# Publications by Dr. Jan Rothhardt

All publications of HI Jena

## 2022

P. Gierschke, C. Grebing, M. Abdelaal, M. Lenski, J. Buldt, Z. Wang, T. Heuermann, M. Mueller, M. Gebhardt, J. Rothhardt, and J. Limpert
Nonlinear pulse compression to 51-W average power GW-class 35-fs pulses at 2-mu m wavelength in a gas-filled multi-pass cell
Optics Letters 47, 3511 (2022)

Abstract: We report on the generation of GW-class peak power, 35-fs pulses at 2-mu m wavelength with an average power of 51 W at 300-kHz repetition rate. A compact, krypton-filled Herriott-type cavity employing metallic mirrors is used for spectral broadening. This multi-pass compression stage enables the efficient post compression of the pulses emitted by an ultrafast coherently combined thulium-doped fiber laser system. The presented results demonstrate an excellent preservation of the input beam quality in combination with a power transmission as high as 80%. These results show that multi-pass cell based post-compression is an attractive alternative to nonlinear spectral broadening in fibers, which is commonly employed for thulium-doped and other mid-infrared ultra-fast laser systems. Particularly, the average power scalability and the potential to achieve few-cycle pulse durations make this scheme highly attractive. (C) 2022 Optica Publishing Group

W. Eschen, L. Loetgering, V. Schuster, R. Klas, A. Kirsche, L. Berthold, M. Steinert, T. Pertsch, H. Gross, M. Krause, J. Limpert, and J. Rothhardt
Material-specific high-resolution table-top extreme ultraviolet microscopy
Light: Science & Applications 11, 2678 (2022)

Abstract: Microscopy with extreme ultraviolet (EUV) radiation holds promise for high-resolution imaging with excellent material contrast, due to the short wavelength and numerous element-specific absorption edges available in this spectral range. At the same time, EUV radiation has significantly larger penetration depths than electrons. It thus enables a nano-scale view into complex three-dimensional structures that are important for material science, semiconductor metrology, and next-generation nano-devices. Here, we present high-resolution and material-specific microscopy at 13.5 nm wavelength. We combine a highly stable, high photon-flux, table-top EUV source with an interferometrically stabilized ptychography setup. By utilizing structured EUV illumination, we overcome the limitations of conventional EUV focusing optics and demonstrate high-resolution microscopy at a half-pitch lateral resolution of 16 nm. Moreover, we propose mixed-state orthogonal probe relaxation ptychography, enabling robust phase-contrast imaging over wide fields of view and long acquisition times. In this way, the complex transmission of an integrated circuit is precisely reconstructed, allowing for the classification of the material composition of mesoscopic semiconductor systems.

L. Lötgering, S. Witte, and J. Rothhardt
Advances in laboratory-scale ptychography using high harmonic sources [Invited]
Optics Express 30, 4133 (2022)

Abstract: Extreme ultraviolet microscopy and wavefront sensing are key elements for nextgeneration ultrafast applications, such as chemically-resolved imaging, focal spot diagnostics in pump-and-probe experiments, and actinic metrology for the state-of-the-art lithography node at 13.5 nm wavelength. Ptychography offers a robust solution to the aforementioned challenges. Originally adapted by the electron and synchrotron communities, advances in the stability and brightness of high-harmonic tabletop sources have enabled the transfer of ptychography to the laboratory. This review covers the state of the art in tabletop ptychography with high harmonic generation sources. We consider hardware options such as illumination optics and detector concepts as well as algorithmic aspects in the analysis of multispectral ptychography data. Finally, we review technological application cases such as multispectral wavefront sensing, attosecond pulse characterization, and depth-resolved imaging.

## 2021

S. Skruszewicz, A. Przystawik, D. Schwickert, M. Sumfleth, M. Namboodiri, V. Hilbert, R. Klas, P. Gierschke, V. Schuster, A. Vorobiov, C. Haunhorst, D. Kip, J. Limpert, J. Rothhardt, and T. Laarmann
Table-top interferometry on extreme time and wavelength scales
Optics Express 29, 40333 (2021)

Abstract: High-resolution Fourier-transform spectroscopy using table-top sources in the extreme ultraviolet (XUV) spectral range is still in its infancy. In this contribution a significant advance is presented based on a Michelson-type all-reflective split-and-delay autocorrelator operating in a quasi amplitude splitting mode. The autocorrelator works under a grazing incidence angle in a broad spectral range (10 nm - 1 µ m) providing collinear propagation of both pulse replicas and thus a constant phase difference across the beam profile. The compact instrument allows for XUV pulse autocorrelation measurements in the time domain with a single-digit attosecond precision resulting in a resolution of E/Δ E=2000. Its performance for spectroscopic applications is demonstrated by characterizing a very sharp electronic transition at 26.6 eV in Ar gas induced by the 11th harmonic of a frequency-doubled Yb-fiber laser leading to the characteristic 3s3p⁶4p¹P¹ Fano-resonance of Ar atoms. We benchmark our time-domain interferometry results with a high-resolution XUV grating spectrometer and find an excellent agreement. The common-path interferometer opens up new opportunities for short-wavelength femtosecond and attosecond pulse metrology and dynamic studies on extreme time scales in various research fields.

W. Eschen, S. Wang, C. Liu, R. Klas, M. Steinert, S. Yulin, H. Meissner, M. Bussmann, T. Pertsch, J. Limpert, and J. Rothhardt
Towards attosecond imaging at the nanoscale using broadband holography-assisted coherent imaging in the extreme ultraviolet
Communications Physics 4, 154 (2021)

Abstract: The inherently broad bandwidth of attosecond pulses conflicts with the coherence requirements of lensless imaging. Here, broadband holography-assisted coherent imaging is demonstrated with a resolution of less than 35 nm. Nanoscale coherent imaging has emerged as an indispensable modality, allowing to surpass the resolution limit given by classical imaging optics. At the same time, attosecond science has experienced enormous progress and has revealed the ultrafast dynamics in complex materials. Combining attosecond temporal resolution of pump-probe experiments with nanometer spatial resolution would allow studying ultrafast dynamics on the smallest spatio-temporal scales but has not been demonstrated yet. To date, the large bandwidth of attosecond pulses poses a major challenge to high-resolution coherent imaging. Here, we present broadband holography-enhanced coherent imaging, which enables the combination of high-resolution coherent imaging with a large spectral bandwidth. By implementing our method at a high harmonic source, we demonstrate a spatial resolution of 34 nm in combination with a spectral bandwidth of 5.5 eV at a central photon energy of 92 eV. The method is single-shot capable and retrieves the spectrum from the measured diffraction pattern.

V. Schuster, V. Hilbert, R. Klas, C. Liu, M. Tschernajew, B. Bernhardt, J. Rothhardt, and J. Limpert
Agile spectral tuning of high order harmonics by interference of two driving pulses
Optics Express 29, 22117 (2021)

Abstract: In this work, the experimental realization of a tunable high photon flux extreme ultraviolet light source is presented. This is enabled by high harmonic generation of two temporally delayed driving pulses with a wavelength of 1030 nm, resulting in a tuning range of 0.8 eV at the 19th harmonic at 22.8 eV. The implemented approach allows for fast tuning of the spectrum, is highly flexible and is scalable towards full spectral coverage at higher photon energies.

V. Schuster, C. Liu, R. Klas, P. Dominguez, J. Rothhardt, J. Limpert, and B. Bernhardt
Ultraviolet dual comb spectroscopy: a roadmap
Optics Express 29, 21859 (2021)

Abstract: Dual Comb Spectroscopy proved its versatile capabilities in molecular fingerprinting in different spectral regions, but not yet in the ultraviolet (UV). Unlocking this spectral window would expand fingerprinting to the electronic energy structure of matter. This will access the prime triggers of photochemical reactions with unprecedented spectral resolution. In this research article, we discuss the milestones marking the way to the first UV dual comb spectrometer. We present experimental and simulated studies towards UV dual comb spectroscopy, directly applied to planned absorption measurements of formaldehyde (centered at 343 nm, 3.6 eV) and argon (80 nm, 16 eV). This will enable an unparalleled relative resolution of up to 10-9 - with a table-top UV source surpassing any synchrotron-linked spectrometer by at least two and any grating-based UV spectrometer by up to six orders of magnitude.

R. Klas, A. Kirsche, M. Gebhardt, J. Buldt, H. Stark, S. Hädrich, J. Rothhardt, and J. Limpert
Ultra-short-pulse high-average-power megahertz-repetition-rate coherent extreme-ultraviolet light source
PhotoniX 2, 4 (2021)

Abstract: High harmonic generation (HHG) enables coherent extreme-ultraviolet (XUV) radiation with ultra-short pulse duration in a table-top setup. This has already enabled a plethora of applications. Nearly all of these applications would benefit from a high photon flux to increase the signal-to-noise ratio and decrease measurement times. In addition, shortest pulses are desired to investigate fastest dynamics in fields as diverse as physics, biology, chemistry and material sciences. In this work, the up-to-date most powerful table-top XUV source with 12.9 ± 3.9 mW in a single harmonic line at 26.5 eV is demonstrated via HHG of a frequency-doubled and post-compressed fibre laser. At the same time the spectrum supports a Fourier-limited pulse duration of sub-6 fs in the XUV, which allows accessing ultrafast dynamics with an order of magnitude higher photon flux than previously demonstrated. This concept will greatly advance and facilitate applications of XUV radiation in science and technology and enable photon-hungry ultrafast studies.

J. White, S. Wang, W. Eschen, and J. Rothhardt
Real-time phase-retrieval and wavefront sensing enabled by an artificial neural network
Optics Express 29, 1 (2021)

Abstract: In this manuscript we demonstrate a method to reconstruct the wavefront of focused beams from a measured diffraction pattern behind a diffracting mask in real-time. The phase problem is solved by means of a neural network, which is trained with simulated data and verified with experimental data. The neural network allows live reconstructions within a few milliseconds, which previously with iterative phase retrieval took several seconds, thus allowing the adjustment of complex systems and correction by adaptive optics in real time. The neural network additionally outperforms iterative phase retrieval with high noise diffraction patterns.

M. Gebhardt, T. Heuermann, R. Klas, C. Liu, A. Kirsche, M. Lenski, Z. Wang, C. Gaida, J. E. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
Bright, high-repetition-rate water window soft X-ray source enabled by nonlinear pulse self-compression in an antiresonant hollow-core fibre
Light: Science & Applications 10, 2021 (2021)

Abstract: Bright, coherent soft X-ray radiation is essential to a variety of applications in fundamental research and life sciences. To date, a high photon flux in this spectral region can only be delivered by synchrotrons, free-electron lasers or high-order harmonic generation sources, which are driven by kHz-class repetition rate lasers with very high peak powers. Here, we establish a novel route toward powerful and easy-to-use SXR sources by presenting a compact experiment in which nonlinear pulse self-compression to the few-cycle regime is combined with phase-matched high-order harmonic generation in a single, helium-filled antiresonant hollow-core fibre. This enables the first 100 kHz-class repetition rate, table-top soft X-ray source that delivers an application-relevant flux of 2.8 x 10(6) photon s(-1) eV(-1) around 300 eV. The fibre integration of temporal pulse self-compression (leading to the formation of the necessary strong-field waveforms) and pressure-controlled phase matching will allow compact, high-repetition-rate laser technology, including commercially available systems, to drive simple and cost-effective, coherent high-flux soft X-ray sources.

L. Loetgering, T. Aidukas, M. Du, D. B. Flaes, D. Penagos, M. Rose, A. Pelekanidis, A. D. Beurs, W. Eschen, J. Hess, T. Wilhein, R. Heintzmann, J. Rothhardt, and S. Witte
ptyLab: a cross-platform inverse modeling toolbox for conventional and Fourier ptychography
(2021)

Abstract: We present a cross-platform software, called ptyLab, enabling both conventional and Fourier ptychographic data analysis. The unified framework will accelerate cross- pollination between the two techniques. The code is available open-source in both MAT-LAB and Python.

W. Eschen, L. Loetgering, V. Schuster, R. Klas, J. Limpert, and J. Rothhardt
Table-top high-resolution ptychographic EUV imaging
(2021)

Abstract: Nanoscale imaging at 13.5 nm provides ideal opportunities for ‘at wavelength’ metrology. We present a setup and the latest results on lensless ptychographic imaging at 92 eV achieving sub 30 nm resolution.

