Fused Silica Optical Components Research Articles

Influence of Thermal Annealing on Mechanical and Optical Property of SiO2 Film Produced by ALD.

The application range of fused silica optical components can be expanded and the cost of fused silica components can be reduced by depositing the same material film on fused silica substrate. However, due to the different manufacturing process, the performance of ALD SiO2 film is lower than that of fused silica substrate, which also limits the use of this process. In this paper, ALD SiO2 film with different thicknesses were deposited, and then the structure and properties were tested. Finally, the ALD SiO2 film was treated via the annealing process. Transmission electron microscopy (TEM) showed that the ALD SiO2 film had good compactness and substrate adhesion. The Raman spectra showed that the ALD SiO2 film and substrate had the same structure, with only slight differences. The XRD pattern showed that ALD-fused silica did not crystallize before or after annealing. The infrared spectra showed that there was an obvious Si-OH defect in the ALD SiO2 film. The laser damage showed that the ALD SiO2 film had a much lower damage threshold than the fused silica substrate. The nanoindentation showed that the mechanical properties of the ALD SiO2 film were much lower than those of the fused silica substrate. After a low-temperature annealing treatment, the ALD SiO2 film Si-OH defect was reduced, the ALD SiO2 film four-member ring content was increased, the elastic modulus of the ALD SiO2 film was increased from 45.025 GPa to 68.025 GPa, the hardness was increased from 5.240 GPa to 9.528 GPa, and the ALD SiO2 film damage threshold was decreased from 5.5 J/cm2 to 1.3 J/cm2.

Three-dimensional reconstruction of subsurface defects in fused silica optical components using cadmium selenide/zinc sulphide quantum dots

Three-dimensional reconstruction of subsurface defects in fused silica optical components using cadmium selenide/zinc sulphide quantum dots

Morphology evolution of Vickers indentation in fused silica glass etched by atmospheric pressure plasma jet

Nondestructive machining of optical components and smoothing of surface/subsurface damage generated by pre-processing is a major challenge for ultra-tight machining. The study analyzes the crack change during smoothing by quantitative layer-by-layer removal of Vickers indentations through atmosphere pressure plasma jet; the indentation change process is modeled and analyzed using Level Set Method (LSM) simulation. The results show that plasma jet processing can smooth the subsurface damage of fused silica optical components, and the LSM simulation verifies that the processing of cracks in fused silica components by plasma jet is dominated by each isotropic etching, and there are also each anisotropic etching during the change of cracks; and the depth of adjacent cracks can be judged by the moving direction of adjacent crack boundaries.

A two-step physical-chemical etching technique to improve laser damage resistance of fused silica

A two-step physical-chemical etching method, namely large incident angle ion beam etching followed by HF etching, is proposed to remove the damage precursors in fused silica surface to achieve good surface quality and significantly improve the laser damage resistance of fused silica. Ion beam etching with large incident angle is utilized to remove the metal impurities and most subsurface defects in fused silica surface, which can reduce the removal amount of subsequent HF etching of fused silica, thus suppressing the replication and expansion of the subsurface defects, deposition of reaction products and deterioration of surface shape during the HF etching process. After that, HF etching is used to remove the atomic-scale damaged layer and reduce the P–V value of the reflection surface shape induced by ion beam etching. Therefore, a better laser damage performance of fused silica can be obtained by the two-step etching method compared with single ion beam etching and single HF etching. The results show that the two-step physical-chemical etching can be developed to fabricate high-quality and high damage-resistant fused silica optical components with small etching amount and good surface quality for high-power laser application.

Influence of ejected SiO2 particles on the laser damage thresholds of fused silica

The SiO2 particles ejected during the laser-induced damage of fused silica will be re-deposited on its surface as secondary contaminants, which seriously reduce the laser damage thresholds of the components. In this article, the influence of the secondary contaminants on the laser damage resistance of the fused silica optical components was investigated by using two test protocols for ion beam etching (IBE), dynamic chemical etching (DCE), and reactive ion etching (RIE)+DCE treated samples, which are the most optimized methods for the surface treatment of fused silica. Results showed that there are obvious differences in the damage thresholds of samples treated by different processes, with the smallest effect on the damage threshold of the RIE+DCE treated sample and the most significant effect on the damage threshold of IBE treated sample. These results may facilitate further optimization of the above surface treatment methods, which will be potentially used for the treatment of fused silica elements applied in high-power laser facilities.

