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Miscellaneous publications


2021

R. Röhlsberger, J. Evers, Y. Yoshida, and G. Langouche
Quantum Optical Phenomena in Nuclear Resonant Scattering
in: Modern Mössbauer Spectroscopy (Springer Singapore) (2021)
No abstract availableLinkBibTeX
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.

T. Aidukas, L. Lötgering, and A. R. Harvey
Segmentation-free, full-field Fourier ptychography
(2021)

Abstract: We present a simple computational method for full-field, segmentation-free Fourier-ptychographic reconstruction, which requires only two multiplications prior and after the reconstruction. This way, quantitative widefield reconstruction is possible even in the presence of illumination curvature.

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.

A. Klenke, A. Steinkopff, C. Aleshire, C. Jauregui, S. Kuhn, J. Nold, N. Haarlammert, T. Schreiber, A. Tunnermann, and J. Limpert
1 kW average power emission from an in-house 4x4 multicore rod-type fiber
(2021)

Abstract: Multicore fibers have the potential to combine the advantages of optical fibers (such as their high average power capability, high efficiency and well-defined beam quality) with those stemming from the large beam areas commonly used in other laser architectures. Coherent combination can then be employed to achieve one single, high-quality, output beam [1] , [2] . To match and even surpass the performance of state-of-the-art lasers systems comprising multiple separate fiber amplifiers, multicore fibers need to leverage the same technological advancements. One example is the use of a rod-type geometry with large core diameters to mitigate detrimental nonlinear effects. In this contribution, we present our high power laser results achieved with an in-house, all-glass, rod-type multicore fiber, whose basic structure is shown in figure 1 . The fiber contains 16 ytterbium-doped cores in a rectangular arrangement with a diameter of 22 µm each, operating at near single-mode output. The core-to-core pitch is 58 µm. An embedded octagonal fluoride ring is used as the guiding mechanism for the pump with a diameter of 310 µm and a NA of 0.22. A device length of 1.1 m was chosen to provide sufficient pump absorption.

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.

M. Muller, J. Buldt, H. Stark, C. Grebing, and J. Limpert
388 W multipass cell broadening supporting few-cycle pulse duration
(2021)

Abstract: High-power, few-cycle lasers are viable tools, e.g. in time-resolved spectroscopy [1] and nanoscale imaging [2] . No laser emits such pulses directly due to its limited gain bandwidth and, hence, either nonlinear pulse compression in fibers [3] or optical parametric amplification [4] are commonly applied to generate few-cycle pulses.

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. Schonberg, H. S. Salman, A. Tajalli, I. Hartl, and C. M. Heyl
7th harmonic generation in gases for coherent 150 nm light production
(2021)

Abstract: High-harmonic-generation in gases (HHG), along with crystal-based nonlinear frequency conversion, represent well established methods for laser-light production, covering spectral regions from soft X-rays down to the terahertz regime. A less-explored intermediate spectral regime can be identified in-between the spectral regions covered by these two methods, characterized by the cut-off wavelength of nonlinear crystals (e.g. KBBF around 160 nm) and the ionization potential of nonlinear gas media (e.g. Xenon at 102 nm). This wavelength regime is currently attracting increasing attention, brought along by measurements of the nuclear clock transition energy of the Thorium-229 isotope at a wavelength of 149.7 ± 3.1 nm [1] . Compared to alternative approaches such as cascaded frequency up-conversion in a hollow capillary [2] or four-wave-mixing processes, the simplest method to producing 150 nm light is the direct generation of the 7 th harmonic of a 1030 nm Ytterbium (Yb) laser source. In particular, it is easily compatible with demonstrated methods for vacuum-ultraviolet frequency comb production via intra-cavity harmonic generation [3] . However, in contrast to HHG, where phase-matching is commonly reached, for 7 th harmonic generation in gases all key phase-mismatch contributions (Gouy phase, neutral- and plasma dispersion) have the same sign. In this work, we demonstrate 7 th harmonic generation in a noble gas-jet driven by a 1025 nm Yb laser source. We measure a maximum conversion efficiency of η ≅ 5 × 10 -6 for two different nozzle orifice diameters for Krypton and Argon, limited mainly by phase-matching effects.

