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Publications by
Dr. Joachim Hein

All publications of HI Jena

2019

I. Tamer, S. Keppler, J. Körner, M. Hornung, M. Hellwing, F. Schorcht, J. Hein, and M. Kaluza
Modeling of the 3D spatio-temporal thermal profile of joule-class Yb³⁺-based laser amplifiers
High Power Laser Science and Engineering 7, E42 (2019)

Abstract: Thermal profile modification of an active material in a laser amplifier via optical pumping results in a change in the material’s refractive index, and causes thermal expansion and stress, eventually leading to spatial phase aberrations, or even permanent material damage. For this purpose, knowledge of the 3D spatio-temporal thermal profile, which can currently only be retrieved via numerical simulations, is critical for joule-class laser amplifiers to reveal potentially dangerous thermal features within the pumped active materials. In this investigation, a detailed, spatio-temporal numerical simulation was constructed and tested for accuracy against surface thermal measurements of various end-pumped Yb³⁺-doped laser-active materials. The measurements and simulations show an excellent agreement and the model was successfully applied to a joule-class Yb³⁺-based amplifier currently operating in the POLARIS laser system at the Friedrich-Schiller-University and Helmholtz-Institute Jena in Germany.

2018

R. Lachheb, A. Herrmann, A. Assadi, J. Reiter, J. Körner, J. Hein, C. Rüssel, R. Maâlej, and K. Damak
Judd–Ofelt analysis and experimental spectroscopic study of erbium doped phosphate glasses
Journal of Luminescence 201, 245 (2018)

Abstract: A series of phosphate glasses including two compositions that are similar to commercial laser glasses and 3 new compositions doped with 2 × 10^20 Er3+/cm3 were prepared by using the classical melt quenching technique. The new glass compositions show much better glass forming properties than the commercially available glasses, lower molecular weights and lower optical basicities which are expected to be advantageous for their luminescence and laser properties. From the UV–vis–NIR absorption spectra, detailed Judd–Ofelt analyses were conducted and the radiative properties of the luminescent levels of Er3+ in these host materials were calculated. In fact all three compositions show longer calculated luminescence lifetimes than the compositions that are based on commercially available laser glasses. The absorption and the emission cross sections, the luminescence lifetimes and the quantum efficiency at 1530 nm were investigated. LiZnLaAPF glass can be suggested as a good host to generate efficient lasing action at 1530 nm. The variation of the Judd–Ofelt intensity parameters Ω2, Ω4 and Ω6 is discussed with respect to the glass compositions and their properties. For this, the calculated Ω2, Ω4 and Ω6 values are compared to the results of numerous publications on Er3+ doped phosphate glasses. From this data a correlation with the symmetry at the local rare earth site (Ω2) and with the theoretical optical basicity (Ω6) of the glass composition can be assumed.

J. Körner, T. Lühder, J. Reiter, I. Uschmann, H. Marschner, V. Jambunathan, A. Lucianetti, T. Mocek, J. Hein, and M. C. Kaluza
Spectroscopic investigations of thulium doped YAG and YAP crystals between 77 K and 300 K for short-wavelength infrared lasers
Journal of Luminescence 202, 427 (2018)

Abstract: We present detailed measurements of laser relevant cross sections of thulium doped yttrium-aluminum-garnet (Tm:YAG) and yttrium-aluminum-perovskite (Tm:YAP), including the absorption cross sections for the H63 to H43 transition near 800nm, and the absorption and emission cross sections for the transitions between the H63 and F43 manifolds in the short-wavelength infrared region. For Tm:YAP we present data for all polarization axes. The measurements were carried out at temperatures ranging from 80 K to 300 K. Furthermore, re-absorption free fluorescence lifetimes of the F43 to H63 transition at 77 K, 200 K and 29 5K were obtained using the pinhole method. We observed a significant enhancement of the fluorescence lifetime when cooling from room temperature to 77 K. The lifetime was increased from 9.42 ms to 15.22 ms in Tm:YAG and from 3.81 ms to 4.93 ms in Tm:YAP. This indicates that lifetime quenching is present at room temperature, which can be overcome, at least partially, by cryogenic cooling. These data are presented with the scope to qualify these materials for their use in a new generation of cryogenically cooled, short-wavelength infrared, high-energy class diode pumped solid state lasers utilizing the cross relaxation mechanism for pumping.

G. A. Becker, S. Tietze, S. Keppler, J. Reislöhner, J. H. Bin, L. Bock, F.-E. Brack, J. Hein, M. Hellwing, P. Hilz, M. Hornung, A. Kessler, S. D. Kraft, S. Kuschel, H. Liebetrau, W. Ma, J. Polz, H.-P. Schlenvoigt, F. Schorcht, M. B. Schwab, A. Seidel, K. Zeil, U. Schramm, M. Zepf, J. Schreiber, S. Rykovanov, and M. C. Kaluza
Ring-like spatial distribution of laser accelerated protons in the ultra-high-contrast TNSA-regime
Plasma Physics and Controlled Fusion 60, 055010 (2018)

Abstract: The spatial distribution of protons accelerated from submicron-thick plastic foil targets using multi-terawatt, frequency-doubled laser pulses with ultra-high temporal contrast has been investigated experimentally. A very stable, ring-like beam profile of the accelerated protons, oriented around the target’s normal direction has been observed. The ring’s opening angle has been found to decrease with increasing foil thicknesses. Two-dimensional particle-in-cell simulations reproduce our results indicating that the ring is formed during the expansion of the proton density distribution into the vacuum as described by the mechanism of target-normal sheath acceleration. Here—in addition to the longitudinal electric fields responsible for the forward acceleration of the protons—a lateral charge separation leads to transverse field components accelerating the protons in the lateral direction.

