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Abstract
We present and investigate a hybrid laser-based method of surface shaping for damage mitigation on fused silica surfaces. Damage sites were removed and precisely shaped into an optically-benign cone by a procedure of femtosecond laser ablation with a subsequent CO2 laser polishing process. The morphology of the cone rim was quantitatively predicted by a numerical model. Since the heat-affected zone (HAZ) of the laser polishing process was effectively confined by the optimization of ablation parameters, the dimensions of the raised rim were reduced by an order of magnitude. The intensity of the on-axis hotspot was positively related to the dimensions of the raised rim, and thus an inapparent downstream intensification was achieved by the rim reduction. Laser-induced damage threshold (LIDT) of the cone was tested to be ∼14 J/cm2 on the input surface. Therefore, the presented method is appropriate to mitigate damage and also provides a promising approach to manufacturing functional microstructures for high-power applications.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
The problem of laser-induced damage on optics emerges as a bottleneck to increasing the output of high-power or -energy laser systems. Particularly for the systems that operate in the ultraviolet regime, for instance, the National Ignition Facility (NIF) in the USA [1], Laser MegaJoule (LMJ) in France [2], and Shenguang in China [3,4], optics are susceptible to laser-induced damage (LID) and damage sites grow rapidly with successive pulses [5–8]. Over the past decades, sub-surface defects, such as micro-cracks and impurities, from the optics preparation process have been discovered as absorbing precursors leading to damage on fused silica optics. Significant efforts were made to improve the finishing process [6,9–12] and develop the post-finishing treatment of the whole-optic wet chemical etching process [13–16] for reducing damage precursors on fused silica optics. Damage density drops by several orders of magnitude and there are only a few damage sites exist on a large-scale fused silica optic when exposed to the fluence of ∼8 J/cm2 [6,17].
A supplementary and efficient approach is selectively mitigating residual damage precursors by laser-based methods [18–20]. In this case, the damaged material is evaporated away and leaves an optically benign cone on the surface. Managing these damage sites using a pulsed CO2 laser with a wavelength of 10.6 µm, for the strong absorption within a depth of a few microns on fused silica [21,22], can improve laser damage resistance of the fused silica optics and realize a dramatic reduction in downstream intensification [19,20]. Such a laser-based method for damage repair has been used for volume production in the optics recycle loop strategy of NIF [6], which supports NIF routinely delivering >1.8 MJ on target. Due to the heat affection during the laser process, a rim arose on the periphery of such a laser-processed cone. The rim could focus the laser and cause an on-axis hotspot which, in turn, would be a potential threat to the downstream optics [19,20]. Elhadj et al. [23] successfully reduced the raised rim by lowering laser treatment temperatures and reducing thermos-capillary flow with the assistance of reactive gases. Given that the size of the raised rim is directly related to the heat-affected zone (HAZ) [6,23,24], the reduction in raised rim can be attributed to the confine of the HAZ. Accordingly, such a cone fabricated by femtosecond (fs) laser ablation may have the potential to reduce the rim because of the non-thermal process [25,26]. However, the roughness of the structure fabricated by fs-laser ablation is too large for optical application, while CO2 laser polishing [27–30] would be an effective post-treatment. Thus, the impact on suppressing the rim via the combination of fs laser ablation and CO2 laser polishing needs to be investigated and proved.
In this paper, fs-laser ablation, in cooperating with CO2-laser polishing, is used for damage mitigation, which brings a reduction in the raised rim by an order of magnitude. Firstly, we discuss the experimental results of the rims around the craters ablated separately using fs-laser ablation and CO2-laser evaporation with single and multiple pulses. Then we focus on the rim formation during the CO2-laser evaporation process and develop a numerical model to quantitatively predict the morphology of the rim. By employing optimum parameters of CO2-laser evaporation from the model, the dimensions of the raised rim of a typical mitigated cone are significantly reduced. Experimental results show that the cone performs well in laser damage resistance and avoids strong intensification of the downstream light. Thus, the presented method is proven capable of creating an optically-benign cone, meanwhile, this method may pave the way to generating microstructure for high-power applications.
