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We introduce and experimentally demonstrate the concept of all-optical beam switching in graded-index multimode optical fibers. Nonlinear coupling between orthogonally polarized seed and signal beams permits to control the spatial beam quality at the fiber output. Remarkably, we show that even a weak few-mode control beam may substantially enhance the quality of an intense, highly multimode signal beam. We propose a simple geometrical representation of the beam switching operation, whose validity is quantitatively confirmed by the experimental mode decomposition of the output beam. All-optical switching of multimode beams may find important applications in high-power beam delivery and fiber lasers.

We analyze the stability and dynamics of dissipative Kerr solitons (DKSs) in the presence of a parabolic potential. This potential stabilizes oscillatory and chaotic regimes, favoring the generation of static DKSs. Furthermore, the potential induces the emergence of new dissipative structures, such as asymmetric breathers and chimera-like states. Based on a mode decomposition of these states, we unveil the underlying modal interactions.

We demonstrate a new practical approach for generating multicolour spiral-shaped beams. It makes use of a standard silica optical fibre, combined with a tilted input laser beam. The resulting breaking of the fibre axial symmetry leads to the propagation of a helical beam. The associated output far-field has a spiral shape, independently of the input laser power value. Whereas, with a high-power near-infrared femtosecond laser, 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. Our method is independent of the laser source wavelength and polarization. Therefore, standard optical fibres may be used for generating spiral beams in many applications, ranging from communications to optical tweezers and quantum optics.

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.

Pioneering works on multimode fiber transmission [1,2], dated 40 years ago, predicted the existence of multimode solitons, providing conditions for the temporal trapping of the input optical modes to form a spatiotemporal soliton [3-5]. Only recently [6-8], multimode solitons were experimentally investigated in graded-index multimode fibers (GRIN), unveiling the complexity of a new, uncharted field. In our work, we experimentally and numerically investigated the propagation of ultrashort pulses over long distances of GRIN fiber. We discovered a new class of spatiotemporal solitons with surprising properties: basically single-mode, they cannot be described by the variational theory; their pulsewidth and energy are independent of the input pulse duration, and appear to depend only on the fiber dispersive parameters and, therefore, the wavelength. The new solitons are promising for the delivery of high-energy laser beams, for high-power spatiotemporal mode-locked multimode fiber lasers, and for high-bit rate multimode fiber networks.

It has been recently demonstrated that multimode solitons are unstable objects which evolve, in the range of hundreds of nonlinearity lengths, into stable single-mode solitons carried by the fundamental mode. We show experimentally and by numerical simulations 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. Controlling the generation of solitons in multimode photonic systems may boost the performance of optical communication networks. Here, the conditions under which an experimentally observed walk-off multimode soliton is generated are revealed.

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.

Experimental and numerical studies of spatiotemporal femtosecond soliton propagation over up to 1 km spans of parabolic graded-index fibers reveal that initial multimode soliton pulses naturally and irreversibly evolve into a single-mode soliton. This is carried by the fundamental mode of the fiber, which acts as a dynamical attractor of the multimode system for up to the record value (for multimode fibers) of 5600 chromatic dispersion distances. This experimental evidence invalidates the use of variational approaches, which intrinsically require that the initial multimode propagation of a self-imaging soliton is indefinitely maintained.

Spatial self-imaging, consisting of the periodic replication of the optical transverse beam profile along the propagation direction, can be achieved in guided wave systems when all excited modes interfere in phase. We exploited material defects photoluminescence for directly visualizing self-imaging in a few-mode, nominal singlemode SMF-28 optical fiber. Visible luminescence was excited by intense femtosecond infrared pulses via multiphoton absorption processes. Our method permits us to determine the mode propagation constants and the cutoff wavelength of transverse fiber modes.

Beam self-imaging of ultrashort pulses in nonlinear graded-index (GRIN) multimode optical fibers is of interest for many applications, including spatiotemporal mode-locking in fiber lasers. We obtained a new analytical description for the nonlinear evolution of a laser beam of arbitrary transverse shape propagating in a GRIN fiber. The longitudinal beam evolution could be directly visualized by means of femtosecond laser pulses, propagating in the anomalous or in the normal dispersion regime, leading to light scattering out of the fiber core via the emission of blue photo-luminescence. As the critical power for self-focusing is approached and even surpassed, a host of previously undisclosed nonlinear effects is revealed, including strong multiphoton absorption by oxygen-deficiency center defects and Germanium inclusions, splitting and shifting of the self-imaging period, filamentation, and conical emission of the guided light bullets. We discovered that nonlinear loss has a profound influence on the process of high-order spatiotemporal soliton fission. The beam energy carried by the fiber is clamped to a fixed value, and nonlinear bullet attractors with suppressed Raman frequency shift and fixed temporal duration are generated, leading to highly efficient frequency conversion of the input near-infrared femtosecond pulses into mid-infrared multimode 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 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.

