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Abstract
We demonstrate the effects of typical bulk defects in fused silica on the bulk damage threshold under nanosecond UV pulse in this study. A new test method is proposed to accurately evaluate laser induced bulk damage performance. The bulk bubble, hydroxyl, metal impurity, and weak absorption of the 355 nm laser are respectively characterized. The effects of bulk defects on bulk damage performance are analyzed statistically based on the correlation principle. For synthetic fused silica, metal impurities and hydroxyl have a weak correlation coefficient with the bulk damage threshold, while there is strong correlation between weak UV absorption and the bulk damage threshold. The influence of bulk damage threshold on surface damage performance is also discussed.
© 2017 Optical Society of America
1.Introduction
Fused silica optical components have excellent transmission performance from ultraviolet to near-infrared, stable chemical properties, high laser damage resistance, and are available for large-scale manufacture. They are commonly applied in high-power laser systems including the National Ignition Facility in the United States, the Megajoule Laser in France, and the ShenGuang series laser facility in China. When fused silica optical surface is exposed at sufficiently high laser intensities, surface or subsurface defects induced by grinding and polishing processes cause laser induced damage (LID), and the damage sites will grow exponentially to unmanageable dimensions upon subsequent pulse exposure [1,2]. LID severely limits the stable operation of laser systems and has been extensively researched in regards to the damage mechanism and novel techniques for enhancing damage resistance. For damage mechanism of fused silica optics, most related studies have been focused on the influence of surface defects on damage performance [3–7]. The Lawrence Livermore National Laboratory (LLNL) reported that damage occurring in the surface of fused silica results from the absorption of sub band-gap light by nanoscale defects in the near-surface [8,9]. The absorption is a common characteristic of various damage precursors under laser irradiation. Many researchers have attempted to enhance damage resistance in fused silica optics by optimizing the polishing process of the large-aperture optics and removing low-threshold polishing defects via post-processing [10–16].
The bulk damage threshold of fused silica is much higher than its surface damage threshold [17–24]. According to the difference of manufacturing technique, fused silica materials mostly includes fused quartz and synthetic fused silica; the former is generally manufactured by high-temperature fusing silica sands in a metallic crucible while syntheticfused silica is typically manufactured by high-temperature (~1500 °C) hydrolyzing SiCl4 in H2/O2 flame, which comes with higher purity and thus is generally used in high-power laser systems for UV transmission. The band gap of the intrinsic material of fused silica exceeds 9 eV, i.e., is far above the photon energy of 355 nm (3.5 eV). According to theoretic calculation, the damage threshold of fused silica bulk material under UV nanosecond pulse(about 10 ns, 355nm) could reach up to 100 J/cm2 or even more [25,26], which is several times higher than that of a fused silica surface. To this effect, there have been many previous studies on the surface damage behavior and mechanism of fused silica and relatively few on its bulk material damage. Noticeably, some studies on bulk damage of thick fused silica windows or lenses used in high-power solid-state laser systems have also been reported, results have shown that the light intensity in the bulk of a transparent medium increases nonlinearly (e.g.,by self-focus effect) and causes filamentation damage in the fused silica material [27]. Additionally, the gap between surface and bulk damage performance can be effectually narrowed by deploying advanced surface treatment processes, especially for the Advanced mitigation process (AMP) [28]. In the case, Researchers have grown increasingly interested in the bulk material damage of fused silica. Yang et al. [29] reported the bulk damage morphologies of fused silica induced by a 1064 pulse laser, thermal stress and mechanical damage were revealed as the main causes of bulk damage. Negres et al. [30,31] investigated the dynamic evolution of bulk material damage under 355 nm UV laser irradiation by a time-resolved microscope system, determined the laws of damage initiation, growth, and final formation; the results indicated that the electron-hole pairs generated by the 2- or 3-photon absorption are an important damage mechanism for fused silica material under UV or deep-UV irradiation. The influence of different fused silica material defect types and their characteristic parameters on bulk damage performance needs to be further investigated, which may provide a useful reference for improving the performance of fused silica material.
