Generation of High-Energy Pulses by Managing the Kerr-Nonlinearity in Fiber-Based Laser Amplifiers

Increasing the pulse energy of ultrafast laser systems is an important field of laser development. High pulse energies simplify and accelerate most applications, such as the stimulation of optical parametric effects or the processing of materials. The amplification of ultrashort pulses in glass fibers is a prominent method, as fiber amplifiers are inexpensive, flexible and highly-integrated. Resulting from the strong confinement in the fiber core and the high peak intensities of the laser pulses, the amplification is often limited by the onset of a nonlinear deterioration of the pulses. Within this thesis, two methods of fiber-based pulse amplification by managing the Kerr-nonlinearity are presented. In the first method, the chirped-pulse amplification, nonlinear effects are suppressed by locally reducing the peak intensity. A chirped-pulse amplifier was realized that generated pulses with energies of 450 nJ and durations of 293 fs, limited by pump power. These pulse parameters were not sufficient for the intended application. In order to further decrease the pulse duration and increase pulse energy, the parameters of the amplified pulses had to be decoupled from the seed pulses. This is achieved in an amplifier based on the second approach. By enforcing the impact of Kerr-nonlinearity, the optical spectrum of self-similar pulses could be broadened by self-phase modulation during the amplification to generate pulses with 1 µJ pulse energy and a compressed duration of 50 fs at which level the amplification was limited by transverse mode instabilities. This improvement of pulse parameters by nonlinear techniques is also exploited in a pulse regenerator. By feeding back a part of the amplified pulse into a second amplifier, a so-called Mamyshev oscillator is formed. Its principle of alternating spectral filtering between sections of gain and spectral regeneration allowed for the generation of mode-locked pulses. This Mamyshev oscillator was optimized for the generation of high-energy pulses by the analysis of optimum band-pass filter parameters and the implementation of a few-mode gain fiber. A pulse with a maximum energy of 650 nJ and a compressed duration of <100 fs was achieved. This was the highest pulse energy achieved by a Mamyshev oscillator based on standard Yb-doped fibers to date, even surpassing the performance of state-of-the-art Titanium:Sapphire lasers. A transfer of the Mamyshev oscillator concept to the regime of Thulium-doped gain fibers with the wavelength 2 µm is challenging due to the anomalous dispersion of the gain fibers which prevents parabolic pulse evolution. Nevertheless, a realization of this design is feasible. Mode-locked pulses with durations of <200 fs and pulse energies of 6.4 nJ were achieved. At this pulse duration it was the highest output power from a Thulium-doped fiber oscillator to date. Due to the alteration of the pulse shape in the glass fibers, a characterization of the final pulses is necessary. A recently developed method for the required complete pulse analysis was transferred from the application in solid-state systems to fiber-based systems in this thesis, which involves the management of less precisely defined amounts of dispersion. The complete characterization by dispersion scans based on a grism compressor was achieved by the use of an adequate retrieval algorithm

Begrüßung durch den Dekan der Fakultät für Rechtswissenschaft

Dieser Band dokumentiert die Reden, die am 30. Oktober 2015 im Rahmen der Akademischen Gedenkfeier zur Würdigung des Strafrechtswissenschaftlers Gerhard Fezer (1938–2014) im Flügelbau West des Hauptgebäudes der Universität Hamburg gehalten wurden. Ergänzt um die Begrüßung durch den Dekan der Fakultät für Rechtswissenschaft vermitteln die fünf Reden mit je unterschiedlichem Akzent ein eindrucksvolles Bild des am 15. August 2014 im Alter von 75 Jahren verstorbenen Kollegen und akademischen Lehrers. Gerhard Fezer, der 1978 eine Professur für Strafrecht und Strafprozessrecht in Hamburg übernommen hatte und hier 2004 emeritiert wurde (aber noch darüber hinaus gelehrt hat), war 36 Jahre lang, nahezu sein halbes Leben, Mitglied der Universität Hamburg.This publication documents the speeches held at the Academic Commemorative Ceremony of Gerhard Fezer (1938-2014) at the University of Hamburg on October 30th, 2015. Supplemented by greetings of the Dean of the Faculty of Law, the five speeches, each with a different accent, convey an impressive picture of the colleagues and academic teachers who died at the age of 75 on August 15th, 2014. Gerhard Fezer had taken up a professorship for criminal law and criminal procedural law in Hamburg in 1978 and emerged as a professor in 2004 (but continued teaching); he was a member of the University of Hamburg for almost thirty years

Amplification of ultrafast pulses in an extended Mamyshev regenerator

We present the characteristics of a high-energy ultrafast Yb-fiber laser system, based on a Mamyshev oscillator and a subsequently arranged fiber amplifier stage. The Mamyshev oscillator emits pulses at a repetition rate of 11 MHz and pulse energies of 31 nJ. These pulses are spectrally filtered and amplified in a Yb-doped fiber up to 1 μJ pulse energy which could be temporally dechirped to less than 50 fs autocorrelation duration. We discuss the scaling as well as limiting options related to pulse energies and duration. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only

Zum Gedenken an Gerhard Fezer (1938-2014). Reden der Akademischen Gedenkfeier der Fakultät für Rechtswissenschaft am 30. Oktober 2015

This publication documents the speeches held at the Academic Commemorative Ceremony of Gerhard Fezer (1938-2014) at the University of Hamburg on October 30th, 2015. Supplemented by greetings of the Dean of the Faculty of Law, the five speeches, each with a different accent, convey an impressive picture of the colleagues and academic teachers who died at the age of 75 on August 15th, 2014. Gerhard Fezer had taken up a professorship for criminal law and criminal procedural law in Hamburg in 1978 and emerged as a professor in 2004 (but continued teaching); he was a member of the University of Hamburg for almost thirty years

Fatal attraction: a critique of Carl Schmitt's international political and legal theory

The ongoing Schmitt revival has extended Carl Schmitt's reach over the fields of international legal and political theory. Neo-Schmittians suggest that his international thought provides a new reading of the history of international law and order, which validates the explanatory power of his theoretical premises – the concept of the political, political decisionism, and concrete-order-thinking. Against this background, this article mounts a systematic reappraisal of Schmitt's international thought in a historical perspective. The argument is that his work requires re-contextualization as the intellectual product of an ultra-intense moment in Schmitt's friend/enemy distinction. It inscribed Hitler's ‘spatial revolution’ into a full-scale reinterpretation of Europe's geopolitical history, grounded in land appropriations, which legitimized Nazi Germany's wars of conquest. Consequently, Schmitt's elevation of the early modern nomos as the model for civilized warfare – the ‘golden age’ of international law – against which American legal universalism can be portrayed as degenerated, is conceptually and empirically flawed. Schmitt devised a politically motivated set of theoretical premises to provide a historical counter-narrative against liberal normativism, which generated defective history. The reconstruction of this history reveals the explanatory limits of his theoretical vocabulary – friend/enemy binary, sovereignty-as-exception, nomos/universalism – for past and present analytical purposes. Schmitt's defective analytics and problematic history compromise the standing of his work for purposes of international theory