Laser-induced Damage Performance Research Articles

Rapid Detection and Elimination of Subsurface Mechanical Damage for Improving Laser-Induced Damage Performance of Fused Silica

The fabrication of SSD-free fused silica optics is a crucial objective for high-power laser applications. To treat the surface of polished fused silica, a combination of RIE/RIBE and deep-controlled etch (DCE) techniques are typically employed. Currently, it is important to consider and study the ideal etching depth and precision while using combined etching techniques to remove the identified SSD. Herein, we present a novel approach to identify the distribution of SSD in fused silica, which corresponds to a specific grinding/polishing process condition. Our method involves using a mobile RIBE to perform cone cutting and remove material from the polished fused silica surface. Afterward, we etch the optical element’s surface with HF to visualize the subsurface cracks and understand their relationship with the RIBE depth. Through a systematic investigation of the combined etching technique, we establish a correlation between the depth of RIBE and DCE and the performance of laser damage. The combined etching technique can be implemented as a dependable approach to treat the surface/subsurface defects in fused silica and has the potential to improve laser damage resistance significantly.

Enhanced laser-induced damage performance ofall-glass metasurfaces for energetic pulsed laserapplications.

To fabricate optical components with surface layers compatible with high-power laser applications that may operate as antireflective coatings, polarization rotators, or harness physical anisotropy for other uses, metasurfaces are becoming an appealing candidate. In this study, large-beam (1.05cm diameter) 351-nm laser-induced damage testing was performed on an all-glass metasurface structure composed of cone-like features with a subwavelength spacing of adjacent features. These structures were fabricated on untreated fused silica glass and damage tested, as were structures that were fabricated on fused silica glass that experienced a preliminary etching process to remove the surface Beilby layer that is characteristic of polished fused silica. The laser-induced damage onset for structures on untreated fused silica glass was 19.3J⋅c m -2, while the sample that saw an initial pretreatment etch exhibited an improved damage onset of 20.4J⋅c m -2, only 6% short of the reference pretreated glass damage onset of 21.7J⋅c m -2. For perspective, the National Ignition Facility operational average fluence at this wavelength and pulse length is about 10J/c m 2. At a fluence of 25.5J⋅c m -2, the reference (pretreated) fused silica initiated 5.2 damage sites per m m 2, while the antireflective metasurface sample with a preliminary etching process treatment initiated 9.8 damage sites per m m 2. These findings demonstrate that substrate-engraved metasurfaces are compatible with high energy and power laser applications, further broadening their application space.

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 (KD2xH2(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.

Statistically Correlating Laser-Induced Damage Performance with Photothermal Absorption for Fused Silica Optics in a High-Power Laser System

Photothermal weak absorption is useful for the diagnosis of absorbing defects on the surface of fused silica optics in high-power lasers. However, how they relate to the laser-induced damage performance remains unclear, especially for a fused silica surface that has been post-treated with different processes (e.g., dynamic chemical etching or magnetorheological finishing). Here, we present a correlation study on the surface defect absorption level and laser-induced damage performance of fused silica optics post-treated with different processes using the photothermal common-path interferometer method. Statistical distribution of the absorbing defects at various absorption levels is obtained. The relationship between the defect density and the laser damage performance was analyzed. We show that the surface absorbing defects of fused silica can be affected by the post-treatment type and material removal amount. Furthermore, we show that the density of the defects with the absorption over 2 ppm is strongly correlated with the damage initiation threshold and damage density. Especially, for high-density defects at this absorption level, the damage density of fused silica optics can be well-predicted. In the low-density range, the density of this kind of defect can reflect the zero-probability damage threshold well. The study exhibits the potential of this methodology to non-destructively detect the key absorbing defects on fused silica surfaces as well as evaluate and optimize the post-treatment level of fused silica optics for high-power laser applications.

Assessing the UV-pulse-laser-induced damage density of fused silica optics using photo-thermal absorption distribution probability curves.

