Fused Silica Optics Research Articles

On the Variation in Surface State of Nominally Identical Fused Silica Optics Surfaces

On the Variation in Surface State of Nominally Identical Fused Silica Optics Surfaces

Combined non-destructive evaluation of the damage precursors in fused silica optics

Combined non-destructive evaluation methods for fused silica’s damage precursors including UV photothermal weak absorption, ultrafast fluorescence, steady-state fluorescence, Raman spectroscopy, and stress distribution, are proposed to explain the damage mechanism from multiple perspectives. The correlation between each non-destructive characterization method and damage behaviors is simultaneously verified, in general, the damage threshold test of R on 1. The results have shown that regions with high photothermal absorption are accompanied by low damage thresholds, because the photothermal conversion process is associated with laser-induced damage. The damage threshold is also lower at locations of bright ultrafast fluorescence signals with a lifetime ranging from 1.2∼3 ns, as ultrafast fluorescence is associated with high densities of defects. A complementary relationship between the distribution of ultrafast fluorescence and the distribution of photothermal absorption signals is constructed. This indicates that the steady-state fluorescence cannot accurately evaluate the precursor’s damage performance. Furthermore, Raman spectroscopy is utilized to identify the structural changes in bond lengths and angles, as well as locations of damages coinciding with stress distribution.

Investigation of the entrapped micro bubble defect as a high-energy laser damage precursor in fused silica optics

Investigation of the entrapped micro bubble defect as a high-energy laser damage precursor in fused silica optics

Role of fictive temperature distribution involved in CO2 laser polishing of fused silica and its optimization for achieving even heat-affected zones

CO2 laser polishing process is a highly effective way for enhancing the surface quality and laser-induced damage threshold of fused silica optics. Nevertheless, due to the thermal history and spatial gradient of the thermodynamic temperature, the heat-affected zones can be formed unevenly on the surface with an increased fictive temperature, which causes the material densification and residual stress. In this work, a 3D model of structural relaxation coupled with temperature and flow fields was innovatively established to explore the role of fictive temperature distribution involved in CO2 laser polishing of fused silica. The Raman spectrum was applied to verify this model. Subsequently, the impacts of laser beam power, scanning speed, track overlapping, substrate temperature on the fictive temperature were discussed. The results indicate that decreasing the scanning speed and increasing the substrate temperature can decrease the fictive temperature. Additionally, a higher track overlapping can reduce the height of the fictive temperature “tool mark”. Moreover, an optimization strategy based on laser beam scanning speed was proposed based on the uniform distribution of fictive temperature. The depth difference of the heat-affected zones was reduced from 164.26 μm to 31.74 μm. This work could be used to quantitatively predict the 3D fictive temperature distribution and proposed an optimization strategy based on scanning speed to suppress the non-uniformity of fictive temperature distribution, which can provide significant theoretical guidance for CO2 laser polishing technology of fused silica optics.

Microsecond-pulsed CO2 laser cleaning of high damage threshold fused silica

Ultraviolet (UV) laser-induced damage from defects and impurities in conventional ground and polished fused silica optics significantly hinders their application in high-power laser systems. Laser cleaning, as a non-contact and polishing fluid-free technology, is likely to be a key technology for improving the damage performance of fused silica optics. However, conventional laser cleaning primarily focuses on surface pollution without removing the material. It overlooks the detrimental effects of multiscale defects and impurities inside the fused silica. In this study, microsecond-pulsed CO2 laser cleaning was proposed to enhance the laser-induced damage threshold (LIDT) by ablating fused silica to nanometer depths. Through multiple-scale simulations, the thermal stress evolution and modulation mechanism of defects and impurities on the damage performance (ranging from the macro- to nanoscale) were revealed. Utilizing multimodal characterization, laser cleaning effectively eliminated and suppressed surface/subsurface defects, absorptive defects, chemical structural defects, and elemental impurities without introducing residual thermal stress and destroying surface roughness. In addition, the evolution of the molecular structures indicated a tendency for intrinsic defects to recombine into shorter-ring structures, which resulted in a densified fused silica surface. The laser-cleaned samples exhibited higher damage thresholds, achieving 47.6 % (0 % probability) and 27.0 % (100 % probability) improvements compared with uncleaned samples. This technology provides a new method for fabricating high damage threshold fused silica optics with superior surface quality for high-power laser applications.

