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Microscopic characterization of bulk damage resistance of DKDP nonlinear crystals
Optics and Laser Technology 121 (2020) 105672
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microscopic characterization of bulk damage resistance of DKDP nonlinear crystals
T
⁎
Yin-Bo Zheng, Xin-Da Zhou, Rong-Sheng Ba, Jie Li, Hong-Lei Xu, Lei Ding , Jin Na, Ya-Jun Li, Jing Yuan, Huan Ren, Xiao-Dong Tang, Li-Qun Chai Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
H I GH L IG H T S
method evaluating the bulk damage resistance of DKDP crystals is presented. • ACrystal has distinct impacts on density and size of pinpoints. • Distincthomogeneity energy deposition mechanisms govern the process of bulk damage. •
A R T I C LE I N FO
A B S T R A C T
Keywords: DKDP crystals Pinpoints densities Size distribution of pinpoints Three-dimensional (3D) distribution of pinpoints Crystal inhomogeneity
DKDP nonlinear crystals in ICF drivers are susceptible to laser-induced bulk damage, therefore it is significant important to characterize bulk damage in order to prevent, reduce, and/or manage bulk damage. This work presents a method evaluating the bulk damage resistance of DKDP crystals, directly providing pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints. This technique can be used for characterizing the crystal inhomogeneity, and the result demonstrates that crystal inhomogeneity have a strong and negligible impact on pinpoints density and the size distribution of pinpoints respectively, suggesting two distinct energy deposition mechanisms governing the damage initiation and the growth of the pinpoints.
1. Introduction Due to the electro-optical properties (birefringence) and the growth rate (in excess of 10 mm/day) at which large (700 kg) crystals can be grown, (D)KDP crystals are currently the only available nonlinear crystals for ICF (Inertial Confinement Fusion) lasers, converting the laser wavelength from 1053 nm to 527 nm and then mixing the generated 527 nm laser pulse with the residual 1053 nm pulse to produce 351 nm pulse [1]. Unfortunately, laser-induced bulk damage of (D)KDP nonlinear crystals becomes a crucial problem and is regarded as a key factor constraining system design and limiting system performance of high-power laser facilities. The understanding and controlling of damage initiation with ultrashort pulses has been achieved to a large extent [2,3], since that it is governed by intrinsic material properties. However, the nature of localized bulk damage initiation under nanosecond laser pulses irradiation still largely defies fundamental understanding despite more than 40 years of research [4].The detriments of bulk damage consisted of pinpoints, whose density is as high as thousand per cubic millimeter, can be described in terms of enhanced scatter
⁎
loss [5,6] as well as beam contrast ratio [7], which in turn can increase the likelihood of damage to downstream optics. Although some advances in the crystal growth [8], laser conditioning [9], and thermal annealing [10] have been achieved, (D)KDP nonlinear crystals are still vulnerable to laser-induced bulk damage. It is therefore of significant important to characterize this phenomenon in order to prevent, reduce, and/or manage bulk damage in these crystals. Previous works to appraise the damage resistance of (D)KDP nonlinear crystals have exclusively focused on measuring their laser-induced-damage-threshold (LIDTs) [11–13], generating curves depicting the probability of initiating at least a single pinpoint as a function of fluence. While such measurements are capable of producing excellent qualitative results, the absolute LIDTs tend to be significantly higher than those found on large-aperture laser systems [14], so damage probability measurements results obtained by testing small optics could not be scalable to larger optics. More recent works have developed an optional damage testing approaches where the bulk damage resistance is quantified with pinpoints density as a function of laser fluence (i.e., ρ(Φ) curve) [6,7,15–17], providing a quantitative assessment of the
Corresponding author. E-mail address: [email protected] (L. Ding).
https://doi.org/10.1016/j.optlastec.2019.105672 Received 31 January 2019; Received in revised form 1 June 2019; Accepted 30 June 2019 Available online 03 September 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
Optics and Laser Technology 121 (2020) 105672
Y.-B. Zheng, et al.