I. Tamer, S. Keppler, M. Hornung, J. Körner, J. Hein, and M. C. Kaluza
Spatio-Temporal Characterization of Pump-Induced Wavefront Aberrations in Yb3 + -Doped Materials
Laser & Photonics Reviews 12, 1700211 (2018)

Abstract: Abstract A comprehensive spatio-temporal characterization is presented describing the pump-induced wavefront aberrations in Yb3 + -doped YAG, CaF2, and fluorophosphate glass. Time-resolved interferometric measurements were performed to reveal the profiles of the total optical path differences (OPDs), which are described by the spatio-temporal superposition of thermal as well as electronic contributions, across the free aperture of the considered diode-pumped active materials. These contributions were individually determined by a COMSOL-based thermal profile model along with a detailed characterization of the electronic changes by measuring the single-pass gain and the spatial fluorescence profile. Due to the low quantum defect, the amplitude of the electronic component becomes comparable for all three materials and, in the case of Yb:CaF2, almost completely compensates the thermal component resulting from a pump pulse during the time frame of laser pulse amplification. Finally, all relevant material constants – such as the photoelastic constant and the polarizability difference – could be determined during this investigation, allowing the accurate modeling of the total pump-induced wavefront aberrations and subsequent optimization for laser systems worldwide employing these Yb3 + -doped materials.

2017

M. Kahle, J. Körner, J. Hein, and M. C. Kaluza
Performance of a quantum defect minimized disk laser based on cryogenically cooled Yb:CaF2
Optics & Laser Technology 92, 19 (2017)

Abstract: Abstract A low quantum defect is the fundamental key to a high efficiency of any laser. To study the anticipated performance boost for a 980 nm-diode pumped cryogenically cooled Yb:CaF2 disk laser we compared its operation at output wavelengths of 991 nm, 996 nm, and 1032 nm. Despite the higher quantum defect a maximum efficiency of 74% (output versus incident power) with an output power of 15.8 W was achieved at the 1032 nm output wavelength. This observation led to a detailed analysis of remaining loss mechanisms we are reporting on in this paper.

H. Liebetrau, M. Hornung, S. Keppler, M. Hellwing, A. Kessler, F. Schorcht, J. Hein, and M. C. Kaluza
Intracavity stretcher for chirped-pulse amplification in high-power laser systems
Optics Letters 42, 326 (2017)

Abstract: We present pulse stretching with an intracavity Offner-type pulse stretcher applied to a high-energy, short-pulse laser system. The compact intracavity design, offering a tunable stretching factor, allows the pulses to be stretched to several nanoseconds and, at the same time, to be amplified to 100 μJ. The stretched pulses have been further amplified with the high-power laser system Polaris and have been recompressed to durations as short as 102 fs, reaching peak powers of 100 TW. Furthermore, the temporal intensity contrast is investigated and compared to the formerly used stretcher setup.

J. Körner, J. Reiter, T. Lühder, J. Hein, V. Jambunathan, A. Lucianetti, T. Mocek, and M. C. Kaluza
Temperature dependent spectroscopic characterization of Tm:YAG crystals as potential laser medium for pulsed high energy laser amplifiers
(2017)

Abstract: Similar to ytterbium doped laser materials laser operation with thulium doped media is possible within a quasi-three level scheme, which especially for pulse pumped lasers is a drawback for efficient laser operation, as a significant amount of energy is required to bleach out the laser medium. Since this energy cannot be extracted, it is lost for the amplification process. Hence, operation of such lasers at cryogenic temperatures seems to be an appropriate solution. For further modeling and derivation of design rules for future laser systems based on such a scheme reliable spectral data is needed. We will present absorption and emission measurements on Tm:YAG as a function of temperature in the range from 80 K to 300 K covering both the absorption bands around 800 nm and the emission bands up to 2.1 μm. The spectral measurements were carried out on two samples of Tm:YAG with doping levels of 2 at.% and 8 at.%. Precautions for reabsorption effects were taken to allow for accurate results over the whole measurement range. From these measurements we have derived absorption and emission cross sections and radiative lifetimes. By comparing the latter values to values obtained by highly accurate measurements of the lifetime using the pinhole method we could also estimate the quantum efficiency.

2016

J. Körner, J. Hein, and M. Kaluza
Compact Aberration-Free Relay-Imaging Multi-Pass Layouts for High-Energy Laser Amplifiers
Applied Sciences 6, 353 (2016)

Abstract: We present the results from a theoretical investigation of laser beam propagation in relay imaging multi-pass layouts, which recently found application in high-energy laser amplifiers. Using a method based on the well-known ABCD-matrix formalism and proven by ray tracing, it was possible to derive a categorization of such systems. Furthermore, basic rules for the setup of such systems and the compensation for low order aberrations are derived. Due to the introduced generalization and parametrization, the presented results can immediately be applied to any system of the investigated kinds for a wide range of parameters, such as number of round-trips, focal lengths and optics sizes. It is shown that appropriate setups allow a close-to-perfect compensation of defocus caused by a thermal lens and astigmatism caused by non-normal incidence on the imaging optics, as well. Both are important to avoid intensity spikes leading to damages of optics in multi-pass laser amplifiers.

M. Hornung, H. Liebetrau, S. Keppler, A. Kessler, M. Hellwing, F. Schorcht, G. A. Becker, M. Reuter, J. Polz, J. Körner, J. Hein, and M. C. Kaluza
54  J pulses with 18  nm bandwidth from a diode-pumped chirped-pulse amplification laser system
Optics Letters 41, 5413 (2016)

Abstract: We report on results from the fully diode-pumped chirped-pulse amplification laser system Polaris. Pulses were amplified to a maximum energy of 54.2 J before compression. These pulses have a full width at half-maximum spectral bandwidth of 18 nm centered at 1033 nm and are generated at a repetition rate of 0.02 Hz. To the best of our knowledge, these are the most energetic broadband laser pulses generated by a diode-pumped laser system so far. Due to the limited size of our vacuum compressor, only attenuated pulses could be compressed to a duration of 98 fs containing an energy of 16.7 J, which leads to a peak power of 170 TW. These pulses could be focused to a peak intensity of 1.3×1021  W/cm2. Having an ultra-high temporal contrast of 1012 with respect to amplified spontaneous emission these laser pulses are well suited for high-intensity laser–matter experiments.

