Controlling the phase matching conditions of optical parametric chirped-pulse amplification using partially deuterated KDP

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1 Controlling the phase matching conditions of optical parametric chirped-pulse amplification using partially deuterated KDP K. Ogawa 1,3, K. Sueda 2,3, Y. Akahane 1,3, M. Aoyama 1,3, K. Tsuji 1, K. Fujioka 2, T. Kanabe 4, K. Yamakawa 1,3, and N. Miyanaga 2,3. 1 Japan Atomic Energy Agency, 8-1 Umemidai, Kizugawa, Kyoto , Japan. 2 Institute of Laser Engineering Osaka University (ILE Osaka) 2-6 Yamadaoka, Suita, Osaka , Japan 3 CREST, Japan Science and Technology Agency, Sanbancho 5, Chiyoda-ku, Tokyo , Japan 4 Graduate School of Engineering, University of Fukui, 3-9-1, Bunkyou, Fukui , Japan * Corresponding author: ogawa.kanade@jaea.go.jp Abstract: Using a partially deuterated KDP crystal for an optical parametric amplifier, we demonstrated ultrabroadband optical parametric chirped-pulse amplification of more than 250 nm bandwidth at a center wavelength of 1050 nm. We numerically show how to control the broadband phase matching conditions at different wavelengths to match center wavelengths of suitable broadband seed sources by adjusting the deuteration level in partially deuterated KDP Optical Society of America OCIS codes: ( ) Ultrafast lasers; ( ) Parametric oscillators and amplifiers; ( ) Nonlinear optics, parametric processes. References and links 1. A. Dubietis, G. Jonusauskas, and A. Piskarskas, "Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal," Opt. Commun. 88, (1992). 2. I. N. Ross, P. Matousek, M. Towrie, A. J. Langley, and J. L. Collier, "The prospects for ultrashort pulse duration and ultrahigh intensity using optical parametric chirped pulse amplifiers," Opt. Commun. 144, (1997). 3. X. Yang, Z.h. Xu, Y.-x. Leng, H.-h. Lu, L.-h. Lin, Z.-q. Zhang, R.-x. Li, W.-q. Zhang, D.-j. Yin, and, B. Tang, "Multiterawatt laser system based on optical parametric chirped pulse amplification," Opt. Lett. 27, (2002). 4. O. V. Chekhlov, J. L. Collier, I. N. Ross, P. K. Bates, M. Notley, C. Hernandez-Gomez, W. Shaikh, C. N. Danson, D. Neely, P. Matousek, and S. Hancock, "35 J broadband femtosecond optical parametric chirped pulse amplification system," Opt. Lett. 31, (2006). 5. I. N. Ross, P. Matousek, G. H. C. New, and K. Osvay, "Analysis and optimization of optical parametric chirped pulse amplification," J. Opt. Soc. Am B 19, (2002). 6. Y. Tang, I. N. Ross, C. Hernandez-Gomez, I. O. Musgrave, J. L. Collier, O. Chekhlov, and P. Matousek, "Novel Ultra-fast broadband laser source at 910nm for Vulcan 10 PW OPCPA laser system," Central Laser Facility Annual Report , N. Ishii, L. Turi, V.S. Yakovlev, T. Fuji, F. Krausz, A. Baltuska, R. Butkus, G. Geitas, V. Smilgevicius, R. Danielius, and A. Piskarskas, "Multimillijoule chirped parametric amplification of few-cycle pulses," Opt. Lett. 30, (2005). 8. S. Witte, R. Th. Zinkstok, W. Hogervorst, and K. S. E. Eikema, "Generation of few-cycle terawatt light pulses using optical parametric chirped pulse amplification," Opt. Express 13, (2005). 9. A. Shirakawa, I. Sakane, M. Takasaka, and T. Kobayashi, "Sub-5-fs visible pulse generation by pulse-frontmatched noncollinear optical parametric amplification," Appl. Phys. Lett. 74, (1999). 10. H. Yoshida, E. Ishii, R. Kodama, H. Fujita, Y. Kitagawa, Y. Izawa, and T. Yamanaka, High-power and high-contrast optical parametric chirped pulse amplification in beta-bab2o4 crystal, Opt. Lett. 28, (2003). 11. N. P. Zaitseva, J. J. De Yoreo, M. R. Dehaven, R. L. Vital, K. E. Montgomery, M. Richardson, and L. J. Atherton, "Rapid growth of large-scale (40-55 cm) KH 2 PO 4 crystals," J. Crystal Growth 180, (1997). 12. K. Fujioka, S. Matsuo, T. Kanabe, H. Fujita, and M. Nakatsuka, "Optical Properties of Rapidly Grown KDP Crystal Improved by Thermal Conditioning," J. Crystal Growth 181, (1997). (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7744