with a pulse width (FWHM) of ~5 ns. As depicted in Fig. 1, the laser pulses were polarized by a polarizer and then the linear polarization orientation was adjusted by a half-wave plate. A sample was exposed to a laser pulse from a slowly focusing beam (~f/30), and a wedge in the beam line was used to produce two ~ 4% reflections. The reflections off the front and rear surface of the wedge were incident on a scientificgrade CCD camera located in a plane equivalent to the rear surface of the sample and a Joule-meter, respectively. The samples used for the experiments were conventional growth DKDP oriented for type II tripling at 1053 nm, and these samples were sliced to100 × 100 × 10 mm3 in size plates polished and uncoated on all sides. polarizing linearly along the ordinary axis Images of the bulk damaged volume were longitudinally scanned along the counter-direction of propagation of the laser pulses, through the rear-surface of the sample using a microscope whose resolution was 1 μm. And a polarizer was placed in front of the objective of microscope to eliminate the effect induced by the birefringence. The sample was first illuminated with an annular illumination composed of a green light (coherent illumination), aiming to debug microscope easily (especially for the case that pinpoints were sparsely distributed in the damaged region) based on interference effects associated with monochromatic scattering to help detecting pinpoints that were too small to observe with white light illumination, and then a coaxial white light was used for scanning (bright filed illumination) to accurately measure the size of pinpoints [6,19]. The above mentioned optical system resulted in transversal field of view of ~1.67 mm × ~1.67 mm with a depth of focus of 100 μm. Therefore, in order to longitudinally scan the whole ~10 mm × 10 mm × 10 mm region with z-steps of 100 μm, a total of approximately ~104 frames where each pinpoint required identifying and measuring were generated. Each frame was analyzed with a method discussed below. Firstly, a mean distance between neighboring pinpoints is determined, and then the original frame is thresholding through OTSU algorithm [20], obtaining the contour and the size of the damage site. Next the position of damage site is calculated with an algorithm called image moments [21], outputting the position of each site. Lastly, the multiple counts of the same site due to being detected by repetitious z-step scanning must be removed by considering the above mentioned mean distance, the position of the site, and the incidence angle of laser pulse. The accuracy of this algorithm is improved and evaluated with manually measuring results, reaching to an
damage performance of a bulk material comparing to statistical measurements of the damage threshold. As a result, the damage threshold is only one point in this curve depicting the observed onset of damage resulting from the precursors initiating damage at the lowest fluence, and what’s more, bulk density is also a preferable measurement of damage because it can be scaled to larger areas easily [7]. The ρ(Φ) curve were firstly obtained through correlating the local damage pinpoints density and corresponding local beam fluence [15], the characteristic of which was establishing direct linkages between bulk damage density and scatter loss. And then, DeMange et al., proposed a skillful method to measure bulk damage performance with single pulses [6] and was capable of providing pinpoints density, size distribution and beam obscuration directly, while it was only applicable to damage densities exceeding about 160 pp/mm3 [7]. In 2006, a new approach suitable for the case with pinpoint densities as low as 0.8 pp/ mm3 was revealed [7], however, it must be consider under what circumstances light scatter was be proportional to damage density. France team presented a method to characterize bulk damage resistance [16], yet it failed to preserve the information of size distribution or threedimensional (3D) distribution of pinpoints. After that, a measurement scanning the entire laser-interaction volume using polarized bright field illumination was demonstrated in 2011, overcoming above-mentioned drawbacks and providing pinpoints density and size distribution accurately [17]. Generally, the number of pinpoints that can be tolerated in high-power laser facilities relays not only on the size distribution and bulk density, but also on the three-dimensional (3D) distribution of pinpoints. However, to the best of our knowledge, the method of obtaining three-dimensional (3D) distribution of pinpoints is almost absent. In this work a method evaluating the bulk damage resistance of (D)KDP crystals is demonstrated, offering pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints together.
2. Experiments The laser pulses used for damage tests are delivered from MODSS (Multipurpose Optical Damage Science System) facility constructed in 2011[18], outputting approximately 100 J and 40 J at fundamental (1053 nm, 1ω) and third harmonic (351 nm, 3ω), respectively. The spatial profiles of pulses were nearly flat top at two wavelengths. The laser pulses used in this experiment had a Gaussian temporal profile
Fig. 1. Schematic diagram of experimental setup used for characterization of bulk damage of DKDP crystals illuminated firstly with an annular illumination composed of a green light (coherent illumination) for debugging microscope easily, especially for the case that pinpoints were sparsely distributed in the whole damaged region, and then a coaxial white light (bright filed illumination) was used for scanning the damage regions. More details are presented in text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2
Optics and Laser Technology 121 (2020) 105672
Y.-B. Zheng, et al.
Fig. 3. The size distribution of pinpoints resulting from 3 neighboring regions of a conventional growth 70% DKDP crystal exposure to a single 351 nm, 5 ns pulse with the different fluencies, and the values in parentheses are the coordinates of peak values.
Fig. 2. Three-dimensional distribution of bulk pinpoints of the damaged region of a conventional growth 70% DKDP crystal exposure to a single 351 nm pulse with the fluence of 9.24 J/cm2, resulting in damage pinpoints density of 27.37/ mm3. The size and the color of representative balls in this 3D distribution picture are proportional to and used for guiding to eye to show the size of corresponding pinpoints, respectively. The space distribution variability is obvious, and the size ranges from ~1 μm to ~40 μm. And more details are presented in text.
pinpoints is ~100 μm and ~40 μm in Ref. [22] and this work, respectively, presuming that the this difference is resulted from microscopy failing to distinguish the overlapping of pinpoints located in different positions in depth-of-focus. Moreover, Fig. 3 reveals that the number of pinpoints exhibiting a stronger dependence on the size than on excitation conditions. In particular, the size distribution profiles show a similar trend. Specifically, the number of pinpoints firstly increases with increasing size until the size reaches a value of ~3.5 μm and then decreases with increasing size, which is similar to the result in Ref. [7]. In addition, it’s noticeable that the change of the sizes corresponding to the peak number of pinpoints can be negligible, consistent with previously reported results for 1ω pulses with fluence ranging from 5 J/ cm2 to 50 J/cm2 [17]. The fact that the size distribution of pinpoints as a function of fluence is nearly unchanged demonstrates the size distribution of pinpoints shows a stronger dependence on the pulse width [17,23,24] than on the fluence.
