Er:CaF2 crystals are crucial gain media for producing 3 μm mid-infrared (MIR) lasers pumped by 976 nm continuous-wave (CW) lasers owing to their low phonon energy and high conversion efficiency. This study investigated the damage characteristics and mechanism of Er:CaF2 crystals irradiated with a 976 nm CW laser. The laser-induced damage threshold of Er:CaF2 crystals with different Er3+ doping levels was tested; the damage morphology consists of a series of regular 70° cracks related to the angle of the crystal slip system on the surface. A finite-element model was used to calculate the temperature and stress fields of the crystals. The results indicated that the damage can be attributed to surface tensile stresses caused by the temperature gradient, and crystals with higher doping concentrations were more susceptible to damage owing to stronger light absorption. These findings provide valuable insights into the development of high-power MIR lasers.

Plasma-enhanced atomic layer deposition (PEALD) is gaining interest in thin films for laser applications, and post-annealing treatments are often used to improve thin film properties. However, research to improve thin film properties is often based on an expensive and time-consuming trial-and-error process. In this study, PEALD-HfO2 thin film samples were deposited and treated under different annealing atmospheres and temperatures. The samples were characterized in terms of their refractive indices, layer thicknesses and O/Hf ratios. The collected data were split into training and validation sets and fed to multiple back-propagation neural networks with different hidden layers to determine the best way to construct the process–performance relationship. The results showed that the three-hidden-layer back-propagation neural network (THL-BPNN) achieved stable and accurate fitting. For the refractive index, layer thickness and O/Hf ratio, the THL-BPNN model achieved accuracy values of 0.99, 0.94 and 0.94, respectively, on the training set and 0.99, 0.91 and 0.90, respectively, on the validation set. The THL-BPNN model was further used to predict the laser-induced damage threshold of PEALD-HfO2 thin films and the properties of the PEALD-SiO2 thin films, both showing high accuracy. This study not only provides quantitative guidance for the improvement of thin film properties but also proposes a general model that can be applied to predict the properties of different types of laser thin films, saving experimental costs for process optimization.

The large-aperture pulse compression grating (PCG) is a critical component in generating an ultra-high-intensity, ultra-short-pulse laser; however, the size of the PCG manufactured by transmission holographic exposure is limited to large-scale high-quality materials. The reflective method is a potential way for solving the size limitation, but there is still no successful precedent due to the lack of scientific specifications and advanced processing technology of exposure mirrors. In this paper, an analytical model is developed to clarify the specifications of components, and advanced processing technology is adopted to control the spatial frequency errors. Hereafter, we have successfully fabricated a multilayer dielectric grating of 200 mm × 150 mm by using an off-axis reflective exposure system with Φ300 mm. This demonstration proves that PCGs can be manufactured by using the reflection holographic exposure method and shows the potential for manufacturing the meter-level gratings used in 100 petawatt class high-power lasers.

Multilayer dielectric gratings (MLDGs) are crucial for pulse compression in picosecond–petawatt laser systems. Bulged nodular defects, embedded in coating stacks during multilayer deposition, influence the lithographic process and performance of the final MLDG products. In this study, the integration of nanosecond laser conditioning (NLC) into different manufacturing stages of MLDGs was proposed for the first time on multilayer dielectric films (MLDFs) and final grating products to improve laser-induced damage performance. The results suggest that the remaining nodular ejection pits introduced by the two protocols exhibit a high nanosecond laser damage resistance, which remains stable when the irradiated laser fluence is more than twice the nanosecond-laser-induced damage threshold (nanosecond-LIDT) of the unconditioned MLDGs. Furthermore, the picosecond-LIDT of the nodular ejection pit conditioned on the MLDFs was approximately 40% higher than that of the nodular defects, and the loss of the grating structure surrounding the nodular defects was avoided. Therefore, NLC is an effective strategy for improving the laser damage resistance of MLDGs.

Maximizing the energy-loading performance of gratings is a universal theme in high-energy pulse compression. However, sporadic grating designs strongly restrict the development of high-power laser engineering. This study proposes an all- and mixed-dielectric grating design paradigm for Nd:glass-based pulse compressors. The solution regions are classified according to the line density. High diffraction efficiency solutions are described in more detail based on the dispersion amount and incident angle. Moreover, an energy scaling factor of 7.09 times larger than that of the National Ignition Facility’s Advanced Radiographic Capability (NIF-ARC) is obtained by taking advantage of the low electric field intensity at transverse magnetic polarization and a small incident angle. These results make a pioneering contribution to facilitate future 20–50-petawatt-class ultrafast laser systems.

