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New frontiers of ultrafast thin-disk laser oscillators for applications in metrology and the generation of THz and XUV radiation

2019, Modsching, Norbert, Südmeyer, Thomas

Les systèmes laser ultra brefs sont des outils versatiles ayant des applications dans l’industrie et la recherche scientifique. Ceux basés sue le Ti:sapphire fonctionnent généralement à des taux de répétition autour du kHz et bénéficient de la large bande spectrale d’amplification disponible pour la génération directe de durée d’impulsions inférieure à 100 fs. Cependant, les effets thermiques délétères dans le milieu amplificateur limitent leur puissance moyenne à quelques watts. De nombreuses d’applications bénéficieraient d’être pilotées à des taux de répétition de l’ordre du MHz en utilisant des systèmes laser simples et puissants. Les systèmes laser ultra brefs basés sur des milieux amplificateurs dopés aux ions ytterbium ont vu une augmentation drastique sans précédent de leur puissance moyenne au cours des dernières décennies. Ces immenses progrès ont été obtenus grâce au faible défaut quantique de ces milieux, combiné à des concepts novateurs utilisant des géométries de milieu amplificateur optimisées pour une extraction de chaleur efficace (fibres optiques, slab, disques minces). Ces systèmes sont habituellement basés sur des architectures présentant plusieurs étages d’amplification et atteignent aujourd’hui des puissances moyennes de l’ordre du kilowatt à des taux de répétition autour du MHz. Cependant, la bande spectrale d’amplification limitée des milieux amplificateurs dopés aux ions ytterbium entraine la génération d’impulsion ayant des durées de plusieurs centaines de femtosecondes. De plus, la post-compression nonlinéaire de ces impulsions devient nécessaire afin d’obtenir des durées d’impulsions inférieures à 100 fs requises par un grand nombre d’applications.
Les études présentées dans cette thèse portent sur le potentiel des oscillateurs lasers à base de disque mince (TDL) puissants et ultra brefs pour la génération d’impulsions de durée inférieure à 100 fs. La géométrie des disques minces a déjà prouvé qu’elle était adaptée à ces besoins en délivrant la plus grande puissance moyenne de toutes les technologies d’oscillateurs laser ultra brefs. À terme, le développement de puissants oscillateurs TDL délivrant des durées d’impulsions inférieures à 100 fs peut surmonter la nécessité d’amplifications et de post-compression nonlinaire additionnelles. De ce fait, en tant que système laser simple et mono étage délivrant des impulsions solitoniques limitées par transformée de Fourier sans piédestal pré ou post impulsions avec une excellente qualité de faisceau et un potentiel d’augmentation de puissance, ces sources sont extrêmement attractives pour un grand nombre d’applications.
L’ingrédient clé des oscillateurs TDL délivrant des impulsions inférieures à 100 fs présentés dans cette thèse réside dans le blocage de mode par effet Kerr combiné à l’utilisation de milieux amplificateurs dopés aux ions ytterbium présentant des bandes spectrale d’amplification plus large que l’Yb:YAG. Un oscillateur laser en de blocage de mode par effet Kerr (KLM) fonctionnant en régime de forte automodulation de phase intra-cavité basé sur l’Yb:Lu2O3 a permis la génération de durées d’impulsions plus courte que celles supportées par la bande spectrale d’amplification correspondante. Ddifférentes configurations ont permis la génération d’impulsions d’une durée de 49 fs avec une puissance moyenne de 4.5 W d’une part et de 95 fs avec 21 W d’autre part. Ces performances représentent les plus importantes puissances moyennes obtenues jusqu’ici par des oscillateurs laser délivrant respectivement des impulsions de durée inférieure à 50 fs et 100 fs. Comparé aux résultats publiés avant cette thèse, la puissance moyenne des oscillateurs TDL a été multipliée par un facteur 4. En utilisant la bande spectrale d’amplification très large de l’Yb :CALGO, des impulsions de 30 fs ont été générées par le premier oscillateur TDL en régime de blocage de mode par effet Kerr basé sur ce milieu amplificateur. À ce jour, ce résultat représente à plus courte durée d’impulsions obtenue par un oscillateur TDL et est 40% plus courte que les durées d’impulsions générées par des oscillateurs TDL avant cette thèse.
La capacité des oscillateurs TDL développés à être utilisé dans des applications en métrologie comme peigne de fréquence optique est démontrée. Le décalage de la fréquence enveloppe-porteuse est détecté et stabilisé avec un asservissement tight de la phase. Une étude préliminaire étudie la stabilisation complète du peigne de fréquence grace à une stabilisation parallèle du taux de répétition.