accuracy exceeding 97%. 3. Results As long as the image processing of original microscopy frames is accomplished, the damage performance under individual wavelength pulse irradiation could be quantified as three-dimensional (3D) distribution of pinpoints, the bulk pinpoints density, and the size distribution of pinpoints. The bulk pinpoints density of damage region is calculated by the ratio of the number of pinpoints with respect to the volume of damage region (the damage region’s cross section containing the pinpoints multiplied the depth through which the damage pinpoints extend). In this experiment, a conventional growth crystal (70% pristine DKDP, type II tripler) irradiated by 351 nm, 5 ns pulse propagating along the z-axis of Fig. 2 and polarizing linearly along the ordinary axis with fluence 9.24 J/cm2, resulting in bulk pinpoints density of 27.37/ mm3. Fig. 2 presents the three-dimensional (3D) distribution of pinpoints through plotting a ball representing each pinpoint according to its position and the size obtained through above mentioned algorithm, where the size and the color of repretative balls are proportional to and used for guiding to eye to show the size of corresponding pinpoints, respectively. It’s obvious that pinpoints are not evenly distributed throughout the damage region and the size of pinpoints ranges from ~1 μm to ~40 μm. One of the adverse effects of bulk damage pinpoints is enhanced beam contrast, resulting from the three-dimensional (3D) distribution of pinpoints, the bulk pinpoints density, and the size distribution of pinpoints together. So the size distribution of pinpoints is another concern of bulk damage of crystal. In this work, the size data is binned into 1 µm bins with the first bin containing 1–2 µm sizes, and the values on the y-axis of Fig. 3 are the numbers of pinpoints counted by the microscope. The size below 1 μm is absent due to the resolution limit of optical system, and the results of size distribution of pinpoints resulting from 3 adjacent regions irradiated with different fluences are shown in Fig. 3, where the values in parentheses are the coordinates of peak values. Results in Fig. 3 are consistent with those in Ref. [22], excepting the resolution limit and the largest size of pinpoints. The largest size of
4. Discussions Firstly, we study the non-uniform distribution of bulk damage pinpoints by plotting the number of pinpoints versus the depth of the crystal. Specifically, the depth of crystal was binned into 1 mm bins with the first bin containing 0–1 mm depth, and the values on the y-axis are the numbers of pinpoints locating within the particular depth-size. Fig. 4 reveals the number variability versus the depth of the damaged region, demonstrating the number of pinpoints is neither monotonically increase nor monotonically decrease versus the depth. Damage behavior in in Fig. 4 can be influence d by spot area, spot intensity profile, and crystal inhomogeneity. In the experiments, the laser pulses were focused into the bulk of DKDP crystal with a focal lens whose focal length was much larger than the thickness of the sample (160:1) to ensure the spot area to be constant along the sample, therefore, the influence of spot area on the measurement can be negligible. For the case of laser pulses polarizing linearly along the extraordinary axis (// e), the spot intensity distribution of a focused laser inside DKDP crystal is complex due to the birefringent effect [25]. While in this experiments, the polarization orientation of laser pulse was collinear to the ordinary axis of crystal (// o), and the change of spot intensity profile can be negligible [25]. In a word, the spot area and spot intensity profile fail to explain the observed changes in damage behavior along the longitudinal direction of DKDP crystal. As a result, the changes in damage behavior may be attributed to the crystal inhomogeneity [26]. Meanwhile, we can obtain the size distribution of pinpoints within each millimeter thickness along the propagation direction of laser pulses by combining the method obtaining the size distribution of 3
Optics and Laser Technology 121 (2020) 105672
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intolerable, while the acceptable upper limit for beam modulation is about ~108[7], therefore it is urgent to give the priority to study the relationship between above mentioned obtained 3 parameters and scatter loss. For sites with size of ~3.5 μm, multiple scattering can be negligible pinpoints below the order of ~103/mm3 [7], so it may be possible to study the relationship, and an investigation is underway. In addition, a laser based system [31] is scheduled to measure defect induced beam modulation of large optics. 5. Conclusions This work proposes a method to scan the damaged region using bright field illumination, directly obtaining the pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints. And this technique is initially used for characterizing the crystal inhomogeneity, demonstrating crystal inhomogeneity having a much stronger impact on pinpoints density than on the size distribution of pinpoints, suggesting two different energy deposition mechanisms governing the damage initiation and the growth of the pinpoints.
Fig. 4. The number of the bulk damage pinpoints resulting from 3 neighboring regions varies with the depth of a conventional growth 70% DKDP crystal exposure to a single 351 nm, 5 ns pulse with the different fluencies, reflecting the crystal inhomogeneity.