In this study, we investigated the influence of bulk defects on the damage performance of fused silica material under a UV nanosecond pulse laser. Unlike the traditional damage testing method, which involves focusing a laser to tens of microns inside a bulk material, we establish a different test method which uses a millimeter-sized test spot for evaluating the bulk damage performance of fused silica material. We then applied correlation statistics to quantitatively compare the influence of different defect types in syntheticfused silica and fused quartz on the bulk damage threshold.
2. Samples and damage threshold testing
Five samples of typical synthetic fused silica available on the market were used in this study to ensure a comprehensive analysis. We also used two fused quartz samples prepared by melted natural quartz powder (FQ-A and FQ-B) for comparison; this kind of fused quartz generally has a high concentration of metal impurities and lower price. All seven samples were square in shape with 50 mm width and 20 mm thickness. Both substrates of each sample were polished by the same mechanical process. Besides, the flanks of the samples were all polished to allow for easy observations of bulk damage. Shortcut names of the samples are defined in Table 1.
Table 1. Description of samples
The laser induced damage threshold (LIDT) of each sample was assessed using a single longitudinal mode pulse laser (2 J) with 355 nm wavelength, 9.3 ns pulse width, and 1 Hz repetition rate. The bulk damage threshold of fused silica can be tested simply by using a focus lens to converge the pulse laser into the bulk material [29–32]. The damage can be induced by an incident laser at the focus with merely micro-joule-scale energy. However, the influence of the material defects on damage performance is difficult to be accurately reflected using this method, because the damage testing region of the material is significantly limited by the focal spot range (about several tens of micrometers); further, the damage induced by discrete bulk defects is difficult to capture.
In this study, we conducted a bulk damage test on fused silica with certain thickness by focusing high energy laser outside of the sample. A 355 nm laser beam with 20 mm diameter was converged to out of the rear surface of the samples; the focus length of the lens is 100 mm, the spot diameter of the rear and front sample surface are1.2 mm and 2.5 mm, respectively. The 100mm short-focus lens also prevented front surface damage during the measurement because the light intensity of the front surface is much less than that of the rear surface. In particular, because of focusing from back reflected beam, the method will also induce bulk damage near to front surface, but it will not to affect our test. The bulk damage near the rear surface can be observed from the flank of the samples via dark field illumination using a microscope with 90mm work distance and 2μm resolution. Because the diameter of test beam is bigger than the focusing depth of the microscope, we executed a Z-axial scanning observation after every laser shot to observe the damage across the whole beam diameter region. Furthermore, the overlay rate of the bulk material defects in the damage test was dramatically increased due to the millimeter size of the test spot area, which can effectively reveal the influence of bulk material defects on bulk damage performance. A sketch of the experimental setup is shown in Fig. 1.
Fig. 1 Bulk damage tests of fused silica samples by focusing high laser energy outside of the materials.
During the laser irradiation process, rear surface damage occurs when the incident laser energy exceeds the surface damage threshold. Because of the strongest light intensity and longest light transmission distance, bulk damage often initiated at the bulk region near the rear surface of the samples. Figure 2 shows the correlation of incident laser energy on bulk damage of materials. When the incident laser energy is below the bulk damage threshold, only surface damage appears; once incident laser energy exceeds the bulk damage threshold, there is initially bulk damage of slight filamentary type. The distance of this initial bulk damage to rear surface is usually within 2mm. If the laser energy far exceeds bulk damage threshold, the filamentary damage extends and density increases rapidly.
Fig. 2 Growth of bulk damage with increasing incident laser energy. (a) Incident laser energy is less than bulk damage threshold. (b) Incident laser energy is just over the bulk damage threshold. (c) Incident laser energy exceeds bulk damage threshold obviously.
