High Energy Laser Systems Research Articles

State-of-the-art Laser Ceramics for the Next-generation Solid-state Laser

Solid-state lasers have aroused many researchers’ interests for a variety of applications in military and industrial fields. Because of the preference for increased output power, Nd:YAG single crystals, which are the most widely used gain media, should be replaced by other more suitable candidates. Polycrystalline sesquioxide ceramics show great potential for use as gain media because their thermal and mechanical characteristics are suitable for use with high-energy laser systems. Recently, novel concepts of the gain media were also introduced. Herein, while briefly looking back on the progress of polycrystalline laser ceramics, we will discuss new interests in host materials and systems.

Near-null interferometric test of aspheric cylinders utilizing a partial null lens

Aspheric cylinders are more advantageous than cylinder with spherical cross section in optical design and aberration correction of high-energy laser systems because additional shape parameters can be introduced. To overcome the limited test accuracy and the insufficient research on the testability of the flexible null test method for aspheric cylinders, a near-null interferometric test method for aspheric cylinders was developed utilizing a partial null lens. A coaxial configuration design was employed. Accordingly, only one translation motion was required for testing various aspheric cylinders in one shot. Using the proposed test configuration and the developed near-null data processing method, test accuracy about λ/20 (λ = 632.8 nm) root-mean-square are easier to guarantee compared with those of the existing stitching-required method with off-axis configuration. Further, the testability of the proposed method was analyzed following the development of the partial null theory. The analysis shows the testable surfaces are equidistant surfaces with nearly constant k·R products (k·R = 211.625 mm). The testability results can serve as a good reference for engineers who intend to use aspheric cylinders in high-energy laser systems and can further promote the development of high-energy laser systems. A near-null test system was established. Its simple configuration, moderate test accuracy, and flexible test capacity were successfully demonstrated based on aspheric cylinders measurements.

Study on the interaction between nanosecond laser and 6061 aluminum alloy considering temperature dependence

The particle pollution caused by the stray light of the high-energy laser system irradiating the frame aluminum alloy is one of the most important reasons that hinder the stable operation of the system. Therefore, accurately revealing the mechanism of the interaction between the stray light and the aluminum alloy frame is an important and challenging task to ensure that inertial confinement fusion (ICF) system can generate clean and sustainable energy. In this paper, based on the secondary development of ABAQUS, a transient-stabilized thermal-mechanical direct coupling model is developed and experimentally investigated for the laser-material interaction in the high-energy nanosecond laser system with an energy density of 2 J/cm2. The mechanical damage, thermal ablation, and especially the temperature dependence of the thermal optical parameters of the aluminum alloy are considered in the modeling. Through comparative analysis, the results indicate that the effect of the variable optical properties must be considered in order to accurately reveal the mechanism of intense laser irradiation. On this basis, we found that the pulsed intense laser is absorbed by the aluminum alloy with a smaller absorption rate, after the aluminum alloy surface reaches the melting point and liquefies. This means more laser is reflected to the laser system, reducing the stability and reliability of the system. The whole process includes thermal expansion, thermal melting and potential mechanical damage, while no vaporization ablation occurs. The simulation conclusions are verified by the characterization of the experimental specimens, and it is verified that the diameter of the melting area is close to the full width at half maximum (FWHM) of the Gaussian laser source. This study provides a more accurate numerical model for precisely revealing the mechanism of high-energy nanosecond laser irradiation on metallic materials, and thus promoting the sustainable and reliable generation of clean energy from inertial confinement fusion.

First experimental results from the high-energy petawatt PETAL laser system

Even a moderately relativistic laser intensity led to unexpectedly high energies, above those expected from early numerical simulations, for proton acceleration.

Enhanced ion acceleration using the high-energy petawatt PETAL laser

The high-energy petawatt PETAL laser system was commissioned at CEA’s Laser Mégajoule facility during the 2017–2018 period. This paper reports in detail on the first experimental results obtained at PETAL on energetic particle and photon generation from solid foil targets, with special emphasis on proton acceleration. Despite a moderately relativistic (<1019 W/cm2) laser intensity, proton energies as high as 51 MeV have been measured significantly above those expected from preliminary numerical simulations using idealized interaction conditions. Multidimensional hydrodynamic and kinetic simulations, taking into account the actual laser parameters, show the importance of the energetic electron production in the extended low-density preplasma created by the laser pedestal. This hot-electron generation occurs through two main pathways: (i) stimulated backscattering of the incoming laser light, triggering stochastic electron heating in the resulting counterpropagating laser beams; (ii) laser filamentation, leading to local intensifications of the laser field and plasma channeling, both of which tend to boost the electron acceleration. Moreover, owing to the large (∼100 μm) waist and picosecond duration of the PETAL beam, the hot electrons can sustain a high electrostatic field at the target rear side for an extended period, thus enabling efficient target normal sheath acceleration of the rear-side protons. The particle distributions predicted by our numerical simulations are consistent with the measurements.

