The Petawatt Field Synthesizer: A new Approach to Ultrahigh Field ...

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The Petawatt Field Synthesizer: A new Approach to Ultrahigh Field Generation ¨ op1,2 , Izhar Ahmad1 , Tie-Jun Wang1 , Stefan Karsch1 , Zsuzsanna Major1 , József Ful¨ 1,2 1 Andreas Henig , Sebastian Kruber , Raphael Weingartner1,2 , Mathias Siebold1 , Joachim Hein3 , Christoph Wandt1 , Sandro Klingebiel1 , Jens Osterhoff1 , Rainer Hörlein1,2 , Ferenc Krausz1,2 1 2

Max-Planck-Institut für Quantenoptik, Hans-Kopfermann-Str. 1, D-85748 Garching, Germany Ludwig-Maximilians-Universität München, Am Coulombwall 1, D-85748 Garching, Germany 3 Friedrich-Schiller-Universit¨ at Jena, Max-Wien-Platz 1, D-07743 Jena, Germany [email protected]

Abstract: The Petawatt Field Synthesizer (PFS) at MPQ will deliver few-cycle pulses at Petawatt power. Short-pulse OPCPA and a diode-pumped, CPA Yb:YAG pump laser are key technologies, and results of the ongoing development will be presented. © 2007 Optical Society of America OCIS codes: 140.7090, 140.3280

1.

Introduction

The Petawatt Field Synthesizer (PFS), currently under construction at the Max-Planck-Institut für Quantenoptik (Garching, Germany), represents a project for developing a light source for a broad variety of fundamental physics. The PFS light-source is expected to deliver wave-form controlled, few-cycle laser pulses with PW-scale peak power. It will support the generation of single intense attosecond extreme ultraviolet (XUV) pulses by relativistically driven high-harmonic generation on a solid surface. A second key application is driving a stable laser-wakefield accelerator in the “bubble” regime for GeV-scale, monoenergetic electron pulses. Laser pulses with the unique PFS parameters are expected to yield stable electron pulses as well as to increase the accelerated charge. These pulses may then be used for seeding a table-top X-ray free-electron laser [1]. Long-term prospects range from advances in attosecond science over material science and biology, to nonlinear nuclear physics and fundamental high-field interactions. 2.

Concept and goals

The Petawatt Field Synthesizer light source is designed to deliver phase stabilized few-cycle (∼ 5 fs) laser pulses in the wavelength band between 800 and 1600 nm with an energy of > 3 J and a repetition rate of 10 Hz. The focussed intensity should reach or exceed 1022 W/cm2 . In order to achieve these ambitious goals, the PFS design is based on a modified optical parametric chirped pulse amplification (OPCPA) scheme [2]. Traditional OPCPA systems operating with ∼100 ps - ns pulses have already demonstrated to deliver pulse energies as high as 35 J in 84-fs pulses [4] as well as 90 mJ in the few-cycle regime (10 fs) [5]. However, the generation of Joule-scale pulse energies in the few-cycle regime requires a modified approach and has yet to be demonstrated. This is the main aim of the PFS project. In [4], large, relatively narrowband DKDP crystals are used to generate high energy pulses with longer duration, whereas in [5] large bandwidth is achieved through the choice of BBO as the nonlinear crystal. Since BBO or LBO are only available in insufficient size for our goals, in the PFS design an alternative approach is applied. Here, short, high-intensity pulses are used to pump the OPA stages, i.e. on the order of ∼ 1 ps. In this case, the large bandwidth can be obtained from ordinary DKDP crystals with small thickness, while the gain is achieved through high intensity, although in our design we stay safely below the threshold for nonlinear propagation and damage. Our modelling suggests that this is the only viable approach for petawatt-scale few-cycle pulse generation, and, moreover, affords several key advantages for high-quality pulse generation. Firstly, the short pulse duration reduces the stretching factor, leading to high stretching and compression fidelity and allowing the use of simple, high-throughput stretchercompressor systems, such as bulk glass or chirped mirrors for. Secondly, the short pump-pulse duration results in a dramatic increase of of pulse contrast due to the sub-ps time-window for parametric fluorescence.


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Compared to other CPA or OPCPA systems, the short-pulse pumped OPCPA in the PFS project requires a sophisticated pump source, delivering 1 ps pulses with 15-20 J (frequency doubled) pulse energy, i.e. ∼ 50 J in the fundamental beam, at 10 Hz repetition rate, which is not commercially available and therefore a challenge for development on its own right. The system will be based on the CPA principle using conventional laser amplification. For reasons of achieving suitable bandwidth for 1 ps operation, efficient heat conduction and efficient diode pumping, Yb:YAG in slab geometry was chosen as the gain material.

Fig. 1. Schematic layout of the PFS system. The planned PFS system layout is shown in Fig. 1. For accurate synchronization of the pump and the seed beam, both beams are derived from the same oscillator. Currently the development is concentrating on the seed generation for both the OPA chain and the pump laser chain, the development of the diode-pumped Yb:YAG pump source, and testing the novel OPCPA concept. Upon completion of a first pump stage, the baseline OPA design will be fixed. The system is expected to start operation in 2010. 3.

