Large Optics for the National Ignition Facility

Laser Megajoule performance status

The Laser Megajoule (LMJ) is among the most energetic inertial confinement fusion laser facilities in the world, together with the National Ignition Facility (NIF) in the USA. The construction of the facility began back in 2003, and the first photons were emitted by the laser bundle #28 in 2014. Today, 11 laser bundles consisting of 88 large aperture 0.35×0.35m 2 laser beams are in operation, delivering daily up to 330 kJ of energy at the wavelength of 351 nm on a target placed in the center of a 10 m diameter vacuum chamber. In this paper, we describe the laser system and its operational performances. We also detail the first laser campaigns carried out to prepare an increase of energy and power on the target. These campaigns, along with the completion of additional bundles mounting, will bring LMJ performance to 1.3 MJ thanks to 22 bundles in operation.

Evolution mechanism of subsurface damage during laser machining process of fused silica

The machining-induced subsurface damage (SSD) on fused silica optics would incur damage when irradiated by intense lasers, which severely restricts the service life of fused silica optics. The high absorption of fused silica to 10.6 µm makes it possible to utilize pulsed CO2 laser to remove and characterize SSD by layer-by-layer ablation, which improves its laser-induced damage threshold. However, thermal stress during the laser ablation process may have an impact on SSD, leading to extension. Still, the law of SSD morphology evolution mechanism has not been clearly revealed. In this work, a multi-physics simulated model considering light field modulation is established to reveal the evolution law of radial SSD during the laser layer-by-layer ablation process. Based on the simulation of different characteristic structural parameters, two evolution mechanisms of radial SSD are revealed, and the influence of characteristic structural parameters on SSD is also elaborated. By prefabricating the SSD by femtosecond laser, the measurements of SSD during CO2 laser layer-by-layer ablation experiments are consistent with the simulated results, and three stages of SSD depth variation under two evolution processes are further proposed. The findings of this study provide theoretical guidance for effectively characterizing SSD based on laser layer-by-layer ablation strategies on fused silica optics.

Nano-imprint lithography of broad-band and wide-angle antireflective structures for high-power lasers

We demonstrate efficient anti reflection coatings based on adiabatic index matching obtained via nano-imprint lithography. They exhibit high total transmission, achromaticity (99.5% < T < 99.8% from 390 to 900 nm and 99% < T < 99.5% from 800 to 1600 nm) and wide angular acceptance (T > 99% up to 50 degrees). Our devices show high laser-induced damage thresholds in the sub-picosecond (>5 J/cm2 at 1030 nm, 500 fs), nanosecond (>150 J/cm2 at 1064 nm, 12 ns and >100 J/cm2 at 532 nm, 12 ns) regimes, and low absorption in the CW regime (<1.3 ppm at 1080 nm), close to those of the fused silica substrate.

Single-test Ritchey-Common interferometry

The Ritchey-Common test is widely adopted to measure large optical flats. The traditional Ritchey-Common test eliminates the defocus error with multiple tests by changing the position of the mirrors, which suffers from cumbersome steps, poor repeatability, coupled system error, extra mirror deformation, and potential overturning. The above problems increase the test time, decrease the reliability and accuracy, increase the test cost, and threaten manufacturing safety. We propose a single-test Ritchey-Common interferometry to avoid the obligatory position change in the traditional method. A sub-aperture of test flat is directly measured by a small-aperture interferometer before the test, which is easy to implement, to replace the extra system wavefront measurement in different positions. The defocus is calculated in sub-aperture at exactly the same position as the full-field measurement without the position change, then the surface form under test can be obtained with accurate optical path modeling. Measurement experiments for 100 mm and 2050 mm aperture flats were performed to demonstrate the feasibility of this method. Compared with a direct test in a standard Zygo interferometer, the peak to valley (PV) and root mean square (RMS) errors were 0.0889 λ and 0.0126 λ (λ=632.8 nm), respectively, which reaches the upper limit of accuracy of the interferometer. To the best of our knowledge, this is the first proposal of the Ritchey-Common test that can eliminate the defocus error and realize high accuracy measurement in a single test. Our work paves the way for reliable and practical optical metrology for large optical flats.

Unveiling sub-bandgap energy-level structures on machined optical surfaces based on weak photo-luminescence

Sub-bandgap defect energy levels (SDELs) introduced by the point defects located in surface defect areas are considered the main factors in decreasing laser-induced damage thresholds (LIDTs). The suppression of SDELs could greatly increase LIDTs. However, no available method could detect SDELs, limiting the characterization and suppression of SDELs. Herein, a self-designed photo-luminescence detection system is developed to explore the weak transient-steady photo-luminescence properties of machined surfaces. Based on the excitation laser wavelength dependence of photo-luminescence properties, a sub-bandgap energy-level structure (SELS) containing SDELs is unveiled for the first time. Based on the developed mathematical model for predicting LIDTs, the feasibility of the detection method was verified. In summary, this work provides a novel approach to characterize SDELs on machined surfaces. This work could construct electronic structures and explore the transition behaviors of electrons, which is vital to laser-induced damage. Besides, this work could predict the LIDTs of the machined surfaces based on their PL properties, which provides convenience for evaluating the LIDTs of various optical elements in industrial production. Moreover, this work provides a convenient method for raising the LIDTs of various optical elements through monitoring and suppressing the SDELs on machined surfaces.

Evolution of intrinsic defects and ring structures on the surface of fused silica optics after CO2 laser conditioning

Recently and interestingly, experiments show that the CO2 laser conditioning can significantly increase the laser-induced damage threshold (LIDT) of fused silica optics, but its underlying mechanism has not been clearly revealed. This Letter reports the experimental studies on the evolution of the intrinsic point defects and intrinsic ring structures on the surface of fused silica optics under the CO2 laser irradiation. The laser conditioning can effectively reduce the intrinsic defect contents in the surface layer of mechanically processed fused silica. However, the suppression effect of defects can be affected by the initial surface state. If there are micro-cracks on the component surface, the effect of the laser conditioning would be limited. The evolution of the intrinsic ring structures indicate that most of the intrinsic defects tend to recombine as short (Si-O)n ring structures during the laser healing of the micro-fractures. The observed recombination behavior and suppression of the intrinsic defects can help find out the reason for the increase of the LIDT of the fused silica optics.

