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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] .

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.

Multimode optical fibers (MMFs) are an excellent alternative to single-mode fibers for applications where high power level is required or for investigating spatiotemporal nonlinear wave propagation [1] . Recent experiments have shown that it is possible to achieve a speckleless output beam in MMF through a novel nonlinear effect called Kerr-induced beam self-cleaning (KBSC) [2] . This effect has been observed in a wide variety of MMFs, including standard graded-index (GRIN) MMFs and photonic crystal fibers (PCFs). Here, we report on the observation of low-peak-power KBSC and supercontinuum generation (SCG) in a few-mode PCF with a GRIN core.

The development of all-fiber format sources delivering high-energy few-cycle pulses at high repetition rate in the near-infrared takes advantage of the availability of telecom-grade optical fibers and components [1] . Similar development in the middle-wave infrared (mid-IR) spectral domain has remained, however, an elusive goal, although such sources are of high significance for applications such as high-precision frequency metrology and molecular spectroscopy [2] . Here, we demonstrate that picosecond pulses at 2 μm from a MHz repetition rate fiber laser can trigger the formation of frequency-shifted solitons up to 2.95 μm with 50 nJ energy and 86 fs duration in a cascade of Large Mode Area (LMA) silica and fluoride fibers. The instantaneous power of the 9-cycle pulse is 580 kW, which represents tenfold improvement compared to previously reported sub-100 fs pulses generated at this wavelength via soliton self-frequency shift from near-infrared lasers [3] , [4] .

Recent experiments have shown that nonlinear wave propagation in multimode optical fibers leads to complex spatio-temporal phenomena. In this talk, we introduce new approaches for the control and optimization of nonlinear beam reshaping in the spatial, temporal and spectral dimensions. The first approach applies to spatial beam self-cleaning the technique of transverse wavefront shaping, which permits to launch an optimized input mode combination, that results in the stable generation of a whole nonlinear mode alphabet at the fiber output. The second approach introduces a longitudinal tapering of the core diameter of multimode active and passive fibers, which permits to generate ultra-wideband and high brightness supercontinuum light, featuring high spatial beam quality.

We overview recent advances in the spatio-temporal nonlinear dynamics of optical pulses propagating in multimode optical fibers. The Kerr effect leads to spatial beam self-cleaning in a graded-index multimode optical fiber, followed by sideband series generation spanning multiple octaves. Effectively single mode supercontinuum spanning from the visible to the mid-infrared was also demonstrated. Enhancement of Kerr beam self-cleaning was observed in active fiber with quasi-step index profile. Moreover, mutual self-cleaning was recently reported for both the fundamental and the second harmonic beams in optically poled multimode fibers with cubic and quadratic nonlinearity.

International audience; The second harmonic signal at the output of an optically poled fiber was studied as a function of the input fundamental frequency power and polarization state. For an injected average power of 3 mW at 1064 nm, the measured output power at 532 nm was 10 μW. The second harmonic intensity profile was a bright green ring even if the pump was injected in the fundamental HE11 mode: we believe that the second harmonic was carried by a combination of the EH11 and HE31 modes.

Ultra Wide Band (UWB) antenna arrays offer the possibility to generate a wide band signal in a sharp direction, which can drastically improve the detection sensitivity. With a configuration including as many antennas as generators, an UWB array presents the advantage of increasing the radiation power on one hand and offering the agility to the array on the other hand. Indeed, the application of time delays between the feeding pulses of the antennas permits to steer the radiated fields in each wanted direction and to realize a coherent sum of each initial power. However, a major difficulty with such a configuration is the minimization of the radiation source jitters, to obtain the best synchronization possible. A solution consists of using pulsed optoelectronic devices, operating in linear switching regime, which permits to bypass this difficulty and to obtain ultra-short electrical waveforms with small temporal jitters (2ps typically). Many applications such as transient radar cross section (RCS) measurements, UWB synthetic aperture radar (SAR) and high power UWB radiation source can be developed by using such systems. In this paper, the design of an innovative autonomous scanning radiation system will be approached. The first experimental results will be also presented. These results are essentially focused on the high voltage pulsed power source which is previously described. This source aims at driving the pulsed optoelectronic devices. It consists of a series-connected IGBTs component.