Observing the bulk damage behavior, we can measured the zero probability damage thresholds of different samples by 1-on-1 testing protocol and determined the anti-damage performance of different fused silica materials accordingly. In the 1-on-1 damage test, we tested bulk damage probability under five different laser fluences with ten sites irradiated for each fluence. Zero probability damage thresholds were obtained by linear fitting of the relationship of test fluence on damage probability. To handily calculate the laser fluence of initial damage sites, we used the beam spot of the rear surface (Fig. 3) to replace that of the actual damage sites; we then obtained the peak fluence of hot spots in the beam via beam modulation (ratio of peak to average of light intensity). The distance of this initial bulk damage to the rear surface is usually within 2 mm, which was considered when calculating the uncertainty of the fluence.
Fig. 3 Distribution of light intensity at rear surface of fused silica optics. The modulation (ratio of peak to average of light intensity) is 3.6, blurred diffraction spot was induced by laser transmission window.
The zero probability damage thresholds of different samples measured by the above method are shown in Fig. 4. The damage thresholds of the synthetic fused silica samples exceed 70 J/cm2, which is higher than that of the fused quartz samples. The SFS-A,SFS-B, and SFS-C samples expressed the maximum damage threshold (~90 J/cm2) while the damage thresholds of the fused quartz samples are only 50 and 29 J/cm2, respectively.
The typical damage morphologies of synthetic fused silica and fused quartz are shown in Fig. 5. The bulk damage of fused quartz often initiates at relatively low laser fluence (less than 50 J/cm2)with discrete and explosive damage points near the rear surface, which are induced by absorptive point defects of discrete distribution in the bulk materials. The bulk damage thresholds of synthetic fused silica often exceed 70 J/cm2 and the bulk damage morphologies present discontinuous filament parallel to the laser transmission direction. This bulk damage behavior of synthetic fused silica is attributable to the nonlinear growth of light intensity at high laser fluence, e.g., self-focusing effect. The filament morphologies reflect intrinsic damage to the fused silica material, but the disparity in the damage threshold of synthetic fused silica suggests that the damage is affected by intrinsic defects in the material. From the present results, no matter for synthetic fused silica or fused quartz, although damage inducement isn’t still clear very much, however, it could be determined that the damage depends on the counterwork of the intrinsic material on thermodynamic effect of the bulk material absorbing the laser energy, and the bulk material defects would have an important effect on the mechanical performance and laser coupling coefficient of bulk material.
Fig. 5 Typical bulk damage morphologies. (a) Fused quartz, induced by 45 J/cm2. (b) Synthetic fused silica, induced by 93 J/cm2.
3. Defects characterization and analysis
3.1 Bulk bubble concentration
Bubbles with various density and scale are well retained inside fused silica during the manufacturing process, especially for fused quartz, natural quartz powder is generally used in the high-temperature melting and cooling shaping of fused silica, which readily induces bubbles to form. Thus, many restrictive standards of the bubbles are established by major manufacturers. In this study, bubbles were characterized by combining laser scattering lighting and microscopic observations. We used a semiconductor laser (473 nm) to irradiate the sample polished flank, the tiny bubbles were scattered by the incident laser and then observed precisely under an optical microscope from the vertical direction of sample surface. The large area observation of the sample could be achieved by moving the object stage of the microscope. Figure 6 shows a sketch map of laser scattering imaging and typical size of bubbles observed from the fused quartz samples. To get statistical bubble distribution results, we ran three-dimensional microscopy mapping scanning of a 8 mm × 6 mm × 2 mm test area, three regions with the test area were chosen randomly from each sample bulk, then, the bubbles were counted for each tested region. Data statistical results show that the bubble diameters in FQ-A and FQ-B samples are between several micrometers and several hundreds of micrometers with a distribution density between 0.1/mm−3 and 100/mm−3. But few bubbles were observed in the other five synthetic fused silica samples.
Fig. 6 Morphologies of bubbles scattered inside fused quartz samples observed by laser scattering lighting. (a) Bubble scattering with high distribution density. (b) Typical bubble morphology.