Investigation of the Multimechanism Laser Cleaning Dynamics for Rough Fused Silica Surfaces with Organic Contaminants: A Computational Simulation and Atomic Analysis

The detrimental impact of organic contaminants on optical components poses a significant obstacle to high-energy laser systems. However, irregularities or defects on the surface of optical components during manufacturing can affect the process of organic contaminant removal. Thus, a comprehensive understanding of the intricate interplay among surface roughness, contaminant absorption, and ablation is essential to effectively address the challenges of laser-induced damage. In this study, a molecular dynamics approach was employed to investigate the interaction between laser-fused silica and contaminants and to analyze the influence of surface roughness on the removal of contaminants from fused silica. Research findings demonstrate that during laser irradiation, organic contaminants on the surface of mechanical components diffuse into the optical elements. As the laser flux increases, the contaminants gradually decompose into smaller molecular clusters. Additionally, the phenomenon of contaminant ablation is observed to consist of two distinct phases: the "Thermal expansion phase" and the "Thermal ablation phase." The study examines the impact of substrate roughness on the contaminant removal in these two phases. It is found that a higher surface roughness leads to stronger thermal expansion and vaporization of contaminants. With increasing roughness of the fused silica substrate, the corresponding van der Waals energy and pressure decrease under the same laser fluence, making the removal of contaminants easier. These results provide valuable insights into the interaction between laser irradiation and organic contaminants.

Research on the influence of the non-stationary effect of the magnetorheological finishing removal function on mid-frequency errors of optical component surfaces

With the continuous development of modern optical systems, the demand for full spatial frequency errors of optical components in the system is increasing. Although computer-controlled sub-aperture polishing technology can quickly correct low-frequency errors, this technology significantly worsens the mid-frequency errors on the surface of the component, which greatly inhibits the improvement of optical system performance. Therefore, we conducted in-depth research on the non-stationary effect of the removal function caused by the fluctuation in magnetorheological polishing and their influence on the mid-frequency errors of the component surface. We established a non-stationary profile model of the removal function and applied this model to simulate the distribution of mid-frequency errors on the surface of the processed component, considering the non-stationary effect. The simulation results showed that the non-stationary effect of the removal function weaken the mid-frequency ripple errors but increase other mid-frequency errors. Therefore, we first proposed the optimal single-material removal thickness corresponding to the non-stationary effect and experimentally verified the effectiveness of the optimal material removal thickness in suppressing mid-frequency errors. The experimental results showed that when the magnetorheological finishing single-material removal thickness is set to the optimal value, both the mid-frequency ripple errors and the mid-frequency RMS on the surface significantly decrease. Therefore, this work provides a basis for improving the existing magnetorheological finishing process and effectively suppressing the mid-frequency errors on the surface of processed components. It also provides theoretical and technical support for the magnetorheological processing and manufacturing of high-precision optical components. At the same time, the non-stationary effect and the corresponding analytical models has the potential to be extended to other polishing tools.

Linear-to-circular polarization conversion with full-silica meta-optics to reduce nonlinear effects in high-energy lasers

High-energy lasers have benefited from intense efforts to bring light-matter interactions to new standards and to achieve laser fusion ignition. One of the main issues to further increasing laser energy is the resistance of optical materials to high laser fluences, in particular at the final stage of the laser beamline where nonlinear Kerr effects can occur in optical materials and provoke laser filamentation. One promising way to mitigate this process is to reduce the nonlinear susceptibility of the material by switching the polarization from a linear to a circular state. Here, we report a significant reduction in the laser filamentation effect on glass by using a full-silica metamaterial waveplateable to switch the linear-to-circular polarization of high fluence laser beams. This result is achieved through the use of a large size full-silica meta-optics exhibiting nominal polarization conversion associated with an excellent transmission efficiency and wavefront quality, as well as a high laser damage resistance.

Interferometric measurement of the radius of curvature based on axial displacement from a confocal position and corresponding defocus wavefront

The radius of curvature (R) is a fundamental parameter of spherical optical surfaces. The measurement range of the widely adopted traditional interferometric method is limited by the length of the precision linear guide rail carrying the measured surface from the cat's eye to the confocal position, and the test result is vulnerable to airflow and vibration in the test environment. An interferometric method is proposed for the radius measurement of spherical surfaces based on a small axial moving distance and the corresponding defocus wavefront to eliminate the dependence on a long guide rail and extend the measuring range. To eliminate the influence of the test environment and calculate the R, a defocus transform algorithm is proposed to instantaneously measure the defocus wavefront from a single interferogram. Numerical simulations theoretically demonstrate that there is no limit to the measurement range of this method because only a short distance of the measured mirror must be moved. A spherical mirror with a radius of curvature of 101.6087 mm is experimentally tested, and the relative measurement error is 0.037%. This method can achieve high accuracy for optical shops and greatly increase the measurement range of the interferometric method without additional equipment.

Formation mechanism of DKDP surface in single point diamond fly-cutting process and the resulting degradation of laser-induced damage performance

Laser damage performance of DKDP (KD_2xH_2(1-x)PO₄) crystal is largely determined by the surface microstructures generated in the manufacturing process, more specifically, single point diamond fly-cutting process. However, because of the lack of knowledge about the formation mechanism and damage performance of the microstructures, laser induced damage of DKDP crystal remains a key issue limiting the output energy of the high power laser systems. In this paper, the influence of fly-cutting parameters on the generation of DKDP surface and the underlying material deformation mechanism have been investigated. Except for cracks, two kinds of new microstructures, namely micro grains and ripples, have been found on the processed DKDP surfaces. GIXRD, nano-indentation and nano-scratch test results prove that the micro grains are generated by the slip motion of the crystal, while the simulation results show that the cracks are induced by the tensile stress formed behind the cutting edge. Moreover, the formation of micro grains can facilitate the plastic chip flow through the mechanism of grain boundary sliding, which will further lead to a periodic fluctuation of the chip separation point and the formation of micro ripples. Finally, laser damage test results demonstrate that cracks will degrade the damage performance of DKDP surface significantly, while the formation of micro grains and micro ripples has little impact. The results of this study can deepen the understanding of the formation mechanism of the DKDP surface during the cutting process and provide guidance to improve the laser-induced damage performance of the crystal.

