Matrix formulation of the Gaussian expansion of coherent multiple beams in arbitrary dimensions.

Modeling the propagation of beams along laser beamlines is very challenging due to the multidimensional and multiscale configuration of the problem. Spatio-temporal couplings are particularly difficult to address with conventional numerical methods. Here we derive the Wigner function of a sum of Gaussian beams by calculating the multidimensional Fourier transform of the intercorrelation function of the fields. The matrix formulation allows for a simple propagation of the Wigner function in the framework of matrix optics. The relevancy of this approach is assessed by applying this model to one-dimensional and multidimensional configurations and by studying the influence of spatio-temporal couplings when considering propagation and dispersion by a diffraction grating.

Wigner matrix formalism for phase-modulated signals.

Laser beams can carry multi-scale properties in space and time that ultimately impact their quality. The study of their evolution along complex optical sequences is of crucial interest, especially in high-intensity laser chains. For such analysis, results obtained with standard numerical methods strongly depend on the sampling. In this paper, we develop an analytic model for a sinusoidal phase modulation inside a sequence of first-order optics elements based on the Wigner matrix formalism. A Bessel decomposition of the Wigner function gives pseudo-Wigner functions that obey the general ABCD matrix law transformation without approximations and sampling considerations. Applied to a Gaussian beam, explicit expressions are obtained for the projections of the Wigner function in the sub-spaces and give a powerful tool for analyzing the laser beam properties. The formalism is established in the spatial and temporal domains and can be used to evaluate the impact of the phase noise on the beam properties and is not limited to small modulation depths. For the sake of illustration, the model is applied to the Talbot effect with the analysis of the propagation in the spatial and phase-space domains. A comparison with full numerical calculations evidences the high accuracy of the analytic model that retrieves all the features of the diffracted beam.

Implications of laser beam metrology on laser damage temporal scaling law for dielectric materials in the picosecond regime.

We report on the implications that the temporal and spatial beam metrologies have on the accuracy of temporal scaling laws of Laser Induced Damage Threshold (LIDT) for dielectric materials in the picosecond regime. Thanks to a specific diagnostic able to measure the temporal pulse shape of subpicosecond and picosecond pulses, we highlight through simulations and experiments how the temporal shape has to be taken into account first in order to correctly understand the temporal dependency of dielectrics LIDT. This directly eases the interpretation of experimental temporal scaling laws of LIDT and improves their accuracy as a prediction means. We also give numerically determined benchmark temporal scaling laws of intrinsic LIDT for SiO2 (thin film) based on the model developed for this work. Finally, we show as well what kind of spatial metrology is needed during any temporal scaling law determination to take into account potential variations of the spatial profile.

Nonlinear FM-AM conversion due to stimulated Brillouin scattering.

We report an effect potentially harmful occurring in regenerative amplifiers due to stimulated Brillouin scattering (SBS). Most high energy laser facilities use phase-modulated pulses to prevent the transverse SBS effect in large optical components and to smooth the focal spot on target. However, this kind of pulse format may undergo a detrimental effect known as frequency modulation to amplitude modulation (FM-AM) conversion in the presence of spectral distortions. In the present letter, we show experimentally and numerically, that SBS can also potentially be created in the regenerative amplifier located in the front-end. In this scenario, some of the side bands of the pulse reflected by regen end-cavity mirror may act as a seed for SBS in an optical component, if the pulse spectrum contains frequency components exactly separated by the Brillouin frequency shift. This self-seeded SBS induces amplitude modulation with a nonlinear dependence that may be detrimental during down-stream propagation. However, we show that a careful choice of the modulation frequencies can mitigate this effect.

A powerful tool for comparing different test procedures to measure the probability and density of laser induced damage on optical materials.

