# Publikationen von Prof. Dr. Jens Limpert

Alle Publikationen des HI Jena

## 2022

H. Stark, C. Grebing, J. Buldt, A. Klenke, and J. Limpert
Divided-pulse nonlinear compression in a multipass cell
Journal of Physics-Photonics 4, 035001 (2022)

Abstract: The pulse-energy and peak-power limitations of a gas-filled multipass cell (MPC) for nonlinear pulse compression are surpassed by applying a burst of four temporally separated pulses instead of a single one. The burst is generated by two birefringent crystals and contains 1 mJ of energy per pulse replica. It is then spectrally broadened in an Argon-filled MPC and recombined into a single pulse by a second set of birefringent crystals. The combined pulse is compressed by chirped mirrors to a pulse duration of 32 fs and a pulse energy of 3.4 mJ. An excellent passive stability and a high system efficiency of >90% are reached. Using the 4-pulse burst, the overall output energy supported by the MPC is doubled in comparison to single-pulse operation.

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

T. Heuermann, Z. Wang, M. Lenski, M. Gebhardt, C. Gaida, M. Abdelaal, J. Buldt, M. Mueller, A. Klenke, and J. Limpert
Ultrafast Tm-doped fiber laser system delivering 1.65-mJ, sub-100-fs pulses at a 100-kHz repetition rate
Optics letters 47, 3095 (2022)

Abstract: High-energy, ultrafast, short-wavelength infrared laser sources with high average power are important tools for industrial and scientific applications. Through the coherent combination of four ultrafast thulium-doped rod-type fiber amplifiers, we demonstrate a Tm-doped chirped pulse amplification system with a compressed pulse energy of 1.65 mJ and 167 W of average output power at a repetition rate of 101 kHz. The system delivers 85 fs pulses with a peak power of 15 GW. Additionally, the system presents a high long- and short-term stability. To the best of our knowledge, this is the highest average output power short wavelength IR, mJ-class source to date. This result shows the potential of coherent beam combining techniques in the short wavelength infrared spectral region for the power scalability of these systems.

A. Steinkopff, C. Aleshire, A. Klenke, A. Klenke, C. Jauregui, J. Limpert, J. Limpert, and J. Limpert
Mitigation of thermally-induced performance limitations in coherently-combined multicore fiber amplifiers
Optics Express 30, 16896 (2022)

Abstract: Multicore fiber (MCF) amplifiers have gained increasing interest over the past years and shown their huge potential in first experiments. However, high thermal loads can be expected when operating such an amplifier at its limit. Especially in short MCF amplifiers that are pumped in counter-propagation, this leads to non-uniform mode-shrinking in the cores and, consequently, to a degradation of the system performance. In this work we show different ways to counteract the performance limitations induced by thermal effects in coherently-combined, multicore fiber amplifiers. First, we will show that pumping MCFs in co-propagation will significantly improve the combinable average power since the thermal load at the fiber end is reduced. However, this approach might not be favorable for high energy extraction. Therefore, we will introduce a new MCF design pumped in counter-propagation that leads to a reduction of the thermal load at the fiber end, which will allow for both high combined output power and pulse energy.

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.

C. Aleshire, A. Steinkopff, A. Klenke, A. Klenke, C. Jáuregui, S. Kuhn, J. Nold, N. Haarlammert, T. Schreiber, J. Limpert, J. Limpert, and J. Limpert
High-energy Q-switched 16-core tapered rod-type fiber laser system
Optics Letters 47, 1725 (2022)

Abstract: High-energy Q-switched master oscillator power amplifier systems based on rod-type 4 × 4 multicore fibers are demonstrated, achieving energy up to 49 mJ in ns-class pulses. A tapered fiber geometry is tested that maintains low mode order in large multimode output cores, improving beam quality in comparison to a similar fiber with no taper. The tapered fiber design can be scaled both in the number of amplifying cores and in the dimensions of the cores themselves, providing a potential route toward joule-class fiber lasers systems.

S. Haedrich, E. Shestaev, M. Tschernajew, F. Stutzki, N. Walther, F. Just, M. Kienel, I. Seres, P. Jojart, Z. Bengery, B. Gilicze, Z. Varallyay, A. Borzsonyi, M. Mueller, C. Grebing, A. Klenke, D. Hoff, G. Paulus, T. Eidam, and J. Limpert
Carrier-envelope phase stable few-cycle laser system delivering more than 100 W, 1 mJ, sub-2-cycle pulses
Optics Letters 47, 1537 (2022)

Abstract: Two-stage multipass-cell compression of a fiber-chirpedpulse amplifier system to the few-cycle regime is presented. The output delivers a sub-2-cycle (5.8 fs), 107W average power, 1.07 mJ pulses at 100kHz centered at 1030nm with excellent spatial beam quality (M-2 =1.1, Strehl ratio S = 0.98), pointing stability (2.3 mu rad), and superior long-term average power stability of 0.1% STD over more than 8 hours. This is combined with a carrier-envelope phase stability of 360mrad in the frequency range from 10Hz to 50kHz, i.e., measured on a single-shot basis. This unique system will serve as an HR1 laser for the Extreme Light Infrastructure Attosecond Light Pulse Source research facility to enable high repetition rate isolated attosecond pulse generation

A. Klenke, A. Steinkopff, C. Aleshire, C. Jauregui, S. Kuhn, J. Nold, C. Hupel, S. Hein, S. Schulze, N. Haarlammert, T. Schreiber, A. Tünnermann, and J. Limpert
500 W rod-type 4 × 4 multicore ultrafast fiber laser
Optics Letters 47, 345 (2022)

Abstract: We present a coherently combined femtosecond fiber chirped-pulse-amplification system based on a rod-type, ytterbium-doped, multicore fiber with 4 × 4 cores. A high average power of up to 500 W (after combination and compression) could be achieved at 10 MHz repetition rate with excellent beam quality. Additionally, \less 500 fs pulses with up to 600 µJ of pulse energy were also realized with this setup. This architecture is intrinsically power scalable by increasing the number of cores in the fiber.

## 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.

C. Jauregui, C. Stihler, S. Kholaif, Y. Tu, and J. Limpert
Control and stabilization of the modal content of fiber amplifiers using traveling waves
Optics Express 29, 34452 (2021)

Abstract: In this work we present a novel way to manipulate the effect of transverse mode instability by inducing traveling waves in a high-power fiber system. What sets this technique apart is the fact that it allows controlling the direction of the modal energy flow, for the first time to the best of our knowledge. Thus, using the method proposed in this work it will be possible to transfer energy from the higher-order mode into the fundamental mode of the fiber, which mitigates the effect of transverse mode instability, but also to transfer energy from the fundamental mode into the higher-order mode. Our simulations indicate that this approach will work both below and above the threshold of transverse mode instability. In fact, our model reveals that it can be used to force a nearly pure fundamental mode output in the fiber laser system almost independently of the input coupling conditions. In this context, this technique represents the first attempt to exploit the physics behind the effect of transverse mode instability to increase the performance of fiber laser systems.

J. Buldt, H. Stark, M. Mueller, C. Grebing, C. Jauregui, and J. Limpert
Gas-plasma-based generation of broadband terahertz radiation with 640 mW average power
Optics Letters 46, 5256 (2021)

Abstract: We present a high-power source of broadband terahertz (THz) radiation covering the whole THz spectral region (0.1-30 THz). The two-color gas plasma generation process is driven by a state-of-the-art ytterbium fiber chirped pulse amplification system based on coherent combination of 16 rod-type amplifiers. Prior to the THz generation, the pulses are spectrally broadened in a multipass cell and compressed to 37 fs with a pulse energy of 1.3 mJ at a repetition rate of 500 kHz. A gas-jet scheme has been employed for the THz generation, increasing the efficiency of the process to 0.1%. The air-biased coherent detection scheme is implemented to characterize the full bandwidth of the generated radiation. A THz average power of 640 mW is generated, which is the highest THz average power achieved to date. This makes this source suitable for a variety of applications, e.g., spectroscopy of strongly absorbing samples or driving nonlinear effects for the studies of material properties.

M. Chambonneau, M. Blothe, Q. Li, V. Fedorov, T. Heuermann, M. Gebhardt, C. Gaida, S. Tertelmann, F. Sotier, J. Limpert, S. Tzortzakis, and S. Nolte
Transverse ultrafast laser inscription in bulk silicon
Physical Review Research 3, 043037 (2021)

Abstract: In-volume ultrafast laser direct writing of silicon is generally limited by strong nonlinear propagation effects preventing the production of modifications. By using advantageous spectral, temporal, and spatial conditions, we demonstrate that modifications can be repeatably produced inside silicon. Our approach relies on irradiation at approximate to 2 mu m wavelength with temporally distorted femtosecond pulses. These pulses are focused in a way that spherical aberrations of different origins mutually balance, as predicted by point spread function analyses and in good agreement with nonlinear propagation simulations. We also establish the laws governing modification growth on a pulse-to-pulse basis, which allows us to demonstrate transverse inscription inside silicon with various line morphologies depending on the irradiation conditions. We finally show that the production of single-pulse repeatable modifications is a necessary condition for reliable transverse inscription inside silicon.

M. Mueller, C. Aleshire, J. Buldt, H. Stark, C. Grebing, A. Klenke, and J. Limpert
Scaling potential of beam-splitter-based coherent beam combination
Optics Express 29, 27900 (2021)

Abstract: The impact of nonlinear refraction and residual absorption on the achievable peakand average power in beam-splitter-based coherent beam combination is analyzed theoretically. While the peak power remains limited only by the aperture size, a fundamental average power limit is given by the thermo-optical and thermo-mechanical properties of the beam splitter material and its coatings. Based on our analysis, 100 kW average power can be obtained with state-of-the-art optics at maintained high beam quality (M-2 <= 1.1) and at only 2% loss of combining efficiency. This result indicates that the power-scaling potential of today\textquotesingle s beam-splitter-based coherent beam combination is far from being depleted. A potential scaling route to megawatt-level average power is discussed for optimized beam splitter geometry.

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.

M. Mueller, J. Buldt, H. Stark, C. Grebing, and J. Limpert
Multipass cell for high-power few-cycle compression
Optics Letters 46, 2678 (2021)

Abstract: A multipass cell for nonlinear compression to few-cycle pulse duration is introduced composing dielectrically enhanced silver mirrors on silicon substrates. Spectral broadening with 388 W output average power and 776 mu J pulse energy is obtained at 82% cell transmission. A high output beam quality (M-2 < 1.2) and a high spatio-spectral homogeneity (97.5%), as well as the compressibility of the output pulses to 6.9 fs duration, are demonstrated. A finite element analysis reveals scalability of this cell to 2 kW average output power.

C. Gaida, M. Gebhardt, T. Heuermann, Z. Wang, C. Jauregui, and J. Limpert
Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads
Optics Express 29, 14963 (2021)

Abstract: We experimentally analyze the average-power-scaling capabilities of ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted that thulium-doped fiber laser systems, with an emission wavelength around 2 mu m, should be able to withstand much higher heat-loads than their Yb-doped counterparts before the onset of transverse mode instability (TMI) is observed. In this work we experimentally verify this theoretical prediction by operating thulium doped fibers at very high heat-load. In separate experiments we analyze the performance of two different large-core, thulium-doped fiber amplifiers. The first experiment aims at operating a short, very-large core, thulium-doped fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this extreme heat-load level, the onset of TMI is not observed. The second experiment maximizes the extractable average-output power from a large-core, thulium-doped, fiber amplifier. We have achieved a pump-limited average output power of 1.15 kW without the onset of TMI. However, during a longer period of operation at this power level the amplifier performance steadily degraded and TMI could be observed for average powers in excess of 847 W thereafter. This is the first time, to the best of our knowledge, that TMI has been reported in a thulium-doped fiber amplifier.

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.

M. Gebhardt, E. Amuah, R. Klas, H. Stark, J. Buldt, A. Steinkopff, and J. Limpert
Investigation of spatiotemporal output beam profile instabilities from differentially pumped capillaries
Optics Express 29, 6957 (2021)

Abstract: Differentially pumped capillaries, i.e., capillaries operated in a pressure gradient environment, are widely used for nonlinear pulse compression. In this work, we show that strong pressure gradients and high gas throughputs can cause spatiotemporal instabilities of the output beam profile. The instabilities occur with a sudden onset as the flow evolves from laminar to turbulent. Based on the experimental and numerical results, we derive guidelines to predict the onset of those instabilities and discuss possible applications in the context of nonlinear flow dynamics. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

H. Stark, J. Buldt, M. Mueller, A. Klenke, and J. Limpert
1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system
Optics Letters 46, 969 (2021)

Abstract: An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

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.

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.

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] .

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] .

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] .

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.

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?

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)
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)
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)
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)
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)
A. Steinkopff, C. Jauregui, C. Aleshire, A. Klenke, and J. Limpert
Optimizing the design of coherently combined multicore fiber amplifiers
(2021)
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)
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

C. Jauregui, C. Stihler, and J. Limpert
Transverse mode instability
Advances in Optics and Photonics 12, 429 (2020)

Abstract: This work presents a review on the effect of transverse mode instability in highpower fiber laser systems and the corresponding investigations led worldwide over the past decade. This paper includes a description of the experimental observations and the physical origin of this effect, as well as some of the proposed mitigation strategies.

J. Buldt, M. Mueller, H. Stark, C. Jauregui, and J. Limpert
Fiber laser-driven gas plasma-based generation of THz radiation with 50-mW average power
Applied Physics B 126, 2 (2020)

Abstract: We present on THz generation in the two-color gas plasma scheme driven by a high-power, ultrafast fiber laser system. The applied scheme is a promising approach for scaling the THz average power but it has been limited so far by the driving lasers to repetition rates up to 1 kHz. Here, we demonstrate recent results of THz generation operating at a two orders of magnitude higher repetition rate. This results in a unprecedented THz average power of 50 mW. The development of compact, table-top THz sources with high repetition rate and high field strength is crucial for studying nonlinear responses of materials, particle acceleration or faster data acquisition in imaging and spectroscopy.

C. Aleshire, A. Steinkopff, C. Jauregui, A. Klenke, A. Tünnermann, and J. Limpert
Simplified design of optical elements for filled-aperture coherent beam combination
Optics Express 28, 21035 (2020)

Abstract: A simplification strategy for segmented mirror splitters (SMS) used as beam combiners is presented. These devices are useful for compact beam division and the combination of linear and 2-D arrays. However, the standard design requires unique thin-film coating sections for each input beam; thus, potential for scaling to high beam-counts is limited due to manufacturing complexity. Taking advantage of the relative insensitivity of the beam combination process to amplitude variations, numerical techniques are used to optimize highly simplified designs with only one, two or three unique coatings. It is demonstrated that with correctly chosen coating reflectivities, the simplified optics are capable of high combination efficiency for several tens of beams. The performance of these optics as beam splitters in multicore fiber amplifier systems is analyzed, and inhomogeneous power distribution of the simplified designs is noted as a potential source of combining loss in such systems. These simplified designs may facilitate further scaling of filled-aperture coherently combined systems in linear array or 2-D array formats.

M. Müller, C. Aleshire, A. Klenke, E. Haddad, F. Légaré, A. Tünnermann, and J. Limpert
10.4 kW coherently combined ultrafast fiber laser
Optics Letters 45, 3083 (2020)

Abstract: An ultrafast laser delivering 10.4 kW average output power based on a coherent combination of 12 step-index fiber amplifiers is presented. The system emits close-to-transform-limited 254 fs pulses at an 80 MHz repetition rate, and has a high beam quality (M2 ≤ 1.2) and a low relative intensity noise of 0.56% in the frequency range of 1 Hz to 1 MHz. Automated spatiotemporal alignment allows for hands-off operation.

A. Steinkopff, C. Jauregui, C. Aleshire, A. Klenke, and J. Limpert
Impact of thermo-optical effects in coherently combined multicore fiber amplifiers
Optics Express 28, 38093 (2020)

Abstract: In this work we analyze the power scaling potential of amplifying multicore fibers (MCFs) used in coherently combined systems. In particular, in this study we exemplarily consider rod-type MCFs with 2 × 2 up to 10 × 10 ytterbium-doped cores arranged in a squared pattern. We will show that, even though increasing the number of active cores will lead to higher output powers, particular attention has to be paid to arising thermal effects, which potentially degrade the performance of these systems. Additionally, we analyze the influence of the core dimensions on the extractable and combinable output power and pulse energy. This includes a detailed study on the thermal effects that influence the propagating transverse modes and, in turn, the amplification efficiency, the combining efficiency, the onset of nonlinear effect, as well as differences in the optical path lengths between the cores. Considering all these effects under rather extreme conditions, the study predicts that average output powers higher than 10 kW from a single 1 m long ytterbium-doped MCF are feasible and femtosecond pulses with energies higher than 400 mJ can be extracted and efficiently recombined in a filled-aperture scheme.