2 13. M. Nakatsuka, K. Fujioka, T. Kanabe, and H. Fujita, "Rapid Growth over 50 mm/day of Water-soluble KDP Crystal," J. Crystal Growth 181, (1997). 14. V. V. Lozhkarev, G. I. Freidman, V. N. Ginzburg, E. V. Katin, E. A. Khazanov, A. V. Kirsanov, G. A. Luchinin, A. N. Mal'shakov, M. A. Martyanov, O. V. Palashov, A. K. Poteomki, A. M. Sergeev, A. A. Shaykin, I. V. Yakovlev, S. G. Garanin, S. A. Sukharev, N. N. Rukavishnikov, A. V. Charukhchev, R. R. Gerke, and V. E. Yashin, "200 TW 45 fs laser based on optical parametric chirped pulse amplification," Opt. Express 14, (2006). 15. J. A. Armstrong, N. Bloembergen, J. Ducuing, and P. S. Pershan, "Interactions between light waves in a nonlinear dielectric," Phys. Rev. 127, (1962). 16. M. S. Webb, D. Eimerl, and S. P. Velsko, "Wavelength insensitive phase-matched second-harmonic generation in partially deuterated KDP," J. Opt. Soc. Am. B 9, 1118 (1992). 17. H. Zhu, T. Wang, W. Zheng, P. Yuan, L. Qian, and D. Fan, "Efficient second harmonic generation of femtosecond laser at one micron," Opt. Express 12, (2004). 18. K. Yamakawa, M. Aoyama, Y. Akahane, K. Ogawa, K. Tsuji, A. Sugiyama, T. Harimoto, J. Kawanaka, H. Nishioka, and M. Fujita, "Ultra-broadband optical parametric chirped-pulse amplification using an Yb: LiYF4 chirped-pulse amplification pump laser," Opt. Express 15, (2007). 19. J. Kawanaka, K. Yamakawa, H. Nishioka, and K.-I. Ueda, 30-mJ, diode-pumped, chirped-pulse Yb:YLF regenerative amplifier, Opt. Lett. 28, (2003). 1. Introduction Optical parametric chirped pulse amplification (OPCPA) is now recognized as a key technique to generate high peak power ultra short laser pulses [1, 2]. Multiterawatt OPCPA systems with a pulse duration of ~100 fs were developed [3, 4]. More recently, an ultrahigh peak power OPCPA system with a peak power of more than 1 PW has been designed [5] and under construction [6]. On the other hands, ultrabroadband OPCPA systems have been developed for the generation of few-cycle laser pulses in small-scale laboratories [7]. Urtlabroad bandwidth optical parametric amplification (OPA) is achieved from 700 nm to 1000 nm in wavelength using a β-bab 2 O 4 (BBO) crystal [8]. Using a pump wavelength of 400 nm, i.e. the second harmonic of the Ti:sapphire laser, and a non-collinear angle α of 3.7, one can observe a magic phase-matching condition at an internal signal angle of ~ It has also a large nonlinear coefficient, high damage threshold, and low absorption. Therefore it is commonly used for ultrabroadband OPA [9] or OPCPA front-end of large-scale Nd:glass laser facilities [10]. Because it is difficult to grow large-aperture BBO crystals, however, maximum output pulse energy is limited. In order to produce a peak power of more than terawatts, hybrid OPCPA design is employed, where high gain OPA such as BBO or LiB 3 O 5 (LBO) crystals are used for preamplification and large size but lower gain OPA such as a Potassium dihydrogen phosphate (KDP) crystal is used as a final amplifier [3-5]. A large-aperture KDP crystal was mainly developed for the frequency conversion of inertial confinement fusion Nd:glass laser drivers [11]. KDP and DKDP crystals of various habits were grown at rates of mm/day [11-13]. Therefore it is suitable for high energy OPCPA. Yang et. al. have developed the system using the KDP crystal which has peak intensity of 3.67 TW and pulse duration of 155 fs in a collinear geometry [3]. In the system, laser pulses with 900 mj pulse energy and 8 nm bandwidth were obtained in 1.2 cm pump diameter in the KDP crystal at a wavelength of 1066 nm. However, amplification bandwidth of KDP in a collinear condition covers roughly 100 nm. Therefore, it is difficult to generate few-cycle laser pulses. In addition, the optimum phase matching wavelength is different between BBO and KDP, i.e. 800 nm and 1000 nm, KDP is unavailable for final amplifier of the high energy, broadband OPCPA system at 800 nm. Meanwhile, broad amplification bandwidth at a center wavelength of 900 nm is obtained with KD 2 PO 4 (DKDP) crystals in a non-collinear geometry with a crossing angle of 0.9 degree at 527 nm pump wavelength. Lozhkarev et. al. have demonstrated a generation of a laser pulse of a peak power of 200 TW using the DKDP crystals with shorter pulse duration of 45 fs than that of the KDP crystal [14]. However, a suitable broadband oscillator as a seed source is unavailable at this wavelength. In addition, it is impossible to obtain an ultrabroad amplification bandwidth more than 200 nm corresponding to the pulse duration of around 10 fs. (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7745

