Search

Transverse mode instability (TMI) represents the main limitation for the power scaling of fiber laser systems with a diffraction-limited beam quality. In this context, it has become increasingly important to find a cheap and reliable way to monitor and characterize TMI and distinguish this effect from other dynamic perturbations. In this work, with the help of a position-sensitive detector, a novel method is developed to characterize the TMI dynamics even in the presence of power fluctuations. The position information of the fluctuating beam is recorded in the X- and Y-axis of the detector, which are used to track the temporal evolution of the center of gravity of the beam. The trajectories described by the beam within a specific time window contain rich information about TMI, which can be used to gain further insight into this phenomenon.

Frequency doubling of a Q-switched Yb-doped rod-type 4 × 4 multicore fiber (MCF) laser system is reported. A second harmonic generation (SHG) efficiency of up to 52% was achieved with type I non-critically phase-matched lithium triborate (LBO), with a total SHG pulse energy of up to 17 mJ obtained at 1 kHz repetition rate. The dense parallel arrangement of amplifying cores into a shared pump cladding enables a significant increase in the energy capacity of active fibers. The frequency-doubled MCF architecture is compatible with high-repetition-rate and high-average-power operation and may provide an efficient alternative to bulk solid-state systems as pump sources for high-energy titanium-doped sapphire lasers.

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

We theoretically and experimentally investigate the optical cross-talk between cores of a multicore fiber. We show that the cross-talk not only depends on the numerical aperture and relative distance between the cores but also, crucially, on the size of the cores.

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.

An investigation on the temperature dependence of spectroscopic parameters of trivalent thulium ions is important for the design of high-power, thulium-doped fiber lasers and amplifiers. In this Letter, the thulium absorption/emission cross sections are determined in the spectral range 700–2200 nm and in the temperature range from −196°C to 300°C. The spectra are obtained from the absorption and emission measurements of a thulium-doped fiber and from measured thulium concentration profiles. Attempts were made to estimate the temperature dependence of the spectra where the measurements are not accessible. Firstly, the spectra are fitted to a multi-Gaussian model with temperature dependent parameters. Secondly, a physically motivated model of the cross section spectra is proposed and analyzed.

We present the generation of broadband terahertz radiation. Driven by a multipass cell compressed high-power fiber laser system at a repetition rate of 200 kHz an average power of 94 mW is generated with the two color gas-plasma scheme. The full bandwidth of the pulses are characterized by a measurement based on the air-biased coherent detection. By using the full potential of the driving laser further power scaling of the generated THz power towards watt-level average power can be expected in the near future.

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.

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] .

Transverse mode instability (TMI) induces detrimental mode coupling in fiber-laser systems at high average powers and still represents the main limitation for the further power scaling of diffraction-limited systems. In this contribution, we describe a new approach to mitigate TMI in fiber amplifiers by dynamically modifying the inversion profile in the fiber. When periodically changing the excitation of the active fiber (e.g. with the help of an acousto-optic deflector), the intensity distribution along the fiber is also dynamically modified. If this is done with a frequency of a few hundreds of kHz (i.e. so that the inversion cannot completely adapt to the new intensity pattern), the inversion grating will be washed out and a homogeneous inversion profile can develop. Consequently, the resulting heat distribution will also be homogenized and the formation of a thermally-induced refractive index grating, which is responsible for the TMI-induced mode coupling, can be largely suppressed. Hence, the presented mitigation approach tackles TMI at an early stage by acting upon the root cause of the detrimental modal energy transfer. At the conference, simulations will be presented which will illustrate the working principle of the new mitigation approach and show its potential to increase the TMI threshold of high-power fiber amplifiers by washing out the inversion profile.

A new approach to characterize the effect of transverse mode instability (TMI) in high-power fiber-laser systems is presented. A position-sensitive detector is employed to detect the trajectories of the center of gravity of the fluctuating beam (in a short time-window) at different average power levels. With an increasing average output power, the area covered by the trajectories increases which is used as a measure for the stability of the system. The new concept allows for a simple, fast, and detailed characterization of TMI, which accurately determines its threshold even in complex operating regimes. This new technique can easily distinguish between spatial and power fluctuations of the beam. Furthermore, the trajectories contain information about the movement of the center of gravity of the beam, which can be used to gain additional insight about the dynamics of TMI. The technique robustness was tested by characterizing the TMI behavior in a complex operating regime. e.g., using a pump-power modulation.

A new passive mitigation strategy for the effect of transverse mode instability is presented in this work. This technique requires the use of a polarization-maintaining fiber in which light is coupled with a polarization state oriented around 45° with respect to the main polarization axes of the fiber. Since the modal beat length in each of the main polarization axes of the fiber is slightly different, the aforementioned coupling condition leads to the modal interference pattern being periodically washed out. Such a situation at the end leads to a weakening of the thermally-induced refractive index grating and to an increase of the threshold for the effect of transverse mode instability that can amount to ~50%.

