High Power Laser Optics Research Articles

Structural evolution of liquid silicates under conditions in Super-Earth interiors

Molten silicates at depth are crucial for planetary evolution, yet their local structure and physical properties under extreme conditions remain elusive due to experimental challenges. In this study, we utilize in situ X-ray diffraction (XRD) at the Matter in Extreme Conditions (MEC) end-station of the Linear Coherent Linac Source (LCLS) at SLAC National Accelerator Laboratory to investigate liquid silicates. Using an ultrabright X-ray source and a high-power optical laser, we probed the local atomic arrangement of shock-compressed liquid (Mg,Fe)SiO3 with varying Fe content, at pressures from 81(9) to 385(40) GPa. We compared these findings to ab initio molecular dynamics simulations under similar conditions. Results indicate continuous densification of the O-O and Mg-Si networks beyond Earth’s interior pressure range, potentially altering melt properties at extreme conditions. This could have significant implications for early planetary evolution, leading to notable differences in differentiation processes between smaller rocky planets, such as Earth and Venus, and super-Earths, which are exoplanets with masses nearly three times that of Earth.

Public exposure to artificial optical radiation in the aesthetics and the entertainment sector in Greece. Risk management actions.

Safety issues are raised from the use of low- and high-power optical radiation sources, both laser and non-laser, by non-experts for aesthetic and entertainment purposes. The Greek Atomic Energy Commission relied on the ISO 31000:2018 framework to manage the public exposure risk to such cases. Risk was evaluated as (1) intolerable for lasers and intense pulsed light sources at aesthetic procedures and in the case of laser pointers, (2) severe for lasers at laser shows and (3) moderate for light-emitting diodes (LEDs) at aesthetic procedures, home-use intense pulsed light sources/LEDs and laser/LED projectors. Operators' training, public awareness campaigns, intensive market surveillance actions and the enhancement of the regulatory framework have been proposed as risk treatment/control measures and have been prioritised in this order, according to their effectiveness in reducing the exposure risk and their urgency of implementation. The Greek Atomic Energy Commission developed public awareness campaigns regarding exposure safety to laser and non-laser light sources at aesthetic procedures and the use of laser pointers.

Heterostructure Films of SiO2 and HfO2 for High-Power Laser Optics Prepared by Plasma-Enhanced Atomic Layer Deposition

Absorption losses and laser-induced damage threshold (LIDT) are considered to be the major constraints for development of optical coatings for high-power laser optics. Such coatings require paramount properties, such as low losses due to optical absorption, high mechanical stability, and enhanced damage resistance, to withstand high-intensity laser pulses. In this work, heterostructures were developed by sub-nanometer thin films of SiO2 and HfO2 using the plasma-enhanced atomic layer deposition (PEALD) technique. Thin-film characterization techniques, such as spectroscopic ellipsometry, spectrophotometry, substrate curvature measurements, X-ray reflectivity, and Fourier transform infrared spectroscopy, were employed for extracting optical constants, residual stress, layer formation, and functional groups present in the heterostructures, respectively. These heterostructures demonstrate tunable refractive index, bandgap, and improved optical losses and LIDT properties. The films were incorporated into antireflection coatings (multilayer stacks and graded-index coatings) and the LIDT was determined at 355 nm wavelength by the R-on-1 method. Optical absorptions at the reported wavelengths were characterized using photothermal common-path interferometry and laser-induced deflection techniques.

Embedded Micro-channel Cooling Technique to Minimize Thermal Deformation of X-ray and High-power Laser Optics

The thermal deformation of silicon-based X-ray and high-power laser optics is a strong function of the beam power and footprint, mirror geometry as well as the location of the “convective” thermal management solution. Previous research carefully optimized the impact of these parameters and arrived at the “Topside cooling” design concept (smart-cut notches) that reduced the thermal slope error (sub-μrads) by 3-5 orders of magnitude compared to the baseline mirror bottom-side cooling [1,2]. The present work introduces internal embedded microchannel cooling which brings the cooling solution to the close proximity of the laser beam footprint and also reduce thermal spreading. The thermomechanical finite element modelling (FEM) results show that embedded microchannel cooling can reduce the thermal deformation in RMS-Uy by a factor of 40 and 13 compared with topside cooling, for Gaussian distribution beam heat flux with full width half maximum (FWHM) 20 mm and 40 mm, respectively.

