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Abstract
We demonstrate an all-solid-state deep-ultraviolet (DUV) laser based on the frequency-quadrupling of a 1 µm, 1.2 ps, Yb: YAG Innoslab solid-state laser at a 10 kHz repetition rate, using LBO and BBO as second-harmonic generation and fourth-harmonic generation crystals, respectively. The DUV laser delivers 20 W, 2.0 mJ, 665 fs, 258 nm DUV pulses, with an overall conversion efficiency of ∼8.7% from 1 µm to DUV. The corresponding peak power of DUV pulses is up to 3 GW, which, to the best of our knowledge, is highest in reported kHz-rate all-solid-state DUV sources driven at 1 µm wavelength.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
High-average-power deep-ultraviolet (DUV) laser sources of below 300 nm with a short pulse duration and a high peak power are attracting extensive attention from researchers due to their applications, such as micromachining in a regime of cold ablations [1], seeding free-electron lasers [2], pumping optical parametric amplification (OPA) and optical parametric chirp-pulse amplification (OPCPA) systems [3], ionizing noble gas and driving coherent soft X-ray radiation through extreme high order harmonic generation (HHG) [4]. In the past decades, with the rapid advances of high-average-power, high-energy 1-µm solid-state and fiber laser technologies, high-average-power, high-energy all-solid-state DUV laser sources with pulse durations from nanosecond (ns) to femtosecond (fs) based on the frequency-quadrupling of those 1-µm lasers have been reported [5–19]. One of the most advanced ns DUV sources was reported by M. Nishioka et al. in 2003 [10], which delivered 40 W average-power DUV pulses at 266 nm, driven by a Nd: YAG laser at a 7 kHz repetition rate. This is the currently reported solid-state DUV laser with the highest average power. In the picosecond (ps) region, a representative high-energy DUV laser was demonstrated by C.-L. Chang et al. in 2015 [7], with a 2.74 mJ pulse energy and a 4.2 ps pulse duration, driven by a ps, 1 kHz, Cryo-Yb: YAG laser. The short pulse duration and high pulse energy make the laser having the highest peak power of 0.56 GW among all reported 1-µm-driven, kHz-rate, all-solid-state DUV lasers. One year later, another ps DUV laser with 6 W average power and an estimated 2 ps pulse duration operating at 100 kHz was reported using a 1030 nm Yb: YAG thin-disk laser as the pump source [13]. The reports of high-average-power DUV sources at sub-ps or fs pulse durations are relatively rare. A recent report is the 4.6 W, 150 fs, 258 nm DUV laser at a high repetition rate of 796 kHz driven by an Yb-doped fiber laser [9]. Despite the laser has a short pulse duration, its peak power is only about 38.5 MW, limited by the relatively low pulse energy. Based on the above literatures, the peak powers of 1-µm-driven, kHz-rate, all-solid-state DUV lasers are wandering around a few tens to a few hundreds of MW either due to the long pulse duration or due to the low pulse energy.
Here, we report a 1-µm-driven, kHz-rate, all-solid-state DUV laser with a peak power beyond 1 GW, simultaneously combined with the characteristics of high average power, high energy and short pulse duration. The DUV laser is based on the frequency-quadrupling of a 1 µm, 1.2 ps, 10 kHz, Yb: YAG Innoslab laser. LBO and BBO crystals are used to implement second-harmonic generation (SHG) and fourth-harmonic generation (FHG), respectively. 20 W, 2.0 mJ, 665 fs, 258 nm DUV pulses are delivered, with a conversion efficiency of ∼8.7% from 1 µm to 258 nm pulses. The corresponding peak power of DUV pulses is up to 3 GW, which is highest among reported kHz-rate all-solid-state DUV lasers driven at 1 µm wavelength. The high peak power of our DUV laser enables researching on nonlinear phenomena under high field which could not be pursued by previously reported kHz-rate, 1-µm-driven, solid-state DUV lasers.
