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L’avènement des lasers à combinaison cohérente de multiples faisceaux est à l’origine d’un changement de paradigme dans le domaine de l’architecture laser. L’opérateur de ce nouveau type de source lumineuse se voit en effet offrir la possibilité d’agir canal par canal sur les caractéristiques des faisceaux lasers individuels afin de façonner la distribution d’énergie optimale du faisceau recombiné requise pour l’application visée. Une telle approche digitale de la mise en oeuvre d’une source cohérente très puissante (GW crête/kW moyen) ouvre un vaste champ applicatif.

International audience; One of the most promising solutions to access high power laser chains is to achieve a coherent combination of a large number of elementary lasers. To interfere constructively, these laser sources should be identical and operate under the same conditions. However, despite these efforts, differential delays appear in the course of time, which must be compensated for. While designing the required correction system, knowing the behavior of a laser as a function of the environmental conditions is not crucial, whereas having access to the differences in the behaviors of identical lasers is, leading to difficulties in modeling. The purpose of this paper is to illustrate how a large set of lasers can be simultaneously analyzed to estimate their variations and optimize a correction system. The X-Coherent Amplified Network laser relies on 61 fiber amplifiers, which are as identical as possible. This state of the art femtosecond digital laser therefore appears as an ideal candidate to study a large number of fiber lasers working under controlled conditions.; L'une des solutions les plus prometteuses pour accéder à des chaînes laser de haute puissance consiste à réaliser une combinaison cohérente d'un grand nombre de lasers élémentaires. Pour interférer de manière constructive, ces sources laser doivent être identiques et fonctionner dans les mêmes conditions. Cependant, malgré ces efforts, des retards différentiels apparaissent au cours du temps, qui doivent être compensés. Lors de la conception du système de correction requis, connaître le comportement d'un laser en fonction des conditions environnementales n'est pas crucial, alors qu'avoir accès aux différences de comportements de lasers identiques l'est, ce qui entraîne des difficultés de modélisation. Le but de cet article est d'illustrer comment un large ensemble de lasers peut être analysé simultanément pour estimer leurs variations et optimiser un système de correction. Le laser XCAN repose sur 61 amplificateurs à fibre, qui sont aussi identiques que possible. Ce laser "numérique" femtoseconde de pointe apparaît donc comme un candidat idéal pour étudier un grand nombre de lasers à fibre fonctionnant dans des conditions contrôlées.

We report on far field energy distribution control using a coherent beam combining femtosecond digital laser employing 61 tiled channels. Each channel is considered as an individual pixel where amplitude and phase are controlled independently. Applying a phase difference between neighboring fibers or neighboring fiber-lines gives high agility for far field energy distribution and paves the way for deeper exploration of phase patterns as a tool to further improve tiled-aperture CBC laser efficiency and far field shaping on demand.

International audience

International audience; Coherent beam combining (CBC) opens the way to a new paradigm in laser architecture [1] . The quest to high peak and average power indeed faces several fundamental limitations (transverse amplified spontaneous emission in wide amplifiers, thermal management issues) which can be addressed when relying on numerous smaller scale reliable lasers coherently combined. Achieving CBC for a large number of amplifiers will not only rely on the use of an efficient phase-locking scheme but also on fiber management inside the laser head (such as accurate angular and lateral positioning, at µm or mrad levels), optics, and amplifiers management as well as laboratory environment control. An accurate diagnosis of the laser array is mandatory in order to get significant feedback on its design, and potentially improve or stabilize its behavior, either in phase-locked loop or in free run.; La combinaison cohérente de faisceaux (CBC) ouvre la voie à un nouveau paradigme dans l'architecture laser [1] . La recherche d'une puissance de crête et d'une puissance moyenne élevées se heurte en effet à plusieurs limitations fondamentales (émission spontanée amplifiée transversale dans les amplificateurs larges, problèmes de gestion thermique) qui peuvent être résolues en s'appuyant sur de nombreux lasers fiables à plus petite échelle combinés de manière cohérente. La réalisation de la CBC pour un grand nombre d'amplificateurs ne dépendra pas seulement de l'utilisation d'un système de verrouillage de phase efficace, mais aussi de la gestion des fibres à l'intérieur de la tête laser (comme le positionnement angulaire et latéral précis, à des niveaux de µm ou de mrad), de la gestion des optiques et des amplificateurs, ainsi que du contrôle de l'environnement du laboratoire. Un diagnostic précis du réseau laser est obligatoire pour obtenir un retour d'information significatif sur sa conception, et potentiellement améliorer ou stabiliser son comportement, que ce soit en boucle à verrouillage de phase ou en fonctionnement libre.