S. Keppler, M. Hornung, P. Zimmermann, H. Liebetrau, M. Hellwing, J. Hein, and M. C. Kaluza
Tunable filters for precise spectral gain control in ultra-short-pulse laser systems
Optics Letters 41, 4708 (2016)

Abstract: We present tunable spectral filters (TSFs) as a variable and precisely adjustable method to control the spectral gain of short-pulse laser systems. The TSFs provide a small residual spectral phase and a high damage threshold, and generate no pre- or post-pulses. The method is demonstrated for two different laser materials and can be applied as an intracavity compensation in regenerative amplifiers as well as a method for pre-compensation in high-energy multipass amplifiers. With this method, a full width at half-maximum bandwidth of 23.9 nm could be demonstrated in a diode-pumped, 50 J Yb:CaF2 amplifier.

J. Krey, A. Herrmann, J. Hein, J. Körner, M. C. Kaluza, and C. Rüssel
Effect of B2O3 and fluoride additions on the Yb3+ luminescence of an alumosilicate glass in the system Li2O/MgO/Al2O3/SiO2
Optical Materials Express 6, 2662 (2016)

Abstract: A series of glasses in the system Li2O/MgO/La2O3/Al2O3/B2O3/ Yb2O3/SiO2 was prepared and doped with 6·1020 Yb3+ ions per cm3 which corresponds to about 1.3 mol% Yb2O3 in the glass composition. In this series, Al2O3 was partially replaced by B2O3 up to a concentration of 15 mol%. Furthermore, Li2O and MgO were partially substituted for LiF and MgF2. All melts were bubbled with argon gas to ensure low OH- concentrations. B2O3 and fluoride additions strongly decrease the glass transition temperatures and viscosities of the melts enabling lower melting temperatures, lower Pt solubility and better homogenization of the melt. Furthermore, both additions slightly decrease the density and refractive indices of the glasses. The relatively low coefficients of thermal expansion remain largely unaffected. Increasing B2O3 concentrations do not negatively affect the luminescence lifetimes of Yb3+ despite their relatively high phonon energy. However, the fluoride addition and the OH- concentration strongly influence the luminescence lifetimes.

H. Liebetrau, M. Hornung, S. Keppler, M. Hellwing, A. Kessler, F. Schorcht, J. Hein, and M. C. Kaluza
High contrast, 86  fs, 35  mJ pulses from a diode-pumped Yb:glass double-chirped-pulse amplification laser system
Optics Letters 41, 3006 (2016)

Abstract: We demonstrate the generation of 86 fs, 35 mJ, high-contrast laser pulses at 1030 nm with a repetition rate of 1 Hz from a diode-pumped double chirped-pulse amplification setup. The pulses exhibit a spectral bandwidth exceeding 27 nm full width at half-maximum. This could be achieved by using a laser architecture comprising two stages of chirped pulse amplification with a cross-polarized wave generation filter in between, by applying spectral shaping and by increasing the spectral hard-clip of the second stretcher. These are, to the best of our knowledge, the shortest pulses at the mJ level with ultra-high contrast generated with a diode-pumped front end at 1030 nm.

J. Körner, F. Yue, J. Hein, and M. C. Kaluza
Spatially and temporally resolved temperature measurement in laser media
Optics Letters 41, 2525 (2016)

Abstract: A technique to measure the spatially resolved temperature distribution in a laser medium is presented. It is based on the temperature dependence of the absorption cross section close to the zero-phonon line of the active medium. Since other materials in the beam path exhibit a high (and constant) transmission at this wavelength, the method can easily be applied in realistic amplifier setups. The method was successfully tested on three different samples, which were pumped by a pulsed laser diode with up to 150 W average power: side-cooled Yb:YAG and Yb:fluoride-phosphate glass at room temperature and face-cooled Yb:CaF2 at 120 K.

S. Keppler, A. Sävert, J. Körner, M. Hornung, H. Liebetrau, J. Hein, and M. Kaluza
The generation of amplified spontaneous emission in high-power CPA laser systems
Laser & Photonics Reviews 10, 264 (2016)

Abstract: An analytical model is presented describing the temporal intensity contrast determined by amplified spontaneous emission in high-intensity laser systems which are based on the principle of chirped pulse amplification. The model describes both the generation and the amplification of the amplified spontaneous emission for each type of laser amplifier. This model is applied to different solid state laser materials which can support the amplification of pulse durations ≤350 fs . The results are compared to intensity and fluence thresholds, e.g. determined by damage thresholds of a certain target material to be used in high-intensity applications. This allows determining if additional means for contrast improvement, e.g. plasma mirrors, are required for a certain type of laser system and application. Using this model, the requirements for an optimized high-contrast front-end design are derived regarding the necessary contrast improvement and the amplified “clean” output energy for a desired focussed peak intensity. Finally, the model is compared to measurements at three different high-intensity laser systems based on Ti:Sapphire and Yb:glass. These measurements show an excellent agreement with the model.

2015

M. Hornung, G. Becker, A. Seidel, J. Reislöhner, H. Liebetrau, L. Bock, S. Keppler, A. Kessler, M. Zepf, J. Hein, and M. Kaluza
Generation of 25-TW Femtosecond Laser Pulses at 515 nm with Extremely High Temporal Contrast
Applied Sciences 5, 1970 (2015)

Abstract: We report on the frequency doubling of femtosecond laser pulses at 1030 nm center wavelength generated from the fully diode-pumped laser system POLARIS. The newly generated pulses at a center wavelength of 515 nm have a pulse energy of 3 J with a pulse duration of 120 fs. On the basis of initially ultra-high contrast seed pulses we expect a temporal intensity contrast better 10^17 200 ps before the peak of the main pulse. We analyzed the temporal intensity contrast from milliseconds to femtoseconds with a dynamic range covering more than 20 orders of magnitude. The pulses were focussed with a f/2-focussing parabola resulting in a peak intensity exceeding 10^20 W/cm2. The peak power and intensity are to the best of our knowledge the highest values for 515 nm-laser-pulses achieved so far.

