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Abstract
Rapid grown KDP and DKDP crystals have obvious phase jump at the pyramid–prism (PY-PR) interfaces, which will give rise to intensity modulation to the propagation beam. In this paper, the basic properties of phase jump at the PY-PR interfaces in rapid grown KDP and DKDP crystals are firsly characterized and compared. Then, the influence of the interface’s phase jump on the third harmonic beam modulation is researched by the third-harmonic-generation (THG) experiment. It is found that there exist two PY-PR interfaces in the KDP crystal (SHG1 and SHG2) shaping a “bamboo hat” and three PY-PR interfaces in the DKDP crystal (THG1, THG2 and THG3) shaping a “basin”. The phase-jump magnitude of the PY-PR interface in DKDP crystal is about 1.8 times larger than that in KDP crystal. The THG1 interface, at the bottom of the “basin”, has the largest root-mean-squared gradient (GRMS) of the phase jump, which arouses about 1.6 times modulations to the THG beam. To suppress the beam modulation, the GRMS of phase jump at the PY-PR interface is suggested below 1.2 λ/mm.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In high-power large-aperture laser system, potassium dihydrogen phosphate (KDP) crystal and its deuterated analog DKDP crystal are the optimal choices for converting high-power Nd:glass laser to the ultraviolet for target irradiation, owing to their excellent optical properties and availability in large sizes [1,2]. Currently, the third harmonic laser is used as the normal working wavelength and DKDP crystal usually performs as the tripler [3,4]. For the tripler, 70% deuteration is needed to avoid laser damage caused by the transverse stimulated Raman scattering, which usually occurs in KDP crystal [5].
Initially, KDP and DKDP crystals are grown by traditional temperature reduction method. The growth rate is only 1–2 mm/d and large opaque regenerated cap is unavailable [6]. Then, the rapid growth method is developed greatly in recent years, and the growth rate can be more than one order of magnitude larger than the traditional technique [6–8]. By the rapid growth method, the crystal grows in all dimensions with a large supersaturation shaping prismatic and pyramidal sectors respectively. It is found that the there are more microdefects in the PY-PR interface and its laser damage threshold is less than that in the bulk [9,10]. In addition, researches find that the refractive index of the prismatic sector is slightly different from that of pyramidal sector especially at the PY-PR interface [10–13]. The refractive index nonuniformity will prohibit the attainment of perfect phase-matching across the beam and the THG efficiency decreases as a result. When a beam of light passes through a medium, the phase of the beam changes and the variation φ is proportional to the refractive index [14]. The refractive index difference at the PY-PR interface will result in a “phase jump” across the interface. The phase jump can be regarded as a “defect”, which will cause intensity modulation to the propagation beam [14,15]. The beam modulation poses a threat to the damage of following components as a result. Thus, the phase jump at the PY-PR interface should be strictly controlled. However, the property of the PY-PR interface remains ambiguous and its influence on the beam modulation has not be given quantificationally. This situation limits the THG design and estimation of its operational condition.
In this paper, the basic properties of phase jump at the PY-PR interfaces in rapid grown KDP and DKDP crystals are firsly characterized and compared. Then, the influence of the interface’s phase jump on the third harmonic beam modulation is researched by the THG experiment.
2. Experiment
2.1 Sample preparation
A large-size KDP and DKDP crystals were grown from aqueous solution by rapid growth method. Crystals grew in the mode of “forward-stop-backward” in about three months. The size of the KDP and DKDP crystal boule is about 600 mm×570 mm×280 mm and 800 mm×660 mm×300 mm, respectively. The grown crystals are all in perfect crystallization and no macroscopic defects are detected. KDP crystal slice with a size of 430 mm×430 mm×11 mm was cut for type-I phase matching and DKDP crystal slice with a size of 430 mm×430 mm×9 mm sample was cut for type-II phase matching. Figure 1 shows the cutting schematic diagram of the KDP and DKDP crystal slices. Crystal slices both contain prismatic sector and pyramidal sectors. They are both fine polished and coated.