J. Buldt, H. Stark, M. Muller, A. Klenke, and J. Limpert
Broadband Ytterbium fiber CPA-system delivering 120fs, 10 mJ pulses at 1 kW average power
(2021)

Abstract: High-power, ultrafast fiber lasers drive a variety of applications in industry and science and the demand for more powerful sources is unabated. Existing performance scaling technologies like chirped-pulse-amplification need to be further improved, driven towards their limits to enable ever growing output performance. In this contribution we present on an ytterbium-fiber chirped-pulse-amplification (Yb:FCPA) system based on coherent combination [1] , [2] of 16 parallel rod-type fiber amplifiers delivering 1 kW average power, 10 mJ pulse energy and 120 fs pulse duration. Multi-stage spectral filtering is applied to maximize the bandwidth.

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] .

R. Hollinger, E. Haddad, M. Zapf, V. Shumakova, P. Herrmann, R. Roder, I. Uschmann, U. Reislohner, A. Pugzlys, A. Baltuska, F. Legare, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Contribution of free carriers to light absorption upon intense light-semiconductor interaction
(2021)

Abstract: In this work we investigated the role of free carriers in the interaction of a wide-band gap semiconductor with strong light fields at long wavelengths. Motivated by the beneficial scaling law of the pondermotive potential (U p ~ Iλ 2 ), the interaction of intense long wavelength laser pulses with condensed matter has attracted huge attention over the last decade [Kruchinin] . After excitation of quasi free electrons in the conduction band (CB) via multiphoton absorption or tunnelling the strong pondermotive force leads to highly energetic free electrons. Bound electrons can be collisionally excited if the energy of the free electrons exceed the band gap energy. Here, we use the onset of near ultraviolet (NUV) stimulated emission in ZnO thin films ( Fig. 1a ) to study off-resonance light absorption and the role of free carriers thereby.

C. Hofer, D. Gerz, M. Gebhardt, T. Heuermann, T. P. Butler, C. Gaida, J. Limpert, F. Krausz, and I. Pupeza
Electro-Optic Sampling with Percent-Level Detection Efficiency
(2021)

Abstract: Electro-optic sampling (EOS) [1] is widely used for broadband and sensitive characterization of mid-infrared (MIR) waveforms and spectroscopy [2] , [3] . GaSe as the nonlinear crystal and ~10-fs gate pulses at ~ 100-mW average power, generated in highly nonlinear fibres from Er-doped fibre lasers, are commonly employed for broadband sampling of MIR fields [4] . Using Yb-based laser systems to generate high-power, 1030-nm gate pulses allowed us to push the detection efficiency to a record-level of 0.5 %, limited by the damage threshold of GaSe [2] . However, the short gate pulse wavelength strongly limits the phase matching bandwidth [5] .

P. Herrmann, R. Hollinger, V. Korolev, M. Zapf, V. Shumakova, R. Roder, I. Uschmann, A. Pugzlys, A. Baltuska, M. Zurch, C. Ronning, C. Spielmann, and D. Kartashov
Ellipticity dependent excitation and high harmonic generation from intense mid-IR laser pulses in ZnO
(2021)

Abstract: In this contribution we experimental examine the ellipticity dependence of strong off resonance absorption and high harmonic generation (HHG) in the wide band gap semiconductor ZnO upon irradiation with intense mid-IR laser pulses. HHG in semiconductors originates from nonlinear intraband currents and interband transitions of the electrons driven by the strong mid-IR fields [Ghimire]. Here, we show that the intra- and interband contribution to HHG in a ZnO thin film are affected differently by the driving laser ellipticity.