M. Gebhardt, T. Heuermann, R. Klas, C. Liu, A. Kirsche, M. Lenski, Z. Wang, C. Gaida, J. E. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
100 kHz water window soft X-ray high-order harmonic generation through pulse self-compression in an antiresonant hollow-core fiber
(2021)

Abstract: Coherent soft X-ray (SXR) sources that provide a high photon flux in the water window are essential tools for advanced spectroscopy (e.g. of magnetic materials [1] and organic compounds [2] ) or for lens-less bio imaging with nm-scale resolution [3] . To date, such sources are mostly large-scale facilities like synchrotrons or free electron lasers. A promising alternative are laser-driven sources, based on high harmonic generation (HHG) in noble gases. Current state-of-the-art SXR HHG uses frequency converted Ti:Sa lasers (to ~2 µm wavelength to increase the phase matching cutoff) with multi-mJ pulse energies at 1 kHz repetition rate [1] . Most recently, there has been a strong push towards increasing the repetition rate of the driving lasers, and the SXR HHG, to enable faster data acquisition, space-charge-reduced SXR photoelectron spectroscopy or coincidence detection [4] , [5] . In this contribution, we present an approach to SXR HHG that is based on nonlinear pulse self-compression and HHG in the same helium gas-filled antiresonant hollow-core fiber (ARHCF). Because of the intensity enhancement resulting from temporal pulse self-compression, the experiments can be driven by moderate-energy, multi-cycle laser pulses, which facilitates repetition rate scaling. We coupled 100 fs-, 250 µJ-pulses centered around 1.9 µm wavelength at a 98 kHz repetition rate to the ARHCF ( Fig. 1a ). When the fiber length (~1.2 m) and the gas pressure at its output (~3.8 bar) were chosen appropriately, the pulses close to its end ( Fig. 1b ) were self-compressed to <20 fs, leading to an on-axis peak intensity >4×10 14 W/cm 2 . At this point, the gas is partially ionized, and the chosen pressure ensures phase matching between the driving laser and the generated SXR light ( Fig. 1b ). It is the first time that this approach is experimentally realized, and we have generated a photon flux >10 6 Ph/s/eV at the carbon K-edge ( Fig. 1c ). To the best of our knowledge, this is the highest photon flux at 300 eV reported to date at a laser repetition rate >1 kHz.

C. Gaida, F. Stutzki, M. Gebhardt, T. Heuermann, S. Breitkopf, T. Eidam, J. Rothhardt, and J. Limpert
4-channel Coherently Combined Long-term-stable Ultrafast Thulium-doped Fiber CPA
(2021)

Abstract: Recent milestones in power-scaling of ultrafast fiber-based lasers were achieved by the simultaneous mitigation of thermal and nonlinear effects through the coherent combination of ultrafast pulses [1] . This technique is based on splitting the light into several spatially separated amplification channels that are subsequently coherently recombined into a single beam. Besides the performance scaling aspect, the laser wavelength is an important parameter for many applications, e.g. for high-field physics due to the quadratic wavelength-dependence of the ponderomotive potential [2] . Tm-doped fiber lasers have proven to be promising and relatively straightforward candidates for the realization of efficient high average- and peak-power ultrafast lasers in the 2-µm wavelength region [3] , [4] . Here we demonstrate the coherent combination [1] , [5] of four thulium-doped fiber amplifiers. The fiber-based chirped pulse amplifier (CPA [6] ) delivers pulses with <120 fs full-width at half-maximum duration with up to 228 µJ of pulse energy at a center wavelength of 1940 nm.

A. Kirsche, R. Klas, M. Gebhardt, L. Eisenbach, W. Eschen, J. Buldt, H. Stark, J. Rothhardt, and J. Limpert
Continuously tunable high photon flux high harmonic source at 50-70 eV
(2021)

Abstract: Table-top extreme ultraviolet (XUV) light sources based on high harmonic generation (HHG) have emerged as a user-friendly, complementary technique to large scale facilities like synchrotrons and free electron lasers [1] . The ultrashort pulse duration (femtosecond to attosecond) and coherence of these laser-driven sources are highly advantageous for many applications, e.g. coherent diffractive imaging [2] or XUV spectroscopy of atoms, molecules and ions [3] . In addition to the precise control of temporal and spatial properties of the HHG radiation, investigations of narrow band resonances and detailed spectral features in the XUV require tuning of the discrete harmonic lines. Therefore, there is a strong application-driven demand for powerful, tunable XUV combs. To date, a large variety of tunable XUV sources have been demonstrated e.g. by shifting the driving wavelength using optical parametric amplifiers [4] or soliton-plasma dynamics [5] , among others [6] .

M. Tschernajew, S. Hadrich, R. Klas, M. Gebhardt, R. Horsten, S. Werdenburg, S. Pyatchenkov, W. Coene, J. Rothhardt, T. Eidam, and J. Limpert
High repetition rate high harmonic generation with ultra-high photon flux
(2021)

Abstract: High-harmonic generation (HHG) driven by ultrashort laser pulses is an established process for the generation of coherent extreme ultraviolet (XUV) to soft X-ray radiation, which has found widespread use in various applications [1] . In recent years photon-hungry applications such as coherent diffractive imaging [2] , [3] and applications based on statistical analysis [3] have required more powerful HHG sources, in particular, at high repetition rates. This need can be addressed by using high average power fiber lasers as the HHG drivers [4] . Here, we present a HHG-based XUV source, capable of providing a large photon flux across a wide range between 66 eV and 150 eV. It is driven by a commercial XUV beamline from Active Fiber Systems GmbH consisting of 100-W average power fiber-laser system, delivering up to 300 µJ at <300-fs pulse duration. For HHG this system is operated at 100 W, 600 kHz. A post-compression unit is part of the device to shorten the pulses to ~35 fs, the average power remains at 63W. The turnkey source can provide unprecedented photon fluxes of >10 11 photons/s in each harmonic between 69 eV and 75 eV (HH57-HH63). All fluxes are given at the generation point, i.e. directly after the source.

M. Gebhardt, T. Heuermann, R. Klas, A. Kirsche, C. Liu, Z. Wang, M. Lenski, C. Gaida, C. Jauregui, J. Antonio-Lopez, A. Schülzgen, R. Amezcua-Correa, J. Rothhardt, and J. Limpert
High repetition rate high-order harmonic generation up to the carbon K-edge in an antiresonant hollow-core fiber
(2021)
W. Eschen, V. Schuster, S. Wang, L. Loetgering, C. Liu, R. Klas, J. Limpert, and J. Rothhardt
Ultrafast nanoscale XUV table-top coherent diffractive imaging
(2021)

## 2020

A. Borovik, G. Weber, V. Hilbert, H. Lin, P. Pfäfflein, B. Zhu, C. Hahn, M. Lestinsky, S. Schippers, Th. Stöhlker, and J. Rothhardt
Development of a detector to register low-energy, charge-changed ions from ionization experiments at CRYRING@ESR
Journal of Physics: Conference Series 1412, 242003 (2020)

Abstract: A detector setup for registering ion species between the poles of a dipole magnet at CRYRING@ESR has been developed. It is based on a scintillator delivering light via a quartz light guide onto a semiconductor photomultiplier. The detector is capable of operating in a strong magnetic field. It can be swiftly retracted from the exposition area during the beam injection into the ring and repositioned back for the measurement cycle to avoid unnecessary exposition and, thus, to increase the scintillator life time.

W. Eschen, G. Tadesse, Y. Peng, M. Steinert, T. Pertsch, J. Limpert, and J. Rothhardt
Single-shot characterization of strongly focused coherent XUV and soft X-ray beams
Optics Letters 45, 4798 (2020)

Abstract: In this Letter, we present a novel, to the best of our knowledge, single-shot method for characterizing focused coherent beams. We utilize a dedicated amplitude-only mask, in combination with an iterative phase retrieval algorithm, to reconstruct the amplitude and phase of a focused beam from a single measured far-field diffraction pattern alone. In a proof-of-principle experiment at a wavelength of 13.5 nm, we demonstrate our new method and obtain an RMS phase error of better than $\lambda /70$. This method will find applications in the alignment of complex optical systems, real-time feedback to adaptive optics, and single-shot beam characterization, e.g., at free-electron lasers or high-order harmonic beamlines.

M. Tschernajew, P. Gierschke, H. Lin, V. Hilbert, J. Kurdal, A. Stancalie, J. Limpert, and J. Rothhardt
Differential pumping unit for windowless coupling of laser beams to ultra high vacuum
Vacuum 178, 109443 (2020)

Abstract: Current laser technology enables table-top high flux XUV sources with photon energies from several tens to several hundreds of eV via high harmonic generation in noble gases. Here we present a compact versatile coupling unit to establish windowless, and thus lossless coupling of such light sources to ultra high vacuum (UHV). The particular coupling unit has been developed to couple a XUV laser source to a heavy ion storage ring. Three-stage differential pumping allows to reduce the input pressure of ~10−6 mbar down to the 10−12 mbar range at the output. The unit particularly reduces the partial pressure of argon, which is used to generate the XUV radiation, by 6 orders of magnitude. Measurements of the pressure distribution inside the different chambers agree well with theoretical simulations. In principle, this unit can also serve for other purposes, where a windowless vacuum coupling is needed, with a transition from High Vacuum (HV) levels to deep UHV, such as coupling to cryogenically cooled detectors, ion traps or to photoelectron emission spectroscopy experiments.

F. Tuitje, W. Eschen, G. Tadesse, J. Limpert, J. Rothhardt, and C. Spielmann
Reliability of ptychography on periodic structures
OSA Continuum 3, 1691 (2020)
V. Hilbert, M. Tschernajew, R. Klas, J. Limpert, and J. Rothhardt
A compact, turnkey, narrow-bandwidth, tunable, and high-photon-flux extreme ultraviolet source

Abstract: We report on a compact high-photon-flux extreme ultraviolet (XUV) source based on high harmonic generation. A high XUV-photon flux (>10¹³ photons/s) is achieved at 21.8 eV and 26.6 eV. The narrow spectral bandwidth (ΔE/E < 10⁻³) of the generated harmonics is in the range of state-of-the-art synchrotron beamlines and enables high resolution spectroscopy experiments. The robust design based on a fiber– laser system enables turnkey-controlled and even remotely controlled operation outside specialized laser laboratories, which opens the way for a variety of applications.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Generation of coherent broadband high photon flux continua in the XUV with a sub-two-cycle fiber laser
Optics Express 28, 6188 (2020)

Abstract: High harmonic sources can provide ultrashort pulses of coherent radiation in the XUV and X-ray spectral region. In this paper we utilize a sub-two-cycle femtosecond fiber laser to efficiently generate a broadband continuum of high-order harmonics between 70 eV and 120 eV. The average power delivered by this source ranges from > 0.2 µW/eV at 80 eV to >0.03 µW/eV at 120 eV. At 92 eV (13.5 nm wavelength), we measured a coherent record-high average power of 0.1 µW/eV, which corresponds to 7 · 109 ph/s/eV, with a long-term stability of 0.8% rms deviation over a 20 min time period. The presented approach is average power scalable and promises up to 1011 ph/s/eV in the near future. With additional carrier-envelop phase control even isolated attosecond pulses can be expected from such sources. The combination of high flux, high photon energy and ultrashort (sub-) fs duration will enable photon-hungry time-resolved and multidimensional studies.

J. Rothhardt, M. Bilal, R. Beerwerth, A. Volotka, V. Hilbert, T. Stöhlker, S. Fritzsche, and J. Limpert
Lifetime measurements of ultrashort-lived excited states in Be-like ions
X-Ray Spectrometry 49, 165 (2020)

Abstract: We propose to measure the lifetime of short-lived excited states in highly charged ions by pump-probe experiments. Utilizing two synchronized and delayed Femtosecond pulses allows accessing these lifetimes with Femtosecond precision. Such measurements could provide sensitive tests of state-of-the art atomic structure calculations beyond the capabilities of established methods.

C. Gaida, F. Stutzki, M. Gebhardt, T. Heuermann, S. Breitkopf, T. Eidam, J. Rothhardt, and J. Limpert
4-channel Coherently Combined Long-term-stable Ultrafast Thulium-doped Fiber CPA
(2020)

Abstract: We present the first coherently combined, thulium-doped fiber CPA delivering >100 W average-power and simultaneously >1 GW of peak-power. Incorporating four amplifier channels the laser delivers pulses with >228 µJ energy and <120 fs duration at 1940 nm center wavelength. Excellent long-term stability is achieved with an average power fluctuation of <0.5% RMS over >48 hours – ideal prerequisites for next-generation industrial and scientific applications.

M. Tschernajew, S. Hädrich, R. Klas, M. Gebhardt, R. Horsten, S. Weerdenburg, S. Pyatchenkov, W. Coene, J. Rothhardt, T. Eidam, and . others
High Repetition Rate High Harmonic Generation with Ultra-high Photon Flux
(2020)

Abstract: A 100W fiber laser system is used to drive a high repetition rate HHG beamline producing record-high photon flux of >10^11 photons/s at 69-75eV and >10^10 photons/s for harmonics between 115eV and 140eV.