3 Here we propose a new scheme where the center wavelengths are adjusted to those of BBO crystal-based preamplifier by changing the deuteration level of KDP crystal. In the OPCPA process, the relation between wave number vectors k p, k s, and k i of the pump, signal and idler is as follows [2, 15]: k p = k s + k i. (1) It has previously been reported that type-i frequency doubling was obtained with the spectrally noncritical phase-matching in partially deuterated KDP [16, 17]. The refractive index of partially deuterated DKDP is as a function of deuteration level (α) follows: n 2 (α) = αn 2 d + (1-α)n 2 h, (2) where n h and n d are the refractive index of KDP and pure DKDP. This equation shows that phase matching condition can be adjusted by changing the deuteration level of KDP. First, we numerically perform the calculations of phase matching the OPA using KDP, DKDP and partially 13% deuterated KDP as shown in Fig. 1 as a function of signal wavelength. In this calculation we assume a pump pulse wavelength of 510 nm that is corresponding to a second harmonic of the Yb:YLF laser [18]. As seen in Fig. 1, broader amplification bandwidth can be obtained with 13% DKDP than those of KDP and DKDP. In this deuteration level, the partially deuterated KDP is suitable for OPCPA seeded by the Yb- or Nd-doped solid-state lasers around a center wavelength of 1 µm. Based on the result, an ultrabroadband OPCPA using the partial dueterated KDP was experimentally studied. Ultrabroad amplification bandwidth of more than 250 nm at a center wavelength of 1050 nm was achieved with the 13% DKDP crystal. In addition, an optimal deuteration level where broader amplification bandwidth at a center wavelength of around 800 nm was numerically investigated in order to achieve high energy few-cycle optical pulses combined with the ultrabroadband Ti:sapphire seed source. 43 Phase matching angle [deg.] KDP 13%DKDP DKDP Fig. 1. Calculated phase matching curves for 13% DKDP, KDP, and DKDP. (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7746

4 Yb:YLF Regen. Yb CPA Stretcher (SM Fiber) Yb CPA Compressor Mode locked Ti:sapphire Oscillator ~ 1020m SHG crystal (Type-I BBO) Beam dump 510nm PCF Delay OPA Crystal 13% DKDP Fig. 2. Experimental setup of optical parametric chirped-pulse amplification with 13% DKDP. PCF: photonic crystal fiber, SMF: single-mode fiber, SHG: second-harmonic generation. 2. Experiments Figure 2 shows the experimental setup of ultrabroadband OPCPA at degeneracy using the 13% DKDP crystal. A seed pulse from Mode-locked Ti:Sapphire oscillator having a 400 mw average power, 80 MHz repetition rate and 1020 nm center wavelength was split into two beams, which were used as signal pulse and seed pulse for pump laser, respectively. One from the oscillator was converted into the white light continuum (WLC) through a photonic crystal fiber (PCF) and temporally stretched with 10.5 cm length glass block to be used as a signal pulse of OPCPA. Another was a pump pulse temporally stretched with a 1.2 km long, polarization-maintained single-mode fiber from 80 fs to ~1 ns and then amplified in the cryogenically-cooled, diode-pumped Yb:YLF regenerative amplifier operating at a 10 Hz repetition rate. The regenerative amplifier is similar to ones presented previously [19]. The laser crystal was 20 at.% Yb:YLF with a thickness of 2 mm and a 5 mm x 5 mm cross section. A fiber-coupled laser diode beam with an emission wavelength of 940 nm was focused to 800 µm diameter on the crystal. A maximum output pulse energy of up to 15.5 mj was obtained at a LD pump power of 116 W with a pulse duration of 4 ms. The amplified chirped pulse was then compressed by two parallel, gold-coated, 1100 grooves/mm, ruled gratings. The duration of the compressed pulse was measured to be 3.7 ps using a scanning second harmonic generation autocorrelator. A fraction of the compressed pulse was down-collimated to a 3 mm diameter by a Galilean telescope. The pulse was then frequency doubled in a 7-mm long, type-i BBO crystal for pumping the parametric amplifier. An output pulse energy of the frequency-doubled pump pulse was measured to be 3.3 mj at a fundamental laser intensity of 36 GW/cm 2 which corresponded to an energy conversion efficiency of ~ 35 %. A duration of the pump pulse was estimated to be 2.6 ps assuming a square root of one half of fundamental pulse. The 15mm thick 13% DKDP crystal is used in a noncollinear geometry with an internal crossing angle between the seed and pump pulses of 0.67º. The seed pulse was