Appl Phys B (2010) 101: 93–102 DOI 10.1007/s00340-010-3952-7
Temporal pulse control of a multi-10 TW diode-pumped Yb:Glass laser M. Hornung · R. Bödefeld · M. Siebold · A. Kessler · M. Schnepp · R. Wachs · A. Sävert · S. Podleska · S. Keppler · J. Hein · M.C. Kaluza
Received: 7 January 2010 / Published online: 7 March 2010 © Springer-Verlag 2010
Abstract At the Institute of Optics and Quantum Electronics in Jena, Germany, the currently most powerful diodepumped solid-state laser system with 25-TW peak power P OLARIS is in operation. In this paper we give an overview about the dispersion management of the chirped pulse amplification in order to minimize the pulse duration and thus to maximize the intensity available for experiments. A detailed description of the stretcher and compressor design with a novel alignment routine is given as well as measurements for the pulse duration and the temporal contrast. The far field measurement of the beam focussed by an off-axis parabola yields a nearly diffraction limited focal spot.
1 Introduction The invention of the technique of chirped pulse amplification (CPA) [1] opened a way to generate laser pulses of highest peak powers and peak intensities for their application in the rapidly growing field of light-matter interaction [2–5]. Nowadays, either high-energy Nd:glass or shortM. Hornung () · R. Bödefeld · A. Kessler · M. Schnepp · R. Wachs · A. Sävert · S. Podleska · S. Keppler · J. Hein · M.C. Kaluza Institute of Optics and Quantum Electronics, FSU Jena, Max-Wien Platz 1, 07743 Jena, Germany e-mail:
[email protected] Fax: +49-3641-947299 R. Bödefeld · M.C. Kaluza Helmholtz-Institute Jena, Max-Wien-Platz 1, 07743 Jena, Germany M. Siebold Forschungszentrum Dresden-Rossendorf, Bautzner Landstrasse 400, 01328 Dresden, Germany
pulse Ti:sapphire laser systems are available for the generation of laser pulses with peak powers in the range of several 100 Terawatt (TW) up to more than 1 Petawatt (PW), which can finally be focussed to peak intensities in excess of 1021 W cm−2 . At those intensities all kind of matter is transformed into the plasma state, and electric fields with amplitudes of the order of several TV/m are generated, which can be used for the acceleration of electrons or ions to kinetic energies of 100 MeV to 1 GeV over extremely short distances [6–8]. So far, flashlamp pumped Nd:Glass amplifiers or Nd:YAG pump sources have been the only option for the generation of high-energy laser pulses with pulse durations of the order of several 100 fs up to ns. However, these systems suffer from very low energy conversion efficiency and can only be operated at very low repetition rates. Alternatively, diodepumped solid-state lasers (DPSSL) are envisioned to be an efficient approach for the high repetition rate amplification of laser pulses to high energies and by using appropriate laser materials to pulse durations of the order of 100 fs too. At present, several diode-pumped laser systems aiming at final pulse energies of 100 J or more are developed worldwide [9–11]. One of those, the P OLARIS project follows the approach of an all-diode-pumped architecture aiming at a final pulse energy in excess of 150 J within a pulse duration of the order of 150 fs [12–14]. In order to achieve efficient energy extraction from the broadband, low-gain Yb-doped fluoride phosphate glass [15] of P OLARIS , the pulses are stretched to 2.6 ns prior to the main amplification. In this paper we focus on the P OLARIS stretcher-compressor system and on issues related to the temporal pulse control such as dispersion, pulse contrast, spectral phase, and spectral amplitude. Since a tiled grating compressor is employed, the present paper also addresses recompression of high-energy femtosecond pulses which have
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Fig. 1 Outline of the P OLARIS laser system. The system is operational up to the fourth amplifier. Compressed and focussed pulses are provided for high-intensity experiments while the fifth amplifier is under construction. The petawattcompressor is in its design phase
been stretched to several nanoseconds before their compression with large beam aperture in a general manner, but also with special emphasis on the application of a phased grating array in the compressor. The paper is structured as follows: In Sect. 2 the current status of the P OLARIS project is reviewed. Sections 3–6 present an overview of the stretcher-compressor design and a detailed calculation of the dispersion throughout the whole P OLARIS system. Finally, in Sect. 7 we present measurements of the pulse duration, temporal contrast, and the far field of the amplified laser pulses.