R. Klas, W. Eschen, A. Kirsche, J. Rothhardt, and J. Limpert
Power scalable fiber laser driven high-harmonic source for broadband high photon flux continua
(2020)

Abstract: A HHG source generating a broadband continuum from 70 eV to 120 eV with an average power of 2 µW is presented. At 92 eV (13.5 nm) 7 10^9 ph/s/eV are generated with an rms deviation of 0.8% over 20 minutes.

R. Klas, J. Buldt, H. Stark, A. Kirsche, M. Gebhardt, J. Rothhardt, and J. Limpert
Sub-20 fs high-energy pulse generation at 515 nm with 50 W of average power
(2020)

Abstract: A nonlinear compression of 515 nm pulses resulting in 17.8 fs-, 50 µJ-pulses at 1 MHz, 50 W average power and near diffraction limited beam quality is presented.

M. Gebhardt, T. Heuermann, Z. Wang, M. Lenski, C. Gaida, R. Klas, A. Kirsche, S. Hädrich, J. Rothhardt, and J. Limpert
Soft x-ray high order harmonic generation driven by high repetition rate ultrafast thulium-doped fiber lasers
(2020)

Abstract: Intense, ultrafast laser sources with an emission wavelength beyond the well-established near-IR are important tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. In particular, such laser systems enable high photon energy cut-off HHG up to, and even beyond, the water window thus enabling a plethora of subsequent experiments. Ultrafast thulium-doped fiber laser systems (providing a broad amplification bandwidth in the 2 μm wavelength region) represent a promising, average-power scalable laser concept in this regard. These lasers already deliver ~100 fs pulses with multi-GW peak power at hundreds of kHz repetition rate. In this work, we show that combining ultrafast thulium-doped fiber CPA systems with hollow-core fiber based nonlinear pulse compression is a promising approach to realize high photon energy cut-off HHG drivers. Herein, we show that thulium-doped, fiber-laser-driven HHG in argon can access the highly interesting spectral region around 90 eV. Additionally, we show the first water window high-order harmonic generation experiment driven by a high repetition rate, thulium-doped fiber laser system. In this proof of principle demonstration, a photon energy cut-off of approximately 400 eV has been achieved, together with a photon flux <105 ph/s/eV at 300 eV. These results emphasize the great potential of exploiting the HHG wavelength scaling laws with 2 μm fiber laser technology. Improvements of the HHG efficiency, the overall HHG yield and further laser performance enhancements will be the subject of our future work.

## 2019

M. Bilal, A. V. Volotka, R. Beerwerth, J. Rothhardt, V. Hilbert, and S. Fritzsche
High-precision calculations of the 1s²2s2p ¹P₁->1s²2s² ¹S₀ spin-allowed E1 transition in C iii
Physical Review A 99, 062511 (2019)

Abstract: Large-scale relativistic calculations are performed for the transition energy and line strength of the 1s22s2p 1P1− 1s22s2 1S0 transition in Be-like carbon. Based on the multiconfiguration Dirac-Hartree-Fock~(MCDHF) approach, different correlation models are developed to account for all major electron-electron correlation contributions. These correlation models are tested with various sets of the initial and the final state wave functions. The uncertainty of the predicted line strength due to missing correlation effects is estimated from the differences between the results obtained with those models. The finite nuclear mass effect is accurately calculated taking into account the energy, wave functions as well as operator contributions. As a result, a reliable theoretical benchmark of the E1 line strength is provided to support high precision lifetime measurement of the 1s22s2p 1P1 state in Be-like carbon.

G. Tadesse, W. Eschen, R. Klas, M. Tschernajew, T. Frederik, M. Steinert, M. Zilk, V. Schuster, M. Zürch, T. Pertsch, C. Spielmann, J. Limpert, and J. Rothhardt
Wavelength-scale ptychographic coherent diffractive imaging using a high-order harmonic source
Scientific Reports 9, 1735 (2019)

Abstract: Ptychography enables coherent diffractive imaging (CDI) of extended samples by raster scanning across the illuminating XUV/X-ray beam, thereby generalizing the unique advantages of CDI techniques. Table- top realizations of this method are urgently needed for many applications in sciences and industry. Previously, it was only possible to image features much larger than the illuminating wavelength with table-top ptychography although knife-edge tests suggested sub-wavelength resolution. However, most real-world imaging applications require resolving of the smallest and closely-spaced features of a sample in an extended field of view. In this work, resolving features as small as 2.5 \lambda (45 nm) using a table-top ptychography setup is demonstrated by employing a high-order harmonic XUV source with record-high photon flux. For the first time, a Rayleigh-type criterion is used as a direct and unambiguous resolution metric for high-resolution table-top setup. This reliably qualifies this imaging system for real-world applications e.g. in biological sciences, material sciences, imaging integrated circuits and semiconductor mask inspection.

## 2018

J. Rothhardt, G. Tadesse, W. Eschen, and J. Limpert
Table-top nanoscale coherent imaging with XUV light
Journal of Optics 20, 113001 (2018)

Abstract: Modern laser-based XUV light sources provide very high photon fluxes which have previously only been available at large scale facilities. This allows high-performance XUV nanoscale imaging to be implemented in a table-top manner, and thus qualifies XUV imaging as a novel imaging technique complementing electron and visible-light microscopy. This article presents the current state-of-the-art in table-top XUV light sources and matched coherent imaging schemes. Selected experiments demonstrate the unique capabilities of XUV imaging—namely, nanoscale (sub-20 nm) resolution, single shot imaging, imaging of extended samples and 3D imaging of µm-sized objects. In addition, future prospects will be discussed, including scaling to few-nm resolution, extension to the soft x-ray spectral region, chemically-specific imaging at absorption edges and time-resolved imaging on femtosecond time-scales.

R. Klas, A. Kirsche, M. Tschernajew, J. Rothhardt, and J. Limpert
Annular beam driven high harmonic generation for high flux coherent XUV and soft X-ray radiation
Optics Express 26, 19318 (2018)

Abstract: Separation of the high average power driving laser beam from the generated XUV to soft-X-ray radiation poses great challenges in collinear HHG setups due to the losses and the limited power handling capabilities of the typically used separating optics. This paper demonstrates the potential of driving HHG with annular beams, which allow for a straightforward and power scalable separation via a simple pinhole, resulting in a measured driving laser suppression of 5⋅10−3. The approach is characterized by an enormous flexibility as it can be applied to a broad range of input parameters and generated photon energies. Phase matching aspects are analyzed in detail and an HHG conversion efficiency that is only 27% lower than using a Gaussian beam under identical conditions is demonstrated, revealing the viability of the annular beam approach for high flux coherent short-wavelength sources and high

G. Tadesse, W. Eschen, R. Klas, V. Hilbert, D. Schelle, A. Nathanael, M. Zilk, M. Steinert, F. Schrempel, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High resolution XUV Fourier transform holography on a table top
Scientific Reports 8, 8677 (2018)

Abstract: Today, coherent imaging techniques provide the highest resolution in the extreme ultraviolet (XUV) and X-ray regions. Fourier transform holography (FTH) is particularly unique, providing robust and straightforward image reconstruction at the same time. Here, we combine two important advances: First, our experiment is based on a table-top light source which is compact, scalable and highly accessible. Second, we demonstrate the highest resolution ever achieved with FTH at any light source (34 nm) by utilizing a high photon flux source and cutting-edge nanofabrication technology. The performance, versatility and reliability of our approach allows imaging of complex wavelength-scale structures, including wave guiding effects within these structures, and resolving embedded nanoscale features, which are invisible for electron microscopes. Our work represents an important step towards real-world applications and a broad use of XUV imaging in many areas of science and technology. Even nanoscale studies of ultra-fast dynamics are within reach.

M. Zurch, A. Guggenmos, R. Jung, J. Rothhardt, C. Spath, J. Tümmler, S. Demmler, S. Haedrich, J. Limpert, A. Tünnermann, U. Kleineberg, H. Stiel, and C. Spielmann
Coherent Diffraction Imaging with Tabletop XUV Sources
(2018)

Abstract: Coherent diffraction imaging (CDI) at wavelengths in the extreme ultraviolet range has become an important tool for nanoscale investigations. Employing laser-driven high harmonic sources allows for lab-scale applications such as cancer cell classification and phase-resolved surface studies in reflection geometry. The excellent beam properties support a spatial resolution below the wavelength, i.e., close to the Abbe limit. Unfortunately, the usually low photon flux of HHG sources limits their applicability. Recent advances in ultrafast fiber laser development cumulated in sources delivering average powers approaching the milliwatt level in the extreme ultraviolet. In comparison, a tabletop soft X-ray laser driven by moderate pump energies was recently employed for CDI featuring excellent temporal coherence and extraordinary high flux allowing for single-shot imaging.

## 2017

J. Rothhardt, S. Hädrich, J. C. Delagnes, E. Cormier, and J. Limpert
High Average Power Near-Infrared Few-Cycle Lasers
Laser and Photonics Reviews 11, 1700043 (2017)

Abstract: Ultra-short laser pulses with only a few optical cycles duration have gained increasing importance during the recent decade and are currently employed in many laboratories worldwide. In addition, modern laser technology nowadays can provide few-cycle pulses at very high average power which advances established studies and opens exciting novel research opportunities. In this paper, the two complementary approaches for providing few-cycle pulses at high average power, namely optical parametric amplification and nonlinear pulse compression, are reviewed and compared. In addition, their limitations and future scaling potential are discussed. Furthermore, selected applications particularly taking advantage of the high average power and high repetition rate are presented.

M. Müller, A. Klenke, T. Gottschall, R. Klas, C. Rothhardt, S. Demmler, J. Rothhardt, J. Limpert, and A. Tünnermann
High-average-power femtosecond laser at 258 nm
Optics Letters 42, 2826 (2017)

Abstract: We present an ultrafast fiber laser system delivering 4.6 W average power at 258 nm based on two-stage fourth-harmonic generation in beta barium borate (BBO). The beam quality is close to being diffraction limited with an M^2 value of 1.3×1.6. The pulse duration is 150 fs, which, potentially, is compressible down to 40 fs. A plain BBO and a sapphire-BBO compound are compared with respect to the achievable beam quality in the conversion process. This laser is applicable in scientific and industrial fields. Further scaling to higher average power is discussed.

## 2016

R. Klas, S. Demmler, M. Tschernajew, S. Hädrich, Y. Shamir, A. Tünnermann, J. Rothhardt, and J. Limpert
Table-top milliwatt-class extreme ultraviolet high harmonic light source
Optica 3, 1167 (2016)

Abstract: Extreme ultraviolet (XUV) lasers are essential for the investigation of fundamental physics. Especially high repetition rate, high photon flux sources are of major interest for reducing acquisition times and improving signal-to-noise ratios in a plethora of applications. Here, an XUV source based on cascaded frequency conversion is presented, which, due to the drastic better single atom response for short wavelength drivers, delivers an average output power of (832±204)  μW at 21.7 eV. This is the highest average power produced by any high harmonic generation source in this spectral range, surpassing previous demonstrations by almost an order of magnitude. Furthermore, a narrowband harmonic at 26.6 eV with a relative energy bandwidth of only ΔE/E=1.8·10−3 has been generated that is of high interest for high-precision spectroscopy experiments.

G. K. Tadesse, R. Klas, S. Demmler, S. Hädrich, I. Wahyutama, M. Steinert, C. Spielmann, M. Zürch, T. Pertsch, A. Tünnermann, J. Limpert, and J. Rothhardt
High speed and high resolution table-top nanoscale imaging
Optics Letters 41, 5170 (2016)

Abstract: We present a table-top coherent diffractive imaging (CDI) experiment based on high-order harmonics generated at 18 nm by a high average power femtosecond fiber laser system. The high photon flux, narrow spectral bandwidth, and high degree of spatial coherence allow for ultrahigh subwavelength resolution imaging at a high numerical aperture. Our experiments demonstrate a half-pitch resolution of 15 nm, close to the actual Abbe limit of 12 nm, which is the highest resolution achieved from any table-top extreme ultraviolet (XUV) or x-ray microscope. In addition, sub-30 nm resolution was achieved with only 3 s of integration time, bringing live diffractive imaging and three-dimensional tomography on the nanoscale one step closer to reality. The current resolution is solely limited by the wavelength and the detector size. Thus, table-top nanoscopes with only a few-nanometer resolutions are in reach and will find applications in many areas of science and technology.