2. Discussion on the rim formation
Laser ablation of dielectric materials such as fused silica contains a series of processes, typically involving energy absorption, heat transfer, evaporation, melt flow, and re-solidification. The fluid dynamics of the molten material play a decisive role in the raised rim [24]. Because the pulse duration of the fs laser is ∼8-order-of-magnitude shorter than the CO2 laser, mechanisms driving the melt flow that determines rim formation should be different during the laser ablation processes. The raised rim surrounding an fs-laser-ablated crater is formed by the high pressure plasma producing a pressure-driven fluid motion of the molten material [24]. Seeing that it is most probably formed over a nanosecond time scale [25], the raised rim would be unaffected by heat accumulation for the repetition frequency of a commercial fs laser is typically below ∼1 MHz. Whereas the rim formation during CO2-laser ablation is associated with the thermo-capillary-driven flow (Marangoni flow) and recoil-pressure-driven flow [6,23,29,31], which depends on temperature, the duration of laser treatment, and the volume of material melted [6,23]. A long pulse and heat accumulation during the CO2-laser ablation process would lead to a significant rim with a height in micrometer scale [19].
A 2mm-diameter cone fabricated layer by layer utilizing fs-laser ablation has a raised rim with a height almost the same as the crater created by a single pulse, though the rim becomes laterally wider (Section 2.1). Different from fs-laser ablation, a 250µm-diameter cone formed layer by layer via CO2-laser ablation has a rim in height of 115 nm and width of 106 ± 2 µm, of which both the height and width are larger than a line formed with the same parameters (Section 2.2). Consequently, a microstructure manufactured by the procedure of femtosecond laser ablation combined with the CO2 laser polishing process, the subsequent polishing process would play a major role in rim formation. To minimize the raised rim, we develop a two-dimensional numerical model to analyze the fluid dynamics of the molten fused silica and quantitatively predict the raised rim during the CO2 laser polishing process (Section 2.3).
2.1 Rim formation during the femtosecond laser ablation process
Fused silica (Corning 7980) samples are processed in a vacuum with a pressure of ∼5 × 10−4 Pa to solve the issue of debris produced by fs-laser ablation. In order to compare the rim height of the pit ablated by a single pulse and multiple pulses, a series of craters are generated by an fs laser (∼260 fs, 1028 nm) with different pulse energy. The laser beam is focused on a spot with a diameter of ∼10 µm@1/e2 on the fused silica surface. Figure 1(a) displays the morphologies of the craters ablated by a single pulse with increased energy and characterized by an atomic force microscope (VEECO, Dimension 3100) with a lateral resolution of 80 nm and vertical resolution better than 0.1 nm. The widths and depths of the craters are both increasing with the energy until reaching saturation (Fig. 1(b)). Compared to the saturated ablation in the air, the material removal volume of a single pulse is increased by ∼30%. When reaching the ablation saturation, the effects of energy instability on the morphology of the crater are negligible, which is available for creating uniform craters.
Fig. 1. (a) Morphology and (b) depth and width of the craters ablated by the single laser pulse with different energy. The morphology of these craters is characterized using an atomic force microscope (VEECO, Dimension 3100) with a lateral resolution of 80 nm and vertical resolution better than 0.1 nm. (c) The definition of the depth and width of a crater.
As depicted in Fig. 2, a cone can be fabricated layer by layer using the fs laser, and δR is the radius reduction between the adjacent layers. In each layer, the route of a laser spot consists of a series of concentric rings, while δr is the radius reduction between the adjacent rings in a layer. Along each ring, the laser spot moves at a velocity of v (typically 80 mm/s at a repetition rate of 10 kHz). The crater created by a single pulse at the energy of 20 µJ, i.e. the average fluence of ∼25.5J/cm2, has a raised rim in height and width of 26 nm and 1.6 ± 0.08 µm, respectively (Fig. 3 (c)). Employing the same average fluence of ∼25.5J/cm2 at the ablation saturation, we manufacture a concave cone in diameter of ∼2 mm (Fig. 3(b) and (d)) layer by layer. Compared to ablation in the air, re-deposited debris around the cone is dramatically reduced. The surface roughness of the cone is 1.386 µm (Ra), characterized by confocal microscopy (Keyence, VK-X3000) with a lateral resolution of 2 µm and vertical resolution better than 1 nm. The profile of the cone on the periphery is measured by an optical profiler (Ametek, Zygo Nomad) with a lateral resolution of 2 µm and vertical resolution better than 0.1 nm, and the result shows that the rim width is accumulated to 42.1 ± 2 µm, but the rim height is 30 nm (Fig. 3(d)), close to the height of a single pulse. Since the rim height keeps unchanged with a large number of pulses though the rim width is accumulated, it is capable of producing a microstructure with a relatively low rim height using fs-laser ablation.