E. Shestaev, S. Haedrich, N. Walther, T. Eidam, A. Klenke, I. Seres, Z. Bengery, P. Jojart, Z. Varallyay, A. Borzsonyi, and J. Limpert
Carrier-envelope offset stable, coherently combined ytterbium-doped fiber CPA delivering 1 kW of average power
Optics Letters 45, 6350 (2020)

Abstract: We present a carrier-envelope offset (CEO) stable ytterbium-doped fiber chirped-pulse amplification system employing the technology of coherent beam combining and delivering more than 1 kW of average power at a pulse repetition rate of 80 MHz. The CEO stability of the system is 220 mrad rms, characterized out-of-loop with an f-to-2f interferometer in a frequency offset range of 10 Hz to 20 MHz. The high-power amplification system boosts the average power of the CEO stable oscillator by five orders of magnitude while increasing the phase noise by only 100 mrad. No evidence of CEO noise deterioration due to coherent beam combining is found. Low-frequency CEO fluctuations at the chirped-pulse amplifier are suppressed by a slow loop feedback. To the best of our knowledge, this is the first demonstration of a coherently combined laser system delivering an outstanding average power and high CEO stability at the same time.

C. Grebing, M. Mueller, J. Buldt, H. Stark, and J. Limpert
Kilowatt-average-power compression of millijoule pulses in a gas-filled multi-pass cell
Optics Letters 45, 6250 (2020)

Abstract: We demonstrate the reliable generation of 1-mJ, 31-fs pulses with an average power of 1 kW by post-compression of 200-fs pulses from a coherently combined Yb:fiber laser system in an argon-filled Herriott-type multi-pass cell with an overall compression efficiency of 96%. We also analyze the output beam, revealing essentially no spatiospectral couplings or beam quality loss.

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
AIP Advances 10, 045227 (2020)

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.

C. Stihler, C. Jauregui, S. Kholaif, and J. Limpert
Intensity noise as a driver for transverse mode instability in fiber amplifiers
PhotoniX 1, 2041 (2020)

Abstract: The effect of transverse mode instability (TMI) is currently the main limitation for the further average-power scaling of fiber laser systems with diffraction-limited beam quality. In this work a main driving force for TMI in fiber amplifiers is identified. Our experiments and simulations illustrate that the performance of fiber laser systems in terms of their diffraction-limited output power can be significantly reduced when the pump or seed radiation exhibit intensity noise. This finding emphasizes the fact that the TMI threshold is not only determined by the active fiber but, rather, by the whole system. In the experiment an artificially applied pump intensity-noise of 2.9% led to a reduction of the TMI threshold of 63%, whereas a similar seed intensity-noise decreased it by just 13%. Thus, even though both noise sources have an impact on the TMI threshold, the pump intensity-noise can be considered as the main driver for TMI in saturated fiber amplifiers. Additionally, the work unveils that the physical origin of this behavior is linked to the noise transfer function in saturated fiber amplifiers. With the gained knowledge and the experimental and theoretical results, it can be concluded that a suppression of pump-noise frequencies below 20 kHz could strongly increase the TMI threshold in high-power fiber laser systems.

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.

E. Shestaev, D. Hoff, A. M. Sayler, A. Klenke, S. Hädrich, F. Just, T. Eidam, P. Jójárt, Z. Várallyay, K. Osvay, G. G. Paulus, A. Tünnermann, and J. Limpert
High-power ytterbium-doped fiber laser delivering few-cycle, carrier-envelope phase-stable 100 µJ pulses at 100 kHz
Optics Letters 45, 97 (2020)

Abstract: We present a carrier-envelope phase (CEP)-stable Yb-doped fiber laser system delivering 100 µJ few-cycle pulses at a repetition rate of 100 kHz. The CEP stability of the system when seeded by a carrier-envelope offset-locked oscillator is 360 mrad, as measured pulse-to-pulse with a stereographic above-threshold ionization (stereo-ATI) phase meter. Slow CEP fluctuations have been suppressed by implementing a feedback loop from the phase meter to the pulse picking acousto-optic modulator. To the best of our knowledge, this is the highest CEP stability achieved to date with a fiber-based, high-power few-cycle laser.

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.

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.

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.

C. Aleshire, A. Steinkopff, C. Jauregui, A. Klenke, A. Tünnermann, and J. Limpert
Simplified Optics for Efficient Filled-Aperture Coherent Beam Combination
(2020)

Abstract: A simplification strategy for Segmented Mirror Splitters used as beam combiners is presented. Numerical methods are used to optimize design parameters and maintain a high theoretical combining efficiency for several tens of beams.

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.

Z. Wang, T. Heuermann, M. Gebhardt, M. Lenski, C. Gaida, C. Jauregui, and J. Limpert
Ultrafast Tm-doped fiber CPA system delivering GW-level peak power pulses at > 100 W average power
(2020)

Abstract: In this contribution, we present a Tm-doped fiber chirped pulse amplifier system delivering 10⁸ W of average output power at 417 kHz repetition rate with 250 fs pulse duration and close to 1 GW of pulse peak power.

T. Nagy, S. Hadrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. Jojart, I. Seres, Z. Varallyay, T. Eidam, and J. Limpert
Pulse Compression to 3-Cycle Duration beyond 300 W Average Power
(2020)
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

T. Saule, S. Heinrich, J. Schötz, N. Lilienfein, M. Högner, O. DeVries, M. Plötner, J. Weitenberg, D. Esser, J. Schulte, P. Russbueldt, J. Limpert, M. Kling, U. Kleineberg, and I. Pupeza
High-flux ultrafast extreme-ultraviolet photoemission spectroscopy at 18.4 MHz pulse repetition rate
Nature Communications 10, 458 (2019)

Abstract: Laser-dressed photoelectron spectroscopy, employing extreme-ultraviolet attosecond pulses obtained by femtosecond-laser-driven high-order harmonic generation, grants access to atomic-scale electron dynamics. Limited by space charge effects determining the admissible number of photoelectrons ejected during each laser pulse, multidimensional (i.e. spatially or angle-resolved) attosecond photoelectron spectroscopy of solids and nanostructures requires high-photon-energy, broadband high harmonic sources operating at high repetition rates. Here, we present a high-conversion-efficiency, 18.4-MHz-repetition-rate cavity-enhanced high harmonic source emitting 5 x 10(5) photons per pulse in the 25-to-60-eV range, releasing 1 x 10(10) photoelectrons per second from a 10-mu m-diameter spot on tungsten, at space charge distortions of only a few tens of meV. Broadband, time-of-flight photoelectron detection with nearly 100% temporal duty cycle evidences a count rate improvement between two and three orders of magnitude over state-of-the-art attosecond photoelectron spectroscopy experiments under identical space charge conditions. The measurement time reduction and the photon energy scalability render this technology viable for next-generation, high-repetition-rate, multidimensional attosecond metrology.

T. Nagy, S. Hädrich, P. Simon, A. Blumenstein, N. Walther, R. Klas, J. Buldt, H. Stark, S. Breitkopf, P. J\'ojárt, I. Seres, Z. Várallyay, T. Eidam, and J. Limpert
Generation of three-cycle multi-millijoule laser pulses at 318  W average power
Optica 6, 1423 (2019)

Abstract: The generation of three-cycle multi-millijoule pulses at 318 W power is reported by compressing pulses of a Yb-fiber chirped pulse amplifier in a 6 m long stretched flexible hollow fiber. This technique brings high-power lasers to the few-cycle regime.

H. Stark, J. Buldt, M. Müller, A. Klenke, A. Tünnermann, and J. Limpert
23  mJ high-power fiber CPA system using electro-optically controlled divided-pulse amplification
Optics Letters 44, 5529 (2019)

Abstract: The pulse-energy scaling technique electro-optically controlled divided-pulse amplification is implemented in a high-power ultrafast fiber laser system based on coherent beam combination. A fiber-integrated front end and a multipass-cell-based back end allow for a small footprint and a modular implementation. Bursts of eight pulses are amplified parallel in up to 12 ytterbium-doped large-pitch fiber amplifiers. Subsequent spatiotemporal coherent combination of the 96 total amplified pulse replicas to a single pulse results in a pulse energy of 23 mJ at an average power of 674 W, compressible to a pulse duration of 235 fs. To the best of our knowledge, this is the highest pulse energy ever accomplished with a fiber chirped-pulse amplification (CPA) system.

P. Gierschke, C. Jauregui, T. Gottschall, and J. Limpert
Relative amplitude noise transfer function of an Yb3+-doped fiber amplifier chain
Optics Express 27, 17041 (2019)

Abstract: In this work we measure the frequency dependent transfer function of the amplitude noise for both the seed and pump power in an Yb3+-doped fiber amplifier chain. In particular, the relative intensity noise transfer function of this amplifier chain in the frequency range of 10 Hz – 100 kHz has been investigated. It is shown that the pump power noise of the pre-amplifier stages is transformed into seed power noise for the next amplification stage. Crucially, the seed power noise in the frequency range of interest is strongly damped by the main-amplifier. This, however, does not happen for the pump power noise. Thus, the noise of the pump of the last amplifier stage is the factor with the strongest impact on the overall noise level of the system. Finally, useful guidelines to minimize the output amplitude noise of an Yb3+-doped fiber amplifier chain are given.

T. P. Butler, D. Gerz, C. Hofer, J. Xu, C. Gaida, T. Heuermann, M. Gebhardt, L. Vamos, W. Schweinberger, J. A. Gessner, T. Siefke, M. Heusinger, U. Zeitner, A. Apolonski, N. Karpowicz, J. Limpert, F. Krausz, and I. Pupeza
Watt-scale 50-MHz source of single-cycle waveform-stable pulses in the molecular fingerprint region
Optics Letters 44, 1730 (2019)

Abstract: We report a coherent mid-infrared (MIR) source with a combination of broad spectral coverage (6--18 µm), high repetition rate (50 MHz), and high average power (0.5 W). The waveform-stable pulses emerge via intrapulse difference-frequency generation (IPDFG) in a GaSe crystal, driven by a 30-W-average-power train of 32-fs pulses spectrally centered at 2 µm, delivered by a fiber-laser system. Electro-optic sampling (EOS) of the waveform-stable MIR waveforms reveals their single-cycle nature, confirming the excellent phase matching both of IPDFG and of EOS with 2-µm pulses in GaSe.

M. Mueller, A. Klenke, A. Steinkopff, E. Shestaev, A. Tünnermann, and J. Limpert
3.5 kW coherently combined ultrafast fiber laser
Proceedings of SPIE 10897, 43 (2019)

Abstract: We present a coherently-combined ultrafast fiber laser system consisting of four amplifier channels delivering 3.5 kW average power at 80 MHz repetition rate with a pulse duration of 430 fs FWHM and a close-to-diffraction-limited beam quality with an M-2 < 1.2. The system incorporates a fully automated self-adjustment of the beam combination, allowing for a quasi-turn-key operation of the system. At the date of publication, this system delivers the highest average power reported from an ultrafast laser.

A. Klenke, M. Müller, H. Stark, F. Stutzki, C. Hupel, T. Schreiber, A. Tünnermann, and J. Limpert
Coherent beam combination of pulses emitted by a 16-core ytterbium-doped fiber
Proceedings of SPIE 10897, 44 (2019)

Abstract: We present a laser amplifier based on coherent combination of 16 channels from a single multicore fiber employing multi-channel components for beam splitting, combination and temporal phasing. Stretched femtosecond pulses (250 fs transform-limit) were combined with an efficiency of 80% at up to 205 W average power.

J. Buldt, M. Mueller, C. Jauregui-Misas, L. Stark, and J. Limpert
Fiber-laser driven THz source based on air-plasma
Proceedings of SPIE 10897, 52 (2019)

Abstract: We present the most recent results of ultrafast fiber-laser driven generation of broadband THz radiation based on two-color gas-plasma. The experiment shows how energetic fiber-lasers can improve on an application today mainly dominated by Ti:sapphire lasers and power-scalability of this kind of THz sources is discussed. With a high-power driving laser THz radiation with more than 4 mW of average power is generated. This is the highest average power using this scheme so far.

E. Shestaev, C. Gaida, T. Heuermann, M. Gebhardt, T. Butler, D. Gerz, N. Lilienfein, P. Sulzer, M. Fischer, R. Holzwarth, A. Leitenstorfer, I. Pupeza, and J. Limpert
High power frequency comb delivered by a Tm-doped fiber laser
Proceedings of SPIE 10897, 17 (2019)

Abstract: Frequency combs are an enabling technology for metrology and spectroscopic applications in fundamental and life sciences. While frequency combs in the 1 lam regime, produced from Yb-based systems have already exceeded the 100 W - level, high power coverage of the interesting mid-infrared wavelength range remains yet to be demonstrated. Tm- and Ho-doped laser systems have recently shown operation at high average power levels in the 2 lam wavelength regime. However, frequency combs in this wavelength range have not exceeded the 5 W-average power level. In this work, we present a high power frequency comb, delivered by a Tm-doped chirped-pulse amplifier with subsequent nonlinear pulse compression. With an integrated phase noise of <320 mrad, low relative intensity noise of <0.5% and an average power of 60 W at 100 MHz repetition rate (and <30 fs FWHM pulse duration), this system demonstrates high stability and broad spectral coverage at an unrivalled average power level in this wavelength regime. Therefore, this laser will enable metrology and spectroscopy with unprecedented sensitivity and acquisition time. It is our ongoing effort to extend the spectral coverage of this system through the utilization of parametric frequency conversion into the mid-IR, thus ultimately enabling high power fingerprint spectroscopy in the entire molecular fingerprint region (2 - 20 mu m).

H. Stark, J. Buldt, M. Mueller, A. Klenke, A. Tünnermann, and J. Limpert
High-power electro-optically controlled divided-pulse amplification
Proceedings of SPIE 10897, 45 (2019)

Abstract: In this contribution, we present the newest results of the recently introduced pulse-energy-scaling technique electrooptically controlled divided-pulse amplification (EDPA) and its implementation in a high-power fiber laser system based on coherent combination. In this experiment, a burst of 8 stretched fs-pulses is amplified in two high-power fiber amplifier channels followed by coherent combination into a single pulse. Afterwards, the signal is compressed to a FWHM pulse duration of 255 fs with a pulse energy of 3 mJ and an average power of 105 W. The additional degrees of freedom provided by EDPA, such as direct access to the amplitudes and phases of all individual pulses in each burst, are exploited to compensate for gain saturation effects. Thus, a great temporal contrast of about 18.5 dB is reached and a very high combining efficiency of nearly 80%, including spatial as well as temporal combining, is reached. Furthermore, the system features three customized multi-pass cells as optical delay lines, minimizing the footprint of the combining stage to 0.5 m2. For the time being, two amplifiers are employed in order to initially optimize the parameters of EDPA and the performance of temporal combining. However, the laser system comprises a total of 16 parallel main amplifier channels, potentially enabling spatio-temporal combination of 128 separately amplified pulses with the currently applied bursts of 8 pulses. This extension is part of upcoming experiments and will allow for significant further scaling of the pulse energy in the near future.

T. Heuermann, M. Gebhardt, C. Gaida, I. Pupeza, and J. Limpert
High-power ultrafast Tm-doped fiber lasers for the generation of mid-infrared radiation in the molecular fingerprint region
Proceedings of SPIE 10897, 16 (2019)

Abstract: Accessing the molecular fingerprint region between 2 and 20 mu m is a key aspect in modern metrology and spectroscopy. While the wavelength range from 2-5 mu m can easily be addressed through nonlinear frequency conversion starting from well-matured 1 mu m driving lasers, access to the deep mid-IR wavelength regime is difficult. This is, because of the limited transmission of non-oxide crystals (that offer high nonlinearity and good transmission for the aspired mid-IR idler) at the pump wavelength and/or multi-photon absorption. Shifting to a longer pump wavelength relieves these limitations. In this work we present an experiment based on intra pulse difference frequency generation (IPDFG) in GaSe driven by an ultrafast Tm-doped chirped pulse amplifier. This experiment led to an octave spanning mid-IR spectrum, covering the wavelength range between 7.2-16.5 mu m (-10 dB width) with 450 mW of average power at 1.25 MHz repetition rate. This result outperforms comparable sources driven at 1 mu m wavelength in average power and conversion efficiency, while providing much broader spectral coverage. To further facilitate the use of these promising sources in real-world spectroscopic applications, we have built a nonlinear amplifier, which, based on its compact and robust design is an ideal candidate in this respect. Optimizing the output ultimately led to high pulse quality 50 fs pulses with 250 nJ of pulse energy at 80 MHz of repetition rate and 20 W average output power, exceeding current designs in the anomalous dispersion regime by 1 order of magnitude. It is our ongoing effort to utilize this laser for parametric downconversion. Covering the wavelength regime beyond 5 mu m wavelength would make it an enabling technology for next generation spectroscopy, fundamental and life sciences.