Les sources laser développées ont également permis la génération de radiation THz de large bande spectrale, pour la première fois par un oscillateur TDL ultra bref. En utilisant le processus de rectification electro-optique in GaP, un spectre optique s’étendant jusqu’à 7 THz a été généré. La génération de THz avec des impulsions de 95 fs à 20 W de puissance moyenne a produit 0.3 mW de puissance moyenne THz avec un spectre optique allant jusqu'à 5 THz. Le spectre d'absorption de la vapeur d'eau a été mesuré avec une résolution inférieure à 20 GHz comme référence pour les applications de spectroscopie linéaire dans le domaine temporel en THz. En outre, une procédure est présentée permettant d'estimer le spectre THz à l'aide de facteurs multiplicatifs dans le domaine de Fourier. Moreover, a procedure is presented allowing to estimate the THz spectrum via multiplicative factors in the frequency domain.
De plus, la première réalisation expérimentale de génération d'harmoniques d’ordres élevés à l'intérieur de la cavité d'un oscillateur TDL à blocage de mode par SESAM est présentée. Ce ouvre la voie à une nouvelle classe de sources de lumière dans l’extrême ultraviolet à un étage fonctionnant à des taux de répétition de l’ordre du MHz., Ultrafast laser systems find application as versatile tool in industry and science. Ultrafast laser systems based on Ti:sapphire – the workhorse in ultrafast science – operate typically at kHz repetition rates and benefit from the available broad gain bandwidth for the direct generation of sub 100-fs pulse durations. However, thermal effects in the gain medium due to a high quantum defect limit their average power to several watt. Many applications would benefit to be driven at MHz repetition rates by simple and powerful laser systems. Ultrafast laser systems based Yb-doped gain materials have experienced an unprecedented scaling of the average power over the last decades. Nowadays, these systems reach the kilowatt level of average power while operating at MHz repetition rates. The tremendous progress was achieved by laser operation at a reduced quantum defect in combination with novel concepts utilizing optimized geometries of the laser gain medium for efficient heat extraction (fiber, slab, thin disk). In general, two basic design approaches for high-power ultrafast laser systems can be distinguished. Either they are based on a low-power master oscillator as seed source followed by multiple amplification stages, or they are directly based on high-power laser oscillators. However, the limited gain bandwidth of Yb-doped gain materials results typically in the generation of pulses with several hundred femtoseconds of duration. Additional nonlinear pulse compression becomes necessary in order to reach the required sub-100-fs pulse durations for many applications. This adds, on the one hand, complexity into the driving laser systems and can, on the other hand, degrade the temporal and spatial quality of the generated pulse train.
This thesis investigates the potential of ultrafast thin-disk laser (TDL) oscillators for laser operation directly in the sub-100-fs pulse duration regime at high average powers. The thin-disk geometry has already proven to be suitable to operate at the highest average power of any ultrafast laser oscillator technology. The development of powerful sub-100-fs TDL oscillators can ultimately overcome the need for amplification and nonlinear pulse compression. By that, these sources are highly attractive as simple single-stage driving laser systems for many applications, featuring transform-limited soliton pulses at excellent beam quality without pre- or post-pulses and potential for further power-scaling.
The key ingredient of the presented sub-100-fs TDL oscillators is the combination of the Kerr lens mode-locking scheme with Yb-doped gain materials of broader gain bandwidth than Yb:YAG. Laser operation of a Kerr lens mode-locked (KLM) Yb:Lu2O3 TDL oscillator in the regime of strong intracavity self-phase modulation (SPM) enabled the generation of shorter pulse durations than directly supported by the corresponding gain bandwidth. The expansion of the optical spectrum up to three times beyond the gain bandwidth resulted in the generation of 35-fs pulses at 1.6 W of average power. In a modified laser configuration slightly longer pulse duration of 49 fs were generated at 4.5 W. Increased round-trip gain by folding the laser cavity a second time on the disk resulted in laser operation with 95 fs pulses at 21 W. These are the highest average powers so far achieved by a sub-50-fs and sub-100-fs laser oscillator, respectively. Compared to the results achieved prior to this thesis, the average power of sub-100 fs TDL oscillators has been increased by a factor of 4. By utilizing the very broad gain bandwidth of Yb:CALGO even shorter pulse durations of 30 fs are generated by the first KLM Yb:CALGO TDL oscillator. This is the shortest pulse duration so far achieved by a TDL oscillator and 40% shorter compared to the pulse durations of TDL oscillators achieved prior to this thesis. At that time, this pulse duration was equal to the shortest one achieved by Yb-based bulk oscillators. The presented results demonstrate that the thin-disk geometry is as well suitable for the generation of shortest pulse durations.