loosely focused to 1.1 mm x 1.0 mm in diameter. The OPA pump pulse was down-collimated to a 1.0-mm diameter. Figure 3 shows measured OPA gain as a function of pump intensity. Maximum OPA gain of 2 x 10 4 was obtained with pump intensities of over 60 GW/cm 2. Figure 4 shows measured OPA spectrum. As seen in Fig. 4, amplification bandwidth from 900 nm to 1200 nm was obtained. The blue line in Fig. 4 indicates the calculated gain spectrum at a same pump intensity as experiment. The amplification bandwidth of 270 nm full-width at halfmaximum (FWHM) can be obtained. The experimental result agrees with the calculation one (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7747

5 Gain Pump intensity [GW/cm 2 ] Fig. 3. Single-pass OPA gain as a function of pump intensity 20 4x10 6 Intensity [A.U.] x10 6 2x10 6 1x10 6 Gain Fig. 4. Measured amplified spectrum (solid line) and oscillator background (dotted line). Calculated amplified spectrum (blue line) is also shown. expecting the behavior of amplified spectrum beyond the wavelength of 1200 nm. It is due to strong absorption of the DKDP crystal above 1200 nm in wavelength. Finally, we show how to control the broadband phase matching conditions at different wavelengths to match center wavelengths of suitable broadband seed sources, such as Ti:sapphire and Yb:glass by adjusting the deuteration level in partially deuterated KDP. Figure 5 shows phase matching curves with various deuteration levels at 510 nm pump wavelength, i.e. the second harmonic of the Yb:YLF laser, and with optimized noncollinear angle. As seen in Fig. 5, broad amplification bandwidth of 320 nm FWHM can be obtained with the 80% deuterated KDP crystal, corresponding to a calculated transform-limited pulse duration of less than 4 fs. It is suitable for amplifying ultrafast Ti:Sapphire seed pulse without reducing its bandwidth. Since it is comparable to that of BBO, the large aperture 80% deuterated KDP crystal can be used as a boost amplifier follow by the series of BBO preamplifiers in the large-scale, high energy OPCPA systems. By further optimizing the deuteration level of DKDP, ultrabroad bandwidth more than 400 nm can be also amplified using 50% deuterated KDP crystal in the 1µm region. It is suitable to amplify the seed pulse from the Yb-doped laser oscillator. Furthermore broadband amplification with different pump wavelengths is also available by adjusting the deuteration level in DKDP. Figure 6 shows phase matching curves optimized for 527 nm pumping. This wavelength is corresponds to the second harmonics of Nd:glass and Nd:YLF lasers, which is generally used as pump lasers in high power OPCPA systems. (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7748

6 Phase matching angle [deg.] Deuteration Crossing level angle 40% % % % % % 1.0 DKDP Gain Fig. 5. Calculated phase matching curves (black lines) with various deuteration levels of DKDP with 510nm pumping. Calculated amplified spectrum with the 80% 42 Phase matching angle [deg.] Deuteration Crossing level angle 60% % % % 0.6 DKDP Fig. 6. Calculated phase matching curves with various deuteration levels of DKDP with 527 nm pumping. As seen in Fig. 6, it is possible to obtain the broadband amplified bandwidth over 400 nm centered at 1µm with KDP crystals from 60% to 80% deuterated level. From these results, we have shown that bandwidth broadening on any high power OPCPA systems is possible by adjusting the deuterated level in KDP crystal with different signal and pump wavelengths. 3. Conclusion Using optimal deuterated KDP, we show that ultrabroad OPA bandwidth can be obtained. In the result of an OPCPA experiment with 13% DKDP, ultrabroad amplification bandwidth of more than 250 nm centered at 1µm wavelength has been achieved. We have found by numerical calculation that OPA bandwidth and center wavelength can be optimized as the high-energy booster amplifier of existing ultrafast OPCPA systems by adjusting deuterated level of KDP. These results are considered to be essential features for the development of high-intensity few-cycle and high-energy ultrafast laser systems in the future. Acknowledgments One of the authors (K.Y.) is indebted to the Japan Society for the Promotion of Science (JSPS) for a financial support through a Grant-in-Aid for Scientific Research (B), No (C) 2009 OSA 11 May 2009 / Vol. 17, No. 10 / OPTICS EXPRESS 7749