Transition between advection and inertial wave propagation in rotating turbulence

In turbulent flows subject to strong background rotation, the advective mechanisms of turbulence are superseded by the propagation of inertial waves, as the effects of rotation become dominant. While this mechanism has been identified experimentally (Dickinson & Long, J. Fluid Mech., vol. 126, 1983, pp. 315–333; Davidson, Staplehurst & Dalziel, J. Fluid Mech., vol. 557, 2006, pp. 135–144; Staplehurst, Davidson & Dalziel, J. Fluid Mech., vol. 598, 2008, pp. 81–105; Kolvin et al.Phys. Rev. Lett., vol. 102, 2009, 014503), the conditions of the transition between the two mechanisms are less clear. We tackle this question experimentally by tracking the turbulent front away from a solid wall where jets enter an otherwise quiescent fluid. Without background rotation, this apparatus generates a turbulent front whose displacement recovers the law classically obtained with an oscillating grid (Dickinson & Long, Phys. Fluids, vol. 21 (10), 1978, pp. 1698–1701) and we further establish the scale independence of the associated transport mechanism. When the apparatus is rotating at a constant velocity perpendicular to the wall where fluid is injected, not only does the turbulent front become mainly transported by inertial waves, but advection itself is suppressed because of the local deficit of momentum incurred by the propagation of these waves. Scale-by-scale analysis of the displacement of the turbulent front reveals that the transition between advection and propagation is local both in space and spectrally, and takes place when the Rossby number based on the considered scale is of order unity, or equivalently, when the scale-dependent group velocity of inertial waves matched the local advection velocity