A photo-thermal absorption distribution probability curve based on a normal distribution model was proposed to describe the distribution of absorptive defects on fused silica surfaces under different processing conditions. Simultaneously, the maximum distribution probability absorption coefficient (MPA) and absorption distribution deviation (ADD) were used to quantitatively describe the overall absorption level and the uniformity of the absorption distribution on the fused silica surface. Based on this, the MPA (μ) and ADD (δ) were used to establish a statistical numerical relationship with the surface damage density of fused silica. The results showed that when μ ≤ 0.095 ± 0.015 and δ ≤ 0.045 ppm, the fused silica optics met the manufacturing process requirements for high laser-induced damage performance. Thus, a non-destructive approximate evaluation of the laser-induced damage density on the fused silica surface was achieved. This evaluation method provides a new, to the best of our knowledge, technology for evaluating the manufacturing process quality related to the damage performance of fused silica optics in high-power solid-state laser facilities and is an important supplement to popular destructive laser-induced damage testing methods.

Formation Mechanism of Dkdp Surface in Single Point Diamond Fly-Cutting Process and the Resulting Degradation of Laser-Induced Damage Performance

Formation Mechanism of Dkdp Surface in Single Point Diamond Fly-Cutting Process and the Resulting Degradation of Laser-Induced Damage Performance

Toward high laser power beam manipulation with nanophotonic materials: evaluating thin film damage performance.

Nanophotonic materials enable unprecedented control of light-matter interactions, including the ability to dynamically steer or shape wavefronts. Consequently, nanophotonic systems such as metasurfaces have been touted as promising candidates for free-space optical communications, directed energy and additive manufacturing, which currently rely on slow mechanical scanners or electro-optical components for beam steering and shaping. However, such applications necessitate the ability to support high laser irradiances (> kW/cm2) and systematic studies on the high-power laser damage performance of nanophotonic materials and designs are sparse. Here, we experimentally investigate the pulsed laser-induced damage performance (at λ ∼ 1 µm) of model nanophotonic thin films including gold, indium tin oxide, and refractory materials such as titanium nitride and titanium oxynitride. We also model the spatio-thermal dissipation dynamics upon single-pulse illumination by anchoring experimental laser damage thresholds. Our findings show that gold exhibits the best laser damage resistance, but we argue that alternative materials such as transparent conducting oxides could be optimized to balance the tradeoff between damage resistance and optical tunability, which is critical for the design of thermally robust nanophotonic systems. We also discuss damage mitigation and ruggedization strategies for future device-scale studies and applications requiring high power beam manipulation.

Laser field manipulation and laser damage resistance property of nanotextures on fused silica optics

Nanotextures, being composed of an array of nanostructures, can manipulate the effective refractive index on the interface between air and optical substrates. Various optical properties, such as antireflection, antifogging, self-cleaning, enhanced absorption, have been achieved by nanotextures. Nanotextures have been considered to have great potential in the high-power laser system because of the single-material configuration. Here, we show design and fabrication of nanotextures on fused silica with high laser-induced damage threshold (LIDT) at the wavelengths 355 nm and 532 nm. The manipulation of light field intensity by nanostructures has been studied. When the average period of nanostructures is less than the 1/3 wavelength, the light field intensity in nanostructures can be lower than that in air. This is the theoretical basis for the high LIDT of fused silica nanotextures. Finally, we study laser induced damage performance of nanotextures. The experimental results show that the LIDT of fused silica can be improved by nanotextures. The LIDT of fused silica substrate was improved form 15 J/cm2 to 22 J/cm2 at the wavelengths 532 nm. This work promotes an application of nanotextures in high power/energy laser area.

Rapid growth of a long-seed KDP crystal

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.

Rapid Growth of a Cuboid DKDP (KDxH2–xPO4) Crystal

The pyramid–prism interface in “point-seed” rapidly grown DKDP (KDxH2–xPO4) crystals affects the quality of tripler-cut samples used in inertial confinement fusion devices. In this study, a cuboid DKDP crystal without a pyramidal sector was rapidly grown. The quality of the cuboid DKDP crystal was evaluated by measuring the deuterium homogeneity, crystalline quality, transmission spectrum, and laser-induced damage performance. The total number N of tripler-cut samples that can be obtained from the cuboid DKDP crystal is easily calculated. The size of the tripler-cut sample is approximately equal to the cross-section of this cuboid DKDP crystal. Moreover, as this cuboid DKDP crystal has a regular shape, calculating the mass of the crystal and accurately controlling the supersaturation of the solution during growth is easy. The successful crystal growth using this proposed approach establishes the possibility of growing large-aperture cuboid DKDP crystals. Currently, research on the rapid growth of a cuboid...