The major impact of the bubbles on fused silica damage performance is the disturbance of the bubbles on the incident laser light field, which can be simulated by the finite difference time domain (FDTD). Figure 7(a) shows the effect of the bubbles in the fused silica material on incident laser electric field intensity, where the field intensification factor reflects the magnification of laser electric field intensity. Base on the results, we can find that the 355 nm laser light field was dramatically modulated and enhanced by the bubbles, larger bubbles induce stronger incident light intensity. An example of typical bubble-induced bulk damage is shown in Fig. 7(b).
Fig. 7 Effect of bubbles on incident laser field intensity of fused silica. (a) Theoretical simulation. (b) Experimental bubble damage induced by 32 J/cm2, 355nm pulse laser.
3.2 Bulk hydroxyl content
Hydroxyl is the intrinsic molecule structure which exists throughout the material system of fused silica. Due to the manufacturing technique of hydrolyzing SiCl4 in H2/O2 flame, synthetic fused silica generally has a higher hydroxyl content. The gap of the hydroxyl content between synthetic fused silica and fused quartz (the lowest level) can be reduced via dehydroxylation. Basing on Lambert-Beer law, hydroxyl content can be obtained by measuring the infrared absorption peak of the hydroxyl. The typical absorption peaks of hydroxyl are centered at 1.4, 2.2, and 2.7 μm, by which the hydroxyl content can be calculated. Figure 8 shows the hydroxyl absorption peaks of our samples near the wavelength of 2.2 μm and the results of calculated hydroxyl content.
Fig. 8 Comparison of the hydroxyl content. (a) Hydroxyl absorption peaks of the samples near the wavelength of 2.2 μm. (b) Computational hydroxyl content.
Figure 8 indicates the different kinds of fused silica significantly differ in hydroxyl content. The hydroxyl contents of the fused quartz (FQ-A and FQ-B) are generally lower than 200 ppm while that of the synthetic fused silica are generally higher than 1200 ppm. By using the new dehydroxylation process, the hydroxyl contents of some synthetic fused silica can be controlled below 200 ppm, such as SFS-A, and SFS-B.
3.3 Bulk metal impurity concentration
Fused silica bulk material generally has a much lower concentration of metal impurities than the polished surface. To distinguish the differences in trace metal impurities, we analyzed these metal elements inside the fused silica material by time of flight secondary ion spectrometry (TOF-SIMS) with a limit of detection of 0.01 ppm. To minimize the influence of polishing-induced impurities on the sample, all the samples were broken up with external force. The fracture surfaces were subjected to testing to ensure the results accurately reflected the intrinsic characters of the bulk material. The results are shown in Table 2 and indicates that the metal impurities in synthetic fused silica samples mainly include Na, Mg, K, Fe, and Cu; the concentrations differed depending on the sample manufacturer. The fused quartz samples were manufactured by fusing silica sands in a metallic crucible, so in addition to the above metal impurities, we observed Al, Ca, Ti, Zn, and other elements. Especially, the metal impurity concentrations of FQ-B are significantly higher than those of other fused silica samples.
Table 2. Relative metal impurity concentrations of seven fused silica samples (intensity counts)
3.4 Weak absorption under UV laser
The photo-thermal common-path interferometer (PCI) technique can be used to accurately measure the weak absorption of transparent optical material [33,34]. The pump beam we used is a 355 nm quasi-continuous laser with 1 W output power focused on the inside of the bulk material with a 60 μm focal spot diameter (1/e2). The probe beam is a modulated He-Ne laser that overlapped with the pump laser on surface of tested samples. When there are absorptive defects in the optical material surface, the pumping laser energy is partly absorbed and induces the change of the optical material reflective index, which will lead to optical axis deflection of the probe laser. We can extract the change of the weak signal using a position sensor and a lock-in amplifier, and then compute the absorption coefficient of the material on the pumping laser. In this study, we tested five sites randomly for each sample to obtain the mean absorption values. The weak absorption per unit length (centimeter) for all seven samples are shown in Fig. 9.