The laser-induced damage threshold (LIDT) of plate laser beam splitter (PLBS) coatings is closely related to the subsurface absorption defects of the substrate. Herein, a two-step deposition temperature method is proposed to understand the effect of substrate subsurface impurity defects on the LIDT of PLBS coatings. Firstly, BK7 substrates are heat-treated at three different temperatures. The surface morphology and subsurface impurity defect distribution of the substrate before and after the heat treatment are compared. Then, a PLBS coating consisting of alternating HfO2–Al2O3 mixture and SiO2 layers is designed to achieve a beam-splitting ratio (transmittance to reflectance, s-polarized light) of approximately 50:50 at 1053 nm and an angle of incidence of 45°, and it is prepared under four different deposition processes. The experimental and simulation results show that the subsurface impurity defects of the substrate migrate to the surface and accumulate on the surface during the heat treatment, and become absorption defect sources or nodule defect seeds in the coating, reducing the LIDT of the coating. The higher the heat treatment temperature, the more evident the migration and accumulation of impurity defects. A lower deposition temperature (at which the coating can be fully oxidized) helps to improve the LIDT of the PLBS coating. When the deposition temperature is 140°C, the LIDT (s-polarized light, wavelength: 1064 nm, pulse width: 9 ns, incident angle: 45°) of the PLBS coating is 26.2 J/cm2, which is approximately 6.7 times that of the PLBS coating deposited at 200°C. We believe that the investigation into the laser damage mechanism of PLBS coatings will help to improve the LIDT of coatings with partial or high transmittance at laser wavelengths.

In this paper, a 2D angle amplifier based on peristrophic multiplexed volume Bragg gratings is designed and prepared, in which a calculation method is firstly proposed to optimize the number of channels to a minimum. The induction of peristrophic multiplexing reduces the performance difference in one bulk of the grating, whereas there is no need to deliberately optimize the fabrication process. It is revealed that a discrete 2D angle deflection range of ±30° is obtained and the relative diffraction efficiency of all the grating channels reaches more than 55% with a root-mean-square deviation of less than 3.4% in the same grating. The deviation of the Bragg incidence and exit angles from the expected values is less than 0.07°. It is believed that the proposed 2D angle amplifier has the potential to realize high-performance and large-angle beam steering in high-power laser beam scanning systems.

Multilayer dielectric gratings typically remove multiple-grating pillars after picosecond laser irradiation; however, the dynamic formation process of the removal is still unclear. In this study, the damage morphologies of multilayer dielectric gratings induced by an 8.6-ps laser pulse were closely examined. The damage included the removal of a single grating pillar and consecutive adjacent grating pillars and did not involve the destruction of the internal high-reflection mirror structure. Comparative analysis of the two damage morphological characteristics indicated the removal of adjacent pillars was related to an impact process caused by the eruption of localized materials from the left-hand pillar, exerting impact pressure on its adjacent pillars and eventually resulting in multiple pillar removal. A finite-element strain model was used to calculate the stress distribution of the grating after impact. According to the electric field distribution, the eruptive pressure of the dielectric materials after ionization was also simulated. The results suggest that the eruptive pressure resulted in a stress concentration at the root of the adjacent pillar that was sufficient to cause damage, corresponding to the experimental removal of the adjacent pillar from the root. This study provides further understanding of the laser-induced damage behavior of grating pillars and some insights into reducing the undesirable damage process for practical applications.

Various coatings in high-power laser facilities suffer from laser damage due to nodule defects. We propose a nodule dome removal (NDR) strategy to eliminate unwanted localized electric-field (E-field) enhancement caused by nodule defects, thereby improving the laser-induced damage threshold (LIDT) of laser coatings. It is theoretically demonstrated that the proposed NDR strategy can reduce the localized E-field enhancement of nodules in mirror coatings, polarizer coatings and beam splitter coatings. An ultraviolet (UV) mirror coating is experimentally demonstrated using the NDR strategy. The LIDT is improved to about 1.9 and 2.2 times for the UV mirror coating without artificial nodules and the UV mirror coating with artificial nodule seeds with a diameter of 1000 nm, respectively. The NDR strategy, applicable to coatings prepared by different deposition methods, improves the LIDT of laser coating without affecting other properties, such as the spectrum, stress and surface roughness, indicating its broad applicability in high-LIDT laser coatings.