Fig. 2. Route of a laser spot for fabricating a cone layer by layer. δR is the radius reduction between the adjacent layers. Each layer consists of a series of concentric rings, and δr is the radius reduction between the adjacent rings in a layer. Along each ring, the laser spot moves at a velocity of v.
Fig. 3. Morphologies of (a) crater ablated using a single pulse and (b) concave cone using multiple pulses. Cross-section views of the (c) crater and (d) periphery of the cone. The insert picture is the cross-section view of the cone.
2.2 Rim formation during the CO2-laser ablation process
For investigating the rim formation during the CO2-laser ablation process, a line and a conical crater are formed on a fused silica sample (Corning 7980) by a pulsed CO2 laser (10.6 µm, ∼10 µs) with the same energy of 0.15 mJ, velocity of 4 mm/s and repetition frequency of 1 kHz. The focused spot on the surface has a diameter of ∼60 µm@1/e2, which means the CO2-laser ablation operates at an average fluence of ∼5.3J/cm2. The cone is also fabricated layer by layer, with a layer radius reduction of 4 µm (δR) and a ring radius reduction of 4 µm (δr). Measurements show that the line has a small raised rim with a height of 15 nm and width of 23 ± 2 µm, while the raised rim size of the cone is dramatically increased and almost an order of magnitude higher in height than that of the line, as illustrated in Fig. 4. The height of the fabricated cone is 115 nm and the width of 106 ± 2 µm, comparable to previous results of other groups [20,19].
Fig. 4. Microscope observations of the (a) line and (b) cone formed by CO2-laser ablation with the same spot separation; (c) cross-section views of the line and periphery of the cone. The rim height of the line and cone is separately 15 nm and 115 nm, characterized using the optical profiler (Ametek, Zygo Nomad) with a lateral resolution of 2 µm and vertical resolution better than 0.1 nm.
Different from the line, more laser spots are overlapped on the periphery of the cone, and more material is evaporated, which may lead to more residual heat accumulation and a larger size of the melted pool during the thermal process of CO2-laser ablation. Thus, reducing material removal and optimizing laser processing parameters may have the ability to reduce the raised rim. Compared to fs-laser ablation, CO2-laser ablation leads to an increase in both the height and width of the raised rim when producing a microstructure with more layers. Thus, it could reduce the rim that the CO2-laser ablation is only used for polishing while employing fs laser to remove the material of a microstructure.
2.3 Simulation and control of the rim formation by CO2 laser ablation
To minimize the raised rim of the microstructure, it still needs to understand the heat and flow process and identify the involved parameters in the CO2-laser polishing process. In previous work, we investigated surface evolution on the wall of a multi-stage microstructure and made a parametrical study on how to reduce the roughness while maintaining its shape using CO2-laser polishing [30]. But the impact of the processing parameters on the rim formation is still not fully understood during the polishing process, and it still needs to figure out whether the optimized parameters for suppressing the raised rim are also appropriate for polishing.
Corresponding to laser polishing with CO2-laser ablation, flux ranges in the regime of ∼0.1-1 MW/cm2, where evaporation mostly dominates the material removal and melt flow mainly determines the rim formation. A schematic of the numerical model used to explore the motion of the molten fused silica and the temporal evolution of the rim formation is drawn in Fig. 5. Considering a reflectivity of 0.15, laser energy is absorbed, leading to a significant increase in the temperature and initiating material melt and evaporation over a microsecond time scale. We assume that the melt flow is incompressible and can be described by the Navier-Stokes equations. This yields [24]
Fig. 5. Schematic of the numerical model used to explore the motion of the molten fused silica and temporal evolution of the rim formation.