The suitability of laser operation in the strongly SPM-broadened regime for applications in metrology as optical frequency comb is investigated by detecting and stabilizing the carrier envelope offset frequency (ƒCEO). In the presented configuration the KLM Yb:Lu2O3 TDL oscillator generated 50-fs pulses at 4.4 W of average power. For ƒCEO detection in a standard ƒ-to-2ƒ interferometer, the short pulse duration enabled the required coherent super continuum generation in a photonic crystal fiber without the need for adjacent nonlinear pulse compression. Only a minor fraction of the available average power was required for ƒCEO detection while the major part remained available for application. In the presented experiment the ƒCEO was stabilized to an external radio-frequency reference up to a bandwidth of 10 kHz by applying an active feedback to the current of the pump diode. A tight phase lock was achieved with a residual in-loop integrated phase noise of 197 mrad (integrated from 1 Hz to 1 MHz). This is the shortest pulse duration and the highest average power of an Yb-based laser oscillator that has been so far ƒCEO stabilized without the need for additional amplification or nonlinear pulse compression. Additionally, a preliminary study presents investigations for full frequency comb stabilization by parallel stabilization of the repetition rate (ƒrep). Stabilization of the ƒrep to an external radio-frequency reference was enabled by mounting one folding cavity mirror onto a controlled piezoelectric transducer. Simultaneous stabilization of ƒCEO and ƒrep was achieved in the range of minutes. Cross-talking between both stabilization loops required to limit the ƒrep stabilization to a bandwidth of 35 Hz while the integrated residual ƒCEO phase noise increased to 745 mrad.
Another application of the developed sub-100-fs KLM TDL oscillators as single-stage driving laser system is presented by driving directly the generation and detection of broadband THz radiation. In an initial configuration broadband THz radiation was generated via optical rectification in GaP utilizing the output of a KLM Yb:Lu2O3 TDL oscillator operating with 50-fs pulses at 4 W of average power. The generated THz spectrum was centered around 3.4 THz and extended from below 1 THz to nearly 7 THz. A spectroscopic characterization of a GaP crystal enabled phase matching calculation which are in good agreement with the observed features in the THz spectrum. As benchmark for applications in linear THz time-domain spectroscopy the absorption spectrum of water vapor was measured with a sub-50-GHz resolution. A presented second study investigates in more detail the impact of the GaP crystal thickness and the driving pulse duration on the generated THz spectrum. Modification of the driving TDL oscillator enabled high-fidelity measurements at discrete pulse durations ranging from 50 fs to 220 fs. Based on the experimental results a procedure is presented allowing to estimate the THz spectrum via multiplicative factors in the frequency domain. Driving THz generation with 95-fs pulses at 20 W of average power enabled the generation of 0.3 mW of THz average power with a spectrum extending up to 5 THz.
Additionally, the first experimental realization of high harmonic generation (HHG) inside the cavity of a mode-locked TDL oscillator is presented. In this proof-of-principle experiment HHG is driven by launching a high-pressure xenon gas jet into the 12 μm radius intracavity focus of a SESAM mode-locked TDL oscillator. The laser operated with a pulse duration of 255 fs at 64 MW of intracavity peak power and 320 W of intracavity average power. At an intracavity peak intensity of ∼2.8 × 1013 W∕cm2 up to the 17th harmonic (61 nm, 20 eV) has been detected. A generated photon flux of 2.6 × 108 photons/s was estimated for the 11th harmonic (94 nm, 13.2 eV). Launching the gas jet into the laser cavity did not disturb the mode-locked operation and noise performance. Further improvement can be expected by utilizing the Kerr lens mode-locking scheme with laser operation in the strongly SPM-broadened regime to reach shorter pulse durations in combination with cavity optimization towards higher intracavity peak powers. The prior presented investigations of ƒCEO and ƒrep stabilization anticipate that full frequency comb stabilization is feasible. As such, TDL driven intra-oscillator HHG is a promising approach for the development of single-stage extreme ultraviolet frequency combs.