Damage analysis and mechanism study of sol-gel coating over KDP crystal under multi-pulse of laser irradiation at low flux

The purpose of this study is to analyze the damage of antireflective (AR) coating over potassium dihydrogen phosphate (KDP) crystal subjected to multi-pulse laser irradiation at low flux under vacuum. Fresh silica AR was characterized as a reference; Atomic Force Microscope (AFM), Scanning Electron Microscopy (SEM), profilometer, and Scanning Near-Field Optical Microscope Photo-induced Force Microscope (SNOM-PiFM) were employed to analyze the characteristics of coatings. The experimental results indicated that the damage of AR coating over the KDP crystal was mainly caused by partial exfoliation, which exposed silica particles beneath the surface. It was found that the accumulated tensile stress led to coating damage with the increase of laser pulse. The initial coating damage was observed to extend and interconnect to form large-area exfoliation. Splitting mechanism of SiO-Si TO₃ was observed at vibration mode peaks of 1064 cm^-1 and 1096 cm^-1showing progressing irradiation damage. Based on this study, it would be helpful to suppress the damage probability of AR coating over KDP crystal applied in high-power laser systems. Moreover, the applicability of SNOM-PiFM method to study the Infrared Radiation (IR) spectra of ultra-thin coatings with transparent substrates was proposed.

Evolution of the point defects involved under the action of mechanical forces on mechanically machined fused silica surfaces

Point defects with different species are concentrated on most mechanically machined fused silica optical surfaces with surface defects, which would sharply decrease the laser damage resistance under intense laser irradiation. Various point defects have distinct roles in affecting the laser damage resistance. Especially, the proportions of various point defects have not been identified, posing the challenge in relating the intrinsic quantitative relationship among various point defects. To fully reveal the comprehensive effect of various point defects, it is necessary to systematically explore the origins, evolution laws and especially the quantitative relationship among point defects. Herein, seven types of point defects are determined. The unbonded electrons in point defects are found to tend to be ionized to induce laser damage and there is a definite quantitative relationship between the proportions of oxygen-deficient point defects and that of peroxide point defects. The conclusions are further verified based on the photoluminescence (PL) emission spectra and the properties (e.g., reaction rule and structural feature) of the point defects. On basis of the fitted Gaussian components and electronic-transition theory, the quantitative relationship between PL and the proportions of various point defects is constructed for the first time. E'-Center accounts for the highest proportion among them. This work is beneficial for fully revealing the comprehensive action mechanisms of various point defects and providing new insights in elucidating the defect-induced laser damage mechanisms of optical components under intense laser irradiation from the atomic scale.

Full-scale optic designed for onsite study of damage growth at the Laser MegaJoule facility

Large fusion scale laser facilities aim at delivering megajoules laser energy in the UV spectrum and nanosecond regime. Due to the extreme laser energies, the laser damage of final optics of such beamlines is an important issue that must be addressed. Once a damage site initiates, it grows at each laser shot which decreases the quality of the optical component and spoil laser performances. Operation at full energy and power of such laser facilities requires a perfect control of damage kinetics and laser parameters. Monitoring damage kinetics involves onsite observation, understanding of damage growth process and prediction of growth features. Facilities are equipped with cameras dedicated to the monitoring of damage site growth. Here we propose to design and manufacture a dedicated full size optical component to study damage growth at increased energy, on the beamline, i.e. in the real environment of the optics on a large laser facility. Used for the first time in 2021, the growth statistics acquired by this approach at the Laser MegaJoule (LMJ) facility provides a new calibration point at a fluence less than 5 J cm^-2 and a flat-in-time pulse of 3 ns.

Optical model-based calibration of gray levels for laser damage size assessment

Fused silica is prone to damage under ultraviolet laser irradiation. Because they are key components to achieve fusion on high energy laser facilities, final fused silica optics are analyzed after each laser shot. The quantification of damage sites is limited by the image resolution. Measurements of scattered light by damage sites allow for sub-pixel detection and growth monitoring after a calibration step based on time-consuming measurements at laser facilities. It is proven herein that modeling laser damage size monitoring based on light scattering is efficient to link gray levels to damage diameters, thereby avoiding any experimental calibration based on reference optics at the facility.

Characteristic of fixed abrasive polishing for KDP in an anhydrous environment

High grade optical components are rapidly being employed in a wide range of applications due to their great performance. Among these, KDP (potassium dihydrogen phosphate) is the preferred choice for frequency doubling components because to its large non-linear optical technique and high laser damage threshold. In this research, KDP is treated employing fixed polishing in an anhydrous environment. Copper oxide is utilized as abrasive, and toner and phenolic resin are combined to form pellets for KDP polishing. Calculation of the temperature field on the surface of the workpiece based on the relationship of the movement. The ideal polishing parameters are also determined from the findings of the temperature field and the optimum process parameters are 5kPa pressure, 150rpm and 8mm eccentricity. A new material removal model is presented and based on this, a unique processing technique to increase the material removal depth is provided. Interestingly, with this form of polishing, the surface roughness of the workpiece remains about Ra 20nm after a quick shift and does not alter with re-pressing.

Stress and wavefront measurement of large-aperture optical components with a ptychographical iterative engine

The clamping stress of large-aperture optical elements has a significant influence on the optical quality of the system. In this study, a comprehensive measurement system combined with ptychographical iterative engine (PIE) wavefront sensors and polarization components is developed to determine the stress distribution of the optical elements and its effect on the transmitted and reflected wavefronts. This system avoids the use of multiple measuring instruments and has low cost and strong anti-interference ability. The experimental results demonstrate that the stress distributions measured at different resolutions are consistent with the finite element analysis, and the wavefront measurement accuracy is 0.1λ. This test configuration is very flexible and provides a useful means for online installation and quality control of large-aperture optical systems.