Acknowledgement The authors acknowledge all colleagues devoutly for their substantial assistance in operation and maintenance for MODSS highpower laser system. This work is supported by Laser Fusion Research Center Funds for Young Talents (No. RCFCZ1-2017-10). References [1] P.J. Wegner, et al., Harmonic conversion of large-aperture 1053nm laser beams for inertial-confinement fusion research, Appl. Opt. 31 (1992) 6414. [2] B.C. Stuart, et al., Nanosecond-to-femtosecond laser-induced breakdown in dielectrics, Phys. Rev. B 53 (1996) 1749. [3] C.B. Schaffer, et al., Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses, Meas. Sci. Technol. 12 (2001) 1784. [4] R.M. Wood, Laser-induced Damage of Optical Materials, IOP, Philadelphia, 2003. [5] M.L. Spaeth, et al., Description of the NIF Laser, Fusion Sci. Technol. 69 (2016) 25–145. [6] P. DeMange, et al., System for evaluation of laser-induced damage performance of optical materials for large aperture lasers, Rev. Sci. Instrum. 75 (10) (2004). [7] C.W. Carr, et al., Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation, Meas. Sci. Technol. 17 (2006) 1958–1962. [8] Yueliang Wang, et al., Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores, Opt. Lett. 41 (2016) 1534–1537. [9] J. Swain, et al., The effect of baking and pulsed laser irradiation on the bulk laser damage threshold of potassium dihydrogen phosphate crystals, Appl. Phys. Lett. 40 (1982) 350. [10] J. Swain, et al., Improving the bulk laser damage resistance of potassium dihydrogen phosphate crystals by pulsed laser irradiation, Appl. Phys. Lett. 40 (1982) 4. [11] J.Y. Natoli, et al., Toward an absolute measurement of LIDT, SPIE 4932 (2003) 224. [12] Hu. Guo-Hang, et al., One-on-one and R-on-one tests on KDP and DKDP crystals with different orientations, Chin. Phys. Lett. 26 (8) (2009) 087801. [13] ISO Standard No.21254, 2011. [14] Y. Tian, et al., Characteristics of laser-induced surface damage on larger-aperture KDP crystals at 351nm, Chin. Phys. Lett. 32 (2015) 027801. [15] Mike Runkel, et al., The results of Pulse-Scaling Experiments on Rapid-Growth DKDP Triplers Using the Optical Sciences Laser at 351 nm, in: Proc.4347, 2000. [16] L. Lamaignere, et al., Accurate measurements of laser-induced bulk damage density, Meas. Sci. Technol. 20 (2009) 095701. [17] David A. Cross, Christopher W. Carr, Analysis of 1ω bulk laser damage in KDP, Appl. Opt. 50 (2011) D7–D11. [18] YinBo Zheng, et al., Spot-shadowing deployment for mitigating damage-growth of optics in high-power lasers based on a programmable spatial beam-shaping system, Opt. Laser Technol. 108 (2018) 602. [19] C.W. Carr, et al., Effect on scattering of complex morphology of DKDP bulk damage sites, LLNL Report, UCRL-PROC-207879, 2004. [20] < https://en.wikipedia.org/wiki/Otsu%27s_method > . [21] < https://en.m.wikipedia.org/wiki/Image_moment > . [22] YinBo Zheng, et al., Characterization of the influence of polarization orientation on bulk damage in KDP crystals at different wavelengths, Opt. Mater. 58 (2016) 248. [23] M. Runkel, et al., The results of pulse-scaling experiments on rapid-growth DKDP triplers using the Optical Sciences Laser at 351nm, SPIE 4347 (2001) 359. [24] C.W. Carr, et al., A summary of recent damage-initiation experiments on KDP crystals, SPIE 5991 (2005) 59911Q–59921Q.
Fig. 5. The size distribution bulk pinpoints evolves with the depth of damaged region of a conventional growth 70% DKDP crystal exposure to a single 351 nm pulse with the fluence of 9.24 J/cm2, exhibiting a similar trend excepting the tenth layer (9–10 mm) influenced by rear-surface damage.
pinpoints distributed in whole damaged region (i.e., Fig. 3) and that obtaining the curve depicting the number of pinpoints versus the depth of crystals (i.e., Fig. 4), and the result is presented in Fig. 5. The size distributions evolving with the depth of damaged region in Fig. 5 shows a similar trend and a nearly unchanged peak values excepting the tenth layer (9–10 mm) which probably be influenced by rear-surface damage, demonstrating the crystal inhomogeneity having a weaker impact on the size distribution than on the number of pinpoints evolving with the depth of damaged region. Nanosecond laser pulse induced bulk damage process can be roughly depicted by 7 stages for crystal [27]. The pinpoint size and its distribution are determined by both pressure and shock wave (the last two stages of damage processes), and the laser parameters plays key role in damage initiation (the second stage of damage processes). While according to nano-absorber model [28–30], the number of pinpoints is both laser parameters and precursors within crystal dependent. As a result, crystal inhomogeneity have a strong impact on the number of pinpoints evolving with the depth of damaged region (i.e., pinpoints density), while have a negligible impact on the size distribution of pinpoints instead, suggesting two distinct energy deposition mechanisms governing the bulk damage initiation and the growth of the pinpoint [24]. If the number of bulk damage pinpoints reaches ~106 in a large optics (~0.5 m × 0.5 m) [5], the enhanced scatter loss considered to be 4
Optics and Laser Technology 121 (2020) 105672
Y.-B. Zheng, et al.
[29] Anthony Dyan, et al., Revistited thermal approach to model laser-induced damage and conditioning in KH2PO4 and D2xKH2(1–x)PO4 crystals, Proc. SPIE 6403 (2007) 640307. [30] Hu. Guo-Hang, et al., A thermal approach to model laser damage in KDP and DKDP crystals, Chin. Phys. B 26 (2009). [31] M. Runkel, et al., A system for measuring defect induced beam modulation on inertial confinement fusion-class laser optic, Proc. SPIE 5991 (2005) 59912H–59921H.
[25] Leimin Deng, et al., Numerical simulation of laser focusing properties inside birefringent crystal, Appl. Opt. 55 (2016) 853–860. [26] P. De Mange, et al., A new damage testing system for detailed evaluation of damage behavior of bulk KDP and DKDP, SPIE 5337 (2005). [27] P. De Mange, et al., Differentiation of defect population responsible for bulk laserinduced damage in potassium dihydrogen phosphate crystals, Opt. Eng. 45 (10) (2006) 104205. [28] M.D. Feit, et al., Implications of nanoabsorber initiators for damage probability curves, pulselength scaling and laser conditioning, Proc. SPIE 5273 (2003) 74.
