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Abstract
Structured conical blockers manufactured on optical input surfaces are being used to shadow, and thereby, arrest growth of exit surface damage on large optics for applications in high-energy laser systems to extend their lifetime usage. However, as the size of the Shadow Cone Blocker (SCB) is increased, the deflected light must be carefully managed to prevent consequential damage from downstream intensification. Here, we design and fabricate a distinct input SCB geometry to alleviate this problem, enabling larger damage site mitigation. The demonstrated ∼1 mm diameter SCB was manufactured with a convergent approach using a CO2 pulsed laser ablative scanning sequence on a fused silica window. The SCB design was refined to maximize resistance to both input and exit surface damage initiations on 1 cm thick fused silica windows when exposed to 351 nm irradiation and validated with laser damage testing. The design showed to prevent damage onset to the exit surface for incident fluences on the SCB of 10.7 ± 1.3 J·cm-2 and is resistant to damage on the input surface exceeding 30 J·cm-2 input fluence.
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1. Introduction
Rapid growth in the field of high-energy laser systems is a catalyst for disruptive technologies. Thermonuclear-ignition systems for energy sources, directed energy systems for distance probing and machining, compact particle sources for medical and civil engineering, and groundbreaking scientific exploration are only part of the potential unlocked in advancing the energetics of these systems [1]. Inertial Confinement Fusion (ICF) systems around the world, such as the National Ignition Facility (NIF), Laser Megajoule, and Shenguang-III, are multi-beam Mega-Joule class laser systems that are designed to deliver megajoules of energy in a few nano-seconds pulse on a target (e.g., see recently produced groundbreaking ignition at the NIF delivering 2.05 MJ of energy on target [2]). In order to deliver such high energies on a target, these systems intentionally operate at fluences that exceed the damage onset threshold for their fused silica components [3]. Consequently, a substantial effort has been conducted over the years to mitigate damage initiation and growth [4–6] and thus extend the lifetime of the optics.
Exit surface damage can be detrimental to the use of optics in high-energy and high-power facilities particularly when it grows from subsequent use. A common growth prevention method used in large-optic recycling is performed by ablation-based laser-machining the damaged site, removing the damaged material, and leaving a cone shape in its place [7,8]. These cones are structured so that the incident light is deflected in such a way that prevents downstream intensification from damaging other optics. However, in this technique, both the area and the depth of the repair cone must be larger than these of the damage site. Since damage extends deeply into the glass, limitations on the producible cone slope frequently require the repair cone area to be much larger than the damage site footprint. Consequently, the footprint of the repair cone area on the optics surface is larger than ideally necessary to mitigate the damage and, therefore, adds excessive light transmission loss; reducing the number of mitigation sites that can be put on an optic before the loss is no longer acceptable and as a result shortens the optic’s lifetime.
Recently, an alternative technique has been proposed and successfully demonstrated which involves laser-machining a cone on the input surface of the optic such that light is deflected away from the damage site located on the exit surface. In this technique, the SCB generates a shadow over the damage and prevents damage growth from subsequent laser shots [6]. Figure 1(a) shows an illustration of a Linear-walled SCB (L-SCB) with radius, Rcone, and maximum depth, h, on the input surface of an optic. The incident beam with intensity Iinc is refracted at the air-glass interface of the SCB structure, propagates as intensity Iexp, and the subsequent light void creates a shadowed area over the exit surface damage with radius equal to Rcone. Since the footprint of the SCB, and thus the shadowed region, could be made to fit tightly around the exit damage, the light transmission loss is minimized and the number of allowable mitigation sites are increased [7].
Fig. 1. Comparative illustration of the incident (solid blue arrows) beam, Iinc, deflection by a L-SCB (a) and conceptual R-SCB presented here (b) creating a shadow over exit surface damage on a fused silica window. Both cones have the same radius Rcone and maximum height, h, but the deflected beam expanding wave (red dashed arrows), Iexp, for the R-SCB differs based on cone geometry resulting in energy distribution over a larger area.