Status

A “Femtopower Compact Pro” system is used as a high-energy “master-oscillator” to provide the seed pulse for both the pump and OPCPA chains. The output is a 20 fs, 1.1 mJ pulse with ∼ 65 nm bandwidth around 790 nm. This pulse is spectrally broadened in a hollow fiber to range from 500 nm to 1000 nm. For seeding the main OPCPA chain, the central wavelength of this pulse has to be shifted to 1100 nm, which in the near future will be achieved by idler generation in a first OPA stage pumped by 400 nm. The pump laser is seeded by a fraction of the Femtopower’s oscillator output, which is shifted to the required wavelength (i.e. 1030 nm) in a photonic crystal fiber (PCF). These seed pulses will first be amplified in a Yb:glass fiber before stretching them to 2 ns and injecting them into a regenerative amplifier delivering few-mJ energy. Currently, in the absence of the fiber amplifier and stretcher, the regenerative amplifier is used as a mJ-oscillator in 6 ns-pulses in order to assess the gain properties in the subsequent amplifier chain. In this test mode, a four-pass and a two-pass Yb:YAG amplification stage boost the energy to the 500 mJ level. Following this the pulses have been successfully amplified to the 1.7-J level using a first two-pass diode-pumped Yb:YAG stage with slab geometry as described in [6]. The energy is currently limited by damage issues arising from growth inhomogeneties of the crystal, but up to 7 J in ms-pulses were extracted from the same amplifier in four passes. The thermal lensing of the crystal was compensated, allowing for 10 Hz operation. The 50 J stage will be a scaled-up version of the 5 J stage. In order to validate the concept of short-pulse OPCPA for energetic pulses, a test experiment was fielded using the ATLAS Ti:Sapphire laser (delivering up to 1 J in 42 fs) as both pump and seed laser. A part of the ATLAS pulse created a white-light continuum in an argon gas cell as a seed pulse. Another part was apodized, spatially filtered and frequency-doubled to provide up to 2 mJ pump energy for an OPCPA stage using between 0.5 mm and 2 mm thick BBO crystals. This system was used to verify our model on short-pulse OPCPA and to test various aspects of the process. Firstly, even after propagating through 20 m of air separately, the timing jitter of the two sub-100 fs pulses was low enough to achieve stable OPCPA gain over a long time. Secondly, noncollinear OPCPA with short pulses requires a matching of the pulse fronts to achieve full spatio-temporal overlap of both beams. This was achived by a pair of detuned transmission gratings in the pump beam, which were image-relayed into the BBO crystal plane (Fig 2 (a)). Thirdly, we could verify the amplification bandwidth and dynamics in both the unsaturated and saturated cases as predicted by the modelling. A substantial effective broadening of the input signal spectrum through the adjustable gain curve allowed compression of the amplified pulses to 8.6 fs (Fig 2 (b)–(e)). Finally, no detrimental influence of the amplification process on the focusability of the beam was observed. For more details, see [7].


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Fig. 2. OPCPA test experiment using ATLAS (cf. [7]). (a) Pulse-front tilt matching of pump and signal pulses. Maximum gain bandwidth (b) and corresponding transform limited pulse duration (c). Bandwidth supported by our chirped mirror compressor (d) along with pulse reconstructed from SPIDER measurements (e).

4.

Outlook

In the near future the 5 J pump source will be seeded from suitable pulses derived from the mode-locked master oscillator. The generation of this seed pulse requires stretching to ∼ 2 ns in a grating stretcher which is currently under construction. After proving the operation of the diode-pumped amplifier chain at the 5 J level, compression to the 1 ps range will be de a grating compressor. Further amplification will follow to 4×12 J energy in the fundamental beam which eventually will be frequency doubled to provide the pump beam for the OPA stages. After a near-future upgrade of the Femtopower Compact Pro system to an output power of 2.5 mJ the frequency shifting of the seed pulse will be accomplished in a noncollinear OPA stage pumped by the second harmonic of a fraction of the Femtopower output pulse. First OPCPA experiments in the PFS parameter regime will commence after the availability of the 5J pump stage and the seed. References 1. F. Gruener, S. Becker, U. Schramm, M. Fuchs, R. Weingartner, D. Habs, J. Meyer-ter-Vehn, M. Geissler, M. Ferrario, L. Serafini, B. Van der Geer, H. Backe, W, Lauth, and S. Reiche, “Design considerations for table-top, laser-based VUV and X-ray free electron lasers”, Appl. Phys. B 86, 431 (2007). 2. A. Dubietis, G. Jonusauskas, and A. Piskarskas, “Powerful femtosecond pulse generation by chirped and stretched pulse parametric amplification in BBO crystal”, Opt. Commun. 88, 437 (1992). 3. D. Strickland and G. Mourou, “Compression of amplified of amplified chirped optical pulses”, Opt. Commun. 56, 219 (1985). 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, S. Hancock, and L. Cardoso, “35 J broadband femtosecond optical parametric chirped pulse amplification system” Opt. Lett. 31, 3665 (2006). 5. F. Tavella, A. Marcinkevicius, and F. Krausz, “90 mJ parametric chirped pulse amplification of 10 fs pulses”, Opt. Expr. 14, 12822 (2006). 6. M. Siebold et al., to be published 7. J. A. Fülöp, Zs. Major, A. Henig, S. Kruber, R. Weingartner, T. Clausnitzer, E.-B. Kley, A. Tünnermann, J. Osterhoff, R. Hörlein, F. Krausz, and S. Karsch, submitted to New. J. Phys. (2007).