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State-of-the-art fiber-laser systems can deliver femtosecond pulses at average powers beyond the kilowatt level and multi-mJ pulse energies by employing advanced large-mode-area fiber designs, chirped-pulse amplification, and the coherent combination of parallel fiber amplifiers. By using sophisticated coherent phase control, one or even several output ports can be modulated at virtually arbitrary power levels and switching speeds. In addition, an all-fiber setup for GHz-burst generation is described allowing to access an even wider range of laser parameters. The combination of all these approaches together with the robustness, efficiency, and excellent beam quality inherent to fiber-laser technology has the potential to strongly improve existing materials-processing applications.

Fiber-based laser sources for Coherent Anti-Stokes Raman scattering (CARS) microscopy and Stimulated Raman Spectroscopy (SRS) have been successfully used for label-free bio-imaging in the last few years. However, these systems were limited so far to a few experimental setups in university labs. We present the first extremely compact commercially available dual-wavelength source for CARS and SRS. With a foot-print of

We present a CEO-stable 1.1 kW CPA system that is designed to drive a few-cycle-generation stage ( 300W of average power at 100kHz repetition-rate providing

We present the first high-power coherently combined thulium-doped fiber-CPA. The laser system delivers >228 µJ pulse energy with 48 hours of operation even at an average-power >120 W. The simultaneous availability of 100 W class average power and 1 GW-class peak power in the 2 µm wavelength regime is not only unique in the world of science but also comes in a fully engineered and commercially available system design optimized for long-term operation..

We present the first thulium-doped fiber CPA delivering >100 W average-power and simultaneously >1 GW of peak-power with >228 µJ energy and 48 hours.

We present a CEP-stable Yb:fiber-based laser system delivering 100 μJ few-cycle pulses at the repetition rate of 100 kHz. The reported system is based on a highly scalable in terms of average power and pulse energy concept of a fiber chirped-pulse amplifier (CPA) combined with a hollow-core fiber compression stage. The same concept and similar components are employed by the ELI-ALPS HR1 [1].

State-of-the-art ultrafast materials processing applications impose strong requirements on the driving laser systems. For novel ablation schemes GHz-bursts are necessary to operate ideally [1,2]. Additionally, fast modulation and control of these pulse trains is required allowing for both high-power operation (kW of average powers and mJ pulse energies) and, ideally, pulse-to-pulse amplitude control. Here we present novel solutions to address these demands based on fiber-laser technology and coherent beam combination.

The generation of three-cycle multi-millijoule pulses at 318W 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. (c) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

High harmonic generation (HHG) of ultrashort laser pulses is a key process for the generation of coherent extreme ultraviolet to soft X-ray radiation, which is routinely employed in a wide variety of applications [1]. Amongst the large variety photoelectron spectroscopy and microscopy particularly demands for high repetition rate and high photon flux sources [2, 3]. As such, fiber laser driven HHG has emerged as a powerful source to provide the highest photon fluxes in combination with unprecedented repetition rates [4].

Among other modern imaging techniques, stimulated Raman scattering (SRS) requires an extremely quiet, widely wavelength tunable laser [1], which, up to now, is unheard of in fiber laser systems. Here we present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber [2]. We employ pulses from an all-fiber frequency and repetition rate tunable laser, which after amplification and subsequent conversion in the linear OPO cavity arrangement are converted to wavelength ranges of 764–960 nm and 1164–1552 nm at a repetition rate of 9.5 MHz. Thus, all biochemically relevant Raman shifts from 922 to 3322 cm−1 may be addressed in combination with an additional output, which is tunable from 1024 to 1052 nm. This ultra-low-noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging.