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.

Parametric study of laser-induced damage growth in fused silica optics with large beams at 351 nm. Part 2: fractal analysis.

The impact of laser fluence and pulse duration on both the rate and probability of growth of laser-induced damage sites has been reported and analyzed statistically in a companion paper. In this paper, we report and analyze the volume morphology of damage sites during the growth process in fused silica optical components, at 351 nm, under various laser fluence and pulse durations. Fractal analysis has been used to quantify the bulk damage morphology. A clear link between the damage morphology and laser pulse duration has been observed. The results from fractal analysis allows for a better understanding of the results from the stochastic approach developed in our companion paper. More specifically, fractal analysis shows how the laser parameters such as fluence and pulse duration impact the phenomenology and the dynamics of the growth process.

熔融石英光学元件亚表面缺陷三维重构技术

熔融石英光学元件亚表面缺陷三维重构技术

Effect of non-linear amplification of phase and amplitude modulations on laser-induced damage of thick fused silica optics with large beams at 351 nm

Nonlinear propagation of high power, large aperture laser beams leads to the amplification of the spatial pre-existing small scale modulations, and therefore locally increases the laser energy density as the beam propagates through the optical component. The measurement of the energy density distribution on the exit face of the tested component is therefore more challenging, as the impact of the propagation must be taken into account. It is for this reason that the damage densities of thick fused silica components have never been precisely measured. To this end, a dedicated imaging setup has been developed to accurately measure the energy density distribution on the exit face of 34-mm thick fused silica optical components during laser damage experiments with a centimeter-scale laser beam. The impact of nonlinear propagation on the beam spatial profile has been studied experimentally and confronted to numerical simulations that solve the nonlinear Schrödinger equation for the experimental spatial and phase profiles. The results clearly indicate that the imaging setup can accurately measure the amplification of the beam spatial modulations. The beam energy density on the exit face is therefore correctly measured and, for the first time, the damage density of thick optics can be accurately determined.

Surface modification and etch process optimization of fused silica during reaction CHF3–Ar plasma etching

The ability to predict and control the influence of reaction ion etching process parameters on laser damage resistance of fused silica optical components is vital for their development in specialized applications such as inertial confinement fusion. In this study, a set of experiments was designed and performed to determine an optimized process condition by characterizing the evolution of the surface roughness and absorption of the fused silica samples treated at various gas flow rates. The macroscopic morphology, carbon concentration and laser damage resistance of the etched surface was measured. The optimized surface was obtained when the gas flow rate R (CHF3/(CHF3+Ar)) decreased down to 38.4%. With the optimized etch process, the presence of subsurface defects known to lead to laser damage initiation was removed and prevented. The C atomic concentration of the sample surface etched was decreased by 64.7%. The laser damage resistance increased dramatically; the average threshold fluence for damage initiation increased from 6.79J/cm2 to 8.99J/cm2.

Effect of residual stress on laser-induced damage characterization of mitigated damage sites in fused silica

The influences of residual stress on damage morphology and laser-induced damage threshold (LIDT) of mitigated sites in fused silica optical components before and after annealing are investigated. The results indicate that the cracks occur either at the inner location or at the maximum retardation location of mitigated sites once the residual stress (retardation) exceeds 26.49±1.79MPa (22.25±1.5nm). Meanwhile, the discrepancy of stress at different locations results in the fluctuation of LIDTs. The results of isochronal and isothermal annealing indicate that annealing temperature is the most important influence factor and the residual stress can be effectively controlled if the residual stress induced by the maximum CO2 laser beam is eliminated. Furthermore, the LIDT of mitigated sites can be enhanced without dramatical fluctuation and the damage growth coefficient that is sharply increasing can be avoided if the retardation of mitigated site is equal to or less than 5nm. The study demonstrates that annealing significantly improves the resistance of fused silica optics.

Subsurface damage distribution characterization of ground surfaces using Abbott–Firestone curves

Measurement of subsurface damage (SSD) induced by grinding process is of major interest in the development of high laser damage fused silica optical components manufacturing processes. Most SSD measurements methods give only access to the peak to peak value. We herein report on the benefit of using Abbott-Firestone curves to get an insight of the SSD distribution inside the optical material. We evidence on various diamond wheel ground fused silica substrates that such an approach is complementary to a classical SSD peak to peak measurement and bring useful information to optimize a grinding process.

