Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity

Appl. Phys. B (2016) 122:297 DOI 10.1007/s00340-016-6574-x Extraction of enhanced, ultrashort laser pulses from a passive 10‑MHz stack‑and‑dump cavity Sven Breitkopf1 · Stefano Wunderlich1,2 · Tino Eidam2 · Evgeny Shestaev1,3 · Simon Holzberger4,5,7 · Thomas Gottschall1 · Henning Carstens4,5 · Andreas Tünnermann1,3,6 · Ioachim Pupeza4 · Jens Limpert1,2,3,6 Received: 30 August 2016 / Accepted: 4 November 2016 © The Author(s) 2016. This article is published with open access at Springerlink.com Abstract Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-anddump scheme with a 30-m-long cavity. Using an acoustooptic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent This article is part of the topical collection “Enlightening the World with the Laser” - Honoring T. W. Hänsch guest edited by Tilman Esslinger, Nathalie Picqué, and Thomas Udem. pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes. 1 Introduction A number of visionary applications like laser wake-field acceleration of elementary particles [1] or space debris removal [2] ask for a dramatically improved performance * Sven Breitkopf sven.breitkopf@uni‑jena.de Stefano Wunderlich stefano.wunderlich@uni‑jena.de Jens Limpert jens.limpert@uni‑jena.de 1 Tino Eidam eidam@afs‑jena.de Institute of Applied Physics, Abbe Center of Photonics, Friedrich-Schiller-Universität Jena, Albert‑Einstein‑Str. 15, 07745 Jena, Germany 2 Evgeny Shestaev evgeny.shestaev@uni‑jena.de Active Fiber Systems GmbH, Wildenbruchstr. 15, 07745 Jena, Germany 3 Simon Holzberger Helmholtz-Institute Jena, Fröbelstieg 3, 07743 Jena, Germany 4 Thomas Gottschall thomas.gottschall@uni‑jena.de Max-Planck-Institute of Quantum Optics, Hans‑Kopfermann‑Str. 1, 85748 Garching, Germany 5 Henning Carstens Department of Physics, Ludwig-Maximilians-Universität München, Am Coulombwall 1, 85748 Garching, Germany 6 Andreas Tünnermann Fraunhofer Institute for Applied Optics and Precision Engineering, Albert‑Einstein‑Str. 7, 07745 Jena, Germany 7 Present Address: Menlo Systems GmbH, Am Klopferspitz 19a, 82152 Martinsried, Germany Ioachim Pupeza 13 297 Page 2 of 7 of femtosecond laser systems with high repetition rates [3]. In particular, Joule-level pulse energies at average powers in the multi-kilowatt regime with diffraction-limited beam quality are required. This combination of parameters greatly exceeds the capabilities of today’s laser systems, and the scalability of the average and of the pulse peak power of single-aperture amplifier solutions does not suffice these demands [4–7]. Current limitations which need to be overcome are mainly caused by thermal or nonlinear effects in the amplifier media [8, 9]. Recently, multiaperture spatial combining approaches have emerged as one possibility to circumvent these limitations [10, 11]. Additionally, temporal combining techniques aimed at artificially extending the stretched pulse duration and, thus, overcoming pulse peak power limitations have been successfully demonstrated. Among those, the most straightforward approach is the so-called divided-pulse amplification (DPA) [12]. Here, in order to reduce the peak power-related limitations, each pulse is split into several temporally separated replicas before the final amplification stage and recombined afterwards. Alternatively, the creation of temporal replicas can be avoided, if a pulse train with a much higher repetition rate is amplified and subsequently temporally combined to achieve the repetition rate demanded by the application. Here, the general idea is to increase the pulse peak power at the cost of a reduced repetition rate by temporally stacking successive pulses after their amplification. One implementation of this approach, which we refer to as stack and dump (SND), is to superpose amplified pulses in an enhancement cavity (EC) and periodically extract them using a fast and efficient switch [13, 14]. Passive ECs have been subject to intensive research and development for several decades [15–17]. They are employed for a multitude of intracavity optical conversion processes such as high-harmonic generation [18, 19] or inverse Compton scattering [20]. Due to the energy enhancement in such a cavity, average powers in the MW range [21] and multi-GW peak power levels [22] are achievable within the cavity at multi-MHz repetition rates. In 2002 and 2003, the extraction of pulses from such an enhancement cavity was proposed [23] and demonstrated at around 80-MHz with nJ-level, picosecond pulses by the Ye and Hänsch groups [24, 25]. In 2004, slightly stretched femtosecond pulses were first enhanced and then extracted from a 100-MHz cavity [26]. Recently, concepts making use of the vast potential of ECs as stacking devices for stretched ultrashort pulses were published [13, 27]. In this paper, we demonstrate the SND scheme in a 30-m-long EC, corresponding to a length increase of a factor of 10 over the state of the art. Towards tapping the full potential of ECs as stacking devices for ultrashort pulses, this constitutes a crucial design criterion relaxing the 13 S. Breitkopf et al. thermal stress in the switch and in the cavity optics [28] and allowing for longer times between successive pulses. The EC supported a steady-state power enhancement factor exceeding 200 and was seeded with a 10-MHz repetitionrate train of 3-µJ pulses. The cavity enabled the enhancement of strongly stretched pulses (~1.5 ns). A systematic investigation of different dumping rates was performed with an intracavity acousto-optic modulator (AOM). Pulses with the accumulated energy of up to 65 input pulses, i.e. 0.2 mJ, were extracted at 30 kHz. These pulses were recompressed to the initial duration of 800 fs, demonstrating the feasibility of SND with strongly stretched pulses and energies surpassing previous results by three orders of magnitude. These results, even if not stating new laser parameter records on their own, constitute the first milestone towards a power-scalable device and, thus, are a necessary step (...truncated)