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A grating-inspection system and a damage-analysis method have been developed to measure in situ laser-induced damage on a 1.5-m tiled-grating assembly of the OMEGA EP pulse compressor during a 15-ps, 2.2-kJ energy ramp. The beam fluence at which significant damage growth occurred was determined. This is the first report on beam fluence versus laser-induced-damage growth of meter-sized multilayer-dielectric-diffraction gratings. This result was correlated to the damage-probability measurement conducted on a small grating sample and is consistent with the fluence, corresponding to 100% damage probability.
©2010 Optical Society of America
1. Introduction
The damage morphology produced by high-power, femtosecond short-pulse laser systems on multilayer-dielectric-diffraction (MLD) gratings exhibits as a periodic microstructure due to the removal of material along the length of grating pillars. The periodicity becomes difficult to identify when the pulse length increases toward 10 ps [1,2]. The diffraction effect caused by the periodic structure or simply the localized holes caused by the removal of the coating material could be detrimental to the downstream optics. Understanding the in situ laser-induced–damage threshold of large-aperture MLD gratings is paramount for petawatt-class laser facilities to reach design energies [3–10]. Until now, short-pulse damage testing has been performed only on small-scale samples [2,11–16]. There are no vacuum-damage test data available on large-scale MLD gratings, and it has not been proven that one can simply transfer the results of the small samples to full-aperture MLD gratings. In this paper, we report the performance and findings of a vacuum-compatible grating-inspection system (GIS) that was deployed on OMEGA EP [3] to detect in situ damages of large-aperture gratings between high-energy shots. The deployment of GIS makes it possible to operate OMEGA EP safely and effectively because the morphological change of the compressor-grating-surface feature can be monitored in real-time without breaking the vacuum. Section 2 describes the mechanism and characterization of the inspection system. The methodology for detecting grating damage and the analysis method for determining the laser-beam fluence causing damage growth are introduced in Sec. 3. Section 4 discusses the accuracy of the determined laser-beam fluence. In Sec. 5, the damage-test result of a large-aperture MLD grating is compared to the damage-probability measurement conducted on a small-grating witness sample.
2. Grating-damage inspection system
The petawatt-class, short-pulse OMEGA EP laser operates on a 90-min shot cycle. The grating-based pulse compressor of the laser consists of four sets of tiled-grating assemblies, each measuring 141 cm × 43 cm [17]. The line density of the gratings is 1740 lines/mm. The incident and diffraction angles of the laser beam on grating 4 are 61.4° and 72.5°, respectively. During a recent 15-ps, 2.2-kJ energy ramp, it was imperative to monitor the damage growth of the final grating (grating 4) between shots to ensure that damage did not grow to a size that would damage downstream optics. A GIS, illustrated in Fig. 1(a) , was deployed in the compressor vacuum chamber to detect damage growth on grating 4 during the energy ramp.
Fig. 1 (a) The grating-inspection system scans through a 1.5 × 0.43-m large-aperture tiled-grating assembly. (b) Optical layout of the line shape illumination generator of the grating-damage inspection system.
The GIS consists of a line-shape illumination generator and an imaging system. The optical layout of the line generator is illustrated in Fig. 1(b). A point source from a fiber-based, 1053-nm continuous-wave laser is projected to form a line-shape illumination pattern on the surface of grating 4 using a multi-element lens assembly. An aspheric singlet collimates the light from the fiber and is followed by a half-wave plate used to adjust polarization of the illumination beam. Two spherical lenses focus the light onto a grating surface ~2 m away. The lens pair was designed to provide a 1/e 2 spot width of approximately 1 mm at the grating. A negative cylindrical lens located after the spherical lens pair spreads the light in a direction parallel to the grating grooves, perpendicular to the scanning direction. The line length, and associated illumination falloff along that direction, are determined by the focal length of the cylindrical lens. Successive iterations of the design resulted in a tradeoff between illumination uniformity and energy loss from light dispersed past the grating edges. The realized illumination pattern is shown in Fig. 2 . The line length at 50% and the 1/e 2 intensity are 243 mm and 450 mm, respectively. Compared to the nominal 370-mm laser’s main beam height, it is sufficient. The angle between the illumination beam and grating normal is 80°. This line-shape illumination is scanned across the grating, and light scattered from any features on the surface is imaged to a 10-bit charge-coupled device (CCD), having 8192 × 1 pixels with 7-μm × 7-μm pixel size. The CCD is scanned across the grating along with the line-shape generator. The scanned images are then combined to generate a composite two-dimensional (2-D) image of the grating surface. The spatial resolution of a GIS image is limited by the modulation transfer function (MTF) of the imaging system, which was measured to be 0.4 cycle/mm and 1 cycle/mm in horizontal and vertical directions, respectively, at a modulation level of 20%.
