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International audience; For the first time, a Zn1.1Ga1.8Ge0.1O4 transparent spinel ceramic has been fully densified by spark plasma sintering. XRD measurements show that this ceramic is composed of a pure cubic spinel phase. SEM analysis revealed a homogeneous and dense microstructure with the average grain size being 200 ± 100 nm. The transmittance of these fine-grained ceramics reached 70 % in the visible range and is very close to 80 % at 2 µm, thus close to the Tmax value deduced from the measurement of the refractive index. The ceramics exhibit excellent mechanical properties with a Young modulus of 222 GPa, a Vickers hardness of 14.25 GPa and a thermal conductivity of 7.3 W.m−1. K−1. By doping with Cr3+ ions, transparent Zn1.1Ga1.8Ge0.1O4 ceramics present both a red luminescence and a long-lasting afterglow during several minutes. Moreover, a near infrared broadband emission at 1.3 µm is also achieved with Ni2+ ions.

We report the experimental demonstration of efficient second-harmonic generation by splicing optically poled fiber segments. A device made from five segments each 20 cm-long exhibits, at a fundamental average power of 4.2 mW, a maximum increase of 5.5 in the conversion efficiency with respect to a single 20 cm poled fiber, corresponding to a conversion efficiency of 5.4%. For any considered fundamental power, the conversion efficiency at the output of a five-segment device is larger than the efficiency of a single poled fiber having the same length.

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 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.

International audience; In this article, a source for generating conducted intentional electromagnetic interference with a mesoband spectrum is presented in order to perform electromagnetic susceptibility tests on electronic equipment. This high-voltage source is based on different optoelectronic generators, which integrate photoconductive semiconductor switches (PCSSs) operating in a linear switching mode and triggered by a laser pulse. These optoelectronic devices generate a damped sinusoidal signal with a center frequency between 20 and 200 MHz, a Q factor between 11 and 12.4, and a maximum peak-to-peak magnitude of 3.12 kV for a bias voltage of 4 kV. An optoelectronic generator model was designed using Advance Design System (ADS) software, and experimental tests were performed with a measurement bench to compare simulated and measured results. A study on a voltage increase of these generators has been performed and has established the experimental limits of generation in planar technology with low-cost PCSS available on the shelf. The highest peak-to-peak magnitude value obtained is 6.10 kV, it is possible, thanks to the design and manufacture of a power combiner, with ADS and CST Suite Studio software tools, to set up a multiway combination.

Here we present the ability of Nd3+-doped zinc-phosphate glasses to be shaped into rectangular core fibers. At first, the physico-chemical properties of the developed P2O5-based materials are investigated for different concentrations of neodymium oxide and core and cladding glass compositions are selected for further fiber development. A modified stack-and-draw technique is used to produce multimode large rectangular-core optical fibers. Self-guided nonlinear effects acting as spatial beam reshaping processes occurring in these newly-developed photonic structures lead to the generation of spectral broadenings in the visible and near-infrared spectral domains. Graphic Abstract

In this article, a damped sinusoids high-voltage source based on the optoelectronic waveform generators principle is presented to perform electromagnetic susceptibility testing. Photoconductive semiconductor switches (PCSS), operating in linear switching mode and triggered via a laser pulse, have been used. The developed conducted source generates damped sinusoids with a center frequency between 30 and 600 MHz, a maximum peak-to-peak amplitude of approximately 3 kV for a bias voltage of 4 kV, a Q factor between 8 and 11, and a transient shape closest to a reference damped sinusoid. The dimensions of the source go from 220 mm × 80 mm to 220 mm × 510 mm. A PCSS characterization has been set up to determine the necessary parameters for the establishment of a damped sinusoids optoelectronic generator model on Keysight Advanced Design System software. Experimental tests have been realized using a measurement bench to compare the obtained and simulated signals.