J. Körner, J. Reiter, J. Hein, and M. Kaluza
Temporal Shaping of High Peak Power Pulse Trains from a Burst-Mode Laser System
Applied Sciences 5, 1790 (2015)

Abstract: It has been shown in the past that pulsed laser systems operating in the so-called “burst mode” are a beneficial approach to generate high peak power laser pulses at high repetition rates suitable for various applications. So far, most high-energy burst-mode laser systems put great effort into generating a homogeneous energy distribution across the burst duration, e.g., by shaping the pump pulse. In this work, we present a new shaping technique, which is able to produce arbitrary energy distributions within the burst by pre-shaping the seed pulse burst with a Pockels cell. Furthermore, this technique allows for the precompensation of any static modulations across the burst, which may be introduced during the subsequent amplification process. Therefore, a pulse burst with a uniform energy distribution can also be generated. The method is tested with an ultra-short pulse burst mode laser amplifier system producing bursts of a 1 ms duration with a pulse repetition rate of 1 MHz and a maximum output power of 800 W during the burst. Furthermore, a method to predict the influence of the amplifier on a non-uniformly shaped burst is presented and successfully tested to produce a pre-defined pulse shape after amplification.

S. Kuhn, M. Tiegel, A. Herrmann, C. Rüssel, S. Engel, C. Wenisch, S. Gräf, F. Müller, J. Körner, R. Seifert, F. Yue, D. Klöpfel, J. Hein, and M. Kaluza
Photo-acoustic spectroscopy and quantum efficiency of Yb3+ doped alumino silicate glasses
Journal of Applied Physics 118, 103104 (2015)

Abstract: In this contribution, we analyze the effect of several preparation methods of Yb3+ doped alumino silicate glasses on their quantum efficiency by using photo-acoustic measurements in comparison to standard measurement methods including the determination via the fluorescence lifetime and an integrating sphere setup. The preparation methods focused on decreasing the OH concentration by means of fluorine-substitution and/or applying dry melting atmospheres, which led to an increase in the measured fluorescence lifetime. However, it was found that the influence of these methods on radiative properties such as the measured fluorescence lifetime alone does not per se give exact information about the actual quantum efficiency of the sample. The determination of the quantum efficiency by means of fluorescence lifetime shows inaccuracies when refractive index changing elements such as fluorine are incorporated into the glass. Since fluorine not only eliminates OH from the glass but also increases the “intrinsic” radiative fluorescence lifetime, which is needed to calculate the quantum efficiency, it is difficult to separate lifetime quenching from purely radiative effects. The approach used in this contribution offers a possibility to disentangle radiative from non-radiative properties which is not possible by using fluorescence lifetime measurements alone and allows an accurate determination of the quantum efficiency of a given sample. The comparative determination by an integrating sphere setup leads to the well-known problem of reabsorption which embodies itself in the measurement of too low quantum efficiencies, especially for samples with small quantum efficiencies.

P. Crump, C. Frevert, F. Bugge, S. Knigge, G. Erbert, G. Tränkle, A. Pietrzak, R. Hüslewede, M. Zorn, J. Sebastian, J. Lotz, W. Fassbender, J. Neukum, J. Körner, J. Hein, and T. Töpfer
Progress in high-energy-class diode laser pump sources
Proceedings of SPIE 9348, 93480U (2015)

Abstract: A new generation of diode-pumped high-energy-class solid-state laser facilities is in development that generate multijoule pulse energies at around 10 Hz. Currently deployed quasi-continuous-wave (QCW) diode lasers deliver average inpulse pump powers of around 300 W per bar. Increased power-per-bar helps to reduce the system size, complexity and cost per Joule and the increased pump brilliance also enables more efficient operation of the solid state laser itself. It has been shown in recent studies, that optimized QCW diode laser bars centered at 940…980 nm can operate with an average in-pulse power of >1000 W per bar, triple that of commercial sources. When operated at pulsed condition of 1 ms, 10 Hz, this corresponds to >1 J/bar. We review here the status of these high-energy-class pump sources, showing how the highest powers are enabled by using long resonators (4…6 mm) for improved cooling and robustly passivated output facets for high reliability. Results are presented for prototype passively-cooled single bar assemblies and monolithic stacked QCW arrays. We confirm that 1 J/bar is sustained for fast-axis collimated stacks with a bar pitch of 1.7 mm, with narrow lateral far field angle (<12° with 95% power) and spectral width (<12 nm with 95% power). Such stacks are anticipated to enable Joule/bar pump densities to be used near-term in commercial high power diode laser systems. Finally, we briefly summarize the latest status of research into bars with higher efficiencies, including studies into operation at sub-zero temperatures (-70°C), which also enables higher powers and narrower far field and spectra.

S. Kuhn, M. Tiegel, A. Herrmann, J. Körner, R. Seifert, F. Yue, D. Klöpfel, J. Hein, M. C. Kaluza, and C. Rüssel
Effect of hydroxyl concentration on Yb3+ luminescence properties in a peraluminous lithium-alumino-silicate glass
Optical Materials Express 5, 430 (2015)

Abstract: Ytterbium doped lithium-alumino-silicate glasses suitable for diode-pumped laser applications were investigated concerning the hydroxyl quenching of the Yb^(3+) fluorescence. Glasses of the nominal composition 18 mol% Li2O, 22 mol% Al2O3 and 60 mol% SiO2 with variable OH concentrations NOH (between 0.04 and 6.01 ∙ 1019 cm−3) and Yb3+ concentrations NYb (between 0.1 and 9 ∙ 1020 cm−3) were produced and a direct correlation between spontaneous emission decay rate and the product NYb ∙ NOH was observed. The radiative spontaneous emission rate in the glass host is around 1,000 s−1 (radiative lifetime 1.0 ms) and the microparameter for Yb-Yb energy migration, CYb-Yb, was found to be 1.358∙10^−38 cm^6 s−1. It was calculated that on average 17% of the OH groups in the glass contribute to the quenching of the Yb3+ fluorescence. By analysis of the UV edge of the glass it was concluded that melting under inert conditions leads to reduction of iron impurities to Fe2+, which can act as quenching sites for the Yb3+ ions and therefore may additionally reduce the energy storage capability of the laser material.

