Microscopic evidence of the role of the near field enhancement in the short pulse damage mechanism of diffraction gratings

International audienceWe investigate the short pulse laser induced damage initiation mechanism on multilayer dielectric (MLD) pulse compression gratings. We report by means of scanning electron microscopy that damages initiate on the edge of the grating pillars opposite to the incoming wave. It demonstrates, at the scale of a grating line, the role of the electric field in the damage process but we also that grating pillars damage is also spatially modulated in the form of a periodic ripple pattern developing along the polarization direction

A discrete element model to investigate sub-surface damage due to surface polishing

International audienceLarge high-power laser facilities such as megajoule laser (LMJ) or National Ignition Facility (NIF) are designed to focus about 2 MJ of energy at the wavelength of 351 nm, in the center of an experiment chamber. The final optic assembly of these systems, operating at 351 nm is made of large fused silica optics working in transmission. When submitted to laser at the wavelength of 351 nm, fused silica optics can exhibit damage, induced by the high amount of energy traversing the part. The created damage is a set of micro-chips that appear on the optic surface. Current researches have shown that this damage could be initiated on pre-existing sub-surface damages created during the optics manufacturing process. It is then very important to understand, for various set of manufacturing parameters, what are the key parameters for sub-surface damage. The presented work details the development of a simplified model to investigate the polishing process. Both silica (the material to be polished) and the abrasive particles are modeled using a discrete element approach. This numerical tool allows following the evolution of micro-cracks inside the material during the abrasion process. It is shown how the mechanical properties (pressure), the abrasive properties (shape and quantity of abrasive particles) and the system properties (filtration) have an influence on the sub-surface properties at the end of the process

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

International audienceWe 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

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

International audienceDetection 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

Chromatism compensation of the PETAL multipetawatt high-energy laser

International audienceHigh-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 ultra-fast 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

Diffraction gratings: from principles to applications in high-intensity lasers

International audienceDiffraction gratings were discovered at the 18 th century and they are now widely used in spectrometry analysis with outstanding achievements spanning from the probing of single molecules in biological samples to the analysis of solar systems in astronomy. The fabrication of high quality diffraction gratings requires a precise control of the period at a nanometer scale. The discovery of lasers in the 1960's gave birth to the optical beam lithography in the 1970's. This technology revolutionized the fabrication of diffraction gratings by offering a highly precise control of the grating period over very large scales. It is surprising to see that a few years after, the unique spectral properties of diffraction gratings revolutionized in turn the field of high energy lasers. We review in this paper the physics of diffraction gratings and detail their interest for the pulse compression of high power laser systems. Light diffraction is a fundamental and emblematic problem in optics. Diffraction gratings are optical components of major importance in the spectral analysis of light. They consist of a periodic modulation at the wavelength scale of an interface between two or more materials (see Fig.1). Their unique spectral properties rely on the fact that the light impinging on the periodically modulated surface is reflected or transmitted in specific angles only, which is not the case if the modulation is aperiodic. The angles of propagation can be predicted by a very simple expression, the so-called grating's law, that can take the simple form: sin θ m + sin θ i = mλ /d. (1) in which d denotes the grating period, λ denotes the wavelength, θ m and θ i denote respectively the angles between the diffracted angles, the incident angle and the normal to the surface. m is a relative integer denoting the diffraction order. When m = 0 (classically called specular order), the diffraction grating does not exhibit spectral properties and acts as a mirror. Diffraction properties appear when m = 0 and it can be observed that in this case, the angle of diffraction θ m depends on the wavelength. This property is at the heart of the spectral analysis since plane-waves of different frequencies will not propagate in the same

Design of a full silica pulse compression grating

A diffraction grating engraved on a two-dimensional photonic crystal composed of square air holes in a silica matrix is numerically studied for the compression of ultra-short pulses. The silica is therefore the only solid material of the grating and the reflection of the incident beam is based on the contrast of the air and silica refractive indices. This optical component enables the single use of silica as a solid material presenting a high laser induced damage threshold. In comparison to gratings engraved on a dielectric stack (MLD), it offers the advantage of avoiding the presence of interfaces between 2 solid materials with different mechanical properties, sources of mechanical constraints whic

Mixed metal dielectric gratings for pulse compression

International audienceWe 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-(SiO 2 /HfO 2) 4-SiO 2 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/cm 2 beam normal are obtained with this new MMLD grating being thus an interesting alternative to gold and pure dielectric gratings for pulse compression applications