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Generation of 35-fs pulses from a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser

, Paradis, Clément, Modsching, Norbert, Wittwer, Valentin J, Deppe, Bastian, Kränkel, Christian, Südmeyer, Thomas

We investigate Kerr lens mode locking of Yb:Lu2O3 thin-disk laser oscillators operating in the sub-100-fs regime. Pulses as short as 35 fs were generated at an average output power of 1.6 W. These are the shortest pulses directly emitted from a thin-disk laser oscillator. The optical spectrum of the 35-fs pulses is almost 3 times broader than the corresponding emission band of the gain crystal. At slightly longer pulse duration of 49 fs, we achieve an average power of 4.5 W. In addition, 10.7 W are obtained in 88-fs pulses, which is twice higher than the previous power record for ultrafast thin-disk lasers generating pulses shorter than 100 fs. Our results prove that Kerr lens mode-locked Yb:Lu2O3 thin-disk lasers are a promising technology for further average power and pulse energy scaling of ultrafast high-power oscillators operating in the sub-100-fs regime.

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Carrier-Envelope Offset Frequency Stabilization of a Thin-Disk Laser Oscillator via Depletion Modulation

, Andrade, José R. C, Modsching, Norbert, Tajalli, Ayhan, Dietrich, Christian M, Kleinert, Sven, Placzek, Fabian, Kreipe, Bernhard, Schilt, Stephane, Wittwer, Valentin J, Südmeyer, Thomas, Morgner, Uwe

We present a novel concept for the stabilization of the carrier-envelope offset (CEO) frequency of femtosecond pulse trains from thin-disk laser oscillators by exploiting gain depletion modulation in the active gain region. We shine a small fraction of the laser output power back onto the thin disk allowing the population inversion in the gain medium to be controlled. We employ this technique in our home-built Kerr-lens mode-locked Yb:YAG thin-disk laser and benchmark the performance against the proven technique of pump current modulation for CEO stabilization, showing that the two techniques have equivalent performance. The new method which only requires an additional AOM demonstrates a scalable and cost-effective method for CEO stabilization of high-power laser oscillators.

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Carrier-envelope offset frequency stabilization of a thin-disk laser oscillator operating in the strongly self-phase modulation broadened regime

, Modsching, Norbert, Paradis, Clément, Brochard, Pierre, Jornod, Nayara, Gürel, Kutan, Kränkel, Christian, Schilt, Stephane, Wittwer, Valentin J, Südmeyer, Thomas

We demonstrate the carrier-envelope offset (CEO) frequency stabilization of a Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating in the strongly self-phase modulation (SPM) broadened regime. This novel approach allows overcoming the intrinsic gain bandwidth limit and is suited to support frequency combs from sub-100-fs pulse trains with very high output power. In this work, strong intra-oscillator SPM in the Kerr medium enables the optical spectrum of the oscillating pulse to exceed the bandwidth of the gain material Yb: Lu2O3 by a factor of two. This results in the direct generation of 50-fs pulses without the need for external pulse compression. The oscillator delivers an average power of 4.4 W at a repetition rate of 61 MHz. We investigated the cavity dynamics in this regime by characterizing the transfer function of the laser output power for pump power modulation, both in continuous-wave and mode-locked operations. The cavity dynamics in mode-locked operation limit the CEO modulation bandwidth to ~10 kHz. This value is sufficient to achieve a tight phase-lock of the CEO beat via active feedback to the pump current and yields a residual in-loop integrated CEO phase noise of 197 mrad integrated from 1 Hz to 1 MHz.

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Optical rectification of ultrafast Yb lasers: pushing power and bandwidth of terahertz generation in GaP

, Drs, Jakub, Modsching, Norbert, Paradis, Clément, Kränkel, Christian, Wittwer, Valentin Johannes, Razskazovskaya, Olga, Südmeyer, Thomas

We demonstrate broadband high-power terahertz (THz) generation at megahertz repetition rates by optical rectification in GaP driven by an ultrafast Yb-based thin-disk laser (TDL) oscillator. We investigate the influence of pulse duration in the range of 50–220 fs and thickness of the GaP crystal on the THz generation. Optimization of these parameters with respect to the broadest spectral bandwidth yields a gap-less THz spectrum extending to nearly 7 THz. We further tailor the driving laser and the THz generation parameters for the highest average power, demonstrating 0.3 mW THz radiation with a spectrum extending to 5 THz. This was achieved using a 0.5 mm thick GaP crystal pumped with a 95 fs, 20 W TDL, operating at 48 MHz repetition rate. We also provide a simple method to estimate the THz spectrum, which can be used for design and optimization of similar THz systems.