2 The P OLARIS laser system The architecture of P OLARIS is introduced in Fig. 1. The frontend consists of a commercial Ti:Sa oscillator (Coherent Mira 900) operating at a center wavelength of 1030 nm, a femtosecond preamplifier, a fast Pockels cell for pulse cleaning, the grating stretcher, and an acousto-optical spectral shaper (Dazzler) [16]. Pulses as short as 85 fs at full width at half maximum (FWHM) are routinely generated in the oscillator running at a repetition rate of 75 MHz. An average
power of 160 mW corresponding to a pulse energy of 2.1 nJ and a bandwidth of 20 nm (FWHM) are routinely achieved at the oscillator output. A pulse train with a repetition rate of 1 Hz is picked out of the 75 MHz train by a Pockels cell. In order to enhance the pulse contrast which is mainly dominated by amplified spontaneous emission (ASE), the pulses from the oscillator are first preamplified to the 20-µJ level, which is close to the damage threshold of the stretcher grating, and temporally cleaned with an ultra-fast Pockels cell with an optical rise time of less than 214 ps before entering the stretcher. During the amplification process, the spectral bandwidth is reduced to 18 nm (FWHM) due to gain narrowing, whereas the pulse duration is increased to >1 ps owing to the material dispersion in the preamplifier. The P OLARIS system comprises five diode-pumped amplifiers (A1–A5). The first two (A1 and A2) are regenerative amplifiers reaching a total gain of 108 . They are followed by three multipass amplifiers, each being designed for a gain of 10. A detailed description of the individual amplifier setups is given in [17]. Recently, we could demonstrate an output pulse energy of 12 J with the first four amplifiers (A1–A4) at a repetition rate of 0.05 Hz, while the final amplifier (A5) is still under construction.
Temporal pulse control of a multi-10 TW diode-pumped Yb:Glass laser
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Fig. 2 Setup of the Stretcher. (a) Top view for the folded P OLARIS stretcher setup. The red and blue rays indicate the red and blue components of the laser pulse. (b) Side view of the P OLARIS stretcher setup. The green ray indicates the first stretcher pass, and the black one the second one, respectively. The corner cube retro-reflects the beam and realizes the third and fourth pass. (c) Deconvolved setup of the stretcher, which is a variation of the 4-fzero-dispersion-compressor
Owing to this first intermediate step, a compressor system for the multi-100-TW level has been built. In order to reach the Petawatt level, the current compressor, designed for 140-mm diameter pulses, will be extended to be able to handle pulses with a beam diameter as large as 300 mm. Currently, gold gratings with a damage threshold of 250 mJ cm−2 at a pulse duration of 150 fs are used. This corresponds to a maximum pulse energy of 48 J assuming a nearly top-hat beam profile. Currently, we have reduced the output energy to 8 J due to inhomogeneities of the near field intensity distribution, which requires further optimization. However, even at this intermediate status, pulses having a peak power of 25 TW have successfully been used for first high-intensity experiments.
3 The stretcher In Fig. 2 the setup of the stretcher is shown. It is the conventional stretcher variation of the 4-f-zero-dispersioncompressor setup (Fig. 2c) [18–20].