2. Pump source and experimental setups
The pump source used in our experiment is a commercial Yb: YAG chirped-pulse amplification (CPA) system based on the Innoslab amplification technology (AMPHOS 300). It delivers 270 W, 27 mJ, 1.2 ps, 10 kHz pulses centered at 1030.8 nm. The typical spectrum (0.4 nm resolution) from the pump source is shown in Fig. 1(a), which has a 2.2 nm full width at half-maximum (FWHM) centered at 1030.8 nm. Figures 1(b)–1(d) depict the temporal profile of the pump pulses, measured using a SHG frequency-resolved optical gating (SHG-FROG). The pulse duration is 1.2 ps, corresponding to 1.7 times the transform-limited (TL) pulse duration. Inset in Fig. 1(a) shows the beam profile of the pump laser at 270 W average power, which has an elliptic shape with two distributed sidelobes surrounding the main beam (∼10% energy is stored in the sidelobes). The main beam has a circularity of 80%. These sidelobes are from the residual high-order modes which are not filtered out completely by the spatial filter (beam slit) in the amplifier.
Fig. 1. (a) The typical measured and retrieved spectra. Inset shows the beam profile of the pump laser. (b) The retrieved pulse shape and phase. The measured (c) and retrieved (d) FROG traces.
Figure 2 shows the optical layout of the frequency quadrupling starting from the 1 µm Yb: YAG Innoslab laser. The collimated pump beam is first sent through an energy tuner comprising of a half-wave plate and a thin-film polarizer for controlling the pump energy on SHG crystal. Taking into account of the high peak power of the pump pulses, a telescope is subsequently used to expand the original pump beam to a larger beam size (11.5 mm at $1/{\textrm{e}^2}$ intensity along the long axis of the main beam) to avoid crystal damage. After the telescope, a 5-mm-thick LBO with anti-reflection (AR) coatings for 1030 nm and 515 nm on both sides and an aperture of $20 \times 20 \;\textrm{mm}$ is used and it is cut at ${\theta } = {90^\circ }\;\textrm{and}\; \varphi = {0^\circ }$, with a non-critical phase-matching temperature of ∼190°C for 1030 nm and 515 nm. Here, LBO is chosen for its largest damage threshold among commercial nonlinear crystals for SHG [7]. The residual pump pulses are removed from green pulses using a 1030 nm/515 nm dichroic mirror which reflects 515 nm and transmits 1030 nm.
Fig. 2. The schematic of the high-average-power, high-energy, sub-ps, all-solid-state deep-ultraviolet (DUV) laser. HWP: half-wave plate. TFP: thin-film polarizer. DM: dichroic mirror.
For FHG setup, a few nonlinear crystals have been used for the FHG pumped by 1-µm lasers in previous works, such as BBO [5–9], CLBO [10–15,19], KBBF [16], YAB [17] and NSBBF [18]. Among those crystals, CLBO and BBO are preferred and have been demonstrated for highly efficient FHG conversion. Compared with BBO, CLBO has large angular acceptance bandwidth and thus suffers less from the spatial walk-off. It is more suitable for the cases where moderate pump laser power and relative long crystal length are used. For high peak-power DUV laser, the nonlinear effects such as two photon absorption (TPA) and self-phase modulation limit the length of the crystal [7,13,20]. BBO with relative higher nonlinear coefficient (${\sim} 1.7\;\textrm{pm}/\textrm{V}$) [21] and lower TPA coefficient [20] provides an alternative choice. Furthermore, CLBO is hydroscopic and requires special precautions. Therefore, the BBO is selected in our experiment. Three BBO crystals (cut at ${\theta } = {50^\circ }\;\textrm{ and}\; \varphi = {0^\circ }$ for Type-I) with the thickness of 0.1 mm, 0.2 mm and 0.4 mm and apertures of 20×20 mm are used. The thin thickness is helpful to mitigate the thermal effects caused by TPA. All crystals have AR coatings for 515 nm and 258 nm on both sides and they are mounted in a tilt and rotation mount for optimizing the phase-matching conditions. Due to the lack of the optical components for the green beam reshaping, the green pulses are directly used to pump BBO crystals. For the optimization of the green polarization in BBO, a half-wave plate is also incorporated into the green beam. The generated DUV pulses are separated from the green pulses using a pair of dichroic mirrors for 515 nm and 258 nm. Then the DUV pulses are characterized using the power meter, the spectrometer, the beam profiler and the autocorrelator with a TPA detector (PulseCheck 15, APE).