C. Jauregui-Misas, C. Stihler, A. Tünnermann, and J. Limpert
Origin and evolution of phase-shifts in high-power fiber laser systems: detailed insights into TMI
Proceedings of SPIE 10897, 3 (2019)

Abstract: In this work, we study the generation and evolution of phase-shifts between the modal interference pattern and the thermally-induced index grating due to pump-power changes. This study is not only important to understand new mitigation strategies based on controlling such phase-shifts, but also to comprehend how pump/signal noise can trigger TMI. Understanding how such a phase-shift can develop from a pump/signal change is not trivial, since the movement of both the MIP and the RIG are thermally driven and, therefore, should have similar time constants. Our simulations show unequivocally that a change of the pump power will lead to the generation of a phase-shift and the physical reason for this behavior is unveiled. The main reason is an increased sensitivity of the MIP to temperature variations because the local beat-length changes of the MIP are accumulated along the whole fiber length. Therefore, the further downstream the fiber a MIP maximum is (i.e. closer to the pump end in a counter-pumped configuration), the stronger and faster its position shift will be. This insight shows a way to obtain more TMI-resilient fiber designs and may help understanding the core area dependence of TMI.

A. Steinkopff, C. Jauregui-Misas, F. Stutzki, J. Nold, C. Hupel, N. Haarlammert, J. Bierlich, A. Tünnermann, and J. Limpert
Single-mode propagation with 205 µm mode-field diameter in a passive large pitch fiber
Proceedings of SPIE 10897, 31 (2019)

Abstract: We present theoretical and experimental investigations on effective single-transverse mode propagation in very large mode area (VLMA) fibers. Upscaling the mode area of fibers is the most effective approach to reduce the nonlinear interaction and, therefore, to allow for the confinement of high-power radiation without detrimental nonlinear effects. Even though the investigations are carried out in a passive large pitch fiber (LPF), they reveal an intrinsic scaling potential of this design which, if unlocked, will be beneficial for active VLMA fibers in the future. A commercial mode solver based on a full-vectorial finite-difference approach has been used to simulate the confinement losses of the fundamental and higher-order transverse modes. These simulations have revealed that the differential loss in one-missing-hole photonic crystal fibers can be tailored to be larger than 10 dB/m for fiber core sizes larger than 200 mu m at 1 mu m wavelength. In order to test the theoretical predictions experimental investigations have been performed. Therefore, a rod-type fiber has been fabricated and effective single-mode operation with unprecedented large mode-field diameters has been demonstrated. We were able to achieve single-mode propagation in a passive 1.3 m long LPF with a pitch of 140 mu m possessing a mode-field diameter of 205 mu m. Even a strong misalignment of the coupling condition did not lead to any significant appearance of higher order modes at the fiber exit, which proves the robustness of the single-mode operation. To the best of our knowledge these results represent the largest dimension of a fundamental transverse mode reported in a waveguide structure at 1 mu m wavelength to date. Compared to previous results the mode area is scaled by a factor of about 4 (with respect to active fibers) and a factor of similar to 8 (with respect to passive fibers).

C. Stihler, C. Jauregui-Misas, A. Tünnermann, and J. Limpert
The impact of pump-power noise on transverse mode instabilities
Proceedings of SPIE 10897, 2 (2019)

Abstract: In this work we have investigated the impact of pump-power noise on transverse mode instabilities (TMI) in high-power fiber laser systems. This is a crucial study since former works have shown that pump-power variations can induce a phase shift between the modal interference pattern and the thermally-induced refractive index grating and, thus, they are the most likely trigger for TMI. To experimentally investigate this behavior, we have generated white noise for different frequency bands with an arbitrary waveform generator and applied it to the pump diode. In a first experiment we have evaluated the frequency range of interest. It was found that only frequency components of the pump noise close to the main frequency of the TMI fluctuations influence the TMI threshold of the fiber laser system. In a second experiment we have measured the TMI threshold of the free-running system and compared it to the ones obtained when applying different pump-noise amplitudes. It was found that the TMI threshold can be decreased by almost a factor of three by increasing the noise of the pump source. This result is in good agreement with former theoretical and experimental studies and suggests that pump-power noise indeed acts as a main trigger for TMI. Furthermore, the findings indicate that the development of pump sources and drivers with a low noise level in a frequency range close to the main frequency of the TMI fluctuations could help to increase the TMI threshold of fiber laser systems.

F. Maes, C. Stihler, L.-P. Pleau, V. Fortin, J. Limpert, M. Bernier, and R. Vallée
3.42 µm lasing in heavily-erbium-doped fluoride fibers
Optics Express 27, 2170 (2019)

Abstract: In this paper, we investigate laser emission at 3.4μm in heavily-erbium-doped fluoride fibers using dual-wavelength pumping. To this extent, a monolithic 7 mol% erbium-doped fluoride fiber laser bounded by intracore fiber Bragg gratings at 3.42 μm is used to demonstrate a record efficiency of 38.6 % with respect to the 1976 nm pump. Through numerical modeling, we show that similar laser performances at 3.4 μm can be expected in fluoride fibers with erbium concentrations ranging between 1 – 7 mol%, although power scaling should rely on lightly-doped fibers to mitigate the heat load. Moreover, this work studies transverse mode-beating of the 1976 nm core pump and its role in the generation of a periodic luminescent grating and in the trapping of excitation in the metastable energy levels of the erbium system. Finally, we also report on the bistability of the 3.42 μm output power of the 7 mol% erbium-doped fluoride fiber laser.

A. Steinkopff, C. Jauregui, F. Stutzki, J. Nold, C. Hupel, N. Haarlammert, J. Bierlich, A. Tünnermann, and J. Limpert
Transverse single-mode operation in a passive large pitch fiber with more than 200  µm mode-field diameter
Optics Letters 44, 650 (2019)

Abstract: In this Letter, we present, to the best of our knowledge, the largest effective single-mode fiber reported to date. The employed waveguide is a passive large pitch fiber (LPF), which shows the core area scaling potential of such a fiber structure. In particular, we achieved stable single-transverse mode transmission at a wavelength of 1.03 µm through a straight passive LPF with a pitch of 140 µm, resulting in a measured mode-field diameter of 205 µm.

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

M. Müller, A. Klenke, A. Steinkopff, H. Stark, A. Tünnermann, and J. Limpert
3.5 kW coherently combined ultrafast fiber laser
Optics Letters 43, 6037 (2018)

Abstract: An ultrafast laser based on the coherent beam combination of four ytterbium-doped step-index fiber amplifiers is presented. The system delivers an average power of 3.5 kW and a pulse duration of 430 fs at an 80 MHz repetition rate. The beam quality is excellent (M2 < 1.24·1.10), and the relative intensity noise is as low as 1% in the frequency span from 1 Hz to 1 MHz. The system is turn-key operable, as it features an automated spatial and temporal alignment of the interferometric amplification channels.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, and J. Limpert
Ultrafast thulium fiber laser system emitting more than 1  kW of average power
Optics Letters 43, 5853 (2018)

Abstract: In this Letter, we report on the generation of 1060 W average power from an ultrafast thulium-doped fiber chirped pulse amplification system. After compression, the pulse energy of 13.2 μJ with a pulse duration of 265 fs at an 80 MHz pulse repetition rate results in a peak power of 50 MW spectrally centered at 1960 nm. Even though the average heat-load in the fiber core is as high as 98 W/m, we confirm the diffraction-limited beam quality of the compressed output. Furthermore, the evolution of the relative intensity noise with increasing average output power has been measured to verify the absence of transversal mode instabilities. This system represents a new average power record for thulium-doped fiber lasers (1150 W uncompressed) and ultrashort pulse fiber lasers with diffraction-limited beam quality, in general, even considering single-channel ytterbium-doped fiber amplifiers.

Y.-G. Jeong, R. Piccoli, D. Ferachou, V. Cardin, M. Chini, S. Hädrich, J. Limpert, R. Morandotti, F. Légaré, B. Schmidt, and L. Razzari
Direct compression of 170-fs 50-cycle pulses down to 1.5 cycles with 70% transmission
Scientific Reports 8, 11794 (2018)

Abstract: We present a straightforward route for extreme pulse compression, which relies on moderately driving self-phase modulation (SPM) over an extended propagation distance. This avoids that other detrimental nonlinear mechanisms take over and deteriorate the SPM process. The long propagation is obtained by means of a hollow-core fiber (HCF), up to 6 m in length. This concept is potentially scalable to TW pulse peak powers at kW average power level. As a proof of concept, we demonstrate 33-fold pulse compression of a 1 mJ, 6 kHz, 170 fs Yb laser down to 5.1 fs (1.5 cycles at 1030 nm), by employing a single HCF and subsequent chirped mirrors with an overall transmission of 70%.

C. Gaida, M. Gebhardt, T. Heuermann, F. Stutzki, C. Jauregui, J. Antonio-Lopez, A. Schülzgen, R. Amezcua-Correa, A. Tünnermann, I. Pupeza, and J. Limpert
Watt-scale super-octave mid-infrared intrapulse difference frequency generation
Light: Science & Applications 7, 94 (2018)

Abstract: The development of high-power, broadband sources of coherent mid-infrared radiation is currently the subject of intense research that is driven by a substantial number of existing and continuously emerging applications in medical diagnostics, spectroscopy, microscopy, and fundamental science. One of the major, long-standing challenges in improving the performance of these applications has been the construction of compact, broadband mid-infrared radiation sources, which unify the properties of high brightness and spatial and temporal coherence. Due to the lack of such radiation sources, several emerging applications can be addressed only with infrared (IR)-beamlines in large-scale synchrotron facilities, which are limited regarding user access and only partially fulfill these properties. Here, we present a table-top, broadband, coherent mid-infrared light source that provides brightness at an unprecedented level that supersedes that of synchrotrons in the wavelength range between 3.7 and 18 µm by several orders of magnitude. This result is enabled by a high-power, few-cycle Tm-doped fiber laser system, which is employed as a pump at 1.9 µm wavelength for intrapulse difference frequency generation (IPDFG). IPDFG intrinsically ensures the formation of carrier-envelope-phase stable pulses, which provide ideal prerequisites for state-of-the-art spectroscopy and microscopy.

C. Gaida, T. Heuermann, M. Gebhardt, E. Shestaev, T. P. Butler, D. Gerz, N. Lilienfein, P. Sulzer, M. Fischer, R. Holzwarth, A. Leitenstorfer, I. Pupeza, and J. Limpert
High-power frequency comb at 2  μm wavelength emitted by a Tm-doped fiber laser system
Optics Letters 43, 5178 (2018)

Abstract: We report on the generation of a high-power frequency comb in the 2 μm wavelength regime featuring high amplitude and phase stability with unprecedented laser parameters, combining 60 W of average power with <30  fs pulse duration. The key components of the system are a mode-locked Er:fiber laser, a coherence-preserving nonlinear broadening stage, and a high-power Tm-doped fiber chirped-pulse amplifier with subsequent nonlinear self-compression of the pulses. Phase locking of the system resulted in a phase noise of less than 320 mrad measured within the 10 Hz–30 MHz band and 30 mrad in the band from 10 Hz to 1 MHz.

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.

A. Klenke, M. Muller, H. Stark, M. Kienel, C. Jauregui, A. Tünnermann, and J. Limpert
Coherent Beam Combination of Ultrafast Fiber Lasers
IEEE Journal of Selected Topics in Quantum Electronics 24, 1 (2018)

Abstract: The performance of fiber laser systems has drastically increased over recent decades which has opened up new industrial and scientific applications for this technology. However, currently a number of physical effects prevents further power scaling. Coherent combination of beams from multiple emitters has been established as a power scaling technique beyond these limitations. It is possible to increase the average power and, for pulsed laser systems, also parameters such as the pulse energy and the peak power. To realize such laser systems, various aspects have to be taken into account which include beam combination elements, stabilization systems and the output parameters of the individual amplifiers. After an introduction to the topic, various ways of implementing coherent beam combination for ultrashort pulses are explored. Besides the spatial combination of beams, the combination of pulses in time will also be discussed. Recent experimental results will be presented, including multi-dimensional (i.e. spatial and temporal) combination. Finally, an outlook on possible further developments is given, focused on scaling the number of combinable beams and pulses.

T. Heuermann, C. Gaida, M. Gebhardt, and J. Limpert
Thulium-doped nonlinear fiber amplifier delivering 50  fs pulses at 20  W of average power
Optics Letters 43, 4441 (2018)

Abstract: In this Letter, we present an optimized nonlinear amplification scheme in the 2 µm wavelength region. This laser source delivers 50 fs pulses at an 80 MHz repetition rate with exceptional temporal pulse quality and 20 W of average output power. According to predictions from numerical simulations, it is experimentally confirmed that dispersion management is crucial to prevent the growth of side pulses and an increase of the energy content in a temporal pedestal surrounding the self-compressed pulse. Based on these results, we discuss guidelines to ensure high temporal pulse quality from nonlinear femtosecond fiber amplifiers in the anomalous dispersion regime.

T. Saule, M. Högner, N. Lilienfein, O. de Vries, M. Plötner, V. Yakovlev, N. Karpowicz, J. Limpert, and I. Pupeza
Cumulative plasma effects in cavity-enhanced high-order harmonic generation in gases
APL Photonics 3, 101301 (2018)

Abstract: Modern ultrafast laser architectures enable high-order harmonic generation (HHG) in gases at (multi-) MHz repetition rates, where each atom interacts with multiple pulses before leaving the HHG volume. This raises the question of cumulative plasma effects on the nonlinear conversion. Utilizing a femtosecond enhancement cavity with HHG in argon and on-axis geometric extreme-ultraviolet (XUV) output coupling, we experimentally compare the single-pulse case with a double-pulse HHG regime in which each gas atom is hit by two pulses while traversing the interaction volume. By varying the pulse repetition rate (18.4 and 36.8 MHz) in an 18.4-MHz roundtrip-frequency cavity with a finesse of 187, and leaving all other pulse parameters identical (35-fs, 0.6-μJ input pulses), we observe a dramatic decrease in the overall conversion efficiency (output-coupled power divided by the input power) in the double-pulse regime. The plateau harmonics (25–50 eV) exhibit very similar flux despite the twofold difference in repetition rate and average power. We attribute this to a spatially inhomogeneous plasma distribution that reduces the HHG volume, decreasing the generated XUV flux and/or affecting the spatial XUV beam profile, which reduces the efficiency of output coupling through the pierced mirror. These findings demonstrate the importance of cumulative plasma effects for power scaling of high-repetition-rate HHG in general and for applications in XUV frequency comb spectroscopy and in attosecond metrology in particular.

K. Bergner, M. Müller, R. Klas, J. Limpert, S. Nolte, and A. Tünnermann
Scaling ultrashort laser pulse induced glass modifications for cleaving applications
Applied Optics 57, 5941 (2018)

Abstract: Ultrashort laser pulses allow for in-volume processing of glass through non-linear absorption. This results in permanent material changes, largely independent of the processed glass, and it is of particular relevance for cleaving applications. In this paper, a laser with a wavelength of 1030 nm, pulse duration of 19 ps, repetition rate of 10 kHz, and burst regime consisting of either four or eight pulses, with an intra-burst pulse separation of 12.5 ns, is used. Subsequently, a Gaussian–Bessel focal line is generated in a fused silica substrate with the aid of an axicon configuration. We show how the structure of the modifications, including the length of material disruptions and affected zones, can be directly influenced by a reasonable choice of focus geometry, pulse energy, and burst regime. We achieve single-shot modifications with 2 μm in diameter and 7.6 mm in length, exceeding an aspect ratio of 1:3800. Furthermore, a maximum length of 10.8 mm could be achieved with a single shot.

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

C. Stihler, C. Jauregui, A. Tünnermann, and J. Limpert
Phase-shift evolution of the thermally-induced refractive index grating in high-power fiber laser systems induced by pump-power variations
Optics Express 26, 19489 (2018)

Abstract: A phase shift between the modal interference pattern and the thermally-induced refractive index grating is most likely the ultimate trigger for the damaging effect of transverse mode instabilities (TMI) in high-power fiber laser systems. By using comprehensive simulations, the creation and evolution of a thermally-induced phase shift is explained and illustrated in detail. It is shown that such a phase shift can be induced by a variation of the pump power. The gained knowledge about the generation and evolution of the phase shift will allow for the development of new mitigation strategies for TMI.