T. Helk, E. Berger, L. Hoffmann, A. Kabacinski, J. Gautier, F. Tissandier, J. P. Goddet, S. Sebban, C. Spielmann, and M. Zurch
Extreme Ultraviolet Second Harmonic Generation using a seeded soft X-ray laser
(2021)

Abstract: Non-linear interactions between light and matter are crucial for widespread applications in physical sciences, life science and engineering. Second harmonic generation (SHG) and sum-frequency generation spectroscopy in the near infrared and optical range have enabled intriguing insights into surface properties and how they influence for instance chemical reactions [1] . Expansion of these methods by developing non-linear X-ray spectroscopies has recently added the capability of studying surfaces [2] , symmetry-breaking [3] and buried interfaces [4] with elemental specificity. However, widespread application is currently limited by access to free-electron laser facilities. Here we report the first generation of second harmonic emission in the extreme ultraviolet (XUV-SHG) at the titanium M-edge. The experiments were carried out with a high-harmonic seeded SXRL [5] bringing nonlinear XUV spectroscopy with atomic specificity to the table-top. The SXRL pulses with an energy of (14 ± 2) nJ, a pulse duration of (1.73 ± 0.13) ps, wavelength of 32.8 nm and a Gaussian-like beam profile is focused down with a gold ellipsoidal mirror down to an elliptical spot with a size of roughly 20 µ m x 40 µ m. The estimated intensity on target is about of (1.0 ± 0.1)•10 10 W/cm 2 . In the focus we exceeded the damage threshold fluence of 2 mJ/cm 2 and observed single-shot damage of 50 nm Ti foils. At these intensities we also generate second harmonic light at 75.6 eV. The fundamental and SHG beams are refocused with a toroidal mirror, spectrally separated by a grating and imaged on a cooled CCD camera.

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.

S. Hadrich, E. Shestaev, N. Walther, T. Nagy, P. Simon, A. Blumenstein, A. Klenke, R. Klas, J. Buldt, H. Stark, M. Gebhardt, S. Breitkopf, P. Jojart, I. Seres, Z. Varallyay, A. Borzsonyi, T. Eidam, and J. Limpert
High-average-power and high-pulse-energy CEP-stable few-cycle pulses: Status of the ELI-ALPS HR2 laser system
(2021)

Abstract: Few-cycle laser systems providing both high repetition rates and high pulse energies are a major focus of next-generation light sources. Consequently, the research facility of the Extreme Light Infrastructure (ELI) that is devoted to the generation of isolated attoseconds pulses, (ELI-ALPS in Szeged, Hungary), has set the demand for a laser system delivering carrier-envelope phase (CEP) stable pulses with 5 mJ of pulse energy, 6 fs pulse duration at 100 kHz repetition rate, which corresponds to an average power of 500 W [1] . This laser system, that is named HR2 (the high-repetition-rate beam line), is currently under development at Active Fiber System GmbH. Achieving these ambitious laser parameters is done by merging the robust techniques of coherent combination as an average-power scaling concept and the use of stretched hollow-core fiber technology for nonlinear pulse compression [2] .

A.-L. Viotti, S. Alisauskas, H. Tunnermann, E. Escoto, M. Seidel, K. Dudde, B. Manschwetus, I. Hartl, and C. Heyl
Intra-Burst Pulse Characterization of a High-Power Post-Compressed Yb:YAG Laser at 100 kHz Repetition Rate
(2021)

Abstract: High-average power Ytterbium (Yb) laser systems are playing an increasingly important role in ultrafast science e.g. as pump lasers for optical parametric amplifiers or directly as ultrafast sources. The gain bandwidth of Yb limits the pulse duration to a few 100 fs up to about 1 ps. However, many applications, such as attosecond physics or X-ray Free Electron Laser (FEL) science, would greatly benefit from the combination of high average powers with much shorter pulses, achievable via post-compression. Nonlinear pulse post-compression of high-average power Yb lasers employing multi-pass cell (MPC) -based spectral broadening [1] , [2] was recently implemented for two burst-mode pump-probe lasers at the FEL facility FLASH in Hamburg [3] , [4] . For such lasers, precise characterization and control of intra-burst pulse dynamics is crucial as the post-compression process couples input pulse energy instabilities with important output pulse parameters such as spectrum, pulse length and temporal contrast. Here, we demonstrate 100 kHz intra-burst spectrum, phase and temporal contrast characterization of a Yb:YAG Innoslab burst-mode amplifier post-compressed in a gas-filled MPC. Our measurements reveal a stable broadened spectrum and compressed pulse duration within the flat part of the burst, yielding a relative energy content of about 80% in the main compressed fs pulse (250 fs window versus 4 ps background pedestal).

