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 a far field energy distribution and paves the way for deeper exploration of phase patterns as a tool to further improve the efficiency achievable with tiled-aperture/filled-aperture setups and far field shaping on demand.
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 λ/100 RMS.
The XCAN project aims at the coherent combination of 61 fiber amplifiers in the femtosecond regime. An important intermediate step towards this goal is the implementation of a seven fiber test setup, which allows to address key scientific and technical challenges which might occur in the scaled version of 61 fibers. This work includes the design and characterization of a support unit able to hold 61 fibers with the high precision required for an efficient coherent combination in tiled aperture configuration. This configuration, in combination with an interferometric phase measurement and active phase control, is particularly well suited for the coherent combination of a very large number of beams. Our first preliminary results with seven fibers include a combination efficiency of 30 % and a residual phase error between two fibers as low as λ/40 rms. Experiments conducted with three fibers in order to evaluate technical improvements revealed an increase of efficiency to 54 %. The combined beam was temporally compressed to 225 fs, which is Fourier transform limited with respect to the measured spectrum.
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.
More than 10 Joules at 2 Hz were recently obtained from the LUCIA laser system based on diode-pumped Yb:YAG
active mirrors. This achievement is the result of careful management of both Amplified Spontaneous Emission and
thermal effects in laser amplifiers. Future developments including a cryogenically-cooled active mirror are also
presented.
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.
We present in this paper the actual status of the LUCIA project, a high average power diode-pumped solid-state laser chain capable to deliver 100 J in nanosecond regime at 10 Hz. In a first step, we deal with the choice of the amplifier medium and the pump and extraction architecture. We present after the thermal management and the cooling architecture. Then, we investigate the damage threshold required. We present last the performances already obtained and the improvements we are working out.
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.
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.
A simple, ultra-compact, four wave achromatic interferometric technique is used to measure with high accuracy and high transverse resolution wavefront of polychromatic lightsource. The wave front to be measured is replicated by a diffraction grating into four copies interfering together leading to an interference pattern very similar to the intensity distribution obtained in the focal plane of a Shack-Hartmann microlens array. The grating is made of optical glass modulated in depth on top of which a chromium mask is printed. The amplitude mask acts like a Hartmann plate. Used in association with the phase mask, it allows suppression of the unwanted zero and second orders. A CCD detector located in the vicinity of the grating records the interference pattern. This new wavefront sensor is able to resolve wavefront spatial frequencies 3 to 4 times higher than a conventional Shack-Hartmann technique using an equivalent CCD detector. Its dynamic is also much higher.
We have implemented on the LULI 100 TW laser facility a closed-loop Adaptive Optics system composed of a large aperature dielectric coated deformable mirror and a home-made shearing interferometer. Significant results have been obtained with this system: not only the beam focusability has been improved to a Strehl ratio of 0.9 but it has also been kept at this level for a sequence full-energy shots, with a repetition rate of 20 minutes, through systematic compensation of the cumulative thermal load generated in the large disk amplifiers.
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.
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.
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 energy levels for fusion energy applications. The primary performance goals include 10 percent electrical efficiencies at 10 Hz and 100J with a 2-10 ns pulse length at 1.047 micrometers wavelength. When completed, Mercury will allow rep-rated target experiments with multiple target chambers for high energy density physics research.
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 Ag19+ 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.
High energy intense lasers are difficult to focus close the diffraction limit because of phase-front aberrations induced in the optics and amplifiers. This may be an issue in applications where the pulses need to be focused near the diffraction limit such as in ultra-intense femtosecond laser-matter interactions. We describe here a new technique that we have implemented and which significantly improves the focal spot quality by correction the wave front even from highly distorted beam. This correction method is based on an adaptive optical technique using an optically addressed light valve and an achromatic three wave lateral sheering interferometer. Results with strongly aberrated beams focused close to the diffraction limit are presented as well as with fluence as high as 100 mJ/cm2.
Space aberration effects which arise in high energy or in high average power laser chains are important parameters to control in order to emit a beam quality close to the diffraction limit. For that purpose we present recent experiments using an original adaptive and programmable module allowing the spatial control of the beam amplitude and the correction of the phase distortions due to the optical components and the gain media of the laser chain. Beam shaping is achieved by an optically addressed photoconductor-liquid crystal light valve. The light valve is addressed in the blue-green spectral range by incoherent projection of a VGA liquid crystal display. This adaptive optics module controls either the amplitude or the phase of near infrared laser beams depending on the liquid crystal operating mode. The other specific characteristics of the module will be detailed: no spurious diffraction effects, up to 10(pi) phase excursion and tri-lateral wavefront sensor. Experimental results of compensation of aberrations introduced on different laser beams will also be presented.
In order to correct wave front distortions, a technique has been developed based on adaptive optics used in a pre- compensation configuration. The system consists of a wave front sensor and an optically driven liquid crystal Spatial Light Modulator as a wave front corrector. The main advantage of the sensor is its ability to detect phase deformation of several tens of wavelengths with an accuracy around (lambda) /10. This approach appears to be a good candidate for phase shaping ultra-intense laser system exhibiting strong wave front distortions. The use of a optically addressed single large liquid crystal cell covering the entire beam aperture makes this device diffraction free. The choice of a pre-compensation geometry is discussed and preliminary results are presented.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.