In this paper, a new design of an optoelectronic system for transient waveforms shaping, with a selection of rejected frequencies, is presented. The generation of short pulses is performed with several optoelectronic generators using photoconductive semiconductor switches (PCSS) operating in linear switching mode and triggered via a laser pulse. With the appropriate number of generators, the system generates a spectrum ranging from 300 MHz to 3 GHz, with rejected frequencies at 900 MHz and 1.8 GHz. A PCSS characterization has been set up to determine the necessary parameters for the establishment of an optoelectronic generator model on Keysight Advanced Design System software (ADS). Experimental tests have been realized using measurement benches to compare the simulated and measured signals.

We experimentally demonstrate 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. As the beam energy grows larger, the output beam from the crystal evolves from a highly speckled intensity pattern into a single, bell-shaped spot, sitting on a low energy background. We demonstrate that quadratic beam cleanup is accompanied by significant self-focusing of the fundamental beam, for both positive and negative signs of the linear phase mismatch close to the phase-matching condition.

We report on all-fiber construction of ultrafast mid-infrared sources with up to 500 kW peak power at MHz repetition rate. We illustrate the potential of the sources by generating supercontinuum in chalcogenide fibers.

International audience; We implement machine learning to optimize spectro-temporal properties of supercontinuum generation during nonlinear fiber propagation. In particular, we numerically study how the suitable adjustment of femtosecond pulse patterns can be leveraged to maximize the output spectral intensities and temporal waveforms at wavelengths relevant for multi-photon imaging.

Recent experimental results have reported the observation of beam self-cleaning or, more generally, nonlinear beam reshaping in active multimode fibers. In this work we present a numerical analysis of these processes, by considering the ideal case of a diode-pumped signal amplifier made of a graded-index multimode fiber with uniform Yb doping. Simulations confirm that beam cleaning of the signal may take place even in amplifying fibers, that is the absence of beam energy conservation. Moreover, we show how the local signal intensity maxima, which are periodically generated by the self-imaging process, may influence the population inversion of the doping atoms, and locally saturate the amplifier gain., Comment: 10 pages, 14 figures

We report on high harmonics generation in zinc oxide thin-films driven by an all-solid fiber-based architecture. The laser delivers 41 nJ 70 fs solitonic pulses at 1764 nm and drives harmonics generation up to H7.

Multi-photon microscopy has played a significant role in biological imaging since it allows to observe living tissues with improved penetration depth and excellent sectioning effect. Multi-photon microscopy relies on multi-photon absorption, enabling the use of different imaging modalities that strongly depends on the properties of the sample structure, the selected fluorophore and the excitation laser. However, versatile and tunable laser excitation for multi-photon absorption is still a challenge, limited by e.g. the narrow bandwidth of typical laser gain medium or by the tunability of wavelength conversion offered by optical parametric oscillators or amplifiers. As an alternative, supercontinuum generation can provide broadband excitationspanning from the ultra-violet to far infrared domains and integrating numerous fluorophore absorption peaks, in turn enabling different imaging modalities or potential multiplexed spectroscopy. Here, we report on the use of machine learning to optimize the spectro-temporal properties of supercontinuum generation in order to selectively enhance multi-photon excitation signals compatible with a variety of fluorophores (or modalities) for multi-photon microscopy. Specifically, we numerically explore how the use of reconfigurable (femtosecond) pulse patterns can be readily exploited to control the nonlinear propagation dynamics and associated spectral broadening occurring in a highly-nonlinear fiber. In this framework, we show that the use of multiple pulses to seed optical fiber propagation can trigger a variety of nonlinear interactions and complex propagation scenario. This approach, exploiting the temporal dimension as an extended degree of freedom, is used to maximize typical multi-photon excitations at selected wavelengths, here obtained in a versatile and reconfigurable manner suitable for imaging applications., Comment: Submitted version - Under review

We demonstrate the characterization of nonlinear noise-driven dynamics with ultra-high sensitivity, resolution, and a theoretically unlimited dynamic range using a novel approach of dispersive Fourier transform-based measurements and multiple single photon detectors.