C. Grebing, M. Muller, J. Buldt, H. Stark, and J. Limpert
Kilowatt-average-power compression of millijoule pulses in a gas-filled multi-pass cell
(2021)

Abstract: In recent years, compression schemes based on distributed spectral broadening in gas-filled multi-pass cells (MPC) have been demonstrated to support millijoule pulse energies [1] , [2] . We experimentally demonstrate the scaling of such a MPC to kilowatt average power. A fiber laser source coherently combining 16 amplifier channels provides ~1-mJ, 200-fs pulses at a repetition rate of 1 MHz resulting in an average power of ~1 kW [3] . The laser output is compressed by distributed spectral broadening in 26 foci of an Argon-filled MPC and subsequent chirped-mirror compression to 31-fs pulses (see Fig. 1 ) with an average power of 1 kW demonstrating a mirror-reflectivity-limited compression efficiency of >95% [4] . To the best of our knowledge, the MPC output represents the highest average power of sub-100-fs pulses ever demonstrated. Sound characterization of the output in terms of spatio-spectral couplings and beam quality factor reveals a close-to lossless compression [4] .

M. Blothe, M. Chambonneau, T. Heuermann, M. Gebhardt, J. Limpert, and S. Nolte
Laying the foundations of ultrafast stealth dicing of silicon with picosecond laser pulses at 2-mu m wavelength
(2021)

Abstract: Numerous applications are enabled by the possibility to modify transparent materials in the bulk with ultrashort laser pulses. In glasses the inscription of waveguides and nanogratings, welding and dicing processes are finding their way into the industrial world. The transfer of these established processes to narrow band-gap materials is subject of current research activities. Here, silicon as the current most important semiconductor material is of particular interest.

A. Steinkopff, C. Aleshire, C. Jauregui, A. Klenke, and J. Limpert
Optimizing rod-type multicore fiber amplifiers in coherently-combined laser systems
(2021)

Abstract: Coherent beam combination of fiber amplifiers is a powerful tool to synthetically increase the effective mode area of the beam while mitigating most of the limitations of fibers. Therefore this technique enables a further power and energy scaling of the radiation emitted by fiber laser systems (even of those delivering ultrafast pulses) [1] , [2] . However, the combination of single emitters leads to very complex, bulky and expensive systems due to the component count growing linearly with the number of channels. To decouple the component count from the channel count, so called multicore fibers (MCFs), that incorporate multiple active cores sharing the same pump cladding, have been developed. First tests with MCF amplifiers have not only shown their linear power scaling potential but also the scaling of the transverse mode instability threshold [3] .

C. Aleshire, A. Steinkopff, M. Karst, A. Klenke, C. Jauregui, S. Kuhn, J. Nold, N. Haarlammert, T. Schreiber, and J. Limpert
Q-Switched Rod-Type Multicore Fibre Laser Delivering 3.1 mJ Pulses
(2021)

Abstract: Multicore fibres (MCFs) with active doped cores are a promising technology to increase effective mode area and therefore available power and energy of fibre amplifiers and lasers, most recently demonstrated in coherently combined systems [1] . With ns-class pulses, energies of up to 26 mJ have been extracted from advanced fibre designs in multi-stage amplifiers [2] . Self-focusing at peak powers of around 5 MW limits the maximum pulse energy extracted from a single fused silica core, but MCF lasers potentially enable continued scaling beyond these limitations by distributing energy over many cores. Modelling of rod-type MCFs show potential to achieve multi-kW average power and Joule-class combined pulses from a 1 m long fibre [3] . In this submission, a Q-switched 16-core rod-type Yb-doped MCF is demonstrated, which can directly seed further MCF amplification stages without the need for fibre arrays or beam-splitting optics.