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

High-power femtosecond laser-oscillators for applications in high-field physics

In dieser Doktorarbeit werden experimentelle Anstrengungen zur Entwicklung eines kompakten Laseroszillators für Femtosekundenimpulse mit hoher Durchschnitts- und Spitzenleistung beschrieben. Dabei zielt dieser Laser auf neuartige Anwendung in der Spektroskopie und Hochfeldphysik ab, insbesondere dem Antreiben von ineffizienten Frequenzkonversionsprozessen wie der Erzeugung von mittlerer Infrarot- und extrem ultravioletter Strahlung. Die entwickelten Strahlquellen bestehen dabei aus einem kerrlinsenmodengekoppelten Hochleistungsscheibenlaser auf Basis von Yb:YAG mit mehreren MHz Wiederholrate und einer anschließenden Impulskompressionsstufe aus massiven Festkörpern. Es wird aufgezeigt, dass Kerrlinsenmodenkopplung sowohl durchschnitts-, als auch spitzenleistungsskalierbar ist und die derzeit einzige Methode zur Modenkopplung, die simultan die effiziente Ausbeute der gesamten Verstärkungsbandbreite des Verstärkungsmediums zulässt. Impulse mit mehr als 60 MW Spitzenleistung und hunderten Watt an Durchschnittsleistung können direkt am Oszillatorausgang erreicht werden mit Impulslängen bis hinab zu 140 fs. Der Hochleistungsoutput des Oszillators wurde in massiven Festkörpermaterialien spektral verbreitert, um die Durchführbarkeit eines effizienten, kompakten und robusten Impulskompressors auszuloten, der sich nicht auf justageempfindliche Fasern verlassen muss. Gestützt durch frühere Arbeiten, sowie neue Experimente, als auch Simulationen konnte festgestellt werden, dass die Nachahmung eines nichtlinearen Quasiwellenleiters zu außerordentlich hoher Effizienz im Durchsatz führen und dabei herausragende Komprimierbarkeit über dem gesamten Strahl sicherstellen kann. Die Impulse mit 60 MW Spitzen- und 150 W Durchschnittsleistung aus einem der entwickelten Oszillatoren wurden in einem sehr kompakten Quasiwellenleiter spektral verbreitert und anschließend mit gechirpten Spiegeln auf 30 fs komprimiert. Durch die hohe Transmission des gesamten Aufbaus von 95 % wurde die Spitzenleistung auf 230 MW hochgetrieben. Simulationen zeigen die Umsetzbarkeit eines Kompressors auf Basis dieser Wellenleiter mit Pulsdauern, die mit 10 fs bis in den Bereich weniger optischer Schwingungszyklen hineinreichen. Untersucht wurde ebenfalls ein anderer Ansatz zur spektralen Verbreiterung, der auf kaskadierten χ(2) Nichtlinearitäten während der Erzeugung der zweiten Harmonischen in BBO mit fehlangepasster Phase beruht. Obgleich die Effizienz nicht vergleichbar mit der des Wellenleiteransatzes ist, machen ihn die faszinierende Möglichkeit zu defokussierenden Phasenschüben und Selbstkompression im Kristall zu einem interessanten Ausgangspunkt für sehr kompakte Impulskompressionsaufbauten. Das Zusammenspiel dieser Entwicklungen zeigt die Realisierbarkeit von unverstärkten, einfachen und kompakten Laserquellen auf, die komplexere und preisintensive Yb- oder Ti:Safir Verstärkersysteme ersetzen können.This thesis describes experimental work in the development of a compact, high average and peak-power femtosecond oscillator. This laser targets new applications in spectroscopy and high-field physics, especially the driving of inefficient frequency-conversion-processes like the generation of mid-infrared and extreme ultraviolet radiation. The developed sources consist of a high-power Kerr-lens mode-locked Yb:YAG thin-disk oscillator with multi-MHz repetition rate and a subsequent all-bulk pulse-compression stage. It is shown that Kerr-lens mode-locking is both average and peak-power scalable and is currently the only mode-locking technique that at the same time allows the efficient use of the full gain-bandwidth of the amplifying medium. Pulses with more than 60 MW peak-power and hundreds of watts in average power can be reached as direct oscillator output with down to 140 fs pulse-duration. The high-power output from the oscillators was spectrally broadened in bulk solids to explore the feasibility of an efficient, compact and robust pulse-compressor that does not have to rely on alignment-sensitive fibers. Leaning on previous work as well as new experiments and computer simulations it was found that the emulation of a nonlinear quasi-waveguide can yield exceptionally high throughput efficiency, while retaining excellent whole-beam compressibility. The 60 MW peak- and 150 W average power pulses from a developed oscillator were spectrally broadened in a very compact quasi-waveguide and subsequently compressed with chirped mirrors to 30 fs pulse duration. By virtue of the 95 % transmission of the whole setup the peak-power was boosted to 230 MW. Simulations show the feasibility of a waveguide based compressor with down to 10 fs pulse duration into the few-optical-cycle regime. A different approach to spectral broadening, relying on cascaded χ(2) nonlinearities from phase-mismatched second-harmonic generation in BBO was also investigated. Although the efficiency is not comparable to the waveguide approach, the intriguing possibility of defocusing phase-shifts and self-compression in the crystal make it an interesting starting point for very compact pulse-compression setups. The combination of these developments demonstrates the feasibility of non-amplified, simple and compact laser sources that can replace more complex and costly Yb or Ti:Sapphire amplifier systems

On the transition from two- to three-dimensional turbulence in the presence of background rotation

We explore the transition from three-dimensional to two-dimensional turbulence in rotating turbulent flows. Inertial waves are thought of as the main mechanism driving physical processes in rotating turbulent flows. However, they can only exist in a limited flow regime and some steady anisotropic phenomena, such as Taylor columns, are driven by wave-free mechanisms. We identify these flow regimes and conditions under which inertial waves play no part with regards to formation of columnar structures, the promotion of anisotropy and the development of a transient turbulent flow field. A new mechanism is proposed by which columnar structures and anisotropy in general develop in rotating flows, which is based on a balance between the Coriolis force and the viscous or inertial forces operating in the flow field. These theories are validated experimentally using a setup where turbulence is forced through fluid injection/withdrawal and both 3D and quasi-2D flow structures develop. In line with the proposed mechanism, the columnar structures are found to scale as _ R

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