Fig. 2 Illumination on the surface of grating 4 exhibits a Gaussian pattern along the groove direction.
3. In situ grating-damage detection and damage analysis
Before the energy ramp, a reference scan of grating 4 was taken to record any existing features on the grating surface. An on-shot near-field fluence map was measured for each high-energy shot, and a GIS image was obtained after each shot. Six shots delivered the following energies to grating 4: 1.12 kJ, 1.33 kJ, 1.58 kJ, 1.82 kJ, 1.95 kJ, and 2.20 kJ. The corresponding laser-beam peak fluences were 2.5 J/cm2, 2.7 J/cm2, 3.3 J/cm2, 3.6 J/cm2, 4.0 J/cm2, and 4.6 J/cm2. The nominal pulse width was 15 ps. The near-field imaging system measuring laser-beam fluence map resides downstream of the grating compressor, as shown in Fig. 3(a) . During each high-energy shot, 0.7% of the main laser beam was delivered to the short-pulse-diagnostics path through a diagnostic mirror. This sample beam was then down-collimated, further attenuated, and sent to a near-field CCD that was imaged to grating 4 for energy and fluence measurement. Figure 3(b) shows the scanned image of grating 4 after the 2.2-kJ shot. A majority of the damage features reside in areas ROI 1 and ROI 2 (ROI: region of interest); these two ROI’s were chosen for damage analysis. Some of the features on this image were intrinsic to the holographic patterning of the gratings, and some existed prior to the energy ramp. Figure 3(c) shows the corresponding beam fluence measured on the 2.2-kJ shot. To prevent the diffraction effect on the edges of grating-tiles, the main beam was shadowed along the gaps of the three individual tiles of grating 4 by a beam apodizer installed in the main beamline. This attributed to the two vertical bands in the image. The two diffraction rings seen at the bottom of the fluence map were caused by the damages on an injection mirror of the main beamline (the rings disappeared after this optic was replaced following the energy ramp). The scaling factor in the x direction between Fig. 3(c) and Fig. 3(b) was 0.3 because the diagnostic beam was 72.5° relative to grating 4 normal. Comparative analysis of the scanned images before and after each shot was used to assess any damage initiation and growth. By spatially registering the scanned image of the grating surface to the fluence map and by correlating the identified feature size and peak beam fluence within a specified ROI for each subsequent shot, the upper fluence limit below which damage growth occurred can be determined.
Fig. 3 (a) The on-shot laser-fluence measurement path in relation to the grating compressor and the main beam path; (b) a GIS image after the 2.2-kJ laser shot; (c) a beam-fluence map of the 2.2-kJ laser shot.
A pattern recognition and processing software was developed using Matlab® [18]. A scanned image of grating 4 was taken after each high-energy shot. Features in the image were identified using various image-processing techniques: A raw image was filtered and properly down-sampled based on the MTF limitation of the imaging system of the GIS. For each specified ROI in the image [shown in Fig. 3(b)], the corresponding portion of the down-sampled image was binarized for subsequent feature identification. The portion of the image within an ROI having intensity higher than the binarization threshold was identified as a feature. The binarization threshold was determined using the statistical intensity information of the background and the scattering patterns within an ROI. The location and area size of each identified feature were calculated. Although the identified features exhibit various shapes, for ease of comparison between shots, the equivalent diameter of an identified feature was defined as the diameter of a circle that has the same area size as the identified feature. The histogram of the identified features was defined as the number of features at each individual equivalent diameter. This property was calculated for each ROI after each high-energy shot. The identified features in ROI 1 prior to the energy ramp are illustrated in Fig. 4(a) . (Note that there are already a significant number of features in this ROI; most of them were the damage patterns from previous shots.) Fig. 4(b) shows the corresponding binarized image of Fig. 4(a). Figures 4(c) and 4(d) illustrate the histograms of the detected features within ROI 1 prior to and after the energy ramp, respectively. Comparing histograms of damage sites of two consecutive shots, one can determine the shot energy that caused significant amounts of damage growth.