Although using L-SCBs have shown to significantly reduce damage growth, scaling up the SCB size has been proven difficult due to downstream intensification from coherent interference between Iinc and Iexp initiating consequential damage on the exit surface [4]. As depicted in Fig. 1(a), Iexp forms an expanding wave and coherently interferes with the uninterrupted Iinc surrounding the shadowed region. One way to reduce Iexp on the exit surface for a given Rcone is to increase h. However, practical fabrication considerations limit the maximal permissible slope and thereby restrict reduction of Iexp. While the peak intensity of the interference for relatively smaller sized L-SCBs (Rcone <300 µm) result in a manageable damage-related fluence on the exit surface, larger radius L-SCBs produce an intensity on the exit surface that exceed the onset damage threshold (nearly 20 J·cm-2 when subjected to multiple shots) [4]. Creation of damage on the optic surface that is susceptible to growth negates long term use of this method forcing new approaches to mitigate large damage sites.
The goal of this work is to develop a new geometric design for a SCB nearly 1 mm in diameter to mitigate damage on third harmonic fused silica optics in the final optics assembly of NIF (e.g., wedge focusing lens, WFL, and grating debris shield, GDS) [3]. These fused silica optics are placed in the final optics assembly after the beam's wavelength conversion to 351 nm and where focusing begins before entering the target chamber. Therefore, the developed SCB should withstand 10 J·cm-2 incident energy of nanosecond pulse lengths at 351 nm wavelength. To this end, an axisymmetric Round-walled SCB (R-SCB) was investigated and tested which stretches the expanding wave such that the exit surface Iexp is significantly lower compared to L-SCBs with the same Rcone and h. An illustration of the impact of this design is shown in Fig. 1(b) where the rounded-wall geometry of the R-SCB redistributes the deflected light over a larger area than that of the L-SCB in Fig. 1(a). The larger area of energy distribution is intended to decrease peak intensification on the exit surface. The presented design is fabricated using a µs pulsed 10.6 µm wavelength CO2 system for precise manufacturing on 1 cm thick fused silica windows similar to the thickness found on NIF final optics and is tested with small-scale, high-energy 351 nm wavelength laser systems as a qualification for use on larger high-energy, high-power systems.
This paper details the design and fabrication process for R-SCBs in Section 2. Section 3.1 demonstrates the characterization of these R-SCBs compared to L-SCBs by imaging the intensification distribution on the exit surface. These intensification measurements for both L- and R-SCBs are compared to the idealized designs and simulated predictions. Damage tests of both the input and exit surfaces results are shown in Section 3.2 to validate the design for damage resistance by the structure.
2. Method
2.1. Design
Several design constraints were considered to optimize both manufacturability and functionality of R-SCBs for use in NIF-like conditions. For the designed structure to be viable for optics recycling, it must both shadow the targeted damage as well as withstand operational fluences without causing damage initiation. Pretreated surfaces on new optics, like the ones used in this work, have been shown to withstand fluences approaching 30 J·cm-2 during small area tests after a single laser shot [9]. However, further studies have shown that after exposure to multiple ns pulses, the surfaces degrade and the damage threshold onset fluence lowers toward 20 J·cm-2 [5]. Therefore, a robust design objective for the R-SCB is that it should not produce an exit surface intensity exceeding 20 J·cm-2 when subjected to 10 J·cm-2 NIF-like incident fluences. Considering an estimated margin of 3 J·cm-2 input fluence due to beam contrast, the intensification, defined as the ratio of exit surface fluence to incident fluence, for the R-SCB should not exceed 1.5 on the fused silica exit surface to remain below the 20 J·cm-2 damage onset fluence.