Table 2. Laser processing parameters of CO2-laser ablation for calculation
2.3.1 Rim formed by a single CO2 laser
In this section, we investigate the rim formation of a single laser pulse and analyze the effect of pulse duration. The pulse energy is adjusted to achieve the high peak temperature required for rapid evaporation, resulting in higher energy for a longer pulse. Simulation results of the rim height and on-axis temperature at the center of the irradiated spot for a pulse duration of 10 µs and 20 µs are shown in Fig. 6. The rim arises, and its height increases in the range that the on-axis temperature is above ∼3500 K, corresponding to the effective time for rim formation. Since the effective time is longer for irradiation using the pulse with a duration of 20 µs, the final rim height is higher than that of 10 µs. Hence, it can be inferred that a shorter pulse can be useful to reduce the rim.
Fig. 6. Variation of the rim heights and on-axis temperatures. The shadow represents the effective time for the raised rim formation, which is ∼12µs and ∼21µs for the two cases with pulse duration of (a) τ=10µs, E = 0.85mJ; and (b) τ=20µs, E = 1.2mJ, respectively.
Figure 7 illustrates the temporal evolution of the surface profile on the laser condition that the pulse duration, energy, and spot diameter are 10 µs, 0.85 mJ, and 120 µm, respectively. The colored backgrounds stand for the temperature, velocity of the melt, and recoil pressure. Black arrows are proportional to the velocity of the melt flow and normalized to the maximum velocity at each moment for clear display. The motion of the fused silica can be divided into three stages according to the temperature fields of the melt pool. Before material evaporation, such as the moment at 5µs, the molten fused silica mostly flows from the cold periphery to the hot center (Fig. 7). But the velocity of the melt flow is small, so there’s no significant rim formation over several microseconds. This inward flow may be driven by the Marangoni effect due to the temperature gradient caused by the Gaussian intensity profile of the laser focal spot [23,24], considering that fused silica has a positive temperature dependence on the surface tension [35]. When the material evaporation begins at the moment of ∼8 µs, the velocity of the melt flow at the edge dramatically increases and the melt flow becomes to arise the rim significantly. Even beyond the end of the laser pulse, the increase in rim height does not stop until the on-axis temperature drops below ∼3500 K again (Fig. 6(a)). That is to say, the rim formation for such a short pulse is associated with material evaporation, which is mostly driven by the recoil pressure. During the evaporation, ranging from ∼8 µs to ∼20 µs, the melt flow velocity is faster in the area where the recoil pressure is high, as shown in Fig. 7 (the third row). Thus, for CO2 laser ablation with short pulse duration, recoil-pressure-induced melt flow would be an important transport mechanism in determining the height of the raised rim while the Marangoni effect would be significant in fused silica evaporation using a continuous-wave (CW) CO2 laser [6,19,23]. It should be noted that the Marangoni effect act through the whole ablation process unless the temperature is low enough, so heat accumulation should be avoided when processing with multiple pulses.
Fig. 7. Temporal evolution of the surface profile for fused silica ablated by a single laser pulse (τ=10 µs, 2ω=120 µm, E = 0.85 mJ). The colored backgrounds from left to right stand for temperature, the velocity of the melt, and recoil pressure. Time is from top to bottom 5 µs, 8 µs, 13 µs, and 20 µs. The black arrows in these pictures indicate the moving directions of the melt flow, which is normalized to the maximum velocity at each moment for clear display.
2.3.2 Rim formed by multiple CO2 laser pulses
For CO2 laser ablation with multiple laser pulses, heat accumulation would facilitate a temperature rise to the ablation threshold in a shorter time under the irradiation of subsequent pulses, leading to an extra on the raised rim. Therefore, the repetition frequency of the pulse should be carefully selected to afford enough cooling between pulses, avoiding significant heat accumulation and rim addition. Figure 8 shows the on-axis temperature on the surface irradiated by laser pulses with a duration of 10µs. The temperature drops exponentially after the end of the laser pulse (Fig. 8(a)) for a single pulse. The variation of on-axis temperature for different repetition frequencies is calculated to determine the appropriate operating range. A repetition frequency of 1kHz is selected for our experiments since the on-axis temperature is below the softening point and sufficiently low to avoid significant heat accumulation, as illustrated in Fig. 8(b).