Ultra-Precision Manufacturing Technology of High Power Laser Optics

Ultra-Precision Manufacturing Technology of High Power Laser Optics

On the selection of materials for high-power laser optics based on analysis of focus shift and aberration induced by thermal gradient

Non-uniform (gradient) heating due to absorption of laser radiation in optical elements causes thermal lensing and paraxial focus shift and aberration leading to change in size and intensity profile of focused spot in optics operating with high-power lasers. Analysis of primary physical effects of geometrical deformation of optical surfaces in the form of aspheric bulges and transformation of the material into a gradient refractive medium makes it possible to estimate quantitatively the focus shift and aberrations for single-mode and multimode lasers, to formulate optimal relationships between the physical properties of optical materials for reduction of thermo-optical effects through compensating the effects from material thermal expansion by the effects due to change in the refractive index—condition of self-compensation or athermalization. Comparison of characteristics Temperature Coefficient of the Optical Pathlength and Thermo-Optical Ratio allows determining the optimal materials for the optics for high-power lasers: athermal crystalline quartz and specialty glasses, sapphire with extremely high thermal conductivity ensuring minimal temperature gradients. Optics made of these materials exhibit minimized thermal focus shift and aberration even when absorption of laser energy in bulk material and coatings, by contamination, scratches, and other surface defects. Weak birefringence of crystalline quartz and sapphire does not prevent their successive use in laser optics.

Parametric study of plasma active medium and gain saturation region in a Ne‐like soft X‐ray laser

AbstractPlasmas created by the interaction of high power optical laser with a target surface can be used as a source of soft X‐ray lasers. Plasma and pump laser characteristics play significant role in achieving high gain coefficient for such plasma based on soft X‐ray lasers. In the present work, the plasma active medium parameters for germanium element at a wavelength of 19.6 nm irradiated by a double‐pulse pump laser have been studied using MED103 hydrodynamic code. For this purpose, first, the effects of laser intensity, pulse width and delay time of two pulses on the gain coefficient have been investigated and the optimum conditions for the maximum gain extent of Ne‐like germanium soft X‐ray laser are obtained. Then, in order to calculate the intensity of such high gain lasers in which Linford equation is invalid, we have adopted the general formula of amplified spontaneous emission intensity, which is valid in all range of intensities even at much higher intensities than saturation intensity. Finally, the soft X‐ray laser intensities in the saturated areas for different plasma lengths have been calculated. The results show that the output of soft X‐ray laser intensity with 294 cm−1 gain coefficient can reach to about several times saturated intensity by applying a 1–2 mm plasma length as the active medium.

Micron-scale phenomena observed in a turbulent laser-produced plasma

Turbulence is ubiquitous in the universe and in fluid dynamics. It influences a wide range of high energy density systems, from inertial confinement fusion to astrophysical-object evolution. Understanding this phenomenon is crucial, however, due to limitations in experimental and numerical methods in plasma systems, a complete description of the turbulent spectrum is still lacking. Here, we present the measurement of a turbulent spectrum down to micron scale in a laser-plasma experiment. We use an experimental platform, which couples a high power optical laser, an x-ray free-electron laser and a lithium fluoride crystal, to study the dynamics of a plasma flow with micrometric resolution (~1μm) over a large field of view (>1 mm2). After the evolution of a Rayleigh–Taylor unstable system, we obtain spectra, which are overall consistent with existing turbulent theory, but present unexpected features. This work paves the way towards a better understanding of numerous systems, as it allows the direct comparison of experimental results, theory and numerical simulations.

2D monochromatic x-ray imaging for beam monitoring of an x-ray free electron laser and a high-power femtosecond laser.