Probability of growth of small damage sites on the exit surface of fused silica optics

Growth of laser damage on fused silica optical components depends on several key parameters including laser fluence, wavelength, pulse duration, and site size. Here we investigate the growth behavior of small damage sites on the exit surface of SiO₂ optics under exposure to tightly controlled laser pulses. Results demonstrate that the onset of damage growth is not governed by a threshold, but is probabilistic in nature and depends both on the current size of a damage site and the laser fluence to which it is exposed. We also develop models for use in growth prediction. In addition, we show that laser exposure history also influences the behavior of individual sites.

Monitoring annealing via CO_2 laser heating of defect populations on fused silica surfaces using photoluminescence microscopy

Photoluminescence (PL) microscopy and spectroscopy under 266 nm and 355 nm laser excitation are explored as a means of monitoring defect populations in laser-modified sites on the surface of fused silica and their subsequent response to heating to different temperatures via exposure to a CO(2) laser beam. Laser-induced temperature changes were estimated using an analytic solution to the heat flow equation and compared to changes in the PL emission intensity. The results indicate that the defect concentrations decrease significantly with increasing CO(2) laser exposure and are nearly eliminated when the peak surface temperature exceeds the softening point of fused silica (approximately 1900K), suggesting that this method might be suitable for in situ monitoring of repair of defective sites in fused silica optical components.

Refractive index of Lithosil and Suprasil 312 at cryogenic temperature

Measurements of the refractive index of two different Lithosil samples and a sample of Suprasil 312 at cryogenic temperature and at 293 K are reported for the spectral range from 480 nm to 894 nm. Such data are useful for the design of fused silica optical components and systems destined for space missions.

Initiation of laser-induced damage sites in fused silica optical components

Significant improvement in the polishing process of fused silica optical components has increased their lifetimes at 351 nm. Nevertheless, for large laser facilities like the LaserMegaJoule (LMJ), zero defect optical components are not yet available. Therefore, a damage mitigation technique has been developed to prevent the growth of the laser-initiated damage sites. Because of the difficulty to produce mitigated sites with sufficiently large depth, the initial morphology of damage to mitigate is a critical issue. The aim of this work is to determine laser parameters (pulse duration, fluence) which permit us to initiate damage sites in accordance with our mitigation process. Confocal microscopy is used to observe damage sites that have sub-surface cracks and consequently to measure precisely the diameter and the depth of the area to mitigate.

Laser-Induced Damage Growth on Larger-Aperture Fused Silica Optical Components at 351 nm

Laser-induced damage is a key lifetime limiter for optics in high-power laser facility. Damage initiation and growth under 351 nm high-fluence laser irradiation are observed on larger-aperture fused silica optics. The input surface of one fused silica component is damaged most severely and an explanation is presented. Obscurations and the area of a scratch on it are found to grow exponentially with the shot number. The area of damage site grows linearly. Micrographs of damage sites support the micro-explosion damage model which could be used to qualitatively explain the phenomena.

Inadequate heating of fused silica components during the coating process

The customary method of heating fused silica optical components during the coating process is shown to result in low temperature and poor coating performance.

Experimental observation and suppression of transverse stimulated Brillouin scattering in large optical components

Using 2.4-nsec pulses at 350 nm, we have observed transverse stimulated Brillouin scattering (SBS) in fused-silica optical components. Transverse SBS sets in when the product of laser fluence and growth time for the scattered optical wave exceeds ∼2.3 J nsec/cm2. An increase in laser bandwidth to 8.3 GHz suppresses SBS losses up to approximately twice the SBS threshold. We review the theory of transverse, broadband SBS and its scaling with experimental parameters.

High laser damage threshold porous silica antireflective coating

A quarterwave-thick narrow-bandwidth antireflective coating for fused silica optical components and KDP crystals has been developed. The coating consists of porous silica prepared from a silica sol in ethanol. It is applied by dip or spin from a solution at room temperature and requires no further treatment. The laser damage threshold levels are about equal to the surface damage thresholds of the uncoated substrates.