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

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

2014

H. Liebetrau, M. Hornung, A. Seidel, M. Hellwing, A. Kessler, S. Keppler, F. Schorcht, J. Hein, and M. C. Kaluza
Ultra-high contrast frontend for high peak power fs-lasers at 1030 nm
Optics Express 22, 24776 (2014)

Abstract: We present the results from a new frontend within a double-chirped pulse amplification architecture (DCPA) utilizing crossed-polarized wave generation (XPW) for generating ultra-high contrast, 150 μJ-level, femtosecond seed pulses at 1030 nm. These pulses are used in the high energy class diode-pumped laser system Polaris at the Helmholtz Institute in Jena. Within this frontend, laser pulses from a 75 MHz oscillator-pulse train are extracted at a repetition rate of 1 Hz, temporally stretched, amplified and then recompressed reaching a pulse energy of 2 mJ, a bandwidth of 12 nm and 112 fs pulse duration at a center wavelength of 1030 nm. These pulses are temporally filtered via XPW in a holographic-cut BaF2 crystal, resulting in 150 μJ pulse energy with an efficiency of 13 %. Due to this non-linear filtering, the relative intensity of the amplified spontaneous emission preceding the main pulse is suppressed to 2×10^−13. This is, to the best of our knowledge, the lowest value achieved in a high peak power laser system operating at 1030 nm center wavelength.

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

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

M. Tiegel, A. Herrmann, S. Kuhn, C. Rüssel, J. Körner, D. Klöpfel, R. Seifert, J. Hein, and M. C. Kaluza
Fluorescence and thermal stress properties of Yb 3+ -doped alumino silicate glasses for ultra high peak power laser applications
Laser Physics Letters 11, 115811 (2014)

Abstract: Various alumino silicate glasses (network modifier ions: Li^+ , Mg^2+ , Zn^2+ and/or La^^3+ ) doped with 1  ×  10^20 Yb^3+  cm^−3 (about 0.2 mol% Yb_2 O_3 ) were prepared. The glasses were studied with respect to their thermo-mechanical and fluorescence properties. Huge differences are found for the coefficients of thermal expansion which determine the thermal shock resistance of the material and hence are required for ultra-high power laser applications. Here, zinc and magnesium alumino silicate glasses show the lowest values. The fluorescence lifetimes of the glasses increase with decreasing average atomic weight of the glass composition (685–1020  µ s). All glasses show broad and smooth emission spectra with little variations due to compositional changes. Mixed lithium zinc or lithium magnesium alumino silicate glasses could be promising new laser materials especially with respect to ultra-high peak power systems or applications with high repetition rates.

J. Körner, V. Jambunathan, J. Hein, R. Seifert, M. Loeser, M. Siebold, U. Schramm, P. Sikocinski, A. Lucianetti, T. Mocek, and M. C. Kaluza
Spectroscopic characterization of Yb3+-doped laser materials at cryogenic temperatures
Applied Physics B 116, 75 (2014)

Abstract: We present measurements of the absorption and emission cross-sections for Yb:YAG, Yb:LuAG and Yb:CaF_{2} as a function of temperature between 80 and 340 K. The cross-sections are determined by the combination of the McCumber relation and the Fuchtbauer–Ladenburg (FL) equation to achieve reliable results in spectral regions of high and low absorption. The experimental setup used for the fluorescence measurements minimizes re-absorption effects due to the measurement from small sample volume, providing nearly undisturbed raw data for the FL approach. The retrieved cross-sections together with the spectral characteristics of the tested materials provide important information for the design of energy efficient, high-power laser amplifiers.

R. Bödefeld, M. Hornung, J. Hein, and M. C. Kaluza
High precision elimination of angular chirp in CPA laser systems with large stretching factors or high bandwidth
Applied Physics B 115, 419 (2014)

Abstract: We present a novel and highly sensitive method to determine the residual angular dispersion of high-power laser pulses after stretching, amplification, and re-compression of the pulses in a chirped-pulse amplification laser system. This method is based on the intentional deflection of a part of the the spectrum within the compressor and aligning the centers of gravity of the two resulting and separated foci with largest possible spectral separation in the far field. Using this technique, we were able to reduce the residual angular dispersion on pulses to less than 0.05 μrad/nm in the vertical plane and less than 0.03 μrad/nm in the horizontal plane, respectively. With this method, it is possible to minimize the deviation of the actual peak intensity for the focused laser pulses to less than 2% of its theoretical limit.

M. Hornung, H. Liebetrau, A. Seidel, S. Keppler, A. Kessler, J. Körner, M. Hellwing, F. Schorcht, D. Klöpfel, A. K. Arunachalam, G. A. Becker, A. Sävert, J. Polz, J. Hein, and M. C. Kaluza
The all-diode-pumped laser system POLARIS – an experimentalist’s tool generating ultra-high contrast pulses with high energy
High Power Laser Science and Engineering 2, e20 (2014)

Abstract: The development, the underlying technology and the current status of the fully diode-pumped solid-state laser system POLARIS is reviewed. Currently, the POLARIS system delivers 4 J energy, 144 fs long laser pulses with an ultra-high temporal contrast of 5×10^12 for the ASE, which is achieved using a so-called double chirped-pulse amplification scheme and cross-polarized wave generation pulse cleaning. By tightly focusing, the peak intensity exceeds 3.5×10^20 W cm^{−2}. These parameters predestine POLARIS as a scientific tool well suited for sophisticated experiments, as exemplified by presenting measurements of accelerated proton energies. Recently, an additional amplifier has been added to the laser chain. In the ramp-up phase, pulses from this amplifier are not yet compressed and have not yet reached the anticipated energy. Nevertheless, an output energy of 16.6 J has been achieved so far.

C. Wandt, S. Klingebiel, S. Keppler, M. Hornung, M. Loeser, M. Siebold, C. Skrobol, A. Kessel, S. Trushin, Z. Major, J. Hein, M. C. Kaluza, F. Krausz, and S. Karsch
Development of a Joule-class Yb:YAG amplifier and its implementation in a CPA system generating 1 TW pulses
Laser & Photonics Reviews 8, 875 (2014)

Abstract: In this paper the development and implementation of a novel amplifier setup as an additional stage for the CPA pump laser of the Petawatt Field Synthesizer, currently developed at the Max-Planck-Institute of Quantum Optics, is presented. This amplifier design comprises 20 relay-imaged passes through the active medium which are arranged in rotational symmetry. As the gain material, an in-house-developed Yb:YAG active-mirror is used. With this setup, stretched 4 ns seed pulses are amplified to output energies exceeding 1 J with repetition rates of up to 2 Hz. Furthermore, a spectral bandwidth of 3.5 nm (FWHM) is maintained during amplification and the compression of the pulses down to their Fourier-limit of 740 fs is achieved. To the best of our knowledge, this is the first demonstration of 1 TW pulses generated via CPA in diode-pumped Yb:YAG.