Fig. 8. The on-axis temperature on the surface for (a) a single pulse and (b) multiple pulses at a repetition frequency of 1kHz. (τ=10 µs, 2ω=120 µm, E = 0.85mJ)
According to the feature dimensions of the roughness, generally, it requires multiple layers to fully polish a micro-structure using CO2 laser ablation. Considering a cone polishing with multiple layers of fixed radius and tapered radius, surface profiles and the rim heights after polishing are calculated. In the two-dimensional model, the radius reduction of each layer can be processed by the movement of the laser spot. Surface profiles and rim heights during the polishing by multiple pulses are shown in Fig. 9, of which the pulse duration and repetition frequency are 10 µs and 1 kHz, respectively. The laser spot moves at a velocity of 20 mm/s, corresponding to a layer radius reduction (δR) of 20 µm. As displayed in Fig. 9 (a), the calculated profiles indicate that the ablation depth at the edge is slightly smaller than that of the center, which agrees with the experimental results described in Section 3.2. Comparatively, fewer pulses are required to achieve an equivalent ablation depth when the spot does not move, but the edge is steeper (Fig. 9 (b)). As drawn in Fig. 9 (c), the rim height gradually increases with the pulse counts and tends to saturation when the rim is located away from the effect zone of the moving spot. The rims are about the same height of ∼23 nm with an ablation depth of ∼9 µm for both cases. Consequently, polishing with multiple layers with fixed size and tapered size could create raised rims of comparable dimensions, while the edge would be a steeper slope for the case with a fixed size.
Fig. 9. The calculated profile after each shot for the laser spot moves at a velocity of (a)20 mm/s and (b) 0 mm/s; (c) variation between the rim height and the number of laser shots during the CO2 laser poling process. (τ=10 µs, 2ω=120 µm, f = 1 kHz)
2.4 Discussion for optimization of ablation parameters
It can be clearly seen that rims formed during the ablation by fs and CO2 laser perform different features. Compared to CO2-laser fabrication, the fs laser can be used for material removal and rapidly create the overall shape of the desired microstructure, with pulse energy at the ablation saturation to avoid inconsistencies in removal between pulses caused by the energy instability. The issue of the re-deposited debris is solved by executing the fs-laser ablation in the vacuum. Microstructure produced by fs laser has the significant advantage of a small raised rim even with large amounts of pulses, for the rim height keeps unchanged though the rim width is accumulated. However, the roughness of the microstructure cannot meet the requirements for its energy dissipation and poor damage resistance. The roughness can be effectively reduced by subsequent polishing with a CO2 laser.
Notably, massive CO2-laser pulses inevitably bring the issue of increased rim size, so the polishing should be accomplished on the optimized condition of pulse duration, energy, repetition frequency, spot size, and moving speed of the spot. The evaporation temperature of the ablation is decided by the laser flux, which depends on pulse duration, energy, and spot size. That is, for a given pulse duration, the energy and spot size are determined.
- 1. Simulation results show that, in the microsecond regime, a shorter pulse duration can reduce the effective time allowed for rim formation and lead to a smaller raised rim, where the energy is adjusted to achieve a high peak temperature ∼4000-5000 K for evaporation with rapid material ejection. Typically, a smaller spot size results in a smaller HAZ, but more pulses are required to evaporate the same volume of material. On the basis of balancing efficiency and HAZ size, in the experiment, the spot diameter is selected to be ∼120 µm @ 1/e2 to meet the flux requirement of CO2-laser polishing for the CO2 laser used has an average power of 100 W. Practically, the energy is insufficient for such evaporation when the pulse duration is below ∼10 µs. Thus, the pulse duration used in the following experiment is selected to be ∼10 µs. Extendedly, a shorter pulse duration would perform better if the pulse energy meets the requirement.
- 2. Calculations indicate that a repetition frequency not more than ∼1 kHz is available to afford sufficient cooling and lead to no significant heat accumulation under the pulse duration of several tens microseconds. Obviously, a low repetition frequency can help to reduce heat accumulation and avoid a rise in the raised rim. Taking account of polishing efficiency, a repetition frequency of 1 kHz is an appropriate choice. Since the moving speed of the laser spot does not affect the variation of on-axis temperature, the speed is experimentally decided by forming a continuous line with an even bottom.