M. Chemnitz, R. Scheibinger, C. Gaida, M. Gebhardt, F. Stutzki, S. Pumpe, J. Kobelke, A. Tünnermann, J. Limpert, and M. A. Schmidt
Thermodynamic control of soliton dynamics in liquid-core fibers
Optica 5, 695 (2018)

Abstract: Liquid-core fibers offer local external control over pulse dispersion due to their strong thermodynamic response, offering a new degree of freedom in accurate soliton steering for reconfigurable nonlinear light generation. Here, we show how to accurately control soliton dynamics and supercontinuum generation in carbon disulfide/silica fibers by temperature and pressure tuning, monitored via the spectral location and the onset energy of non-solitonic radiation. Simulations and phase-matching calculations based on an extended thermodynamic dispersion model of carbon disulfide confirm the experimental results, which allows us to demonstrate the potential of temperature detuning of liquid-core fibers for octave spanning recompressible supercontinuum generation in the near-infrared.

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.

C. Stihler, C. Jauregui, A. Tünnermann, and J. Limpert
Modal energy transfer by thermally induced refractive index gratings in Yb-doped fibers
Light: Science & Applications 7, 59 (2018)

Abstract: Thermally induced refractive index gratings in Yb-doped fibers lead to transverse mode instability (TMI) above an average power threshold, which represents a severe problem for many applications. To obtain a deeper understanding of TMI, the evolution of the strength of the thermally induced refractive index grating with the average output power in a fiber amplifier is experimentally investigated for the first time. This investigation is performed by introducing a phase shift between the refractive index grating and modal interference pattern, which is obtained by applying a pump power variation to the fiber amplifier. It is demonstrated that the refractive index grating is sufficiently strong to enable modal energy coupling at powers that are significantly below the TMI threshold if the induced phase shift is sufficiently large. The experiments indicate that at higher powers, the refractive index grating becomes more sensitive to such phase shifts, which will ultimately trigger TMI. Furthermore, the experimental results demonstrate beam cleaning above the TMI threshold via the introduction of a positive phase shift. This finding paves the way for the development of a new class of mitigation strategies for TMI that are based on controlling the phase shift between the thermally induced refractive index grating and modal interference pattern.

S. Breitkopf, N. Lilienfein, T. Achtnich, C. Zwyssig, A. Tünnermann, I. Pupeza, and J. Limpert
Velocity- and pointing-error measurements of a 300 000-r/min self-bearing permanent-magnet motor for optical applications
Review of Scientific Instruments 89, 063110 (2018)

Abstract: Compact, ultra-high-speed self-bearing permanent-magnet motors enable a wide scope of applications including an increasing number of optical ones. For implementation in an optical setup, the rotors have to satisfy high demands regarding their velocity and pointing errors. Only a restricted number of measurements of these parameters exist and only at relatively low velocities. This manuscript presents the measurement of the velocity and pointing errors at rotation frequencies up to 5 kHz. The acquired data allow us to identify the rotor drive as the main source of velocity variations with fast fluctuations of up to 3.4 ns (RMS) and slow drifts of 23 ns (RMS) over ∼120 revolutions at 5 kHz in vacuum. At the same rotation frequency, the pointing fluctuated by 12 μrad (RMS) and 33 μrad (peak-to-peak) over ∼10 000 round trips. To our best knowledge, this states the first measurement of velocity and pointing errors at multi-kHz rotation frequencies and will allow potential adopters to evaluate the feasibility of such rotor drives for their application.

C. Jauregui, C. Stihler, A. Tünnermann, and J. Limpert
Pump-modulation-induced beam stabilization in high-power fiber laser systems above the mode instability threshold
Optics Express 26, 10691 (2018)

Abstract: A new way of stabilizing the output beam of a fiber laser system operating above the mode instability threshold is described and the first proof-of-principle experimental results are presented. This technique, which relies on a modulation of the pump power, works by washing the thermally-induced refractive index grating out, which weakens the coupling efficiency between transverse modes. One of the main advantages of this simple, yet powerful, approach is that it can be easily incorporated in already existing fiber laser systems since it does not require any additional optical elements. Using this beam stabilization strategy, a significant pointing stability and beam quality improvement has been demonstrated up to an average power of \~600W, which is a factor of 2 above the mode instability threshold.

A. Klenke, M. Müller, H. Stark, A. Tünnermann, and J. Limpert
Sequential phase locking scheme for a filled aperture intensity coherent combination of beam arrays
Optics Express 26, 12072 (2018)

Abstract: We present a novel phase locking scheme for the coherent combination of beam arrays in the filled aperture configuration. Employing a phase dithering mechanism for the different beams similar to LOCSET, dithering frequencies for sequential combination steps are reused. By applying an additional phase alternating scheme, this allows for the use of standard synchronized multichannel lock-in electronics for phase locking a large number of channels even when the frequency bandwidth of the employed phase actuators is limited.

Y. Hua, W. Liu, M. Hemmer, L. E. Zapata, G. Zhou, D. N. Schimpf, T. Eidam, J. Limpert, A. Tünnermann, F. X. Kärtner, and G. Chang
87-W 1018-nm Yb-fiber ultrafast seeding source for cryogenic Yb: yttrium lithium fluoride amplifier
Optics Letters 43, 1686 (2018)

Abstract: We demonstrate a compact and robust Yb-fiber master-oscillator power-amplifier system operating at 1018 nm with 2.5-nm bandwidth and 1-ns stretched pulse duration. It produces 87-W average power and 4.9-µJ pulse energy, constituting a powerful seed source for cryogenically cooled ultrafast Yb: yttrium lithium fluoride (Yb:YLF) amplifiers.

A. Klenke, M. Müller, H. Stark, F. Stutzki, C. Hupel, T. Schreiber, A. Tünnermann, and J. Limpert
Coherently combined 16-channel multicore fiber laser system
Optics Letters 43, 1519 (2018)

Abstract: We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled-aperture beam splitting and combination, together with temporal phasing, is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses is achieved with combination efficiencies around 80%.

C. Jauregui, F. Stutzki, A. Tünnermann, and J. Limpert
Thermal analysis of Yb-doped high-power fiber amplifiers with Al:P co-doped cores
Optics Express 26, 7614 (2018)

Abstract: It has been recently shown that photodarkening can significantly reduce the mode instability threshold in high power Yb-doped fiber amplifiers, thus resulting in an even more severe limitation to the scaling of the output average power of these systems. Therefore, an efficient reduction of photodarkening in an Yb-doped active fiber will lead to very significant gains in the output average power delivered by such systems. In this context, it has been reported that photodarkening can be significantly mitigated when co-doping a fiber core with Al and P, which makes this approach potentially appealing to increase the TMI threshold. Unfortunately co-doping the fiber core with Al and P also alters the effective cross-sections of the fiber, which has repercussion in the amplification efficiency. Thus, a fiber with a higher P concentration will exhibit lower cross-sections, therefore requiring a higher Yb-ion concentration to reach a certain desired amplification efficiency. However, increasing the Yb-ion concentration leads to higher photodarkening losses, which might potentially counteract the benefits of using P co-doping. In this paper we present a comparative analysis of the expected performance of different fiber amplifiers for a given constant average heat-load and amplification efficiency as a function of the ratio of Al:P concentration in the fiber core. This study indicates which core compositions are more beneficial for increasing the mode instability threshold in Yb-doped high-power fiber amplifier systems.

M. Chemnitz, C. Gaida, M. Gebhardt, F. Stutzki, J. Kobelke, A. Tünnermann, J. Limpert, and M. A. Schmidt
Carbon chloride-core fibers for soliton mediated supercontinuum generation
Optics Express 26, 3221 (2018)

Abstract: We report on soliton-fission mediated infrared supercontinuum generation in liquid-core step-index fibers using highly transparent carbon chlorides (CCl4, C2Cl4). By developing models for the refractive index dispersions and nonlinear response functions, dispersion engineering and pumping with an ultrafast thulium fiber laser (300 fs) at 1.92 µm, distinct soliton fission and dispersive wave generation was observed, particularly in the case of tetrachloroethylene (C2Cl4). The measured results match simulations of both the generalized and a hybrid nonlinear Schrödinger equation, with the latter resembling the characteristics of non-instantaneous medium via a static potential term and representing a simulation tool with substantially reduced complexity. We show that C2Cl4 has the potential for observing non-instantaneous soliton dynamics along meters of liquid-core fiber opening a feasible route for directly observing hybrid soliton dynamics.

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.

J. Limpert
Toward multi-kW femtosecond fiber lasers based on mutlicore fibers
(2018)

Abstract: The achievements and the potential of coherent pulse addition as a performance scaling approach of ultrafast laser systems will be reviewed. Active multicore arrangements pursue that concept in a most compact way. First very promising results will be presented.

## 2017

M. Gebhardt, C. Gaida, T. Heuermann, F. Stutzki, C. Jauregui, J. Antonio-Lopez, A. Schulzgen, R. Amezcua-Correa, J. Limpert, and A. Tünnermann
Nonlinear pulse compression to 43  W GW-class few-cycle pulses at 2  μm wavelength
Optics Letters 42, 4179 (2017)

Abstract: High-average power laser sources delivering intense few-cycle pulses in wavelength regions beyond the near infrared are promising tools for driving the next generation of high-flux strong-field experiments. In this work, we report on nonlinear pulse compression to 34.4 μJ-, 2.1-cycle pulses with 1.4 GW peak power at a central wavelength of 1.82 μm and an average power of 43 W. This performance level was enabled by the combination of a high-repetition-rate ultrafast thulium-doped fiber laser system and a gas-filled antiresonant hollow-core fiber.

J. Buldt, M. Müller, R. Klas, T. Eidam, J. Limpert, and A. Tünnermann
Temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra
Optics Letters 42, 3761 (2017)

Abstract: We present a novel approach for temporal contrast enhancement of energetic laser pulses by filtered self-phase-modulation-broadened spectra. A measured temporal contrast enhancement by at least seven orders of magnitude in a simple setup has been achieved. This technique is applicable to a wide range of laser parameters and poses a highly efficient alternative to existing contrast-enhancement methods.

M. Kozák, P. Beck, H. Deng, J. McNeur, N. Schönenberger, C. Gaida, F. Stutzki, M. Gebhardt, J. Limpert, A. Ruehl, I. Hartl, O. Solgaard, J. S. Harris, R. L. Byer, and P. Hommelhoff
Acceleration of sub-relativistic electrons with an evanescent optical wave at a planar interface
Optics Express 25, 19195 (2017)

Abstract: We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element’s surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

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.

M. Chemnitz, M. Gebhardt, C. Gaida, F. Stutzki, J. Kobelke, J. Limpert, A. Tünnermann, and M. Schmidt
Hybrid soliton dynamics in liquid-core fibres
Nature Communications 8, 42 (2017)

Abstract: The discovery of optical solitons being understood as temporally and spectrally stationary optical states has enabled numerous innovations among which, most notably, supercontinuum light sources have become widely used in both fundamental and applied sciences. Here, we report on experimental evidence for dynamics of hybrid solitons—a new type of solitary wave, which emerges as a result of a strong non-instantaneous nonlinear response in CS2-filled liquid-core optical fibres. Octave-spanning supercontinua in the mid-infrared region are observed when pumping the hybrid waveguide with a 460 fs laser (1.95 μm) in the anomalous dispersion regime at nanojoule-level pulse energies. A detailed numerical analysis well correlated with the experiment uncovers clear indicators of emerging hybrid solitons, revealing their impact on the bandwidth, onset energy and noise characteristics of the supercontinua. Our study highlights liquid-core fibres as a promising platform for fundamental optics and applications towards novel coherent and reconfigurable light sources.

H. Stark, M. Müller, M. Kienel, A. Klenke, J. Limpert, and A. Tünnermann
Electro-optically controlled divided-pulse amplification
Optics Express 25, 13494 (2017)

Abstract: A novel technique for divided-pulse amplification is presented in a proof-of-principle experiment. A pulse burst, cut out of the pulse train of a mode-locked oscillator, is amplified and temporally combined into a single pulse. High combination efficiency and excellent pulse contrast are demonstrated. The system is mostly fiber-coupled, enabling a high interferometric stability. This approach provides access to the amplitude and phase of the individual pulses in the burst to be amplified, potentially allowing the compensation of gain saturation and nonlinear phase mismatches within the burst. Therefore, this technique enables the scaling of the peak power and pulse energy of pulsed laser systems beyond currently prevailing limitations.

M. Gebhardt, C. Gaida, F. Stutzki, S. Hädrich, C. Jauregui, J. Limpert, and A. Tünnermann
High average power nonlinear compression to 4 GW, sub-50  fs pulses at 2 μm wavelength
Optics Letters 42, 747 (2017)

Abstract: The combination of high-repetition-rate ultrafast thulium-doped fiber laser systems and gas-based nonlinear pulse compression in waveguides offers promising opportunities for the development of high-performance few-cycle laser sources at 2 μm wavelength. In this Letter, we report on a nonlinear pulse compression stage delivering 252 μJ, sub-50 fs-pulses at 15.4 W of average power. This performance level was enabled by actively mitigating ultrashort pulse propagation effects induced by the presence of water vapor absorptions.

J. Limpert
High Power Fiber Lasers
(2017)

Abstract: Fiber lasers enjoy an excellent reputation as power-scalable diode-pumped solid-state laser concept. Their immunity against thermo-optical issues is combined with high efficiency and performance. The properties, challenges and perspectives of fiber lasers will be discussed.

M. Mueller, M. Kienel, A. Klenke, T. Gottschall, E. Shestaev, M. Plötner, J. Limpert, and A. Tünnermann
12 mJ pulse energy 8-channel divided-pulse ultrafast fiber-laser system
(2017)

Abstract: State-of-the-art ultrafast fiber lasers currently are limited in peak power by excessive nonlinearity and in average power by modal instabilities. Coherent beam combination in space and time is a successful strategy to continue power scaling by circumventing these limitations. Following this approach, we demonstrate an ultrafast fiber-laser system featuring spatial beam combination of 8 amplifier channels and temporal combination of a burst comprising 4 pulses. Active phase stabilization of this 10-armed interferometer is achieved using LOCSET and Hänsch-Couillaud techniques. The system delivers 1 kW average power at 1 mJ pulse energy, being limited by pump power, and delivers 12 mJ pulse energy at 700 W average power, being limited by optically induced damage. The system efficiency is 91% and 78%, respectively, which is due to inequalities of nonlinearity between the amplifier channels and to inequality of power and nonlinearity between the pulses within the burst. In all cases, the pulse duration is ~260 fs and the M2-value is better than 1.2. Further power scaling is possible using more amplifier channels and longer pulse bursts.

C. Stihler, C. Jauregui, H.-J. Otto, J. Limpert, and A. Tünnermann
Controlling mode instabilities at 628 W average output power in an Yb-doped rod-type fiber amplifier by active modulation of the pump power
(2017)

Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Thus, it is of high interest to develop efficient mitigation strategies to further enhance the performance of fiber laser systems. By actively modulating the pump power of an Yb-doped rod-type fiber amplifier, it was possible to weaken the thermally-induced refractive index grating along the fiber and, thus, to mitigate TMI to a large extent. A significant advantage of this approach is that it can be easily integrated in any existing fiber-laser system since no further optical components are needed. A function generator connected to the pump diode driver was used to achieve the modulation. With this setup we were able to extract a fully stabilized beam at ~ 1.5 times above the TMI threshold. Furthermore, a stabilization of the beam was still feasible at an average output power of 628 W, which is more than three times higher than the free-running TMI threshold of that particular fiber under identical conditions (e.g. seed power). This is the highest average output power reported from a single-channel rod-type fiber amplifier with a high-quality stabilized beam, to the best of our knowledge.

H. Stark, M. Müller, M. Kienel, A. Klenke, J. Limpert, and A. Tünnermann
Divided-pulse amplification with fiber-coupled electro-optic modulators
(2017)

Abstract: n this contribution, we present a spatio-temporal coherent beam combining setup in a proof-of-principle experiment with an entirely fiber-coupled front-end. Unlike in previous experiments, where the temporal pulse division was achieved using free-space optical delay lines, the pulses are taken directly from the pulse train of the oscillator. Thereby, the free-space paths and the alignment requirement are cut in half. The combination inevitably remains in free-space considering application in high-power lasers. For the combination of 4 temporally separated pulses, a combining efficiency larger than 95% is demonstrated. The efficiency is largely independent of the combined pulse energy and temporal contrasts close to the theoretically estimated maximum are reached. Potentially, this approach allows for self-optimization of the combination due to the many degrees of freedom accessible with the electro-optic modulators.

C. Stihler, H.-J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann
Experimental investigation of transverse mode instabilities in a double-pass Yb-doped rod-type fiber amplifier
(2017)

Abstract: The phenomenon of transverse mode instabilities (TMI) is currently the most limiting effect for the scaling of the average output power of fiber laser systems with nearly diffraction-limited beam quality. Even though a significant amount of knowledge on TMI in single-pass fiber amplifiers has been generated in the last years, relatively little is known about this effect in multi-pass amplifiers and oscillators. In this contribution TMI is experimentally investigated in a double-pass fiber amplifier, for the first time to the best of our knowledge. The TMI threshold was found to be significantly lower in the double-pass configuration than in the single-pass arrangement. Furthermore, the investigations unveiled a complex dynamic behavior of the instabilities in the double-pass fiber amplifier.