3. Experimental results and discussion
For the SHG setup, ∼230 W, 23.0 mJ pump pulses at 1 µm reach the LBO owing to some losses caused by routing optics. The pump pulses have a peak intensity of 37 $\textrm{GW}/\textrm{c}{\textrm{m}^2}$ on the LBO. By optimizing the oven temperature, 108 W, 10.8 mJ green pulses are obtained, corresponding to a conversion efficiency of 47%. Figures 3(a) and 3(b) show the dependence of the green average power and conversion efficiency on the pump average power. Due to the high average power, the heat caused by linear absorption of the pump and green pulses in LBO is non-negligible, which affects the phase-matching conditions of LBO. Therefore, the green average power at every pump level is optimized by adjusting the oven temperature to compensate the phase-mismatching caused by the heat. When the pump average power approaches 110 W, the efficiency saturation is found. The walk-off length between pump and green pulses is 12 mm, which is much longer than the thickness of LBO, indicating it does not make contribution for the efficiency saturation. The saturation may be caused by the wavefront aberrations, pulse chirp, beam shape of the pump pulses as well as the longitudinal temperature gradient (LTG) in LBO. A systematic investigation on the impact of LTG on the conversion efficiency was reported in [22]. The inset of Fig. 3(a) shows that the green beam profile at 108 W average power has an elliptic shape with a circularity of 70%. It is worth mentioning that the sidelobes appearing in the pump beam have been filtered out in the green beam owing to their lower conversion efficiency than the main beam. Figure 3(c) shows the green spectrum at 108 W average power with 0.2 nm resolution. It has a FWHM of 1.6 nm, centered at 515.4 nm, which corresponds to 244 fs TL pulse duration. The measured pulse duration of green pulses, shown in Fig. 3(d), is 910 fs with the assumption of a Gaussian pulse shape. It is slightly wider than the estimated (850 fs, SH pulse duration reduces by a factor of $\sqrt 2 $) probably due to the chirp in the pump pulses. Considering the pulse energy of 10.8 mJ, the peak power of green pulses is up to 11.9 GW. Such a high-average-power, high-peak-power, sub-ps green laser can find various applications, not only for material processing with minimized thermal effect [23], but also for pumping OPCPA systems at 0.8 or 1 µm.
Fig. 3. The green average power (a) and conversion efficiency (b) versus the pump average power. Inset shows the green beam profile at 108 W average power. The green spectrum (c) and autocorrelation trace (d) at 108 W average power.
Regarding the FHG, the green beam size at $1/{\textrm{e}^2}$ intensity is ∼9 mm on BBO and it is slightly converged due to the self-focusing effect in LBO. The pump intensity on BBO is up to 37 $\textrm{GW}/\textrm{c}{\textrm{m}^2}$ when 108 W, 10.8 mJ green pulses are used. Strong TPA of the generated DUV pulses must be considered for 3 different thicknesses of crystals. Because it causes LTG inside the crystal and affects the phase-matching conditions [24]. In order to obtain the optimized DUV output, the phase-mismatching caused by the LTG is partially mitigated through finely adjusting the phase-matching angle of BBO. Figure 4(a) shows the dependence of DUV average power on the crystal thickness. It can be found that under our experimental conditions, the crystal thickness is still a key parameter to improve the DUV output average power. This is because the DUV gain can still overcome the DUV loss induced by TPA with the increase of the crystal thickness from 0.1 mm to 0.4 mm. A thicker BBO crystal is possible to improve the DUV output average power further. The maximum DUV average power of 20 W is obtained in a 0.4-mm-thick BBO crystal, corresponding to a conversion efficiency of 18.5% from green to DUV and 8.7% from 1 µm to DUV.
Fig. 4. (a) The DUV average power for different BBO thicknesses. Three BBO crystals produce 6.7 W (0.1 mm-thick), 16 W (0.2 mm-thick), 20 W (0.4 mm-thick) DUV output, respectively. The inset shows the beam profile for 0.4-mm-thick BBO. (b) The measured DUV beam quality in the long-axis direction.
