Large Laser Systems Research Articles

Study on the influence of local optical field amplification effect on laser-induced damage of fused silica materials

Fused silica is an important part of optical components in large laser systems. Due to the limitation of manufacturing process, impurities and defects in the optical element would greatly reduce the service life of the optical element, and significantly reduce the final output of the laser performance. Aiming at the modulation effect of the internal defect of the component on the internal optical field of the component, the theoretical, simulation and experimental research are carried out. The results show that in the double-bubble impurity coupling, under the same radius R, when the impurity spacing is 1 λ, the local optical field amplification has a maximum value. The effect is equivalent to single-bubble modulation with radius 2 R–3 R. There is an extreme point of the modulated optical field between the air and fused silica crossing line for bubble impurities of different radii. The optical field modulation of small radius impurities is distributed behind the bubble impurities, and the modulation effect of large radius impurities is the maximum when the spacing is 2 λ. The temperature distribution curve of the fused silica element modulated by bubble impurities is consistent with the optical field distribution curve, showing a trend of decreasing slope. The presence of bubble impurities will cause the surface combustion wave of the component to flash and accelerate, and the bubble impurities will increase the generation and expansion rate of the surface combustion wave. This study provides a basis for reducing the uneven distribution of laser energy during the interaction between laser and fused silica, improving the lifetime of the overall optical system, and experimental measurement and analysis.

Design of Pauci-Adaptive, Spirally Tiled Mirrors for Large Size Optical Vortex Generation

We propose the optical design of an adaptive and flexible 4-quadrant, or N-facet, spirally tiled mirror setup, to generate an optical vortex on large diameter laser systems. From a far-field analysis, we derive the optimal angular parameters that generate the highest contents of the targeted Laguerre-Gaussian mode. The modal purities thus obtained are close to those obtained with standard segmented spiral phase plates or mirrors. The intensity behavior in the focal Rayleigh zone is regular, and the effect of a small laser astigmatism, known to break the circularity of the ring intensity profile at focus, can be corrected. This pauci-adaptive faceted mirror setup appears well suited for use on ultra-high intensity or high energy laser systems.

Large dispersion-managed broadband high-energy fiber femtosecond laser system with sub 300 fs pulses and high beam quality output

We experimentally demonstrate a high-energy sub 300 fs polarization maintaining fiber chirped pulse amplification (CPA) system. The preamplifier is a monolithic fiberized system that uses two cascaded temperature-assisted dispersion-tuning broadband chirped fiber Bragg gratings (CFBGs) with a reflected bandwidth of 20 nm as stretchers. To make full use of the stretcher to lower the system’s nonlinearity accumulation, a homemade mode locked fiber laser with a spectral width of 14.8 nm (full width at half maximum) is selected as the seeder to offer a stretched pulse width of 1.69 ns. The main amplifier is based on a one-stage simple Yb:YAG single crystal fiber amplifier with an amplified output power of 40.6 W at a repetition rate of 200 kHz, and the beam quality is conserved in a single mode beam profile with beam quality of 1.246 and 1.142 in the horizontal direction and vertical direction, respectively. During amplification, the spectral gain narrowing effect is observed. To achieve the high-speed switch of the laser, an acoustical optical modulator (AOM) is inserted before the compressor to achieve high-speed turn-on/off control. The compressor is based on a diffraction grating pair with a groove density of 1600 line/mm to offer a dispersion match with the stretcher of the CFBGs. With the CFBG’s fine-tuned capacity of second-order dispersion and higher-order dispersion, the compressed average power of 29.6 W and pulse duration of 278 fs, corresponding to a pulse energy of 148 µJ and a peak power of 532 MW with ignoring the wings of the pulse, is obtained. The beam quality is well conserved after compression, and the beam quality is 1.250 and 1.196 in the horizontal direction and vertical direction, respectively. A power fluctuation of 0.1% (root mean square) and a beam pointing drift of 8.47 µrad/°C over 8 h are realized. This high peak power and high beam quality femtosecond laser is promising in science and industrial applications.