M. Gebhardt, C. Gaida, F. Stutzki, S. Hädrich, C. Jauregui, J. Limpert, and A. Tünnermann
High-average power 4 GW pulses with sub-8 optical cycles from a Tm-doped fiber laser driven nonlinear pulse compression stage
(2017)

Abstract: Thulium-doped fiber lasers are an attractive concept for the generation of mid-infrared (mid-IR) ultrashort pulses around 2 μm wavelength with an unprecedented average power. To date, these systems deliver >150 W of average power and GW-class pulse peak powers with output pulse durations of a few hundreds of fs. As some applications can greatly benefit from even shorter pulse durations, the spectral broadening and subsequent temporal pulse compression can be a key enabling technology for high average power few-cycle laser sources around 2 μm wavelength. In this contribution we demonstrate the nonlinear compression of ultrashort pulses from a high repetition rate Tm-doped fiber laser using a nitrogen gas-filled hollow capillary. Pulses with 4 GW peak power, 46 fs FWHM duration at an average power of 15.4 W have been achieved. This is, to the best of our knowledge, the first 2 μm laser delivering intense, GW-pulses with sub 50-fs pulse duration and an average power of >10 W. Based on this result, we discuss the next steps towards a 100 W-level, GW-class few-cycle mid-IR laser.

C. Jauregui, M. Müller, M. Kienel, F. Emaury, C. J. Saraceno, J. Limpert, U. Keller, and A. Tünnermann
Optimizing the noise characteristics of high-power fiber laser systems
(2017)

Abstract: The noise characteristics of high-power fiber lasers, unlike those of other solid-state lasers such as thin-disks, have not been systematically studied up to now. However, novel applications for high-power fiber laser systems, such as attosecond pulse generation, put stringent limits to the maximum noise level of these sources. Therefore, in order to address these applications, a detailed knowledge and understanding of the characteristics of noise and its behavior in a fiber laser system is required. In this work we have carried out a systematic study of the propagation of the relative intensity noise (RIN) along the amplification chain of a state-of-the-art high-power fiber laser system. The most striking feature of these measurements is that the RIN level is progressively attenuated after each amplification stage. In order to understand this unexpected behavior, we have simulated the transfer function of the RIN in a fiber amplification stage (~80μm core) as a function of the seed power and the frequency. Our simulation model shows that this damping of the amplitude noise is related to saturation. Additionally, we show, for the first time to the best of our knowledge, that the fiber design (e.g. core size, glass composition, doping geometry) can be modified to optimize the noise characteristics of high-power fiber laser systems.

F. Stutzki, C. Gaida, M. Gebhardt, C. Jauregui, J. Limpert, A. Tünnermann, and I. Pupeza
Self-protecting nonlinear compression in a solid fiber for long-term stable ultrafast lasers at 2 µm wavelength
(2017)

Abstract: Ultrashort-pulse laser systems are an enabling technology for numerous applications. The stability of such systems is especially crucial for frequency metrology and high precision spectroscopy. Thulium-based fiber lasers are an ideal starting point as a reliable and yet powerful source for the nonlinear conversion towards the mid-IR region. Recently, we have demonstrated that nonlinear self-compression in a fused silica solid-core fiber allows for few-cycle pulse duration with up to 24 MW peak power using a high-repetition rate thulium-based fiber laser system operating at around 2 μm wavelength [1]. This experiment operates near the self-focusing limit of about 24 MW for circular polarization, which increases the requirements for the system stability due to the risk of a fiber damage. Here, we present a self-protecting nonlinear compression regime allowing for long-term operation and high output-pulse stability with very similar output performance.

C. Jauregui, H.-J. Otto, C. Stihler, J. Limpert, and A. Tünnermann
The impact of core co-dopants on the mode instability threshold of high-power fiber laser systems
(2017)

Abstract: Transverse mode instabilities (TMI) have become a very serious problem for the further scaling of the average power of fiber laser systems. Recently the strong impact that photodarkening (PD) has on the TMI threshold of Yb-doped fiber laser systems has been revealed. This is a remarkable finding since it opens the door to a significant increase of the average power of fiber laser systems in the near future. The key to achieve this is to reduce the amount of PD losses in the fiber, which can be done with an optimization of the glass composition in the fiber. In this work we perform a theoretical study on the impact that co-dopants such as Al and P have on PD and on the TMI threshold. This analysis tries to find the optimum glass composition from the point of view of TMI. It is shown that in a short rod type fiber, changing the glass composition only leads to a modest increase of the TMI threshold due to the degradation of the cross-sections. This demonstrates that the optimization of the glass cannot be done attending only to the PD losses at the cost of the laser cross-sections. In spite of this, changing the glass composition can bring benefits in pulsed operation in terms of the stored energy. Additionally, other fiber geometries different from the rod-type can benefit in a greater degree by introducing co-dopants in the glass.

C. Gaida, M. Gebhardt, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Towards sub-100 fs multi-GW pulses directly emitted from a Thulium-doped fiber CPA system
(2017)

Abstract: Experimental demonstrations of Tm-doped fiber amplifiers (typically in CW- or narrow-band pulsed operation) span a wavelength range going from about 1700 nm to well beyond 2000 nm. Thus, it should be possible to obtain a bandwidth of more than 100 nm, which would enable sub-100 fs pulse duration in an efficient, linear amplification scheme. In fact, this would allow the emission of pulses with less than 20 optical cycles directly from a Tm-doped fiber system, something that seems to be extremely challenging for other dopants in a fused silica fiber. In this contribution, we summarize the current development of our Thulium-doped fiber CPA system, demonstrate preliminary experiments for further scaling and discuss important design factors for the next steps. The current single-channel laser system presented herein delivers a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. Special care has been taken to reduce the detrimental impact of water vapor absorption by placing the whole system in a dry atmosphere housing (<0.1% rel. humidity) and by using a sufficiently long wavelength (1920-1980 nm). The utilization of a low-pressure chamber in the future will allow for the extension of the amplification bandwidth. Preliminary experiments demonstrating a broader amplification bandwidth that supports almost 100 fs pulse duration and average power scaling to < 100W have already been performed. Based on these results, a Tm-doped fiber CPA with sub-100 fs pulse duration, multi-GW pulse peak power and >100 W average power can be expected in the near future.

## 2016

S. Breitkopf, S. Wunderlich, T. Eidam, E. Shestaev, S. Holzberger, T. Gottschall, H. Carstens, A. Tünnermann, I. Pupeza, and J. Limpert
Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity
Applied Physics B 122, 297 (2016)

Abstract: Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.

T. Saule, S. Holzberger, O. De Vries, M. Plötner, J. Limpert, A. Tünnermann, and I. Pupeza
Phase-stable, multi-textmuJ femtosecond pulses from a repetition-rate tunable Ti:Sa-oscillator-seeded Yb-fiber amplifier
Applied Physics B 123, 17 (2016)

Abstract: We present a high-power, MHz-repetition-rate, phase-stable femtosecond laser system based on a phase-stabilized Ti:Sa oscillator and a multi-stage Yb-fiber chirped-pulse power amplifier. A 10-nm band around 1030 nm is split from the 7-fs oscillator output and serves as the seed for subsequent amplification by 54 dB to 80 W of average power. The textmuJ-level output is spectrally broadened in a solid-core fiber and compressed to textasciitilde30 fs with chirped mirrors. A pulse picker prior to power amplification allows for decreasing the repetition rate from 74 MHz by a factor of up to 4 without affecting the pulse parameters. To compensate for phase jitter added by the amplifier to the feed-forward phase-stabilized seeding pulses, a self-referencing feed-back loop is implemented at the system output. An integrated out-of-loop phase noise of less than 100 mrad was measured in the band from 0.4 Hz to 400 kHz, which to the best of our knowledge corresponds to the highest phase stability ever demonstrated for high-power, multi-MHz-repetition-rate ultrafast lasers. This system will enable experiments in attosecond physics at unprecedented repetition rates, it offers ideal prerequisites for the generation and field-resolved electro-optical sampling of high-power, broadband infrared pulses, and it is suitable for phase-stable white light generation.

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.

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.

C. Gaida, M. Gebhardt, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Thulium-doped fiber chirped-pulse amplification system with 2 GW of peak power
Optics Letters 41, 4130 (2016)

Abstract: Thulium-doped fibers with ultra large mode-field areas offer new opportunities for the power scaling of mid-IR ultrashort-pulse laser sources. Here, we present a laser system delivering a pulse-peak power of 2 GW and a nearly transform-limited pulse duration of 200 fs in combination with 28.7 W of average power. This performance level has been achieved by optimizing the pulse shape, reducing the overlap with atmospheric absorption lines, and incorporating a climate chamber to reduce the humidity of the atmospheric environment.

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.

M. Müller, M. Kienel, A. Klenke, T. Gottschall, E. Shestaev, M. Plötner, J. Limpert, and A. Tünnermann
1 kW 1 mJ eight-channel ultrafast fiber laser
Optics Letters 41, 3439 (2016)

Abstract: An ultrafast fiber chirped-pulse amplifier comprising eight coherently combined amplifier channels is presented. The laser delivers 1 kW average power at 1 mJ pulse energy and 260 fs pulse duration. Excellent beam quality and low-noise performance are confirmed. The laser has proven suitable for demanding scientific applications. Further power scaling is possible right away using even more amplifier channels.

M. Kienel, M. Müller, A. Klenke, J. Limpert, and A. Tünnermann
12 mJ kW-class ultrafast fiber laser system using multidimensional coherent pulse addition
Optics Letters 41, 3343 (2016)

Abstract: An ultrafast fiber-chirped-pulse amplification system using a combination of spatial and temporal coherent pulse com- bination is presented. By distributing the amplification among eight amplifier channels and four pulse replicas, up to 12 mJ pulse energy with 700 W average power and 262 fs pulse duration have been obtained with a system efficiency of 78% and excellent beam quality. To the best of our knowledge, this is the highest energy achieved by an ultrafast fiber-based laser system to date.

G. Matthäus, S. Demmler, M. Lebugle, F. Küster, J. Limpert, A. Tünnermann, S. Nolte, and R. Ackermann
Ultra-broadband two beam CARS using femtosecond laser pulses
Vibrational Spectroscopy 85, 128 (2016)

Abstract: Femtosecond (fs)-CARS is a promising approach for gas spectroscopy under high pressure and temperature conditions, as it allows probing molecular states on a time scale which is significantly shorter than the typical decay time induced by interfering collisions. Usually, fs-CARS is performed in a three beam setup, which requires maintaining spatial and temporal overlap of the pulses at the focal point. This is a challenging task, especially in harsh environments such as in a combustion chamber. In this study, we present an alternative approach, which uses two beams in a collinear configuration. An ultra-broadband, sub 7 fs laser pulse acts as pump and Stokes pulse, and a ∼500 fs pulse is used for probing. We show that this configuration is suitable for measuring the gas temperature and concentration. Furthermore, possible single shot measurements of the gas temperature are evaluated.

H. Carstens, M. Högner, T. Saule, S. Holzberger, N. Lilienfein, A. Guggenmos, C. Jocher, T. Eidam, D. Esser, V. Tosa, V. Pervak, J. Limpert, A. Tünnermann, U. Kleineberg, F. Krausz, and I. Pupeza
High-harmonic generation at 250  MHz with photon energies exceeding 100  eV
Optica 3, 366 (2016)

Abstract: Ultrafast spectroscopy in the extreme ultraviolet demands for ever-higher pulse repetition rates and photon energies. Here, we drive cavity-enhanced high-order harmonic generation (HHG) at a repetition rate of 250 MHz, with 30 fs pulses and an average power of 10 kW. Employing an optimized cavity geometry and a high-pressure gas target, we couple out nanowatt-level harmonics at photon energies around 100 eV. This constitutes an improvement of more than two orders of magnitude over previous megahertz-repetition-rate HHG experiments and paves the way toward high-photon-energy frequency-comb spectroscopy and toward pump-probe photoelectron microscopy and spectroscopy at unprecedented repetition rates.

C. Jauregui, H.-J. Otto, S. Breitkopf, J. Limpert, and A. Tünnermann
Optimizing high-power Yb-doped fiber amplifier systems in the presence of transverse mode instabilities
Optics Express 24, 7879 (2016)

Abstract: The average output power of Yb-doped fiber amplifier systems is currently limited by the onset of transverse mode instabilities. Besides, it has been recently shown that the transverse mode instability threshold can be significantly reduced by the presence of photodarkening in the fiber. Therefore, reducing the photodarkening level of the core material composition is the most straightforward way to increase the output average power of fiber amplifier systems but, unfortunately, this is not always easy or possible. In this paper we present guidelines to optimize the output average power of fiber amplifiers affected by transverse mode instabilities and photodarkening. The guidelines derived from the simulations do not involve changes in the composition of the active material (except for its doping concentration), but can still lead to a significant increase of the transverse mode instability threshold. The dependence of this parameter on the active ion concentration and the core conformation, among others, will be studied and discussed.

M. Mueller, M. Kienel, A. Klenke, T. Eidam, J. Limpert, and A. Tünnermann
Phase stabilization of spatiotemporally multiplexed ultrafast amplifiers
Optics Express 24, 7893 (2016)

Abstract: Actively stabilized, simultaneous spatial and temporal coherent beam combination is a promising power-scaling technique for ultrafast laser systems. For a temporal combination based on optical delay lines, multiple stable states of operation arise for common stabilization techniques. A time resolved Jones’ calculus is applied to investigate the issue. A mitigation strategy based on a temporally gated error signal acquisition is derived and demonstrated, enabling to stabilize laser systems with arbitrary numbers of amplifier channels and optical delay lines.

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.

C. Gaida, M. Gebhardt, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Self-compression in a solid fiber to 24  MW peak power with few-cycle pulses at 2  μm wavelength
Optics Letters 40, 5160 (2015)

Abstract: We report on the experimental realization of a compact, fiber-based, ultrashort-pulse laser system in the 2 μm wavelength region delivering 24 fs pulse duration with 24 MW pulse peak power and 24.6 W average power. This performance level has been enabled by the favorable quadratic wavelength-dependence of the self-focusing limit, which has been experimentally verified to be at approximately 24 MW for circular polarization in a solid-core fused-silica fiber operated at a wavelength around 2 μm. The anomalous dispersion in this wavelength region allows for a simultaneous nonlinear spectral broadening and temporal pulse compression. This makes an additional compression stage redundant and facilitates a very simple and power-scalable approach. Simulations that include both the nonlinear pulse evolution and the transverse optical Kerr effect support the experimental results.

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.

T. Eidam, M. Kienel, A. Klenke, J. Limpert, and A. Tünnermann
Divided-pulse amplification for terawatt-class fiber lasers
European Physical Journal Special Topics 224, 2567 (2015)

Abstract: The coherent combination of ultra short laser pulses is a promising approach for scaling the average and peak power of ultrafast lasers. Fiber lasers and amplifiers are especially suited for this technique due to their simple singe-pass setups that can be easily parallelized. Here we propose the combination of the well-known approach of spatially separated amplification with the technique of divided-pulse amplification, i.e. an additionally performed temporally separated amplification. With the help of this multidimensional pulse stacking, laser systems come into reach capable of emitting 10’s of joules of energy at multi-kW average powers that simultaneously employ a manageable number of fibers.

S. Breitkopf, T. Eidam, A. Klenke, H. Carstens, S. Holzberger, E. Fill, T. Schreiber, F. Krausz, A. Tünnermann, I. Pupeza, and J. Limpert
Stack and dump: Peak-power scaling by coherent pulse addition in passive cavities
European Physical Journal Special Topics 224, 2573 (2015)

Abstract: During the last decades femtosecond lasers have proven their vast benefit in both scientific and technological tasks. Nevertheless, one laser feature bearing the tremendous potential for high-field applications, delivering extremely high peak and average powers simultaneously, is still not accessible. This is the performance regime several upcoming applications such as laser particle acceleration require, and therefore, challenge laser technology to the fullest. On the one hand, some state-of-the-art canonical bulk amplifier systems provide pulse peak powers in the range of multi-terawatt to petawatt. On the other hand, concepts for advanced solid-state-lasers, specifically thin disk, slab or fiber systems have shown their capability of emitting high average powers in the kilowatt range with a high wall-plug-efficiency while maintaining an excellent spatial and temporal quality of the output beam.

In this article, a brief introduction to a concept for a compact laser system capable of simultaneously providing high peak and average powers all along with a high wall-plug efficiency will be given. The concept relies on the stacking of a pulse train emitted from a high-repetitive femtosecond laser system in a passive enhancement cavity, also referred to as temporal coherent combining. In this manner, the repetition rate is decreased in favor of a pulse energy enhancement by the same factor while the average power is almost preserved. The key challenge of this concept is a fast, purely reflective switching element that allows for the dumping of the enhanced pulse out of the cavity. Addressing this challenge could, for the first time, allow for the highly efficient extraction of joule-class pulses at megawatt average power levels and thus lead to a whole new area of applications for ultra-fast laser systems.