M. Lestinsky, V. Andrianov, B. Aurand, V. Bagnoud, D. Bernhardt, H. Beyer, S. Bishop, K. Blaum, A. Bleile, At. Borovik, F. Bosch, C. Bostock, C. Brandau, A. Bräuning-Demian, I. Bray, T. Davinson, B. Ebinger, A. Echler, P. Egelhof, A. Ehresmann, M. Engström, C. Enss, N. Ferreira, D. Fischer, A. Fleischmann, E. Förster, S. Fritzsche, R. Geithner, S. Geyer, J. Glorius, K. Göbel, O. Gorda, J. Goullon, P. Grabitz, R. Grisenti, A. Gumberidze, S. Hagmann, M. Heil, A. Heinz, F. Herfurth, R. Heß, P.-M. Hillenbrand, R. Hubele, P. Indelicato, A. Källberg, O. Kester, O. Kiselev, A. Knie, C. Kozhuharov, S. Kraft-Bermuth, T. Kühl, G. Lane, Y. Litvinov, D. Liesen, X. Ma, R. Märtin, R. Moshammer, A. Müller, S. Namba, P. Neumayer, T. Nilsson, W. Nörtershäuser, G. G. Paulus, N. Petridis, M. Reed, R. Reifarth, P. Reiß, J. Rothhardt, R. Sanchez, M. Sanjari, S. Schippers, H. Schmidt, D. Schneider, P. Scholz, R. Schuch, M. Schulz, V. Shabaev, A. Simonsson, J. Sjöholm, Ö. Skeppstedt, K. Sonnabend, U. Spillmann, K. Stiebing, M. Steck, T. Stöhlker, A. Surzhykov, S. Torilov, E. Träbert, M. Trassinelli, S. Trotsenko, X. Tu, I. Uschmann, P. Walker, G. Weber, D. Winters, P. Woods, H. Zhao, and Y. Zhang
Physics book: CRYRING@ESR
European Physical Journal Special Topics 225, 797 (2016)

Abstract: The exploration of the unique properties of stored and cooled beams of highly-charged ions as provided by heavy-ion storage rings has opened novel and fascinating research opportunities in the realm of atomic and nuclear physics research. Since the late 1980s, pioneering work has been performed at the CRYRING at Stockholm and at the Test Storage Ring (TSR) at Heidelberg. For the heaviest ions in the highest charge-states, a real quantum jump was achieved in the early 1990s by the commissioning of the Experimental Storage Ring (ESR) at GSI Helmholtzzentrum für Schwerionenforschung (GSI) in Darmstadt where challenging experiments on the electron dynamics in the strong field regime as well as nuclear physics studies on exotic nuclei and at the borderline to atomic physics were performed. Meanwhile also at Lanzhou a heavy-ion storage ring has been taken in operation, exploiting the unique research opportunities in particular for medium-heavy ions and exotic nuclei.

I. S. Wahyutama, G. K. Tadesse, A. Tünnermann, J. Limpert, and J. Rothhardt
Influence of detector noise in holographic imaging with limited photon flux
Optics Express 24, 22013 (2016)

Abstract: Lensless coherent diffractive imaging usually requires iterative phase-retrieval for recovering the missing phase information. Holographic techniques, such as Fourier-transform holography (FTH) or holography with extended references (HERALDO), directly provide this phase information and thus allow for a direct non-iterative reconstruction of the sample. In this paper, we analyze the effect of detector noise on the reconstruction for FTH and HERALDO with linear and rectangular references. We find that HERALDO is more sensitive to this type of noise than FTH, especially if rectangular references are employed. This excessive noise, caused by the necessary differentiation step(s) during reconstruction in case of HERALDO, additionally depends on the numerical implementation. When considering both shot-noise and detector noise, we find that FTH provides a better signal-to-noise ratio (SNR) than HERALDO if the available photon flux from the light source is low. In contrast, at high photon flux HERALDO provides better SNR and resolution than FTH. Our findings will help in designing optimum holographic imaging experiments particularly in the photon-flux-limited regime where most ultrafast experiments operate.

S. Hädrich, M. Kienel, M. Müller, A. Klenke, J. Rothhardt, R. Klas, T. Gottschall, T. Eidam, A. Drozdy, P. Jójárt, Z. Várallyay, E. Cormier, K. Osvay, A. Tünnermann, and J. Limpert
Energetic sub-2-cycle laser with 216 W average power
Optics Letters 41, 4332 (2016)

Abstract: Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

S. Hädrich, J. Rothhardt, M. Krebs, S. Demmler, A. Klenke, A. Tünnermann, and J. Limpert
Single-pass high harmonic generation at high repetition rate and photon flux
Journal of Physics B: Atomic, Molecular and Optical Physics 49, 172002 (2016)

Abstract: Sources of short wavelength radiation with femtosecond to attosecond pulse durations, such as synchrotrons or free electron lasers, have already made possible numerous, and will facilitate more, seminal studies aimed at understanding atomic and molecular processes on fundamental length and time scales. Table-top sources of coherent extreme ultraviolet to soft x-ray radiation enabled by high harmonic generation (HHG) of ultrashort pulse lasers have also gained significant attention in the last few years due to their enormous potential for addressing a plethora of applications, therefore constituting a complementary source to large-scale facilities (synchrotrons and free electron lasers). Ti:sapphire based laser systems have been the workhorses for HHG for decades, but are limited in repetition rate and average power. On the other hand, it has been widely recognized that fostering applications in fields such as photoelectron spectroscopy and microscopy, coincidence detection, coherent diffractive imaging and frequency metrology requires a high repetition rate and high photon flux HHG sources. In this article we will review recent developments in realizing the demanding requirement of producing a high photon flux and repetition rate at the same time. Particular emphasis will be put on suitable ultrashort pulse and high average power lasers, which directly drive harmonic generation without the need for external enhancement cavities. To this end we describe two complementary schemes that have been successfully employed for high power fiber lasers, i.e. optical parametric chirped pulse amplifiers and nonlinear pulse compression. Moreover, the issue of phase-matching in tight focusing geometries will be discussed and connected to recent experiments. We will highlight the latest results in fiber laser driven high harmonic generation that currently produce the highest photon flux of all existing sources. In addition, we demonstrate the first promising applications and discuss the future direction and challenges of this new type of HHG source.

J. Rothhardt, S. Hädrich, Y. Shamir, M. Tschnernajew, R. Klas, A. Hoffmann, G. K. Tadesse, A. Klenke, T. Gottschall, T. Eidam, J. Limpert, A. Tünnermann, R. Boll, C. Bomme, H. Dachraoui, B. Erk, M. D. Fraia, D. A. Horke, T. Kierspel, T. Mullins, A. Przystawik, E. Savelyev, J. Wiese, T. Laarmann, J. Küpper, and D. Rolles
High-repetition-rate and high-photon-flux 70 eV high-harmonic source for coincidence ion imaging of gas-phase molecules
Optics Express 24, 18133 (2016)

Abstract: Unraveling and controlling chemical dynamics requires techniques to image structural changes of molecules with femtosecond temporal and picometer spatial resolution. Ultrashort-pulse x-ray free-electron lasers have significantly advanced the field by enabling advanced pump-probe schemes. There is an increasing interest in using table-top photon sources enabled by high-harmonic generation of ultrashort-pulse lasers for such studies. We present a novel high-harmonic source driven by a 100 kHz fiber laser system, which delivers 10^11 photons/s in a single 1.3 eV bandwidth harmonic at 68.6 eV. The combination of record-high photon flux and high repetition rate paves the way for time-resolved studies of the dissociation dynamics of inner-shell ionized molecules in a coincidence detection scheme. First coincidence measurements on CH3I are shown and it is outlined how the anticipated advancement of fiber laser technology and improved sample delivery will, in the next step, allow pump-probe studies of ultrafast molecular dynamics with table-top XUV-photon sources. These table-top sources can provide significantly higher repetition rates than the currently operating free-electron lasers and they offer very high temporal resolution due to the intrinsically small timing jitter between pump and probe pulses.

J. Rothhardt, C. Rothhardt, M. Müller, A. Klenke, M. Kienel, S. Demmler, T. Elsmann, M. Rothhardt, J. Limpert, and A. Tünnermann
100  W average power femtosecond laser at 343  nm
Optics Letters 41, 1885 (2016)

Abstract: We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2<1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.

S. Hädrich, J. Rothhardt, S. Demmler, M. Tschernajew, A. Hoffmann, M. Krebs, A. Liem, O. d. Vries, M. Plötner, S. Fabian, T. Schreiber, J. Limpert, and A. Tünnermann
Scalability of components for kW-level average power few-cycle lasers
Applied Optics 55, 1636 (2016)

Abstract: In this paper, the average power scalability of components that can be used for intense few-cycle lasers based on nonlinear compression of modern femtosecond solid-state lasers is investigated. The key components of such a setup, namely, the gas-filled waveguides, laser windows, chirped mirrors for pulse compression and low dispersion mirrors for beam collimation, focusing, and beam steering are tested under high-average-power operation using a kilowatt cw laser. We demonstrate the long-term stable transmission of kW-level average power through a hollow capillary and a Kagome-type photonic crystal fiber. In addition, we show that sapphire substrates significantly improve the average power capability of metal-coated mirrors. Ultimately, ultrabroadband dielectric mirrors show negligible heating up to 1 kW of average power. In summary, a technology for scaling of few-cycle lasers up to 1 kW of average power and beyond is presented.

D. Davydova, A. de la Cadena, S. Demmler, J. Rothhardt, J. Limpert, T. Pascher, D. Akimov, and B. Dietzek
Ultrafast transient absorption microscopy: Study of excited state dynamics in PtOEP crystals
Journal of Chemical Physics 464, 69 (2016)

Abstract: We report a novel transient absorption microscope based on a tailor-made femtosecond fiber laser system operating at 250 kHz. The setup is applied to study PtOEP crystals embedded in a PBMA polymer matrix by analyzing the excited state dynamics in specific points of the sample as well as by spatially resolved excited state dynamics of the crystals. The results reveal the impact of the distortions of the crystal lattice, such as microcracks or amorphous regions caused by non-thermal melting on a lifetime of the excited triplet states of PtOEP crystals. Although transient absorption studies without any spatial resolution of PtOEP in solution and thin films were reported before, the study of spatially resolved excited state dynamics of micrometer-sized PtOEP crystals is performed for the first time to the best of our knowledge.

## 2015

Y. Shamir, J. Rothhardt, S. Hädrich, S. Demmler, M. Tschernajev, J. Limpert, and A. Tünnermann
High average power 2 mum few-cycle optical parametric chirped pulse amplifier at 100-kHz repetition rate
Optics letters 40, 5546 (2015)

Abstract: Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 µm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 µJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.

J. Rothhardt, S. Hädrich, S. Demmler, M. Krebs, D. Winters, T. Kuehl, T. Stöhlker, J. Limpert, and A. Tünnermann
Prospects for laser spectroscopy of highly charged ions with high-harmonic XUV and soft x-ray sources
Physica Scripta T166, 14030 (2015)

Abstract: We present novel high photon flux XUV and soft x-ray sources based on high harmonic generation (HHG). The sources employ femtosecond fiber lasers, which can be operated at very high (MHz) repetition rate and average power (>100 W). HHG with such lasers results in ∼1013 photons s−1 within a single harmonic line at ∼40 nm (∼30 eV) wavelength, a photon flux comparable to what is typically available at synchrotron beam lines. In addition, resonant enhancement of HHG can result in narrow-band harmonics with high spectral purity—well suited for precision spectroscopy. These novel light sources will enable seminal studies on electronic transitions in highly-charged ions. For example, at the experimental storage ring 2s1/2– 2p1/2 transitions in Li-like ions can be excited up to Z=47 (∼100 eV transition energy), which provides unique sensitivity to quantum electro-dynamical effects and nuclear corrections. We estimate fluorescence count rates of the order of tens per second, which would enable studies on short-lived isotopes as well. In combination with the Doppler up-shift available in head-on excitation at future heavy-ion storage rings, such as the high energy storage ring, even multi-keV transitions can potentially be excited. Pump–probe experiments with femtosecond resolution could also be feasible and access the lifetime of short-lived excited states, thus providing novel benchmarks for atomic structure theory.

S. Hädrich, M. Krebs, A. Hoffmann, A. Klenke, J. Rothhardt, J. Limpert, and A. Tünnermann
Exploring new avenues in high repetition rate table-top coherent extreme ultraviolet sources
Light: Science & Applications 4, e320 (2015)

Abstract: The process of high harmonic generation (HHG) enables the development of table-top sources of coherent extreme ultraviolet (XUV) light. Although these are now matured sources, they still mostly rely on bulk laser technology that limits the attainable repetition rate to the low kilohertz regime. Moreover, many of the emerging applications of such light sources (e.g., photoelectron spectroscopy and microscopy, coherent diffractive imaging, or frequency metrology in the XUV spectral region) require an increase in the repetition rate. Ideally, these sources are operated with a multi-MHz repetition rate and deliver a high photon flux simultaneously. So far, this regime has been solely addressed using passive enhancement cavities together with low energy and high repetition rate lasers. Here, a novel route with significantly reduced complexity (omitting the requirement of an external actively stabilized resonator) is demonstrated that achieves the previously mentioned demanding parameters. A krypton-filled Kagome photonic crystal fiber is used for efficient nonlinear compression of 9 mJ, 250 fs pulses leading to ,7 mJ, 31 fs pulses at 10.7 MHz repetition rate. The compressed pulses are used for HHG in a gas jet. Particular attention is devoted to achieving phase-matched (transiently) generation yielding .10^13 photons s-1 (.50 mW) at 27.7 eV. This new spatially coherent XUV source improved the photon flux by four orders of magnitude for direct multi-MHZ experiments, thus demonstrating the considerable potential of this source.