Towards Rapid Fabrication of Superhydrophobic Surfaces by Multi-Beam Nanostructuring with 40,401 Beams.

Superhydrophobic surfaces attract a lot of attention due to many potential applications including anti-icing, anti-corrosion, self-cleaning or drag-reduction surfaces. Despite a list of attractive applications of superhydrophobic surfaces and demonstrated capability of lasers to produce them, the speed of laser micro and nanostructuring is still low with respect to many industry standards. Up-to-now, most promising multi-beam solutions can improve processing speed a hundred to a thousand times. However, productive and efficient utilization of a new generation of kW-class ultrashort pulsed lasers for precise nanostructuring requires a much higher number of beams. In this work, we introduce a unique combination of high-energy pulsed ultrashort laser system delivering up to 20 mJ at 1030 nm in 1.7 ps and novel Diffractive Laser-Induced Texturing element (DLITe) capable of producing 201 × 201 sub-beams of 5 µm in diameter on a square area of 1 mm2. Simultaneous nanostructuring with 40,401 sub-beams resulted in a matrix of microcraters covered by nanogratings and ripples with periodicity below 470 nm and 720 nm, respectively. The processed area demonstrated hydrophobic to superhydrophobic properties with a maximum contact angle of 153°.

Investigation of UV, ns-laser damage resistance of hafnia films produced by electron beam evaporation and ion beam sputtering deposition methods

Laser-induced damage in coating materials with a high index of refraction, such as hafnia, limits the performance of high power and high energy laser systems. Understanding the underlying physics responsible for laser damage holds the key for developing damage-resistant optical films. Previous studies have reported a substantial difference in laser damage onset for hafnia films produced by different deposition methods, yet the underlying mechanisms for the observed difference remain elusive. We combined laser damage testing with analytical characterizations and theoretical simulations to investigate the response of hafnia films produced by electron (e-) beam evaporation vs ion beam sputtering (IBS) methods upon UV ns-laser exposure. We found that e-beam produced hafnia films were overall more damage resistant; in addition, we observed a polarization anisotropy associated with the onset of damage in the e-beam films, while this effect was absent in the latter films. The observed differences can be attributed to the stark contrast in the pressure inside the pores inherent in both films. The high pressure inside the IBS-induced nanobubbles has been shown to reduce the threshold for laser-induced plasma breakdown leading to film damage. The polarization effects in the e-beam coatings can be related to the asymmetric electric field intensification induced by the columnar void structure. Our findings provide a fundamental basis for developing strategies to produce laser damage-resistant coatings for UV pulsed laser applications.

A Yb:YAG dual-crystal regenerative amplifier

An Yb:YAG dual-crystal regenerative amplifier was developed. The thermal effects were largely decreased by the dual-crystal configuration and optimized cavity design. The chirped pulse was amplified to 3.5 mJ with excellent energy stability (root mean square ∼0.95%) and beam quality (M2 < 1.1) at a repetition rate of 1 kHz using chirped pulse amplification (CPA) systems. A single space-saving simple-to-align chirped volume Bragg grating (CVBG) was used as a stretcher/compressor, which yielded 2-mJ pulses with a pulse duration of 1.9 ps after compression with an efficiency of 94.3%. A low-cost highly compact kilohertz high-energy ultrafast laser system solution was obtained.

Interferometric stitching method for testing cylindrical surfaces with large apertures.

Cylindrical surfaces widely used in high-energy laser systems can have nearly semi-meter-scale dimensions, and aperture angles can exceed R/3. State-of-the-art interferometric stitching test methods involve stitching only along the arc direction, and the reported dimensions of ∼50 × 50 mm2 are far smaller than those required in high-energy laser systems. To rectify this limitation, an interferometric stitching method for cylindrical surfaces with large apertures is proposed. Moreover, a subaperture stitching algorithm that can stitch along both the linear and arc directions is developed. An interferometric stitching workstation equipped with a six-axis motion stage and a series of computer-generated holograms is established, where cylindrical surfaces with R/# values as large as R/0.5 and apertures up to 700 mm can be tested based on the theoretical analysis. A convex cylindrical surface with a 350 × 380 mm2 aperture is tested to validate the proposed method's feasibility in enlarging the testable aperture of cylindrical surfaces significantly from Ф50 mm to Ф700 mm, thereby promoting the application of large cylindrical surfaces in high-energy laser systems.