Effect of pad elastic modulus on polishing-induced subsurface damages distribution and laser-induced damage performance of fused silica optics.

The plastic subsurface damages distribution and their influence on the laser induced damage performance of fused silica optics polished with different pads are investigated. The elastic interaction model, plastic indentation model and wear relationships are combined together to theoretically characterize the plastic subsurface damages distribution in different polishing processes, which shows consistent results with experiments. It reveals that most of the polishing-induced subsurface damages are plastic damages. A few largest polishing particles in the tail end distribution mainly decide the final depth distribution and density of the polishing-induced plastic subsurface damages. The larger pad elastic modulus will make the few largest polishing particles bear much larger load and generate larger proportion of observable plastic subsurface damages. Using polishing pad with lower elastic modulus is prominent for restricting the generation of fractures and plastic damages and finally makes the polished optics show higher laser induced damage threshold, lower damage density and smaller slope damage probability curve. This research is meaningful for further establishing the quantitative relationships between polishing parameters, subsurface damages distribution and laser induced damage performance in fused silica optics.

Simulation of internal stress waves generated by laser-induced damage in multilayer dielectric gratings.

Multilayer dielectric (MLD) gratings used in ultrahigh-intensity laser systems often exhibit a laser-induced damage performance below that of their constituent materials. Reduced performance may arise from fabrication- and/or design-related issues. Finite element models were developed to simulate stress waves in MLD grating structures generated by laser-induced damage events. These models specifically investigate the influence of geometric and material parameters on how stress waves can lead to degradation of material structural integrity that can have adverse effects on its optical performance under subsequent laser irradiation: closer impedance matching of the layer materials reduces maximum interface stresses by ~20% to 30%; increasing sole thickness from 50 nm to 500 nm reduces maximum interface stresses by ~50%.

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

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

Influences of surface defects on the laser-induced damage performances of KDP crystal.

When potassium dihydrogen phosphate (KDP) crystals are exposed to high-energy laser irradiation, the pre-existing surface defects may act as damage precursors and will reduce the lifespan of the crystal components. Although it has been found that different kinds of surface defects exhibit distinct damage characteristics, the influence of surface defects on the laser-induced damage performance of KDP crystal is not yet clear. In this paper, KDP surface defects have been characterized by multiple measuring methods and classified into five categories according to their structure features. Laser-induced damage tests were then carried out to investigate the laser-induced damage thresholds of different kinds of KDP surface defects as well as the evolution of the morphology of damage sites. The results of the experiment indicate that the damage thresholds of cracks, fracture pits, and surface protuberances are between 6 and 11 J/cm2 (355 nm, 3 ns, similarly hereinafter), which are much lower than the thresholds of plastic scratches, discontinuous scratches, and a defect-free KDP surface. In addition, it has been found that fluorescence enhancement is just a necessary condition for reduction of damage thresholds. Finally, reasons for the formation of the most threatening KDP surface defects have been analyzed and corresponding suppression measures have been proposed for increasing the surface damage thresholds of the crystal components.

Study on Brewster angle thin film polarizer using hafnia–silica mixture as high-refractive-index material

Two kinds of polarizer coatings were prepared by electron beam evaporation, using HfO2–SiO2 mixture and HfO2 as the high-refractive-index materials, respectively. The HfO2–SiO2 mixture layer was implemented by coevaporating SiO2 and metal Hf, the materials were deposited at an oxygen atmosphere to achieve stoichiometric coatings. The certain HfO2 and SiO2 content ratio is controlled by adjusting the deposition rate of HfO2 and SiO2 using individual quartz crystal monitor. The spectral performance, surface and interfacial properties, as well as the laser-induced damage performance were studied and compared. Comparing with polarizer coating using HfO2 as high-refractive-index material, the polarizer coating using HfO2–SiO2 mixture as high-refractive-index material shows better performance with broader polarizing bandwidth, lower surface roughness, better interfacial property while maintaining high laser-induced damage threshold.