S. Keppler, M. Hornung, R. Bödefeld, A. Sävert, H. Liebetrau, J. Hein, and M. C. Kaluza
Full characterization of the amplified spontaneous emission from a diode-pumped high-power laser system
Optics Express 22, 11228 (2014)

Abstract: We present the first complete temporal and spatial characterization of the amplified spontaneous emission (ASE) of laser radiation generated by a diode-pumped high-power laser system. The ASE of the different amplifiers was measured independently from the main pulse and was characterized within a time window of \minus10ms \leq t \leq 10ms and an accuracy of up to 15fs around the main pulse. Furthermore, the focusability and the energy of the ASE from each amplifier was measured after recompression. Using our analysis method, the laser components, which need to be optimized for a further improvement of the laser contrast, can be identified. This will be essential for laser-matter interaction experiments requiring a minimized ASE intensity or fluence.

A. Herrmann, S. Kuhn, M. Tiegel, C. Russel, J. Korner, D. Klopfel, J. Hein, and M. Kaluza
Structure and fluorescence properties of ternary aluminosilicate glasses doped with samarium and europium
Journal of Materials Chemistry C 2, 4328 (2014)

Abstract: Various ternary aluminosilicate glasses with the molar compositions 20 Al2O3-60 SiO2-20 R2O (R = Li or Na), 20 Al2O3-60 SiO2-20 RO (R = Mg, Ca or Zn) and 23.1 Al2O3-69.2 SiO2-7.7 R2O3 (R = Y or La) doped with 1 [times] 1020 Sm3+ cm-3 or 1 [times] 1020 Eu3+ cm-3 (about 0.2 mol% Sm2O3 or Eu2O3) were prepared. The glasses were studied with respect to their molecular structure, and their thermo-mechanical and fluorescence properties. All glasses show relatively broad fluorescence excitation and only a weak effect of the glass composition on the emission spectra is observed. Although the glasses should be structurally very similar, huge differences are found in the coefficients of thermal expansion and the glass transition temperatures. The fluorescence lifetime increases steadily with decreasing mean atomic weight and decreasing refractive index of the glasses, which may be explained by local field effects. The only exception from this rule is the zinc aluminosilicate glass, which shows a relatively high fluorescence lifetime. The highest fluorescence lifetime is found for the lithium aluminosilicate glass. The lowest coefficients of thermal expansion are found for zinc- and magnesium aluminosilicate glasses. A low coefficient of thermal expansion is a prerequisite for a high thermal shock resistance of the material and hence favorable for high-power laser applications.

A. Kessler, M. Hornung, S. Keppler, F. Schorcht, M. Hellwing, H. Liebetrau, J. Körner, A. Sävert, M. Siebold, M. Schnepp, J. Hein, and M. C. Kaluza
16.6 J chirped femtosecond laser pulses from a diode-pumped Yb:CaF2 amplifier
Optics Letters 39, 1333 (2014)

Abstract: We report the amplification of laser pulses at a center wavelength of 1034 nm to an energy of 16.6 J from a fully diode-pumped amplifier using Yb:CaF2 as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g=6.1 was achieved in a nine-pass amplifier configuration agreeing with numerical simulations. A measured spectral bandwidth of 10 nm full width at half-maximum promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are, to our knowledge, the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.

2013

S. Kuhn, A. Herrmann, J. Hein, M. C. Kaluza, and C. Rüssel
Sm3+-doped La2O3–Al2O3–SiO2-glasses: structure, fluorescence and thermal expansion
Journal of Materials Science 48, 8014 (2013)

Abstract: This paper reports on the effect of the chemical composition on the glass structure, the coefficients of thermal expansion and the fluorescence properties of Sm3+-doped La2O3–Al2O3–SiO2-glasses. The silica concentration was varied between 50 and 70 mol% and the La2O3:Al2O3 ratio between 50:50 and 30:70. The glass formation and the densities are evaluated and FTIR reflectance spectra, coefficients of thermal expansion and fluorescence lifetimes are determined. It is shown that high SiO2 concentrations and low La2O3:Al2O3 ratios result in relatively high fluorescence lifetime (2.19 ms, 4G5/2) and low coefficients of thermal expansion (4.6 × 10^{−6} / K). The coefficients of thermal expansion and the fluorescence lifetimes show a linear dependency on the ratio LaO3/2/(AlO3/2 + SiO2).

J. Körner, J. Hein, H. Liebetrau, R. Seifert, D. Klöpfel, M. Kahle, M. Loeser, M. Siebold, U. Schramm, and M. C. Kaluza
Efficient burst mode amplifier for ultra-short pulses based on cryogenically cooled Yb3+:CaF2
Optics Express 21, 29006 (2013)

Abstract: We present a novel approach for the amplification of high peak power femtosecond laser pulses at a high repetition rate. This approach is based on an all-diode pumped burst mode laser scheme. In this scheme, pulse bursts with a total duration between 1 and 2 ms are be generated and amplified. They contain 50 to 2000 individual pulses equally spaced in time. The individual pulses have an initial duration of 350 fs and are stretched to 50 ps prior to amplification. The amplifier stage is based on Yb3+:CaF2 cooled to 100 K. In this amplifier, a total output energy in excess of 600 mJ per burst at a repetition rate of 10 Hz is demonstrated. For lower repetition rates the total output energy per burst can be scaled up to 915 mJ using a longer pump duration. This corresponds to an efficiency as high as 25% of extracted energy from absorbed pump energy. This is the highest efficiency, which has so far been demonstrated for a pulsed Yb3+:CaF2 amplifier.