5
Contents lists available at ScienceDirect
Optics and Laser Technology journal homepage: www.elsevier.com/locate/optlastec
Full length article
Microscopic characterization of bulk damage resistance of DKDP nonlinear crystals
T
⁎
Yin-Bo Zheng, Xin-Da Zhou, Rong-Sheng Ba, Jie Li, Hong-Lei Xu, Lei Ding , Jin Na, Ya-Jun Li, Jing Yuan, Huan Ren, Xiao-Dong Tang, Li-Qun Chai Research Center of Laser Fusion, China Academy of Engineering Physics, Mianyang 621900, China
H I GH L IG H T S
method evaluating the bulk damage resistance of DKDP crystals is presented. • ACrystal has distinct impacts on density and size of pinpoints. • Distincthomogeneity energy deposition mechanisms govern the process of bulk damage. •
A R T I C LE I N FO
A B S T R A C T
Keywords: DKDP crystals Pinpoints densities Size distribution of pinpoints Three-dimensional (3D) distribution of pinpoints Crystal inhomogeneity
DKDP nonlinear crystals in ICF drivers are susceptible to laser-induced bulk damage, therefore it is significant important to characterize bulk damage in order to prevent, reduce, and/or manage bulk damage. This work presents a method evaluating the bulk damage resistance of DKDP crystals, directly providing pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints. This technique can be used for characterizing the crystal inhomogeneity, and the result demonstrates that crystal inhomogeneity have a strong and negligible impact on pinpoints density and the size distribution of pinpoints respectively, suggesting two distinct energy deposition mechanisms governing the damage initiation and the growth of the pinpoints.
1. Introduction Due to the electro-optical properties (birefringence) and the growth rate (in excess of 10 mm/day) at which large (700 kg) crystals can be grown, (D)KDP crystals are currently the only available nonlinear crystals for ICF (Inertial Confinement Fusion) lasers, converting the laser wavelength from 1053 nm to 527 nm and then mixing the generated 527 nm laser pulse with the residual 1053 nm pulse to produce 351 nm pulse [1]. Unfortunately, laser-induced bulk damage of (D)KDP nonlinear crystals becomes a crucial problem and is regarded as a key factor constraining system design and limiting system performance of high-power laser facilities. The understanding and controlling of damage initiation with ultrashort pulses has been achieved to a large extent [2,3], since that it is governed by intrinsic material properties. However, the nature of localized bulk damage initiation under nanosecond laser pulses irradiation still largely defies fundamental understanding despite more than 40 years of research [4].The detriments of bulk damage consisted of pinpoints, whose density is as high as thousand per cubic millimeter, can be described in terms of enhanced scatter
⁎
loss [5,6] as well as beam contrast ratio [7], which in turn can increase the likelihood of damage to downstream optics. Although some advances in the crystal growth [8], laser conditioning [9], and thermal annealing [10] have been achieved, (D)KDP nonlinear crystals are still vulnerable to laser-induced bulk damage. It is therefore of significant important to characterize this phenomenon in order to prevent, reduce, and/or manage bulk damage in these crystals. Previous works to appraise the damage resistance of (D)KDP nonlinear crystals have exclusively focused on measuring their laser-induced-damage-threshold (LIDTs) [11–13], generating curves depicting the probability of initiating at least a single pinpoint as a function of fluence. While such measurements are capable of producing excellent qualitative results, the absolute LIDTs tend to be significantly higher than those found on large-aperture laser systems [14], so damage probability measurements results obtained by testing small optics could not be scalable to larger optics. More recent works have developed an optional damage testing approaches where the bulk damage resistance is quantified with pinpoints density as a function of laser fluence (i.e., ρ(Φ) curve) [6,7,15–17], providing a quantitative assessment of the
Corresponding author. E-mail address: [email protected] (L. Ding).
https://doi.org/10.1016/j.optlastec.2019.105672 Received 31 January 2019; Received in revised form 1 June 2019; Accepted 30 June 2019 Available online 03 September 2019 0030-3992/ © 2019 Published by Elsevier Ltd.
Optics and Laser Technology 121 (2020) 105672
Y.-B. Zheng, et al.