E. Escoto, A.-L. Viotti, S. Alisauskas, H. Tunnermann, M. Seidel, K. Dudde, B. Manschwetus, I. Hartl, and C. M. Heyl
Role of dispersion and compression ratio on the temporal contrast of SPM-broadened post-compressed pulses
(2021)

Abstract: Nonlinear pulse post-compression, mainly enabled by self-phase modulation (SPM), opens new avenues towards high peak power laser pulses at high average power while bypassing the need for a gain medium with large bandwidth. However, SPM-induced spectral broadening typically introduces spectral amplitude modulations as well as a chirp of third and higher orders, limiting the temporal contrast of the compressed pulse. While some recent works address this issue and discuss mitigation strategies [1] , [2] , not much attention has been devoted to the physical processes and limitations that determine the temporal contrast of post-compressed pulses. As novel compression techniques expand the achievable compression ratio [3] , it is increasingly important to fully understand the underlying pulse quality limitations. Here, we outline the role of two important characteristics — dispersion and compression ratio — on the temporal quality of post-compressed pulses. Using both numerical simulations as well as experimental tests employing a gas-filled multi-pass cell (MPC), we study the temporal contrast of post-compressed pulses over large compression-ratio and dispersion range. Using a 730 fs input pulse we were able to generate a 55 fs post-compressed pulse with up to 78% energy contained in the main compressed pulse (defined via the first local minima near the highest peak) against its picosecond background.

C. Jauregui, C. Stihler, S. Kholaif, Y. Tu, and J. Limpert
Transverse Mode Instability in High-Power Fiber Laser Systems: a "Hot Topic"
(2021)

Abstract: Roughly a decade ago the future of fiber laser technology looked brighter than ever with enticing power scaling prospects [1] . These predictions seemed to be supported by the unprecedented exponential rate of power increase sustained over the previous two decades [2] . At those times, multi ten-kW fiber laser systems seemed within reach. So what could possibly go wrong?

R. Hollinger, N. Harshitha, V. Korolev, Z. Gan, A. George, V. Shumakova, M. Zürch, T. Vogl, A. P. zlys, A. B. ska, F. Eilenberger, C. Spielmann, A. Turchanin, and D. Kartashov
Ellipticity controlled high-order harmonic generation in 2D materials
(2021)

Abstract: High harmonic generation (HHG) in a single-atomic-layer non-centrosymmetric semiconductor is investigated experimentally for different driving laser field polarizations. The ellipticity enhanced even-order HHG for certain crystal orientations reveals linked laser and valley polarizations.

A.-L. Viotti, S. Alisauskas, E. Escoto, H. Tünnermann, K. Dudde, M. Seidel, B. Manschwetus, I. Hartl, and C. M. Heyl
Intra-Burst Temporal Pulse Contrast of a High-Power Post-Compressed Picosecond Yb:YAG Laser
(2021)

Abstract: We report on pulse contrast characterization of the output of a gas-filled multi-pass cell employed for 20-fold compression of a high-power Yb:YAG laser. We demonstrate an energy content of 80% in the compressed fs pulse.

C. Heyl
Multi-pass post-compression of ultrashort laser pulses at extreme parameter scales
(2021)

Abstract: This talk provides a review of laser pulse post-compression leveraged by the advent of the multi-pass spectral broadening scheme, including perspectives on expanding the limits of pulse duration and energy.

Y. Ma, S. Salman, C. Li, C. Mahnke, Y. Hua, J. Fellinger, A. S. Mayer, O. H. Heckl, C. Heyl, and I. Hartl
Sub-fs timing jitter of an 88 fs all-PM fiber integrated ultrafast Yb NALM oscillator
(2021)

Abstract: We demonstrate an all-PM fiber integrated femtosecond Yb NALM oscillator with 88 fs compressed pulse duration and sub-fs free-running timing jitter [25 kHz to 5 MHz].

C. Mahnke, Y. Hua, Y. Ma, S. Salman, T. Lamb, S. Schulz, C. M. Heyl, H. Cankaya, and I. Hartl
Synchronizable, low-jitter, picosecond Ho:fiber NALM oscillator for Ho:YLF amplifier driven electron acceleration
(2021)

Abstract: We demonstrate a 41.6 MHz, 1.3 ps, 140 pJ Ho:fiber oscillator centered at 2050 nm for seeding Ho:YLF amplifiers. RIN and timing jitter of the oscillator are characterized while comparing two commercial Tm pump lasers.