We did not find the obvious degradation of beam profiles and any crystal damage for all three crystals. The beam profile at 20 W average power for 0.4 mm BBO, as shown in the inset of Fig. 4(a), is more elliptical than that of green beam, with a circularity of 45%. Although the spatial walk-off makes contribution to the worse circularity [25], its contribution is very little due to the large green beam size and thin crystal thickness. The circularity degradation is mainly attributed to the axis-dependent thermal lensing effect in BBO. It is worth mentioning that this elliptical beam can be shaped to a circle shape using suitable cylindrical optics [25]. The measured beam quality (${\textrm{M}^2}$) in the long-axis direction at 20 W is 1.7, as shown in Fig. 4(b). We have not recorded the data of the beam quality in the short-axis direction of the beam, but the beam quality should be worse than that in the long-axis direction based on the beam profile at 20 W.
The dependence of the DUV average power and conversion efficiency on the green average power for the 0.4-mm-thick BBO is shown in Figs. 5(a) and 5(b). Here, DUV output at every green average power is optimized by adjusting the phase-matching conditions of BBO. The maximized DUV conversion efficiency is limited below 24%, indicating TPA loss and TPA induced phase-mismatching play important roles for the conversion efficiency. The quadratic dependence of the conversion efficiency on green average powers is not observed in Fig. 5(b) due to the lack of sampling points with lower green powers. Figures 5(c) and 5(d) depict the spectrum and autocorrelation trace of DUV pulses at 20 W average power. The DUV spectrum measured with 0.1 nm resolution is centered at 257.7 nm with a FWHM of 1.05 nm, which supports 93 fs TL pulse duration. The measured pulse duration is 665 fs, assuming a Gaussian pulse shape, and agrees well with the estimated (643 fs, SH pulse duration reduces by a factor of $\sqrt 2 $). The calculated group delay dispersion (GDD) amount is $2.21 \times {10^4}\;\textrm{f}{\textrm{s}^2}$, based on the assumption of a Gaussian pulse shape. We are not sure about GDD sign which needs to be further confirmed. It is worth mentioning that the peak power of the DUV pulses is up to 3 GW, indicating its potential for researching on nonlinear phenomena under high field. The compression of these pulses down to shorter pulse duration can be implemented by compensating the residual GDD using chirped mirrors in future to further improve the peak power.
Fig. 5. The DUV average power (a) and conversion efficiency (b) versus the green average power. The DUV spectrum (c) and autocorrelation trace (d) at 20 W average power.
At last, we measured the power fluctuation of DUV pulses within a 20-min measurement duration, as shown in Fig. 6. A 2.5% rms instability is measured, which is higher than that of the pump pulses (0.3% rms) and green pulses (0.7% rms). This shows the thermal effects (due to TPA and induced dynamic color center) in BBO degrade the power stability, as discussed in [25]. It should be mentioned that for a longer-term operation of the DUV laser at 20 W average power, it usually needs the human intervention to keep the average power at 20 W by slightly adjusting the phase-matching angle of BBO with a time interval of 25-30 mins. The decrease of DUV output average power is not due to the BBO damage, but related to the thermal effects in BBO. A cooling system for the BBO crystal will be designed in future to overcome this issue.
4. Conclusions
We have demonstrated the frequency-quadrupling of a 1 µm, 1.2 ps, 10 kHz, Yb: YAG Innoslab solid-state laser using LBO and BBO as SHG and FHG crystals. Up to 108 W, 10.8 mJ, 515.4 nm green pulses with a pulse duration of 910 fs and 20 W, 2 mJ, 257.7 nm DUV pulses with a pulse duration of 665 fs are obtained. The DUV pulses have up to 3 GW peak power. This is highest peak-power kHz-rate all-solid-state DUV source driven at 1 µm to date, to the best of our knowledge. In future studies, the improvement of the average power and peak power of our DUV laser can be expected by further optimizing the thicknesses of LBO and BBO and the corresponding pump intensities on them.
Funding
Agency for Science, Technology and Research (1426500050, 1426500051, A1890b0049).
Disclosures
The authors declare no conflicts of interest.