M. Tiegel, A. Herrmann, C. Russel, J. Korner, D. Klopfel, J. Hein, and M. C. Kaluza
Magnesium aluminosilicate glasses as potential laser host material for ultrahigh power laser systems
Journal of Materials Chemistry C 1, 5031 (2013)

Abstract: Magnesium aluminosilicate glasses doped with 0.2 mol% Sm2O3 (1 x 10^{20} Sm3+ cm^{-3}) have been prepared in a very broad compositional range. The effect of the MgO, Al2O3 and SiO2 concentrations as well as the effect of partial substitution of MgO by CaO, SrO, BaO, ZnO or MgF2 have been studied. Increasing the network modifier concentration results in decreasing the glass transformation temperature and increasing the coefficient of thermal expansion due to the formation of non-bridging oxygen sites and decreasing glass network connectivity. Although the network connectivity is changed substantially by the addition of network modifier oxides, the maximum phonon energy and the fluorescence lifetime of Sm3+ are not affected. Equimolar replacement of up to 9 mol% MgO by MgF2 results in increasing Sm3+ fluorescence lifetimes without increasing the coefficient of thermal expansion or decreasing the glass forming ability. Glasses with fairly small thermal expansion coefficients (≤ 3.2 x 10^{-6} K^{-1}), low thermal stress values (≤ 0.5 MPa K^{-1}), broad fluorescence emission peaks and fluorescence lifetimes in the range from 2.4 to 2.8 ms are obtained. Such glasses are interesting candidates for laser host materials in ultrahigh peak power laser systems.

S. Banerjee, J. Koerner, M. Siebold, Q. Yang, K. Ertel, P. D. Mason, P. J. Phillips, M. Loeser, H. Zhang, S. Lu, J. Hein, U. Schramm, M. C. Kaluza, and J. L. Collier
Temperature dependent emission and absorption cross section of Yb3+ doped yttrium lanthanum oxide (YLO) ceramic and its application in diode pumped amplifier
Optics Express 21, 726 (2013)

Abstract: Temperature dependent absorption and emission cross-sections of 5 at-% Yb3+ doped yttrium lanthanum oxide (Yb:YLO) ceramic between 80K and 300K are presented. In addition, we report on the first demonstration of ns pulse amplification in Yb:YLO ceramic. A pulse energy of 102mJ was extracted from a multi-pass amplifier setup. The amplification bandwidth at room temperature confirms the potential of Yb:YLO ceramic for broad bandwidth amplification at cryogenic temperatures.

J. Körner, J. Hein, H. Liebetrau, M. Kahle, R. Seifert, D. Klöpfel, and M. C. Kaluza
Diode-pumped, cryogenically cooled, femtosecond burst mode laser
Proceedings of SPIE 8780, 878008 (2013)

Abstract: We present a novel approach for the construction of a high energy, high power burst mode laser system, based on diode pumped cryogenically cooled Yb:CaF2. The system consists of a frontend producing pulses of 300 fs duration with 1 MHz. Bursts of 1000 subsequent pulses are cut from the continuous train by an electro optical modulator. Afterwards the duration of the individual pulses is stretched to 50 ps.

The amplifier system consists of two amplifiers. Both amplifiers utilize mirror based relay imaging schemes to allow for a sufficient number of extraction passes for achieving efficient energy extraction. The goal parameters of the system are to achieve a total energy of 5 J per burst with a repetition rate of 10Hz.

Amplification results for the first of two amplifiers are demonstrated. A total output energy of 480 mJ was achieved corresponding to an optical to optical efficiency from absorbed pump energy to extracted energy of more than 17%. Single pulse energies of up to 7.5mJ are generated when changing to less pulses per burst.

To achieve a constant energy from pulse to pulse during the burst we present a technique based on the modulation of the laser diode current during one pulse. With this technique the gain variation during the burst was than 5% peak to peak.

S. Keppler, C. Wandt, M. Hornung, R. Bödefeld, A. Kessler, A. Sävert, M. Hellwing, F. Schorcht, J. Hein, and M. C. Kaluza
Multipass amplifiers of POLARIS
Proceedings of SPIE 8780, 87800I (2013)

Abstract: Advanced high intensity laser matter interaction experiments always call for optimized laser performance. In order to further enhance the POLARIS laser system, operational at the University of Jena and the Helmholtz-Institute Jena, in particular its energy, bandwidth and focusability, new amplifier technologies have been developed and are reported here. Additionally, existing sections were considerably improved. A new multi-pass amplification stage, which is able to replace two currently used ones, was developed in close collaboration with the MPQ (Garching). The new basic elements of this amplifier are well homogenized pump modules and the application of a successive imaging principle. By operating the amplifier under vacuum conditions a top hat beam profile with an output energy of up to 1.5 J per pulse is foreseen. The already implemented POLARIS amplifier A4 was further improved by adapting an advanced method for the homogenization of the multi-spot composed pump profile. The new method comprises a computer-based evolutionary algorithm which optimizes the position of the different spots regarding its individual size, shape and intensity. The latter allowed a better homogenization of the POLARIS near field profile.

M. Hornung, S. Keppler, R. Bödefeld, A. Kessler, H. Liebetrau, J. Körner, M. Hellwing, F. Schorcht, O. Jäckel, A. Sävert, J. Polz, A. K. Arunachalam, J. Hein, and M. C. Kaluza
High-intensity, high-contrast laser pulses generated from the fully diode-pumped Yb:glass laser system POLARIS
Optics Letters 38, 718 (2013)

Abstract: We report on the first generation of high-contrast, 164 fs duration pulses from the laser system POLARIS reaching focused peak intensities in excess of 2×10^20  W/cm2. To our knowledge, this is the highest peak intensity reported so far that has been achieved with a diode-pumped, solid-state laser. Several passive contrast enhancement techniques have been specially developed and implemented, achieving a relative prepulse intensity smaller than 10^−8 at t=−30  ps before the main pulse. Furthermore a closed-loop adaptive-optics system has been installed. Together with angular chirp compensation, this method has led to a significant reduction of the focal spot size and an increase of the peak intensity.