The determination of the laser damage resistance of optics in the nanosecond regime is based on statistical tests and approaches because the response of the components is mainly related to the presence of defects randomly distributed in the optics and is therefore probabilistic in nature. For practical reasons, the tests are mostly carried out with beams of small dimensions (several tens of micrometer), that make it possible to determine a damage probability from which a laser damage threshold is extracted. This threshold is, however, highly correlated with the size of the test beam and the sampling of the test procedure. Some measurements are also made with beams of large dimensions (several millimeters) from which a damage density is determined. However, the relationship between the damage probability and the damage density is not trivial. It is based on assumptions that are difficult to verify because the experimental validations are carried out on different laser installations. In order to study accurately the coherence between these tests with small and large beams, as well as the link between damage probability and damage density, it is necessary to perform measurements on the same laser installation. We propose here, to compare for the first time, the results obtained with the same laser source with a large beam and also with small beams. The small beams are shaped from phase objects specifically implemented to obtain several small beams from a single larger beam. The consistency of the laser damage that results from both sets of measurements is demonstrated here. It validates the assumptions made and the specific mathematical treatment implemented to establish the link between the two approaches. In fine, it also validates and strengthens the approach previously developed from the rasterscan procedure [Lamaignère et al., Rev. Sci. Instrum. 78, 103105 (2007)] used to measure damage densities from the scanning of optics with beams of small dimensions. The reported original work based on phase objects thus makes it possible to replicate small beam tests with a large beam facility. The comparison between the results from the small beams and the results from the large beam experiments definitively makes the link between damage probabilities and damage densities. This also shows that small beam tests are reasonable representative of tests carried out with large beams.

Biphasic lung injury during Streptococcus pneumoniae infection in a murine model.

OBJECTIVES: Streptococcus pneumoniae is the leading cause of community-acquired pneumonia. We aimed to analyze the epithelial response to S. pneumoniae-induced lung injury. METHODS: Using an in vitro model with 16HBE cells and experimental in vivo murine model of acute lung injury, we analyzed the epithelial response to S. pneumoniae. Lung epithelial cell monolayers were exposed to S. pneumoniae and permeability was assessed by transepithelial resistance (TER) measurement and organization and expression of junction proteins. Functional consequences were studied with an in vivo murine model measuring alveolar permeability, distal alveolar fluid clearance (DAFC), and the alveolar inflammatory response. RESULTS: In vitro, S. pneumoniae induced a dose-dependent decrease in transepithelial resistance, which was associated with significant modifications in the organization of junction proteins assessed by immunofluorescence staining and expression after 6hours of exposure. In vivo, S. pneumoniae induced a transient increase in alveolar permeability with an adequate increase in DAFC 6hours post infection. In a second phase, a permanent increased permeability was associated with a major decrease in DAFC. CONCLUSION: Overall, the epithelial response to S. pneumoniae followed a biphasic pattern with an initial reversible increase in permeability related to the alteration of tight and adherens junctions and a second phase associated with an epithelial injury with a major increase in permeability with a decreased DAFC reflecting an injured alveolar capillary barrier.

Modeling of the petawatt PETAL laser chain using Miró code.

Miró software has been used intensively to simulate the Laser Megajoule (LMJ) with the treatment of amplification, frequency conversion, and both temporal/spatial smoothing of the beam for nanosecond pulses. We show that the software is able to model most relevant aspects of the petawatt PETAL laser chain in the subpicosecond regime, from the front-end to the focal spot with a broadband treatment of the amplification and compression stages, including chromatism compensation in the laser chain, segmentation and recombination of the beams on the second compression stage, and focusing by an off-axis parabola.

1.15 PW-850 J compressed beam demonstration using the PETAL facility.

The Petawatt Aquitaine Laser (PETAL) facility was designed and constructed by the French Commissariat à l'énergie atomique et aux énergies alternatives (CEA) as an additional PW beamline to the Laser MegaJoule (LMJ) facility. PETAL energy is limited to 1 kJ at the beginning due to the damage threshold of the final optics. In this paper, we present the commissioning of the PW PETAL beamline. The first kJ shots in the amplifier section with a large spectrum front end, the alignment of the synthetic aperture compression stage and the initial demonstration of the 1.15 PW @ 850 J operations in the compression stage are detailed. Issues encountered relating to damage to optics are also addressed.

Experimental demonstration of a synthetic aperture compression scheme for multi-Petawatt high-energy lasers.

We present the experimental demonstration of a subaperture compression scheme achieved in the PETAL (PETawatt Aquitaine Laser) facility. We evidence that by dividing the beam into small subapertures fitting the available grating size, the sub-beam can be individually compressed below 1 ps, synchronized below 50 fs and then coherently added thanks to a segmented mirror.

Direct spectral phase measurement with spectral interferometry resolved in time extra dimensional.

The complete spectral characterization of ultrashort pulses is demonstrated with a new diagnostic called Spectral Interferometry Resolved in Time Extra Dimensional. This method, based on spectral shearing interferometry, is self-referenced and self-calibrated. It yields directly to an interferogram pattern displaying an intuitive representation of the derivative of the spectral phase. No iterative algorithm is needed for phase measurement making this method suitable for real time and easy characterization. This technique is highlighted by the spectral phase characterization of pulses out of a folded nondispersive line and the pulse shape is compared with a trace recorded with an intensity autocorrelator.

