Search

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; 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.

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 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.

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.

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.

With ongoing efforts in Europe, the United States, and Asia on power production through inertial fusion, intense work has been focused on proposing and studying Diode Pumped Solid State Lasers (DPSSLs). Such drivers should be able to deliver 1 to 10 kJ at a repetition rate in the 10-Hz range and a wall-plug efficient nearing 10%. Recent achievements will be presented with emphasis on 100 J-class prototypes, which are currently being built.

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.

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.

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.

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 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.

The aim of the LUCIA program (laser ultra-court intense and applications) is to set up a diode pumped solid state laser chain resulting in a pulse duration in the order of nanoseconds, up to 100 J per pulse and a repetition rate of 1 to 10 Hz. This chain consists of an oscillator, called "Pilote LUCIA" (P.L.) and two amplifiers with Yb:YAG directly pumped by laser diodes. Possible future applications for LUCIA are laser-matter interactions in the field of physics or surface treatment for industrial purposes.

The Lucia laser will deliver a 100 Joules-10 ns-10 Hz pulse train from ceramic/crystal Yb:YAG using 4400 QCW diode laser bars (440 Joules). Petwatt-class laser chain scheme using Lucia as a pump source is described.

We present the current status of the Lucia laser being built at the LULI laboratory, the national civil facility for intense laser matter interaction in France. This diode pumped laser will deliver a 100 Joules, 10 ns, 10 Hz pulse train from Yb:YAG using 4400 power diode laser bars. We first focus on the amplifier stage by describing the reasons for selecting our extraction architecture. Thermal issues and solutions for both laser and pumping heads are then described. Finally, we emphasize more specifically the need for long-lifetime high-laser-damage-threshold coatings and optics.

A diode-pumped Yb:YAG laser has been demonstrated. A V-shape unstable resonator with a Super Gaussian coupling mirror was chosen. We describe the model that permits to choose the parameters of the cavity and predict the laser performances. A diode pumping architecture is used in which 941 nm radiation is homogenously delivered to the laser crystal. We present here the Pumping Delivery Optics and the laser performances.

Multiple-wave achromatic interferometric techniques are used to measure, with high accuracy and high transverse resolution, wave fronts of polychromatic light sources. The wave fronts to be measured are replicated by a diffraction grating into several copies interfering together, leading to an interference pattern. A CCD detector located in the vicinity of the grating records this interference pattern. Some of these wave-front sensors are able to resolve wave-front spatial frequencies 3 to 4 times higher than a conventional Shack-Hartmann technique using an equivalent CCD detector. Its dynamic is also much higher, 2 to 3 orders of magnitude.

The LUCIA (French acronym for "Laser Ultra-Court Intenses, et Applications") laser system consists in an oscillator delivering around 1 Joule in about 10 ns, followed by a multipass two-heads amplifier stage. In our design, oscillator and amplifier stages are longitudinally pumped at 10 Hz with respectively 15 and 10 kW/cm/sup 2/ intensity with 1 ms pulses, allowing close to 20% optical efficiency. The diode pumped Yb:YAG lasers are particularly good candidate for such an application, because centimeter-scale size aperture YAG crystal are easily available, because of reduced heat dissipation. it is concluded that the pulsed solid state lasers able to deliver large energy in the nanosecond regime with a high repetition rate, 10 Hz at least, will be the basic component for the new generation of high energy lasers. These characteristics lead indeed to diode pumped devices and rare-earth doped media based materials.

We demonstrate the wave-front correction of the LULI 100TW, 300 fs/30 J high power laser facility for a sequence of shots. Excellent beam focusability close to diffraction limit has been obtained using an adaptive optic system, composed of a large aperture dielectric coated deformable mirror and a home-made shearing interferometer. This correction allows to produce reproducible focal spots with Strehl ratios close to 0.9 at a repetition rate of a shot every 20 minutes, despite of wave front distortions generated by cumulative thermal effects in the large disc amplifiers.

With the goal to set up a high average power Diode Pumped Solid State Laser (100 Joules/10 Hz/10 ns), the Laboratory for Use of Intense Laser (LULI) is now studying various solutions concerning the amplifier medium, the cooling, the pumping and the extraction architectures. In this paper, we present the last states of these developments and the solutions already chosen.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We have implemented on one beam of the LULI six-beam high-energy (6×100 J, 1 ns) Nd:glass laser facility a closed-loop Adaptive Optics (AO) system to compensate for thermal distortions onto the wave front. Using the AO system composed of a dielectric coated deformable mirror and of a wave front sensor, we are able to improve the wave front quality in order to obtain a focal spot close to the diffraction limit. This allows not only to improve the reproducibility of the experiments but also to increase by at least two orders of magnitude the peak intensity as compared with what usual laser smoothing techniques can achieve.

Summary form only given. Ultra intense laser systems with peak power in the 10 TW-1 PW range are difficult to focus close to the diffraction limit because of spatial phase aberrations. Reaching this goal is an important task when trying to get the highest possible intensity (typically >10/sup 20/ w/cm/sup 2/). In this paper, we describe an adaptive optics (AO) technique which has significantly improved the wavefront quality of the LULI 100 TW CPA Nd:glass laser. This AO system relies on a feed-back loop coupling a wavefront sensor and a wavefront corrector.