To reduce the seed length while maintaining the advantages of the cuboid KDP-type crystal, a long-seed KDP crystal with size $471~\text{mm}\times 480~\text{mm}\times 400~\text{mm}$ is rapidly grown. With almost the same high cutting efficiency to obtain third harmonic generation oriented samples, this long-seed KDP-type crystal can be grown with a shorter seed than that of the cuboid KDP-type crystal. The full width at half maximum of the high-resolution X-ray diffraction of the (200) crystalline face is 28.8 arc seconds, indicating that the long-seed KDP crystal has good crystalline quality. In the wavelength range of 377–1022 nm, the transmittance of the long-seed KDP crystal is higher than 90%. The fluence for the 50% probability of laser-induced damage (LID) is $18.5~\text{J}/\text{cm}^{2}$ (3 ns, 355 nm). Several test points survive when the laser fluence exceeds $30~\text{J}/\text{cm}^{2}$ (3 ns, 355 nm), indicating the good LID performance of the long-seed KDP crystal. At present, the growth of a long-seed DKDP crystal is under way.

The nonlinear absorption (NLA) properties of potassium dideuterium phosphate crystals at 515 nm under different excitation laser intensities are investigated with the Z-scan technique. Two critical intensities are highlighted: the critical intensity for exciting the NLA and the critical intensity of the multiphoton absorption mechanism transition. Experimental results indicate the existence of defect states located in the band gap, which can be manipulated by varying laser intensity. A model based on the change of multiphoton absorption mechanism induced by the transformation of defect species is proposed to interpret the experiments. Modeling results are in good agreement with the experiment data.

Rapid growth processing of KDP crystals was improved by employing continuous filtration to eliminate bulk defects. The performances of the KDP crystals, including scattering defects, laser damage resistance and transmittance, were measured and analyzed. Compared with rapid-grown KDP without continuous filtration, the transmittance in the near-infrared was increased by at least 2%, almost all of ‘micron size’ defects were eliminated and ‘sub-micron size’ defects were decreased by approximately 90%. Laser damage testing revealed that the laser-induced damage thresholds (LIDTs), as well as the consistency of the LIDTs from sample to sample, were improved greatly. Moreover, it identified that ‘micron size’ defects were the precursors which initiated laser damage at relative lower laser fluence (4–6 J cm−2), and there was a lower correlation between smaller size scattering defects and laser damage initiation. The improved consistency in the LIDTs, attributed to elimination of ‘micron size’ defects, and LIDT enhancement originated from the decreased absorption of the KDP crystals.

The characteristic parameters of initiating defects in HfO2/SiO2 high-reflector film at wavelength of 1064 nm are investigated through the combined application of laser-conditioning process and the multispot damage threshold test. The laser damage tests before and after laser-conditioning are carried out within three different spot diameters. The damage threshold and the density are considered to characterize the initiating defects which are shown to be susceptible to laser damage. These characteristic parameters are obtained through fitting the damage data with the spot size effect. The influence of laser-conditioning effect on the initiating defects is analyzed. The initiating defects are specified by a Gaussian distribution after laser-conditioning. It is discussed the essential roles of the initiating defects to the damage threshold of the film. The investigation on characteristic parameters of initiating defects has potential to be applied in the mechanisms of laser induced damage and laser-conditioning, as well as in the direction the preparation of the films.

Laser resistance and stress-free mirrors, windows, polarizers, and beam splitters up to $400~\mathrm{mm} \times 400~\mathrm{mm} $ are required for the construction of the series SG facilities. In order to improve the coating quality, a program has been in place for the last ten years. For the small-aperture pick-off mirror, the laser-induced damage threshold (LIDT) is above $60~\mathrm{J} / {\mathrm{cm} }^{2} $ (1064 nm, 3 ns), and the reflected wavefront is less than $\lambda / 4~(\lambda = 633~\mathrm{nm} )$. The Brewster-angle polarizing beam splitter ($\Phi 50\times 10~\mathrm{mm} $) shows the best LIDT result, up to $29. 8~\mathrm{J} ~{\mathrm{cm} }^{- 2} $ (1064 nm, 10 ns) for a p-polarized wave in the 2012 damage competition of the XLIV Annual Boulder Damage Symposium. For the larger-aperture mirror and polarizer, the LIDT is above $23~\mathrm{J} ~{\mathrm{cm} }^{- 2} $ (1064 nm, 3 ns) and $14~\mathrm{J} ~{\mathrm{cm} }^{- 2} $ (1064 nm, 3 ns), respectively. The reflected wavefront is less than $\lambda / 3~(\lambda = 633~\mathrm{nm} )$ at the used angle.