M. Seidel, P. Balla, G. Arisholm, L. Winkelmann, I. Hartl, and C. M. Heyl
Ultrafast Pulse Compression in Bulk with > 20 Times Spectral Broadening Factor from a Single Stage
(2021)

Abstract: We introduce the combination of multi-pass cell and multi-plate spectral broadening. We demonstrate the compression of 110-μJ pulses from 900-fs to 60-fs in a single stage and report broadening to 38-fs transform-limit by nonlinear mode-matching.

H. Stark, J. Buldt, M. Müller, A. Klenke, and J. Limpert
100 fs pulses directly from a kW-class mJ-level ytterbium-doped fiber CPA laser system
(2021)
No abstract availableLinkBibTeX
S. E. Kholaif, Y. Tu, C. Stihler, C. Jauregui, and J. Limpert
Characterization of transverse mode instability in fiber-laser systems using a position-sensitive detector
(2021)
No abstract availableLinkBibTeX
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)
No abstract availableLinkBibTeX
C. Jauregui-Misas, C. Stihler, S. E. Kholaif, Y. Tu, and J. Limpert
Mitigation of transverse mode instability in polarization maintaining, high-power fiber amplifiers
(2021)
No abstract availableLinkBibTeX
C. Stihler, C. Jauregui, S. E. Kholaif, Y. Tu, and J. Limpert
Mitigation of transverse mode instability through a dynamic modification of the inversion in high-power fiber amplifiers
(2021)
No abstract availableLinkBibTeX
A. Steinkopff, C. Jauregui, C. Aleshire, A. Klenke, and J. Limpert
Optimizing the design of coherently combined multicore fiber amplifiers
(2021)
No abstract availableLinkBibTeX
G. Palma-Vega, C. Hupel, J. Nold, S. Kuhn, J. Limpert, N. Haarlammert, and T. Schreiber
Simplified manufacturing of advanced microstructured fibers for laser applications
(2021)
No abstract availableLinkBibTeX
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)
No abstract availableLinkBibTeX

2020

M. Mueller, C. Aleshire, H. Stark, J. Buldt, A. Steinkopff, A. Klenke, A. Tünnermann, and J. Limpert
10.4 kW coherently-combined ultrafast fiber laser
Proceedings of SPIE 11260, 112600B (2020)

Abstract: We present a coherently-combined ultrafast fiber laser system consisting of twelve amplifier channels delivering 10.4 kW average power at 80 MHz repetition rate with a pulse duration of 240 fs FWHM and an almost diffraction-limited beam quality of M2 ≤ 1.2. The system incorporates an automated self-adjustment of the beam combination with 3 degrees of freedom per channel. The system today is, to the best of our knowledge, the world's most average-powerful femtosecond laser. Thermographic analysis indicates that power scaling to 100 kW-class average power is feasible.

Z. Wang, T. Heuermann, M. Gebhardt, C. Gaida, C. Jauregui, and J. Limpert
108 W average power ultrashort pulses with GW-level peak power from a Tm-doped fiber CPA system
Proceedings of SPIE 11260, 112600K (2020)

Abstract: Applications such as material processing, spectroscopy, particle acceleration, high-harmonic and mid-IR generation can greatly benefit from high repetition rate, high power, ultrafast laser sources emitting around 2 μm wavelength. In this contribution we present a single-channel Tm-doped fiber chirped-pulse amplifier delivering 108 W of average output power at 417 kHz repetition rate with 250 fs pulse duration and 0.73 GW of pulse peak power. To the best of our knowledge, this is the first demonstration of an ultrafast Tm-doped fiber laser with more than 100 W of average power and GW-level peak power.

A. Steinkopff, C. Jauregui, A. Klenke, C. Aleshire, A. Tünnermann, and J. Limpert
Investigation of the thermo-optical behavior of multicore fibers used in coherently combined fiber laser systems
Proceedings of SPIE 11260, 112600D (2020)

Abstract: In this work we present theoretical investigations of the power scaling potential of multicore fibers. In principle it is widely accepted that increasing the number of active cores helps to overcome current challenges such as transversal mode instabilities and non-linear effects. However, in order to do a proper analysis of the average power scaling potential of multicore fibers it is required to pay particular attention to thermal effects arising in such fibers. Therefore, a simulation tool has been developed that is capable of solving the laser rate equations, taking into account the resulting temperature gradient and the distortions in the mode profiles that it causes. In the study several different multicore fibers possessing a rectangular core position layout of 2×2 to 7×7 of active cores have been analyzed. Moreover, we have investigated the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. This includes a study in the maximum achievable coherent combination efficiency of the multicore channels (that is strongly influenced by the distorted mode profile at the fiber end facet), the impact on nonlinear effects, the optical path differences between the cores and the amplification efficiency which are all triggered by thermal effects. Finally the scaling potential as well as the challenges of such fibers will be discussed.