C. Jauregui, H.-J. Otto, F. Stutzki, J. Limpert, and A. Tünnermann
Simplified modelling the mode instability threshold of high power fiber amplifiers in the presence of photodarkening
Optics Express 23, 20203 (2015)

Abstract: In this paper we present a simple model to predict the behavior of the transversal mode instability threshold when different parameters of a fiber amplifier system are changed. The simulation model includes an estimation of the photodarkening losses which shows the strong influence that this effect has on the mode instability threshold and on its behavior. Comparison of the simulation results with experimental measurements reveal that the mode instability threshold in a fiber amplifier system is reached for a constant average heat load value in good approximation. Based on this model, the expected behavior of the mode instability threshold when changing the seed wavelength, the seed power and/or the fiber length will be presented and discussed. Additionally, guidelines for increasing the average power of fiber amplifier systems will be provided.

O. de Vries, T. Saule, M. Plötner, F. Lücking, T. Eidam, A. Hoffmann, A. Klenke, S. Hädrich, J. Limpert, S. Holzberger, T. Schreiber, R. Eberhardt, I. Pupeza, and A. Tünnermann
Acousto-optic pulse picking scheme with carrier-frequency-to-pulse-repetition-rate synchronization
Optics Express 23, 19586 (2015)

Abstract: We introduce and experimentally validate a pulse picking technique based on a travelling-wave-type acousto-optic modulator (AOM) having the AOM carrier frequency synchronized to the repetition rate of the original pulse train. As a consequence, the phase noise characteristic of the original pulse train is largely preserved, rendering this technique suitable for applications requiring carrier-envelope phase stabilization. In a proof-of-principle experiment, the 1030-nm spectral part of an 74-MHz, carrier-envelope phase stable Ti:sapphire oscillator is amplified and reduced in pulse repetition frequency by a factor of two, maintaining an unprecedentedly low carrier-envelope phase noise spectral density of below 68 mrad. Furthermore, a comparative analysis reveals that the pulse-picking-induced additional amplitude noise is minimized, when the AOM is operated under synchronicity. The proposed scheme is particularly suitable when the down-picked repetition rate is still in the multi-MHz-range, where Pockels cells cannot be applied due to piezoelectric ringing.

M. Gebhardt, C. Gaida, F. Stutzki, S. Hädrich, C. Jauregui, J. Limpert, and A. Tünnermann
Impact of atmospheric molecular absorption on the temporal and spatial evolution of ultra-short optical pulses
Optics Express 23, 13776 (2015)

Abstract: We present a rigorous study on the impact of atmospheric molecular absorption on the linear propagation of ultrashort pulses in the mid-infrared wavelength region. An ultrafast thulium-based fiber laser was employed to experimentally investigate ultrashort-pulse propagation through the atmosphere in a spectral region containing several strong molecular absorption lines. The atmospheric absorption profile causes a significant degradation of the pulse quality in the time domain as well as a distortion of the transverse beam profile in the spatial domain. Numerical simulations carried out in the small signal limit accurately reproduce the experimental observations in the time domain and reveal that the relative loss in peak power after propagation can be more than twice as high as the relative amount of absorbed average power. Although their nature is purely linear (i.e. the intensities considered are sufficiently low) the discussed effects represent significant challenges to performance-scaling of mid-infrared ultrafast lasers operating in spectral regions with molecular absorption bands. Guidelines for an efficient mitigation of the pulse quality degradation and the beam profile distortion are discussed.

H.-J. Otto, N. Modsching, C. Jauregui, J. Limpert, and A. Tünnermann
Impact of photodarkening on the mode instability threshold
Optics Express 23, 15265 (2015)

Abstract: The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber laser systems with diffraction limited beam quality. In this contribution, the impact of a wavelength shift of the seed signal on the mode instability threshold has been investigated. Against expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal. This finding implies that the quantum defect is not the only source of thermal heating in the fiber. Systematic experiments and simulations have helped in identifying photodarkening as the most likely second heat source in the fiber. It is shown that even a negligible photodarkening-induced power loss can lead to a decrease of the mode instabilities threshold by a factor of two. Consequently, reduction of photodarkening is a promising way to mitigate mode instabilities.

M. Gebhardt, C. Gaida, S. Hädrich, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Nonlinear compression of an ultrashort-pulse thulium-based fiber laser to sub-70  fs in Kagome photonic crystal fiber
Optics Letters 40, 2770 (2015)

Abstract: Nonlinear pulse compression of ultrashort pulses is an established method for reducing the pulse duration and increasing the pulse peak power beyond the intrinsic limits of a given laser architecture. In this proof-of-principle experiment, we demonstrate nonlinear compression of the pulses emitted by a high-repetition-rate thulium-based fiber CPA system. The initial pulse duration of about 400 fs has been shortened to <70  fs with 19.7 μJ of pulse energy, which corresponds to about 200 MW of pulse peak power.

C. Gaida, M. Kienel, M. Müller, A. Klenke, M. Gebhardt, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Coherent combination of two Tm-doped fiber amplifiers
Optics Letters 40, 2301 (2015)

Abstract: The efficient coherent combination of two ultrafast Tm-doped fiber amplifiers in the 2-µm wavelength region is demonstrated. The performance of the combined amplifiers is compared to the output characteristics of a single amplifier being limited by the onset of detrimental nonlinear effects. Nearly transform-limited pulses with 830- fs duration, 22-µJ pulse energy, and 25-MW peak power have been achieved with a combining efficiency greater than 90%. Based on this result, it can be expected that 2-µm-ultrafast-fiber-laser systems will enter new performance realms in the near future.

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.

C. Jauregui, H.-J. Otto, N. Modsching, J. Limpert, and A. Tünnermann
Recent progress in the understanding of mode instabilities
Proceedings of SPIE 9344, 93440J (2015)

Abstract: In this work the latest progress in the understanding of mode instabilities is reviewed. Particular emphasis is put on the recently established influence of photodarkening on the mode instability threshold and its behavior. It is shown, for example, that even degradations of the output power in the order of a few percent can lead to very significant reductions of the mode instability threshold. Moreover, our analysis shows that photodarkening also alters the expected behavior of the mode instability threshold with respect to the signal wavelength and the seed power. Thus photodarkening is revealed as one of the main effects shaping the behavior of the mode instability threshold observed in experiments.

C. Jauregui, F. Stutzki, J. Limpert, and A. Tünnermann
Self-efficiency improvement and cooling in thulium-doped fibers
Proceedings of SPIE 9344, 93441Q (2015)

Abstract: A way to increase the efficiency of Thulium-doped fiber systems and simultaneously prevent the generation of heat by pumping the excited state around 1460 nm has been recently proposed by the authors. In this contribution we show that a Thulium-doped fiber amplifier can lase around 1460nm while simultaneously amplifying signals around 2 μm. Such an operation results in considerably higher amplification efficiencies and in lower operating temperatures without the need for an external pump around 1460 nm.

C. Gaida, F. Stutzki, M. Gebhardt, F. Jansen, A. Wienke, U. D. Zeitner, F. Fuchs, C. Jauregui, D. Wandt, D. Kracht, J. Limpert, and A. Tünnermann
Sub-700 fs pulses at 152 W average power from a Tm-doped fiber CPA system
Proceedings of SPIE 9344, 93441K (2015)

Abstract: Thulium-based fiber lasers potentially provide for the demand of high average-power ultrafast laser systems operating at an emission wavelength around 2 μm. In this work we use a Tm-doped photonic-crystal fiber (PCF) with a mode field diameter of 36 μm enabling high peak powers without the onset of detrimental nonlinear effects. For the first time a Tm-doped PCF amplifier allows for a pump-power limited average output power of 241 W with a slope efficiency above 50%, good beam quality and linear polarization. A record compressed average power of 152 W and a pulse peak power of more than 4 MW at sub-700 fs pulse duration are enabled by dielectric gratings with diffraction efficiencies higher than 98% leading to a total compression efficiency of more than 70%. A further increase of pulse peak power towards the GW-level is planned by employing Tm-doped large-pitch fibers with mode field diameters well above 50 μm. The coherent combination of ultrafast pulses might eventually lead to kW-level average power and multi-GW peak power.

H.-J. Otto, N. Modsching, C. Jauregui, J. Limpert, and A. Tünnermann
Wavelength dependence of maximal diffraction-limited output power of fiber lasers
Proceedings of SPIE 9344, 93441Y (2015)

Abstract: The threshold-like onset of mode instabilities is currently the main limitation for the scaling of the average output power of fiber-laser systems with diffraction limited beam quality. In this contribution wavelength shifting of the seed signal has been experimentally investigated in order to mitigate mode instabilities. Against the expectations, it is experimentally shown that the highest mode instabilities threshold is reached around 1030 nm and not for the smallest wavelength separation between pump and signal wavelength. This finding implies that the quantum defect is not the sole significant source for thermal heating in the fiber.

M. Kienel, M. Müller, A. Klenke, T. Eidam, J. Limpert, and A. Tünnermann
Multidimensional coherent pulse addition of ultrashort laser pulses
Optics Letters 40, 522 (2015)

Abstract: Spatially and temporally separated amplification and subsequent coherent addition of femtosecond pulses is a promising performance-scaling approach for ultrafast laser systems. Herein we demonstrate for the first time the application of this multidimensional scheme in a scalable architecture. Applying actively controlled divided-pulse amplification producing up to four pulse replicas that are amplified in two ytterbium-doped step-index fibers (6 μm core), pulse energies far beyond the damage threshold of the single fiber have been achieved. In this proof-of-principle experiment, high system efficiencies are demonstrated at both high pulse energies (i.e., in case of strong saturation) and high accumulated nonlinear phases.

C. João, F. Wagner, J. Körner, J. Hein, T. Gottschall, J. Limpert, and V. Bagnoud
A 10-mJ-level compact CPA system based on Yb:KGW for ultrafast optical parametric amplifier pumping
Applied Physics B 118, 401 (2015)

Abstract: A compact diode-pumped Yb:KGW regenerative amplifier producing 10 Hz, 10-mJ-level picosecond pulses at 1,040 nm is demonstrated. The system is used at the new front end of the PHELIX petawatt laser system to pump an ultrafast optical parametric amplifier for temporal contrast enhancement. Before frequency doubling, a pulse length of ∼1 ps is obtained by using a stretcher/compressor system based on a single large-aperture chirped volume Bragg grating.

W. Liu, D. N. Schimpf, T. Eidam, J. Limpert, A. Tünnermann, F. X. Kärtner, and G. Chang
Pre-chirp managed nonlinear amplification in fibers delivering 100 W, 60 fs pulses
Optics Letters 40, 151 (2015)

Abstract: We demonstrate a pre-chirp managed Yb-doped fiber laser system that outputs 75 MHz, 130 W spectrally broadened pulses, which are compressed by a diffraction-grating pair to 60 fs with average powers as high as 100 W. Fine tuning the pulse chirp prior to amplification leads to high-quality compressed pulses. Detailed experiments and numerical simulation reveal that the optimum pre-chirp group-delay dispersion increases from negative to positive with increasing output power for rod-type high-power fiber amplifiers. The resulting laser parameters are suitable for extreme nonlinear optics applications such as frequency conversion in femtosecond enhancement cavities.

F. Stutzki, C. Gaida, M. Gebhardt, F. Jansen, C. Jauregui, J. Limpert, and A. Tünnermann
Tm-based fiber-laser system with more than 200  MW peak power
Optics Letters 40, 9 (2015)

Abstract: Tm-based fiber-laser systems are an attractive concept for the development of high-performance laser sources in the spectral region around 2 μm wavelength. Here we present a system delivering a pulse-peak power higher than 200 MW in combination with 24 W average power and 120 μJ pulse energy. Key components enabling this performance level are a Tm-doped large-pitch fiber with a mode-field diameter of 65 μm, highly efficient dielectric gratings, and a Tm-based fiber oscillator operating in the stretched-pulse regime.

## 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.

H.-J. Otto, F. Stutzki, N. Modsching, C. Jauregui, J. Limpert, and A. Tünnermann
2  kW average power from a pulsed Yb-doped rod-type fiber amplifier
Optics Letters 39, 6446 (2014)

Abstract: This Letter reports on a fiber-laser system that, employing a 1 m long rod-type photonic-crystal fiber as its main-amplifier, emits a record average output power of 2 kW, by amplifying stretched ps-pulses. A further increase of the output power was only limited by the available laser-diode pump power. The energy of the pulses is 100 μJ, corresponding to MW-level peak powers extracted directly from the fiber of the main amplifier. The corresponding M2 at the maximum output power is <3, due to the onset of mode instabilities. The Letter covers the influence of this effect on the evolution of the beam quality with the output power. The numerical results show that the M2 value settles at around 3, even if the output average power is further increased.

S. Breitkopf, T. Eidam, A. Klenke, L. v. Grafenstein, H. Carstens, S. Holzberger, E. Fill, T. Schreiber, F. Krausz, A. Tünnermann, I. Pupeza, and J. Limpert
A concept for multiterawatt fibre lasers based on coherent pulse stacking in passive cavities
Light: Science & Applications 3, e211 (2014)

Abstract: Since the advent of femtosecond lasers, performance improvements have constantly impacted on existing applications and enabled novel applications. However, one performance feature bearing the potential of a quantum leap for high-field applications is still not available: the simultaneous emission of extremely high peak and average powers. Emerging applications such as laser particle acceleration require exactly this performance regime and, therefore, challenge laser technology at large. On the one hand, canonical bulk systems can provide pulse peak powers in the multi-terawatt to petawatt range, while on the other hand, advanced solid-state-laser concepts such as the thin disk, slab or fibre are well known for their high efficiency and their ability to emit high average powers in the kilowatt range with excellent beam quality. In this contribution, a compact laser system capable of simultaneously providing high peak and average powers with high wall-plug efficiency is proposed and analysed. The concept is based on the temporal coherent combination (pulse stacking) of a pulse train emitted from a high-repetition-rate femtosecond laser system in a passive enhancement cavity. Thus, the pulse energy is increased at the cost of the repetition rate while almost preserving the average power. The concept relies on a fast switching element for dumping the enhanced pulse out of the cavity. The switch constitutes the key challenge of our proposal. Addressing this challenge could, for the first time, allow the highly efficient dumping of joule-class pulses at megawatt average power levels and lead to unprecedented laser parameters.

F. Stutzki, F. Jansen, H.-J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann
Designing advanced very-large-mode-area fibers for power scaling of fiber-laser systems
Optica 1, 233 (2014)

Abstract: Fiber lasers are a highly regarded solid-state laser concept due to their high efficiency, beam quality, and easy thermal management. Unfortunately, the performance of high-power fiber-laser systems is challenged by the onset of detrimental nonlinear effects. Their impact can be reduced dramatically by employing fibers with larger mode-field areas. Even though this is an efficient way to mitigate nonlinear effects, maintaining effective single-mode operation, and with it high beam quality, becomes increasingly difficult as the core is enlarged. In this paper the demands and challenges for the design of a very-large-mode-area (VLMA) fiber are discussed. The benefits of using higher-order mode delocalization as the working principle of active double-clad VLMA fibers are described. Finally, a new low-symmetry large-pitch fiber, which is expected to improve the performance of state-of-the-art fiber-laser systems by increasing higher-order mode delocalization, is proposed and thoroughly analyzed.

J. Limpert, A. Klenke, M. Kienel, S. Breitkopf, T. Eidam, S. Hädrich, C. Jauregui, and A. Tünnermann
Performance Scaling of Ultrafast Laser Systems by Coherent Addition of Femtosecond Pulses
IEEE Journal of Selected Topics in Quantum Electronics 20, 0901810 (2014)

Abstract: In the last decades, ultrafast lasers and amplifiers have achieved an extraordinary power increase and have enabled a plethora of scientific, medical or industrial applications. However, especially in recent years, it has become more and more challenging to keep up with this pace since intrinsic physical limitations are becoming difficult to avoid. A promising way to get around this problem is the technique of spatially and/or temporally separated amplification and subsequent coherent addition of ultrashort pulses. It turns out that fiber amplifiers are perfect candidates for this concept due to their outstanding average-power capability and their simple single-pass setups, which can be easily parallelized. Herein we provide an overview of the most important experimental implementations of this concept and recent results. We discuss the ability of these approaches to generate laser parameters that, only a few years ago, seemed impossible to achieve.

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.