Fig. 4 (a) Detected features in ROI 1 prior to the energy ramp; (b) binarized image of the detected features in ROI 1 prior to the energy ramp; (c) histogram of the detected features after the 1120-J shot; (d) histogram of the detected features after the 2210-J shot.
Owing to the fact that it is very challenging to register the grating-damage map and the beam-fluence map within high precision, the peak fluence within a ROI was used to estimate the upper limit of the fluence causing damage growth within that area. ROI 2 [also shown in Fig. 3(b)], measuring 76 mm × 24 mm of grating 4 (corresponding to 23 mm × 24 mm of the laser main beam), had no GIS-detectable features before the energy ramp and exhibited significant change after the completion of the energy ramp. This region was chosen to determine the upper limit of the incident fluence causing damage initiation and growth. The binarized damage image, dimensions scaled to the laser-beam space, and the corresponding fluence map within ROI 2 for 2.2-kJ shot energy are shown in Figs. 5(a) and 5(b). The shot numbers for the series energy-ramping shots were 5864, 5869, 5874, 5881, 5895, and 5900. The corresponding laser-beam peak fluences within ROI 2 were 2.5 J/cm2, 2.7 J/cm2, 3.3 J/cm2, 3.7 J/cm2, 4.3 J/cm2, and 4.6 J/cm2. The growth in damage versus peak fluence in ROI 2 is shown in Fig. 5(c). The fluence at which significant growth in damage occurred was determined to be 3.3 J/cm2. This fluence was measured normal to the laser beam (72.5° relative to the grating normal). The corresponding shot energy was 1.58 kJ.
Fig. 5 (a) Detected features in ROI 2 after the energy ramp; (b) fluence map of ROI 2 of the 2.2-kJ shot; (c) damage size versus beam fluence; (d) Fractions of ROI 2 seeing fluence larger than 3.3 J/cm2 during a shot and exhibiting damage growth after the shot.
Figure 5(d) shows that the fraction of ROI 2 exhibiting newly induced damage growth after each shot proportionally increased with the fraction of ROI 2, seeing a fluence larger than 3.3 J/cm2 during the shot. This indicated that the fluence threshold for a damage growth to occur with certainty was around 3.3 J/cm2. A similar analysis was performed for ROI 1 and the result was consistent. The signal on the streak camera used for pulse measurement was saturated on this shot; therefore, no valid pulse measurement was obtained. Our on-shot pulse-prediction model, taking into account the measured on-shot spectrum, stretcher, compressor configurations, system input energy, and glass amplifier gain information, predicted a narrowed, 12.9-ps (FWHM intensity) pulse due to dispersion induced by nonlinear phase accumulations in the glass amplifiers [19].
4. Accuracy of peak-fluence measurement
The spatial resolution of the fluence measurement [Fig. 3(b)] was limited by the coherent transfer function (CTF) of the imaging system. The CTF cutoff frequency of the near-field imaging system was measured to be 0.25 cycles/mm. In addition, this imaging system is downstream of grating 4; therefore, the measurement of the beam fluence at a given location could have been affected by damage that existed prior to a particular shot. It is worth verifying that the peak-fluence measurement was not distorted by any damage on the optics of the diagnostics chain. To estimate the accuracy of peak fluence determined for a given ROI, the image from a different on-shot near-field imaging system is used. This system is located just upstream of the grating compressor [illustrated as IR near-field CCD in Fig. 3(a)], and its spatial resolution was measured to be 3× better than the one used to measure fluence. We then used a propagation model of the compressor set up in Miró [20], which includes the estimated phase of the beam at the input to the compressor and the phase and intensity effects due to the compressor, to propagate the fluence map measured at the input to the compressor to the grating-4 plane. Figure 6(a) shows the modeled fluence map for the 2.2-kJ shot. Comparing this fluence map to the measured beam fluence shown in Fig. 3(c) [for ease of comparison, Fig. 3(c) is repeated as Fig. 6(b)], the measured on-shot peak fluence agrees with the modeled result within 3% and represents the accurate on-shot fluence at grating 4.
Fig. 6 (a) The modeled fluence map using the measured fluence at the input of the compressor; (b) measured beam-fluence map of the 2.2-kJ laser shot.