Development of a confocal line-scan laser scattering probe for dark-field surface defects detection of transmissive optics

Dark-field detection has long been used to identify micron/submicron-sized surface defects benefiting from the broadening effect of the actual defect size caused by light scattering. However, the back-side scattering of a transmissive optical slab is inevitably confused with the front-side scattering phenomenon, resulting in deterioration of the signal-to-noise ratio (SNR) of the scattering signal and false alarms for real defect detection. To this end, a confocal line-scan laser scattering probe equipped with optical sectioning ability is proposed to separate the back-side scattering from the front-side scattering. The optical sectioning ability is realized through a confocal light scattering collector, which overcomes the restriction imposed on the numerical aperture (NA) and the field of view (FOV), reaching an FOV length of 90 mm and NA of 0.69. The line-scan principle of the probe protects itself from crosstalk because it produces only a laser spot on the tested surface in an instant. Experimental results verified that the probe has a line-scan length of 90 mm with a uniformity better than 98%, an rms electronic noise of 3.4 mV, and an rms background noise of 6.4 mV with laser on. The probe can reject the false back-side scattering light for a 2 mm thick fused silica slab at 17.1 dB SNR and operate at a high imaging efficiency of 720 mm²/s with a minimum detectability limit of 1.4 µm at 12 dB SNR. This work put forward an effective method with great application value for submicron-sized defect detection in transmissive optics.

Progress on deuterated potassium dihydrogen phosphate (DKDP) crystals for high power laser system application

In this review, we introduce the progress in the growth of large-aperture DKDP crystals and some aspects of crystal quality including determination of deuterium content, homogeneity of deuterium distribution, residual strains, nonlinear absorption, and laser-induced damage resistance for its application in high power laser system. Large-aperture high-quality DKDP crystal with deuteration level of 70% has been successfully grown by the traditional method, which can fabricate the large single-crystal optics with the size exceeding 400 mm. Neutron diffraction technique is an efficient method to research the deuterium content and 3D residual strains in single crystals. More efforts have been paid in the processes of purity of raw materials, continuous filtration technology, thermal annealing and laser conditioning for increasing the laser-induced damage threshold (LIDT) and these processes enable the currently grown crystals to meet the specifications of the laser system for inertial confinement fusion (ICF), although the laser damage mechanism and laser conditioning mechanism are still not well understood. The advancements on growth of large-aperture high-quality DKDP crystal would support the development of ICF in China.

An end-to-end laser-induced damage change detection approach for optical elements via siamese network and multi-layer perceptrons

With the presence of complex background noise, parasitic light, and dust attachment, it is still a challenging issue to perform high-precision laser-induced damage change detection of optical elements in the captured optical images. For resolving this problem, this paper presents an end-to-end damage change detection model based on siamese network and multi-layer perceptrons (SiamMLP). Firstly, representative features of bi-temporal damage images are efficiently extracted by the cascaded multi-layer perceptron modules in the siamese network. After that, the extracted features are concatenated and then classified into changed and unchanged classes. Due to its concise architecture and strong feature representation ability, the proposed method obtains excellent damage change detection results efficiently and effectively. To address the unbalanced distribution of hard and easy samples, a novel metric called hard metric is introduced in this paper for quantitatively evaluating the classification difficulty degree of the samples. The hard metric assigns a classification difficulty for each individual sample to precisely adjust the loss assigned to the sample. In the training stage, a novel hard loss is presented to train the proposed model. Cooperating with the hard metric, the hard loss can up-weight the loss of hard samples and down-weight the loss of easy samples, which results in a more powerful online hard sample mining ability of the proposed model. The experimental results on two real datasets validate the effectiveness and superiority of the proposed method.

Novel abrasive-free jet polishing for Bulk single-crystal KDP with a low viscosity microemulsion

In present work, the abrasive-free jet polishing (AFJP) of bulk single-crystal KDP was first fulfilled, when using a newly-designed low-viscosity microemulsion as the AFJP fluid. The novel AFJP fluid shows a typical water-in-oil structure, in which the water cores uniformly distribute in the BmimPF6 IL, with a particle size of about 20-25 nm. What's more, the AFJP fluid is a controllable and selective non-abrasive jet fluid that the shape of the removal function is regular and smooth, presenting a similar Gaussian function, meanwhile, the dispersion coefficient of the removal rate is only 1.9%. Finally, the surface quality of the bulk single-crystal KDP is further improved by AFJP, meanwhile, the subsurface damage is first obviously mitigated.

The Cause of Ribbon Fluctuation in Magnetorheological Finishing and Its Influence on Surface Mid-Spatial Frequency Error

In the high-power laser system, the mid-spatial frequency error of the surface of the high-power laser component will affect the normal operation of the high-power laser system. In order to improve the mid-spatial frequency error of the high-power laser component after magnetorheological finishing, the causes and influencing factors of the ribbon fluctuation in magnetorheological finishing are studied, and the influence of different ribbon fluctuation on the mid-spatial frequency error of the surface is studied. Firstly, the influence of different ribbon fluctuations on the mid-spatial frequency error of the machined surface is simulated by a computer. Secondly, the magnetic field in the circumferential direction of the polishing wheel, the fluctuation amount and frequency of the magnetorheological polishing ribbon are measured, and then the causes of the fluctuation of the magnetorheological polishing ribbon are analyzed. Moreover, through the principle of a single variable, the influence of process parameters on the fluctuation of magnetorheological polishing ribbon is explored. Finally, the fused silica component is scanned uniformly under the process parameters of magnetorheological polishing ribbon fluctuation of 40 μm, 80 μm, 150 μm, and 200 μm. The experimental results show that the greater the ribbon fluctuation, the greater the surface mid-spatial frequency error of the component, and the ribbon fluctuation is approximately linear with the RMS of the PSD2 in the mid-spatial frequency band on the surface of the component. Therefore, the fluctuation of the ribbon can be controlled by controlling the magnetorheological processing parameters, and the mid-spatial frequency band error on the surface of the high-power laser component can be significantly reduced by optimizing process parameters after magnetorheological finishing.

Virtual-real combination Ritchey-Common interferometry

Large optical flats play a remarkable role in advanced large-aperture optical systems and the testing of the surface shape error is indispensable for the fabrication. The widely adopted Ritchey-Common test for large optical flats will fail without the rigorous test configurations including a large F/# prerequisition and a flat-to-interferometer distance invariance. A virtual-real combination Ritchey-Common interferometry is proposed to avoid the large F/# prerequisition by accurately modelling the optical path in a virtual interferometer. Furthermore, a virtual-real combination iterative algorithm is proposed in this method to break the flat-to-interferometer distance invariance. Measurement experiments for 100 mm and 422 mm aperture flats were performed to demonstrate the feasibility of this method. Compared with a direct testing in a standard Zygo interferometer, the peak to valley (PV) and root mean square (RMS) errors were less than 0.1 λ and 0.01 λ (λ=632.8 nm), respectively, in different Ritchey angles and flat-to-interferometer distances. Further numerical simulations demonstrate that RMS errors for various Zernike aberrations in arbitrary F/# are less than 0.01 λ. This method can break the distance invariance restriction and achieve high accuracy with an arbitrary F/#, thus providing substantial freedom in the design of test configurations to accommodate various test scenarios.