In this communication, we present a novel hybrid Q-switched laser source designed to emit short pulses with high peak power and low timing jitter at increased repetition rate, for LIDAR, CARS microscopy or flow cytometry applications.

Summary form only given. Dispersive wave (DW) generation in microstructured optical fibers (MOFs) is a hotly debated topic in supercontinuum generation. DWs are a typical outcome of strong nonlinear wave interactions, however their spectral positions are mainly ruled by the linear dispersive properties of the fiber: DWs appear at those spectral positions where wavevectors of DWs and soliton-like pulses are matched [1]. In this work we experimentally and numerically discuss how super-bright sideband generation around 1550 nm can be achieved in dual concentric core MOF (DCC-MOF) with long pulses typically delivered by subnanosecond lasers working in the near infrared. Strong conversion efficiencies (up to 30% of the injected input power) were observed thanks to the exotic guiding properties of these DCC-MOFs [2]. Differently from conventional MOFs, whose dispersion is in general anomalous in the optical communications band, the linear coupling of DCC-MOF drastically changes the guiding properties of the fundamental mode, thus causing an abrupt variation of the group delay between the pumping and the DW generation spectral regions. In this work we emphasize the role of group velocity difference between DWs and solitons. For this reason we have carried out a time-resolved spectral analysis of the supercontinuum generated by a 4-m long DCC-MOF by combining a diffractive grating and a single-slit giving a spectral resolution of 6 nm, and two 12.5 GHz bandwidth photodiodes. The laser pump was a microchip laser with pulse duration of 900 ps at 1064 nm and 4 kW peak power. We show on the left panel of Fig. 1 the pulse-width measured at different spectral regions (finest temporal structures cannot be unveiled with this method). We observe a negative time delay for the pulse center of mass when moving the 6 nm spectral window toward the infrared side, which qualitatively agrees well with the theoretical group delay and it is confirmed by our numerical predictions. In our DCC-MOF the DW overtakes the supercontinuum between 1064 nm and 1200 nm, in striking contrast with the case of single-core conventional MOFs, where the infrared part of the spectrum is in the trailing edge of the pulse envelope. Another key point discussed in the present work is the role of pump wavelength and pulse duration. On the top right panel of Fig. 1 we show the spectrum obtained with the 1064 nm microchip laser: an intense and spectrally wide DW around 1575 nm is easily recognized. Whereas the spectrum in the bottom right panel was obtained with a mode-locked laser emitting 120 ps pulses at 1030 nm. The pulse peak power had the same order of magnitude in both cases. In this case the DW is shifted towards 1505 nm, and its maximum spectral density is about 20 dB lower than in the previous case. Numerical simulations confirm the formation of the DW starting from the solitons bunch generated by the quasi-CW pump pulse break-up.

Intended for the next generation of optical access networks, OCDMA is of great interest to meet the demand of increasing the number of users per access fiber, especially as spectral phase coding increases its performance in the optical domain. This, however, requires handling broad spectra and short pulses, which are best dealt with using opto-electronic or all-optical devices instead of slower electronics. Among others, we demonstrate spectral-phase-coded OCDMA using a fiber-based saturable absorber as thresholding in the receiver.

We take advantage of nonlinear effects in a low birefringence fiber to reshape subnanosecond laser pulses. Both polarization and spectral modulation allow pulse shortening by a factor higher than 10 in the normal dispersion regime.

In this article, the generation of a visible ps polarized-supercontinuum in a new highly birefringent fiber was demonstrated together with the mechanisms involved.

This work has built a compact broad-band femtosecond spectrometer based on a probe light supercontinuum generated in a birefringent photonic crystal fiber. The near-ultraviolet spectrometer system is based on a Ti:sapphire oscillator with extended cavity operating at 27 MHz and delivering 40 fs pulses of 15-18 nJ energy.