Polycrystalline simulation and experimental study of spatiotemporal anisotropy aluminum alloy irradiated by nanosecond laser

Revealing the mechanism of the interaction between nanosecond lasers and metallic materials is an important and challenging task. This paper focuses on model development and experimental studies of the interaction of AA6061 and nanosecond lasers in high-energy laser systems. A numerical model based on the secondary development of ABAQUS is developed for the irradiation of aluminum alloy specimens by nanosecond lasers with an energy density of 2 J/cm2. The model takes into account the spatiotemporal anisotropy of the laser irradiation, including the temperature dependence of the physical parameters, the inhomogeneous distribution of the material, and the differential laser ablation threshold at different positions. The results indicate that at an energy density of 2 J/cm2, the interaction of the laser and aluminum alloy is accompanied by thermal expansion, thermal melting, thermal ablation, and mechanical damage, which are randomly distributed in a circular area with a diameter of one-half of the facula. In addition, the experimental results obtained by laser targeting are consistent with the simulation results. This study provides a more accurate numerical model and experimental guidance for precisely revealing the interaction mechanism of the irradiation of metallic materials by high-energy nanosecond lasers.

Efficient generation of turbulent collisionless shocks in laser-ablated counter-streaming plasmas

Laser-ablated high-energy-density (HED) plasmas offer a promising route to study astrophysically relevant processes underlying collisionless shock formation, magnetic field amplification, and particle acceleration in the laboratory. Using large-scale, multi-dimensional particle-in-cell simulations, we explore the interpenetration of laser-ablated counter-streaming plasmas for realistic experimental flow profiles. We find that the shock formation and its structure are substantially different from those of more idealized and commonly considered uniform flows: shock formation can be up to 10 times faster due to the transition from small-angle scattering to magnetic reflection and the shock front develops strong corrugations at the ion gyroradius scale. These findings have important consequences for current experimental programs and open exciting prospects for studying the microphysics of turbulent collisionless shocks with currently available high-energy laser systems.

Energetic α-particle sources produced through proton-boron reactions by high-energy high-intensity laser beams.

In an experiment performed with a high-intensity and high-energy laser system, α-particle production in proton-boron reaction by using a laser-driven proton beam was measured. α particles were observed from the front and also from the rear side, even after a 2-mm-thick boron target. The data obtained in this experiment have been analyzed using a sequence of numerical simulations. The simulations clarify the mechanisms of α-particle production and transport through the boron targets. α-particle energies observed in the experiment and in the simulation reach 10-20 MeV through energy transfer from 20-30 MeV energy incident protons. Despite the lower cross sections for protons with energy above the sub-MeV resonances in the proton-boron reactions, 10^{8}-10^{9}α particles per steradian have been detected.

Optimizing laser focal spot size using self-focusing in a cone-guided fast-ignition ICF target

This paper presents a scheme for strong self-focusing of a laser beam interacting with a cone-guided fast-ignition inertial confinement fusion target using cone pre-plasma filling as an optical medium for reducing the laser beam waist. The objective is to reduce the focal spot size at the interior of the tip of the re-entrant cone to that required for efficient coupling to the dense imploded fuel core. This is challenging to achieve in a large laser system using the standard optical components of a chirped-pulse-amplified (CPA) laser-beam chain where the spot sizes produced are often significantly larger than would be desirable for fast ignition. The approach described also makes use of the presence of pre-plasma in the cone. Such pre-plasma filling is difficult to avoid entirely when illuminating a cone with a high-energy CPA laser system due to the challenges of reducing laser pre-pulse to below the threshold for plasma production. For deriving the differential equation which governs the progress of the laser beam-width with propagation distance, paraxial theory in a WKB approximation has been used. A simulation is performed assuming strong self-focusing in accordance with the laser parameters and plasma density profile chosen.

Design and Management of Stray Light for Compact Final Optics Assembly on the High Energy Laser System

In this study, a model is proposed to design and manage the stray light of a compact final optics assembly (FOA) for a high energy laser system. Based on the method we proposed, the high-order stray light can be managed to optimizing the position and angle of the optical elements. A light trap is designed to manage the first-order stray light with high fluence. Applying the method, we provide an experimental demonstration to designing a compact FOA. By comparing the cleaning results with no management testing result, it proves that using the above design and management, it can achieve the great improvement of cleanliness from ISO Class 5 to Class 3, which is significant to improve the output capability of the high energy laser system. In addition, we also verify the stray light by an optical field paper. It demonstrates that the field characteristics and position calculation of stray light are reliable.