The design presented here minimizes the peak intensity on the exit surface of the optic by manipulating the geometry of the air-glass interface to control the ray deflection and thus the redistribution of light on the exit surface. As a plane wave hits the incident surface and structure, the light is redirected according to Snell’s law. When the coherent laser light meets at the exit surface, as illustrated in Fig. 1, Iinc and Iexp interfere. The combined intensity, I, on the exit surface can be approximated, due to small angles of the deflected beam, as a coherent interference between the electrical fields following:
In this work, the R-SCB depth profile is derived from a combination of the two separately controlled components: a cone whose slope controls the “aim” direction for Iexp and parabolic rounding of its linear sidewall which both controls the amount of defocus of the expanding wave and planarizes the intensity distribution on the exit surface of the 10 mm thick optic. The equation for the axially symmetric R-SCB was designed so that the depth, d, varies along the radius, r, following:
Fig. 2. Geometric optics model design considerations for R-SCBs (a) Axisymmetric depth profiles for designed R-SCBs with varying eccentricity, η. (b) corresponding angle of incidence to R-SCB surfaces for light illuminated perpendicular to the unaltered optical surface (r > Rcone). (c) Calculated maximum intensification distribution result from R-SCBs on the exit surface of 1 cm thick fused silica windows using a geometrical optics model.
A constraint in creating an implementable design of a SCB is the limit on the maximum attainable wall slope. For both the 10.6 µm wavelength laser-machining and 351 nm wavelength end-use lasers, reflection intensity varies with sidewall slope due to Fresnel reflections at the interface. Figure 2(b) shows the calculated incident angle between the incident plane wave and the surface normal along the radius of SCB with varying η. It elucidates those designs with higher eccentricity result in a larger incident angle near the SCB rim. This forces a tradeoff between lowering Iexp by providing larger defocus and maximum fabricable wall-slope when determining the value of η.
The manufacturing process consists of removing multiple layers of material by scanning a laser over the optical surface. At these designed wall slopes, Fresnel reflections become an important manufacturing consideration and motivates the usage of circularly polarized laser light over linearly polarized light. Increased reflections of the laser light result in less energy absorption in the material, and thus lowers the removal capabilities. Additionally, stray light reflection at high enough intensity could cause errant material removal. At normal incidence, nearly 17% of the CO2 fabrication laser light is reflected, while at 60 degrees, circularly polarized light reflection increases to ∼21.5%, and with linear polarization the reflection around the annulus differs directionally by 39.4% (assuming fused silica glass index is 2.4 at the 10.6 µm fabrication laser wavelength). Therefore, an anisotropic deformation can occur when using linearly polarized light for fabrication, further motivating the use of circularly polarized light.
Similarly, reflections by the end-use laser can cause damage to the SCB structure itself, especially if the reflection focuses on the structure walls. At surface normal, about 3.7% of the 351 nm beam is reflected and at 60 degrees, up to 18% of light can be reflected. While polarization for the fabrication laser is adjustable, the NIF lasers use linearly polarized light, and therefore the fabricated shadower geometry must consider and minimize reflection focusing effects. As will be detailed in Section 3.2.1, input surface damage testing verifies that the demonstrated shadowers address such an effect.
Regarding design efficacy, a geometrical optics model was used to assess the SCB depth profile and calculate maximum exit surface intensification. To illustrate the design considerations, Fig. 2(c) shows the exit surface intensification predicted by the model for the depth profiles given in Fig. 2(a). The model is based on geometrical ray tracing since the light deflection is substantially off-axis and therefore beam propagation methods that assume paraxial beam propagation cannot be justified. The ray tracing method maps the deflected light from a solid angle of the conical structure onto a solid angle of the exit surface and is coherently added to the unperturbed light; evaluating Eq. (1) at Δϕ=0, to approximate maximum radially varying intensification depicted in Fig. 2(c).
As expected, by increasing η, the peak intensification is reduced to result in near-uniform distribution of the intensification. However, two factors limit the maximum value of η. As η increases, the extent of the expanding wave grows radially both inward and outward. The first limiting factor is preventing light leakage into the shadowed area due to the expanding wave extending toward the center. The second factor which further limits η is shadow-edge intensification ringing due to diffractive effects at the boundary between the SCB and surface which is not present in the geometrical optics model but is seen in beam propagation simulations and has been observed in literature [10]. The interference between the expanding wave and edge-ringing are avoided by reducing η so that the minimal extent of the expanding wave annulus exceeds the shadower area radius by the typical length of the edge-ringing.
After considering slope manufacturability due to incident angle limitations and limitations on η to avoid high interference intensification, the designed R-SCB with Rcone = 450 µm and h = 350 µm requires η=55 µm. This design gives a maximum wall-sloping angle 51.24° which is achievable using the 10.6 µm fabrication laser and results in a maximum exit surface intensification of 1.2848 which is lower than the 1.5 intensification goal.