with a pulse width (FWHM) of ~5 ns. As depicted in Fig. 1, the laser pulses were polarized by a polarizer and then the linear polarization orientation was adjusted by a half-wave plate. A sample was exposed to a laser pulse from a slowly focusing beam (~f/30), and a wedge in the beam line was used to produce two ~ 4% reflections. The reflections off the front and rear surface of the wedge were incident on a scientificgrade CCD camera located in a plane equivalent to the rear surface of the sample and a Joule-meter, respectively. The samples used for the experiments were conventional growth DKDP oriented for type II tripling at 1053 nm, and these samples were sliced to100 × 100 × 10 mm3 in size plates polished and uncoated on all sides. polarizing linearly along the ordinary axis Images of the bulk damaged volume were longitudinally scanned along the counter-direction of propagation of the laser pulses, through the rear-surface of the sample using a microscope whose resolution was 1 μm. And a polarizer was placed in front of the objective of microscope to eliminate the effect induced by the birefringence. The sample was first illuminated with an annular illumination composed of a green light (coherent illumination), aiming to debug microscope easily (especially for the case that pinpoints were sparsely distributed in the damaged region) based on interference effects associated with monochromatic scattering to help detecting pinpoints that were too small to observe with white light illumination, and then a coaxial white light was used for scanning (bright filed illumination) to accurately measure the size of pinpoints [6,19]. The above mentioned optical system resulted in transversal field of view of ~1.67 mm × ~1.67 mm with a depth of focus of 100 μm. Therefore, in order to longitudinally scan the whole ~10 mm × 10 mm × 10 mm region with z-steps of 100 μm, a total of approximately ~104 frames where each pinpoint required identifying and measuring were generated. Each frame was analyzed with a method discussed below. Firstly, a mean distance between neighboring pinpoints is determined, and then the original frame is thresholding through OTSU algorithm [20], obtaining the contour and the size of the damage site. Next the position of damage site is calculated with an algorithm called image moments [21], outputting the position of each site. Lastly, the multiple counts of the same site due to being detected by repetitious z-step scanning must be removed by considering the above mentioned mean distance, the position of the site, and the incidence angle of laser pulse. The accuracy of this algorithm is improved and evaluated with manually measuring results, reaching to an
damage performance of a bulk material comparing to statistical measurements of the damage threshold. As a result, the damage threshold is only one point in this curve depicting the observed onset of damage resulting from the precursors initiating damage at the lowest fluence, and what’s more, bulk density is also a preferable measurement of damage because it can be scaled to larger areas easily [7]. The ρ(Φ) curve were firstly obtained through correlating the local damage pinpoints density and corresponding local beam fluence [15], the characteristic of which was establishing direct linkages between bulk damage density and scatter loss. And then, DeMange et al., proposed a skillful method to measure bulk damage performance with single pulses [6] and was capable of providing pinpoints density, size distribution and beam obscuration directly, while it was only applicable to damage densities exceeding about 160 pp/mm3 [7]. In 2006, a new approach suitable for the case with pinpoint densities as low as 0.8 pp/ mm3 was revealed [7], however, it must be consider under what circumstances light scatter was be proportional to damage density. France team presented a method to characterize bulk damage resistance [16], yet it failed to preserve the information of size distribution or threedimensional (3D) distribution of pinpoints. After that, a measurement scanning the entire laser-interaction volume using polarized bright field illumination was demonstrated in 2011, overcoming above-mentioned drawbacks and providing pinpoints density and size distribution accurately [17]. Generally, the number of pinpoints that can be tolerated in high-power laser facilities relays not only on the size distribution and bulk density, but also on the three-dimensional (3D) distribution of pinpoints. However, to the best of our knowledge, the method of obtaining three-dimensional (3D) distribution of pinpoints is almost absent. In this work a method evaluating the bulk damage resistance of (D)KDP crystals is demonstrated, offering pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints together.
2. Experiments The laser pulses used for damage tests are delivered from MODSS (Multipurpose Optical Damage Science System) facility constructed in 2011[18], outputting approximately 100 J and 40 J at fundamental (1053 nm, 1ω) and third harmonic (351 nm, 3ω), respectively. The spatial profiles of pulses were nearly flat top at two wavelengths. The laser pulses used in this experiment had a Gaussian temporal profile
Fig. 1. Schematic diagram of experimental setup used for characterization of bulk damage of DKDP crystals illuminated firstly with an annular illumination composed of a green light (coherent illumination) for debugging microscope easily, especially for the case that pinpoints were sparsely distributed in the whole damaged region, and then a coaxial white light (bright filed illumination) was used for scanning the damage regions. More details are presented in text. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 2
Optics and Laser Technology 121 (2020) 105672
Y.-B. Zheng, et al.
Fig. 3. The size distribution of pinpoints resulting from 3 neighboring regions of a conventional growth 70% DKDP crystal exposure to a single 351 nm, 5 ns pulse with the different fluencies, and the values in parentheses are the coordinates of peak values.
Fig. 2. Three-dimensional distribution of bulk pinpoints of the damaged region of a conventional growth 70% DKDP crystal exposure to a single 351 nm pulse with the fluence of 9.24 J/cm2, resulting in damage pinpoints density of 27.37/ mm3. The size and the color of representative balls in this 3D distribution picture are proportional to and used for guiding to eye to show the size of corresponding pinpoints, respectively. The space distribution variability is obvious, and the size ranges from ~1 μm to ~40 μm. And more details are presented in text.
pinpoints is ~100 μm and ~40 μm in Ref. [22] and this work, respectively, presuming that the this difference is resulted from microscopy failing to distinguish the overlapping of pinpoints located in different positions in depth-of-focus. Moreover, Fig. 3 reveals that the number of pinpoints exhibiting a stronger dependence on the size than on excitation conditions. In particular, the size distribution profiles show a similar trend. Specifically, the number of pinpoints firstly increases with increasing size until the size reaches a value of ~3.5 μm and then decreases with increasing size, which is similar to the result in Ref. [7]. In addition, it’s noticeable that the change of the sizes corresponding to the peak number of pinpoints can be negligible, consistent with previously reported results for 1ω pulses with fluence ranging from 5 J/ cm2 to 50 J/cm2 [17]. The fact that the size distribution of pinpoints as a function of fluence is nearly unchanged demonstrates the size distribution of pinpoints shows a stronger dependence on the pulse width [17,23,24] than on the fluence.