International audience; We report on the coherent beam combining of 61 femtosecond fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beam recombination in the far field, this technique appears suitable for the combination of a large number of fiber amplifiers. The 61 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal micro-lens array. The residual phase error between two fibers is as low as λ/90 RMS, while a combining efficiency of ∼50% is achieved.

International audience; A 61 beamlets Coherent Beam Combination femtosecond laser is used as a programmable phase source to generate high power Orbital Angular Momentum beams and open the path for higher order non-symmetrical user-defined far field distributions.

International audience; We report on the first coherent beam combining of 60 fiber chirped-pulse amplifiers in a tiled-aperture configuration along with an interferometric phase measurement technique. Relying on coherent beams recombination in the far field, this technique appears well suited for the combination of a large number of fiber amplifiers. The 60 output beams are stacked in a hexagonal arrangement and collimated through a high fill factor hexagonal microlens array. The measured residual errors within the fiber array yields standard deviations of 4.2 μm for the fiber pitch and 3.1 mrad for the beam-to-beam pointing, allowing a combining efficiency of 50 %. The phasing of 60 fiber amplifiers demonstrates both pulse synchronization and phase stabilization with a residual phase error as low as lambda/100 RMS.

International audience; we report on the qualification of an all-fiber counter propagating pumped amplifier by using a narrow-linewidth single-frequency nanosecond fiber laser. The proposed amplifier design is tailored to be used for coherent beam combining of fiber amplifiers in tiled-aperture configuration.

High power laser science and engineering 7, e54 (2019). doi:10.1017/hpl.2019.36, Published by Cambridge Univ. Press, Cambridge

International audience; We report on the coherent beam combining of seven fiber chirped-pulse amplifiers using a highly scalable architecture. A combining efficiency of 48% is obtained in linear regime, yielding 71 W pump-limited average power after compression, with a phase noise between two fibers as low as λ/38 RMS. The experiment is operated at high B-integral, with only little impact on the combining efficiency as well as on the temporal and spatial properties of the combined beam.

International audience; We report on the use of a 61 beamlets Coherent Beam Combination femtosecond fiber amplifiers as a digital laser source to generate high power Orbital Angular Momentum beams. Such approach opens the path for higher order non-symmetrical user-defined far field distributions.

International audience; For the last decade, Coherent Beam Combining (CBC) has proven to be a successful method for increasing the output peak and average powers of femtosecond fiber amplifiers [1]. Within the framework of the XCAN project, we aim at the demonstration of the scalability of such laser systems by coherently combining 61 fiber amplifiers. Therefore, we use a highly scalable approach relying on a tiled-aperture geometry along with an interferometric phase measurement and control which can theoretically handle the coherent combination of up to 10 000 beams [2]. After very promising results obtained with 19 passive fibers [3], we present the first results obtained using seven active fibers. Figure 1: (a) Experimentally obtained far field. (b) Combined beam with corresponding M²-values. (c) Measured temporal profile of the combined beam. (d) Corresponding spectrum. The seven beams are arranged in a hexagonal array in the near field, collimated by a microlens array, and the combined beam is found in the far field. Based on a characterization of our experimental fiber array, the maximum efficiency is calculated to be η = 0.56. By filtering the central lobe, we obtain an experimental efficiency of η = 0.48 (fig. 1a,1b), which corresponds to 86 % of the theoretical value stated above. The measured M² of 1.15 on each axis proves a good quality of the combined beam. Finally, the beam is temporally compressed to 244 fs with a time-bandwidth product of 0.55, close to the simulated Fourier transform limit of 0.48 (fig. 1c,1d). The average power of the output beam is measured to be 71 W, corresponding to a compression efficiency of 0.86. The next steps will include the reduction of the pulse repetition rate and the resulting increase in peak power. Finally, the number of beams will be increased to 61 and the spatial and spectral characteristics of the combined beam will be studied.

International audience; We report on the coherent combination of seven fiber amplifiers in the femtosec-ond regime using an interferometric phase measurement technique. A combination efficiencyof 54 % and a compressed pulse length of 225 fs were achieved.

Coherent beam combining in the femtosecond regime of a record number of 19 fibers is demonstrated. The interferometric phase measurement technique, particularly well suited to phase-lock a very large number of fibers, is successfully demonstrated in the femtosecond regime. A servo loop is implemented to control piezoelectric fiber stretchers for both phase and delay variation compensation. The residual phase errors are below λ/60 rms. Nearly 50% of the total energy is contained in the far-field central lobe. After compression, we obtain a combined pulse width of 300 fs identical to the master oscillator pulse width.