Large Dispersion-Managed Broadband High-Energy Fiber Femtosecond Laser System with Sub 300 Fs Pulses and High Beam Quality Output

Large Dispersion-Managed Broadband High-Energy Fiber Femtosecond Laser System with Sub 300 Fs Pulses and High Beam Quality Output

Spatial mode control based on photonic lanterns

We demonstrate two critical rules of designing photonic lanterns for applications in adaptive spatial mode control: (1) optimized input fiber arrangements to effectively control modes; (2) appropriate input fiber core-cladding ratio to expand the optional range of the output fiber. The 3×1 and 5×1 photonic lanterns according to above design requirements have been fabricated. Using stochastic parallel gradient descent algorithm, the phases of the inputs are actively modulated to stabilize the output of novel 5×1 photonic lantern with 30/125 µm output fiber. When the control target is the fundamental mode, the M2 factor of output beam is below 1.2 stably, which will provide a possible technical solution to increase the mode instability threshold in large mode area fiber laser systems. Furthermore, we obtain single orbital angular momentum mode (OAM01 or OAM02 mode) and high order linearly polarized mode (LP11 or LP21 mode) with the purity of the corresponding modes over 0.85 by altering evaluation function, which will be of benefit in optical communication and atomic optics.

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.

Propagation characteristics of a spatially chirped Gaussian laser beam propagating through multiple lenses

Variation of curvature of laser beam changes characteristics of spatially chirped pulsed Gaussian (SCPG) laser beam primarily due to varying spectral divergence. Parametric dependence of different pulse parameters, e.g. angular chirp, pulse front tilt, pulse width, beam radius, radius of curvature etc., is presented for a SCPG laser beam propagating through single and pair of reflective or refractive optical system. Effect of chromatic aberration on characteristic of SCPG laser beam have also been presented under different propagation conditions. Different expressions for various parameters of pulsed Gaussian beam are extended using recursive relation for a sequence of multiple chromatic lenses generally used in large scale high energy high power laser system. Discussion is supported with few examples of lens system.

Effects of water adsorption on properties of electron-beam HfO2/SiO2 high-reflection coatings

The moisture-induced instability of electron-beam (e-beam) coating can even result in the performance instability of a large laser system. A dense plasma-ion-assisted-deposition SiO2 capping layer was employed to isolate the top surface of the coating from ambient humidity and extend the originally rapid water adsorption process, allowing a deeper understanding of the instability of e-beam coatings caused by moisture. Spectral measurements suggested that four water adsorption steps, including filling, stabilization, swelling and refilling, led to the overall redshift and short-term fluctuations in spectra. The stress and Fourier transform infrared analysis revealed that water adsorption and ice-like/ice structure water was responsible for the increasing compressive stress evolution at the first stage and subsequent transition from compressive to tensile stress evolution, respectively. A model based on the finite element method was proposed to calculate the water vapor transmission rate of e-beam coatings. Our study on the underlying mechanism of moisture-induced instability of e-beam coating is helpful to ensure the reliability and stability of large laser systems.

Effect of Polishing-Induced Subsurface Impurity Defects on Laser Damage Resistance of Fused Silica Optics and Their Removal with HF Acid Etching

Laser-induced damage on fused silica optics remains a major issue that limits the promotion of energy output of large laser systems. Subsurface impurity defects inevitably introduced in the practical polishing process incur strong thermal absorption for incident lasers, seriously lowering the laser-induced damage threshold (LIDT). Here, we simulate the temperature and thermal stress distributions involved in the laser irradiation process to investigate the effect of impurity defects on laser damage resistance. Then, HF-based etchants (HF:NH4F) are applied to remove the subsurface impurity defects and the surface quality, impurity contents and laser damage resistance of etched silica surfaces are tested. The results indicate that the presence of impurity defects could induce a dramatic rise of local temperature and thermal stress. The maximum temperature and stress can reach up to 7073 K and 8739 MPa, respectively, far higher than the melting point and compressive strength of fused silica, resulting in serious laser damage. The effect of impurity defects on laser damage resistance is dependent on the species, size and spatial location of the defects, and CeO2 defects play a dominant role in lowering the LIDT, followed by Fe and Al defects. CeO2 defects with radius of 0.3 μm, which reside 0.15 μm beneath the surface, are the most dangerous defects for incurring laser damage. By HF acid etching, the negative effect of impurity defects on laser damage resistance could be effectively mitigated. It is validated that with HF acid etching, the number of dangerous CeO2 defects is decreased by more than half, and the LIDT could be improved to 27.1 J/cm2.

Ultra-intense laser pulses in near-critical underdense plasmas – radiation reaction and energy partitioning

Although, for current laser pulse energies, the weakly nonlinear regime of laser wakefield acceleration is known to be the optimal for reaching the highest possible electron energies, the capabilities of upcoming large laser systems will provide the possibility of running highly nonlinear regimes of laser pulse propagation in underdense or near-critical plasmas. Using an extended particle-in-cell (PIC) model that takes into account all the relevant physics, we show that such regimes can be implemented with external guiding for a relatively long distance of propagation and allow for the stable transformation of laser energy into other types of energy, including the kinetic energy of a large number of high energy electrons and their incoherent emission of photons. This is despite the fact that the high intensity of the laser pulse triggers a number of new mechanisms of energy depletion, which we investigate systematically.