Precise refractive index measurement of fused silica optics

Abstract We have developed an experimental platform that non-destructively and precisely measures the Refractive Index (RI) and dispersion of ultra-polished fused silica optics. Using Total Internal Reflection Digital Holographic Microscopy (TIRDHM), we exploit the phase change of reflected light in Total Internal Reflection (TIR) mode. This phase change depends on the incident angle at the TIR interface and the refractive indices of the involved media. We have optimized a combination of higher TIR phase sensitivity, considerable penetration depth, and minimized phase measurement inaccuracies through simulations to design our experiment. Key features include a custom-made precision Right-Angle Prism (RAP) of Astrositall material, a seamless interface with fused silica optics on TIR interface through optical contact, and single-shot measurement. We have demonstrated the accuracy of measuring fused silica optics through proof of concept and experimental results. Our measurements on two different samples show accuracy better than ±3 × 10−4 compared to those obtained using a commercially available critical angle Refractometer (Metricon). Importantly, the setup offers the advantage of spatially mapping the refractive index, unlike point measurements by available Refractometers.

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.

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.

Material migration and damage characteristics of fused silica optical surface treated by CO2 laser/short – pulse ultraviolet laser

Fused silica optics, as key components in high - power laser systems, are prone to be damaged under short - pulse ultraviolet (UV) laser irradiation. CO2 laser is usually used to repair the laser damage and extend the service life of optics. However, the anti - laser damage characteristics of repaired substrates sometimes can’t meet the expectation. To facilitate better application of CO2 laser repairing method, this work studied the CO2 laser irradiating mechanism of fused silica in detail and investigated the damage characteristics of repaired optical surface through experiments. Firstly, finite element level - set (FELS) method is adopted to simulate CO2 laser irradiating process. The simulation showed that CO2 laser irradiation would cause molten material migration, vapor material deposition and affect the formation of residual stress. Various tests of morphology, residual stress, and photo - thermal absorption were conducted to study the changes of laser damage characteristics of repaired craters. The experimental results presented that molten materials migration and vapor materials re deposition occurred in CO2 laser irradiation, which would promote the formation of residual stress, cause stronger laser absorption, or even induce laser damage. Through the experiment, accuracy of simulation was verified further. In this work, the CO2 laser repairing mechanism of fused silica and laser damage characteristics of repaired optics are clarified through FELS simulation and experiments. Relative results can provide a certain technical support for the performance improvement of repaired fused silica optics.

Research on the mitigation of redeposition defects on the fused silica surface during wet etching process.

The laser-induced damage of ultraviolet fused silica optics is a critical factor that limits the performance enhancement of high-power laser facility. Currently, wet etching technology based on hydrofluoric acid (HF) can effectively eliminate absorbing impurities and subsurface defects, thereby significantly enhancing the damage resistance of fused silica optics. However, with an increase in the operating fluence, the redeposition defects generated during wet etching gradually become the primary bottleneck that restricts its performance improvement. The composition and morphology of redeposition defects were initially identified in this study, followed by an elucidation of their formation mechanism. A mitigation strategy was then proposed, which combines a reduction in the generation of precipitation with an acceleration of the precipitation dissolution process. Additionally, we systematically investigated the influence of various process parameters such as extrinsic impurity, etching depth, and megasonic excitation on the mitigation of deposition defects. Furthermore, a novel multiple-step dynamic etching method was developed. Through comprehensive characterization techniques, it has been confirmed that this new etching process not only effectively mitigate redeposition defects under low fluence conditions but also exhibits significant inhibition effects on high fluence precursors. Consequently, it significantly enhances the laser damage resistance performance of fused silica optics.