B. Manschwetus, N. Lin, J. Rothhardt, R. Guichard, T. Auguste, A. Camper, P. Breger, J. Caillat, M. Géléoc, T. Ruchon, R. Taïeb, B. Carré, and P. Salières
Self-Probing Spectroscopy of the SF 6 Molecule: A Study of the Spectral Amplitude and Phase of the High Harmonic Emission
The Journal of Physical Chemistry A 119, 6111 (2015)

Abstract: We present characterizations of the attosecond pulse train produced in the high harmonic generation (HHG) from SF6 molecules irradiated by a strong pulsed laser field at 800 nm. At harmonic order 17, we observe a minimum in the amplitude of the emitted spectrum and a corresponding distortion in the phase. Our experimental results are compared to two models: a multicenter interference model focused on the effect of the structure of the SF6 molecule in HHG and a model focused on the interferences between multiple ionization channels in HHG. We find that the experimental results agree very well with the multiple ionization channels model, illustrating that HHG in molecules can be very complex and that it provides insights of the intramolecular electron dynamics during the interaction process.

## 2014

A. Klenke, S. Hädrich, T. Eidam, J. Rothhardt, M. Kienel, S. Demmler, T. Gottschall, J. Limpert, and A. Tünnermann
22  GW peak-power fiber chirped-pulse-amplification system
Optics Letters 39, 6875 (2014)

Abstract: In this Letter, we report on a femtosecond fiber chirped-pulse-amplification system based on the coherent combination of the output of four ytterbium-doped large-pitch fibers. Each single channel delivers a peak power of about 6.2 GW after compression. The combined system emits 200 fs long pulses with a pulse energy of 5.7 mJ at 230 W of average power together with an excellent beam quality. The resulting peak power is 22 GW, which to the best of our knowledge is the highest value directly emitted from any fiber-based laser system.

M. Zürch, J. Rothhardt, S. Hädrich, S. Demmler, M. Krebs, J. Limpert, A. Tünnermann, A. Guggenmos, U. Kleineberg, and C. Spielmann
Real-time and Sub-wavelength Ultrafast Coherent Diffraction Imaging in the Extreme Ultraviolet
Scientific Reports 4, 7356 (2014)

Abstract: Coherent Diffraction Imaging is a technique to study matter with nanometer-scale spatial resolution based on coherent illumination of the sample with hard X-ray, soft X-ray or extreme ultraviolet light delivered from synchrotrons or more recently X-ray Free-Electron Lasers. This robust technique simultaneously allows quantitative amplitude and phase contrast imaging. Laser-driven high harmonic generation XUV-sources allow table-top realizations. However, the low conversion efficiency of lab-based sources imposes either a large scale laser system or long exposure times, preventing many applications. Here we present a lensless imaging experiment combining a high numerical aperture (NA = 0.8) setup with a high average power fibre laser driven high harmonic source. The high flux and narrow-band harmonic line at 33.2 nm enables either sub-wavelength spatial resolution close to the Abbe limit (Δr = 0.8λ) for long exposure time, or sub-70 nm imaging in less than one second. The unprecedented high spatial resolution, compactness of the setup together with the real-time capability paves the way for a plethora of applications in fundamental and life sciences.

S. Hädrich, A. Klenke, J. Rothhardt, M. Krebs, A. Hoffmann, O. Pronin, V. Pervak, J. Limpert, and A. Tünnermann
High photon flux table-top coherent extreme-ultraviolet source
Nature Photonics 8, 779 (2014)

Abstract: High harmonic generation (HHG) enables extreme-ultraviolet radiation with table-top set-ups. Its exceptional properties, such as coherence and (sub)-femtosecond pulse durations, have led to a diversity of applications. Some of these require a high photon flux and megahertz repetition rates, for example, to avoid space charge effects in photoelectron spectroscopy. To date, this has only been achieved with enhancement cavities. Here, we establish a novel route towards powerful HHG sources. By achieving phase-matched HHG of a megahertz fibre laser we generate a broad plateau (25 eV – 40 eV) of strong harmonics, each containing more than 1 × 10^12 photons s–1, which constitutes an increase by more than one order of magnitude in that wavelength range. The strongest harmonic (H25, 30 eV) has an average power of 143 μW (3 × 10^13 photons s–1). This concept will greatly advance and facilitate applications in photoelectron or coincidence spectroscopy, coherent diffractive imaging or (multidimensional) surface science.

J. Rothhardt, S. Hädrich, A. Klenke, S. Demmler, A. Hoffmann, T. Gotschall, T. Eidam, M. Krebs, J. Limpert, and A. Tünnermann
53 W average power few-cycle fiber laser system generating soft x rays up to the water window
Optics Letters 39, 5224 (2014)

Abstract: We report on a few-cycle laser system delivering sub-8-fs pulses with 353 µJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.

R. Riedel, J. Rothhardt, K. Beil, B. Gronloh, A. Klenke, H. Höppner, M. Schulz, U. Teubner, C. Kränkel, J. Limpert, A. Tünnermann, M. Prandolini, and F. Tavella
Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification
Optics Express 22, 17607 (2014)

Abstract: The potential of borate crystals, BBO, LBO and BiBO, for high average power scaling of optical parametric chirped-pulse amplifiers is investigated. Up-to-date measurements of the absorption coefficients at 515 nm and the thermal conductivities are presented. The measured absorption coefficients are a factor of 10–100 lower than reported by the literature for BBO and LBO. For BBO, a large variation of the absorption coefficients was found between crystals from different manufacturers. The linear and nonlinear absorption coefficients at 515 nm as well as thermal conductivities were determined for the first time for BiBO. Further, different crystal cooling methods are presented. In addition, the limits to power scaling of OPCPAs are discussed.

M. Kienel, M. Müller, S. Demmler, J. Rothhardt, A. Klenke, T. Eidam, J. Limpert, and A. Tünnermann
Coherent beam combination of Yb:YAG single-crystal rod amplifiers
Optics Letters 39, 3278 (2014)

Abstract: Coherent combination of ultrashort laser pulses emitted from spatially separated amplifiers is a promising power-scaling technique for ultrafast laser systems. It has been successfully applied to fiber amplifiers, since guidance of the signal provides the advantage of an excellent beam quality and straightforward superposition of beams as compared to bulk-type amplifier implementations. Herein we demonstrate, for the first time to our knowledge, a two-channel combining scheme employing Yb:YAG single-crystal rod amplifiers as an energy booster in a fiber chirped-pulse amplification system. In this proof-of-principle experiment, combined and compressed pulses with a duration of 695 fs and an energy of 3 mJ (3.7 GW of peak power) are obtained. The combining efficiency is as high as 94% and the beam quality of the combined output is characterized by a measured M2-value of 1.2.

J. Rothhardt, S. Hädrich, S. Demmler, M. Krebs, S. Fritzsche, J. Limpert, and A. Tünnermann
Enhancing the Macroscopic Yield of Narrow-Band High-Order Harmonic Generation by Fano Resonances
Physical Review Letters 112, 233002 (2014)

Abstract: Resonances in the photoabsorption spectrum of the generating medium can modify the spectrum of high-order harmonics. In particular, window-type Fano resonances can reduce photoabsorption within a narrow spectral region and, consequently, lead to an enhanced emission of high-order harmonics in absorption-limited generation conditions. For high harmonic generation in argon it is shown that the 3s3p6np1P1 window resonances (n=4, 5, 6) give rise to enhanced photon yield. In particular, the 3s3p64p1P1 resonance at 26.6  eV allows a relative enhancement up to a factor of 30 in a 100 meV bandwidth compared to the characteristic photon emission of the neighboring harmonic order. This enhanced, spectrally isolated, and coherent photon emission line has a relative energy bandwidth of only ΔE/E=3×10^−3. Therefore, it might be very useful for applications such as precision spectroscopy or coherent diffractive imaging. The presented mechanism can be employed for tailoring and controlling the high harmonic emission of manifold target materials.

C. Rothhardt, J. Rothhardt, A. Klenke, T. Peschel, R. Eberhardt, J. Limpert, and A. Tünnermann
BBO-sapphire sandwich structure for frequency conversion of high power lasers
Optical Materials Express 4, 1092 (2014)

Abstract: We report on successful joining of a beta barium borate crystal by plasma-activated direct bonding. Based on this technology, a sandwich structure consisting of a beta barium borate crystal, joined with two sapphire heat spreaders has been fabricated. Due to the high thermal conductivity of sapphire, the sandwich structure possesses superior thermal properties compared to the single crystal. Simulations based on the finite element method indicate a significant reduction of thermal gradients and the resulting mechanical stresses. A proof of principle experiment demonstrates the high power capability of the fabricated structure. A pulsed fiber laser emitting up to 253 W average power has been frequency doubled with both a single BBO crystal and the fabricated sandwich structure. The bonded stack showed better heat dissipation and less thermo-optical beam distortion than the single crystal. The work demonstrates the huge potential of optical sandwich structures with enhanced functionality. In particular, frequency conversion at average powers in the kW range with excellent beam quality will be feasible in future.

J. Rothhardt, M. Krebs, S. Hädrich, S. Demmler, J. Limpert, and A. Tünnermann
Absorption-limited and phase-matched high harmonic generation in the tight focusing regime
New Journal of Physics 16, 033022 (2014)

Abstract: High harmonic generation (HHG) at a high repetition rate requires tight focusing of the moderate peak power driving pulses. So far the conversion efficiencies that have been achieved in this regime are orders of magnitude behind the values that have been demonstrated with loose focusing of high energy (high peak power) lasers. In this contribution, we discuss the scaling laws for the main physical quantities of HHG and in particular analyze the limiting effects: dephasing, absorption and plasma defocusing. It turns out that phase-matched and absorption-limited HHG can be achieved even for very small focal spot sizes using a target gas provided with an adequately high density. Experimentally, we investigate HHG in a gas jet of argon, krypton and xenon. By analyzing the pressure dependence we are able to disentangle the dephasing and absorption effects and prove that the generated high order harmonics are phase-matched and absorption-limited. The obtained conversion efficiency is as high as 8 × 10^−6 for the 17th harmonic generated in xenon and 1.4 × 10^−6 for the 27th harmonic generated in argon. Our findings pave the way for highly efficient harmonic generation at megahertz repetition rates.

## 2013

S. Demmler, J. Rothhardt, S. Hädrich, M. Krebs, A. Hage, J. Limpert, and A. Tünnermann
Generation of high photon flux coherent soft x-ray radiation with few-cycle pulses
Optics Letters 38, 5051 (2013)

Abstract: We present a tabletop source of coherent soft x-ray radiation with high-photon flux. Two-cycle pulses delivered by a fiber-laser-pumped optical parametric chirped-pulse amplifier operating at 180 kHz repetition rate are upconverted via high harmonic generation in neon to photon energies beyond 200 eV. A maximum photon flux of 1.3 x 10^{8}  photons/s is achieved within a 1% bandwidth at 125 eV photon energy. This corresponds to a conversion efficiency of ~10^{−9}, which can be reached due to a gas jet simultaneously providing a high target density and phase matching. Further scaling potential toward higher photon flux as well as higher photon energies are discussed.

S. Hädrich, A. Klenke, A. Hoffmann, T. Eidam, T. Gottschall, J. Rothhardt, J. Limpert, and A. Tünnermann
Nonlinear compression to sub-30-fs, 0.5  mJ pulses at 135  W of average power
Optics Letters 38, 3866 (2013)

Abstract: Incorporation of coherent combination into a state-of-the-art fiber-chirped pulse amplification system obtains 1.1 mJ, 340 fs pulses with up to 280 W of average power at 250 kHz repetition rate. Propagation of this laser pulse inside a krypton-filled hollow-core fiber results in significant spectral broadening. Chirped mirrors are used to compress the pulses to 26 fs, 540 μJ (135 W) leading to a peak power of more than 11 GW. This unprecedented combination of high peak and average power ultrashort pulses opens up new possibilities in multidimensional surface science and coherent soft x-ray generation.