Characteristics and performance of a two-lens slit spatial filter for high power lasers

The characteristics of a two-lens slit spatial filtering system on image relay and spatial filtering are discussed with detailed theoretical calculation and numerical simulation. The slit spatial filter can be used as the cavity spatial filter in large laser systems, such as National Ignition Facility, which can significantly decrease the focal intensity in cavity spatial filter and suppress or even avoid the pinhole (slit) closure while keeping the output power and beam quality. Additionally, the overall length of the cavity spatial filter can be greatly reduced with the use of the two-lens slit spatial filter.

Determination of ultra-short laser induced damage threshold of KH2PO4 crystal: Numerical calculation and experimental verification

Rapid growth and ultra-precision machining of large-size KDP (KH2PO4) crystals with high laser damage resistance are tough challenges in the development of large laser systems. It is of high interest and practical significance to have theoretical models for scientists and manufacturers to determine the laser-induced damage threshold (LIDT) of actually prepared KDP optics. Here, we numerically and experimentally investigate the laser-induced damage on KDP crystals in ultra-short pulse laser regime. On basis of the rate equation for free electron generation, a model dedicated to predicting the LIDT is developed by considering the synergistic effect of photoionization, impact ionization and decay of electrons. Laser damage tests are performed to measure the single-pulse LIDT with several testing protocols. The testing results combined with previously reported experimental data agree well with those calculated by the model. By taking the light intensification into consideration, the model is successfully applied to quantitatively evaluate the effect of surface flaws inevitably introduced in the preparation processes on the laser damage resistance of KDP crystals. This work can not only contribute to further understanding of the laser damage mechanisms of optical materials, but also provide available models for evaluating the laser damage resistance of exquisitely prepared optical components used in high power laser systems.

Optics Recycle Loop Strategy for NIF Operations above UV Laser-Induced Damage Threshold

The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL) houses the world’s largest laser system, composed of 192 individual, 40-cm-aperture beamlines. The NIF laser routinely operates at ultraviolet (UV) fluences above 8 J/cm2, more than twice the (3ω only) damage threshold of commercially available UV-grade fused silica. NIF is able to maintain such high fluence operation by using an optics recycling loop strategy. Successful operation of the loop relies on a number of technologies specifically developed for NIF. One of the most important is the capability developed by LLNL and their vendors for producing highly damage-resistant optics. Other technologies developed for the optics recycle loop raise the operating point of NIF by keeping damage growth in check. LLNL has demonstrated the capability to sustain UV fused silica optic recycling rates of up to 40 optics per week. The optics are ready for reinstallation after a 3-week trip through a recycle loop where the damage state of each optic is assessed and repaired. The impact of the optics recycle loop has been profound, allowing the experimental program to routinely employ energies and fluences that would otherwise have been unachievable. Without the recycle loop, it is likely that the NIF fluence would need to be kept below the UV threshold for damage growth, ~4 J/cm2, thus keeping the energy delivered to the target significantly below 1 MJ. With the recycle loop implemented during the National Ignition Campaign, NIF can routinely deliver >1.8 MJ on target, an increase in operational capability of more than 100%. In this review, the enabling technological advances, optical performance, and operational capability implications of the optics recycle loop are discussed.

Real-time and sub-wavelength ultrafast coherent diffraction imaging in the extreme ultraviolet.

Coherent Diffraction Imaging is a technique to study matter with nanometer-scale spatial resolution based on coherent illumination of the sample with hard X-ray, soft X-ray or extreme ultraviolet light delivered from synchrotrons or more recently X-ray Free-Electron Lasers. This robust technique simultaneously allows quantitative amplitude and phase contrast imaging. Laser-driven high harmonic generation XUV-sources allow table-top realizations. However, the low conversion efficiency of lab-based sources imposes either a large scale laser system or long exposure times, preventing many applications. Here we present a lensless imaging experiment combining a high numerical aperture (NA = 0.8) setup with a high average power fibre laser driven high harmonic source. The high flux and narrow-band harmonic line at 33.2 nm enables either sub-wavelength spatial resolution close to the Abbe limit (Δr = 0.8λ) for long exposure time, or sub-70 nm imaging in less than one second. The unprecedented high spatial resolution, compactness of the setup together with the real-time capability paves the way for a plethora of applications in fundamental and life sciences.