2.2 Raman spectrum measurement
A Raman spectroscopy (Renishaw Invia-Reflex) was used to measure the Raman spectrum with excitation wavelength of 532 nm. The measuring system was stabilized for 2 hours after starting up to acquire a high repeatability. The repetition of the measuring system is about 0.02 cm−1 (root-mean-square). The exciting light focus on 2 mm depth beneath the sample’s surface to avoid the proton exchange layer profiles in DKDP crystal. The Raman spectrum of KDP crystal was also measured as a reference. Raman spectra are collected from 840 cm−1 to 940 cm−1 with a step of 1 cm−1.
2.3 Transmitted wavefront measurement
A large-aperture interferometer Zygo (VeriFire MST) was used to attain the transmitted wavefront distribution of the large-size KDP and DKDP crystal slices. The wavelength of the test is 632.8 nm. The diameter of the test aperture is as large as 800 mm with a precision of 0.02 λ. The detection aperture of the crystal is 410 mm×410 mm.
2.4 THG experiment
The experiment was carried out on a large-aperture high-power laser system called Integrated Test Bed. The fundamental light (1ω) is a Nd:glass laser with a wavelength of 1053 nm, pulse width of 1.5 ns and beam aperture of 360 mm. The 1ω laser is in flat-top distributions in space and time. The experimental setup and the spatial distribution of fundamental light are shown in Fig. 2. The KDP crystal converts approximately two thirds of the 1ω laser energy to the second harmonic (2ω) laser. Then the 2ω laser mixes with the residual 1ω laser to produce the third harmonic (3ω) laser in DKDP crystal [16]. The generated 3ω laser is focused by a wedged focus lens passing through the vacuum window to the target. The target here is made of aluminum alloy to absorb and scatter the 3ω laser. A small amount of 3ω laser is separated by the splitter and monitored by the measuring system including energy meter and CCD camera. The image face of the CCD camera is set at the rear surface of the vacuum window, which is about 5.3 meters away from the DKDP crystal.
Fig. 2. Schematic diagram of the experimental setup and the spatial distribution of fundamental light
3. Results and discussion
3.1 Raman spectrum
In DKDP crystal, two points are chosen for measuring the Raman spectrum as shown in Fig. 3(a). It can be seen that Point A is at the pyramidal sector and Point B is at the prismatic sector. Figure 3(b) shows the measured Raman spectra of KDP and DKDP crystals. It is reported that the Raman shift of the v1 mode, assigned to the totally symmetric vibration of the PO4 ion, can be used to characterize the deuterium degree and 1 cm−1 shift represents about 2.78% deuteration degree [17,18]. Comparing with KDP crystal, the v1 mode of DKDP crystal shifts about 25 cm−1 to lower wavenumbers. Thus, the deutertation degree of the DKDP crystal is about 70%. Besides, it is found that the v1 mode of Point A is about 0.3 cm−1 lower than Point B indicating that the deuteration degree of Point A is 0.8% higher than Point B. The result in this paper is perfectly coincident with our earlier report that the deuterium content of prismatic sector is about 1% lower than that of pyramidal sector [19]. The deuterium inhomogeneity is mainly due to the large variation of the supersaturation in solution.
Fig. 3. (a) Measuring points for Raman spectrum in DKDP crystal, (b) Raman spectra of KDP and DKDP crystals
3.2 Transmitted wavefront
Figure 4 shows the transmitted wavefront distribution of the KDP and DKDP crystals. Two features of this distribution are notable. The weak circular arcs running across the full aperture correspond to diamond turning grooves that are incurred during the finishing of the surfaces of the crystal. Another is the strip which means the discontinuity of the phase distribution. As can be seen, there exist two strips in the KDP crystal (SHG1 and SHG2) shaping a “bamboo hat” and three strips in the DKDP crystal (THG1, THG2 and THG3) shaping a “basin”. Compared to Fig. 1, it can be concluded that the strips are ascribed to the PY-PR interfaces.
