Kerr-Lens Mode-Locked High-Power Thin-Disk Oscillators

Femtosecond Kerr-lens mode-locked thin-disk oscillators constitute a peak- and average power scalable oscillator concept. Over last several years, they were developed directly to provide unprecedentedly high average and peak power levels of more than 200 W and more than 50 MW, respectively—the parameter range of more complex amplification systems. These developments were accompanied by many challenges, including the initiation of mode-locking, thermal lensing and the oscillator stability. These challenges were successfully overcome, resulting in a better understanding of power scaling of this technology. We offer an overview over these diverse aspects and show that this technology has a very bright future not only for further power scaling but also in terms of applications. In particular, this type of oscillator can enable a novel class of compact, table-top powerful extreme-ultraviolet and infrared radiation sources paving the way towards new spectroscopic applications

Towards a compact thin-disk-based femtosecond XUV source

The goal of this thesis is to develop a compact high-power solid-state oscillator capable of superseding existing ultrafast technology based on low-power Ti:sapphire oscillators. Different applications such as extra- or intra-cavity XUV generation, seeding of high-energy low-repetition-rate amplifier systems and femtosecond enhancement cavities can be dramatically influenced by the availability of such a reliable, compact femtosecond source. We applied, for the first time, Kerr-lens mode-locking to a thin-disk Yb:YAG oscillator, resulting in an unprecedented combination of an average power 45 W and pulse duration of 250 fs directly available from the oscillator with repetition rate of 40 MHz and a footprint of only 1*0.4 m^2. Even shorter emission-bandwidth-limited 200-fs pulses have been generated with the reduced output coupler transmission of 5.5% at an average power of 17 W. Moreover, the oscillator was operating not only in the negative dispersion regime common to solid-state oscillators but also in the positive dispersion regime, resulting in a spectrum spanning a range of 20 nm, which is the broadest hitherto reported for Yb:YAG material in high-power operation. First attempts towards CE phase-stabilized high-power pulses from such an oscillator are also described. State-of-the-art XUV generation driven by high-power NIR femtosecond systems requires methods of separating generated XUV from NIR radiation. Such a method has been proposed and realized. It constitutes a glass substrate having a low-loss anti-reflection coating for NIR wavelengths at grazing incidence of >70° and serving simultaneously as a high reflector for radiation in the range of 1-100 nm with reflectivity >60%. The device can be used for both extra- and intra-cavity XUV generation.Das Ziel dieser Arbeit ist die Entwicklung eines kompakten Hochleistungs-Laseroszillators auf Festkörperbasis. Dieser soll in der Lage sein, die schon existierende ultraschnelle Technologie der Oszillatoren mit niedriger Leistung, basierend auf Ti:Saphir, zu ersetzen. Verschiedene Anwendungen, wie resonatorinterne und -externe Erzeugung von XUV-Strahlung, das Seeden von Hochenergie-Verstärkersystemen mit niedrigen Repetitionsraten und Femtosekunden-Überhöhungsresonatoren, können durch eine solche zuverlässige und kompakte Quelle von Femtosekundenpulsen wesentlich effektiver werden. Wir haben Kerrlinsen-Modenkopplung erstmalig auf einen Yb:YAG-Dünnscheibenoszillator angewandt und eine bisher unerreichte Kombination von 45 W mittlerer Leistung und 250 fs Pulsdauer bei 40 MHz Repetitionsrate direkt vom Oszillator erreicht. Die Grundfläche des Lasers war nur 1*0.4 m^2. Noch kürzere, durch die Emissionsbandbreite begrenzte Pulse mit 200 fs konnten mit reduzierter Auskopplertransmission von 5,5% bei 17 W mittlerer Leistung erzeugt werden. Darüber hinaus lief der Oszillator nicht nur im für Festkörperoszillatoren üblichen Regime negativer Dispersion, sondern auch im Regime positiver Dispersion mit einem 20 nm breiten Spektrum - dem bisher breitesten für Yb:YAG im Hochleistungsbetrieb. Erste Anläufe zu CE-phasenstabilisierten Pulsen hoher Leistung aus einem derartigen Oszillator werden ebenfalls beschrieben. Moderne Ansätze zur XUV-Erzeugung mit NIR-Femtosekundensystemen als Treiber erfordern Methoden zur Trennung der XUV- von der NIR-Strahlung. Eine solche Methode wurde vorgestellt und umgesetzt. Sie verwendet ein Glassubstrat mit Antireflektionsbeschichtung für NIR-Wellenlängen unter einem Grazing-Einfallswinkel von >70°, das gleichzeitig als hoch-reflektierender Spiegel für Strahlung im Bereich 1-100 nm mit einer Reflektivität >60% dient. Diese optische Komponente kann sowohl für resonatorexterne, wie auch -interne XUV-Erzeugung genutzt werden