In pump-probe experiments with an X-ray Free Electron Laser (XFEL) and a high-power optical laser, spatial overlap of the two beams must be ensured to probe a pumped area with the x-ray beam. A beam monitoring diagnostic is particularly important in short-pulse laser experiments where a tightly focused beam is required to achieve a relativistic laser intensity for generation of energetic particles. Here, we report the demonstration of on-shot beam pointing measurements of an XFEL and a terawatt class femtosecond laser using 2D monochromatic Kα imaging at the Matter in Extreme Conditions end-station of the Linac Coherent Light Source. A thin solid titanium foil was irradiated by a 25-TW laser for fast electron isochoric heating, while a 7.0 keV XFEL beam was used to probe the laser-heated region. Using a spherical crystal imager (SCI), the beam overlap was examined by measuring 4.51 keV Kα x rays produced by laser-accelerated fast electrons and the x-ray beam. Measurements were made for XFEL-only at various focus lens positions, laser-only, and two-beam shots. Successful beam overlapping was observed on ∼58% of all two-beam shots for 10 μm thick samples. It is found that large spatial offsets of laser-induced Kα spots are attributed to imprecise target positioning rather than shot-to-shot laser pointing variations. By applying the Kα measurements to x-ray Thomson scattering measurements, we found an optimum x-ray beam spot size that maximizes scattering signals. Monochromatic x-ray imaging with the SCI could be used as an on-shot beam pointing monitor for XFEL-laser or multiple short-pulse laser experiments.

Application and analysis of three-layer-core structure in single-mode large-mode-area fiber with low bending loss

A three-layer-core single-mode large-mode-area fiber with low bending loss is investigated in this paper. The three-layer structure in the core which is comprised of core-index layer, cladding-index layer and depression-index layer, can achieve large-effective-area Aeff while maintaining low-bending-loss without deteriorating cutoff behaviors. The large-mode-area of 100–330 μm<sup>2</sup> can be achieved in the fiber. The effective area Aeff can be further enlarged by adjusting the layer parameters. Furthermore, the bending property can be improved in this three-layer-core structure. The bending loss can decrease by 2–4 orders of magnitude compared with the bending loss of the conventional step-index fiber with the same Aeff. These characteristics of three-layer-core fiber suggest that it can be used in large-mode-area wide-bandwidth high-capacity transmission, or high-power optical fiber laser and amplifier in the optical communications, which can be conducive to studying the basic physical layer structure of big data storage, reading, calculation and transmission applications and so on.

Seventh User Workshop on High-Power Lasers at the Linac Coherent Light Source

We report on a seventh annual workshop in a series focused on science realized by the combination of hard X-ray free electron lasers with high power optical lasers, hosted at the SLAC National Accelerator Laboratory in Menlo Park, CA. Members from the user community of the Matter in Extreme Conditions (MEC) endstation of the Linac Coherent Light Source (LCLS) and other scientists met with local scientists to discuss developments at LCLS and MEC and related facilities, including experimental results and future plans.

Thermomechanical response of thickly tamped targets and diamond anvil cells under pulsed hard x-ray irradiation

In the laboratory study of extreme conditions of temperature and density, the exposure of matter to high intensity radiation sources has been of central importance. Here, we interrogate the performance of multi-layered targets in experiments involving high intensity, hard x-ray irradiation, motivated by the advent of extremely high brightness hard x-ray sources, such as free electron lasers and 4th-generation synchrotron facilities. Intense hard x-ray beams can deliver significant energy in targets having thick x-ray transparent layers (tampers) around samples of interest for the study of novel states of matter and materials’ dynamics. Heated-state lifetimes in such targets can approach the microsecond level, regardless of radiation pulse duration, enabling the exploration of conditions of local thermal and thermodynamic equilibrium at extreme temperature in solid density matter. The thermal and mechanical responses of such thick layered targets following x-ray heating, including hydrodynamic relaxation and heat flow on picosecond to millisecond timescales, are modeled using radiation hydrocode simulation, finite element analysis, and thermodynamic calculations. Assessing the potential for target survival over one or more exposures and resistance to damage arising from heating and resulting mechanical stresses, this study doubles as an investigation into the performance of diamond anvil high pressure cells under high x-ray fluences. Long used in conjunction with synchrotron x-ray radiation and high power optical lasers, the strong confinement afforded by such cells suggests novel applications at emerging high intensity x-ray facilities and new routes to studying thermodynamic equilibrium states of warm, very dense matter.