F. Wagner, C. João, J. Fils, T. Gottschall, J. Hein, J. Körner, J. Limpert, M. Roth, T. Stöhlker, and V. Bagnoud
Temporal contrast control at the PHELIX petawatt laser facility by means of tunable sub-picosecond optical parametric amplification
Applied Physics B 116, 429 (2014)

Abstract: We report on the development of a preamplifier module for temporal contrast enhancement and control at petawatt-class lasers. The module is based on an ultrafast optical parametric amplifier (uOPA), which produces temporally clean pulses at the 60 μJ level for seeding a chirped pulse amplification (CPA) system, namely the petawatt facility PHELIX. The amplifier module allows for gain reduction in the following amplifiers, resulting in an attenuation of amplified spontaneous emission (ASE) by more than 4 orders of magnitude. Since the ASE of a CPA system linearly depends on the seeding energy, we were able to demonstrate a continuous variation of the temporal contrast by tuning the gain of the uOPA.

F. Stutzki, C. Gaida, M. Gebhardt, F. Jansen, A. Wienke, U. Zeitner, F. Fuchs, C. Jauregui, D. Wandt, D. Kracht, J. Limpert, and A. Tünnermann
152  W average power Tm-doped fiber CPA system
Optics Letters 39, 4671 (2014)

Abstract: A high-power thulium (Tm)-doped fiber chirped-pulse amplification system emitting a record compressed average output power of 152 W and 4 MW peak power is demonstrated. This result is enabled by utilizing Tm-doped photonic crystal fibers with mode-field diameters of 35 μm, which mitigate detrimental nonlinearities, exhibit slope efficiencies of more than 50%, and allow for reaching a pump-power-limited average output power of 241 W. The high-compression efficiency has been achieved by using multilayer dielectric gratings with diffraction efficiencies higher than 98%.

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.

A. Klenke, S. Hädrich, M. Kienel, T. Eidam, J. Limpert, and A. Tünnermann
Coherent combination of spectrally broadened femtosecond pulses for nonlinear compression
Optics Letters 39, 3520 (2014)

Abstract: The coherent combination of ultrashort pulses has recently been established as a technique to overcome the limitations of laser amplifiers regarding pulse peak-power, pulse energy, and average power. Similar limitations also occur in nonlinear compression setups. In a proof-of-principle experiment, we show that the techniques developed for the combination of amplifiers can be adapted to nonlinear compression. We create two spatially separated pulse replica that undergo self-phase modulation in independent optical fibers and are recombined afterwards. Using this technique we demonstrate operation above the self-focusing threshold of a single pulse. Furthermore, we prove that the recombined pulses can be temporally compressed. This experiment paves the way for higher energy or average power operation of various nonlinear compression setups.

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.

H.-J. Otto, A. Klenke, C. Jauregui, F. Stutzki, J. Limpert, and A. Tünnermann
Scaling the mode instability threshold with multicore fibers
Optics Letters 39, 2680 (2014)

Abstract: Mode instabilities (MIs) have quickly become the most limiting effect for the average power scaling of nearly diffraction-limited beams from state-of-the-art fiber laser systems. In this work it is shown that, by using an advanced multicore photonic crystal fiber design, the threshold power of MIs can be increased linearly with the number of cores. An average output power of 536 W, corresponding to 4 times the threshold power of a single core, is demonstrated.

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.

M. Kienel, A. Klenke, T. Eidam, S. Hädrich, J. Limpert, and A. Tünnermann
Energy scaling of femtosecond amplifiers using actively controlled divided-pulse amplification
Optics Letters 39, 1049 (2014)

Abstract: Divided-pulse amplification is a promising method for the energy scaling of femtosecond laser amplifiers, where pulses are temporally split prior to amplification and coherently recombined afterwards. We present a method that uses an actively stabilized setup with separated stages for splitting and combining. The additional degrees of freedom can be employed to mitigate the limitations originating from saturation of the amplifier that cannot be compensated in passive double-pass configurations using just one common stage for pulse splitting and combining. In a first proof-of-principle experiment, actively controlled divided pulses are applied in a fiber chirped-pulse amplification system resulting in combined and compressed pulses with an energy of 1.25 mJ and a peak power of 2.9 GW.

C. Gaida, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, O. d. Vries, J. Limpert, and A. Tünnermann
Triple-clad large-pitch fibers for compact high-power pulsed fiber laser systems
Optics Letters 39, 209 (2014)

Abstract: We present a novel ytterbium (Yb)-doped large-pitch fiber design with significantly increased pump absorption and higher energy storage/gain per unit length, which enables high-peak-power fiber laser systems with smaller footprints. Up to now index matching between core and surrounding material in microstructured fibers was achieved by co-doping the active core region with fluorine. Here we carry out the index matching by passively doping the cladding with germanium, thus raising its index of refraction. Hence, the fluorine in the core can be omitted, which leads to an effective increase of the core doping concentration, while detrimental effects such as photo-darkening and lifetime quenching are avoided by maintaining the bulk Yb concentration. Experiments and simulations show that a gain higher than 50  dB/m and an output average power higher than 100 W with excellent beam quality are feasible even with a fiber length of only 40 cm.

## 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.

M. Kienel, A. Klenke, T. Eidam, M. Baumgartl, C. Jauregui, J. Limpert, and A. Tünnermann
Analysis of passively combined divided-pulse amplification as an energy-scaling concept
Optics Express 21, 29031 (2013)

Abstract: The energy scaling of ultrashort-pulse systems employing simultaneously the techniques of chirped-pulse amplification and passively combined divided-pulse amplification is analyzed both experimentally and numerically. The maximum achievable efficiency is investigated and fundamental limitations originating from gain saturation, self-phase modulation and depolarization are discussed. A solution to these limitations could be an active stabilization scheme, which would allow for the operation of every single fiber amplifier at higher pulse energies.

R. Riedel, M. Schulz, M. J. Prandolini, A. Hage, H. Höppner, T. Gottschall, J. Limpert, M. Drescher, and F. Tavella
Long-term stabilization of high power optical parametric chirped-pulse amplifiers
Optics Express 21, 28987 (2013)

Abstract: The long-term stability of optical parametric chirped-pulse amplifiers is hindered by thermal path length drifts affecting the temporal pump-to-signal overlap. A kilowatt-pumped burst amplifier is presented delivering broadband 1.4 mJ pulses with a spectral bandwidth supporting sub-7 fs pulse duration. Active temporal overlap control can be achieved by feedback of optical timing signals from cross-correlation or spectral measurements. Using a balanced optical cross-correlator, we achieve a pump-to-signal synchronization with a residual jitter of only (46 ± 2) fs rms. Additionally, we propose passive pump-to-signal stabilization with an intrinsic jitter of (7.0 ± 0.5) fs rms using white-light continuum generation.

A. Klenke, M. Kienel, T. Eidam, S. Hädrich, J. Limpert, and A. Tünnermann
Divided-pulse nonlinear compression
Optics Letters 38, 4593 (2013)

Abstract: We report on the nonlinear pulse compression of temporally divided pulses, which is presented in a proof-of-principle experiment. A single 320 fs pulse is divided into four replicas, spectrally broadened in a solid-core fiber, and subsequently recombined. This approach makes it possible to reduce the nonlinearities in the fiber and therefore to use total input peak power of about 13.3 MW, which is more than three times higher than the self-focusing threshold. Finally, the combined output pulse could be compressed to sub-100 fs pulse duration. This general and universal approach holds promise for overcoming fundamental limitations of the pulse peak power that lead to destruction of the fiber or ionization limitations in high-energy hollow-core compression.

C. Jauregui, J. Limpert, and A. Tünnermann
High-power fibre lasers
Nature Photonics 7, 861 (2013)

Abstract: Fibre lasers are now associated with high average powers and very high beam qualities. Both these characteristics are required by many industrial, defence and scientific applications, which explains why fibre lasers have become one of the most popular laser technologies. However, this success, which is largely founded on the outstanding characteristics of fibres as an active medium, has only been achieved through researchers around the world striving to overcome many of the limitations imposed by the fibre architecture. This Review focuses on these limitations, both past and current, and the creative solutions that have been proposed for overcoming them. These solutions have enabled fibre lasers to generate the highest diffraction-limited average power achieved to date by solid-state lasers.

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.

C. Jauregui, H.-J. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann
Passive mitigation strategies for mode instabilities in high-power fiber laser systems
Optics Express 21, 19375 (2013)

Abstract: Mode instabilities have quickly become the most limiting effect when it comes to scaling the output average power of fiber laser systems. In consequence, there is an urgent need for effective strategies to mitigate it and, thus, to increase the power threshold at which it appears. Passive mitigation strategies can be classified into intrinsic, which are related to the fiber design, and extrinsic, which require a modification of the setup. In order to evaluate the impact of mitigation strategies, a means to calculate its power threshold and predict its behavior is required. In this paper we present a simple semi-analytic formula that is able to predict the changes of the mode instability threshold by analyzing the strength of the thermally-induced waveguide perturbations. Furthermore, we propose two passive mitigation strategies, one intrinsic and one extrinsic, that should lead to a significant increase of the power threshold of mode instabilities.

H.-J. Otto, C. Jauregui, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann
Controlling mode instabilities by dynamic mode excitation with an acousto-optic deflector
Optics Express 21, 17285 (2013)

Abstract: We demonstrate an approach to actively stabilize the beam profile of a fiber amplifier above the mode instability threshold. Both the beam quality and the pointing stability are significantly increased at power levels of up to three times the mode instabilities threshold. The physical working principle is discussed at the light of the recently published theoretical explanations of mode instabilities.

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.

H. Otto, F. Jansen, F. Stutzki, C. Jauregui, J. Limpert, and A. Tunnermann
Improved Modal Reconstruction for Spatially and Spectrally Resolved Imaging
Journal of Lightwave Technology 31, 1295 (2013)

Abstract: Spatially and spectrally resolved imaging (S2) is a very sensitive, robust and elegant method to measure the power of excited modes in a fiber. For the common reconstruction technique an approximation is necessary, which is based on dominant fundamental mode content. In this work we present several algorithms that significantly improve the accuracy of modal reconstruction for weak excited fundamental mode content. We show that in some cases general analytical solutions exist that can completely overcome the former limitation. In addition, we introduce an iterative procedure to improve the mode modal reconstruction independent of the specific used algorithm.

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.

F. Jansen, F. Stutzki, H.-J. Otto, C. Jauregui, J. Limpert, and A. Tünnermann
High-power thermally guiding index-antiguiding-core fibers
Optics Letters 38, 510 (2013)

Abstract: We investigate high-power operation of a very-large-mode-area (VLMA) fiber concept based on an index-antiguiding, thermally guiding core in which an ytterbium-doped region is completely surrounded by silica with a slightly higher refractive index. Experimentally, regimes of antiguidance, single-mode operation, and mode instabilities predominantly with radially symmetric higher-order modes are observed. Fundamental limitations for conventional VLMA step-index fibers are discussed.

S. Fuchs, C. Rödel, M. Krebs, S. Hädrich, J. Bierbach, A. E. Paz, S. Kuschel, M. Wünsche, V. Hilbert, U. Zastrau, E. Förster, J. Limpert, and G. G. Paulus
Sensitivity calibration of an imaging extreme ultraviolet spectrometer-detector system for determining the efficiency of broadband extreme ultraviolet sources
Review of Scientific Instruments 84, 023101 (2013)

Abstract: We report on the absolute sensitivity calibration of an extreme ultraviolet (XUV) spectrometer system that is frequently employed to study emission from short-pulse laser experiments. The XUV spectrometer, consisting of a toroidal mirror and a transmission grating, was characterized at a synchrotron source in respect of the ratio of the detected to the incident photon flux at photon energies ranging from 15.5 eV to 99 eV. The absolute calibration allows the determination of the XUV photon number emitted by laser-based XUV sources, e.g., high-harmonic generation from plasma surfaces or in gaseous media. We have demonstrated high-harmonic generation in gases and plasma surfaces providing 2.3 μW and μJ per harmonic using the respective generation mechanisms.

F. Stutzki, F. Jansen, C. Jauregui, J. Limpert, and A. Tünnermann
2.4 mJ, 33 W Q-switched Tm-doped fiber laser with near diffraction-limited beam quality
Optics Letters 38, 97 (2013)

Abstract: We report on a high pulse energy and high average power Q-switched Tm-doped fiber oscillator. The oscillator produces 2.4 mJ pulses with 33 W average power (at a repetition rate of 13.9 kHz) and nearly diffraction-limited beam quality. This record performance is enabled by a Tm-doped large-pitch fiber, which allows for large core diameters in combination with effective single-mode operation.

## 2012

S. Breitkopf, A. Klenke, T. Gottschall, H. Otto, C. Jauregui, J. Limpert, and A. Tünnermann
58 mJ burst comprising ultrashort pulses with homogenous energy level from an Yb-doped fiber amplifier
Optics Letters 37, 5169 (2012)

Abstract: We report on a laser system producing a burst comprising femtosecond pulses with a total energy of 58 mJ. Every single pulse within this burst has an energy between 27 and 31 μJ. The pump is able to rebuild the inversion fast enough between the pulses, resulting in an almost constant gain for every pulse during the burst. This causes a very homogenous energy distribution during the burst. The output burst has a repetition frequency of 20 Hz, is 200 μs long and, therefore, contains 2000 pulses at a pulse repetition rate of 10 MHz.

F. Jansen, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
High-power very large mode-area thulium-doped fiber laser
Optics Letters 37, 4546 (2012)

Abstract: Large-pitch photonic-crystal fibers have demonstrated their unique capability of combining very large mode areas, high output powers and robust single-mode operation at a wavelength of 1 μm. In this Letter, we present the experimental realization of thulium-doped very large mode-area fibers based on the large-pitch fibers with record mode-field diameters exceeding 60 μm and delivering more than 52 W of output power.

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.

C. Jocher, T. Eidam, S. Hädrich, J. Limpert, and A. Tünnermann
Sub 25 fs pulses from solid-core nonlinear compression stage at 250 W of average power
Optics Letters 37, 4407 (2012)

Abstract: We report on a highpower femtosecond fiber chirped-pulse amplification system with an excellent beam quality (M^2 = 1.2) operating at 250 MHz repetition rate. We demonstrate nonlinear compression in a solid-core photonic crystal fiber at unprecedented average power levels. By exploiting self-phase modulation with subsequent chirped-mirror compression we achieve pulse shortening by more than one order of magnitude to 23 fs pulses. The use of circular polarization allows higher than usual peak powers in the broadening fiber resulting in compressed 0.9 μJ pulse energy and a peak power of 34 MW at 250 W of average power (M^2 = 1.3). This system is well suited for driving cavity-enhanced high-repetition rate high-harmonic generation.

C. Jauregui, A. Steinmetz, J. Limpert, and A. Tünnermann
High-power efficient generation of visible and mid-infrared radiation exploiting four-wave-mixing in optical fibers
Optics Express 20, 24957 (2012)

Abstract: We report on the generation of 17.6 W of visible radiation at 650 nm using four-wave-mixing in an endlessly single-mode silica fiber. The conversion efficiency was as high as ~ 30%. This high efficiency could be obtained by exploiting the natural absorption of silica for the mid-infrared radiation > 2.5µm. In a separate experiment 1.6 W of mid-IR radiation at 2570 nm were generated simultaneously with 14.4 W at 672 nm. These power levels of picosecond red radiation are among the highest reported so far for a diffraction limited beam quality in this wavelength region.

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.

H.-J. Otto, F. Stutzki, F. Jansen, T. Eidam, C. Jauregui, J. Limpert, and A. Tünnermann
Temporal dynamics of mode instabilities in high-power fiber lasers and amplifiers
Optics Express 20, 15710 (2012)

Abstract: The temporal behavior of mode instabilities in active large mode area fibers is experimentally investigated in detail. Thus, apart from the onset threshold of mode instabilities, the output beam is characterized using both high-speed camera measurements with 20,000 frames per second and photodiode traces. Based on these measurements, an empiric definition of the power threshold of mode instabilities is introduced. Additionally, it is shown that the temporal dynamics show a transition zone between the stable and the unstable regimes where well-defined periodic temporal fluctuations on ms-timescale can be observed. Finally, it is experimentally shown that the larger the mode-field area, the slower the mode-instability fluctuation is. The observations support the thermal origin of mode instabilities.

C. Jauregui, T. Eidam, H.-J. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann
Physical origin of mode instabilities in high-power fiber laser systems
Optics Express 20, 12912 (2012)

Abstract: Mode instabilities, i.e. the rapid fluctuations of the output beam of an optical fiber that occur after a certain output power threshold is reached, have quickly become one of the most limiting effects for the further power scaling of fiber laser systems. Even though much work has been done over the last year, the exact origin of the temporal dynamics of this phenomenon is not fully understood yet. In this paper we show that the origin of mode instabilities can be explained by taking into account the interplay between the temporal evolution of the three-dimensional temperature profile inside of the active fiber and the related waveguide changes that it produces via the thermo-optical effect. In particular it is proposed that non-adiabatic waveguide changes play an important role in allowing energy transfer from the fundamental mode into the higher order mode. As it is discussed in the paper, this description of mode instabilities can explain many of the experimental observations reported to date.