Assessing low laser damage densities of fused silica optics by means of a functional raster scan protocol.

The knowledge of the laser damage resistance of fused silica optics for their use in high-power lasers is of primary importance for the operation and maintenance of these facilities. Among the control procedures developed, one of the most relevant to date is the raster scan procedure [Lamaignère etal., Rev. Sci. Instrum. 78, 103105 (2007)]. This procedure is used to determine the damage density of optical components as a function of fluence. To date, this procedure takes into account all triggered damage sites, regardless of their size and/or morphology. We have added a step to this procedure, which consists in irradiating again all the damage sites with a series of shots to ascertain their ability to grow. This allows us to estimate the densities of growing damage sites, which are most critical for the safe operation of lasers. This pragmatic approach can be considered a functional test procedure. By applying this procedure to large optical areas, we were then able to measure extremely low damage densities (∼10-4 damage cm-2) over a wide range of fluences [0-18Jcm-2], corresponding to fluences to which the optics are irradiated during the operation of high-power lasers. We can then determine the damage law of a given set of optical components. This reference law makes it possible, on the one hand, to predict the lifetime of the optics. On the other hand, any deviation can then be analyzed with regard to this reference law. Thanks to the determination of confidence intervals, this functional procedure can also be used to delimit the reference law by upper and lower bounds.

Laser-induced damage growth of fused silica optics near growth threshold at 351 nm.

Laser-induced damage growth on the exit surface of fused silica optics triggered by nanosecond pulses at 351 nm is widely described with exponential dynamics. In this Letter, a particular experimental setup allowed us to study damage growth with a large beam and fluences near damage growth threshold for a high number of shots. This allowed us to observe and characterize a regime with a slow and linear growth dynamic not documented in the literature and yet fundamental for the operation of high-power laser installations.

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.

Precise and Rapid Replication of Complex‐Shaped Fused Silica Optics

AbstractThe high hardness, brittleness, and thermal resistance of fused silica glasses extremely challenge the mass production of complex‐shaped fused silica optics. This paper reports a new process chain for rapidly replicating complex‐shaped fused silica optics from complex molds at ambient temperature. The process chain mainly consists of the ultraprecision diamond turning of the complex‐shaped molds, the rapid shape replication through the silica precursor photopolymerization, and the transparent complex‐shaped optics derivation from the debinding and sintering. The directional shrinkages and surface texture evolutions in the process are comprehensively characterized for the replicated fused silica optics. After the shrinkage compensation, transparent fused silica optics with the spherical, microlens array, and hierarchical freeform surfaces are precisely replicated, exhibiting micrometric form accuracy, nanometric surface roughness, and high imaging quality. The proposed process chain provides a revolutionary approach for the mass production of precise complex‐shaped fused silica optics with fundamentally improved production efficiency and complex shape formation capability.

Smoothing of fused silica with less damage by a hybrid plasma process combining isotropic etching and atom-migration

In order to effectively reduce the subsurface damage of fused silica optics and obtain an ultra-smooth surface at atomic scale, we proposed a hybrid manufacturing process based on inductively coupled plasma (ICP), which combined isotropic etching polishing (IEP) and atom-migration manufacturing (AMM). In the plasma-IEP process, a large number of isotropic etching pits with ultra-smooth inner surface formed, enlarged, overlapped and merged, resulting in a smooth surface. The continuous downward etching process was accompanied by the continuous removal of the subsurface damage layer. With optimized process parameters, plasma-IEP could efficiently generate a less damage surface of fused silica with a material removal rate of 0.8 μm/min, and reduce the Sa roughness from 97.1 nm to 31.3 nm. The surface roughness could be further reduced to less than 0.15 nm by plasma-AMM, which was a non-subtractive finishing approach. The result of laser induced damage threshold (LIDT) test showed that the fused silica surface after the IEP-AMM hybrid manufacturing could withstand higher laser fluence, which implied more effective applications of this technique in high-power laser systems. This study proposed and verified the hybrid plasma manufacturing process combining plasma-IEP and plasma-AMM, which provided an ultra-smooth surface with less damage manufacturing process for fused silica.