- 3. In addition, polishing with multiple layers of fixed radius and tapered radius are separately preferred for treating cones with steep and gentle walls, and a slightly larger layer radius than the original cone is also beneficial to eliminating the raised rim caused by fs-laser ablation.
- 4. The above optimized parameters for reducing the raised rim are consistent with that for polishing [30].
3. Laser-based mitigation of damage precursors
On the basis of the above simulation results, the experimental laser-based ablation is optimized in the process of sample preparation, damage mitigation, and performance evaluation. The advantages, optical performance, and operational capability of the hybrid laser ablation procedure are discussed.
3.1 Sample preparation
Employing raster-scan by a UV-laser, sub-surface defects, i.e. damage precursors, on a large-aperture fused silica optics can be screened out and exposed as damage initiations. Equivalently, initial damage sites on a small-aperture fused silica sample are prepared at a higher fluence (>20 J/cm2) by a damage test station (Fig. 10) since there’s little low-fluence damage precursor on a sample. Fused silica (Corning, 7980) samples used in the experiment have a diameter of 50.8 mm and a thickness of 12.7 mm. The damage test station is equipped with a homemade Nd:YAG laser system, which is frequency-tripled to 355 nm. The laser pulse is allowed to run at the repetition of 10 Hz, with a maximum energy of 16 mJ and a pulse duration of 1.6 ns. We focus the laser beam to a diameter of 185 µm @ 1/e2 to achieve high fluence. Wedges are used for sampling and parametrical measurement. The temporal profile of each pulse is analyzed by the photomultiplier tube (PMT), combined with an oscilloscope which is not drawn. The fluence on the sample is calculated by the pulsed energy measured via the energy meter and spatial profile recorded on the CCD. The optical microscope is used for watching damage initiations in situ. Damage sites of different sizes, typically ranging from ∼50 µm to ∼500 µm in lateral width, can be prepared by varying the fluence and number of shots. The damage growth thresholds of these damage sites are below 8 J/cm2.
Fig. 10. (a) Experimental configuration of the damage test station. (b) Microscope observation of a typical laser-induced damage site on fused silica @ 355 nm.
3.2 Mitigation of the Damaged site
A critical requirement for mitigating a damaged site is that the intensification of the laser passing through the processed site must remain below the damage threshold of its own and downstream optical surfaces [6,19,20,38]. Shaping a damaged site into a concave cone with an angle larger than 140° is optically benign to restrict the self and downstream intensification to lower than 1.6 [38], while a raised rim surrounding the cone would focus the light and raise the downstream intensification [19,20].
Figure 11 illustrates the schematic of laser-based damage mitigation via the combination of fs laser and CO2 laser. Damage material is removed, and the site is finally formed into an optically benign cone. The fs laser operating at a high-frequency repetition of 10 kHz is tightly focused on the damaged site for layered removal of the damaged material with a tiny depth of δh. The initial layer radius is determined by the cone radius, and it decreases gradually by an amount of δR (δR=δh·cotα, typically 4 µm) until a value of zero is obtained for deeper layers. After all the layers are completed, a precisely shaped cone with a somewhat rough wall is generated, and then the wall is flexibly polished by a CO2 laser. The CO2 laser is chopped into a pulse with a duration of 10µs and a repetition frequency of 1kHz by the acoustic-optic modulator (Isomet, AOM740) for enough cooling. During the CO2 laser polishing process, the initial layer radius is 10 µm larger than that of fs laser fabrication to fully cover the multi-staged cone. The number of layers for polishing is decided by the feature dimensions of the roughness, and the layers with tapered radius help to form the cone with a gentle wall.
Fig. 11. Schematic of laser-based damage mitigation. The damaged material is removed and the site is precisely shaped into a cone layer by layer with a tiny layer depth of δh and a radius reduction of δR (δR=δh·cotα) using the fs laser. Subsequently, the cone is polished using a CO2 laser and formed into an optically benign cone.