This paper summarises our recent progress achieved in the characterisation and understanding of the Ni‐like Ag transient X‐ray laser pumped under traveling wave irradiation. We carried out two experiments at the LULI CPA laser facility. Several diagnostics of the plasma emission at the XRL wavelength or in the keV range indicate the presence of small‐scale spatial structures in the emitting XRL source. Single‐shot Fresnel interference patterns at 13.9 nm were successfully obtained with a good fringe visibility. For the first time we obtained plasma images with a high spatial resolution about 1 μm, showing the effects of pumping parameters on the X‐ray laser far‐field.

We have developed a high-resolution programmable adaptive-optic device based on an optically addressed liquid-crystal electro-optic valve controlled by an achromatic three-wave lateral shearing interferometer. We apply this phase-only filter and loop to shape the far-field pattern of laser beams. As a first application, we theoretically compute and experimentally verify the focus along a line longer than tens of Rayleigh ranges.

Rare Earth doped YAG crystals with controlled variable doping, are laser gain materials recently attracting much attention due to the unique properties it offer in terms of laser engineering. It is very important to perform an accurate and reliable doping spatial distribution measurement in order to be capable to fully map the doping values in the obtained material. We therefore cross-evaluated most common methods. We also mapped doping distribution in a Bagdasarov grown Yb:YAG crystal.

This paper summarizes our recent progress achieved in the characterization and understanding of the Ni-like Ag transient x-ray laser pumped under traveling wave irradiation. At the Rutherford Laboratory CPA laser facility, we measured the temporal history of the 13.9 nm laser pulse with a high-resolution streak camera. A very short, approximately 2 ps x-ray laser pulse was directly demonstrated for the first time. More recently we carried out an experiment at the LULI CPA laser facility. Several diagnostics that recorded the plasma emission at the XRL wavelength or in the keV range indicate the presence of small-scale spatial structures in the emitting XRL source. Single-shot Fresnel interferograms at 13.9 nm were successfully obtained with a good fringe visibility. Strong lasing was also observed on the Ni-like 4f-4d line at 16 nm.

We show wave front correction of a 300 fs/60J laser pulse serie. This correction is based on an optically addressed liquid crystal optical valve (OASLM) which induces high resolution phase modulations. When performed before complete thermal relaxation of the laser Nd:glass amplifiers, this correction allows to increase the system repetition rate by a factor three.

Initial measurements are reported for the Mercury laser system, a scalable driver for rep-rated high energy density physics research. The performance goals include 10% electrical efficiency at 10 Hz and 100 J with a 2-10 ns pulse length. This laser is an angularly multiplexed 4-pass gas-cooled amplifier system based on image relaying to minimize wavefront distortion and optical damage risk at the 10 Hz operating point. The efficiency requirements are fulfilled using diode laser pumping of ytterbium doped strontium fluorapatite crystals.

The scope of the Mercury Laser project encompasses the research, development, and engineering required to build a new generation of diode-pumped solid-state lasers for Inertial Confinement Fusion (ICF). The Mercury Laser will be the first integrated demonstration of laser diodes, crystals, and gas cooling within a scalable laser architecture. This report is intended to summarize the progress accomplished during the first three years of the project. Due to the technological challenges associated with production of 900 nm diode-bars, heatsinks, and high optical-quality Yb:S-FAP crystals, the initial focus of the project was primarily centered on the R&D in these three areas. During the third year of the project, the R&D continued in parallel with the development of computer codes, partial activation of the laser, component testing, and code validation where appropriate.

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 1005 with a 2-10 ns pulse length at 1.047 {micro}m wavelength. When completed, Mercury will allow rep-rated target experiments with multiple chambers for high energy density physics research.

Summary form only given.Gas-cooled, diode-pumped, Yb:crystal lasers are envisioned to be the next-generation inertial confinement fusion (ICF) solid state laser system producing high energy per pulse at modest rep-rates. Innovative solutions for building ICF lasers with high repetition rate and efficiency include: trading the flashlamps for large, low-cost laser diode arrays; using Yb:crystals for greater energy storage and thermal conductivity than Nd:glass; and employing near-sonic helium for cooling of the laser slabs.

Recent experiments, performed at the C.E.A./Limeil-Valenton P102 laser facility on the Ni-like transient collisional scheme, are reported in this paper. They mainly aimed at enhancing the efficiency and improving the optical properties of the already demonstrated 4d J equals 0/4p J equals 1 Ag 19+ x-ray laser at 13.9 nm. The now classical 2- stage traveling-wave irradiation of slab targets was used, the illumination sequence being constituted of a long (600 ps) low-flux (0.5 - 11 J) laser pulse followed (200 ps later) by a short (< 1 ps) high intensity (1 - 20 J) one. The work novelty was the use of frequency-doubled pulses, either for the pre-forming or the pumping one. Various combinations ((omega) -(omega) , 2(omega) -(omega) , (omega) - 2(omega) ) have been investigated in terms of lasing performances. High gains, around 34/cm, have been measured and saturation achieved for target lengths above 4 mm. A strong enhancement, up to a few (mu) J, of the x-ray laser output has been observed, due to traveling-wave irradiation method, while the emission duration was decreased to less than 10 ps, resulting in a 300 kW source. Moreover, under specific laser conditions, a second lasing line at 16 nm was detected. Finally, the possibility of cavity operating transient collisional x-ray lasers has been demonstrated.