References
1. G. Račiukaitis, M. Brikas, M. Gedvilas, and G. Darčianovas, “Patterning of ITO layer on glass with high repetition rate picosecond lasers,” J. Laser Micro/Nanoeng. 2(1), 1–6 (2007). [CrossRef]
2. E. Allaria, D. Castronovo, P. Cinquegrana, P. Craievich, M. Dal Forno, M. B. Danailov, G. D’Auria, A. Demidovich, G. De Ninno, S. Di Mitri, B. Diviacco, W. M. Fawley, M. Ferianis, E. Ferrari, L. Froehlich, G. Gaio, D. Gauthier, L. Giannessi, R. Ivanov, B. Mahieu, N. Mahne, I. Nikolov, F. Parmigiani, G. Penco, L. Raimondi, C. Scafuri, C. Serpico, P. Sigalotti, S. Spampinati, C. Spezzani, M. Svandrlik, C. Svetina, M. Trovo, M. Veronese, D. Zangrando, and M. Zangrando, “Two-stage seeded soft-X-ray free-electron laser,” Nat. Photonics 7(11), 913–918 (2013). [CrossRef]
3. G. Kurdi, K. Osvay, M. Csatári, I. N. Ross, and J. Klebniczki, “Optical parametric amplification of femtosecond ultraviolet laser pulses,” IEEE J. Sel. Top. Quantum Electron. 10(6), 1259–1267 (2004). [CrossRef]
4. D. Popmintchev, C. Hernández-García, F. Dollar, C. Mancuso, J. A. Pérez-Hernández, M.-C. Chen, A. Hankla, X. Gao, B. Shim, A. L. Gaeta, M. Tarazkar, D. A. Romanov, R. J. Levis, J. A. Gaffney, M. Foord, S. B. Libby, A. Jaron-Becker, A. Becker, L. Plaja, M. M. Murnane, H. C. Kapteyn, and T. Popmintchev, “Ultraviolet surprise: Efficient soft x-ray high-harmonic generation in multiply ionized plasmas,” Science 350(6265), 1225–1231 (2015). [CrossRef]
5. Q. Liu, X. P. Yan, X. Fu, M. Gong, and D. S. Wang, “High power all-solid-state fourth harmonic generation of 266 nm at the pulse repetition rate of 100 kHz,” Laser Phys. Lett. 6(3), 203–206 (2009). [CrossRef]
6. X. Délen, L. Deyra, A. Benoit, M. Hanna, F. Balembois, B. Cocquelin, D. Sangla, F. Salin, J. Didierjean, and P. Georges, “Hybrid master oscillator power amplifier high-power narrow-linewidth nanosecond laser source at 257 nm,” Opt. Lett. 38(6), 995–997 (2013). [CrossRef]
7. C.-L. Chang, P. Krogen, H. Liang, G. J. Stein, J. Moses, C.-J. Lai, J. P. Siqueira, L. E. Zapata, F. X. Kärtner, and K.-H. Hong, “Multi-mJ, kHz, ps deep-ultraviolet source,” Opt. Lett. 40(4), 665–668 (2015). [CrossRef]
8. L. Goldberg, B. Cole, C. McIntosh, V. King, A. D. Hays, and S. R. Chinn, “Narrow-band 1 W source at 257 nm using frequency quadrupled passively Q-switched Yb:YAG laser,” Opt. Express 24(15), 17397–17405 (2016). [CrossRef]
9. M. Müller, A. Klenke, T. Gottschall, R. Klas, C. Rothhardt, S. Demmler, J. Rothhardt, J. Limpert, and A. Tünnermann, “High-average-power femtosecond laser at 258 nm,” Opt. Lett. 42(14), 2826–2829 (2017). [CrossRef]
10. M. Nishioka, S. Fukumoto, F. Kawamura, M. Yoshimura, Y. Mori, and T. Sasaki, “Improvement of laser-induced damage tolerance in CsLiB6O10 for high-power UV laser source,” In Proceedings of the Lasers and Electro-Optics/Quantum Electronics and Laser Science Conference, Baltimore, MD, USA, 1–6 June 2003.