The XCAN project, which is a three years project and began in 2015, carried out by Thales and the Ecole Polytechnique aims at developing a laser system based on the coherent combination of laser beams produced through a network of amplifying optical fibers. This technique provides an attractive mean of reaching simultaneously the high peak and high average powers required for various industrial, scientific and defense applications. The architecture has to be compatible with very large number of fibers (1000-10000). The goal of XCAN is to overcome all the key scientific and technological barriers to the design and development of an experimental laser demonstrator. The coherent addition of multiple individual phased beams is aimed to provide tens of Gigawatt peak power at 50 kHz repetition rate. Coherent beam combining (CBC) of fiber amplifiers involves a master oscillator which is split into N fiber channels and then amplified through series of polarization maintaining fiber pre-amplifiers and amplifiers. In the so-called tiled aperture configuration, the N fibers are arranged in an array and collimated in the near field of the laser output. The N beamlets then interfere constructively in the far field, and give a bright central lobe. CBC techniques with active phase locking involve phase mismatch detection, calculation of the correction and phase compensation of each amplifier by means of phase modulators. Interferometric phase measurement has proven to be particularly well suited to phase-lock a very large number of fibers in continuous regime. A small fraction of the N beamlets is imaged onto a camera. The beamlets interfere separately with a reference beam. The phase mismatch of each beam is then calculated from the interferences’ position. In this presentation, we demonstrate the phase locking of 19 fibers in femtosecond pulse regime with this technique. In our first experiment, a master oscillator generates pulses of 300 fs (chirped at 200 ps). The beam is split into 19 passive channels. Prior to phase locking, the optical path differences are adjusted down to 10 μm with optical delay lines. Interferograms of the 19 fibers are recorded at 1 kHz with a camera. A dedicated algorithm is developed to measure both the phase and the delay between the fibers on a measurement path. The delay and phase shift are thus calculated collectively from a single image and piezo-electric fiber stretchers are controlled in order to ensure compensation of time-varying phase and delay variations. The residual phase shift error is below λ/60 rms. The coherent beam combining is obtained after propagation and compression. The combined pulse width is measured at 315fs. A second experiment was done to coherently combine two amplified channels of the XCAN demonstrator. A residual phase shift error of λ/30 rms was measured in this case.

Coherent beam combining of a record number of 19 fibers is demonstrated with 300fs pulses. This result is achieved by controlling both phase and delay variations on each fiber channel in an interferometric measurement setup.

International audience; An experimental study of a static helium gas gap heat switch concept for laser amplification is presented. High single pass gains with large co-sintered ceramic Yb:YAG disks are recorded in the 80-200K temperature range on a diode pumped active mirror amplifier.

Experimental demonstration of the first controlled gradient doped ytterbium YAG crystal boules grown via modified Horizontal Direct Crystallization technique is presented. Yb3+:YAG crystals extracted from these boules exhibit a controlled continuous doping gradient, offering opportunities to explore laser concepts with greatly improved characteristics such as thermal and amplified spontaneous emission management; but also in terms of gain medium volume requirements. The growth process with its specificities compared to known growth techniques used for homogeneously doped crystal growth, is described in detail. The Yb3+ ions distribution within the extracted crystals is quantified. Optical transmission and fluorescence measurements will also be presented.

The Yb:Y3Al5O12/Y3Al5O12 (Yb:YAG/YAG) composite crystals were prepared by thermal bonding method with different technological parameters. The bonding interface of the composite crystals were observed by optical microscope, scanning electron microscope, and atom force microscope. The light scattering experiments for bonding interface of the composite crystals were measured by the laser and transmission spectra. All experiments show that high-quality Yb:YAG/YAG composite crystals without space transition layer and light scattering on the bonding interface can be obtained by thermal bonding method under appropriate technological parameters.

Yb:Y3Al5O12 (Yb:YAG) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 25 at.%, 50 at.%, 100 at.% and Yb:YAlO3 (Yb:YAP) single crystals with Yb doping concentration 0.5 at.%, 5 at.%, 15 at.%, 30 at.% were grown by the Czochralski process. The fluorescence spectra of these crystals and the effects of self-absorption on the shape of the fluorescence spectra were studied. Through comparing the fluorescence spectra of Yb:YAG and Yb:YAP, all results indicate that the effects of self-absorption on the fluorescence spectra of Yb:YAP are remarkably stronger than that of Yb:YAG at the same Yb concentration. (c) 2006 Elsevier B.V. All rights reserved.