Figure 5(a) shows the typical phase jump of the PY-PR interfaces. The data showed in the figure is the average of the transmitted wavefront values along the PY-PR interface shown in Fig. 4. As can be seen, phase difference gradually increases from the prismatic sector to the pyramidal sector. Phase jumps of the PY-PR interfaces have different characteristics. The magnitude of phase jump Δφ and the distance of phase jump Δd have been used to characterize the PY-PR interface. It is found that the Δφ of all the interfaces in DKDP crystal are close to each other, and the same is true for KDP crystal. Furthermore, the Δφ in DKDP crystal is about 1.8 times larger than that in KDP crystal. The schematic diagram of the PY-PR interface in the crystal is shown in Fig. 5(b). The width of the distinct boundaries in crystal boule and the the cutting angle of the crystal slice are signed as Δd0 and α, respectively. The width of the PY-PR interface can be expressed as Δd=Δd0/cosα. Parameters of the PY-PR interfaces are summarized in Table 1.
Fig. 5. (a) Typical phase jump of the PY-PR interfaces in KDP and DKDP crystals, (b) Schematic diagram of the PY-PR interface in the crystal
Table 1. Parameters of the PY-PR interfaces
The magnitude of phase jump Δφ is related to the refractive index difference Δn, the crystal thickness L and the interferometer wavelength λ by the relation [20,21]
where θ is the angle between the direction of propagation and the crystal optic axis Z and x is the deuterium content. Thus, the Δφ represents the refractive index deviation. By the rapid growth method, the crystal grows along the pyramidal and prismatic faces simultaneously. The pyramidal face has the K+ ions on the outside of the crystal, while the prismatic face has both the positive K+ ions and the negative (H/D)2PO4− at the surface [22]. The difference in polarity of the layers and size of the ions will result in a different stacking manner of the growth unit, which may influence local direction of the crystal Z axis [11,22,23]. The θ at the pyramidal sector is different from the prismatic sector as a result. Thus, for KDP crystal the phase jump at the interface is mainly due to the local direction deviation of the Z axis. For DKDP crystal as mentioned above, the deuterium content across the PY-PR interface is different, which aggravates the refractive index deviation. As a reslut, a larger phase jump at the interface is obtained in DKDP crystal. In addition, the Δd of the PY-PR interface is different from each other. However, the α variation makes just about 5% contribution to the difference of Δd according the equation Δd=Δd0/cosα. The results indicate that the variation of the boundary width Δd0 is responsible for the difference of Δd. Theoretically, the SHG1 and SHG2 should have the same feature. In practical application, however, the SHG1 has obviously smaller Δd. The inconformity indicates that the characteristics of the interface are related to the growth process. The increasing of the Δφ with Δd is approximately linear at the interface. It is clear that the THG1 has the largest slope.The distribution of transmitted wavefront in KDP and DKDP crystal can be describe as WSHG(X,Y) and WTHG(X,Y) respectively. And the root-mean-squared gradient (GRMS) is an important parameter to characterize the phase jump. In the process of THG, the laser beam propagates through the KDP and DKDP crystals in succession. The total GRMS of the phase jump denoted as g(X,Y) can be calculated according to the formula (2)–(3) [24].
Fig. 6. (a) Total GRMS of the wavefront in KDP and DKDP crystals, (b) Profile data of the GRMS at Section A of THG1
3.3 3ω beam modulation
The energy of 1ω laser is about 4000 J and about 2500 J of 3ω laser is obtained by the THG process. Figure 7(a) displays the near-field intensity distribution of 3ω laser. As can be seen, the intensity distribution is inhomogeneous. Generally, the intensity in the pyramidal sector is larger than that in the prismatic sector. The phase-matching angle depends on the deuterium content and 1% higher deuterium content will cause 230 µrad increasing of the phase-matching angle [19]. The 0.8% deuterium inhomogeneinty in the DKDP crystal induces 184 µrad deviation of the phase-matching angle. Therefore, the intensity inhomogeneity is mainly induced by the nonuniformity of deuterium content in DKDP crystal.