5. Correlation to the damage-test results obtained on small grating samples
Damage probability versus beam fluence was measured in vacuum on a small-scale, 100-mm-diam MLD-grating sample. This piece was the fabrication witness sample of the large-scale gratings deployed on OMEGA EP. The compressed pulse used for damage excitation was generated by a multiterawatt laser based on an LBO-crystal optical parametric chirped-pulse amplifier (OPCPA) [21] and a gold grating-pulse-compression system housed in a large vacuum chamber. This 1053-nm OPCPA system was virtually identical to the OMEGA EP laser front end, guaranteeing damage-test excitation pulses similar to those experienced by OMEGA EP transport optics. Figure 7(a) shows the layout of the damage-test setup for sample gratings. The excitation beam with a pulse width of 11.5 ps (FWHM) for inducing damage and a 1053-nm continuous-wave illumination beam for detecting damage were co-aligned and co-focused to a 420-μm (width at FWHM) focal spot on the grating surface. The incidence angle of the two beams was 61.4° relative to grating normal. The OPCPA was operated at 5 Hz; however, sample irradiation occurred at only a rate of one shot every 10 s, enabled by a mechanical shutter. The illumination beam was shuttered during the damage excitation process and was turned on afterward for damage detection. The illumination light was scattered by the induced-damage sites and was imaged to a CCD. The resolution of the CCD was 10 μm/pixel. Change in the scattered image was used to determine the initiation of damage. The sample grating was translated along and across the grating-groove directions for damage tests at various locations. For each fluence, ten sites across the grating sample were chosen, and the percentage of sites damaged within 20 shots was used to determine damage probability. Figure 7(b) shows damage probability versus peak beam fluence. To compare with damage-test results of the large-aperture grating, the fluence was scaled to the beam space 72.5° relative to grating normal. As shown in Fig. 7(b), all ten sites under test were damaged at a fluence of 3.4 J/cm2.
Fig. 7 (a) Damage-test setup for damage probability at various beam fluences; (b) damage probability versus beam fluence measured on a 100-mm MLD-grating sample in vacuum.
For large-aperture grating-damage detection obtained during the energy ramp, the upper fluence limit causing damage growth within the two ROI’s under study was determined to be around 3.3 J/cm2 for a 12.9-ps pulse; i.e., damage growth was seen at a peak fluence not exceeding 3.3 J/cm2. Figure 5(d) further demonstrated that the fraction of an area showing new damage growth after a shot was proportional to the fraction of the same area experiencing a beam fluence higher than 3.3 J/cm2 during the shot. This indicates that there is a certainty for damage growth to occur when the fluence exceeds 3.3 J/cm2. This result is consistent with the determined fluence corresponding to 100% damage probability on small-scale grating-damage testing (3.4 J/cm2), taking into account that damage threshold scales approximately with the square root of the pulse length [22]. This is the first damage test comparison between a small-sized witness sample and a large-scale grating. More tests will be conducted and results will be reported in a subsequent publication.
6. Conclusions
A vacuum-compatible grating-inspection system has been developed to measure the in situ laser-induced damage of a 1.5-m tiled-grating assembly of the OMEGA EP pulse compressor during a 15-ps, 2.2-kJ energy ramp. The grating surface scanning image after each high-energy shot was correlated to the on-shot laser-beam fluence map to determine the relation between damage growth and beam fluence. The upper limit of the fluence at which significant grating-damage growth occurred was determined to be 3.3 J/cm2. This result is consistent with damage-probability measurement conducted on a 100-mm-diam witness-grating sample: the measured peak fluence at which 100% selected sites on the small sample were damaged was 3.4 J/cm2. The deployment of GIS makes it possible to operate OMEGA EP safely and effectively because the morphological change of the compressor-grating-surface feature can be monitored in real-time without breaking the vacuum. For future work, the damage-probability measurement on a large-scale grating can be conducted using the damage-detection analysis technique described here. The correlation between grating-damage growth, morphological change, and grating diffraction efficiency degradation should be investigated.
Acknowledgment
We thank Dr. J. H. Kelly for his support on characterizing the coherent transfer functions of the imaging systems for measuring on-shot laser-beam fluence. OPCPA laser support by Dr. J. D. Zuegel and his group at LLE is gratefully acknowledged. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC52-08NA28302, the University of Rochester, and the New York State Energy Research and Development Authority. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.
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22. LLE grating-sample test result provided by A. W. Schmid, University of Rochester.