The XCAN collaboration program between the Ecole Polytechnique and Thales aims at developing a laser system based on the coherent combination of several tens of laser beams produced through a network of amplifying optical fibers [1]. As a first step this project aspires to demonstrate the scalability of a combining architecture in the femtosecond regime providing high peak power with high repetition rate and high efficiency. The initial system will include 61 individual phased beams aimed to provide 10 mJ, 350 fs pulses at 50 kHz.

A comprehensive experimental benchmarking of Yb(3+):YAG crystalline and co-sintered ceramic disks of similar thickness and doping level is presented in the context of high average power laser amplifier operation. Comparison is performed considering gain, depolarization and wave front deformation quantitative measurements and analysis.

Gains as high as 20 for a 77mm ceramic Yb:YAG disks are recorded in the 80-200K temperature range on a laser amplifier cooled through a static low pressure helium gas cell operated in the 1 to 10 Hz regime.

Infrared (1.2-1.6 mu m) luminescence in a ytterbium aluminium garnet (YbAG) crystal, doped with Cr (0.05 at.%) ions, was investigated under CW laser diode pumping (lambda = 940 nm). The Cr4+ emission band was observed with its peak at 1.34 mu m and measured to be about 1.3 times with respect to Yb3+ IR luminescence (lambda = 1.03 mu m). We demonstrate that for the excitation wavelength of 940 nm Yb3+ ions act as sensitizers of the B-3(2)(T-3(2))-B-3(1)((3)A(2)) emission of Cr4+ ions. This crystal is promising as a high-efficient system for tunable laser (1.2-1.6 mu m) output. (c) 2004 Elsevier B.V. All rights reserved.

In this paper, we present results on upconversion luminescence performed on Yb3+-doped yttrium aluminum garnets under 940 nm excitation. The upconversion luminescence was ascribed to Yb3+ cooperative luminescence and the presence of rare earth impurity ions. The cooperative luminescence spectra as a function of Yb concentration were measured and the emission intensity variation with Yb concentration was discussed. Yb3+ energy migration quenched the cooperative luminescence of Yb:YAG crystals with doping level over 15 at%. (c) 2004 Elsevier B.V. All rights reserved.

Single pass gain of 77mm crystal and ceramic Yb:YAG disks are compared in the 100-200K temperature range. Experiments are performed on a laser amplifier cooled through a static low pressure helium gas cell.

International audience

We have begun building the Mercury laser system as the first in a series of new generation diode-pumped solid-state lasers for inertial fusion research. Mercury will integrate three key technologies: diodes, crystals and gas cooling, within a unique laser architecture that is scalable to kilojoules and megajoule energy levels for fusion energy applications. The primary near-term performance goals include 10% electrical efficiencies at 10 Hz and 100 J with a 2–10 ns pulse length at 1.047 μ m wavelength. When completed, Mercury will allow rep-rated target experiments with multiple chambers for high energy density physics research.

We have begun building the “Mercury” laser system as the first in a series of new generation diode-pumped solid-state lasers for inertial fusion research. Mercury will integrate three key technologies: diodes, crystals, and gas cooling, within a unique laser architecture that is scalable to kilojoule and megajoule energy levels for fusion energy applications. The primary near-term performance goals include 10% electrical efficiencies at 10 Hz and 100J with a 2–10 ns pulse length at 1.047 μm wavelength. When completed, Mercury will allow rep-rated target experiments with multiple chambers for high energy density physics research.

If further developments in high-field lasers are to be accessible to universities and institutes, new laser materials and phase control techniques, which will result in compact, reliable systems with higher peak power, must be adopted. The choice of high-saturation-fluence gain material and the measurement and active control of temporal and spatial phase distortions for compact chirped-pulse amplification (CPA) systems of the future are discussed. Using the proper material and phase control a focused intensity of 10/sup 25/ W/cm/sup 2/ is theoretically possible.

HiPER (High Power laser Energy Research) is a European project aiming at exploring laser inertial fusion. An overview of the program with its time schedule is presented while emphasis is given on the laser driver studies.

The Lucia laser chain is a Diode Pumped Solid State Laser system based on Yb3+ doped YAG disks used in an active mirror scheme. Front-end and amplifier stages are presented with recent energetic performances (14 J / 2 Hz) achieved with improved pumping and extraction architectures. Emphasis is given on the crucial role of ASE and thermal mitigation considerations in engineering the amplifier head.