Fig. 7. (a) Near-field intensity distribution of 3ω laser, (b) Profile data of the 3ω intensity at Section A of the THG1 interface
In addition, it is found that the phase jump at the interface gives rise to intensity modulation of the propagation beam. The phenomenon can be explained by Fresnel diffraction by a straight edge [15,25]. Clearly, the interfaces of DKDP crystal, especially the THG1, are obvious in the near-field intensity distribution of 3ω laser. While the interfaces of KDP crystal are indistinctive. The results are coincident with the distribution of phase-jump GRMS shown in Fig. 6(a). As mentioned above, the Section A of THG1 has the largest GRMS of phase jump. However, it seems that the 3ω intensity at Section A is not the largest. This contradiction is attribute to the influence of THG efficiency difference induced by the deuterium content inhomogeneity in DKDP crystal. Figure 7(b) displays the profile data of the 3ω intensity at Section A. As can be seen, the 3ω intensity at the interface oscillates with the space position and the width is also about 4 mm. The intrinsic modulation of the beam is about 1.2, which is induced by the inhomogeneity of input energy and phase distortion of optical elements. The intensity modulation at the Section A is about 1.6, which is about 30% larger than the intrinsic modulation of the beam.
Comparing Fig. 6(a) with Fig. 7(a), we can find that larger GRMS of phase jump will induce larger intensity modulation of the 3ω beam. The relationship between the intensity modulation and the GRMS of phase jump is statisticed in Fig. 8. The result indicates the intensity modulation increases more rapidly at larger GRMS of phase jump. When the GRMS is 1.2 λ/mm, the beam modulation is 1.2 corresponding to the intrinsic modulation of the beam. And the GRMS of 2.4 λ/mm arouses about 1.6 times modulations to the 3ω beam. The 1.6 times modulations at the THG1 interface poses serious damage threat to the vacuum window and followed components. In the experiment, the laser damage induced by the THG1 interface is observed on the splitter. For higher 3ω laser the beam modulation will be more larger acorrding to the diffraction theory. To reduce the influence of the PY-PR interface, the beam modulation induced by the PY-PR interface should be below the intrinsic modulation. Therefore, the GRMS of phase jump at the interface should be controlled below 1.2 λ/mm.
4. Summary
In summary, large-size KDP and DKDP crystals were grown from aqueous solution by rapid growth method. The basic properties of phase jump at the PY-PR interfaces in rapid grown KDP and DKDP crystals are characterized and compared. And the influence of PY-PR interfaces on the 3ω beam modulation is obtained by the THG experiment. The results show that the phase-jump magnitude of the PY-PR interface in DKDP crystal is about 1.8 times larger than that in KDP crystal, which is mainly induced by the deuterium content difference across the interface in DKDP crystal. Among all the interfaces, the THG1 is the most notable with a phase-jump GRMS of 2.4 λ/mm. The phase jump of the PY-PR interface gives rise to intensity modulation of the 3ω beam. The 3ω intensity at the interface oscillates with the space position. The intensity modulation increases more rapidly at larger phase-jump GRMS. The intensity modulation induced by the THG1 is about 1.6, which poses serious damage threat to the vacuum window and followed components. However, the PY-PR interfaces of the KDP crystal do not affect significantly on the 3ω beam. In view of the intrinsic modulation of the beam, the GRMS of phase jump at the PY-PR interface should be controlled below 1.2 λ/mm. Besides, it seems that the feature of PY-PR interface is related to the growth process. In a word, the PY-PR interfaces of the rapid grown DKDP crystal should be paid more attention and the procedure of rapid growth should be optimized in view of lowering the phase jump.
Funding
Open Project of State Key Laboratory of Crystal Materials, Shandong University (KF1601).
Acknowledgements
We would like to thank the Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science for providing the KDP and DKDP crystals.