Ultra high damage threshold optics for high power lasers

The output energies of lasers have increased year-by-year since their invention. Compared to this increase of laser energies, the damage threshold of optical components has not strongly changed. Therefore, the size of optics in high-energy laser system increases. This situation could change dramatically if optics with higher damage threshold were developed. Here, we propose a high damage threshold optics using a neutral gas as an active medium. More than 95% diffraction efficiency has been achieved. The damage threshold for a 6 ns laser pulse is measured to be 1.6 kJ/cm2. The aperture size of the present system is about 60 mm2. Based on this result, we anticipate that control of a 1 kJ laser beam may be achievable using 1 cm sized optics, driven by a < 50 mJ ultraviolet laser, making this scheme promising in high power laser applications.

Antireflective coatings with high damage threshold prepared by laser ablation

Latest developments in the field of high power ultra-short pulse lasers have led to intensive studies dedicated to the fabrication possibility of new antireflective coatings which exhibit high damage threshold. Therefore, this study is focused on the deposition and characterization of metal oxide heterostructures followed by laser-induced damage threshold tests which evidence their application in high power laser optics. Al2O3, SiO2, and HfO2 layers are combined to obtain different heterostructures, i.e. HfO2/Al2O3/HfO2/Al2O3/HfO2 and HfO2/SiO2/HfO2/SiO2/HfO2. The metal oxide heterostructures are deposited in a controllable oxygen atmosphere, either at room temperature or high temperatures (600 °C) by pulsed laser deposition (PLD). The morphological, structural and optical properties of the as-deposited heterostructures are first investigated. Atomic force microscopy and spectroscopic ellipsometry investigations reveal a lower roughness of the heterostructures based on HfO2/Al2O3 layers grown at 600 °C as compared to those grown at room temperature. Furthermore, following the laser-induced damage threshold (LIDT) tests carried out with a Ti–Sapphire laser, higher LIDT values are obtained for the HfO2/Al2O3-based heterostructures than for the HfO2/SiO2-based heterostructures. The ability to control the morphological and structural properties of the antireflective coatings by modifying the deposition parameters of the metal oxide heterostructures demonstrates that PLD is a suitable technique for the manufacturing of antireflective coatings for high power ultra-short laser systems.

Large optics metrology for high-power lasers.

The Laser MégaJoule (LMJ) is a high-power laser dedicated to laser-plasma experiments. At the beginning of the project in the mid-1990s, an optical metrology laboratory was created at CEA to help accomplish all the steps in the construction of this laser. This paper proposes an overview of the capabilities of this metrology laboratory in four main fields: surface imperfections, photometry, laser damage measurement, and wavefront measurement. The specificities for high-power laser optics in each domain are highlighted as well as the specific features that make our instruments unique.

Dual-Mode Large-Mode-Area Multicore Fiber With Air-Hole Structure

A novel dual-mode large-mode-area multicore fiber with air-hole structure, which is a quasi-37 core structured fiber with 20 air holes and 17 cores, is proposed in this paper. This fiber can realize strict dual-mode operation, that is, the fundamental mode HE11 and the second-order mode HE21 operation. The air-hole structure makes the second-order modes TE01 and TM01 cut off. Effects of structural parameters on effective refractive index and effective area of fundamental mode are investigated systematically. By optimizing the structural parameters of the fiber, effective area of fundamental mode is up to 1557.91 μm2 and maintaining a low bending loss (less than 10−4 dB/m at R > 0.47 m) while keeping dual-mode operation. The fiber can be used for large-mode-area high-power optical fiber laser, amplifier, FTTH and so on.