2.2. Fabrication
The geometric model predictions for the idealized depth-profile serve as goal for the laser-ablation based micro-machining process. Figure 3 indicates our approach to manufacturing the design on fused silica (Corning 7980). As a baseline, we use the fabrication process for L-SCBs, which follow a series of evaporative pulses using a 130 µm e-2 diameter circularly polarized CO2 beam rastered in successive circular scan patterns as further detailed in [7] with each circular scan separated by 10 µm in the radial direction. For the ablation process, we used a 20 µm spacing between pulses which is measured azimuthally at each radius. Executing this process with a constant input power, 2 kHz rep rate, and 12.5 µs pulsewidth results in a L-SCB shape as shown in the measured depth profile in Fig. 3(a).
Fig. 3. Fabrication process to achieve R-SCB design. (a) Confocal depth measurement of a L-SCB site with Rcone = 450 µm and h = 350 µm manufactured with constant power. For the R-SCB: (b) Finalized radially varying fractional power function for fabrication (c) confocal depth measurement, and (d) comparison between average depth of the fabricated and designed radial profile.
To obtain the rounded sidewalls of the newly designed R-SCB, we vary the applied fractional power radially while keeping the number of pulses and pulse distribution the same (Fig. 3(b) shown normalized by maximum 33 W power). Although the depth of shallow layers using constant power can produce repeatable/linear material removal [11], there is additional complexity from the cone scan pattern and varied power resulting in a nonlinear effect on material removal. Nonlinear material removal complicates the manufacturing process and calls for a unique solution to the required R-SCB design. To overcome this nonlinearity, an iterative manufacturing process was used to find the optimal radially varying power function shown in Fig. 3(b) to achieve the designed R-SCB.
For the first iteration of the process, several L-SCBs were made using constant powers between 20 W to 33 W from a 10.6 µm commercially available CO2 laser (Synrad Firestar T100) running at 50% duty cycle to create L-SCBs with varying depths. The geometry of these cones was measured with a confocal microscope 3D surface profiler (Keyence VK-X3000) to find the depth and slope of the cone profile. The radially varying power function was calculated by interpolating the applied fractional power needed to provide the designed depth at each radial position used in manufacturing. The calculated radially dependent power function was then applied to the scan sequence (applied in previous step with constant power across all radii) along with constant perturbations above and below the calculated fractional power to create a variety of arched cones which served as a basis for the next iteration.
The resulting fabricated cones were measured and compared to the ideal design. The RMS error was calculated along the radius to quantify the deviation of the realized depth from the design depth in Eq. (2). The power function was modified using a linear interpolation method along the radius according to the depth measurement of the newly fabricated cones. The process of fabrication and measurement was repeated until the RMS error was minimized resulting in an optimized radially varying power function, shown in Fig. 3(b), for the prescribed design. Figure 3(c) shows a measurement of the final R-SCB manufactured using the power function in Fig. 3(b). Figure 3(d) shows the average radial profile of the fabricated cone compared to the designed depth. The final measured R-SCB geometry was within 5% of the designed cone profile at each radius below 418 µm where the fabricated cone approaches the surface.
3. Results and discussion
3.1. Characterization
While the intensification on the exit surface from the idealized cone could be calculated using an axisymmetric geometrical optics model, scatter and microlensing of light from residual artifacts in the fabricated structure needed to be analyzed experimentally to determine the full intensity distribution and whether finer adjustments to the fabrication process needed to be made.
The intensity distribution from a planar wave incident on the input surface was measured on the exit surface using an imaging system consisting of a ∼2 cm diameter 351 nm, collimated incident laser beam, microscope objective, and a Charge Coupled Device (CCD) camera as described in [4]. The light interfering on the exit surface was captured with a montage of various images of the exit surface. The sample was rastered to capture the entire area of the expanding wave with the camera and images were stitched together to form a single composition. Periodic stitching artifacts and high frequency noise were filtered using image processing by way of a Fourier transform.