2012

J. Körner, C. Vorholt, H. Liebetrau, M. Kahle, D. Klöpfel, R. Seifert, J. Hein, and M. C. Kaluza
Measurement of temperature-dependent absorption and emission spectra of Yb:YAG, Yb:LuAG, and Yb:CaF_2 between 20 °C and 200 °C and predictions on their influence on laser performance
Journal of the Optical Society of America B 29, 2493 (2012)

Abstract: Accurate values of the emission and absorption cross sections of Yb:YAG, Yb:LuAG, and Yb:CaF_2 as a function of temperature between room temperature and 200 °C are presented. For this purpose, absorption and fluorescence spectra were measured using a setup optimized to reduce the effect of radiation trapping. From these data, emission cross sections were retrieved by combining the Fuchtbauer–Ladenburg equation and the reciprocity method. Based on our measurements, simple estimations illustrate the effect of temperature shifts that are likely to occur in typical laser setups. Our results show that even minor temperature variations can have significant impact on the laser performance using Yb:YAG and Yb:LuAG as an active medium, while Yb:CaF_2 appears to be rather insensitive.

S. Keppler, M. Hornung, R. Bödefeld, M. Kahle, J. Hein, and M. C. Kaluza
All-reflective, highly accurate polarization rotator for high-power short-pulse laser systems
Optics Express 20, 20742 (2012)

Abstract: We present the setup of a polarization rotating device and its adaption for high-power short-pulse laser systems. Compared to conventional halfwave plates, the all-reflective principle using three zero-phase shift mirrors provides a higher accuracy and a higher damage threshold. Since plan-parallel plates, e.g. these halfwave plates, generate postpulses, which could lead to the generation of prepulses during the subsequent laser chain, the presented device avoids parasitic pulses and is therefore the preferable alternative for high-contrast applications. Moreover the device is easily scalable for large beam diameters and its spectral reflectivity can be adjusted by an appropriate mirror coating to be well suited for ultra-short laser pulses.

2011

S. Keppler, R. Bödefeld, M. Hornung, A. Sävert, J. Hein, and M. C. Kaluza
Prepulse suppression in a multi-10-TW diode-pumped Yb:glass laser
Applied Physics B 104, 11 (2011)

Abstract: We describe a novel method to improve the temporal intensity contrast (TIC) between the main pulse and prepulses in a high-power chirped-pulse amplification (CPA) laser system. Pre- and post-pulses originating from the limited extinction ratio of the polarization gating equipment are suppressed by carefully adjusting the round-trip times of the regenerative amplifiers (RAs) with respect to the oscillator. As a result, leaking pulses from earlier or later round-trips in the RAs are hidden below the temporal shape of the main pulse. The synchronization can easily be controlled by a contrast measurement on a picosecond time scale using a third-order cross-correlator that enables a sub-mm precise adjustment of the cavity lengths. Finally, a method based on spectral interference is introduced that can be used for a fine-adjustment of the cavity lengths for the daily operation, making this new method easy to implement into existing laser systems.

S. Keppler, R. Bödefeld, M. Hornung, A. Sävert, J. Hein, and M. C. Kaluza
Contrast improvement by prepulse suppression in cascaded amplifier cavities
Proc. SPIE (2011)

Abstract: We introduce a method to suppress prepulses of pulse picking systems due to the limited extinction ratio of polarization gating systems. By matching the round trip times of the oscillator and the subsequent regenerative amplifiers, leaking pulses are hidden below the temporal intensity pedestal of the main pulse. With this method, prepulses at the temporal position equal to the time difference of the round trip times of the cavities could be suppressed completely.

2010

M. Hornung, R. Bödefeld, M. Siebold, A. Kessler, M. Schnepp, R. Wachs, A. Sävert, S. Podleska, S. Keppler, J. Hein, and M. C. Kaluza
Temporal pulse control of a multi-10 TW diode-pumped Yb:Glass laser
Applied Physics B 101, 93 (2010)

Abstract: At the Institute of Optics and Quantum Electronics in Jena, Germany, the currently most powerful diode-pumped solid-state laser system with 25-TW peak power Polaris is in operation. In this paper we give an overview about the dispersion management of the chirped pulse amplification in order to minimize the pulse duration and thus to maximize the intensity available for experiments. A detailed description of the stretcher and compressor design with a novel alignment routine is given as well as measurements for the pulse duration and the temporal contrast. The far field measurement of the beam focussed by an off-axis parabola yields a nearly diffraction limited focal spot.

M. Hornung, R. Bödefeld, A. Kessler, J. Hein, and M. C. Kaluza
Spectrally resolved and phase-sensitive far-field measurement for the coherent addition of laser pulses in a tiled grating compressor
Optics Letters 35, 2073 (2010)

Abstract: We describe a method that can be used for the coherent addition of laser pulses. As different laser pulses are initially generated in a laser-pulse compressor equipped with a tiled grating, such a coherent addition is indispens able in order to maximize the intensity in the laser far field. We present measurements in this context where, up to now, an unavoidable difference in the grating constants between the phased gratings reduced the maximum achievable intensity. The method significantly facilitates the high-precision alignment of a tiled grating compressor and could also be used for a coherent addition of laser pulses.

V. Bagnoud, J. Fils, J. Hein, M. C. Kaluza, G. G. Paulus, Th. Stöhlker, and M. Wolf
High‐Contrast Ultrafast OPA Module For The PHELIX Facility
AIP Conference Proceedings 1228, 217 (2010)

Abstract: In this paper, a new system for improvement of the pulse contrast in CPA laser systems by use of an ultrafast OPA is reviewed together with a scheme to create sub‐picosecond synchronized OPA pump pulses. The scheme is being implemented at the PHELIX facility at GSI‐Darmstadt, Germany.

J. Hein, M. Hornung, R. Bödefeld, S. Podleska, A. Sävert, R. Wachs, A. Kessler, S. Keppler, M. Wolf, J. Polz, O. Jäckel, M. Nicolai, M. Schnepp, J. Körner, M. C. Kaluza, and G. G. Paulus
Multiterawatt peak power generated by the all diode pumped laser—POLARIS
AIP Conference Proceedings 1228, 159 (2010)

Abstract: At the Institute of Optics and Quantum Electronics, University of Jena, a fully diode pumped ultrahigh peak power laser system—POLARIS—has been realized. Presently, this laser system reaches a peak power of some ten terawatt. The last amplifier, which will boost the output energy to the 100 J level, is nearly completed and will be soon commissioned. The applied technologies and the basic design are reviewed here.