C. Jauregui, C. Stihler, Y. Tu, S. Kholaif, and J. Limpert
Mitigation of transverse mode instability with travelling waves in high-power fiber amplifiers
Proceedings of SPIE 11260, 112601A (2020)

Abstract: In this work we present a novel way to mitigate the effect of transverse mode instability in high-power fiber amplifiers. In this technique a travelling wave is induced in the modal interference pattern by seeding the amplifier with two modes that have slightly different frequencies. The interference pattern thus formed will travel up-or downstream the fiber (depending on the sign of the frequency difference between the modes) with a certain speed (that depends on the absolute value of the frequency difference). If the travelling speed is chosen properly, the thermally-induced index grating will follow the travelling modal interference pattern creating a constant phase shift between these two elements. Such a constant controllable phase shift allows for a stable energy transfer from the higher-order modes to the fundamental mode or viceversa. Thus, this technique can be adjusted in such a way that, at the output of the fiber almost all the energy is concentrated in the fundamental mode, regardless of the excitation conditions. Moreover, this technique represents one of the first examples of the new family of mitigation strategies acting upon the phase shift between the modal interference pattern and the refractive index grating. Additionally, it even exploits the effect of transverse mode instability for gaining control over the beam profile at the output of the amplifier. Therefore, by adjusting the frequency difference between the seed modes, it is possible to force that the beam at the output acquires the shape of the fundamental mode or that of a higher order mode.

C. Aleshire, A. Klenke, M. Müller, H. Stark, A. Tünnermann, and J. Limpert
Simplification strategies for segmented-mirror splitters in multicore fiber CBC systems
Proceedings of SPIE 11260, 112600E (2020)

Abstract: A simplification of segmented-mirror splitters for coherent beam combination based on numerical optimization of coating designs is presented. The simplified designs may facilitate the production of such elements for coherent beam combination while maintaining high combination efficiency. The achievable efficiency and error tolerance, and additional performance characteristics are analyzed in the context of coherently combined multicore fiber laser systems.

C. Jauregui, A. Steinkopff, and J. Limpert
Talbot Fiber: a poorman's approach to coherent combining
Proceedings of SPIE 11260, 1126011 (2020)

Abstract: In this work we present a multicore fiber design that exploits the Talbot effect to carry out the beam splitting and recombination inside of the fiber. This allows reducing the complexity of coherent combining systems since it makes the splitting and combining subsystems together with the active stabilization redundant. In other words, such a multicore fiber behaves for the user as a single core fiber, since the energy is coupled just in a single core and it is extracted from the same core. This work describes the operating principle of this novel fiber design and analyzes its performance in high power operation using a simulation model based on the supermode theory. This includes a study on the impact on non-linear effects, on the amplification efficiency, on the thermal resilience of this design and on the performance dependence on the pump direction. Moreover, some design guidelines will be provided to tailor the characteristics of the fiber. Finally, it will be discussed how these fibers can be used to increase the TMI threshold of fiber laser systems.

C. Stihler, C. Jauregui, S. Kholaif, and J. Limpert
The sensitivity of the mode instability threshold to different types of intensity noise
Proceedings of SPIE 11260, 1126018 (2020)