Sparse bi-step raster path for suppressing the mid-spatial-frequency error by fluid jet polishing

The periodic ripple errors (mid-spatial-frequency (MSF) error) produced by computer-controlled sub-aperture polishing severely limit the improvement of high-performance optical systems. At the same time, the fluid jet polishing (FJP) method is non-destructive and non-contact, but it is still hard to widely use it due to the defect of small spot-size and low efficiency. In this paper, we found that FJP has a significant advantage in removing the residual periodic ripples in sub-aperture polishing. The mathematical model developed by complex spectrum optimization verifies the existence of the sparse "bi-step raster path" (BSRP), which can achieve efficient periodic ripple error removal by suppressing the first two-order peaks of the error spectrum. In the experiments, it was observed that the MSF error has been significantly reduced after BSRP processing while the surface form and surface roughness have not been deteriorated, which demonstrates the validity of the proposed method. The proposal of the BSRP provides a new approach for the application of FJP and the suppression of the MSF error.

Evolution law of comet-shaped defects in magnetorheological finishing

Ultraprecision optical systems such as high-energy laser systems have very strict requirements for surface defects of optical components. Comet-shaped defects are common in magnetorheological finishing, which is one of the main ultraprecision manufacturing technologies. Computational fluid dynamics is used to simulate the pressure and velocity of the magnetorheological fluid in pits during magnetorheological finishing for the first time, to the best of our knowledge. The simulation model analyzes the distribution of velocity and pressure in the process of the formation of comet-shaped defects. After the simulation, magnetorheological polishing experiments were carried out on the fused quartz silica glass. The polishing experiments use the rotation speed and immersion depth of the polishing wheel to verify the velocity and pressure distribution in the simulation experiment. The polishing experiment results are consistent with the simulation results, which show that the high polishing speed will increase the depth of the defect, and the uneven velocity and pressure distribution at the pits make the pits evolve into comet-shaped defects. We explain the cause of comet-shaped defects from the perspective of material removal and provide an optimization method of process parameters for high-efficiency removal of comet-shaped defects.

Electron paramagnetic resonance spectroscopy and first-principals calculations of Cr3+ doped KDP crystals

The characteristics of potassium dihydrogen phosphate (KDP) crystals with the Cr(NO3)3 doping at ～10 ppm level were studied by the ultraviolet-visible (UV-Vis) absorption spectrum, electron paramagnetic resonance spectroscopy (EPR), and...

Study on the Absorption Characteristics and Laser Damage Properties of Fused Silica Optics under Flexible Polishing and Shallow DCE Process

The enhancement of laser damage resistance of fused silica optics was a hotspot in scientific research. At present, a variety of modern processes have been produced to improve the laser induced damage threshold (LIDT) of fused silica optics. They included pre-treatment processes represented by flexible computer controlled optical surfacing (CCOS), magnetorheological finishing (MRF), ion beam finishing (IBF), and post-treatment processes represented by dynamic chemical etching (DCE). These have achieved remarkable results. However, there are still some problems that need to be solved urgently, such as excessive material removal, surface accuracy fluctuation in the DCE process, and the pollution in MRF process, etc. In view of above problems, an MRF, CCOS, IBF and shallow DCE combined technique was used to process fused silica optics. The surface morphology could be greatly controlled and chemical etching depth was reduced, while the LIDT increased steadily. After processing by this combined technique, the LIDT increased to 12.1 J/cm2 and the laser damage resistance properties of fused silica were significantly enhanced. In general, the MRF, IBF, CCOS and shallow DCE combined technique brought much help to the enhancement of laser damage resistance of fused silica, and could be used as a process route in the manufacturing process of fused silica.

Formation mechanism of bubbles in the crack healing process of fused silica using a CO2 laser

The machining-induced cracks and other defects on the surface of fused silica would incur damage when irradiated by intense lasers, which greatly shortens the service life of the fused silica optical components. The high absorption coefficient of fused silica for far-infrared lasers makes it possible to use low-energy CO₂ lasers to melt and heal micro defects on the surface, and hence improve its damage threshold under the service conditions of extremely intense laser. However, the air in the cracks may evolve into bubbles during the laser healing process, but the law of crack morphology evolution and the bubble formation mechanism have not been clearly revealed. In this work, a simulation model of the healing process of fused silica surface cracks under the effect of low-energy CO₂ laser is established. Three bubble formation mechanisms (i.e., the uneven fluidity caused by temperature gradient, the collapse effect caused by inclined cracks, and the internal cracks) are identified based on the simulation results of cracks with various original morphologies and characteristic structural parameters. The simulated fused silica morphology is consistent with the results of the laser healing experiment. This work can provide theoretical guidance for the optimization of optical manufacturing parameters of fused silica, as well as the CO₂ laser healing and polishing strategies.

High-Accuracy Surface Topography Manufacturing for Continuous Phase Plates Using an Atmospheric Pressure Plasma Jet

The continuous phase plate (CPP) is the vital diffractive optical element involved in laser beam shaping and smoothing in high-power laser systems. The high gradients, small spatial periods, and complex features make it difficult to achieve high accuracy when manufacturing such systems. A high-accuracy and high-efficiency surface topography manufacturing method for CPP is presented in this paper. The atmospheric pressure plasma jet (APPJ) system is presented and the removal characteristics are studied to obtain the optimal processing parameters. An optimized iterative algorithm based on the dwell point matrix and a fast Fourier transform (FFT) is proposed to improve the accuracy and efficiency in the dwell time calculation process. A 120 mm × 120 mm CPP surface topography with a 1326.2 nm peak-to-valley (PV) value is fabricated with four iteration steps after approximately 1.6 h of plasma processing. The residual figure error between the prescribed surface topography and plasma-processed surface topography is 28.08 nm root mean square (RMS). The far-field distribution characteristic of the plasma-fabricated surface is analyzed, for which the energy radius deviation is 11 μm at 90% encircled energy. The experimental results demonstrates the potential of the APPJ approach for the manufacturing of complex surface topographies.