The expanding wave on the exit surface contains high frequency fringes resulting from the interference between Iexp and Iinc propagating at high relative angles to each other. Since the period of these fringes is smaller than the imaging system’s resolvable pixels, the expected intensity in the captured image can be approximated as the average between constructive and destructive interference. An approximation of the measured intensity can be calculated using Eq. (1) at a phase difference of Δϕ=0 and Δϕ=π and averaging the spatially varying intensity, or more readily, the expression cos(Δϕ) = 0.
For fair comparison, a L-SCB with the same nominal Rcone and h was made on the same input surface to demonstrate improvement of the exit surface intensification from the newly designed R-SCB. Figure 4(a) shows the measured intensification on the exit surface of a 10 mm thick window from a Rcone = 450 µm L-SCB with h = 362.0 ± 0.2 µm. Figure 4(b) shows the expanding wave intensification of the fabricated R-SCB with Rcone = 450 µm and h = 358.7 ± 0.2 µm, at the exit of the same 10 mm thick window. Figure 4(c) shows a line profile taken along the dashed lines in Fig. 4 a and b of the L-SCB and R-SCB, respectively. Additionally, the averaged fluence in the ideal designed R-SCB as determined by ray tracing software and calculated using Eq. (1) with cos(Δϕ) = 0 is plotted in Fig. 4(c) for comparison.
Fig. 4. Images of the measured exit surface intensification from a L-SCB (a), R-SCB (b), and horizontal line through the center of the experimental L-SCB, R-SCB corresponding to the blue and orange dashed lines in a and b, respectively, and simulated exit surface intensification of the idealized R-SCB (c).
Periodic-like features can be seen on the exit surface from the R-SCB within the stretched expanding wave in Fig. 4(b). The radial scan of the fabrication sequence has finite resolution resulting in radial-rings from imperfections on the cone walls, which can be observed on both the L-SCB in Fig. 4(a) and (R)-SCB in 4b. These structural imperfections result in very subtle intensity ripples from the R-SCB that are nonconsequential to this work due to low intensification, and thus, further minimizing them is outside the scope of this study.
The exit surface intensification measurement validates that the R-SCB design substantially reduces the peak intensification with respect to the L-SCB of the same diameter and depth. The shadow radius for the L- and R-SCB are 0.50 ± 0.01 mm and 0.47 ± 0.01 mm, respectively as expected, corresponding to the cone radius. The intensification from the L-SCB is localized to a 2.57 ± 0.06 mm annulus averaging 1.55 ± 0.2 intensification. The rounded cone presents a decreased intensification spanning between 1.6 mm to 3.6 mm, spreading out the expanding wave to an overall average intensification of 1.05 ± 0.05. For reference, a unity intensification on the exit surface represents the incident intensity, (I = 1), therefore the R-SCB reduces the added peak intensification by 91% compared to the L-SCB peak intensification. The exit surface intensification measurement of the L- and R-SCB with the same diameter and depth validate the design, providing the anticipated shadow and reduced intensification.
3.2. Laser-induced damage testing
After validating that the fabricated R-SCB fits the design requirements for exit light intensification, the structure was laser-damage tested. These tests include small-beam input damage testing and overall damage testing with a ∼3 cm diameter beam at fluences slightly above the intended operational conditions. The input surface test of the R-SCB is meant to identify and resolve potential residual structural imperfections that may cause sub-surface micro-lensing resulting in bulk damage or surface damage by unintended focusing of Fresnel reflections. The small size of the laser allows for higher input fluences than allowable in the larger beam test. The larger ∼3 cm beam testing is a more comprehensive test emulating real use conditions and allows us to test exit surface damage initiation caused by the expanding wave intensification, which requires exposure to a beam larger than the expanding wave footprint on the exit surface. Both tests use setups with nano-second pulsed, 351 nm wavelength laser irradiation. The designed cones were made on 50.8 mm diameter windows that were 10 mm thick and were prepared using the acid etch process described in [12] to remove any impurities from the polishing process which serve as damage precursors, and is standard procedure for NIF optics.