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.

The 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. By using this time-resolved mapping, we provide the first unambiguous experimental observation of instantaneous intrapulse nonlinear coupling processes among the modes of a graded index fiber., 4 pages, 6 figures

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 experimentally demonstrate spatial beam self-cleaning in an Yb-doped graded-index multimode fiber taper, 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 counterdirectional configuration. The Kerr effect permits to obtain high-beam quality amplification with no accompanying frequency conversions. As a result, our nonlinear taper amplifier may provide an important building block for multimode fiber lasers and amplifiers.

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.

The process of high-energy soliton fission is experimentally and numerically investigated in a graded-index multimode fiber. Fission dynamics is analyzed by comparing experimental observations and simulations. A novel nonlinear propagation regime is 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.

Shedding light on nonlinear loss occurring close to multimode optical fiber damage threshold, we demonstrate that multiple photons are absorbed by material defects and upconverted to shorter wavelength luminescence. An array of blue emitting points results from the spatial self-imaging in graded-index fibers.

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.

The dynamics of high-energy Raman solitons in graded-index multimode fibers is both numerically and experimentally investigated. The propagation of high-power pulses produces nonlinear losses, that quench up to 80% of the fiber transmission. In such a regime, several solitons arising from the fission of ultra-short femtosecond pulses manifest unique features: pulse width, Raman self-frequency shift and soliton order remain nearly constant over a broad range of energies.

A new generation of inhomogeneous nonimaging optical concentrators is proposed, able to achieve simultaneously high optical efficiency and acceptance solid angle at a given geometrical concentration factor. General design methods are given, and concentrators are numerically investigated and optimized.

We report a theory on the four wave mixing that permits to evaluate the signal degradation in WDM systems with a bandwidth located at the zero dispersion wavelength of the fiber link, taking into account the pulse duration of the signal. Numerical simulations confirm this theory showing how systems based on 16×2.5 and 16×10 Gbit/s can operate also at the zero dispersion wavelength if the pulse overlapping is limited by the use of suitable values of the pulse duration and of the delay among the input signals located at different frequencies.

The observation of a dragging interaction in a glass waveguide, between two self-trapped beams of orthogonal polarisation, is reported. A numerical investigation on dragging is also presented. The two beams enter a planar waveguide at a small angle between each other, having Gaussian overlapping transverse profiles and different powers. At solitonic powers they trap each other, and the strong pump beam is dragged by the weaker signal beam, even in the presence of strong linear losses. The interaction is phase-insensitive, and is promising for implementing an all optical NOT gate.

We propose and study a novel dispersion management method that permits the propagation of soliton pulses with a very low time jitter. The scheme assumes a chromatic dispersion that on average decreases with a 1/z law, where z is the propagation distance. The advantage of this method consists in the fact that at the end of the link the mean chromatic dispersion is very small and as a consequence the jitter can be deeply reduced, while at the beginning of the link the chromatic dispersion is high, hence the system can operate with a large input power that permits a good signal to noise ratio. The scheme is simply implemented by periodically compensating the chromatic dispersion with fiber pieces having different lengths.

A new optical modulation format is described, based on the simultaneous modulation of the amplitude and the phase of an optical signal. The proposed modulation format offers significant advantages in wavelength-division-multiplexing transmission, offering high spectral efficiency values, high extinction ratio, and requiring electronics with reduced bandwidth.

Spatial solitons in planar glass structures can create reconfigurable guiding channels where low-intensity light bits can propagate. By using 3-ps-long pulses the solitonic regime has been reached and, within this solitonic channel, a picosecond signal at the typical intensities of the fibre telecommunications was carried. At the output the carried signal had the same size of the solitonic channel, keeping its hyperbolic secant shape.

A good agreement has been found between numerical and experimental investigation of a 20-Gb/s dispersion-managed soliton transmission system on standard fiber. More than 6 dB of power margin over 30 nm has been obtained at 1100 km by improving the dispersion slope compensation and the prechirping.

Summary form only given. Dispersion management (DM) techniques to periodically compensate for fiber chromatic dispersion, /spl beta//sub 2/, has been shown to be a useful strategy for increasing the capacity of optical communication systems. We show that optimal propagation conditions can be obtained by simultaneously minimizing nonlinear distortion on single pulses and nonlinear interactions among the pulses. As a result, a 40 Gbit/s soliton signal can be transmitted over a record distance of 1300 km in a link encompassing step-index fibers, with 100 km amplifier spacing and GVD compensation by gratings. Theoretically, minimization of nonlinear distortion can be obtained by analyzing the pulse bandwidth variation along the link.