M. Baumgartl, B. Ortac, J. Limpert, and A. Tünnermann
Impact of dispersion on pulse dynamics in chirped-pulse fiber lasers
Applied Physics B 107, 263 (2012)

Abstract: We report on a systematic study of an environmentally stable mode-locked Yb-doped fiber laser operating in the chirped-pulse regime. The linear cavity chirped-pulse fiber laser is constructed with a saturable absorber mirror as nonlinear mode-locking mechanism and a nonlinearity-free transmission-grating-based stretcher/compressor for dispersion management. Mode-locked operation and pulse dynamics from strong normal to strong anomalous total cavity dispersion in the range of +2.5 to -1.6 ps^2 is experimentally studied. Strongly positively chirped pulses from 4.3 ps (0.01 ps^2) to 39 ps (2.5 ps^2) are obtained at normal net-cavity dispersion. In the anomalous dispersion regime, the laser generates average soliton feature negatively chirped pulses with autocorrelation pulse durations from 0.8 ps (−0.07 ps^2) to 3.9 ps (-1.6 ps^2). The lowered peak power due to the pulse stretching allows one to increase the double pulse threshold. Based on the numerical simulation, different regimes of mode locking are obtained by varying the intra-cavity dispersion, and the characteristics of average soliton, stretched-pulse, wave-breaking-free and chirped-pulse regimes are discussed.

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.

M. Baumgartl, C. Lecaplain, A. Hideur, J. Limpert, and A. Tünnermann
66 W average power from a microjoule-class sub-100 fs fiber oscillator
Optics Letters 37, 1640 (2012)

Abstract: Performance scaling of passively mode-locked ultrashort-pulse fiber oscillators in terms of average power, peak power, and pulse energy is demonstrated. A very-large-mode-area fiber laser in an all-positive group-velocity-dispersion ring cavity configuration with intracavity spectral filter, mode-locked by nonlinear polarization evolution, emits 66 W of average power at 76 MHz repetition rate, corresponding to 0.9 μJ pulse energy. The pulses are dechirped to 91 fs outside the cavity with an average power of 60 W remaining after the compressor. The generated pulse peak power is as high as 7 MW.

J. Limpert, F. Stutzki, F. Jansen, H.-J. Otto, T. Eidam, C. Jauregui, and A. Tünnermann
Yb-doped large-pitch fibres: effective single-mode operation based on higher-order mode delocalisation
Light: Science & Applications 1, e8 (2012)

Abstract: Rare earth-doped fibres are a diode-pumped, solid-state laser architecture that is highly scalable in average power. The performance of pulsed fibre laser systems is restricted due to nonlinear effects. Hence, fibre designs that allow for very large mode areas at high average powers with diffraction-limited beam quality are of enormous interest. Ytterbium-doped, rod-type, large-pitch fibres (LPF) enable extreme fibre dimensions, i.e., effective single-mode fibres with mode sizes exceeding 100 times the wavelength of the guided radiation, by exploiting the novel concept of delocalisation of higher-order transverse modes. The non-resonant nature of the operating principle makes LPF suitable for high power extraction. This design allows for an unparalleled level of performance in pulsed fibre lasers.

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.

F. Stutzki, F. Jansen, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann
26 mJ, 130 W Q-switched fiber-laser system with near-diffraction-limited beam quality
Optics Letters 37, 1073 (2012)

Abstract: We demonstrate a Q-switched fiber laser system emitting sub-60 ns pulses with 26 mJ pulse energy and near-diffraction-limited beam quality (M^2 < 1.3). In combination with a repetition rate of 5 kHz, a corresponding average output power of 130 W is achieved. This record performance is enabled by a large-pitch fiber with a core diameter of 135 µm. This fiber allows for effective single-mode operation with mode field diameters larger than 90 µm even at average output powers exceeding 100 W.

M. Baumgartl, M. Chemnitz, C. Jauregui, T. Meyer, B. Dietzek, J. Popp, J. Limpert, and A. Tünnermann
All-fiber laser source for CARS microscopy based on fiber optical parametric frequency conversion
Optics Express 20, 4484 (2012)

Abstract: A novel approach for an all-fiber mono-laser source for CARS microscopy is presented. An Yb-fiber laser generates 100 ps pulses, which later undergo narrowband in-fiber frequency conversion based on degenerate four-wave-mixing. The frequency conversion is optimized to access frequency shifts between 900 and 3200 cm^(−1), relevant for vibrational imaging. Inherently synchronized pump and Stokes pulses are available at one fiber end, readily overlapped in space and time. The source is applied to CARS spectroscopy and microscopy experiments in the CH-stretching region around 3000 cm^(−1). Due to its simplicity and maintenance-free operation, the laser scheme holds great potential for bio-medical applications outside laser laboratories.

F. Jansen, F. Stutzki, H.-J. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann
Thermally induced waveguide changes in active fibers
Optics Express 20, 3997 (2012)

Abstract: Thermally induced waveguide changes become significant for very large mode area fibers. This results in a reduction of the mode-field diameter, but simultaneously in an improvement of the beam quality. In this work the first systematic experimental characterization of the reduction of the mode-field diameter in various fibers during high-power operation is carried out. It is shown that the reduction of the mode-field diameter shows a characteristic behavior that scales with the core size but that is independent of the particular fiber design. Furthermore, the strength of the actual index change is experimentally estimated, and its use to overcome avoided crossings is discussed and experimentally demonstrated.

C. Jauregui, T. Eidam, H.-J. Otto, F. Stutzki, F. Jansen, J. Limpert, and A. Tünnermann
Temperature-induced index gratings and their impact on mode instabilities in high-power fiber laser systems
Optics Express 20, 440 (2012)

Abstract: Mode-interference along an active fiber in high-power operation gives rise to a longitudinally oscillating temperature profile which, in turn, is converted into a strong index grating via the thermo-optic effect. In the case of mode beating between the fundamental mode and a radially anti-symmetric mode such a grating exhibits two periodic features: a main one which is radially symmetric and has half the period of the modal beating, and a second one that closely follows the mode interference pattern and has its same period. In the case of modal beating between two radially symmetric modes the thermally induced grating only has radially symmetric features and exhibits the same period of the mode interference. The relevance of such gratings in the context of the recently observed mode instabilities of high-power fiber laser systems is discussed.

## 2011

A. Klenke, E. Seise, J. Limpert, and A. Tünnermann
Basic considerations on coherent combining of ultrashort laser pulses
Optics Express 19, 25379 (2011)

Abstract: Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining. The results show that, in spite of the temporal and spectral effects mentioned above, for a carefully adjusted and stabilized system an excellent efficiency of the combining process can still be achieved.

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.

F. Stutzki, H. Otto, F. Jansen, C. Gaida, C. Jauregui, J. Limpert, and A. Tünnermann
High-speed modal decomposition of mode instabilities in high-power fiber lasers
Optics Letters 36, 4572 (2011)

Abstract: A high-speed mode analysis technique is required to gain fundamental understanding of mode instabilities in high-power fiber laser systems. In this work a technique, purely based on the intensity profile of the beam, is demonstrated to be ideally suited to analyze fiber laser dynamics. This technique, together with a high-speed camera, has been applied to the study of the temporal dynamics of mode instabilities at high average powers with up to 20,000 frames per second. These measurements confirm that energy transfer between the fluctuating transversal modes takes place in millisecond-time-scale.

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.

E. Seise, A. Klenke, S. Breitkopf, J. Limpert, and A. Tünnermann
88 W 0.5 mJ femtosecond laser pulses from two coherently combined fiber amplifiers
Optics Letters 36, 3858 (2011)

Abstract: The generation of 0.5 mJ femtosecond laser pulses by coherent combining of two high power high energy fiber chirped-pulse amplifiers is reported. The system is running at a repetition frequency of 175 kHz producing 88 W of average power after the compressor unit. Polarizing beam splitters have been used to realize an amplifying Mach–Zehnder interferometer, which has been stabilized with a Hänsch–Couillaud measurement system. The stabilized system possesses a measured residual rms phase difference fluctuation between the two branches as low as λ/70 rad at the maximum power level. The experiment proves that coherent addition of femtosecond fiber lasers can be efficiently and reliably performed at high B-integral and considerable thermal load in the individual amplifiers.

M. Baumgartl, B. Ortac, T. Schreiber, J. Limpert, and A. Tünnermann
Ultrashort pulse formation and evolution in mode-locked fiber lasers
Applied Physics B 104, 523 (2011)

Abstract: Passive mode-locking in fiber lasers is investigated by numerical and experimental means. A non-distributed scalar model solving the nonlinear Schrödinger equation is implemented to study the starting behavior and intra-cavity dynamics numerically. Several operation regimes at positive net-cavity dispersion are experimentally accessed and studied in different environmentally stable, linear laser configurations. In particular, pulse formation and evolution in the chirped-pulse regime at highly positive cavity dispersion is discussed. Based on the experimental results a route to highly energetic pulse solutions is shown in numerical simulations.

C. Jocher, C. Jauregui, C. Voigtländer, F. Stutzki, S. Nolte, J. Limpert, and A. Tünnermann
Fiber based polarization filter for radially and azimuthally polarized light
Optics Express 19, 19582 (2011)

Abstract: We demonstrate a new fiber based concept to filter azimuthally or radially polarized light. This concept is based on the lifting of the modal degeneracy that takes place in high numerical aperture fibers. In such fibers, the radially and azimuthally polarized modes can be spectrally separated using a fiber Bragg grating. As a proof of principle, we filter azimuthally polarized light in a commercially available fiber in which a fiber Bragg grating has been written by a femtosecond pulsed laser.

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.

F. Jansen, F. Stutzki, C. Jauregui, J. Limpert, and A. Tünnermann
Avoided crossings in photonic crystal fibers
Optics Express 19, 13578 (2011)

Abstract: The impact of avoided crossings (also known as anti-crossings) in single and double-clad large mode area Photonic Crystal Fibers (PCFs) suitable for high-power laser systems is evaluated numerically. It is pointed out that an inappropriate choice of pump core diameter, bending radius and/or index depression may lead to avoided crossings that manifest themselves in unwanted deformations of the output beam.

T. Eidam, C. Wirth, C. Jauregui, F. Stutzki, F. Jansen, H. Otto, O. Schmidt, T. Schreiber, J. Limpert, and A. Tünnermann
Experimental observations of the threshold-like onset of mode instabilities in high power fiber amplifiers
Optics Express 19, 13218 (2011)

Abstract: We report on the observation and experimental characterization of a threshold-like onset of mode instabilities, i.e. an apparently random relative power content change of different transverse modes, occurring in originally single-mode high-power fiber amplifiers. Although the physical origin of this effect is not yet fully understood, we discuss possible explanations. Accordingly, several solutions are proposed in this paper to raise the threshold of this effect.

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.

M. Schulz, R. Riedel, A. Willner, T. Mans, C. Schnitzler, P. Russbueldt, J. Dolkemeyer, E. Seise, T. Gottschall, S. Hädrich, S. Duesterer, H. Schlarb, J. Feldhaus, J. Limpert, B. Faatz, A. Tünnermann, J. Rossbach, M. Drescher, and F. Tavella
Yb:YAG Innoslab amplifier: efficient high repetition rate subpicosecond pumping system for optical parametric chirped pulse amplification
Optics Letters 36, 2456 (2011)

Abstract: We report on a Yb:YAG Innoslab laser amplifier system for generation of subpicsecond high energy pump pulses for optical parametric chirped pulse amplification (OPCPA) at high repetition rates. Pulse energies of up to 20 mJ (at 12.5 kHz) and repetition rates of up to 100 kHz were attained with pulse durations of 830 fs and average power in excess of 200 W. We further investigate the possibility to use subpicosecond pulses to derive a stable continuum in a YAG crystal for OPCPA seeding.

F. Stutzki, F. Jansen, C. Jauregui, J. Limpert, and A. Tünnermann
Non-hexagonal Large-Pitch Fibers for enhanced mode discrimination
Optics Express 19, 12081 (2011)

Abstract: Photonic-Crystal Fibers (PCF) are among the most promising concepts to achieve large mode field areas suitable for the reduction of nonlinearities in fibers. Differential mode propagation loss is the cornerstone of effective single-mode behavior in passive and core-pumped active PCFs. In this work, we explore non-hexagonal PCF designs with increased mode discrimination in comparison to the classical hexagonal PCF designs. It is shown that a pentagonal design can increase the mode discrimination and, simultaneously, also improve the beam quality of Large-Pitch Fibers with mode field diameters well beyond 100 µm.

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.

F. Stutzki, F. Jansen, T. Eidam, A. Steinmetz, C. Jauregui, J. Limpert, and A. Tünnermann
High average power large-pitch fiber amplifier with robust single-mode operation
Optics Letters 36, 689 (2011)

Abstract: Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294 W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62 µm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3.

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.

E. Seise, A. Klenke, S. Breitkopf, M. Plötner, J. Limpert, and A. Tünnermann
Coherently combined fiber laser system delivering 120 mu J femtosecond pulses
Optics Letters 36, 439 (2011)

Abstract: We report on the coherent combination of two chirped pulsed fiber lasers. The beams coming from two 100 μm core diameter ytterbium-doped rod-type fibers were coupled in a Mach-Zehnder-type interferometer by means of a polarization beam splitter. Active stabilization of the interferometer was achieved by using a piezo-mounted mirror driven by a Hänsch-Couillaud polarization detection system. Pulses with 120 μJ energy and a compressed duration of 800 fs were obtained. This, compared with the 66 μJ coming from each single amplifier, results in a combining efficiency as high as 91%.

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.

M. Baumgartl, F. Jansen, F. Stutzki, C. Jauregui, B. Ortac, J. Limpert, and A. Tünnermann
High average and peak power femtosecond large-pitch photonic-crystal-fiber laser
Optics Letters 36, 244 (2011)

Abstract: We report on the generation of high-average-power and high-peak-power ultrashort pulses from a mode-locked fiber laser operating in the all-normal-dispersion regime. As gain medium, a large-mode-area ytterbium-doped large-pitch photonic-crystal fiber is used. The self-starting fiber laser delivers 27 W of average power at 50.57 MHz repetition rate, resulting in 534 nJ of pulse energy. The laser produces positively chirped 2 ps output pulses, which are compressed down to sub - 100 fs , leading to pulse peak powers as high as 3.2 MW.

## 2010

E. Seise, A. Klenke, J. Limpert, and A. Tünnermann
Coherent addition of fiber-amplified ultrashort laser pulses
Optics Express 18, 27827 (2010)

Abstract: We report on a novel approach of performance scaling of ultrafast lasers by means of coherent combination. Pulses from a single mode-locked laser are distributed to a number of spatially separated fiber amplifiers and coherently combined after amplification. Splitting and combination is achieved by polarization cubes, i.e. the approach bases on polarization combining. A Hänsch-Couillaud detector measures the polarization state at the output. The error signal (deviation from linear polarization) is used to stabilize the synchronization of different channels. In a proof-of-principle experiment the combination of two femtosecond fiber-based CPA systems is presented. A combining efficiency as high as 97% has been achieved. The technique offers a unique scaling potential and can be applied to all ultrafast amplification schemes independent of the architecture of the gain medium.

D. N. Schimpf, J. Limpert, and A. Tünnermann
Optimization of high performance ultrafast fiber laser systems to > 10 GW peak power
Journal of the Optical Society of America B 27, 2051 (2010)

Abstract: We show that the peak powers of ytterbium-doped fiber chirped pulse amplification (CPA) can be scaled by at least 1 order of magnitude (in the transform limit) as compared to current systems by using a different spectral region of operation. A simple and fast model for saturated broadband fiber CPA systems is developed and applied to study the impact of the interplay between the spectrally dependent small signal gain and the saturation on the output bandwidth. The influence of self-phase modulation on the recompression of the pulse is discussed. It can be shown that the novel operation regime exhibits superior performance even if nonlinear effects are considered. The numerical results are significant for the design of the next generation of ultrafast high power fiber lasers.

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.

M. Baumgartl, B. Ortac, C. Lecaplain, A. Hideur, J. Limpert, and A. Tünnermann
Sub-80 fs dissipative soliton large-mode-area fiber laser
Optics Letters 35, 2311 (2010)

Abstract: We report on high-energy ultrashort pulse generation from an all-normal-dispersion large-mode-area fiber laser by exploiting an efficient combination of nonlinear polarization evolution (NPE) and a semiconductor-based saturable absorber mode-locking mechanism. The watt-level laser directly emits chirped pulses with a duration of 1 ps and 163 nJ of pulse energy. These can be compressed to 77 fs, generating megawatt-level peak power. Intracavity dynamics are discussed by numerical simulation, and the intracavity pulse evolution reveals that NPE plays a key role in pulse shaping.

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.