Maximizing magnetic field generation in high power laser–solid interactions

In order to understand the transport of fast electrons within solid density targets driven by an optical high power laser, we have numerically investigated the dynamics and structure of strong self-generated magnetic fields in such experiments. Here we present a systematic study of the bulk magnetic field generation due to the ponderomotive current, Weibel-like instability and resistivity gradient between two solid layers. Using particle-in-cell simulations, we observe the effect of varying the laser and target parameters, including laser intensity, focal size, incident angle, preplasma scale length, target thickness and material and experimental geometry. The simulation results suggest that the strongest magnetic field is generated with laser incident angles and preplasma scale lengths that maximize laser absorption efficiency. The recent commissioning of experimental platforms equipped with both optical high power laser and X-ray free electron laser (XFEL), such as European XFEL-HED, LCLS-MEC and SACLA beamlines, provides unprecedented opportunities to probe the self-generated bulk magnetic field by X-ray polarimetry via Faraday rotation with simultaneous high spatial and temporal resolution. We expect that this systematic numerical investigation will pave the way to design and optimize near future experimental setups to probe the magnetic fields in such experimental platforms.

Surface ripple suppression in subaperture polishing with fragment-type tool paths.

In order to reduce the ripple-induced effect in subaperture polishing of high-power laser optics, the tool paths are investigated and optimized in this study. Various fragment-type curves (i.e., fractal and fractal-like unicursal curves) are designed as tool paths, and most importantly, a novel algorithm for generating a random fractal-like tool path is proposed in detail. Compared with scanning paths, fragment-type paths possess multi-directionality during multiple polishing iterations, which could better smooth the surface texture and restrain the polishing-induced surface ripples. In particular, the random fractal-like path exhibits high randomness, boundary adaption, and step-length arbitrariness, which make it more flexible and powerful for iterative polishing. Experiments were conducted on a computer numerical control machine, with conclusions that during iterative polishing, (1)the repetitive usage of a single fractal path would aggravate surface ripples; (2)the combination of multiple fractal paths or using the random fractal-like path can restrain surface ripples efficiently; and (3)pitch pads exhibit a better ripple-smoothing effect than polyurethane. The proposed random fractal-like path is a good prospect for the manufacture of high-power laser optics, which demands very low surface ripples.

Sapphire/Nd:YAG composite by pulsed electric current bonding for high-average-power lasers.

As a new bonding technique for high-power laser optics, pulsed electric current bonding (PECB) of sapphire and Nd:YAG ceramics was demonstrated. The optical properties of the composite were measured, and its microstructure at the interface and laser performance was analyzed. The optical transmittance was equal to the theoretical value, and the transmitted wavefront was λ/3 (λ=633 nm); both are appropriate values for laser applications. The microstructural analysis indicated an absence of scattering sources such as pores or non-contact points at the sapphire/Nd:YAG interface, and the distance of yttrium diffusion into the sapphire was theoretically expected to be less than 10nm, much smaller than that of ceramic materials bonded by conventional thermal diffusion techniques. The laser performance of the composite material showed an 18% higher output power with almost the same threshold power and slope efficiency as a Nd:YAG ceramic due to the sapphire-conductive cooling effect. This new PECB technique for different transparent materials has the potential to bond large aperture optical materials over 100mm in diameter and could be especially effective for fabricating active laser media for high-average-power lasers having both high-pulse energy and high repetition rates.

Oxygen plasma etching of fused silica substrates for high power laser optics

Laser damage resistance of a transparent optical component significantly depends on its substrate surface quality. In this work low energy oxygen plasma etching of fused silica (FS) substrates was investigated in terms of plasma ion energy and etching depth. Laser induced damage threshold (LIDT) and surface roughness of experimental samples were measured and compared. The LIDT of plasma treated uncoated FS substrate increased from 5.0 ± 0.3 J/cm2 up to 75 ± 4 J/cm2 for 355 nm laser ns pulses without any surface roughness deterioration. Anti-reflective (AR) optical interference coating for 355 nm wavelength was deposited on plasma etched and non-etched substrates. Measured LIDT of plasma etched and AR coated component was 3.4 times higher comparing to coated non-etched substrate case and reached 14 ± 0.7 J/cm2. These results demonstrate successful application of low energy oxygen plasma etching for improving laser induced damage performance of uncoated and AR coated fused silica substrate without deterioration of surface roughness. This application could be also successfully extended for production of other transparent high power UV laser optics like polarizers, beam splitters, etc.