11. T. Kojima, S. Konno, S. Fujikawa, K. Yasui, K. Yoshizawa, Y. Mori, T. Sasaki, M. Tanaka, and Y. Okada, “20-W ultraviolet-beam generation by fourth-harmonic generation of an all-solid-state laser,” Opt. Lett. 25(1), 58–60 (2000). [CrossRef]
12. G. Wang, A. Geng, Y. Bo, H. Li, Z. Sun, Y. Bi, D. Cui, Z. Xu, X. Yuan, and X. Wang, “28.4 W 266 nm ultraviolet-beam generation by fourth-harmonic generation of an all-solid-state laser,” Opt. Commun. 259(2), 820–822 (2006). [CrossRef]
13. O. Novák, H. Turčičová, M. Smrž, T. Miura, A. Endo, and T. Mocek, “Picosecond green and deep ultraviolet pulses generated by a high-power 100 kHz thin-disk laser,” Opt. Lett. 41(22), 5210–5213 (2016). [CrossRef]
14. H. Turcicova, O. Novak, L. Roskot, M. Smrz, J. Muzik, M. Chyla, A. Endo, and T. Mocek, “New observations on DUV radiation at 257 nm and 206 nm produced by a picosecond diode pumped thin-disk laser,” Opt. Express 27(17), 24286–24299 (2019). [CrossRef]
15. H. Xuan, C. Qu, S. Ito, and Y. Kobayashi, “High-power and high-conversion efficiency deep ultraviolet (DUV) laser at 258 nm generation in the CsLiB6 O10 (CLBO) crystal with a beam quality of M2< 1.5,” Opt. Lett. 42(16), 3133–3136 (2017). [CrossRef]
16. L. Wang, N. Zhai, L. Liu, X. Wang, G. Wang, Y. Zhu, and C. Chen, “High-average-power 266 nm generation with a KBe2BO3 F2 prism-coupled device,” Opt. Express 22(22), 27086–27093 (2014). [CrossRef]
17. Q. Liu, X. Yan, M. Gong, H. Liu, G. Zhang, and N. Ye, “High-power 266 nm ultraviolet generation in yttrium aluminum borate,” Opt. Lett. 36(14), 2653–2655 (2011). [CrossRef]
18. Z. Fang, Z.-y. Hou, F. Yang, L.-j. Liu, X.-y. Wang, Z.-y. Xu, and C.-t. Chen, “High-efficiency UV generation at 266 nm in a new nonlinear optical crystal NaSr3Be3B3O9F4,” Opt. Express 25(22), 26500–26507 (2017). [CrossRef]
19. K. Kohno, Y. Orii, H. Sawada, D. Okuyama, K. Shibuya, S. Shimizu, M. Yoshimura, Y. Mori, J. Nishimae, and G. Okada, “High-power DUV picosecond pulse laser with a gain-switched-LD-seeded MOPA and large CLBO crystal,” Opt. Lett. 45(8), 2351–2354 (2020). [CrossRef]
20. M. Divall, K. Osvay, G. Kurdi, E. Divall, J. Klebniczki, J. Bohus, Á Péter, and K. Polgár, “Two-photon-absorption of frequency converter crystals at 248 nm,” Appl. Phys. B 81(8), 1123–1126 (2005). [CrossRef]
21. SNLO software: see the website https://www.as-photonics.com/snlo.
22. K.-H. Hong, C.-J. Lai, A. Siddiqui, and F. X. Kärtner, “130-W picosecond green laser based on a frequency-doubled hybrid cryogenic Yb: YAG amplifier,” Opt. Express 17(19), 16911–16919 (2009). [CrossRef]
23. D. C. Brown and J. W. Kuper, “Solid-state lasers: steady progress through the decades,” Opt. Photonics News 20(5), 36–41 (2009). [CrossRef]
24. S. Wu, G. A. Blake, S. Sun, and J. Ling, “A multicrystal harmonic generator that compensates for thermally induced phase mismatch,” Opt. Commun. 173(1-6), 371–376 (2000). [CrossRef]
25. S. C. Kumar, J. C. Casals, J. Wei, and M. Ebrahim-Zadeh, “High-power, high-repetition-rate performance characteristics of β-BaB2O4 for single-pass picosecond ultraviolet generation at 266 nm,” Opt. Express 23(21), 28091–28103 (2015). [CrossRef]