3.2.1. Input damage test
The laser damage onset fluence for the shadower’s input surface was tested using a 650 µm e-2 diameter laser spot size described in [13]. The sample consisted of 11 R-SCBs spaced 2.5 mm apart to prevent transverse effects. The laser beam was rastered over the cone aperture, delivering multiple 7.5 ns pulses in 54 µm steps distributed over a 2 mm square grid. The initial peak laser fluence used was 15 J·cm-2 over the entire area. After all 11 of the tested cones were exposed to the beam scan, images of the cones were taken to examine if any damage occurred. For all cones without damage, the process was repeated at higher fluence with 5 J·cm-2 increments up to and including 45 J·cm-2.
Figure 5(a) shows a graph of the cumulative percentage of damaged cones with applied fluences between 15 J·cm-2 to 45 J·cm-2 for the final revision of the cone fabrication process. The input surface for all cones withstood all successive fluences up to and including 30 J·cm-2, on par with unmodified surfaces for small area testing. Figure 5(b) shows the damage observed on a cone which was subjected to 35 J·cm-2. The same damage mechanism occurred in various locations along the rim of all 11 cones indicating the damage was not caused by systematic form-figure errors. Previous versions of the R-SCB presented sub-surface damage in specific locations on the cones due to micro-lensing and form errors which have been removed through the development process [13]. While the cause for the damage presented in Fig. 5(a) is not yet determined, the experiment shows that the rounded cone input surface can withstand over 30 J·cm-2. Since the degradation effect alongside the exit surface intensification are likely to be limiting the maximal permissible input fluences to well below 20 J·cm-2, the demonstrated input damage threshold fluence of 30 J·cm-2 in this work suffices.
Fig. 5. Cumulative percentage of input-damaged R-SCBs at corresponding input fluences (a). Image of R-SCB exhibiting a typical input damage from sub-aperture laser testing at 35 J·cm-2 input fluence (b).
3.2.2. Exit damage test
The damage onset from the intensification on the exit surface was tested using the Optical Sciences Laser (OSL) facility at LLNL [14]. The system was arranged to provide a 30 mm diameter, nominally uniform, 10.9 ± 2.7 J·cm-2 fluence on the input surface of the sample using a 5 ns flat in time pulse width. The test sample was made with 6 R-SCBs placed so that the anticipated expanding waves on the exit surface for the different shadowers did not overlap. A reference cone with a Rcone = 300 µm and h = 220 µm depth L-SCB, which has been tested previously in literature [4,6], was used as a control site. The sample was exposed to 3 shots averaging 10.6 ± 1.3 J·cm-2 within the area of the cones and microscopy was conducted after each shot to examine the surfaces and bulk material for damage.
No damage was observed on the R-SCBs from any of the 3 laser shots, while there was damage on the exit surface from the smaller L-SCB reference cone. Since there was no damage observed on the R-SCBs, the damage metric ρ(ϕ) could not be used as common practice for large beam testing in literature [9]. To better estimate the exit intensification that shadowers survived without damaging, the exit fluence was evaluated from the measured input fluence distribution and using a 3D ray tracing model derived from the geometrical measurements of the rounded cones. The maximum interference was calculated using Eq. (1) with the phase difference of Δϕ=0 and averaged over the area of the expanding wave on the exit surface. Further calculations of exit fluence are demonstrated in the Supplement 1.
Figure 6(a) shows both the measured average incident fluence within the SCB area and the calculated exit surface intensity based on the input fluence map (shown in Fig. 6(b)) over all 3 laser shots. The R-SCBs numbered 1-6 and the L-SCB reference cone, R, corresponding to the solid blue circle locations noted in Fig. 6(b), are presented on the x-axis in Fig. 6(a). The expected locations of the expanding wave intensification on the exit surface are overlayed as red dashed lines on the measured fluence map image in Fig. 6(b) (we note that there is a shift in their location from center due to a 7.5-degree OSL vertical rotation described in [4]).