Model Development for Nanosecond Laser-Induced Damage Caused by Manufacturing-Induced Defects on Potassium Dihydrogen Phosphate Crystals

Nanosecond laser-induced damage on (potassium dihydrogen phosphate) KDP crystals is a complex process, which involves coupled actions of multi-physics fields. However, the mechanisms governing the laser damage behaviors have not been fully understood and there have been no available models to accurately describe this complex process. In this work, based on the theories of electromagnetic, thermodynamic, and hydrodynamic fields, a coupled multi-physics model is developed to describe the transient behavior of laser-supported energy deposition and diffusion accompanied by the surface defect (e.g., surface cracks)-initiated laser damage process. It is found that the light intensification caused by the defects near the crystal surface plays a significant role in triggering the laser-induced damage, and a large amount of energy is quickly deposited via the light intensity-activated nonlinear excitation. Using the developed model, the maximum temperature of the crystal material irradiated by a 3 ns pulse laser is calculated, which agrees well with previously reported experimental results. Furthermore, the modeling results suggest that physical processes such as material melting, boiling, and flowing have effects on the evolution of the laser damage process. In addition, the experimentally measured morphology of laser damage sites exhibits damage features of boiling cores, molten regions, and fracture zones, which are direct evidence of bowl-shaped high-temperature expansion predicted by the model. These results well validate that the proposed coupled multi-physics model is competent to describe the dynamic behaviors of laser damage, which can serve as a powerful tool to understand the general mechanisms of laser interactions with KDP optical crystals in the presence of different defects.

Phase defect detection of large-aperture optics with static multiplanar coherent diffraction imaging

Phase defect detection with micrometer scale on large aperture optical elements is one of the challenges in precision optical systems. An efficient scheme is proposed to detect phase defects. First, the defects are positioned in a large aperture by dark-field imaging based on large aperture photon sieves to improve the detection efficiency with a relatively low cost. Second, static multiplanar coherent diffraction imaging is used to retrieve the phase of the positioned defects in a small field of view. Here, a spatial light modulator is used as a multifocal negative lens to eliminate the mechanical errors in multiplanar imaging. The use of a negative lens instead of a positive lens has the advantage of a larger imaging space for the system configuration. Compared to the traditional interferometry system, this diffraction detection system has a simpler optical path and doesn't require sparse distribution of the defects. Experiment results demonstrate the success of the proposed scheme with a detection resolution better than 50 µm. We believe this work provides an effective method to rapidly detect phase defects on large aperture optics with high accuracy and high resolution.

Testing the mid-spatial frequency error of a large aperture long-focal-length lens with CGH

We present the use of a computer-generated hologram (CGH) to test the mid-spatial frequency error of a large aperture long-focal-length lens. In order to verify this test approach, a 450 mm × 450 mm reflective CGH is designed and fabricated for testing the 440 mm × 440 mm spatial filter lens. Both 0^th and 1^st order diffraction wavefront of CGH are measured, and the 0^th order diffraction wavefront is used to calibrate the substrate error. The mid-spatial frequency wavefront error caused by the CGH fabrication errors are evaluated using the binary linear grating model and power spectral density (PSD) theory. Experimental results and error analysis show that the CGH test approach is also feasible for the measurement of mid-spatial frequency error, and the measurement accuracy of PSD1 can reach 0.8832 nm RMS.

Three-dimensional laser damage positioning by a deep-learning method

A holographic and deep learning-based method is presented for three-dimensional laser damage location. The axial damage position is obtained by numerically focusing the diffraction ring into the conjugate position. A neural network Diffraction-Net is proposed to distinguish the diffraction ring from different surfaces and positions and obtain the lateral position. Diffraction-Net, which is completely trained by simulative data, can distinguish the diffraction rings with an overlap rate greater than 61% which is the best of results reported. In experiments, the proposed method first achieves the damage pointing on each surface of cascade slabs using diffraction rings, and the smallest inspect damage size is 8µm. A high precision result with the lateral positioning error less than 38.5µm and axial positioning error less than 2.85mm illustrates the practicability for locating the damage sites at online damage inspection.

Combined modulation of incident laser light by multiple surface scratches and their effects on the laser damage properties of KH2PO4 crystal

Manufacturing-induced surface defects are deemed as a potential source, leading the laser-induced damage threshold (LIDT) of the actual KDP crystal optics to be much lower than the intrinsic one. However, the underlying mechanisms have not been fully recognized. We explore the combined modulation of incident laser light by multiple scratches and their effects on laser damage performance of KDP optics by modeling the light intensifications and performing a laser damage test. Under the combined modulation of multiple scratches, enhanced hot spots are generated due to the focusing effects of convex lens profiles surrounded by the neighboring scratches. The combined modulation actions are much stronger than that of a single scratch. The relative light intensities (I_Rs) caused by multiple scratches can reach up to two times, and the number of hot spots (IPs) are four times as large as those by a single scratch. The I_Rs exhibit a general, increasing tendency as the scratch density increases. But for the case of double total reflections of rear-surface scratches, the totally reflected lights are transmitted through neighboring scratches, resulting in decreasing tendency of I_Rs. The tested LIDTs and optical transmittances of multiple scratches present a gradual, decreasing tendency with the increase of scratch density, which agrees with the simulation results. Besides, the simulated light intensifications could well explain the locations of laser damage, which further verify the role of multiple scratches in lowering the laser damage resistance.

Template-Free Fabrication of Refractive Index Tunable Polysiloxane Coating Using Homogeneous Embedding Strategy: Application in High-Power Laser System

A refractive index (RI) tunable polysiloxane coating was fabricated based on the cross-linked network structure embedded with mesoporous silica nanoparticles (MSNs), in which the MSNs were utilized to modulate the RI as well as to support the interior structure of the polysiloxane coating. The Si-O-Si inorganic backbone structure in combination with characteristics from the photopolymerization of active bonds produced the main cross-linked network structure, and controllable embedding of MSNs constructed the network-sphere structure. This approach eliminated the high-temperature post-treatment that was needed to remove the template, which ensures the safe application for temperature-sensitive laser crystal substrates and avoids coating structure collapse. In addition, degradation of the resulting coating can be minimized due to the similar chemical formation between MSN and polysiloxane coating. Hereby, a polysiloxane coating with expected spectral and laser damage-resistant properties can be obtained. This will facilitate the fabrication and application of a laser component with both high-transmission and high-flux capability for a high-power laser system.