Overcoming Technical Challenges and Moving into the Future with Laser Glass

The long‐term commercial potential for solid‐state laser gain materials based on glass has only been possible by constant technological developments that have overcome otherwise “market lethal” performance and cost issues. We will discuss a few examples that resulted in the development of completely new manufacturing processes that expanded the laser glass operation window and made possible the construction of large laser systems such as the National Ignition Facility in the United States and the French Laser Mégajoule. In parallel, through compositional modifications and identification of special postprocessing treatments, new active glasses with tailored properties have been continuously developed for specific laser architectures. We will also discuss current research activity directed at finding customized laser glass compositions for the next generation of high‐peak‐power (e.g., Exawatt class) laser systems.

Output temporal contrast simulation of a large aperture high power short pulse laser system

Abstract The work presented in this paper is a study of output temporal contrast degradation by near-field quality deterioration, such as intensity modulation and wavefront deviation, in a large aperture high power short pulse laser system. A two-step focusing algorithm with a coordinate transform based on the Fresnel approximation in the space domain is used for simulating the output focused by an off-axis parabolic mirror. The temporal contrast degradation by intensity modulation and wavefront deviation is analyzed and the influence of the diameter on the temporal contrast degradation is revealed. The simulation and assumption results based on the parameters of the Shen Guang-II laser system are compared with the online experimental temporal contrast data. The near-field quality deterioration might lead to temporal contrast degradation, hindering higher temporal contrast in large aperture high power short pulse laser systems.

Direct fluorescence characterisation of a picosecond seeded optical parametric amplifier

The temporal intensity contrast of high-power lasers based on optical parametric amplification (OPA) can be limited by parametric fluorescence from the non-linear gain stages. Here we present a spectroscopic method for direct measurement of unwanted parametric fluorescence widely applicable from unseeded to fully seeded and saturated OPA operation. Our technique employs simultaneous spectroscopy of fluorescence photons slightly outside the seed bandwidth and strongly attenuated light at the seed central wavelength. To demonstrate its applicability we have characterised the performance of a two-stage picosecond OPA pre-amplifier with 2.8×105 gain, delivering 335μJ pulses at 1054nm. We show that fluorescence from a strongly seeded OPA is reduced by ~500× from the undepleted to full pump depletion regimes. We also determine the vacuum fluctuation driven noise term seeding this OPA fluorescence to be 0.7±0.4photonsps−1nm−1 bandwidth. The resulting shot-to-shot statistics highlights a 1.5% probability of a five-fold and 0.3% probability of a ten-fold increase of fluorescence above the average value. Finally, we show that OPA fluorescence can be limited to a few-ps pedestal with 3×10−9 temporal intensity contrast 1.3ps ahead of an intense laser pulse, a level highly attractive for large scale chirped-pulse OPA laser systems.

Performance of a simplified slit spatial filter for large laser systems.

A new-type slit spatial filter system with three lenses was proposed, in which the focal spot was turned into focal line by adding cylindrical lenses to increase focal area and then lower the focal intensity. Its performances on image relay, aperture matching and spatial filtering are comprehended by detailed theoretical calculations and numerical simulation. According to transmission spatial filter in national ignition facility, we present a replaceable slit spatial filter, which can reduce the overall length of laser system, improve the beam quality and suppress or even avoid the pinhole (slit) closure in the spatial filter.

Possibility of measuring photon-photon scattering via relativistic mirrors

Photon-photon scattering predicted by quantum electrodynamics has not been experimentally confirmed in the optical frequency range. This is due to its extremely small cross section in this energy range. Since the cross section rapidly increases with photon energy, we propose measuring the photon-photon scattering cross section by upshifting optical lasers to the 100-eV to keV range using four relativistic mirror schemes and colliding them. In two of the schemes we find that although the number of scattering events is small with realistic laser parameters, measurement could be possible with an accumulation of a large number of laser shots. In the two other schemes a large number of scattering events is possible, however, the use of large future laser systems will be necessary.

Micro-vibrating spatial filters-induced beam positioning stability in large laser system

A dynamic beam propagation model of micro-vibrating spatial filters in inertial confinement fusion (ICF) facilities is built based on the additional beam in SG-II facility. The transfer matrix is then deduced, and the sensitivities of the beam positioning to the pellet in the target area to the vibrations of every spatial filter are analyzed, which indicates that the vibrations of spatial filters in the pre-amplify zone has less effects on beam positioning stability at the target. In addition, the vibrations of spatial filters in the main amplify zone dominates the beam positioning stability of the target, especially the vibration of the spatial filter SF7.