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.

In this work we optimize the design of coherently-combined multicore fiber amplifiers. It has been shown that increasing the number of cores in such fibers helps to increase the combinable output power. However, in counter-pumped multicore fibers, thermal effects will finally lead to strong non-uniform mode-shrinking in each core. This, in turn, will result in a significant reduction of the combining efficiency. In this study we will examine the power and energy scaling potential for different pumping schemes and different fiber designs. To this purpose, a simulation tool is used that solves the laser rate equations taking into account the resulting temperature gradient and the transverse mode distortions caused by it. In the simulation co- and counter pumped multicore fibers with a square core arrangement and a core number ranging from 2x2 up to 10x10 will be considered. Moreover, we investigate the influence of the active core size in terms of thermal effects as well as the extractable output power and energy. Particular attention is paid to the mitigation of non-uniform mode-shrinking at the fiber end-facet. By comparing the co- and counter-pumped cases, we will show that a combinable output power of 26 kW (co-pump) instead of 14 kW (counter-pump) with a 10x10 MCF and 30 μm cores should be achievable.

Intense, ultrafast laser sources with an operation wavelength beyond the well-established near-IR are valuable tools for exploiting the wavelength scaling laws of strong-field, light-matter interactions. Such laser systems enable the scaling of the phase matching photon energy cut-off in high-order harmonic generation, which allows for the generation of coherent soft X-ray radiation up to, and even beyond, the water window. Such laser-driven sources enable a plethora of subsequent applications. A number of these applications can significantly benefit from an increase in repetition rate. In that regard, 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 for driving high-order harmonic generation. These lasers are capable of delivering ~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 HHG in an antiresonant hollow-core fiber is a promising approach to realize high photon energy cut-off HHG from a compact setup. The realization is based on combining nonlinear pulse self-compression (leading to strong-field waveforms) and phase-matched high-order harmonic generation in a single antiresonant hollow-core fiber. In this demonstration, a photon energy cut-off of approximately 330 eV has been achieved, together with a photon flux >106 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 subjects of our future work.

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.

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 2x2 up to 10x10 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., Comment: This is a post-peer-review, pre-copyedit version of an article published in Optics Express. The final authenticated version is available online at: https://doi.org/10.1364/OE.410614

Nanosecond-class pulse energies up to 32 mJ are obtained from a tapered rod-type multicore fiber. The tapered fiber improves output beam quality in large multimode cores, with targeted application in ultrafast coherently combined systems.

The effect of transverse mode instability is a limitation for the power scaling of fiber laser systems, that can originate due to heat caused by stimulated Raman scattering. In this contribution, we experimentally investigate the threshold of transverse mode instability caused by stimulated Raman scattering in a passive fiber. Both, the Stokes seed power and the fiber length of a core-pumped Raman fiber amplifier are varied to systematically study this effect. Mode resolved measurements reveal that the threshold occurs at approximately the same Stokes output power for all tested configurations, independent of the total Raman conversion efficiency. These results increase the understanding of this type of mode instability and show which parameters are important for a further power scaling of high-power Raman fiber amplifiers.

A simplification strategy for Segmented Mirror Splitters (SMS) used as beam combiners is presented. These devices are useful for compact beam division and combination of linear and 2-D arrays. However, the standard design requires unique thin-film coating sections for each input beam and 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 beamsplitters 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., Comment: 11 pages, 7 figures

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.

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.

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.

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.

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.

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.

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.

We report on soft x-ray HHG driven by a thulium-doped fiber laser. It is the first time that a photon energy cut-off ~400 eV has been demonstrated using this highly scalable laser technology.

With active multicore fibers (MCFs), many parallel amplifying waveguides can be densely assembled into a common glass cladding. A tapered fiber geometry applied to MCFs enhances the power and energy scalability of these systems by increasing the doped waveguide volume and reducing peak irradiance while maintaining low output mode order. Recent high-energy experiments have achieved 37 mJ ns-class pulse energies with Yb-doped tapered MCFs, with potential application to a new generation of compact MCF-based coherently-combined laser systems. In this submission, latest experimental results with tapered MCFs and flexible fabrication of taper profiles as a post-draw processing step will be discussed. Numerical analyses of MCF tapers will be presented, using beam propagation method (BPM) and mode-decomposition techniques to study mode coupling and inter-core crosstalk. These simulations are used to guide the tapering of existing fibers and aid the design of future “taper-ready” MCFs.