3D dark-field confocal microscopy for subsurface defects detection

The three-dimensional (3D) precision measurement of subsurface defects (SSDs) remains a long-term, critical, and urgent challenge in advanced manufacturing technology. In this study, we present a 3D dark-field confocal microscopy technique with complementary illumination and detection apertures to detect the SSD in ultraprecise optical components, which are widely employed at laser fusion facilities. Under an annular illumination generated using a pair of axicons, the specular reflected beam from the surface can be blocked by a diaphragm placed in the detection path, while the scattered beam from the SSD can be effectively collected by the detector. Both surface topography and subsurface defects distribution can be measured simultaneously by this method. We constructed a dark-field confocal microscope that could readily detect the SSD 60 µm beneath the surface in neodymium glass. Furthermore, the 3D volume distributions of the SSD were also reconstructed.

A New Stitching Method for Dark-Field Surface Defects Inspection Based on Simplified Target-Tracking and Path Correction

A camera-based dark-field imaging system can effectively detect defects of microns on large optics by scanning and stitching sub-apertures with a small field of view. However, conventional stitching methods encounter problems of mismatches and location deviations, since few defects exist on the tested fine surface. In this paper, a highly efficient stitching method is proposed, based on a simplified target-tracking and adaptive scanning path correction. By increasing the number of sub-apertures and switching to camera perspective, the defects can be regarded as moving targets. A target-tracking procedure is firstly performed to obtain the marked targets. Then, the scanning path is corrected by minimizing the sum of deviations. The final stitching results are updated by re-using the target-tracking method. An experiment was carried out on an inspection of our specially designed testing sample. Subsequently, 118 defects were identified out of 120 truly existing defects, without stitching mismatches. The experiment results show that this method can help to reduce mismatches and location deviations of defects, and it was also effective in increasing the detectability for weak defects.

Self-trapped holes (small polarons) in ferroelectric KH2PO4 crystals

Density functional theory is used to establish the ground-state structure of the self-trapped hole (STH) in KH₂PO₄ crystals. The STHs in this nonlinear optical material are free small polarons, a fundamental intrinsic point defect. They are produced with ionizing radiation in the low-temperature orthorhombic structure of KH₂PO₄ and are only stable (i.e. long-lived) below approximately 70 K. A large 129-atom cluster, K₁₉H₄₀P₁₄O₅₆, is constructed to model the STH. The ωB97XD functional with the 6-31+G^* basis set is used and geometry optimization is performed. Our results show that two of the oxygen ions in a PO₄ unit relax toward each other and equally share the hole. These two oxygen ions do not initially have close hydrogen neighbors. This equal sharing of the hole is related to the presence of isolated, slightly distorted, PO₄ units and is significantly different from the small-polaron behavior often observed in other oxide crystals where the hole is localized on only one oxygen ion. The computational results provide a detailed description of the lattice relaxation occurring during formation of the STH. Characteristic spectral features of this defect are a larger hyperfine interaction with one ³¹P nucleus and equal, but smaller, hyperfine interactions with two ¹H nuclei. The computed values for these isotropic and anisotropic hyperfine coupling constants are in excellent agreement with results obtained from electron paramagnetic resonance experiments.

A simple high-precision wide-spectrum interferometric system

This paper presents a simple system capable of detecting wavefront information in the wavelength range of 488-1064 nm based on an interferometric system consisting of a ball lens and an aspheric collimator. The design is conducive to short-distance Gaussian beam expansion and collimation at multiple wavelengths. The design principles and performance of the system are described in detail; the measurement precision of the system is verified experimentally by measuring the surface profile of a piece of uncoated N-BK7 glass at various test wavelengths with the proposed setup and with a digital 4D FizCam 2000 interferometer. The highly consistent measurement results of the two methods reveal that the proposed system operates precisely and effectively at various wavelengths. Subsequently, the system is applied to measure the transmitted-wavefront error of a solar filter at a wavelength of 808.0 nm; the measurement results show superb repeatability. The proposed technique avoids the shortcomings of conventional single-working-wavelength interferometric systems, which can neither work in a wide spectrum with high precision nor measure the wavefront properties of many optical bandpass elements in one device; this technique also provides technical support for future applications of high-precision and low-cost wavelength-switchable laser interferometers.

Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces

A key challenge for metasurface research is locally controlling at will the nanoscale geometric features on meter-scale apertures. Such a technology is expected to enable large aperture meta-optics and revolutionize fields such as long-range imaging, lasers, laser detection and ranging (LADAR), and optical communications. Furthermore, these applications are often more sensitive to light-induced and environmental degradation, which constrains the possible materials and fabrication process. Here, we present a relatively simple and scalable method to fabricate a substrate-engraved metasurface with locally printed index determined by induced illumination, which, therefore, addresses both the challenges of scalability and durability. In this process, a thin metal film is deposited onto a substrate and transformed into a mask via local laser-induced dewetting into nanoparticles. The substrate is then dry-etched through this mask, and selective mask removal finally reveals the metasurface. We show that masking by the local nanoparticle distribution, and, therefore, the local index, is dependent on the local light-induced dewetting temperature. We demonstrate printing of a free-form pattern engraved into a fused silica glass substrate using a laser raster scan. Large-scale spatially controlled engraving of metasurfaces has implications on other technological fields beyond optics, such as surface fluidics, acoustics, and thermomechanics.

Theoretical and experimental comparisons of the smoothing effects for different multi-layer polishing tools during computer-controlled optical surfacing

Optical smoothing is a very effective method to restrict mid-to-high spatial frequency errors generated by computer-controlled sub-aperture polishing technologies. According to Preston's equation, pressure distribution is the key factor in describing the smoothing effect of a tool. Although the classic bridging model can solve pressure distribution during the smoothing process, it is only suitable for a tool that consists of a rigid base, compliant interlayer, thin metal plate, and pitch polishing layer. In this paper, a mathematical model applicable for any multi-layer polishing tool is proposed, which helps to calculate the pressure distribution dependent on the thickness of the layer, mid-spatial frequency errors at different periods, and the structures of polishing tools. Based on the model, the pressure distributions and smoothing rates of a rigid tool and a semi-flexible tool are deduced and compared in detail. Validity and improvement in accuracy are verified by comparison with the finite element model. In addition, three groups of experiments using a rigid tool and a semi-flexible tool for smoothing 4 mm and 8 mm period errors generated by magnetorheological finishing are carried out to validate the model. The simulation result of the smoothing rates of the errors after every smoothing process matches well with the experiment results.