Figure 9 shows that the bulk absorptions in fused quartz samples (FQ-A, FQ-B) are three orders of magnitude higher (~10000 ppm/cm) than those in synthetic fused silica samples. Apart from the bulk absorption of SFS-D (~500 ppm/ cm), the bulk absorption values in other synthetic fused silica samples are below 100 ppm/cm. Such results provide useful information regarding the trace absorption of the bulk material at the 355 nm UV laser wavelength. The results also reflect defects responding to the pumping wavelength in the whole testing region but neglect the defect types and distribution patterns. According to the intrinsic laser damage mechanism of avalanche ionization and multi-photon free ionization, the amount of free electrons associated with bulk damage will increase with the enhancement of bulk material to laser energy absorption. Thereby, weak laser absorption behavior can significantly affect the bulk damage of fused silica material, which will be discussed in Section 4.
4.Analysis and discussion
As discussed in Section 3, we systematically characterized the typical defects associated with bulk damage including bubble concentration, hydroxyl content, metal impurity concentration, and weak absorption (355 nm) in samples from different suppliers. For bubbles inside the bulk material, it can be believed to be one of the defects absolutely associated with the bulk damage of fused silica due to the intensity modulation for light field. Moreover, the network structure of SiO2 around the bubble positions is broken, which weakens the structural strength of the Si-O tetrahedron and may initiate explosive bulk damage; this may explain why the damage thresholds of fused quartz are lower than those of synthetic fused silica. We observed bubble defects only in fused quartz in this study, so more attention should be paid to hydroxyl content, metal impurities, and other absorbing defects to determine the key factors affecting the damage resistance of synthetic fused silica.
Statistical correlation is useful for analyzing the interaction between two sets of numerical values. In a previous study, we successfully used this method to establish correlation between defects and damage resistance in fused silica surfaces [16]. Similarly, since there is no specific numerical distribution characteristic for bulk defect parameter and bulk damage performance, but a rank correlation definitely exists between such two variables. So we can use Spearman rank correlation function to estimate the correlation coefficient between bulk defect distribution and bulk damage performance. The Spearman rank correlation coefficient is calculated according to the following equation [35]:
Where di is the difference between ranks for each xi, yi data pair and n is the number of data pairs. The calculated results are shown in Table 3.
Table 3. Correlations among characterization parameters of three defects and bulk damage threshold at 355 nm pulse laser
The Spearman correlation coefficient is used to estimate the monotonic increasing or decreasing relationship between two variables. When the value is 1, the relation of two variables is monotonic increasing; when the value is −1, the relation of two variables is monotonic decreasing; and when the value is 0, the two variables are completely uncorrelated. The value of Sig indicates the reliability of the correlation analysis results. A smaller value of Sig means higher reliability, which should be less than 0.01 in the analysis. The correlation results shown in Table 3 indicate that hydroxyl content has a weak correlation with the bulk damage threshold at 355 nm (correlation coefficient < 0.2). This suggests that hydroxyl is not a factor (at least not a major factor) influencing the bulk damage of fused silica. Although hydroxyl exists throughout the SiO2 network structure and may decrease the structural strength of the Si-O tetrahedral, there are likely other defects that have a greater influence on the bulk damage of fused silica when various defects coexist. Metal impurities can directly absorb UV laser energy (355 nm) and are likely an important defect initiating damage in fused silica. As shown in Table 3, however, this type of defect has weak correlation with the bulk damage (correlation coefficient of about 0.3), possibly because metal impurity concentrations in the bulk material of synthetic fused silica are fairly low (often lower than 0.01 ppm). In this case, the sort of defect is difficult to affect laser induced damage of synthetic fused silica. Unlike the aforementioned defect types, the UV absorption coefficient at 355 nm has a strong correlation with the bulk damage threshold (the absolute value of correlation coefficient reaches up to 0.72). Fused quartz showed the strongest UV absorption and the lowest bulk damage thresholds. The high UV absorption may correspond to high bubble density (FQ-A, FQ-B) and high metallic impurity content (FQ-B). All synthetic fused silica has low metallic impurity content, so we can’t deduce the cause of inducing UV absorption difference. Regardless, our statistical correlation analysis shows that 355 nm absorption defects are an important factor influencing the UV bulk damage performance of fused silica optics. Weak UV laser absorption can be used as a reference point for evaluating the LIDT of fused silica material under 355 nm nanosecond pulse laser. To clearly illustrate the correlations of the three defects and bulk damage threshold, we express them in histogram form in Fig. 10.