Among other modern imaging techniques, stimulated Raman Scattering (SRS) requires an extremely quiet, widely wavelength tunable laser, which, up to now, is unheard of in fiber laser systems. We present a compact all-fiber laser system, which features an optical parametric oscillator (OPO) based on degenerate four-wave mixing (FWM) in an endlessly single-mode photonic-crystal fiber. We employ an all-fiber frequency and repetition rate tunable laser in order to enable wideband conversion in the linear OPO cavity arrangement, the signal and idler radiation can be tuned between 764 and 960 nm and 1164 and 1552 nm at 9.5 MHz. Thus, all biochemically relevant Raman shifts between 922 and 3322 cm-1 may be addressed in combination with a secondary output, which is tunable between 1024 and 1052 nm. This ultra-low noise output emits synchronized pulses with twice the repetition rate to enable SRS imaging. We measure the relative intensity noise of this output beam at 9.5 MHz to be between -145 and -148 dBc, which is low enough to enable high-speed SRS imaging with a good signal-to-noise ratio. The laser system is computer controlled to access a certain energy differences within one second. Combining FWM based conversion, with all-fiber Yb-based fiber lasers enables the construction of the first automated, turn-key and widely tunable fiber laser. This laser concept could be the missing piece to establish CRS imaging as a reliable guiding tool for clinical diagnostics and surgical guidance.

Coherently enhancing laser pulses in a passive cavity provides ideal conditions for high-order harmonic generation in a gas, with repetition rates around 100 MHz (refs 1,2,3). Recently, extreme-ultraviolet radiation with photon energies of up to 30 eV was obtained, which is sufficiently bright for direct frequency-comb spectroscopy at 20 eV (ref. 4). Here, we identify a route to scaling these radiation sources to higher photon energies. We demonstrate that the ionization-limited attainable intracavity peak intensity increases with decreasing pulse duration. By enhancing nonlinearly compressed pulses of an Yb-based laser and coupling out the harmonics through a pierced cavity mirror, we generate spatially coherent 108 eV (11.45 nm) radiation at 78 MHz. Exploiting the full potential of the demonstrated techniques will afford high-photon-flux ultrashort-pulsed extreme-ultraviolet sources for a number of applications in science and technology, including photoelectron spectroscopy, coincidence spectroscopy with femtosecond to attosecond resolution5,6 and characterization of components and materials for nanolithography7. Spatially coherent 11.45 nm radiation is produced by outcoupling the harmonics of cavity-enhanced nonlinearly compressed pulses from a Yb-based laser through a pierced cavity mirror. This technique may lead to high-photon-flux ultrashort-pulse extreme-ultraviolet sources for use in a wide range of applications.

Here, we present a passive 30-m long enhancement cavity that supports a steady-state enhancement of 198, which is the highest enhancement that has ever been reached in such a long cavity. Furthermore, we demonstrate the extraction of a short burst with a total energy of 53.6 μJ employing an acousto-optic modulator (AOM) as a switching device. The cavity was seeded with pulses of 1.49 μJ energy at 10 MHz repetition rate. The individual output coupled pulses showed an energy enhancement of up to 8.5 while the whole burst contained the entire energy of 36 input pulses. In the last section theoretical considerations for the single pulse extraction are presented and briefly discussed.

We demonstrate a power improvement of two orders of magnitude to the nW-level for multi-MHz-repetition-rate high-harmonic generation in the 100-eV range, driven in an enhancement cavity by 30-fs, 10-kW average-power pulses at 250 MHz.

Optics letters 43(8), 1686 - 1689 (2018). doi:10.1364/OL.43.001686, 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., Published by OSA, Washington, DC

We proposed and demonstrated pre-chirp managed amplification (PCMA), in which the seeding pulse was nonlinearly amplified such that the amplified spectrum was substantially broadened. By properly pre-chirping the seeding pulse, the amplified pulse can be compressed with the duration much shorter than the transform-limited duration allowed by the seeding spectrum. Using an Yb-doped rod-type large-pitch fiber as the power amplifier, PCMA has enabled us to generate 75 MHz, ~60 fs, linearly-polarized pulses with >100-W average power. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We demonstrate that seeding pre-chirp managed amplification by circularly polarized seeding pulses can increase the amplified pulse energies. Using this method, we generate 100-W, 26-fs pulses at 23.7-MHz repetition rate.

This chapter reviews the fundamentals and achievements of ultrashort pulse generation and amplification in ytterbium-doped fibers. Compact and ultrastable passively mode-locked fiber oscillators represent an ideal seed source for high performance femtosecond fiber amplification systems, which have been scaled towards kW-level average power and pulse energies well above the mJ-level. These laser systems will have significant impact in numerous scientific and industrial applications.

