Difference-frequency generation of ultrashort pulses in the mid-IR using Yb-fiber pump systems and AgGaSe2 Marcus Beutler,1 Ingo Rimke,1 Edlef Büttner,1 Paolo Farinello,2 Antonio Agnesi,2 Valeriy Badikov,3 Dmitrii Badikov,3 and Valentin Petrov4,* 1
APE Angewandte Physik und Elektronik GmbH, Haus N, Plauener Str. 163-165, D-13053, Berlin, Germany 2 University of Pavia, Laser Source Laboratory, via Ferrata 5, IT-27100 Pavia, Italy 3 High Technologies Laboratory, Kuban State University, 149 Stavropolskaya, 350040 Krasnodar, Russia 4 Max-Born-Institute for Nonlinear Optics and Ultrafast Spectroscopy, 2A Max-Born-Str., D-12489 Berlin, Germany *
[email protected]
Abstract: We employ AgGaSe2 for difference-frequency generation between signal and idler of synchronously-pumped picosecond / femtosecond OPOs at 80 / 53 MHz. Continuous tuning in the picosecond regime is achieved from 5 to 18 µm with average power of 140 mW at 6 µm. In the femtosecond regime the tunability extends from 5 to 17 µm with average power of 69 mW at 6 µm. Maximum single pulse energies of >1 nJ in both cases represent the highest values at such high repetition rates. ©2015 Optical Society of America OCIS codes: (190.2620) Harmonic generation and mixing; (160.4330) Nonlinear optical materials.
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M. H. Dunn and M. Ebrahimzadeh, “Parametric generation of tunable light from continuous-wave to femtosecond pulses,” Science 286(5444), 1513–1517 (1999). 2. V. Petrov, “Parametric down-conversion devices: The coverage of the mid-infrared spectral range by solid-state laser sources,” Opt. Mater. 34(3), 536–554 (2012). 3. V. Petrov, “Frequency down-conversion of solid-state laser sources to the mid-infrared spectral range using nonoxide nonlinear crystals,” Prog. Quantum Electron. in press. 4. J. M. Fraser, D. Wang, A. Haché, G. R. Allan, and H. M. van Driel, “Generation of high-repetition-rate femtosecond pulses from 8 to 18 µm,” Appl. Opt. 36(21), 5044–5047 (1997). 5. R. Hegenbarth, A. Steinmann, S. Sarkisov, and H. Giessen, “Milliwatt-level mid-infrared (10.5-16.5 μm) difference frequency generation with a femtosecond dual-signal-wavelength optical parametric oscillator,” Opt. Lett. 37(17), 3513–3515 (2012). 6. A. Harasaki and K. Kato, “New data on the nonlinear optical constant, phase-matching, and optical damage of AgGaS2,” Jpn. J. Appl. Phys. 36(1), 700–703 (1997). 7. M. Beutler, I. Rimke, E. Büttner, V. Petrov, and L. Isaenko, “Difference-frequency generation of fs and ps midIR pulses in LiInSe2 based on Yb-fiber laser pump sources,” Opt. Lett. 39(15), 4353–4355 (2014). 8. Ch. Grässer, S. Marzenell, J. Dörring, R. Beigang, and R. Wallenstein, “Continuous-wave mode-locked operation of a picosecond AgGaSe2 optical parametric oscillator in the mid infrared,” OSA TOPS on Advanced Solid-State Lasers (1996), Vol. 1, S. A. Payne and C. Pollock (eds), OSA, 1996, paper OP6, pp. 158–163. 9. S. Marzenell, R. Beigang, and R. Wallenstein, “Synchronously pumped femtosecond optical parametric oscillator based on AgGaSe2 tunable from 2 µm to 8 µm,” Appl. Phys. B 69(5-6), 423–428 (1999). 10. R. Hegenbarth, A. Steinmann, S. Mastel, S. Amarie, A. J. Huber, R. Hillenbrand, S. Y. Sarkisov, and H. Giessen, “High-power femtosecond mid-IR sources for s-SNOM applications,” J. Opt. 16(9), 094003 (2014).