110 MW Thin-Disk Oscillator

A compact Kerr-lens mode-locked thin-disk oscillator delivering 110 MW output peak power, the highest among all oscillators, is reported. A pulse train with a repetition rate of 14 MHz carries 115 fs long, 14.4 uJ pulses resulting in 202 W of average power. This compact, simple, and stable oscillator is a suitable driver and an important milestone for further high harmonics generation and the development of extreme ultraviolet transportable frequency comb sources.Comment: 14 pages, 4 figure

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator

Lasers based on Cr$^{2+}$-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with super-octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr$^{2+}$-doped II-VI oscillator pumped by a single InP diode, providing average powers of over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 $\mu$m. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step towards a more accessible alternative to synchrotron-like infrared radiation, and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.Comment: 8 pages, 5 figure

Free-space quasi-phase matching

We report a new approach to phase matching of nonlinear materials based on the free space multipass cells. This concept quasi-phase matches crystalline quartz and increases the second harmonic generation efficiency by a factor 40

High photon flux table-top coherent extreme ultraviolet source

High harmonic generation (HHG) enables extreme ultraviolet radiation with table-top setups. Its exceptional properties, such as coherence and (sub)-femtosecond pulse durations, have led to a diversity of applications. Some of these require a high photon flux and megahertz repetition rates, e.g. to avoid space charge effects in photoelectron spectroscopy. To date this has only been achieved with enhancement cavities. Here, we establish a novel route towards powerful HHG sources. By achieving phase-matched HHG of a megahertz fibre laser we generate a broad plateau (25 eV - 40 eV) of strong harmonics, each containing more than $10^{12}$ photons/s, which constitutes an increase by more than one order of magnitude in that wavelength range. The strongest harmonic (H25, 30 eV) has an average power of 143 $\mu$W ($3\cdot10^{13}$ photons/s). This concept will greatly advance and facilitate applications in photoelectron or coincidence spectroscopy, coherent diffractive imaging or (multidimensional) surface science

Efficient High-Power Ultrashort Pulse Compression in Self-Defocusing Bulk Media

Peak and average power scalability is the key feature of advancing femtosecond laser technology. Today, near-infrared light sources are capable of providing hundreds of Watts of average power. These sources, however, scarcely deliver pulses shorter than 100fs which are, for instance, highly beneficial for frequency conversion to the extreme ultraviolet or to the mid-infrared. Therefore, the development of power scalable pulse compression schemes is still an ongoing quest. This article presents the compression of 90 W average power, 190 fs pulses to 70 W, 30 fs. An increase in peak power from 18 MW to 60 MW is achieved. The compression scheme is based on cascaded phase-mismatched quadratic nonlinearities in BBO crystals. In addition to the experimental results, simulations are presented which compare spatially resolved spectra of pulses spectrally broadened in self-focusing and self-defocusing media, respectively. It is demonstrated that balancing self-defocusing and Gaussian beam convergence results in an efficient, power-scalable spectral broadening mechanism in bulk material

Multi-watt, multi-octave, mid-infrared femtosecond source

Spectroscopy in the wavelength range from 2 to 11 mu m (900 to 5000 cm(-1)) implies a multitude of applications in fundamental physics, chemistry, as well as environmental and life sciences. The related vibrational transitions, which all infrared-active small molecules, the most common functional groups, as well as biomolecules like proteins, lipids, nucleic acids, and carbohydrates exhibit, reveal information about molecular structure and composition. However, light sources and detectors in the mid-infrared have been inferior to those in the visible or near-infrared, in terms of power, bandwidth, and sensitivity, severely limiting the performance of infrared experimental techniques. This article demonstrates the generation of femtosecond radiation with up to 5 W at 4.1 mu m and 1.3 W at 8.5 mu m, corresponding to an order-of-magnitude average power increase for ultrafast light sources operating at wavelengths longer than 5 mu m. The presented concept is based on power-scalable near-infrared lasers emitting at a wavelength near 1 mu m, which pump optical parametric amplifiers. In addition, both wavelength tunability and supercontinuum generation are reported, resulting in spectral coverage from 1.6 to 10.2 mu m with power densities exceeding state-of-the-art synchrotron sources over the entire range. The flexible frequency conversion scheme is highly attractive for both up-conversion and frequency comb spectroscopy, as well as for a variety of time-domain applications