The laboratory XLIM has developed an impulse optoelectronic ultra-wide band antenna array for UWB radar applications. This paper presents this system in which photoconductive switches generating high voltage monopolar pulses and UWB antennas are associated. The control of the releasing delay of the sources via optic provides to sum the radiation power and to steer the transient radiation beam accurately. The experiments realized with this optoelectronic array validate these two concepts. We also propose another way of improvement of these systems by using bipolar pulse generators.

We present the integration of deformable micro-mirror in switchable optical systems. The goal is to associate laser optics with the high and inexpensive replication of MEMS to create compact laser sources delivering intense pulsed radiation

We propose an ultra wide band radar system based on a coherent emission of an ultra-wide band antenna array using photoconductive switching devices. The triggering process is obtained by excitation of semiconductor samples in linear mode using a picosecond laser source. The emitting antenna system and the receiving antenna developed by our research laboratory, present some specific qualities suitable for radiation and measurement of ultra-short pulses

We studied numerically and experimentally an ultrafast passive nonlinear temporal reshaping mechanism based on spatial soliton emission in Ti periodically poled lithium niobate waveguides combined with spatial filtering. We demonstrated efficient temporal reshaping of 4-ps pulses at 1548 nm.

Synchronization of actively Q-switched Erbium and Ytterbium doped fiber lasers with a single electrostatically actuated deformable metallic micro-mirror is reported. Sum frequency generation in PPLN crystal leading to a tunable red radiation is demonstrated.

We demonstrated active Q-switching of an erbium-doped fiber laser by using an electrostatically actuated deformable metallic micro-mirror. Q-switch operation has been demonstrated with frequencies up to 120 kHz and short pulses down to 300 ns.

We observed spatial addressing of light by bidirectional spatial emission of beams at a quadratically nonlinear interface in periodically poled lithium niobate waveguides. The interface consisted of the boundary between two quasi-phase-matched regions with different poling period. We show the intensity and phase-mismatch (temperature or wavelength) dependence of the phenomena.

We report on modulational instability process in large normal dispersion regime leading to supercontinuum generation at visible wavelengths (350-700 nm) in air-silica microstructured optical fibers.

We experimentally and numerically studied, in the picosecond regime, a fast nonlinear saturable absorber based on quadratic spatial trapped propagation in a Ti:PPLN waveguide at 1550 nm combined with spatial filtering.

We experimentally and numerically studied the excitation of quadratic spatially self-trapped beams in a Ti:PPLN film waveguide in presence of large group velocity mismatch.

We experimentally and numerically investigated spatial switching and steering of a soliton beam in periodically poled LiNbO/sub 3/ (PPLN) film waveguides. We demonstrated the operation of an ultra-fast spatial switch based on the non collinear excitation at a repetition rate of 125 Gbit/s.

We proposed and demonstrated the use of a self-imaging confocal (SIG) cavity to obtain high efficiencies in a simple way with diode-pumped solid-state lasers. The mode of this cavity fits the pumped volume at best, and optimizes the efficiency. The same SIC resonator permits unusual mode profile selection thanks to the use of proper spatial filterings. These ones occur in two intracavity planes for which the transverse fields are connected by a Fourier transform. There exist SIC resonators without internal lens exhibiting the same properties.

Multimode optical fibers (MMFs) have recently regained interest because of the degrees of freedom associated with their different eigenmodes. In the nonlinear propagation regime in particular, new phenomena have been unveiled in graded-index (GRIN) MMFs such as geometric parametric instabilities and Kerr beam self-cleaning [1, 2]. The speckled pattern observed at the output of the MMF at low powers, is transformed at high powers into a bell-shaped beam close to the fundamental mode. Recent work has also demonstrated that Kerr beam self-cleaning can lead to a low-order spatial mode, different from a bell-shape, by adjusting the laser beam in-coupling conditions [3]. An attractive way to systematically control the spatial excitation conditions at the fiber input is provided by the use of a spatial light modulator (SLM) which permits to profile the beam wavefront entering the MMF. In most cases, experiments involving adaptive optics consider linear propagation through scattering plates or MMFs [4]. So far, few works have dealt with the nonlinear propagation regime[5, 6].