Polarization-dependent dissipative soliton intensity modulation enabled repetition-rate-switchable CPA system

We report a watt-level, repetition-rate-switchable all-fiber chirped pulse amplification (CPA) system by using polarization-dependent dissipative soliton intensity modulation effect. The intensity modulation for pulse repetition rate tuning is realized by combination of the polarization discrimination device (i.e., polarizer) and the polarization-rotating vector soliton (PRVS). Up to ~100% intensity modulation depth with a period of two cavity roundtrips for the PRVS seed was obtained by adjusting the dissipative soliton polarization state that was injected into the polarizer. Relying on this mechanism, the repetition rate of the output pulse from the CPA system can be dynamically switched between 1.37 MHz and 2.74 MHz. The pulse energy of ~2.0 μJ and ~2.2 μJ were achieved for the repetition rates of 1.37 MHz and 2.74 MHz, corresponding to 1.12 ps and 970 fs pulse duration, respectively. These findings would provide an alternative method for dynamic tuning of pulse repetition rate in high power laser systems, which also underpins the significance of fundamental research of vector dissipative solitons in practical applications such as flexible high-energy pulse laser systems.

Creation of high-fluence precursors by 351-nm laser exposure on SiO2 substrates

The onset of laser-induced damage on the exit surface of fused silica optics when exposed to UV ns pulses is a limiting factor in the design and operation of most high-energy laser systems. As such, significant effort has been expended in developing laser damage testing protocols and procedures to inform laser system design and operating limits. These tests typically rely on multiple laser exposures for statistical validation. For large aperture systems, testing an area equal to that of the optical components in the system is functionally impossible; instead, sub-scale witness samples are interrogated with elevated fluences. We show that, under certain circumstances, the laser exposure used to test one location on a sample will generate additional, laser-induced damage precursors in regions beyond that exposed to laser light and hence degrade the damage performance observed on subsequent exposures. We hypothesize the precursors result from condensation of vaporized silica, which re-condenses as SiOx particles deposited on the exit surface. In addition, we will outline the conditions under which this phenomenon occurs, as well as methods for mitigating or eliminating the effect.

Spectral shaping of picosecond petawatt laser system based on lithium niobate birefringent crystal

In recent years, chirped pulse amplification (CPA) technology injects vitality into the development of ultra-strong and ultra-short lasers. However, in the CPA based gain media, the gain narrowing effect limits the higher output of ultrashort pulse in energy, power, signal-to-noise ratio. In order to compensate for the gain narrowing caused by the broadband amplification of Nb:glass in picosecond pewter laser system, a method of high-energy spectral shaping is proposed based on LiNbO&lt;sub&gt;3&lt;/sub&gt; birefringent crystal, and the spectral phase introduced by the crystal is analysed for the first time. Based on the strict Jones matrix, the transmittance function of birefringent crystal and the spectral phase introduced by the crystal are obtained. Further, three kinds of birefringent crystals are compared among each other, and the results show that the higher birefringence and the smaller thickness are required to achieve the same intensity modulation. For the laser pulse at 1053 nm, LiNbO&lt;sub&gt;3&lt;/sub&gt; is selected as the spectral shaping crystal due to its high birefringence, large diameter, and non-deliquescent. The influences of crystal thickness, tilt angle, and in-plane rotation angle on the spectral intensity modulation are simulated theoretically, and the results show the above parameters affect the modulation bandwidth, center wavelength, and modulation depth of the shaping. By analyzing the spectral phase introduced by the crystal, it is found that the dispersion of each order changes with the thickness of the crystal, the tilt angle, and the in-plane rotation angle, and it is the most sensitive to the change of thickness. In addition, by controlling the dispersion of each order, the influence on the pulse signal-to-noise ratio can be weakened during spectrum shaping. On the basis of theoretical analysis, the shaping experiment with a center wavelength of 1053 nm, modulation bandwidth of 10 nm, and modulation depth of 80% is carried out. And the phase introduced by the LiNbO&lt;sub&gt;3&lt;/sub&gt; is measured. The experimental results are consistent with the theoretical analysis. For the Shenguang Ⅱ high-energy petawatt laser system, by the above-mentioned shaping scheme, a high-energy broadband laser output of 1700 J and 6 nm (FWHM) is realized for the first time in China, which is 2 times that at 3.2 nm when it is not shaped. The research effectively compensates for the Nb:glass gain narrowing effect, and will provide references for the parameter design, material selection and spectral phase compensation in the birefringent spectral shaping.