accuracy exceeding 97%. 3. Results As long as the image processing of original microscopy frames is accomplished, the damage performance under individual wavelength pulse irradiation could be quantified as three-dimensional (3D) distribution of pinpoints, the bulk pinpoints density, and the size distribution of pinpoints. The bulk pinpoints density of damage region is calculated by the ratio of the number of pinpoints with respect to the volume of damage region (the damage region’s cross section containing the pinpoints multiplied the depth through which the damage pinpoints extend). In this experiment, a conventional growth crystal (70% pristine DKDP, type II tripler) irradiated by 351 nm, 5 ns pulse propagating along the z-axis of Fig. 2 and polarizing linearly along the ordinary axis with fluence 9.24 J/cm2, resulting in bulk pinpoints density of 27.37/ mm3. Fig. 2 presents the three-dimensional (3D) distribution of pinpoints through plotting a ball representing each pinpoint according to its position and the size obtained through above mentioned algorithm, where the size and the color of repretative balls are proportional to and used for guiding to eye to show the size of corresponding pinpoints, respectively. It’s obvious that pinpoints are not evenly distributed throughout the damage region and the size of pinpoints ranges from ~1 μm to ~40 μm. One of the adverse effects of bulk damage pinpoints is enhanced beam contrast, resulting from the three-dimensional (3D) distribution of pinpoints, the bulk pinpoints density, and the size distribution of pinpoints together. So the size distribution of pinpoints is another concern of bulk damage of crystal. In this work, the size data is binned into 1 µm bins with the first bin containing 1–2 µm sizes, and the values on the y-axis of Fig. 3 are the numbers of pinpoints counted by the microscope. The size below 1 μm is absent due to the resolution limit of optical system, and the results of size distribution of pinpoints resulting from 3 adjacent regions irradiated with different fluences are shown in Fig. 3, where the values in parentheses are the coordinates of peak values. Results in Fig. 3 are consistent with those in Ref. [22], excepting the resolution limit and the largest size of pinpoints. The largest size of
4. Discussions Firstly, we study the non-uniform distribution of bulk damage pinpoints by plotting the number of pinpoints versus the depth of the crystal. Specifically, the depth of crystal was binned into 1 mm bins with the first bin containing 0–1 mm depth, and the values on the y-axis are the numbers of pinpoints locating within the particular depth-size. Fig. 4 reveals the number variability versus the depth of the damaged region, demonstrating the number of pinpoints is neither monotonically increase nor monotonically decrease versus the depth. Damage behavior in in Fig. 4 can be influence d by spot area, spot intensity profile, and crystal inhomogeneity. In the experiments, the laser pulses were focused into the bulk of DKDP crystal with a focal lens whose focal length was much larger than the thickness of the sample (160:1) to ensure the spot area to be constant along the sample, therefore, the influence of spot area on the measurement can be negligible. For the case of laser pulses polarizing linearly along the extraordinary axis (// e), the spot intensity distribution of a focused laser inside DKDP crystal is complex due to the birefringent effect [25]. While in this experiments, the polarization orientation of laser pulse was collinear to the ordinary axis of crystal (// o), and the change of spot intensity profile can be negligible [25]. In a word, the spot area and spot intensity profile fail to explain the observed changes in damage behavior along the longitudinal direction of DKDP crystal. As a result, the changes in damage behavior may be attributed to the crystal inhomogeneity [26]. Meanwhile, we can obtain the size distribution of pinpoints within each millimeter thickness along the propagation direction of laser pulses by combining the method obtaining the size distribution of 3
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intolerable, while the acceptable upper limit for beam modulation is about ~108[7], therefore it is urgent to give the priority to study the relationship between above mentioned obtained 3 parameters and scatter loss. For sites with size of ~3.5 μm, multiple scattering can be negligible pinpoints below the order of ~103/mm3 [7], so it may be possible to study the relationship, and an investigation is underway. In addition, a laser based system [31] is scheduled to measure defect induced beam modulation of large optics. 5. Conclusions This work proposes a method to scan the damaged region using bright field illumination, directly obtaining the pinpoints density, size distribution, and three-dimensional (3D) distribution of pinpoints. And this technique is initially used for characterizing the crystal inhomogeneity, demonstrating crystal inhomogeneity having a much stronger impact on pinpoints density than on the size distribution of pinpoints, suggesting two different energy deposition mechanisms governing the damage initiation and the growth of the pinpoints.
Fig. 4. The number of the bulk damage pinpoints resulting from 3 neighboring regions varies with the depth of a conventional growth 70% DKDP crystal exposure to a single 351 nm, 5 ns pulse with the different fluencies, reflecting the crystal inhomogeneity.