M. Krebs, S. Hädrich, S. Demmler, J. Rothhardt, A. Zaïr, L. Chipperfield, J. Limpert, and A. Tünnermann
Towards isolated attosecond pulses at megahertz repetition rates
Nature Photonics 7, 555 (2013)

Abstract: The strong-field process of high-harmonic generation is the foundation for generating isolated attosecond pulses, which are the fastest controllable events ever induced. This coherent extreme-ultraviolet radiation has become an indispensable tool for resolving ultrafast motion in atoms and molecules. Despite numerous spectacular developments in the new field of attoscience the low data-acquisition rates imposed by low-repetition-rate (maximum of 3 kHz) laser systems hamper the advancement of these sophisticated experiments. Consequently, the availability of high-repetition-rate sources will overcome a major obstacle in this young field. Here, we present the first megahertz-level source of extreme-ultraviolet continua with evidence of isolated attosecond pulses using a fibre laser-pumped optical parametric amplifier for high-harmonic generation at 0.6 MHz. This 200-fold increase in repetition rate will enable and promote a vast variety of new applications, such as attosecond-resolution coincidence and photoelectron spectroscopy, or even video-rate acquisition for spatially resolved pump–probe measurements.

A. Klenke, S. Breitkopf, M. Kienel, T. Gottschall, T. Eidam, S. Hädrich, J. Rothhardt, J. Limpert, and A. Tünnermann
530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system
Optics Letters 38, 2283 (2013)

Abstract: We report on a femtosecond fiber laser system comprising four coherently combined large-pitch fibers as the main amplifier. With this system, a pulse energy of 1.3 mJ and a peak power of 1.8 GW are achieved at 400 kHz repetition rate. The corresponding average output power is as high as 530 W. Additionally, an excellent beam quality and efficiency of the combination have been obtained. To the best of our knowledge, such a parameter combination, i.e., gigawatt pulses with half a kilowatt average power, has not been demonstrated so far with any other laser architecture.

J. Rothhardt, S. Demmler, S. Hädrich, T. Peschel, J. Limpert, and A. Tünnermann
Thermal effects in high average power optical parametric amplifiers
Optics Letters 38, 763 (2013)

Abstract: Optical parametric amplifiers (OPAs) have the reputation of being average power scalable due to the instantaneous nature of the parametric process (zero quantum defect). This Letter reveals serious challenges originating from thermal load in the nonlinear crystal caused by absorption. We investigate these thermal effects in high average power OPAs based on beta barium borate. Absorption of both pump and idler waves is identified to contribute significantly to heating of the nonlinear crystal. A temperature increase of up to 148 K with respect to the environment is observed and mechanical tensile stress up to 40 MPa is found, indicating a high risk of crystal fracture under such conditions. By restricting the idler to a wavelength range far from absorption bands and removing the crystal coating we reduce the peak temperature and the resulting temperature gradient significantly. Guidelines for further power scaling of OPAs and other nonlinear devices are given.

## 2012

S. Hädrich, J. Rothhardt, M. Krebs, S. Demmler, J. Limpert, and A. Tünnermann
Improving carrier-envelope phase stability in optical parametric chirped-pulse amplifiers by control of timing jitter
Optics Letters 37, 4910 (2012)

Abstract: It is shown that timing jitter in optical parametric chirped-pulse amplification induces spectral drifts that transfer to carrier-envelope phase (CEP) instabilities via dispersion. Reduction of this effect requires temporal synchronization, which is realized with feedback obtained from the angularly dispersed idler. Furthermore, a novel method to measure the CEP drifts by utilizing parasitic second harmonic generation within parametric amplifiers is presented. Stabilization of the timing allows the obtainment of a CEP stability of 86 mrad over 40 min at 150 kHz repetition rate.

S. Demmler, J. Rothhardt, S. Hädrich, J. Bromage, J. Limpert, and A. Tünnermann
Control of nonlinear spectral phase induced by ultra-broadband optical parametric amplification
Optics Letters 37, 3933 (2012)

Abstract: Optical parametric amplifiers (OPAs) impose an optical parametric phase (OPP) onto the amplified signal. It manifests itself as a spectral phase in the case of broadband signals and, therefore, hampers pulse compression. Here we present, for the first time, a complete experimental characterization of this OPP for different ultra-broadband noncollinear OPA configurations. This measurement allows us to compensate the OPP and to achieve Fourier-limited pulses as short as 1.9 optical cycles. A numerical model is in excellent agreement with our measurements and reveals the importance of high order phase compensation in the case of noncollinear phase matching. In contrast, operation at degeneracy enables almost complete compensation of the OPP by second-order dispersion only.

J. Rothhardt, A. M. Heidt, S. Hädrich, S. Demmler, J. Limpert, and A. Tünnermann
High stability soliton frequency-shifting mechanisms for laser synchronization applications
Journal of the Optical Society of America B 29, 1257 (2012)

Abstract: We analyze frequency-shifting mechanisms in photonic crystal fibers (PCFs). In contrast to the generally used approach of launching pulses in the negative group velocity dispersion (GVD) region of PCFs, we suggest employing a fiber with a higher zero dispersion wavelength that is pumped in the positive GVD region. Results of a numerical optimization reveal that the amplitude stability of the frequency-shifted pulses can be improved by more than 1 order of magnitude and the timing jitter arising from input fluctuations by 2 orders of magnitude by a proper choice of the fiber dispersion. The presented approach and optimization will improve the performance of timing- and amplitude-sensitive applications, such as nonlinear microscopy and spectroscopy or optical synchronization for optical parametric chirped pulse amplification significantly.

T. Gottschall, M. Baumgartl, A. Sagnier, J. Rothhardt, C. Jauregui, J. Limpert, and A. Tünnermann
Fiber-based source for multiplex-CARS microscopy based on degenerate four-wave mixing
Optics Express 20, 12004 (2012)

Abstract: We present a fiber-based laser source for multiplex coherent anti-Stokes Raman scattering (CARS) microscopy. This source is very compact and potentially alignment-free. The corresponding pump and Stokes pulses for the CARS process are generated by degenerate four-wave mixing (FWM) in photonic-crystal fibers. In addition, an ytterbium-doped fiber laser emitting spectrally narrow 100 ps pulses at 1035 nm wavelength serves as pump for the FWM frequency conversion. The FWM process delivers narrow-band pulses at 648 nm and drives a continuum-like spectrum ranging from 700 to 820 nm. With the presented source vibrational resonances with energies between 1200 cm^(−1) and 3200 cm^(−1) can be accessed with a resolution of 10 cm^(−1). Additionally, the temporal characteristics of the FWM output have been investigated by a cross-correlation setup, revealing the suitability of the emitted pulses for CARS microscopy. This work marks a significant step towards a simple and powerful all-fiber, maintenance-free multiplex-CARS source for real-world applications outside a laboratory environment.

J. Rothhardt, S. Demmler, S. Hädrich, J. Limpert, and A. Tünnermann
Octave-spanning OPCPA system delivering CEP-stable few-cycle pulses and 22 W of average power at 1 MHz repetition rate
Optics Express 20, 10870 (2012)

Abstract: We report on an OPCPA system delivering CEP-stable pulses with a pulse duration of only 1.7 optical cycles at 880 nm wavelength. This pulse duration is achieved by the generation, optical parametric amplification and compression of a full optical octave of bandwidth. The system is pumped by a high average power Yb-fiber laser system, which allows for operation of the OPCPA at up to 1 MHz repetition rate and 22 W of average output power. Further scaling towards single-cycle pulses, higher energy and output power is discussed.

## 2011

J. Rothhardt, S. Hädrich, H. Carstens, N. Herrick, S. Demmler, J. Limpert, and A. Tünnermann
1 MHz repetition rate hollow fiber pulse compression to sub-100-fs duration at 100 W average power
Optics Letters 36, 4605 (2011)

Abstract: We report on nonlinear pulse compression at very high average power. A high-power fiber chirped pulse amplification system based on a novel large pitch photonic crystal fiber delivers 700 fs pulses with 200 μJ pulse energy at a 1 MHz repetition rate, resulting in 200 W of average power. Subsequent spectral broadening in a xenon-filled hollow-core fiber and pulse compression with chirped mirrors is employed for pulse shortening and peak power enhancement. For the first time, to our knowledge, more than 100 W of average power are transmitted through a noble-gas-filled hollow fiber. After pulse compression of 81 fs, 93 μJ pulses are obtained at a 1 MHz repetition rate.

A. Klenke, E. Seise, S. Demmler, J. Rothhardt, S. Breitkopf, J. Limpert, and A. Tünnermann
Coherently-combined two channel femtosecond fiber CPA system producing 3 mJ pulse energy
Optics Express 19, 24280 (2011)

Abstract: We present a fiber CPA system consisting of two coherently combined fiber amplifiers, which have been arranged in an actively stabilized Mach-Zehnder interferometer. Pulse durations as short as 470 fs and pulse energies of 3 mJ, corresponding to 5.4 GW of peak power, have been achieved at an average power of 30 W.

S. Demmler, J. Rothhardt, A. M. Heidt, A. Hartung, E. G. Rohwer, H. Bartelt, J. Limpert, and A. Tünnermann
Generation of high quality, 1.3 cycle pulses by active phase control of an octave spanning supercontinuum
Optics Express 19, 20151 (2011)

Abstract: Nonlinear pulse compression based on the generation of ultra-broadband supercontinuum (SC) in an all-normal dispersion photonic crystal fiber (ANDi PCF) is demonstrated. The highly coherent and smooth octave-spanning SC spectra are generated using 6 fs, 3 nJ pulses from a Ti:Sapphire oscillator for pumping a 13 mm piece of ANDi PCF. Applying active phase control has enabled the generation of 4.5 fs pulses. Additional spectral amplitude shaping has increased the bandwidth of the SC spectra further leading to nearly transform-limited pulses with a duration of 3.64 fs, which corresponds to only 1.3 optical cycles at a central wavelength of 810 nm. This is the shortest pulse duration achieved via compression of SC spectra generated in PCF to date. Due to the high stability and the smooth spectral intensity and phase distribution of the generated SC, an excellent temporal pulse quality exhibiting a pulse contrast of 14 dB with respect to the pre- and post-pulses is achieved.

S. Hädrich, M. Krebs, J. Rothhardt, H. Carstens, S. Demmler, J. Limpert, and A. Tünnermann
Generation of µW level plateau harmonics at high repetition rate
Optics Express 19, 19374 (2011)

Abstract: The process of high harmonic generation allows for coherent transfer of infrared laser light to the extreme ultraviolet spectral range opening a variety of applications. The low conversion efficiency of this process calls for optimization or higher repetition rate intense ultrashort pulse lasers. Here we present state-of-the-art fiber laser systems for the generation of high harmonics up to 1 MHz repetition rate. We perform measurements of the average power with a calibrated spectrometer and achieved µW harmonics between 45 nm and 61 nm (H23-H17) at a repetition rate of 50 kHz. Additionally, we show the potential for few-cycle pulses at high average power and repetition rate that may enable water-window harmonics at unprecedented repetition rate.

J. Bromage, J. Rothhardt, S. Hädrich, C. Dorrer, C. Jocher, S. Demmler, J. Limpert, A. Tünnermann, and J. D. Zügel
Analysis and suppression of parasitic processes in noncollinear optical parametric amplifiers
Optics Express 19, 16797 (2011)

Abstract: The influence of parasitic processes on the performance of ultra-broadband noncollinear optical parametric amplifiers (NOPA’s) is investigated for walk-off and non-walk-off compensating configurations. Experimental results with a white-light–seeded NOPA agree well with numerical simulations. The same model shows that 10% of the output energy of an amplified signal can be transferred into a parasitic second harmonic of the signal. These findings are supported by quantitative measurements on a few-cycle NOPA, where a few percent of the signal energy is converted to its second harmonic in the walk-off compensating case. This effect is reduced by an order of magnitude in the non-walk-off compensating configuration. A detailed study of the phase-matching conditions of the most common nonlinear crystals provides guidelines for designing NOPA systems.

A. Heidt, J. Rothhardt, A. Hartung, H. Bartelt, E. Rohwer, J. Limpert, and A. Tünnermann
High quality sub-two cycle pulses from compression of supercontinuum generated in all-normal dispersion photonic crystal fiber
Optics Express 19, 13873 (2011)

Abstract: We demonstrate nonlinear pulse compression based on recently introduced highly coherent broadband supercontinuum (SC) generation in all-normal dispersion photonic crystal fiber (ANDi PCF). The special temporal properties of the octave-spanning SC spectra generated with 15 fs, 1.7 nJ pulses from a Ti:Sapphire oscillator in a 1.7 mm fiber piece allow the compression to 5.0 fs high quality pulses by linear chirp compensation with a compact chirped mirror compressor. This is the shortest pulse duration achieved to date from the external recompression of SC pulses generated in PCF. Numerical simulations in excellent agreement with the experimental results are used to discuss the scalability of the concept to the single-cycle regime employing active phase shaping. We show that previously reported limits to few-cycle pulse generation from compression of SC spectra generated in conventional PCF possessing one or more zero dispersion wavelengths do not apply for ANDi PCF.