A novel method to additively repair micro-damaged fused silica using localized CO2 laser

A novel method to additively repair localized laser-damaged sites in fused silica optics using CO2 laser was developed with coating nanomaterials. Optimal composition of coating solution for filling damaged sites was engineered. The composition of repaired sites was found typical fused silica structure, and there is no obvious difference than commercial fused silica KN7980 substrate. The developed technique has repaired damage points up to 35 % of their depth, while the localized treatment could generate smooth repaired surfaces. The repair depth of the damaged points was affected by the volume of fused silica nano-powder solution used and depth of the original damage sites.

Effect of H2O2 Treatment on Mechanical and Mechanochemical Properties of Fused Silica

The surface properties of fused silica (FS) change after H2O2 treatment, but the surface and subsurface damage behaviors and their mechanisms under various physical contact conditions have not been elucidated yet. This work investigated the effect of H2O2 treatment on mechanical and mechanochemical properties of FS surface. The results show that the hydrophilicity and adsorbed water film thickness of the FS surface increase with the concentration of H2O2 solution. The surface damage, nanowear, and subsurface deformation of FS caused by indentation increase with the concentration of H2O2 solution, while the nanohardness and reduced modulus decrease. Further analysis revealed that the water activity on the FS surface plays a critical role in reducing the mechanical and mechanochemical properties. In addition, the treatment with H2O2 solution on the FS surface shows a weakly corrosive effect, which implies the H2O2 treatment can be an alternative method to remove the surface defects on FS optics.

Estimation of laser-induced damage depth from surface image features.

In laser damage experiments, damage initiation and growth are typically monitored by imaging the surface of the tested fused silica sample, ignoring their bulk morphology. The depth of a damage site in fused silica optics is considered to be proportional to its equivalent diameter. However, some damage sites experience phases with no diameter changes but growth in the bulk independently from their surface. A proportionality relationship with the damage diameter does not accurately describe the growth of such sites. In the following, an accurate estimator for damage depth is proposed, which is based on the hypothesis that the light intensity scattered by a damage site is proportional to its volume. Such an estimator, using the pixel intensity, describes the change of damage depth through successive laser irradiations, including phases in which depth and diameter variations are uncorrelated.

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.

Generation of chemical deposits on fused silica optics and their modulation on the light intensity distribution.

Deposited substances generated from hydrofluoric acid based (HF-based) etching are found to be the precursors that deteriorate the resistance of fused silica optics to laser damage. In this paper, the surface of polished fused silica was treated with a buffer oxide etchant (BOE, 5-10%wt. HF + 10%wt. N H 4 F+80-85% w t.H 2 O). The optic surface area affected by the etching-induced deposits (N H 4)S i F 6 was found to increase significantly with the amount of material removal and sensitivity to the post-cleaning procedure. Three shapes of deposited particles are simultaneously identified on the treated samples. The 3D finite-difference time-domain (FDTD) was used to simulate the light field distribution near the deposited particles, which demonstrates that fused silica is subject to more light modulation when a particle is exposed on the front surface laser than on the rear surface laser. In addition, dense particles barely increase the light intensification factor (LIF) while markedly increase the light focused probability inside fused silica. Moreover, a multiple linear regression (MLR) analysis for the LIF builds a fitting plane model of the LIF and the particle height as well as the horizontal size, revealing that the LIF increases with the size of the deposited particle, especially its height. The laser damage testing results indicate that a deposited layer of ∼76n m in height had little influence on the laser-induced damage threshold (LIDT) of the optics and larger deposited particles with up to 9µm thickness may deteriorate the LIDT to 41.62% that of the reference sample. Both the simulation and experimental results demonstrate that deposited particles with a height of more than 0.1µm should be inhibited to achieve fused silica with superior laser damage performance.