References
1. D. Yoreo, A. Burnham, and P. Whitman, “Developing KH2PO4 and KD2PO4 crystals for the world’s most powerful laser,” Int. Mater. Rev. 47(3), 113–152 (2002). [CrossRef]
2. C. Maunier, P. Bouchut, S. Bouillet, H. Cabane, R. Courchinoux, P. Defossez, J. C. Poncetta, and N. Ferriou-Daurios, “Growth and characterization of large KDP crystals for high power lasers,” Opt. Mater. 30(1), 88–90 (2007). [CrossRef]
3. C. A. Haynam, P. J. Wegner, J. M. Auerbach, M. W. Bowers, S. N. Dixit, G. V. Erbert, G. M. Heestand, M. A. Henesian, M. R. Hermann, K. S. Jancaitis, K. R. Manes, C. D. Marshall, N. C. Mehta, J. Menapace, E. Moses, J. R. Murray, M. C. Nostrand, C. D. Orth, R. Patterson, R. A. Sacks, M. J. Shaw, M. Spaeth, S. B. Sutton, W. H. Williams, C. C. Widmayer, R. K. White, S. T. Yang, and B. M. Van Wonterghem, “National Ignition Facility laser performance status,” Appl. Opt. 46(16), 3276–3303 (2007). [CrossRef]
4. L. L. Ji, B. Q. Zhu, T. Y. Zhan, Y. P. Dai, J. Zhu, W. X. Ma, and Z. Q. Lin, “The third harmonics generation with large aperture and high fluency,” Acta Phys. Sin. 60(9), 1–6 (2001).
5. S. G. Demos, R. N. Raman, S. T. Yang, R. A. Negres, K. I. Schaffers, and M. A. Henesian, “Measurement of the Raman scattering cross section of the breathing mode in KDP and DKDP crystals,” Opt. Express 19(21), 21050–21059 (2011). [CrossRef]
6. G. Dhanaraj, K. Byrappa, V. Prasad, and M. Dudley, “Spinger handbook of crystal growth,” Published by Springer, 759–789 (2010).
7. X. X. Zhuang, L. W. Ye, G. Z. Zheng, G. B. Su, Y. P. He, X. Q. Lin, and Z. H. Xu, “The rapid growth of large-scale KDP single crystal in brief procedure,” J. Cryst. Growth 318(1), 700–702 (2011). [CrossRef]
8. S. K. Sharma, S. Verma, Y. Singh, K. S. Bartwal, M. K. Tiwari, G. S. Lodha, and G. Bhagavannarayana, “Investigations of structural defects, crystalline perfection, metallic impurity concentration and optical quality of flat-top KDP crystal,” Opt. Mater. 46, 329–338 (2015). [CrossRef]
9. M. Yan, R. A. Torres, M. J. Runkel, B. W. Woods, I. D. Hutcheon, N. P. Zaitseva, and J. J. De Yoreo, “Investigation of impurities and laser-induced damage in the growth sectors of rapidly grown KDP crystals,” Proc. SPIE 2966(1996), 11–16 (1996). [CrossRef]
10. D. Y. Chen, B. Wang, H. Wang, Y. B. Bai, N. Xu, B. Z. Li, H. J. Qi, and J. D. Shao, “Investigation of the pyramid-prism boundary of a rapidly grown KDP crystal,” CrystEngComm 21(9), 1482–1487 (2019). [CrossRef]
11. J. M. Auerbach, P. J. Wegner, S. A. Couture, D. Eimerl, R. L. Hibbard, D. Milam, M. A. Norton, P. K. Whitman, and L. A. Hackel, “Modeling of frequency doubling and tripling with measured crystal spatial refractive-index nonuniformities,” Appl. Opt. 40(9), 1404–1411 (2001). [CrossRef]
12. W. Han, L. D. Zhou, F. Wang, F. Q. Li, K. Y. Li, L. Q. Wang, B. Feng, Q. H. Zhu, J. Q. Su, and M. L. Gong, “Effect of spatial refractive-index nonuniformities existed in a large-scale rapid growth KDP crystal on third-harmonic conversion,” Optik 124(24), 6506–6508 (2013). [CrossRef]
13. J. H. Campbell, R. A. Hawley-Fedder, C. J. Stolz, J. A. Menapace, M. R. Borden, P. K. Whitman, J. Yu, M. Runkel, M. O. Riley, M. D. Feit, and R. P. Hackel, “NIF optical materials and fabrication technologies: An overview,” Proc. SPIE 5341, 84–101 (2004). [CrossRef]