Acknowledgement The authors acknowledge all colleagues devoutly for their substantial assistance in operation and maintenance for MODSS highpower laser system. This work is supported by Laser Fusion Research Center Funds for Young Talents (No. RCFCZ1-2017-10). References [1] P.J. Wegner, et al., Harmonic conversion of large-aperture 1053nm laser beams for inertial-confinement fusion research, Appl. Opt. 31 (1992) 6414. [2] B.C. Stuart, et al., Nanosecond-to-femtosecond laser-induced breakdown in dielectrics, Phys. Rev. B 53 (1996) 1749. [3] C.B. Schaffer, et al., Laser-induced breakdown and damage in bulk transparent materials induced by tightly focused femtosecond laser pulses, Meas. Sci. Technol. 12 (2001) 1784. [4] R.M. Wood, Laser-induced Damage of Optical Materials, IOP, Philadelphia, 2003. [5] M.L. Spaeth, et al., Description of the NIF Laser, Fusion Sci. Technol. 69 (2016) 25–145. [6] P. DeMange, et al., System for evaluation of laser-induced damage performance of optical materials for large aperture lasers, Rev. Sci. Instrum. 75 (10) (2004). [7] C.W. Carr, et al., Techniques for qualitative and quantitative measurement of aspects of laser-induced damage important for laser beam propagation, Meas. Sci. Technol. 17 (2006) 1958–1962. [8] Yueliang Wang, et al., Laser damage dependence on the size and concentration of precursor defects in KDP crystals: view through differently sized filter pores, Opt. Lett. 41 (2016) 1534–1537. [9] J. Swain, et al., The effect of baking and pulsed laser irradiation on the bulk laser damage threshold of potassium dihydrogen phosphate crystals, Appl. Phys. Lett. 40 (1982) 350. [10] J. Swain, et al., Improving the bulk laser damage resistance of potassium dihydrogen phosphate crystals by pulsed laser irradiation, Appl. Phys. Lett. 40 (1982) 4. [11] J.Y. Natoli, et al., Toward an absolute measurement of LIDT, SPIE 4932 (2003) 224. [12] Hu. Guo-Hang, et al., One-on-one and R-on-one tests on KDP and DKDP crystals with different orientations, Chin. Phys. Lett. 26 (8) (2009) 087801. [13] ISO Standard No.21254, 2011. [14] Y. Tian, et al., Characteristics of laser-induced surface damage on larger-aperture KDP crystals at 351nm, Chin. Phys. Lett. 32 (2015) 027801. [15] Mike Runkel, et al., The results of Pulse-Scaling Experiments on Rapid-Growth DKDP Triplers Using the Optical Sciences Laser at 351 nm, in: Proc.4347, 2000. [16] L. Lamaignere, et al., Accurate measurements of laser-induced bulk damage density, Meas. Sci. Technol. 20 (2009) 095701. [17] David A. Cross, Christopher W. Carr, Analysis of 1ω bulk laser damage in KDP, Appl. Opt. 50 (2011) D7–D11. [18] YinBo Zheng, et al., Spot-shadowing deployment for mitigating damage-growth of optics in high-power lasers based on a programmable spatial beam-shaping system, Opt. Laser Technol. 108 (2018) 602. [19] C.W. Carr, et al., Effect on scattering of complex morphology of DKDP bulk damage sites, LLNL Report, UCRL-PROC-207879, 2004. [20] < https://en.wikipedia.org/wiki/Otsu%27s_method > . [21] < https://en.m.wikipedia.org/wiki/Image_moment > . [22] YinBo Zheng, et al., Characterization of the influence of polarization orientation on bulk damage in KDP crystals at different wavelengths, Opt. Mater. 58 (2016) 248. [23] M. Runkel, et al., The results of pulse-scaling experiments on rapid-growth DKDP triplers using the Optical Sciences Laser at 351nm, SPIE 4347 (2001) 359. [24] C.W. Carr, et al., A summary of recent damage-initiation experiments on KDP crystals, SPIE 5991 (2005) 59911Q–59921Q.
Fig. 5. The size distribution bulk pinpoints evolves with the depth of damaged region of a conventional growth 70% DKDP crystal exposure to a single 351 nm pulse with the fluence of 9.24 J/cm2, exhibiting a similar trend excepting the tenth layer (9–10 mm) influenced by rear-surface damage.
pinpoints distributed in whole damaged region (i.e., Fig. 3) and that obtaining the curve depicting the number of pinpoints versus the depth of crystals (i.e., Fig. 4), and the result is presented in Fig. 5. The size distributions evolving with the depth of damaged region in Fig. 5 shows a similar trend and a nearly unchanged peak values excepting the tenth layer (9–10 mm) which probably be influenced by rear-surface damage, demonstrating the crystal inhomogeneity having a weaker impact on the size distribution than on the number of pinpoints evolving with the depth of damaged region. Nanosecond laser pulse induced bulk damage process can be roughly depicted by 7 stages for crystal [27]. The pinpoint size and its distribution are determined by both pressure and shock wave (the last two stages of damage processes), and the laser parameters plays key role in damage initiation (the second stage of damage processes). While according to nano-absorber model [28–30], the number of pinpoints is both laser parameters and precursors within crystal dependent. As a result, crystal inhomogeneity have a strong impact on the number of pinpoints evolving with the depth of damaged region (i.e., pinpoints density), while have a negligible impact on the size distribution of pinpoints instead, suggesting two distinct energy deposition mechanisms governing the bulk damage initiation and the growth of the pinpoint [24]. If the number of bulk damage pinpoints reaches ~106 in a large optics (~0.5 m × 0.5 m) [5], the enhanced scatter loss considered to be 4
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[29] Anthony Dyan, et al., Revistited thermal approach to model laser-induced damage and conditioning in KH2PO4 and D2xKH2(1–x)PO4 crystals, Proc. SPIE 6403 (2007) 640307. [30] Hu. Guo-Hang, et al., A thermal approach to model laser damage in KDP and DKDP crystals, Chin. Phys. B 26 (2009). [31] M. Runkel, et al., A system for measuring defect induced beam modulation on inertial confinement fusion-class laser optic, Proc. SPIE 5991 (2005) 59912H–59921H.
[25] Leimin Deng, et al., Numerical simulation of laser focusing properties inside birefringent crystal, Appl. Opt. 55 (2016) 853–860. [26] P. De Mange, et al., A new damage testing system for detailed evaluation of damage behavior of bulk KDP and DKDP, SPIE 5337 (2005). [27] P. De Mange, et al., Differentiation of defect population responsible for bulk laserinduced damage in potassium dihydrogen phosphate crystals, Opt. Eng. 45 (10) (2006) 104205. [28] M.D. Feit, et al., Implications of nanoabsorber initiators for damage probability curves, pulselength scaling and laser conditioning, Proc. SPIE 5273 (2003) 74.
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