielectric of the external electromagnetic field. Moving from frequencies and wavenumbers to wavelengths by the formulas 0  2c /0 , W  2 / kW , defined, for quartz 16 1 0 3.0 10     c ,0  0.06 microns. and 0.02 microns  l  0.41 of microns. Consequently, one-dimensional optical solitons can be formed in such a medium at a given wave length onedimensional optical solitons

Multimode optical fibers are attracting a growing interest for their capability to transport high-power laser beams, coupled with novel nonlinear optics-based applications. However, optical fiber breakdown occurs when beam intensities exceed a certain critical value. Optical breakdown associated with irreversible modifications of the refractive index, triggered by multiphoton absorption, has been largely exploited for fiber material micro-structuration. Here we show that, for light beam intensities slightly below the breakdown threshold, nonlinear absorption strongly affects the dynamics of a propagating beam as well. We experimentally analyze this subthreshold regime and highlight the key role played by spatial self-imaging in graded-index fibers for enhancing nonlinear optical losses. We characterize the nonlinear power transmission properties of multimode fibers for femtosecond pulses propagating in the near-infrared spectral range. We show that an effective N-photon absorption analytical model is able to describe the experimental data well.

We demonstrate dynamical topological phase transitions in evolving Su-Schrieffer-Heeger (SSH) lattices made of interacting soliton arrays, which are entirely driven by nonlinearity and thereby exemplify emergent nonlinear topological phenomena. The phase transitions occur from topologically trivial-to-nontrivial phase in periodic succession with crossovers from topologically nontrivial-to-trivial regime. The signature of phase transition is gap-closing and re-opening point, where two extended states are pulled from the bands into the gap to become localized topological edge states. Crossovers occur via decoupling of the edge states from the bulk of the lattice., Comment: are welcome!

We demonstrate how spatial beam self-cleaning and supercontinuum generation in graded-index multimode optical fibers can be directly applied in multiplex coherent anti-Stokes Raman Scattering (M-CARS) spectroscopy. Although supercontinuum generation causes pump depletion mainly in the center of the beam, the partial recovery of the pump brightness due to self-cleaning may enable self-referenced M-CARS, with no additional delay lines to synchronize pump and Stokes waves. As a proof-of-principle, we report examples of imaging of single chemical compounds and polystyrene beads. The new scheme paves the way towards simpler M-CARS systems based on multimode fiber sources.

Since its first demonstration in graded-index multimode fibers, spatial beam self-cleaning has attracted a growing research interest. It allows for the propagation of beams with a bell-shaped spatial profile, thus enabling the use of multimode fibers for several applications, from biomedical imaging to high-power beam delivery. So far, beam self-cleaning has been experimentally studied under several different experimental conditions. Whereas it has been theoretically described as the irreversible energy transfer from high-order modes towards the fundamental mode, in analogy with a beam condensation mechanism. Here, we provide a comprehensive theoretical description of beam self-cleaning, by means of a semi-classical statistical mechanics model of wave thermalization. This approach is confirmed by an extensive experimental characterization, based on a holographic mode decomposition technique, employing laser pulses with temporal durations ranging from femtoseconds up to nanoseconds. An excellent agreement between theory and experiments is found, which demonstrates that beam self-cleaning can be fully described in terms of the basic conservation laws of statistical mechanics.

We demonstrate broadband resonant supercontinuum generation based on stable bright soliton formation in Si 3 N 4 microresonators with either anomalous or normal dispersion. The frequency combs have an electronically detectable repetition rate of 28 GHz, and span over 2,000 comb teeth.

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.

Beam self-imaging in nonlinear graded-index multimode optical fibers is of interest for many applications, such as implementing a fast saturable absorber mechanism in fiber lasers via multimode interference. We obtain a new exact solution for the nonlinear evolution of first and second order moments of a laser beam of arbitrary transverse shape carried by a graded-index multimode fiber. We have experimentally directly visualized the longitudinal evolution of beam self-imaging by means of femtosecond laser pulse propagation in both the anomalous and the normal dispersion regime of a standard telecom graded-index multimode optical fiber. Light scattering out of the fiber core via visible photo-luminescence emission permits us to directly measure the self-imaging period and the beam dynamics. Spatial shift and splitting of the self-imaging process under the action of self-focusing are also revealed. Funding Agencies|European Research Council (740355); Agence Nationale de la Recherche (ANR-18-CE080016-01); CILAS Company (ArianeGroup); Vetenskapsrådet (2017-05309); Russian Ministry of Science and Education (14.Y26.31.0017)

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.

A two-level iterative algorithm for finding stationary solutions of coupled nonlinear Schrödinger equations describing the propagation dynamics of an electromagnetic pulse in multimode and multicore optical fibers of various structures was developed and tested. Using as an example the proposed analytical soliton solution which is localized in space and time, test calculations were performed, and the convergence of the algorithm was demonstrated.

International audience; We experimentally demonstrate 2D quadratic extreme events, appearing and disappearing without a trace. Besides the spontaneous formation of self-guided bicolor solitary waves, ultrafast spatial switching, cascaded events and pulse reshaping are observed.

We experimentally demonstrate the fission of femtosecond pulses in a GRIN standard fiber, leading to high-energy multimode solitons that undergo huge Raman frequency shifts, and exhibit complex multisoliton dynamics.

We observe and analyze formation, decay, and subsequent regeneration of ring-shaped clusters of (2+1)-dimensional spatial solitons (filaments) in a medium with the cubic-quintic (focusing-defocusing) self-interaction and strong dissipative nonlinearity. The cluster of filaments, that remains stable over ~17.5 Rayleigh lengths, is produced by the azimuthal modulational instability from a parent ring-shaped beam with embedded vorticity l = 1. In the course of still longer propagation, the stability of the soliton cluster is lost under the action of nonlinear losses. The annular cluster is then spontaneously regenerated due to power transfer from the reservoir provided by the unsplit part of the parent vortex ring. A (secondary) interval of the robust propagation of the regenerated cluster is identified. The experiments use a laser beam (at wavelength 800 nm), built of pulses with temporal duration 150 fs, at the repetition rate of 1 kHz, propagating in a cell filled by liquid carbon disulfide. Numerical calculations, based on a modified nonlinear Schr\"odinger equation which includes the cubic-quintic refractive terms and nonlinear losses, provide results in close agreement with the experimental findings., Comment: To be published in Phys. Rev. A

We introduce the concept of distributed Kerr-lens mode-locking in GRIN fiber lasers. Our analysis demonstrates that proper control of spectral and spatial graded dissipation results in the self-emergence of stable dissipative solitons.

All-fiber Raman lasers have demonstrated their potential for efficient conversion of highly multimode pump beams into high-quality Stokes beams. However, the modal content of these beams has not yet been investigated. In this work, based on a mode decomposition technique, we are able to reveal the details of intermodal interactions in the different operation regimes of continuous wave multimode graded-index fiber Raman lasers. We observed that, above the laser threshold, the residual pump beam is strongly depleted in its transverse modes with principal quantum number below 10. However, the generated Stokes signal beam mainly consists of the fundamental mode, but higher-order modes are also present, albeit with exponentially decreasing population.