13.7 J-8 ns pulses were extracted at 2 Hz in four passes from Lucia active mirror Yb3+: YAG diode pumped laser amplifier. A second amplifier head relying on cryogenic cooling will allow reaching the 30 J level.

We will present growth process and laser experimental studies on Yb3+:YAG single crystals exhibiting controlled distribution of the doping ions in the matrix. A 3 at%/cm gradient was obtained with Horizontal Direct Crystallization method.

Demonstrating ignition and net energy gain in the near future on MJ-class laser facilities will be a major step towards determining the feasibility of Inertial Fusion Energy (IFE), in Europe as in the United States. The current status of the French Laser MégaJoule (LMJ) programme, from the laser facility construction to the indirectly driven central ignition target design, is presented, as well as validating experimental campaigns, conducted, as part of this programme, on various laser facilities. However, the viability of the IFE approach strongly depends on our ability to address the salient questions related to efficiency of the target design and laser driver performances. In the overall framework of the European HiPER project, two alternative schemes both relying on decoupling target compression and fuel heating—fast ignition (FI) and shock ignition (SI)—are currently considered. After a brief presentation of the HiPER project's objectives, FI and SI target designs are discussed. Theoretical analysis and 2D simulations will help to understand the unresolved key issues of the two schemes. Finally, the on-going European experimental effort to demonstrate their viability on currently operated laser facilities is described.

We illustrate the benefits of a thin, low pressure helium cell for efficient and safe heat removal in cryogenically-cooled active mirror laser amplifiers operating in the [100 J-1 kJ]/[1-10 Hz] range. A homogeneous gain medium temperature distribution averaging 160 K is obtained with a sub-mm helium-filled gap between the gain medium and a copper plate at 77 K. A significant degree of flexibility for tuning the temperature in the amplifier can be achieved by varying the pressure of the helium gas in the 10(2) to 10(5) Pa range.

We discuss the deformation of a partially pumped active mirror amplifier as a free standing disk, as implemented in several laser systems. We rely on the Lucia laser project to experimentally evaluate the analytical and numerical deformation models.

A major challenge the HiPER project is facing is to derive laser architectures satisfying simultaneously all HiPER requirements; among them, high wall-plug efficiency (~ 10%) and repetition rate (5 to 10 Hz) are the most challenging constraints. The active mirror Yb:YAG amplifier proposal from LULI is described.

With careful Amplified Spontaneous Emission and thermal management, 6.6 Joules 7 ns pulses were extracted at 2 Hz in four passes from Lucia active mirror Yb:YAG Diode Pumped Laser amplifier. 15% optical to optical efficiency was achieved.

A time resolved experiment on the lowest lying states of barium fluoride has been performed by applying laser resonance methods in the near infrared and green-blue wavelength regions. The A2Π and B2Σ+ states have been examined using Ti:sapphire laser excitations in the 710–850 nm wavelength region. The C2Π state was examined using the 496·5 nm Ar+ laser line and the lifetime of this state was in accordance with earlier measurements. The lifetimes obtained for the BaF molecule were: A2Π3/2, 46·1 ± 0·9 ns; B2Σ+, 41·7 ± 0·3 ns.

Amplified spontaneous emission (ASE) in large size, high gain Yb(3+):YAG slabs severely impacts the gain/energy storage capability. We will discuss numerical simulations and experimental results obtained on large size Yb(3+):YAG slabs. The spatial distribution and temporal evolution is shown under different conditions.

We present the latest investigations performed on the LUCIA Yb:YAG diode pumped oscillator (260 mJ, 49 ns, 2 Hz). Thermal distortions as well gain measurements have been explored.

We will present a three-dimensional Monte-Carlo model calculating the impact of amplified spontaneous emission on large size, high gain quasi-three level laser materials, especially Yb3+:YAG and compared it to experimental results, currently under progress.

Wave-front correction and focal spot improvement of femtosecond laser beams have been achieved, for the first time to our knowledge, with a deformable mirror with an on-line single-shot three-wave lateral shearing interferometer diagnostic. Wave-front distortions of a 100-fs laser that are due to third-order nonlinear effects have been compensated for. This technique, which permits correction in a straightforward process that requires no feedback loop, is also used on a 10-TW Ti:sapphire-Nd:phosphate glass laser in the subpicosecond regime. We also demonstrate that having a focal spot close to the diffraction limit does not constitute a good criterion for the quality of the laser in terms of peak intensity.