Non-destructive evaluation of UV pulse laser-induced damage performance of fused silica optics

The surface laser damage performance of fused silica optics is related to the distribution of surface defects. In this study, we used chemical etching assisted by ultrasound and magnetorheological finishing to modify defect distribution in a fused silica surface, resulting in fused silica samples with different laser damage performance. Non-destructive test methods such as UV laser-induced fluorescence imaging and photo-thermal deflection were used to characterize the surface defects that contribute to the absorption of UV laser radiation. Our results indicate that the two methods can quantitatively distinguish differences in the distribution of absorptive defects in fused silica samples subjected to different post-processing steps. The percentage of fluorescence defects and the weak absorption coefficient were strongly related to the damage threshold and damage density of fused silica optics, as confirmed by the correlation curves built from statistical analysis of experimental data. The results show that non-destructive evaluation methods such as laser-induced fluorescence and photo-thermal absorption can be effectively applied to estimate the damage performance of fused silica optics at 351 nm pulse laser radiation. This indirect evaluation method is effective for laser damage performance assessment of fused silica optics prior to utilization.

Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores.

We investigate the laser-induced damage performance at 1064 nm of potassium dihydrogen phosphate (KDP) crystals grown using filters of different pore sizes. The aim is to explore a novel method for understanding laser-matter interactions with regard to physical parameters affecting the ability of damage precursors to initiate damage. By reducing the pore size of filters in continuous filtration growth, we can improve laser damage resistance. Furthermore, we develop a model based on a Gaussian distribution of precursor thresholds and heat transfer to obtain a size distribution of the precursor defects. Smaller size and/or lower concentration of precursor defects could lead to better damage resistance.

Subsurface defect characterization and laser-induced damage performance of fused silica optics polished with colloidal silica and ceria

This paper mainly focuses on the influence of colloidal silica polishing on the damage performance of fused silica optics. In this paper, nanometer sized colloidal silica and micron sized ceria are used to polish fused silica optics. The colloidal silica polished samples and ceria polished samples exhibit that the root-mean-squared (RMS) average surface roughness values are 0.7 nm and 1.0 nm, respectively. The subsurface defects and damage performance of the polished optics are analyzed and discussed. It is revealed that colloidal silica polishing will introduce much fewer absorptive contaminant elements and subsurface damages especially no trailing indentation fracture. The 355-nm laser damage test reveals that each of the fused silica samples polished with colloidal silica has a much higher damage threshold and lower damage density than ceria polished samples. Colloidal silica polishing is potential in manufacturing high power laser optics.

Impact of substrate pits on laser-induced damage performance of 1064-nm high-reflective coatings.

The laser damage resistance of coatings in high-power laser systems depends significantly on the surface quality of the substrate. In our experiment, pits were precisely fabricated on the surface of fused silica substrate using a femtosecond laser processing bench. The HfO2/SiO2 high-reflective coatings at 1064nm were deposited by conventional e-beam evaporation onto fused silica substrates with and without pits, respectively. The internal crack that was induced by the substrate geometrical structure was first observed in our experiment. The laser-induced damage threshold test showed negative effects of the substrate pits on the laser resistance of high-reflective coatings. Simulations by the finite element method were carried out, and results demonstrated that the modulation of a high reflector multilayer geometry could lead to electrical-field amplification and reduce laser damage resistance. Combined with its poor mechanical properties, the pits on substrate could contribute to the occurrence of damages.

Subsurface defects characterization and laser damage performance of fused silica optics during HF-etched process

Subsurface defects of fused silica optics would vary with HF-etched process. In this paper, the subsurface defects characteristics of HF-etched fused silica optics and their effects on laser induced damage were investigated. The results suggest that most of metal impurities defects (especially Ce element) of fused silica optics can be dissolved in strong acid solution. Subsurface damage can be removed by submerging fused silica optics in HF-based etchants. Laser damage resistance is improved through the removal of metal impurities and subsurface damage. Optical thermal absorption describes the laser absorption characterization of fused silica subsurface layer. A good correlation between optical thermal absorption and laser-induced damage performance is shown in this paper. Laser damage performance deteriorates when the HF-based etching depth exceeds one value, which can be explained by the impurities redeposition and surface roughness increase. Research results have a guiding significance for HF-based etching process technology of fused silica optics.