14. A. Lahiri, Basic optics: principles and concepts, Published by Elsevier Inc, 355–430 (2016).
15. Q. Q. Su, G. W. Zhang, H. Tao, and J. X. Pu, “Modulation of nonlinear medium with surface defects on intensity properties of laser,” Qiangjiguang Yu Lizishu 24(11), 2585–2590 (2012). [CrossRef]
16. M. A. Henesian, P. J. Wegner, D. R. Speck, C. Bibeau, R. B. Ehrlich, C. W. Laumann, J. K. Lawson, and T. L. Weiland, “Modeling of large aperture third harmonic frequency conversion of high power Nd: glass laser systems,” Proc. SPIE 1415, 90–103 (1991). [CrossRef]
17. M. A. Yakshin, D. Kim, Y. S. Kim, Y. Broslavets, O. E. Sidoryuk, and S. Goldstein, “Determination of the deuteration degree of DKDP crystals by Raman spectroscopy technique,” Laser Phys. 7, 941–945 (1997).
18. T. Huser, C. W. Hollars, W. J. Siekhaus, J. J. de Yoreo, T. I. Suratwala, and T. A. Land, “Characterization of proton exchange layer profiles in KD2PO4 crystals by micro-Raman spectroscopy,” Appl. Spectrosc. 58(3), 349–351 (2004). [CrossRef]
19. X. X. Chai, F. Wang, B. Feng, X. Feng, L. S. Zhang, F. Q. Li, W. Han, L. Q. Wang, P. Li, D. Y. Zhu, Y. K. Jing, and G. Z. Wang, “Deuterium homogeneity investigation of large-size DKDP crystal,” Opt. Mater. Express 8(5), 1193–1201 (2018). [CrossRef]
20. C. Ai and J. C. Wyant, “Testing an optical window of a small wedge angle: effect of multiple reflections,” Appl. Opt. 32(25), 4904–4912 (1993). [CrossRef]
21. W. J. Liu, X. Yin, S. L. Wang, Z. P. Wang, J. X. Ding, Y. Sun, and G. X. Liu, “Dependence of refractive indices on the deuterium content in K(H1-xDx)2PO4 crystals,” Opt. Laser Technol. 44(6), 1769–1772 (2012). [CrossRef]
22. S. A. de Vries, P. Goedtkindt, S. L. Bennett, W. J. Huisman, M. J. Zwanenburg, D. M. Smilgies, J. J. De Yoreo, W. J. P. van Enckevort, P. Bennema, and E. Vlieg, “Surface atomic structure of KDP crystals in aqueous solution: an explanation of the growth shape,” Phys. Rev. Lett. 80(10), 2229–2232 (1998). [CrossRef]
23. M. F. Reedijk, J. Arsic, F. F. A. Hollander, S. A. de Vries, and E. Vlieg, “Liquid order at the interface of KDP crystals with water: evidence for icelike layers,” Phys. Rev. Lett. 90(6), 066103 (2003). [CrossRef]
24. L. Q. Liu, F. Jing, Z. T. Peng, Q. H. Zhu, X. F. Cheng, D. B. Jiang, Q. Q. Zhang, and H. J. Liu, “Study of the static wavefront distortion of optics elements in the multi-pass amplification system,” High Power Laser and Particle Beams 15(3), 241–244 (2003).
25. J. H. Wang, W. Zhang, Y. W. Cui, and S. Y. Teng, “Fresnel diffraction by a square aperture with rough edge,” Optik 126(21), 3066–3071 (2015). [CrossRef]