S. Hädrich, H. Carstens, J. Rothhardt, J. Limpert, and A. Tünnermann
Multi-gigawatt ultrashort pulses at high repetition rate and average power from two-stage nonlinear compression
Optics Express 19, 7546 (2011)

Abstract: We present simple and compact (1.5 m x 0.5 m footprint) post-compression of a state-of-the-art fiber chirped pulse amplification system. By using two stage nonlinear compression in noble gas filled hollow core fibers we shorten 1 mJ, 480 fs, 50 kHz pulses. The first stage is a 53 cm long, 200 µm inner diameter fiber filled with xenon with subsequent compression in a chirped mirror compressor. A 20 cm, 200 µm inner diameter fiber filled with argon further broadens the spectrum in a second stage and compression is achieved with another set of chirped mirrors. The average power is 24.5 W / 19 W after the first / second stage, respectively. Compression to 35 fs is achieved. Numerical simulations, agreeing well with experimental data, yield a peak power of 5.7 GW at a pulse energy of 380 µJ making this an interesting source for high harmonic generation at high repetition rate and average power.

T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, J. Rothhardt, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann
Preferential gain photonic-crystal fiber for mode stabilization at high average powers
Optics Express 19, 8656 (2011)

Abstract: We report on the design and experimental investigation of a preferential gain photonic-crystal fiber with a mode-field diameter of 47 µm. This few-mode fiber design confines the doping of Ytterbium-ions just to the center of the core and, therefore, promotes fundamental mode operation. In a chirped-pulse amplification system we extracted up to 303 W of average power from this fiber with a measured M^2 value of 1.4.

J. Limpert, S. Hädrich, J. Rothhardt, M. Krebs, T. Eidam, T. Schreiber, and A. Tünnermann
Ultrafast fiber lasers for strong-field physics experiments
Laser & Photonics Reviews 5, 634 (2011)

Abstract: The recent demonstration of rare-earth-doped fiber lasers with a continuous-wave output power approaching the 10-kW level with diffraction-limited beam quality proves that fiber lasers constitute a scalable solid-state laser concept in terms of average power. In order to generate high peak power pulses from a fiber several fundamental limitations have to be overcome. This can be achieved by novel experimental strategies and fiber designs that offer an enormous potential towards ultrafast laser systems combining high average powers (> kW) and high peak power (> GW). In this paper the challenges, achievements and perspectives of ultrashort pulse generation and amplification in fibers are reviewed. This kind of laser system will have a tremendous impact on strong-field physics experiments, such as the generation of coherent light by high-harmonic generation. So far, applications in the interesting EUV spectral range suffer from the very low photon count leading to nonrelevant integration times with highly sophisticated detection schemes. High repetition rate high average power fiber lasers can potentially solve this issue. First demonstrations of high repetition-rate strong-field physics experiments using novel fiber laser systems will be discussed.

O. Schmidt, M. Rekas, C. Wirth, J. Rothhardt, S. Rhein, A. Kliner, M. Strecker, T. Schreiber, J. Limpert, R. Eberhardt, and A. Tünnermann
High power narrow-band fiber-based ASE source
Optics Express 19, 4421 (2011)

Abstract: In this paper we describe a high power narrow-band amplified spontaneous emission (ASE) light source at 1030 nm center wavelength generated in an Yb-doped fiber-based experimental setup. By cutting a small region out of a broadband ASE spectrum using two fiber Bragg gratings a strongly constrained bandwidth of 12±2 pm (3.5±0.6 GHz) is formed. A two-stage high power fiber amplifier system is used to boost the output power up to 697 W with a measured beam quality of M2≤1.34. In an additional experiment we demonstrate a stimulated Brillouin scattering (SBS) suppression of at least 17 dB (theoretically predicted ~20 dB), which is only limited by the dynamic range of the measurement and not by the onset of SBS when using the described light source. The presented narrow-band ASE source could be of great interest for brightness scaling applications by beam combination, where SBS is known as a limiting factor.

J. Rothhardt, T. Eidam, S. Hädrich, F. Jansen, F. Stutzki, T. Gottschall, T. Andersen, J. Limpert, and A. Tünnermann
135 W average-power femtosecond pulses at 520 nm from a frequency-doubled fiber laser system
Optics Letters 36, 316 (2011)

Abstract: We present a high-average-power femtosecond laser system at 520 nm central wavelength. The laser system delivers sub- 500 fs pulses with 135 W average power at a pulse repetition rate of 5.25 MHz. Excellent beam quality is provided by high power fiber amplifiers and maintained during frequency doubling, resulting in a beam quality factor of M^2 < 1.2. To our knowledge, the system presented here is the highest average power green laser source generating femtosecond pulses with diffraction-limited beam quality.

S. Hädrich, S. Demmler, J. Rothhardt, C. Jocher, J. Limpert, and A. Tünnermann
High-repetition-rate sub-5-fs pulses with 12 GW peak power from fiber-amplifier-pumped optical parametric chirped-pulse amplification
Optics Letters 36, 313 (2011)

Abstract: An optical parametric chirped-pulse amplification system delivering pulses with more than 12 GW peak power is presented. Compression to sub- 5 fs, 87 μJ and 5.4 fs, 100 μJ is realized at the 30 kHz repetition rate. A high-energy fiber chirped-pulse amplification system operating at 1 mJ pulse energy and nearly transform-limited pulses is used to achieve ultrabroadband amplification in two 2mm beta-barium borate crystals. Precise pulse shaping is used to compress the pulses to a few percentages of their transform limit. Assuming diffraction limited focusing (d < 2 μm), peak intensities as high as 10^(18) W/cm2 can be reached.

T. Eidam, J. Rothhardt, F. Stutzki, F. Jansen, S. Hädrich, H. Carstens, C. Jauregui, J. Limpert, and A. Tünnermann
Fiber chirped-pulse amplification system emitting 3.8 GW peak power
Optics Express 19, 255 (2011)

Abstract: We report on the experimental demonstration of a fiber chirped- pulse amplification system capable of generating nearly transform-limited sub 500 fs pulses with 2.2 mJ pulse energy at 11 W average power. The resulting record peak power of 3.8 GW could be achieved by combining active phase shaping with an efficient reduction of the acquired nonlinear phase. Therefore, we used an Ytterbium-doped large-pitch fiber with a mode field diameter of 105 µm as the main amplifier.

## 2010

S. Hädrich, J. Rothhardt, M. Krebs, F. Tavella, A. Willner, J. Limpert, and A. Tünnermann
High harmonic generation by novel fiber amplifier based sources
Optics Express 18, 20242 (2010)

Abstract: Significant progress in high repetition rate ultrashort pulse sources based on fiber technology is presented. These systems enable operation at a high repetition rate of up to 500 kHz and high average power in the extreme ultraviolet wavelength range via high harmonic generation in a gas jet. High average power few-cycle pulses of a fiber amplifier pumped optical parametric chirped pulse amplifier are used to produce µW level average power for the strongest harmonic at 42.9 nm at a repetition rate of 96 kHz.

J. Rothhardt, S. Hädrich, E. Seise, M. Krebs, F. Tavella, A. Willner, S. Duesterer, H. Schlarb, J. Feldhaus, J. Limpert, J. Rossbach, and A. Tünnermann
High average and peak power few-cycle laser pulses delivered by fiber pumped OPCPA system
Optics Express 18, 12719 (2010)

Abstract: We report on a high power optical parametric amplifier delivering 8 fs pulses with 6 GW peak power. The system is pumped by a fiber amplifier and operated at 96 kHz repetition rate. The average output power is as high as 6.7 W, which is the highest average power few-cycle pulse laser reported so far. When stabilizing the seed oscillator, the system delivered carrier-envelop phase stable laser pulses. Furthermore, high harmonic generation up to the 33th order (21.8 nm) is demonstrated in a Krypton gas jet. In addition, the scalability of the presented laser system is discussed.

F. Tavella, A. Willner, J. Rothhardt, S. Hädrich, E. Seise, S. Duesterer, T. Tschentscher, H. Schlarb, J. Feldhaus, J. Limpert, A. Tünnermann, and J. Rossbach
Fiber-amplifier pumped high average power few-cycle pulse non-collinear OPCPA
Optics Express 18, 4689 (2010)

Abstract: We report on the performance of a 60 kHz repetition rate sub-10 fs, optical parametric chirped pulse amplifier system with 2 W average power and 3 GW peak power. This is to our knowledge the highest average power sub-10 fs kHz-amplifier system reported to date. The amplifier is conceived for applications at free electron laser facilities and is designed such to be scalable in energy and repetition rate.

S. Hädrich, T. Gottschall, J. Rothhardt, J. Limpert, and A. Tünnermann
CW seeded optical parametric amplifier providing wavelength and pulse duration tunable nearly transform limited pulses
Optics Express 18, 3158 (2010)

Abstract: An optical parametric amplifier that delivers nearly transform limited pulses is presented. The center wavelength of these pulses can be tuned between 993 nm and 1070 nm and, at the same time, the pulse duration is varied between 206 fs and 650 fs. At the shortest pulse duration the pulse energy was increased up to 7.2 microJ at 50 kHz repetition rate. Variation of the wavelength is achieved by applying a tunable cw seed while the pulse duration can be varied via altering the pump pulse duration. This scheme offers superior flexibility and scaling possibilities.

## 2009

D. Nodop, J. Rothhardt, S. Hädrich, J. Limpert, and A. Tünnermann
Wavelength-independent all-optical synchronization of a Q-switched 100-ps microchip laser to a femtosecond laser reference source
Applied Physics B 94, 399 (2009)

Abstract: We present a Q-switched microchip laser emitting 1064-nm pulses as short as 100 ps synchronized to a cavity dumped femtosecond laser emitting 800-nm pulses as short as 80 fs. The synchronization is achieved by presaturating the saturable absorber of the microchip laser with femtosecond pulses even though both lasers emit at widely separated wavelengths. The mean timing jitter is 40 ps and thus considerably shorter than the pulse duration of the microchip laser.

J. Rothhardt, S. Hädrich, T. Gottschall, T. Clausnitzer, J. Limpert, and A. Tünnermann
Compact fiber amplifier pumped OPCPA system delivering Gigawatt peak power 35 fs pulses
Optics Express 17, 24130 (2009)

Abstract: We report on a compact Gigawatt peak power OPCPA system which is pumped by the second harmonic of an Yb-doped fiber amplifier and seeded by a cavity dumped Ti:Sapphire oscillator. Picosecond pump pulses for the OPCPA are generated by spectral filtering and directly amplified to 1 mJ pulse energy in several fiber amplifiers, without the need of chirped pulse amplification. Since no stretcher and compressor is required, the pump laser is very compact and easy to operate. The two stage optical parametric amplifier delivers 35 fs pulses with 53 microJ pulse energy and 1.1 GW peak power at 40 kHz repetition rate. Additionally, the scaling potential of this approach is discussed.

J. Rothhardt, S. Hädrich, T. Gottschall, J. Limpert, A. Tünnermann, M. Rothhardt, M. Becker, S. Brückner, and H. Bartelt
Generation of flattop pump pulses for OPCPA by coherent pulse stacking with fiber Bragg gratings
Optics Express 17, 16332 (2009)

Abstract: We present a simple and robust pulse shaping device based on coherent pulse stacking. The device is embedded in a polarisation maintaining step index fiber. An input pulse is sent through a fiber optical circulator. Up to four pulse replicas are reflected by fiber Bragg gratings and interfere at the output. Temperature control allows tuning of the relative pulse phases of the sub-pulses. Additionally fine tuning of the sub-pulse amplitudes is demonstrated. We experimentally generated 235 ps and 416 ps long flattop pulses with rising and falling edges shorter than 100 ps. In contrast to other pulse shaping techniques the presented setup is robust, alignment free, provides excellent beam quality and is also suitable for pulse durations up to several nanoseconds.

C. Aguergaray, O. Schmidt, J. Rothhardt, D. Schimpf, D. Descamps, S. Petit, J. Limpert, and E. Cormier
Ultra-wide parametric amplification at 800 nm toward octave spanning.
Optics Express 17, 5153 (2009)

Abstract: We report on a significant improvement of the total bandwidth amplified in an optical parametric process. By pumping a parametric amplifier with a broadband pump, we demonstrate amplification of a supercontinuum whose spectrum expands over nearly an octave ranging from less than 600 nm up to 1200 nm. Our amplifier stage is set to provide amplification at degeneracy in the quasi-collinear configuration with a temporally as well as angularly dispersed pump.