A damaged site with a lateral width of ∼430 µm is removed and replaced with a conical crater having a diameter of 800 µm, a slope angle of 12°, and a raised rim of low height, as described in Fig. 12. It can be seen that the cone is well shaped and there is almost no silica debris nearby. The typical depth of a layer ablated by the fs laser is ∼1 µm in the experiment. Generally, the layer depth would become shorter than 1 µm as the crater becomes deeper because of defocusing. Thus, we compensated to guarantee the dimensions of the cone and designed a 100-layer ablation, instead of synchronously moving the sample to maintain the ablated depth. As expected, the cone has a depth of 84.70 µm when the wall is jagged after processing by the fs laser, as illustrated by the red line in Fig. 12(c), measured via a mechanical stylus profiler (Bruker, DektaXT) with a lateral resolution of 0.167 µm and vertical resolution of 8 nm. Next, with additive CO2-laser polishing by using optimized parameters from the numerical model, the wall of the cone becomes smooth, and the angle remains the same despite the depth increase of 10 µm, as illustrated by the blue line in Fig. 12(c). As shown in Fig. 12(d), taking advantage of the limited heat-affected zone during the whole process, the width and height of the raised rim of the cone are both approximately an order of magnitude smaller than that processed by only CO2 laser (Fig. 4).
Fig. 12. Microscope observation of a typical cone applied to the damage site (a) manufactured by fs-laser and (b) after polishing by CO2 laser. (c) Measured profiles of the fabricated cone after processing by fs laser (red) and CO2 laser (blue) via a stylus profiler with a lateral and vertical resolution of 0.167 µm and 8 nm, respectively. (d) Profile of the raised rim measured by the optical profiler.
3.3 Downstream intensity profile beyond the cone
When the incident beam size is larger than the conical crater, the downstream field intensification should be as small as possible to protect the optics from laser-induced damage. The downstream intensity is featured by a Q-switched laser operating at 351 nm (Crystalaser, QL351-025). The emitted Gaussian beam is expanded so that the intensity is nearly uniform inside a serrated aperture with a diameter of 8 mm. The fused silica optics with a fabricated cone on the output surface is placed behind the serrated aperture and expander. Afterward, the downstream intensity profiles are captured by a CCD (Ophir, LT665) at a series of propagation distances ranging from 18 mm to 1000 mm. Figure 13 (a) and (b) show the profile at the propagation distance of 18 mm and 70 mm, respectively, where the maximum intensities constitute a ring around the cone, as indicated by red arrows in the pictures. As a result of interference, mainly between the input beam and its diffraction via the cone, the diameter of the maximum ring expands with the propagation distance. The maximum relative intensity is less than 1.6, of which the relative intensity is the local intensity divided by the average intensity of the region beyond the maximum ring, as shown in Fig. 13(c). Moreover, as a benefit of the reduction in the raised rim, the relative intensity of the central spot is not more than 1, as predicted [19,20]. Furthermore, it is notable that an obscure ring-shaped pattern farther away from the center is embedded in the profile, indicated by blue arrows in Fig. 13 (a). The diameter of the outer ring is nearly proportional to the distance, which is related to refraction through the wall of the cone [39].
Fig. 13. Downstream intensity profile at propagation distances of (a) 18 mm and (b) 70 mm beyond the cone. The inserted red curves are the 1D profile of the center lines. Red arrows indicate the rings that consist of the maximum intensities and blue arrows indicate an obscure ring related to the refraction. (c) Downstream relative intensity at propagation distances beyond the cone manufactured by the combination of fs and CO2 laser. Blue circles are the maximum intensities, and red squares are the peak intensities of the on-axis hotspots.
Additionally, a small-aperture beam traveling through the cone is transferred into a hollow beam. A Gaussian beam (632.8 nm, Thorlabs, HNLS008R) with a diameter of 288 µm on the cone is employed to achieve a beam with an aperture smaller than the conical crater. As shown in Fig. 14(a), the Gaussian beam is expanded by a pair of lenses (L1 & L2), then symmetrically travels through the cone and is finally transferred into a hollow beam by a lens (L3). The ring focused on the CCD has a diameter of 8.61 mm and a width of 160 µm (Fig. 14(b)). Namely, the fabricated cone can act as a concave axicon that can convert a Gaussian beam into an approximation of a Bessel beam [40], which is ideal for applications in the fields of microdrilling [41], high-speed 3D live fluorescence imaging [42], and enhanced nonlinear processes [43].
