Multiphysics model of liquid-cooled Nd:phosphate split-slabs in large aperture optical amplifiers

High repetition rates in high energy solid-state laser systems can yield to a rise of temperature in amplifiers despite the use of cooling systems. This effect can significantly impact the performance of amplifiers by inducing thermal stress, birefringence or thermal lensing. Here, we develop a multiphysics model to support the design, optimization and commissioning of a liquid-cooled large aperture split-slab laser glass amplifier. This multiphysics model includes optical pumping in the amplifying medium, heat loading, hydraulic effects induced by the liquid coolant, mechanical deformation and their potential coupled effects on the optical wavefront. The accuracy of each model is assessed by carrying out specific experimental measurements and characterizations. We show that this set of models allows the prediction of performance of a liquid-cooled amplifier from the flash-lamp emission to the amplified wavefront at a repetition rate of one shot per minute.

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.

Relations between subsurface damage depth and surface roughness of grinded fused silica

Relationships between subsurface damage (SSD) depth and peak to valley surface roughness (Rt) have been widely studied and present a major interest for an easy assessment of the SSD depth. We look at the relation between SSD depth and other surface roughness parameters using the Abbott-Firestone curve on a large campaign of grinding tests (with different abrasive grain size, grinding speed and grinding mode). The results reveal that Abbott-Firestone parameters are better for an assessment of SSD depth and that relationships between SSD depth and surface roughness are not universal but depend on the grinding process.

Mixed metal dielectric gratings for pulse compression

We report on manufacturing and testing results of high efficiency mixed metal dielectric gratings (MMLD) for high power pulse compression applications. The gratings with 1780 l/mm are etched in the top low index layer of a Au-(SiO2/HfO2)4-SiO2 mirror stack. Various grating profiles manufactured in order to modify the near electric field distribution are damage tested on a facility operating at 1.053 µm, 500 fs pulse duration. We evidence that damage threshold is governed by the value of the maximum electric field intensity inside the grating pillar. Moreover thresholds close to 3 J/cm² beam normal are obtained with this new MMLD grating being thus an interesting alternative to gold and pure dielectric gratings for pulse compression applications.

Subsurface damage measurement of ground fused silica parts by HF etching techniques

Detection and measurement of subsurface damage of ground optical surfaces are of major concern in the assessment of high damage thresholds fused silica optics for high power laser applications. We herein detail a new principle of SSD measurement based on the utilization of HF acid etching. We also review and compare different subsurface damage (SSD) characterization techniques applied to ground and fine ground fused silica samples. We demonstrate good concordance between the different measurements.

High reflection mirrors for pulse compression gratings

We report an experimental investigation of high reflection mirrors used to fabricate gratings for pulse compression application at the wavelength of 1.053microm. Two kinds of mirrors are studied: the mixed Metal MultiLayer Dielectric (MMLD) mirrors which combine a gold metal layer with some e-beam evaporated dielectric bilayers on the top and the standard e-beam evaporated MultiLayer Dielectric (MLD) mirrors. Various samples were manufactured, damage tested at a pulse duration of 500fs. Damage sites were subsequently observed by means of Nomarski microscopy and white light interferometer microscopy. The comparison of the results evidences that if MMLD design can offer damage performances rather similar to MLD design, it also exhibits lower stresses; being thus an optimal mirror substrate for a pulse compression grating operating under vacuum.

Imaging subsurface damage of grinded fused silica optics by confocal fluorescence microscopy

We report an experimental investigation of fluorescence confocal microscopy as a tool to measure subsurface damage on grinded fused silica optics. Confocal fluorescence microscopy was performed with an excitation at the wavelength of 405 nm on fixed abrasive diamond grinded fused silica samples. We detail the measured fluorescence spectrums and compare them to those of oil based coolants and grinding slurries. We evidence that oil based coolant used in diamond grinding induces a fluorescence that marks the subsurface damages and eases its observation. Such residual traces might also be involved in the laser damage process.

Effect of electric field on laser induced damage threshold of multilayer dielectric gratings

This paper studies gratings engraved in multilayer dielectric stacks for ultra high intensity laser compressors application. We design various grating profiles with high reflected efficiencies for 1780 l/mm multilayer dielectric gratings (MLD). Each grating is defined to exhibit a different electric field maximum value in the pillars of the grating. A damage testing facility operating at 1.053 mum, 500 fs pulse duration is used to damage test the parts manufactured from these designs. It is evidenced that for fixed incident angle and materials the damage of the grating is directly related to the electric field intensity maximum in the material, which depends on the groove profile. Laser induced damage thresholds of 5 J/ cm(2) are experimentally reached with very high and uniform efficiencies.

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.

Polishing-induced contamination of fused silica optics and laser induced damage density at 351 nm

In this paper we study the effect of contamination induced by fabrication process on laser damage density of fused silica polished parts at 351 nm in nanosecond regime. We show, owing to recent developments of our raster scan metrology, that a good correlation exists between damage density and concentration of certain contaminants for the considered parts.