1. Introduction Picosecond and femtosecond synchronously-pumped optical parametric oscillators (SPOPOs) based on oxide nonlinear crystals pumped by mode-locked Ti:Sapphire lasers represent a unique class of ultrafast coherent sources operating at high (50-200 MHz) repetition rates, primarily in the near-IR part of the spectrum [1]. SPOPOs usually rely on uncritical phasematching in periodically-poled ferroelectric oxide crystals such as lithium niobate or lithium tantalate setting an upper limit for the idler wavelength of 4-5 µm in the mid-IR. Their
#230801 - $15.00 USD © 2015 OSA
Received 16 Dec 2014; revised 21 Jan 2015; accepted 22 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002730 | OPTICS EXPRESS 2730
wavelength coverage can be extended into the mid-IR by difference-frequency generation (DFG) with convenient tuning possible thanks to the simultaneous variation of signal and idler wavelengths. For wavelengths exceeding 4-5 µm, non-oxide nonlinear crystals have to be employed in the DFG stage which exhibit smaller band-gap and two-photon absorption (TPA) limitations will come into play at tight focusing [2]. AgGaS2 (AGS), AgGaSe2 (AGSe), GaSe, GaS0.4Se0.6, and LiInSe2 (LISe) have been employed for DFG with such 800nm pumped SPOPOs but operation was in all cases in the femtosecond regime, to obtain reasonable conversion efficiency [3]. The recent development of diode-pumped mode-locked solid-state and fiber lasers based on the Yb3+ ion provides new, more stable and cost-effective pump sources for picosecond and femtosecond SPOPOs. In contrast to Ti:Sapphire laser pumping such technology is scalable in average power. From optical damage considerations, it is more suitable for pumping SPOPOs built with oxide materials which then emit at longer wavelengths (degeneracy point around 2 µm) and are compatible with DFG schemes based on nonlinear crystals possessing higher nonlinearity and extended mid-IR transparency. AGSe is such a nonlinear crystal and its application in DFG schemes based on ~800-nm Ti:Sapphire laser sources was indeed limited to long wavelengths due to birefringence and TPA restrictions: Mixing the signal and idler of a femtosecond SPOPO, tuning from 8 to 18 µm was achieved using AGSe but the maximum energy was only 12 fJ at 84 MHz [4]. In [5] a diode-pumped mode-locked at 42 MHz Yb:KGd(WO4)2 laser was used to pump a dual wavelength SPOPO and DFG between the two signal outputs was studied in GaSe and AGSe crystals. High conversion was achieved with AGSe in a narrow spectral range with maximum average power of 4.3 mW at 13.2 µm. All-fiber systems as ultrafast pump sources, however, have the advantage that they can be power scaled by Yb-fiber amplifiers, maintaining the high repetition rate. Here we study such a DFG scheme based on AGSe, mixing the signal and idler from a SPOPO pumped by picosecond / femtosecond Yb-fiber based sources emitting near 1 µm and demonstrate unprecedented average powers and single pulse energies in the mid-IR. 2. Relevant properties of AGSe The commercially available AGSe is attractive for DFG with low energy tightly focused beams because of its high nonlinear coefficient (d36~35 pm/V for frequency doubling of 5.3 µm radiation), small spatial walk-off and broad transparency extending from ~0.78 up to ~18 µm. The band-gap of this chalcopyrite crystal is 1.83 eV which means that no TPA should be observed above 1355 nm. With an index of refraction of n~2.6 a typical figure of merit (d2/n3) is ~70 pm2/V2, roughly 6 times higher compared to its sulphide analogue AGS. The advantage of AGSe against GaSe which exhibits a similar transparency window but higher nonlinearity is the much lower spatial walk-off, the capability of directed growth and cutting at the desired orientation, as well as the availability of antireflection coatings.
Fig. 1. (a) Internal phase-matching angle θ (black) and spatial walk-off angles ρ1,3 (red) for type-I (solid lines) and type-II (dashed lines) DFG in AGSe. (b) GVM parameters mixing signal and idler pulses from a 1034 nm pumped SPOPO. The indices 1,2,3 denote DFG, idler, and signal pulses. Sellmeier equations used are from [6].
#230801 - $15.00 USD © 2015 OSA
Received 16 Dec 2014; revised 21 Jan 2015; accepted 22 Jan 2015; published 30 Jan 2015 9 Feb 2015 | Vol. 23, No. 3 | DOI:10.1364/OE.23.002730 | OPTICS EXPRESS 2731
Type-II eo-e phase-matching exhibits higher effective nonlinearity for DFG wavelengths exceeding 8 µm (deff~d36sinθ for type-I while deff~d36sin2θ for type-II). However, for shorter DFG wavelengths, deff approaches zero and there is no phase-matching below ~6 µm, see Fig. 1(a). For type-I eo-o phase-matching, the critical angle θ varies from 44° to 60° in the 5-18 µm DFG tuning range while deff changes from ~70% of its maximum value (~d36) near 18 µm to about 87% near 5 µm. In AGSe, the birefringence walk-off angle ρ3 remains between 9.8 and 11.8 mrad in type-I phase-matching, see Fig. 1(a), while ρ1 = 0. The spatial walk-off is similar in type-II phasematching except at short DFG wavelengths (