To reduce the space required by the stretcher, the system is folded with a plane mirror, and the two lenses are replaced by a spherical mirror. The stretcher top- and side view are displayed in Figs. 2a and 2b, respectively. The stretcher is used in a four-pass configuration where the two additional passes are realized by height separation with a roof mirror. In Fig. 2b the green ray represents the first pass, and the black ray the second pass. The beam is then retroreflected by a corner cube to provide the third and fourth pass. This configuration minimizes the abberations of the stretcher significantly [21]. Raytracing calculations have been carried out and predict abberations having peak-to-valley values smaller than 8 × 10−7 µrad. Such small abberations can be neglected for the P OLARIS setup. In the stretcher we use a 1480-lines/mm gold coated grating with an angle of incidence of 60.7°. This results in a diffracted angle of 40.7° for the center wavelength of 1030 nm. The separation between the grating and the spherical mirror is 560 mm, which leads to a group delay dispersion (GDD) of 8.50 × 107 fs2 , a thirdorder dispersion (TOD) of −3.81 × 108 fs3 , and a fourthorder dispersion (FOD) of 2.81 × 109 fs4 . This stretcher arrangement hence stretches an initially bandwidth-limited
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Table 1 Dispersion for the P OLARIS amplifier stages and the transport beamline between each device. Ax = amplifier x, Mi = oscillator (Mira), St = stretcher, Da = Dazzler, Co = compressor, i = number of round trips or passes Device
GDD in fs2
TOD in fs3
FOD in fs4
Mi to A1
7.1 × 103
6.7 × 103
−3.2 × 103
A1 (i = 37)
6.5 × 104
8.8 × 104
−7.2 × 104
A1 to St
4.0 × 103
4.0 × 103
−2.3 × 103
St
8.50 × 107
−3.81 × 108
Da
3.5 × 104
0
Da to A2
3.5 × 103
3.4 × 103
−1.9 × 103
A2 (i = 50)
8.8 × 104
1.2 × 105
−9.8 × 104
A2 to A3
4.8 × 103
5.4 × 103
−3.7 × 103
A3 (i = 12)
5.3 × 103
3.1 × 103
4.3 × 101
A3 to A4
1.8 × 104
1.9 × 104
−1.2 × 104
A4 (i = 10)
4.4 × 103
2.6 × 103
1.0 × 101
A4 to Co
1.2 × 103
2.2 × 103
−2.5 × 103
Sum
8.52 × 107
−3.81 × 108
2.81 × 109 0
2.81 × 109
pulse having a spectral width of 18 nm (FWHM) at a central wavelength of 1030 nm by a factor of ≈30000 to a pulse duration of 2.6 ns.
Fig. 3 Calculated temporal intensity profile of a bandwidth-limited 150-fs pulse (black curve) with a spectral clip due to the finite size of the stretcher optics of ±16 nm around the central wavelength at 1030 nm. The red dots indicate a pulse with a residual fourth-order dispersion of −1.1 × 107 fs4 . No significant difference between the two graphs is observed. I0 indicates the maximum intensity, and I1 is the intensity of the side peak generated by the spectral clipping
expected. The laser pulse propagates a distance of 500 m in air [26] before it enters the compressor chamber. This chamber and the subsequent beam line guiding the pulses into the interaction chamber have all been evacuated to a pressure of 1 × 10−6 mbar. Once the pulses have entered the vacuum, the group velocity dispersion introduced by the residual gas is below 10−10 ) (Sequoia) [39]. The measured trace of the temporal pulse contrast is shown in Fig. 10. This contrast ratio was measured after the second amplifier and with the fast Pockels cell (cf. Fig. 1) to minimize the amplified spontaneous emission. All prepulse-like features in the correlation trace are peaks generated by post pulses which have their corresponding peak (with higher signal) at the corresponding delay after the central peak. The remaining prepulse level at 10−9 is likely to be caused by amplified spontaneous emission generated in the frontend of the laser. 7.4 Far field measurement In Fig. 11 the far field generated by a 300-mm parabolic mirror (the effective numerical aperture of the focussing geometry for our laser pulses was f/2.7 in this case) in the target chamber is shown for the first two (a) and the first four (b) amplifiers in operation. The FWHM focus area (i.e., the area in which the local intensity is larger than Imax /2) in both cases is 8.1 µm2 . The far field is imaged
8 Outlook and conclusions In conclusion, we have presented results characterizing the laser pulses generated by a fully diode-pumped multi-TW CPA laser system deploying a high stretching factor and recompression in a tiled grating compressor. So far, the output power has been limited to 25 TW by the output energy of the fourth amplifier in the laser chain. Limited by the damage threshold of the last compressor grating the maximum peak power supported by the compressor is presently 300 TW assuming a top hat beam profile. The actively controlled tiled grating technology was successfully implemented in this system. With the completion of the fifth amplifier and an upgraded compressor P OLARIS will soon run at a multi100-TW level and reach the PW-level in the near future. Using our alignment procedure for the tiled-grating compressor, we achieve nearly diffraction limited far field profiles of the laser pulse and an almost bandwidth-limited pulse duration. With this laser system, we reproducibly generate peak intensities in excess of 8.1 × 1019 W/cm2 , which defines P OLARIS as a powerful tool for high-intensity laser-matter interaction experiments.
102 Acknowledgements We gratefully acknowledge the support by the German Federal Ministry of Education and Research (BMBF) under contracts No. 03ZIK052 (ultra optics) and No. 03ZIK445 (oncooptics), by the German Research Foundation (DFG) under contract no. TR18 and by the European Union (Laserlab Europe).
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