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., Mid-infrared photonics: bright broadband source A table-top-sized, coherent light source that offers a compact and bright alternative to a synchrotron in the 4−18 µm spectral range has been developed by a German-US research team. The team used a novel ultrashort (16 fs) pulse, high power Tm-doped fiber laser operating at 1.9 µm to induce a nonlinear frequency downconversion process called intrapulse difference frequency generation in a crystal of GaSe. The broad spectral coverage and high brightness render this mid-infrared source a unique tool for state-of-the art spectroscopy and microscopy. The team says that the compactness and simplicity of the presented approach brings exciting prospects for the future accessibility, in particular for emerging applications that are currently addressed only with mid-infrared beamlines in large-scale synchrotron facilities.

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., More power to fibre optic lasers Researchers have cracked a ‘did the chicken come before the egg’ dilemma in fibre optic lasers. The new understanding of ‘transverse mode instability’ could lead to even stronger lasers. Jens Limpert and colleagues at Germany’s Friedrich-Schiller Universität Jena provided experimental evidence that increasing the power of an optic fibre beyond a certain point degrades laser output because interference patterns between light waves within the fibre core become ‘out of sync’ with the core’s refractive index grating; called a phase shift. Researchers had not been sure if the instability of light waves within the fiber core was initially triggered by this phase shift or by the growing core’s refractive index grating with increasing power. A poor understanding of transverse mode instability has halted further improvements in optic fibre technologies, which produce high power lasers for industry, medicine and defence. The new information could lead to strategies that mitigate the phenomenon.

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.

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.

Optical fibers have demonstrated, against all early predictions, that their geometry is extremely well-suited for handling high optical powers. However, over the years, many refinements have been done to the basic structure of these waveguides to allow guiding and generating high-powers. As a result of this development the fibers employed in high-power laser systems differ significantly from those typically used in other fields such as telecommunications and/or sensors. Therefore, this chapter is devoted to describing and explaining the main intricacies of those optical fiber designs presently used for high-power operation. In this context, the main goal of this chapter is that the reader gets an overview of the different fiber designs and building blocks of high-power optical fibers. The chapter is divided into three main parts: a brief historical overview of the evolution of this technology, a description of the main constituents of fibers for high-power operation (including the core, the pump cladding and materials), and, finally, an outlook in which novel trends in the design of these waveguides are briefly described.

We present the power scaling of broadband THz radiation generated by the two-color gas plasma scheme driven by a state-of-the-art, ultrafast, high average-power fiber laser system to an unprecedented average power of 50 mW. The possibilities for further scaling are discussed.

In this work we measure the frequency dependent transfer function of the amplitude noise for both the seed and pump power in an Yb

High-power few-cycle laser systems enable table-top coherent XUV and soft X-ray sources via high harmonic generation (HHG) and, consequently, subsequent applications in advanced spectroscopy and imaging [1, 2]. As a result of the application-driven demand for bright, laser-like soft X-ray sources with spectral coverage up to the water window (280–530 eV), the community is developing ultrafast lasers at carrier wavelengths beyond the emission bands of Yb- or Ti:Sa-based gain media. The reason for this wavelength shift is the fundamental upscaling of the HHG-driving laser's ponderomotive force and with it, the photon energy cut-off. However, the increased cut-off comes at the cost of conversion efficiency, which is why the long-wavelength driving lasers must deliver pulses as short as possible to allow for optimizing the phase-matched peak intensity (and therefore the efficiency [3]). Additionally, high repetition rates are aspired to increase the overall yield.

Ultrafast lasers that deliver intense femtosecond pulses at carrier wavelengths beyond the well-established emission bands of Yb- or Ti:Sa-based sources have become extremely popular drivers for scientific applications. It is widely known that the long wavelength is a key property to enable new strong-field experiments or to significantly improve existing ones, e.g. in the case of extreme nonlinear frequency conversion to otherwise hardly accessible spectral regions. Those spectral regions of interest range from the deep mid-infrared (based on parametric amplification in non-oxide crystals) up to the soft X-ray regime (via high-harmonic generation). In order to turn such laboratory experiments into real-world applications with improved signal-to-noise-ratios and fast data acquisition and to unlock applications within the industry sector as well, the long wavelength driving laser sources need to deliver high repetition rates and high average powers.

Electromagnetic radiation in the THz gap between 0.1 THz and 10 THz has hsa been attracting more and more interest in recent years. A growing number of applications for THz-radiation in industry, science, security, biology and medicine demand for high power sources within this spectral region to push the fundamental detection limits and enable unprecedented sensitivity, precision and measurement speed [1]. A possibility to develop high-power THz sources is to transfer the progress achieved in the development of high-power ultrafast lasers to the THz sources.