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Publications by
Arno Klenke

All publications of HI Jena


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.

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.

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.

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%.


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.

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. 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

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.

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

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.


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

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

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

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

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

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

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.

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.

A. Klenke
Performance scaling of laser amplifiers via coherent combination of ultrashort pulses
Doctoral thesis
Friedrich-Schiller-Universität Jena, Physikalisch-Astronomische Fakultät (2016)

Abstract: Laser systems emitting ultrashort pulses have become an indispensable tool in science. However, the performance of a single amplifier is limited by a variety of physical effects. Hence, the coherent combination of ultrashort pulses has been investigated as a way to provide a new power-scaling opportunity. This concept can provide a simultaneous increase of the average power, pulse energy and peak power while preserving the beam quality and temporal pulse profile of a single-amplifier system. Theoretical considerations were carried out to investigate the impact of differences between the pulses on the combination process. It could be shown that their impact is small enough to realize laser systems based on coherent combination experimentally with a good combination efficiency. Additionally, the total combination efficiency converges to a fixed values for an increasing number of channels. The coherent combination concept was demonstrated experimentally with a fiber-CPA system comprising four parallel state-of-the-art amplifiers. In these experiments, the highest peak power emitted from a fiber laser system so far (22GW) could be achieved. Finally, for future systems with a large channel count, the compact integration of these channels will play a major role in reducing the footprint and component count and, therefore, the cost. Experimentally, this was demonstrated by employing a multicore fiber together with a compact beam-splitter design.


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.

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.

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.

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.


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.

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.

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.

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.

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.

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.


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.

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.

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.

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.


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.


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.

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.

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.

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%.


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.