Laser energy absorption prediction of silicon substrate surface from a mid- and high-spatial frequency error.

Additional laser energy absorption of optical elements limits the further development of high-energy laser systems. In engineering, inexpensive and precise absorption test technology is essential. We attempt to predict energy absorption via surface spatial error value based on the roughness-induced absorption (RIA) theory. However, the absorption coefficients cannot match roughness values measured with an atomic force microscope or white light interferometer. We find three influencing factors and optimize the definition of RIA to spatial error-induced absorption (SEIA). SEIA is proportional to δ2 of a mid- and high-spatial frequency error in a certain frequency range. This range depends on laser diameter, wavelength, and coating. Excluding the absorption induced by fabrication defects, the total absorption can be classified into SEIA and background absorption (BGA). BGA is decided by material and process technology, which can be obtained by calculations. The sum of SEIA and BGA is predictable because both can be estimated. The substrate absorption of high-energy optics can be semi-quantificationally predicted. SEIA provides a new angle to research element-absorbed laser energy for high-power laser technologies.

Enhancement of the Load Capacity of High-Energy Laser Monocrystalline Silicon Reflector Based on the Selection of Surface Lattice Defects.

Various defects during the manufacture of a high-energy laser monocrystalline silicon reflector will increase the energy absorption rate of the substrate and worsen the optical properties. Micron-scale or larger manufacturing defects have been inhibited by mechanism study and improvement in technology, but the substrate performance still fails to satisfy the application demand. We focus on the changes in the optical properties affected by nanoscale and Angstrom lattice defects on the surface of monocrystalline silicon and acquire the expected high reflectivity and low absorptivity through deterministic control of its defect state. Based on the first principles, the band structures and optical properties of two typical defect models of monocrystalline silicon—namely, atomic vacancy and lattice dislocation—were analyzed by molecular dynamics simulations. The results showed that the reflectivity of the vacancy defect was higher than that of the dislocation defect, and elevating the proportion of the vacancy defect could improve the performance of the monocrystalline silicon in infrared (IR) band. To verify the results of simulations, the combined Ion Beam Figuring (IBF) and Chemical Mechanical Polishing (CMP) technologies were applied to introduce the vacancy defect and reduce the thickness of defect layer. After the process, the reflectivity of the monocrystalline silicon element increased by 5% in the visible light band and by 12% in the IR band. Finally, in the photothermal absorption test at 1064 nm, the photothermal absorption of the element was reduced by 80.5%. Intense laser usability on the monocrystalline silicon surface was achieved, and the effectiveness and feasibility of deterministic regulation of optical properties were verified. This concept will be widely applied in future high-energy laser system and X-ray reflectors.

Weak absorption and scattering losses from the visible to the near-infrared in single-crystal sapphire materials

Durable window materials with minimal optical loss are important for future high-energy laser (HEL) systems that will operate at or near the megawatt level. Sapphire is recognized as a promising HEL window candidate due to its outstanding optical transparency and mechanical properties. However, its weak scattering characteristics and absorption levels near the 1-μm wavelength region need to be lowered further for future HEL applications. In our study, the weak absorption and scattering of sapphire samples provided by three vendors were measured. Ultraviolet–visible spectroscopy measurements were made in the wavelength range 190 to 600 nm and several absorption bands due to vacancies and impurities were detected. The bulk absorption of several samples at wavelengths of 355, 532, and 1064 nm were measured using photothermal common-path interferometry and the absorption coefficient values obtained were in the range of 10 − 5 to 10 − 2 cm − 1 and increased as the wavelength decreased. An empirical weak absorption tail model was used to fit the measured data. In addition, scattering measurements on all samples were made at 405, 532, 633, 1064, and 1550 nm using an instrument developed to assess the bidirectional scatterance probability distribution function. The total integrated scatterance was in the range of 10 − 4 to 10 − 2 and increased for all samples as the wavelength decreased. Surface roughness was found to contribute insignificantly to the scattering loss, while bulk defects along with subsurface damage have major impact. A simple single-scatter model was developed and applied to the measured bulk scattering data. The model suggests that impurity particles, porosity, and other density variations exist with a range of sizes that contribute to scattering. Overall, the measurements indicate that both weak absorption and scattering losses are strongly related to defect structures such as lattice disorder and impurities that were introduced during crystal growth or postgrowth processing. Understanding these defects and their contributions to optical loss can lead to improved manufacturing and processing methods.