Fiber-based sources delivering high-energy few-cycle pulses at high repetition rates are currently being developed in the near-infrared spectral range, thanks to the wide availability of telecommunication-grade optical fibers and components. Similar sources in the middle-wave infrared (mid-IR) spectral domain, however, are scarce, although such sources are of high interest for applications such as high-precision frequency metrology and molecular spectroscopy or as a seed source to reach further into the mid-IR via coherent nonlinear processes. Here we report on the design of a fiber-based source of 50-nJ energy 90 fs duration pulses up to 2950 nm, corresponding to 500 kW peak power. To obtain this level of peak power we exploit multi-solitonic fission and soliton self-frequency shift in large mode area fibers excited by picosecond pulses emitted at 2 µm from a megahertz repetition rate fiber laser. We leverage mature silica-based fiber technology up to 2.4 µm and restrict the use of fluoride fiber to the very last frequency-shifting stage. The level of instantaneous power and ultra-short duration achieved in this Letter pave the way to all-fiber format generation of an ultra-broadband coherent continuum in the mid-IR with profound implications for applications such as high-resolution molecular spectroscopy and imaging.

International audience; Understanding the mechanism at play during the partial crystallization of a parent glass remains crucial for controlling the optical properties of the final glass-ceramics. In this work, we study the crystallization of bismuth borotellurite glasses, where a specific investigation on the 60TeO2–20B2O3–20Bi2O3 composition is reported. Under adapted heat treatment conditions, highly transparent glass-ceramics can be obtained: the crystallization of the unique anti-glass Bi2Te4O11 phase is evidenced by X-ray diffraction and Raman spectroscopy data confirm its disordered nature. While the quenched glass appears homogeneous, the observation of the early stages glass-ceramic samples by transmission electron microscopy reveals the formation of isolated polycrystalline Bi2Te4O11 entities scattered in a predominant glassy matrix. However, longer heat-treatment of samples induce some chemical demixtion of the residual glass matrix, where two separate amorphous regions of a different composition coexist. The resulting material is finally constituted of the aforementioned Bi2Te4O11 polycrystalline clusters dispersed within a majority of regions with a Te/Bi ratio larger than the nominal 1.5 ratio, separated by tiny “venules” strongly impoverished in tellurium and also likely containing boron element. Photoluminescence properties of Eu3+-doped samples indicate that tiny spectral and temporal modifications happen with the crystallization, reflecting the persistent disordered surrounding of the rare-earth ions. © 2022

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] .

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] .

We performed label-free imaging of human-hair medulla using multi-modal nonlinear optical microscopy. Intra-medulla lipids (IMLs) were clearly visualized by ultra-multiplex coherent anti-Stokes Raman scattering (CARS) spectroscopic imaging. Two groups of IMLs were found: second harmonic generation (SHG) active and inactive. By combining SHG analysis with CARS, the two groups were identified as free fatty acids and wax esters, respectively.

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.

Among the various imaging methods, the multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) technique has received a special attention from the researchers. Indeed, its capability to provide quickly, simultaneous information about various molecular vibrations is of high interest. Over the past years M-CARS microscopy has typically relied on using a supercontinuum, which is generated in photonics crystals fibers (PCFs) with the cubic nonlinearity. This technique requires an additional optical delay line to temporally synchronize the pump and the Stokes pulses [1] . Significant simplification could be introduced by exploiting other nonlinear processes that allows to generate ultra-broadband spectrum without temporal desynchronization between the pump and the supercontinuum pulses. In this work we demonstrate three direct, self-referenced M-CARS systems that do not require any delay line, by using different nonlinear components such as a PPLN crystal (χ (2) and χ (3) ); a YAG:Nd crystal (χ (3) ) and a GRIN multimode fiber (χ (3) ) under the Kerr beam self-cleaning regime [2] .

Optical poling is a very simple technique for inducing a quadratic nonlinearity in germanium-doped silica fibers [1] , [2] . By injecting into the fiber fundamental frequency (FF) pulses together with a low power second harmonic (SH) seed, a multi-photon quantum interference process naturally generates a quadratic nonlinearity whose period satisfies the quasi-phase matching (QPM) condition. The nonlinear grating writing process takes a few hours before showing a saturation of the SH conversion efficiency, which is always lower than the best values achievable by resorting to thermal poling [3] . Also, the maximum length of the poled region exhibits saturation, and this hampers the SH generation [4] .