Abstract: In this work we experimentally and theoretically investigate the impact of seed intensity-noise on the threshold of transverse mode instability (TMI) in Yb-doped, high-power fiber laser systems and compare it to the impact of pump intensity-noise. Former studies have shown that pump intensity-noise significantly decreases the TMI threshold due to the introduction of a phase shift between the modal interference pattern and the thermallyinduced refractive index grating in the fiber. However, it can be expected that fluctuations of the seed power will also induce such phase shifts due to a change of the extracted energy and the heat load in the fiber. Thus, it is important to investigate which one, i.e. the seed-or the pump intensity-noise, has a severer impact on the TMI threshold. Our experiments have shown that the TMI threshold of a fiber amplifier was decreased by increasing the seednoise amplitude. However, contrary to conventional belief, the impact of seed intensity-noise was much weaker than the one of pump intensity-noise. The measurements are in good agreement with our simulations and can be well explained with previous studies about the noise transfer function. The reason for the weaker impact of seed intensity-noise on the TMI threshold is the attenuation of its frequency components below 20 kHz in saturated fiber amplifiers, which includes the frequencies relevant for TMI. Thus, the main trigger for TMI in saturated fiber amplifiers can be considered to be pump intensity-noise. A suppression of this noise below 20 kHz represents a promising way to increase the TMI threshold of fiber laser systems.

T. Heuermann, M. Gebhardt, Z. Wang, C. Gaida, F. Maes, C. Jauregui, and J. Limpert
Watt-class optical parametric amplification driven by a thulium doped fiber laser in the molecular fingerprint region
Proceedings of SPIE 11260, 112600I (2020)

Abstract: Numerous molecules important for environmental and life sciences feature strong absorption bands in the molecular fingerprint region from 3 μm-20 μm. While mature drivers at 1 μm wavelength are the workhorse for the generation of radiation up to 5 μm (utilizing down-conversion in nonlinear crystals) they struggle to directly produce radiation beyond this limit, due to impeding nonlinear absorption in non-oxide crystals. Since only non-oxide crystals provide transmission in the whole molecular fingerprint region, a shift to longer driving wavelengths is necessary for a power scalable direct conversion of radiation into the wavelength region beyond 5 μm. In this contribution, we present a high-power single-stage optical parametric amplifier driven by a state of the art 2 μm wavelength, thulium-doped fiber chirped pulse amplifier. In this experiment, the laser system provided 23 W at 417 kHz repetition rate with 270 fs pulse duration to the parametric amplifier. The seed signal is produced by supercontinuum generation in 3 mm of sapphire and pre-chirped with 3 mm of germanium. Combining this signal with the pump radiation and focusing it into a 2 mm thick GaSe crystal with a pump intensity of 160 GW/cm2 lead to an average idler power of 700 mW with a spectrum spanning from 9 μm-12 μm. To the best of our knowledge, this is the highest average power reported from a parametric amplifier directly driven by a 2 μm ultrafast laser in the wavelength region beyond 5 μm. Employing common multi-stage designs, this approach might in the future enable multi-watt high-power parametric amplification in the long wavelength mid infrared.

M. Müller, C. Aleshire, L. Stark, J. Buldt, A. Steinkopff, E. Haddad, F. Legare, A. Klenke, A. Tünnermann, and J. Limpert
10.4 kW 12-channel ultrafast fibre laser
(2020)

Abstract: We present a twelve-channel coherently-combined ultrafast fibre laser delivering 10.4 kW average power at 80 MHz repetition rate with 254 fs pulse duration and excellent beam quality (M2 ≤ 1.2). Further power scaling is discussed.

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.

J. Fellinger, A. Mayer, W. Grosinger, G. Winkler, L. Perner, S. Droste, S. Salman, C. Li, C. Heyl, and I. Hartl
Flexible Figure-9 Yb: Fiber Laser: Operation Regimes and Noise Performance
(2020)

Abstract: We present a flexible figure-9 Yb: fiber-laser and investigate the impact of intra-cavity group delay dispersion on amplitude/phase noise. We show that the free-running carrier-envelope-offset frequency short-term linewidth can range from several MHz to <10 kHz.

S. Hädrich, N. Walther, E. Shestaev, T. Nagy, P. Simon, A. Blumenstein, R. Klas, J. Buldt, H. Stark, S. Breitkopf, and . others
High Pulse Energy CEP-stable Few-cycle Pulses at High Average Power: Status of the ELI-ALPS HR2 System
(2020)

Abstract: We demonstrate a two-pulse Bessel beam scheme for generating plasma waveguides guiding high intensity laser pulses over 30 cm with on-axis plasma densities as low as 5 × 10^16cm−3.