Quantitative Evaluation of Subsurface Damage by Improved Total Internal Reflection Microscopy

Subsurface damage (SSD), having a great impact on the laser-induced damage threshold (LIDT) of ultra-smooth optics applied in high-power laser systems, should be tightly controlled. An improved total internal reflection microscopy (TIRM), combined with digital image processing techniques, is proposed to quantitatively inspect SSD. With the characteristic that there is a relatively small depth of field (DOF) for a microscope at high magnification (50×), a series of SSD images are captured along with the microscope focusing at different depths under the surface by means of micro-focusing control. The definition of each image is calculated through wavelet transformation. By simulation, the relationship between the definition of TIRM images and the depth of the SSD has been established. According to the definition curve, the SSD depth is acquired. Fused silica glasses polished after fine grinding are measured non-destructively by our TIRM setup. The results show that the improved TIRM is a useful method to evaluate SSD. It is helpful to improve the efficiency of optical fabrication.

Binocular vision profilometry for large-sized rough optical elements using binarized band-limited pseudo-random patterns

In this paper, a non-contact binocular vision profilometry method is proposed to measure a rough lens with aperture of around 300mm. A series of binarized band-limited pseudo-random patterns (BBPPs) are projected onto the rough lens, we utilize the temporal encoding method so that each pixel in the captured images has its specific code word. Homologous points could be matched via stereo matching procedure, then the surface of the rough lens will be reconstructed based on triangulation method according to the previous calibration data. Compared with the three coordinate measuring machine (CMM), this method achieves a fast and cheap measurement of the large-sized rough lens, which might be highly interesting for fast and overall measurement of metre-sized rough elements in the future.

Theoretical modeling and analysis of material removal characteristics for KDP crystal in abrasive-free jet processing

Traditional KDP polishing methods, such as magnetorheological finishing (MRF), ion-beam figuring (IBF), and chemical mechanical polishing (CMP), are limited by either hard-to-remove residual particles, unavoidable heating effect, or applicability that is restricted to large-sized KDP. The abrasive-free jet polishing (AFJP) is regarded as a promising polishing method that can circumvent the above issues. KDP AFJP makes use of a thermodynamically and kinetically stable ionic liquid (IL) microemulsion that contains nanometer range water droplets evenly dispersed in the non-aqueous carrier liquid. The sprayed out nanoscale water droplets can remove material through dissolution. In this paper, the normal impinging of a nanoscale water droplet on the KDP surface is investigated. And then a materials removal model is proposed for water droplets. This model considers two major modes, namely deformation of a water droplet in compressing and deformation restoring of a water droplet in slipping process. Finally, KDP AFJP spot experiments were then conducted to validate the model veracity. The proposed model fits well with the simulation and experimental results which further suggest KDP AFJP's feasibility. This proposed model provides a good explanation for KDP AFJP's removal mechanism.

Measurement of mid-frequency wavefront error for large optical components with ptychography

In a high power laser system, the wavefront quality of large optical components in the mid spatial frequency band plays a critical role in system performance and safe operation. A simple and efficient method based on ptychography is used to measure mid-frequency wavefront error, which has the advantages of simple structure, strong anti-interference ability, flexible frequency-range selection, and low cost. It has excellent frequency response in the entire mid-frequency region. The transfer function is demonstrated to be greater than 0.7 at 1/2 Nyquist frequencies (8.33  mm^-1) for 60 mm field-of-view experimentally. The method has been successfully applied to the wedged focused lens (WFL) to achieve high-fidelity measurement. Without any auxiliary lens, the power spectrum of the WFL at the frequency of interest is obtained through large-aperture measurement. This technique is especially suitable for optical components difficult to be measured using interferometers and opens up a new perspective for measuring mid-frequency wavefronts.

Effect of temperature on surface error and laser damage threshold for self-healing BK7 glass

Cracks caused during the lapping and polishing process can decrease the laser-induced damage threshold (LIDT) of the BK7 glass optical elements, which would shorten the lifetime and limit the output power of the high-energy laser system. When BK7 glass is heated under appropriate conditions, the surface cracks can exhibit a self-healing phenomenon. In this paper, based on thermodynamics and viscous fluid mechanics theory, the mechanisms of crack self-healing are explained. The heat-healing experiment was carried out, and the effect of water was analyzed. The multi-spatial-frequency analysis was used to investigate the effect of temperature on surface error for self-healing BK7 glass, and the lapped BK7 glass specimens before and after heat healing were detected by an interferometer and atomic force microscopy. The low-spatial-frequency error was analyzed by peak to valley and root mean square, the mid-spatial-frequency error was analyzed by power spectral density, and the high-spatial-frequency error was analyzed by surface roughness. The results showed that the optimal heating temperature for BK7 was 450°C, and when the heating temperature was higher than the glass transition temperature (555°C), the surface quality decreased a lot. The laser damage test was performed, and the specimen heated at 450°C showed an improvement in LIDT.

Line-scanning laser scattering system for fast defect inspection of a large aperture surface

Inspection of defects with micrometer level on large aperture surfaces with hundreds of millimeters is one of the challenges in surface quality evaluation. Various microscopic imaging methods have been applied to inspecting the surface defects, while they are time-consuming for the small field of view and the sub-aperture stitching. To tackle this problem, a high-speed line scanning system based on the dark-field laser scattering method is proposed. The laser beam is scanned by the rotating polygon mirror to a laser line for high throughput and then the telecentric F-theta lens converges each incoming laser beam to a focused spot that creates a high intensity to enhance the signal-to-noise ratio. The scattered light from surface defect is collected by the designed integrating sphere for low background noise and the scattering signal is detected for each focused spot at a proper acquisition rate by a photomultiplier (PMT) detector with extremely short response time. In the meanwhile, the tested surface is moving perpendicular to the laser line to realize high-speed large area inspection. The defect inspection system is confirmed experimentally with laser line length of 60 mm, minimum detectable size less than 0.5 μm, and figure of merit of 9.6  cm² s^-1 μm^-1. The work put forward an effective method for automatic discovery of surface defects such as scratches, digs, and contaminants on large aperture surfaces.