Over the last decade, the performance of femtosecond fiber laser systems has been rapidly improved. However, further improvements might be held back due to different physical limitations such as nonlinearities or optically induced damage. We demonstrate that with the coherent combination of four parallel fiber amplifiers record pulse energies and peak-powers of 5.7 mJ and 22 GW, respectively, could be achieved. These values could be realized with a chirped-pulse-amplification (CPA) laser system running at a repetition rate of 40 kHz and delivering a compressed average power of 230 W. A high combination efficiency of 89% was achieved demonstrating the scalability of the combining approach to a larger number of channels.

Pre-chirp managed nonlinear amplification delivering high quality 60 fs pulses is compared to conventionally employed CPA. We present a compact rod-type system that outputs >100 W at 75 MHz.

The coherent combining of ultrashort pulses is a concept for scaling the pulse energy and average power of laser systems emitting ultrashort pulses. This concept has already been demonstrated for two-channel systems. In this contribution we report on a four-channel system that delivers 2.1 mJ pulse energies at a repetition rate of 100 kHz and 210 W of average power after compression. A combination efficiency of 88% was achieved and this demonstrates the scalability of the combining approach to a larger number of channels.

We present a high-power ultrashort-pulse laser that generates 200fs pulses at 132W average power and more than 18GW of peak-power, which is the highest power value directly emitted from a fiber-based system.

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. Coherent pulse stacking may allow compact fibre-laser systems to be used in laser-based particle accelerators, report scientists in Germany. Such accelerators require lasers that can simultaneously provide exceedingly high peak and average powers—a tall order indeed. Now, Sven Breitkopf and co-workers propose that it could be achieved by employing temporal coherent combination of a pulse train emitted by a high-repetition-rate femtosecond fibre laser in a passive cavity. Following pulse stacking, a fast switching element dumps the enhanced high-peak power pulse from the cavity. Although this process increases the pulse peak power at the expense of a lower repetition rate, it retains a high average power. Ultimately, the scheme could lead to Joule-class pulses with terawatt peak powers and megawatt average powers.

A scalable concept based on ultrafast fiber lasers followed by nonlinear compression is presented, demonstrating 350 µJ, sub-8 fs pulses at an unprecedented average power of 53 W. Efficient High Harmonic Generation produces record-high photon flux soft X-ray and extreme ultraviolet radiation.

We demonstrate the enhancement of 250-fs pulses in a passive resonator to 400 kW of average power. Thermal effects are investigated and mitigated by custom mirrors and optimized cavity designs.

We demonstrate a pre-chirp managed Yb-fiber laser system that outputs 75-MHz, 130-W spectrally broadened pulses, which are compressed to 60 fs with 100-W average power. The source is suitable for cavity-enhanced ultrafast optics.

Coherent combination is a promising power-scaling approach for ultrafast lasers and can be performed both in the spatial and in the temporal domain. We demonstrate for the first time the implementation of both techniques combining four temporal and two spatial channels.

We present a promising concept to reach femtosecond pulses with over 1 J energy at repetition rates of 15 kHz using a combination of fiber-amplifiers and ultra-long, non-steady-state enhancement-cavities. We show first preliminary results for a 10 MHz cavity.

We present a method for energy-scaling of fiber chirped-pulse-amplification systems using the concept of actively controlled divided-pulse amplification incorporating free-space division and recombination stages. In a first experiment femtosecond pulses with pulse energies of 1mJ have been achieved.

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.

Coherent combination of pulses emitted from spatially separated amplifiers allows for the scaling of the pulse energy and average power compared to a single amplifier system. This was already demonstrated for femtosecond pulses in a two-channel configuration [1,2].

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.

Thermal waveguide changes in high power fiber lasers can have a significant effect on the waveguide properties of large mode area fibers. We demonstrate this by overcoming avoided crossings and measuring the fundamental mode shrinking.

Harmonics up to order 21 generated in Xe are coupled out of a 78-MHz enhancement cavity with a fused silica Brewster plate. An irreversible degradation of the plate, accelerated by the generated harmonics, is observed.

We present results of ultrafast fiber amplifiers reaching multi-GW peak powers and average powers close the kW-level. Additionally, we describe two concepts of coherent combining, which will enable unseen laser parameters in the near future.