J. Rothhardt, S. Hädrich, J. Limpert, and A. Tünnermann
80 kHz repetition rate high power fiber amplifier flat-top pulse pumped OPCPA based on BIB₃O₆
Optics Express 17, 2508 (2009)

Abstract: We present a high peak power optical parametric chirped pulse amplifier (OPCPA) seeded by a cavity dumped Ti:Sapphire oscillator. A frequency doubled high power Ytterbium-doped fiber amplifier is pumping the device. Temporal synchronization of the pump pulses is done via soliton generation in a highly nonlinear photonic crystal fiber. This soliton is fiber amplified and spectrally filtered in several fiber amplifiers. A simple birefringent pulse shaper generates a flat-top temporal pump pulse profile. Direct amplification of these pulses in large mode area fibers without using a stretcher and compressor provides significantly reduced complexity. For the first time to our knowledge broadband amplification around 800 nm central wavelength is demonstrated in BIB(3)O(6) (BIBO) crystals. The stretched Ti:Sapphire oscillator pulses are amplified up to a pulse energy of 25 microJ. Recompression with a grating compressor yields 50.7 fs pulses with 16.2 microJ pulse energy.

S. Hädrich, J. Rothhardt, T. Eidam, J. Limpert, and A. Tünnermann
High energy ultrashort pulses via hollow fiber compression of a fiber chirped pulse amplification system
Optics Express 17, 3913 (2009)

Abstract: A simple, robust and compact pulse compressor for a high-repetition rate high-peak power fiber chirped pulse amplification system is presented. We use noble-gas-filled hollow fibers for spectral broadening of the optical pulses via self-phase modulation. Subsequent compression with chirped mirrors shortens the pulses by more than a factor of 10. Pulses shorter than 70 fs with pulse energies of the order of 100 µ J have been obtained resulting in a peak power up to 1GW at 30.3kHz. Additionally, nonlinear polarization rotation has been used for temporal pulse cleaning during the nonlinear compression at 30.3kHz and 100kHz, respectively.

T. Eidam, S. Hädrich, F. Röser, E. Seise, T. Gottschall, J. Rothhardt, T. Schreiber, J. Limpert, and A. Tünnermann
A 325-W-Average-Power Fiber CPA System Delivering Sub-400 fs Pulses
IEEE Journal of Selected Topics in Quantum Electronics 15, 187 (2009)

Abstract: A high-average-power Yb-doped fiber chirped-pulse amplification system is presented. Compressed average power of 325 W at 40 MHz repetition rate corresponding to 8.2 muJ pulse energy is extracted. Compression in a highly efficient dielectric-grating-based compressor yields pulses as short as 375 fs, resulting in 22 MW of peak power.

J. Limpert, R. Fabian, D. Schimpf, E. Seise, T. Eidam, S. Hädrich, J. Rothhardt, C. Jauregui, and A. Tünnermann
High Repetition Rate Gigawatt Peak Power Fiber Laser Systems: Challenges, Design, and Experiment
IEEE Journal of Selected Topics in Quantum Electronics 15, 159 (2009)

Abstract: We review the main challenges and give design guidelines for high-peak-power high-average-power fiber-based chirped-pulse amplification (CPA) systems. It is clearly pointed out that the lowest order fiber nonlinearity (NL), namely the self-phase modulation, limits the scalability of high-energy ultrashort pulse fiber amplifiers. Therefore, a distinguished difference arises between the consequences of accumulated nonlinear phase originating from the pulse envelope and initial weak modulations, resulting in a strong recommendation to operate an amplification system as linearly as possible in order to generate high-contrast pulses. Low-NL rare-earth-doped fibers, such as the recently available designs of photonic crystal fibers, are the key element for successful peak power scaling in fiber laser systems. In this paper, we present a detailed analysis and optimization of the extraction characteristics in connection with the accumulated nonlinear phase in such extreme fiber dimensions. Consequently, millijoule pulse energy femtosecond pulses at repetition rates in the 100 kHz range have already been demonstrated experimentally in a Yb-fiber-based CPA system that has even further scaling potential.

## 2008

S. Hädrich, J. Rothhardt, F. Röser, T. Gottschall, J. Limpert, and A. Tünnermann
Degenerate optical parametric amplifier delivering sub 30 fs pulses with 2GW peak power
Optics Express 16, 19812 (2008)

Abstract: Degenerated optical parametric amplification (OPA) is a well known technique to achieve broadband amplification necessary to generate ultrashort pulses. Here we present a parametric amplifier pumped by the frequency doubled output of a state-of-the-art fiber chirped pulse amplification system (FCPA) delivering mJ pulse energy at 30 kHz repetition rate and 650 fs pulse duration. The parametric amplifier and the FCPA system are both seeded by the same Yb:KGW oscillator. Additional spectral broadening of the OPA seed provides enough bandwidth for the generation of ultrashort pulses. After amplification in two 1mm BBO crystals a pulse energy of 90 microJ is yielded at 30 kHz. Subsequent compression with a sequence of chirped mirrors shortens the pulses to 29 fs while the pulse energy is as high as 81 µJ resulting in 2 GW of peak power.

J. Rothhardt, S. Hädrich, F. Röser, J. Limpert, and A. Tünnermann
500 MW peak power degenerated optical parametric amplifier delivering 52 fs pulses at 97 kHz repetition rate
Optics Express 16, 8981 (2008)

Abstract: We present a high peak power degenerated parametric amplifier operating at 1030 nm and 97 kHz repetition rate. Pulses of a state-of-the art fiber chirped-pulse amplification (FCPA) system with 840 fs pulse duration and 410 µJ pulse energy are used as pump and seed source for a two stage optical parametric amplifier. Additional spectral broadening of the seed signal in a photonic crystal fiber creates enough bandwidth for ultrashort pulse generation. Subsequent amplification of the broadband seed signal in two 1 mm BBO crystals results in 41 µJ output pulse energy. Compression in a SF 11 prism compressor yields 37 µJ pulses as short as 52 fs. Thus, pulse shortening of more than one order of magnitude is achieved. Further scaling in terms of average power and pulse energy seems possible and will be discussed, since both concepts involved, the fiber laser and the parametric amplifier have the reputation to be immune against thermo-optical effects.

O. Schmidt, J. Rothhardt, T. Eidam, F. Röser, J. Limpert, A. Tünnermann, K. P. Hansen, C. Jakobsen, and J. Broeng
Single-polarization ultra-large-mode-area Yb-doped photonic crystal fiber
Optics Express 16, 3918 (2008)

Abstract: We report on an ytterbium-doped single-transverse-mode rod-type photonic crystal fiber that combines the advantages of low nonlinearityand intrinsic polarization stability. The mode-field-area of the fundamental mode is as large as 2300 µm2. An output power of up to 163 W with adegree of polarization better than 85% has been extracted from a simple fiber laser setup without any additional polarizing element within the cavity than the fiber itself. The beam quality has been characterized by a M2 value of 1.2. The single-polarization window ranges from 1030 to 1080 nm, hence possesses an excellent overlap with the gain profile of ytterbium-doped silica fibers. To the best of our knowledge this fiber design has the largest mode-field-diameter ever reported for polarizing or even polarization maintaining rare-earth-doped double-clad fibers.

## 2007

J. Rothhardt, S. Hädrich, D. Schimpf, J. Limpert, and A. Tünnermann
High repetition rate fiber amplifier pumped sub-20 fs optical parametric amplifier
Optics Express 15, 16729 (2007)

Abstract: We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) based on a cavity dumped Ti:Sapphire oscillator providing the signal, and an Ytterbium-doped fiber amplifier pumping the device. Temporally synchronized NOPA pump pulses are created via soliton generation in a highly nonlinear photonic crystal fiber. This soliton is fiber amplified to high pulse-energies at high repetition rates. The broadband Ti:Sapphire laser pulses are parametrically amplified either directly or after additional spectral broadening. The approach of fiber-based pump-pulse generation from a femtosecond laser, that emits in the spectral region of NOPA-gain, offers enhanced long-term stability and pulse quality compared to conventional techniques, such as signal pulse generation from a high power laser system via filamentation in bulk media. The presented system produces high-energy ultra-short pulses with pulse-durations down to 15.6 fs and pulse-energies up to 500 nJ at a repetition rate as high as 2 MHz.

F. Röser, T. Eidam, J. Rothhardt, O. Schmidt, D. Schimpf, J. Limpert, and A. Tünnermann
Millijoule pulse energy high repetition rate femtosecond fiber chirped-pulse amplification system
Optics Letters 32, 3495 (2007)

Abstract: We report on an ytterbium-doped fiber chirped-pulse amplification (CPA) system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses are as short as 800 fs. As the main amplifier, an 80 µm core diameter short length photonic crystal fiber is employed, which allows the generation of pulse energies up to 1.45 mJ with a B-integral as low as 7 at a stretched pulse duration of 2 ns. A stretcher-compressor unit consisting of dielectric diffraction gratings is capable of handling the average power without beam and pulse quality distortions. To our knowledge, we present the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.

D. N. Schimpf, J. Rothhardt, J. Limpert, A. Tünnermann, and D. C. Hanna
Theoretical analysis of the gain bandwidth for noncollinear parametric amplification of ultrafast pulses
Journal of the Optical Society of America B 24, 2837 (2007)

Abstract: The choice of optimum phase-matching conditions for noncollinear optical parametric amplifiers is usually made on the basis of the linear spectral dispersion characteristics of the anisotropic nonlinear crystal. However, for high-peak-power operation, where pump depletion is involved, it is shown that the tolerance of the parametric gain with regard to k-vector mismatch is to change the optimum phase-matching parameters. Our calculations show that, with the revised parameters, an enhancement in peak power approaching 50% could be achieved.

O. Schmidt, J. Rothhardt, F. Röser, S. Linke, T. Schreiber, K. Rademaker, J. Limpert, S. Ermeneux, P. Yvernault, F. Salin, and A. Tünnermann
Millijoule pulse energy Q-switched short-length fiber laser
Optics Letters 32, 1551 (2007)

Abstract: We report on a Q-switched short-length fiber laser producing 100 W of average output power at 100 kHz repetition rate and pulse durations as short as 17 ns. Up to 2 mJ of energy and sub-10-ns pulse duration are extracted at lower repetition rates. This performance is obtained by employing a rod-type ytterbium-doped photonic crystal fiber with a 70 microm core as gain medium, allowing for very short pulse durations, high energy storage, and emission of a single-transverse-mode beam.

C. Aguergaray, T. Andersen, D. Schimpf, O. Schmidt, J. Rothhardt, T. Schreiber, J. Limpert, E. Cormier, and A. Tünnermann
Parametric amplification and compression to ultrashort pulse duration of resonant linear waves
Optics Express 15, 5699 (2007)

Abstract: We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation. Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal generation is a promising alternative to white light generation in bulky glass or sapphire plates. We demonstrate a novel and successful signal engineering implemented in a setup for parametric amplification and subsequent recompression of resonant linear waves resulting from soliton fission in a highly nonlinear photonic crystal fiber. The applied pump source is a high repetition rate ytterbium-doped fiber chirped pulse amplification system. The presented approach results in the generation of \~50 fs pulses at MHz repetition rate. The potential of generating even shorter pulse duration and higher pulse energies will be discussed.

## 2006

J. Limpert, O. Schmidt, J. Rothhardt, F. Röser, T. Schreiber, A. Tünnermann, S. Ermeneux, P. Yvernault, and F. Salin
Extended single-mode photonic crystal fiber lasers
Optics Express 14, 2715 (2006)

Abstract: We report on an ytterbium-doped photonic crystal fiber with a core diameter of 60 microm and mode-field-area of ~2000 µm^2 of the emitted fundamental mode. Together with the short absorption length of 0.5 m this fiber possesses a record low nonlinearity which makes this fiber predestinated for the amplification of short laser pulses to very high peak powers. In a first continuous-wave experiment a power of 320 W has been extracted corresponding to 550 W per meter. To our knowledge this represents the highest power per unit length ever reported for fiber lasers. Furthermore, the robust single-transverse-mode propagation in a passive 100 microm core fiber with a similar design reveals the potential of extended large-mode-area photonic crystal fibers.

## 2005

F. Röser, J. Rothhardt, B. Ortac, A. Liem, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann
131 W 220 fs fiber laser system
Optics Letters 30, 2754 (2005)

Abstract: We report on an ytterbium-doped photonic-crystal-fiber-based chirped-pulse amplification system delivering 131 W average power 220 fs pulses at 1040 nm center wavelength in a diffraction-limited beam. The pulse repetition rate is 73 MHz, corresponding to a pulse energy of 1.8 µJ and a peak power as high as 8.2 MW.