Fig. 10 Histogram form of correlation between the three defect parameters and bulk damage threshold at 355 nm.
We next investigated the influence of bulk damage threshold on the surface damage resistance of fused silica. The surface fabrication and post-processing capabilities for fused silica remain fairly limited, so the damage threshold of fused silica surfaces is far below the intrinsic damage threshold. We sought to determine whether the bulk damage threshold correlates with the surface damage threshold, so we conducted surface damage threshold tests based on 1-on-1 irradiation by changing the focal distance of the lens to1.5 m. The spot used in this test is near flat distribution with a modulation degree of 2.2, spot diameter at sample rear surface is 2.5 mm. The 0% probability damage thresholds of the seven samples are shown in Fig. 11. For comparison, the corresponding bulk damage thresholds are also given.
The average 0% probability damage threshold of all the sample surfaces is approximately 13.5J/cm2 with mean deviation of 20%, which is far below bulk damage threshold. Fluctuations in the surface damage threshold were probably caused by the polishing difference among the sample surfaces. The fused quartz (FQ-A, FQ-B) has a much lower bulk damage threshold, but the disadvantage does not decrease their surface damage resistance obviously. SFS-A and SFS-B have the highest bulk damage threshold, but this advantage is not reflected in their surface damage thresholds.
We also evaluated the Spearman correlation coefficient between surface and bulk damage performance using the same method and failed to identify any significant correlation; the coefficient was only 0.11. The result indicates that the surface damage threshold isn't correlated with the bulk damage threshold. In the current mechanical polishing processes, the surface damage precursor with low threshold is the maximum bottleneck limiting the damage resistance of optics, which far surpass the influence of defects in bulk material on surface damage resistance.
5. Conclusions
The effects of typical fused silica bulk defects on the bulk damage threshold under a UV laser were investigated in this study. The bulk damage thresholds of typical fused silica samples, including synthetic fused silica and fused quartz, were evaluated by focusing a high energy laser outside the bulk material. The bulk bubble concentration, hydroxyl content, metal impurity concentration, and weak absorption of the 355 nm laser were characterized by laser scattering lighting, infrared absorption spectrum analysis, TOF-SIMS, and PTD techniques, respectively. The effects of bulk defects in the fused silica on bulk damage performance were analyzed statistically based on the correlation principle. The results can be summarized as follows.
- (1) The difference in bulk damage thresholds among the synthetic fused silica samples manufactured by different vendors is not significant (75-90 J/cm2), and the bulk damage threshold of fused quartz is only about half that of synthetic fused silica.
- (2) Bulk bubbles have a significant impact on the bulk damage performance of fused quartz due to the light field modulation of the incident laser, and can be enhanced with larger-scale bubbles.
- (3) Metal impurities and hydroxyl have weak correlation with the bulk damage threshold of synthetic fused silica. There is strong correlation between weak absorption and the bulk damage threshold, suggesting that 355nm UV laser absorption significantly impacts bulk damage performance and thus represents a useful reference parameter for evaluating the LIDT of fused silica material with a 355 nm laser.
- (4) In the presence of low threshold damage precursors induced by surface finishing processing, there is no significant correlation of fused silica bulk damage threshold with surface damage threshold.
The results presented here may serve as a useful reference for the performance evaluation of UV fused silica materials used in high power laser systems.
Funding
National Natural Science Foundation of China (Grant No. 51602296).
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