We demonstrate a high-finesse femtosecond enhancement cavity with an on-axis obstacle. By inserting a wire with a width of 5% of the fun-damental mode diameter, the finesse of F = 3400 is only slightly reduced to F = 3000. The low loss is due to the degeneracy of transverse modes, which allows for exciting a circulating field distribution avoiding the obstacle. We call this condition quasi-imaging. The concept could be used for output coupling of intracavity-generated higher-order harmonics through an on-axis opening in one of the cavity mirrors.

Large Pitch photonic crystal Fibers (LPFs) have enabled the output power increase of fiber lasers with mode field diameters exceeding 50 µm [1,2]. However, these fibers need strict refractive index matching between the glass matrix and the doped material in the signal core. On the one hand a higher refractive index for the doped material will lead to a step-index guiding structure, on the other hand a strong depression results in huge deformations of the modes. This effect is even more critical in double clad structures, necessary for high power diode pumping. In this case, core guided modes can interact with the modes of the highly multimode double clad structure. An index depression of the doped area can lead to mode anticrossings, as the effective refractive indices of both modes converge and both modes deform strongly. In this contribution the refractive index change in high power fiber lasers is investigated. Therefore, a fiber with a mode anticrossing is experimentally studied. By pumping this fiber, the mode anticrossing can be passed and a quantitative estimate for the refractive index change can be given.

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.

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.

Mode-locked lasers have an undisputed position in the ultrafast domain, though they are fairly expensive for miscellaneous applications. Thus, laser consumers revert to more cost-effective systems like Q-switched lasers. Here we report on the nonlinear compression of passively Q-switched laser pulses that allows accessing the time domain of sub-10-picoseconds, which has been so far the realm of mode-locked lasers. Laser pulses with an initial duration of 100 ps from a passively Q-switched microchip laser are amplified in a photonic crystal fiber and spectrally broadened from 20 pm to 0.68 nm by self-phase modulation. These pulses are compressed in a grating compressor to a duration of 6 ps with a pulse energy of 13 µJ.

We report on high-power, ultrafast enhancement cavity designs with enlarged laser spots on the mirrors. Together with a novel seeding Yb-fiber based CPA, MW-level intra-cavity average powers with sub-ps pulse durations come into reach.

We experimentally demonstrate that circular polarization state is beneficial if the Kerr-nonlinearity has to be lowered during the amplification of laser pulses. It can be shown that in a fiber-based chirped pulse amplification (CPA) system, the use of circularly and linearly polarized light result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally confirmed. Circularly polarized light facilitates peak-power scaling, moreover, the self-focussing threshold can be enhanced.

Based on an analytical model for the impact of SPM on stretched ultrashort pulses, the influence of SPM is experimentally controlled in a fiber-based CPA-system. Nearly transform-limited pulses with energies as high as 1 mJ are produced at a B-integral as high as 8 rad. The value of the B-integral is measured by the phase-compensation method. The technique neither relies on complex phase-measurements nor on time-consuming optimization routines.

We experimentally present the advantage of using circularly polarized light during laser amplification to reduce the accumulated Kerr-nonlinearity.The theoretical value of 2/3 for the reduction of the B-integral is verified in a fiber CPA-system. The demonstrated technique is very attractive since it can be employed in nearly all kind of ultrafast laser systems to scale their performance, when they are limited by Kerr-nonlinearity.

We report on the cavity enhancement of a 78MHz, 200fs ytterbium-fiber laser system. Linear enhancement up to a record intra-cavity power of 18kW has been observed. In a non-linear enhancement regime 40kW have been reached.

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 mu J 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.

Nonlinear pulse compression by more than a factor of 10 in an 200 µm xenon-filled hollow fiber with subsequent chirped mirror compressor is presented yielding sub-60 fs pulses at repetition rates up to 100 kHz.

We show that the classic Raman threshold formula is unsuitable to accurately predict the onset of Raman scattering in high-power CW fiber amplifiers. We propose a new analytical formula which accuracy is tested with simulations.

We report on the cavity enhancement of a 78-MHz ytterbium-fiber laser to a record intra-cavity power of 14 kW. For the first time, pulse energies are comparable to CPA-systems, thus enabling efficient high harmonic generation.

Combining fiber- and Innoslab technology enables up to 420W average output power at almost diffraction limited beam quality. Pulses compressible to τ=720 fs at 1MHz and 100kHz repetition rate have been achieved so far.