Chromatism compensation of the PETAL multipetawatt high-energy laser.

High-energy petawatt lasers use series of spatial filters in their amplification section. The refractive lenses employed introduce longitudinal chromatism that can spatially and temporally distort the ultrafast laser beam after focusing. To ensure optimum performances of petawatt laser facilities, these distortions need to be corrected. Several solutions using reflective, refractive, or diffractive optical components can be addressed. We give herein a review of these various possibilities with their application to the PETAL (Petawatt Aquitaine Laser at the Laser Integration Line facility) laser beamline and show that diffractive-based corrections appear to be the most promising.

Delay interferometric single shot measurement of a petawatt-class laser longitudinal chromatism corrector.

In this paper we present a self-referenced interferometric single-shot measurement technique that we use to evaluate the longitudinal chromatism compensation made by a diffractive lens corrector. A diffractive lens with a delay of 1 ps is qualified for a 60 mm beam aperture. This corrector was implemented on the Alisé Nd:glass power chain. We qualify the corrector and the Alisé power chain chromatism, demonstrating the potential of this measuring principle as well as the interest of diffractive lenses to correct longitudinal chromatism of petawatt-class lasers.

Synthetic aperture compression scheme for a multipetawatt high-energy laser.

High-energy petawatt lasers using the chirped-pulse amplification technique require meter-sized gratings to limit the beam fluence on the surface of the grating. An alternative, studied by many groups, is a mosaic grating consisting of smaller, coherently added gratings. We propose what we believe to be a new compression scheme consisting of beam phasing instead of grating mosaic phasing. This synthetic aperture compression scheme allows us to control the beam thanks to a unique segmented mirror equipped with three degrees of freedom. With this configuration, the beam is divided into small subapertures adapted to the classical grating size. After compression, these subapertures are coherently added before the focusing stage. Therefore the alignment processes are simplified.

3D numerical model for a focal plane view in case of mosaic grating compressor for high energy CPA chain.

An important issue, mosaic grating compressor, is studied to recompress pulses for multiPetawatt, high energy laser systems. Alignment of the mosaic elements is crucial to control the focal spot and thus the intensity on target. No theoretical approach analyses the influence of compressor misalignment on spatial and temporal profiles in the focal plane. We describe a simple 3D numerical model giving access to the focal plane view after a compressor. This model is computationally inexpensive since it needs only 1D Fourier transforms to access to the temporal profile. We present simulations of monolithic and mosaic grating compressors.

Kerr-like nonlinearity induced via terahertz generation and the electro-optical effect in zinc blende crystals.

A model is proposed to account for Kerr-like nonlinearity induced by femtosecond pulses via terahertz generation and electro-optical effect. This phenomenon, so far overlooked, is evidenced in a zinc blende single crystal with a heterodyne optical Kerr effect setup. The spectral evolution of this phenomenon as well as its noninstantaneous response character are reported. Its competition with a third-order optical Kerr effect is demonstrated.

Diode-pumped regenerative amplifier delivering 100-mJ single-mode laser pulses.

We report on a side-pumped Nd:phosphate laser regenerative amplifier that delivers laser pulses of as much as 100 mJ in a single TEM mode. The laser beam is mode matched to the amplification medium by an intracavity fused-silica phase plate for mode shaping and a telescope for adjustment of the beam mode to the amplification rod section such that most of the energy stored in the rod is transferred to the laser pulses. As a result of the good overlap and the low loss, an optical-to-optical conversion efficiency of as much as 10% was measured for a pumping current of 80 A and greater than 100-mJ output pulses.

All-optical programmable shaping of narrow-band nanosecond pulses with picosecond accuracy by use of adapted chirps and quadratic nonlinearities.

We experimentally demonstrate pure optical pulse picosecond shaping of narrow-bandwidth nanosecond pulses. The method used is based on the manipulation in the spectral domain of strongly chirped femtosecond pulses and on the use of either frequency addition or frequency difference.

Ultrashort, intense ultraviolet pulse generation by efficient frequency tripling and adapted phase matching.

We demonstrate efficient frequency tripling of 1057-chirped pulses, using adapted chirping and thick KDP crystals. These millijoules broadband pulses at 352 nm have been compressed to 220-fs duration by use of a UV grating-pair compressor. The technique is scalable to kilojoule petatwatt lasers.