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Sub-100-fs Kerr lens mode-locked Yb:Lu2O3 thin-disk laser oscillator operating at 21 W average power

, Modsching, Norbert, Drs, Jakub, Fischer, Julien, Paradis, Clément, Labaye, François, Gaponenko, Maxim, Kränkel, Christian, Wittwer, Valentin J, Südmeyer, Thomas

We investigate power-scaling of a Kerr lens mode-locked (KLM) Yb:Lu2O3 thin-disk laser (TDL) oscillator operating in the sub-100-fs pulse duration regime. Employing a scheme with higher round-trip gain by increasing the number of passes through the thin-disk gain element, we increase the average power by a factor of two and the optical-to-optical efficiency by a factor of almost three compared to our previous sub-100-fs mode-locking results. The oscillator generates pulses with a duration of 95 fs at 21.1 W average power and 47.9 MHz repetition rate. We discuss the cavity design for continuous-wave and mode-locked operation and the estimation of the focal length of the Kerr lens. Unlike to usual KLM TDL oscillators, an operation at the edge of the stability zone in continuous-wave operation is not required. This work shows that KLM TDL oscillators based on the gain material Yb:Lu2O3 are an excellent choice for power-scaling of laser oscillators in the sub-100-fs regime, and we expect that such lasers will soon operate at power levels in excess of hundred watts.

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Kerr lens mode-locked Yb:CALGO thin-disk laser

, Modsching, Norbert, Paradis, Clément, Labaye, François, Gaponenko, Maxim, Graumann, Ivan J, Diebold, Andreas, Emaury, Florian, Wittwer, Valentin J, Südmeyer, Thomas

We demonstrate the first Kerr lens mode-locked Yb:CaGdAlO4 (Yb:CALGO) thin-disk laser oscillator. It generates pulses with a duration of 30 fs at a central wavelength of 1048 nm and a repetition rate of 124 MHz. The laser emits the shortest pulses generated by a thin-disk laser oscillator, equal to the shortest pulse duration obtained by Yb-doped bulk oscillators. The average output power is currently limited to 150 mW by the low gain and limited disk quality. We expect that more suitable Yb:CALGO disks will enable substantially higher power levels with similar pulse durations.

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Broadband terahertz pulse generation driven by an ultrafast thin-disk laser oscillator

, Clément, Paradis, Drs, Jakub, Modsching, Norbert, Razskazovskaya, Olga, Meyer, Frank, Kränkel, Christian, Saraceno, Clara J, Wittwer, Valentin J, Südmeyer, Thomas

We demonstrate broadband THz generation driven by an ultrafast thin-disk laser (TDL) oscillator. By optical rectification of 50-fs pulses at 61 MHz repetition rate in a collinear geometry in crystalline GaP, THz radiation with a central frequency at around 3.4 THz and a spectrum extending from below 1 THz to nearly 7 THz are generated. We realized a spectroscopic characterization of a GaP crystal and a benchmark measurement of the water-vapor absorption spectrum in the THz range. Sub-50-GHz resolution is achieved within a 5 THz bandwidth. Our experiments show the potential of ultrafast TDL oscillators for driving MHz-repetition-rate broadband THz systems.

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XUV Sources Based on Intra-Oscillator High Harmonic Generation with Thin-Disk Lasers: Current Status and Prospects

, Labaye, François, Gaponenko, Maxim, Modsching, Norbert, Brochard, Pierre, Paradis, Clément, Schilt, Stephane, Wittwer, Valentin Johannes, Südmeyer, Thomas

Ultrafast thin-disk laser (TDL) oscillators provide higher intracavity pulse energy, average power, and peak power levels than any other femtosecond laser oscillator technology. They are suitable for driving extreme nonlinear interactions directly inside the laser oscillator. High harmonic generation (HHG) driven inside ultrafast TDL oscillators is a very recent approach for the generation of coherent extreme ultraviolet (XUV) light at multi-megahertz repetition rates. In this paper, we review the current state of the development, discuss the technological potential, and give an outlook toward the future developments. We compare the current performance to established technologies and evalu-ate possible limitations. We discuss future improvements, such as reduction of the driving pulse duration and increase of the intracavity peak power, efficient extraction of the XUV light from the cavity, and carrier-envelope offset frequency stabilization of the generated XUV light. Due to the power scalability of the TDL concept and the possibility to operate in a spectrally broadened regime with pulse durations below the gain bandwidth limitation, intra-oscillator HHG with TDLs has a high potential for powerful table-top multi-megahertz coherent XUV light sources for science and applications.