Fig. 6. Average measured incident and calculated exit surface fluence for R-SCB (Rcone = 450 µm, h = 350 µm) 1-6, and L-SCB (Rcone = 300 µm, h = 220 µm) reference cone, R, for all 3 pulse exposures (a). Fluence distribution map for shot 1 used for exit damage testing (b). Site location and size are indicated by solid blue circles and labels corresponding to data in a. Shifted expanding wave locations are shown as red dashed lines indicating the maximum extent of the expanding wave around each SCB on the exit surface based on 7.5-degree incident angle rotation. Damage locations are indicated with yellow asterisks and a 20X image of each damage location (c-e) is shown. No damage was observed on the R-SCBs.
The image in Fig. 6(b) shows relative location between the SCBs, their expected exit surface intensification, and observed damage c, d, and e locations noted in yellow asterisks. Images of the observed damage on the reference cone are shown in Fig. 6- c, d, and e. The damage on the exit surface from the reference cone occurred on the third shot, consistent with what was observed using the same shadow cone geometry in the surface degradation experiments conducted by I. Bass, et.al. [4]. This indicates that the expanding wave exit fluence from the R-SCB did not exceed the degradation precursors’ damage onset threshold, while the L-SCB reference cone exit intensification did, consistent with the postulated mode of damage. The smaller L-SCB reference cone geometry was used here as a control, as it was used in past experiments (see [4]) and thus serves to validate the experimental conditions. The damage mechanism validated here predicts that if we were to use a L-SCB with the same radius and depth as the R-SCBs, the damage initiation rate would be much higher than observed with the smaller reference L-SCB (R) used in this study.
The R-SCBs in sites 1-5 show a similar calculated intensification on the exit surface based on the input surface fluence in Fig. 6(a). However, it should be noted that site 6 deviates from this pattern due to the fact that the average input fluence for a site, Ī0, is calculated from the measured fluence within the cone area (solid blue circle) and the exit fluence for that site is calculated using Eq. (1) (with Δϕ=0) where Iint is the input fluence at each point within the footprint of the expanding wave (extending to the red dashed circles) and Iexp is the projected exit surface intensification as calculated by a ray tracing model and multiplied by Ī0. It is clear in the fluence map in Fig. 6(b) that the fluence within the site 6 expanding wave area is not uniform, with high fluences observed near the beam edge. There is an increase in the measured Iint in this location which affects the calculation of expected exit surface fluence. Nevertheless, even that predicted increase in exit fluence did not cause damage to the exit surface which supports the robustness of the R-SCB design.
4. Conclusion
A newly designed R-SCB was created and manufactured using a pulsed CO2 laser. The improved design was tested using 351 nm lasers to evaluate the exit surface intensification and input and exit damage performance of the design compared to L-SCBs with same shadowing aperture. The carefully crafted shape of the Rcone = 450 µm, h = 350 µm, η=55 µm R-SCB reduces average added intensification by 91% compared to a L-SCB of the same nominal radius and depth. Small beam damage testing shows that the onset fluence of input surface damage of the manufactured R-SCB exceeds 30 J·cm-2 of repeated sub aperture shots. For a large 30 mm diameter input beam, greater than both the SCB aperture and the exit surface expanding wave, no observable damage was detected on the exit surface showing that these designs can be implemented in high-energy laser systems when subjected to input fluences averaging 10.6 J·cm-2 without causing further damage to optical surfaces. We expect that the process detailed here should be extendable to other diameters enabling a variety of damage site dimensions to be shadowed with minimal transmission loss. The optical and laser-induced damage performance of these R-SCBs present a path for extending the optical lifetime on high-energy laser systems. The approach for fabrication optimization presented here for the R-SCBs could be utilized for manufacturing further abstract geometries on surfaces, beyond the scope of shadowing elements. The considerations and the approach to achieve a design that meets constraints on the input and downstream intensification could be employed to other SCB geometries as well. Furthermore, the approach illustrated here for development of laser-induced damage resistance of such sites could be utilized for other micro-machined sites on high-energy and high-power laser systems with other functions. Further testing of the R-SCBs presented here is being conducted to evaluate the function of damage growth suppression with regards to size, input fluence, and fatigue implemented in high energy laser systems at NIF.
Acknowledgements
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL-JRNL-845806.
Disclosures
The author declares 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.
Supplemental document
See Supplement 1 for supporting content.
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