Fig. 14. (a)Setup for cone characterization using a Gaussian beam with a diameter smaller than the cone. (b) Ring-shaped beam captured on the CCD. The inserted red curve is the 1D profile of the centerline.
Fig. 15. (a)Laser-induced damage probabilities on the input and output surface of the cones manufactured by the combination of fs laser and CO2 laser @ 355 nm, 1.6 ns; (b) damage typically emerges at the edge of the cone.
3.4 Performance in the laser-induced damage resistance
Subsequently, we employ a raster-scan procedure [44,45] to evaluate the damage resistance of the mitigated cones on both the input and output surfaces, via the 3ω Nd:YAG laser system @ 355 nm and 1.6 ns mentioned before. During the damage test procedure, the actual incident of the test beam is 12° deviating from the normal to protect the optics of the damage test station. The scan area is 2 mm ×2 mm, including the cone and its surroundings, with an interval of 40 µm between successive shots. The average fluence of each shot is calculated based on the region of intensity higher than 90% maximum (fluence uncertainty ±5%). Due to the measurement uncertainty of the laser spot area (±2.5%), and laser energy fluctuation during the measurement (±2.5%), the relative error of the average fluence amounts to ±8%. Similar to the R-on-1 test method, several mitigated cones on each surface are executed with this test, which begins with a fluence of 6 J/cm2 and gradually increases the fluence until a damage event occurs. The fluence that damage occurs is referred to as the LIDT. Statistically, the damage probabilities, fractions of the mitigated cones that are damaged, are determined at a given fluence. As depicted in Fig. 15(a), the cones for damage mitigation improve the thresholds of the damage sites on both surfaces. Test results show that the LIDT of the mitigated sites on the output surface is around 10 J/cm2. As shown in Fig. 15(b), the damage typically initials at the edge of the cone, which may result from the laser electric field enhancement at the edge [38] and the ejected silica debris on the edge [20]. Additionally, the damage emerges asymmetrically at the cone’s edges. This is probably caused by the fact that the electric field enhancement is slightly asymmetrical at the left and right of the cone’s edge owing to the laser incident angle of 12°. The cone works better on the input surface than on the output surface in terms of the threshold. The difference between the input and output LIDT may lie in that the local electrical field intensity at the interface of the output surface is higher than the input surface due to the electric field enhancement [46] and the asymmetry in the plasma-ball growth that causes damage [5,47]. The sites on the input surface have a threshold around 14 J/cm2, and most sites are survived at this fluence. For comparison, cones processed using only a CO2 laser are also tested and results show the LIDTs are almost at the same level as the proposed method using a combination of fs laser and CO2 laser. Compared with damage sites before mitigation, the damage resistance is improved and capable of mitigating damage growth in the high-power or -energy laser systems.
4. Conclusion
In conclusion, combining the advantages of the fs laser and CO2 laser, we present the laser-based method to evaporate damaged material away and form the site into optically-benign cones for improving its damage resistance and lowering the downstream intensification. The parameters of laser-based ablation are optimized for restraining the HAZ, which prevents the formation of a significant rim and consequently leads to an intensity reduction of the on-axis hotspot. Herein, the downstream intensification is below a level that would damage downstream optics in high-power laser systems. LIDTs of the cone are tested to be ∼14 J/cm2 and ∼10 J/cm2 on the input and output surfaces. These results indicate that this method could effectively mitigate damage sites on fused silica surfaces, which could increase the operational capability to deliver high energy to targets of high-power laser systems. Also, this is a promising approach to fabricating micro-optical elements on transparent materials such as sapphire and glass for applications including imaging and high-power beam shaping.
Funding
The Strategic Priority Research Program of Chinese Academy of Sciences (XDA25020000); National Natural Science Foundation of China (11604350).
Acknowledgments
We would like to acknowledge Yibin Zhang and Yonglu Wang for their help in the profile measurement via stylus profiler, and Rong Wu for providing the Q-switched laser @ 351 nm.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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