Generation of energetic femtosecond pulses at high average power

Abstract

The development of more powerful and more energetic femtosecond laser systems builds the foundation for an even wider application of lasers. Powerful attosecond sources for sensitive experiments and compact high brilliance X-ray sources are only two examples of a vast field of possibilities that can be made accessible. The envisioned lasers with kilowatt scale average powers, hundreds of millijoule of pulse energy and pulse durations down to the few-cycle regime require significant technological advances that are partially introduced in this thesis. The amplification of laser pulses to an energy of 200 mJ with a repetition rate of 5 kHz, a duration of about 1 ps and an average power of 1 kW is demonstrated for the first time using a thin-disk regenerative amplifier. The excellent thermal properties of the thin-disk scheme in combination with a thorough investigation of the amplifier properties and a careful optimization of the design provides a highly stable laser output with an excellent virtually diffraction limited beam quality. This amplifier can be used as nearly ideal pump source for an OPA chain, which is predicted to facilitate few-cycle pulses with energies up to 20 mJ and a multi-kilohertz repetition rate. In a second part an alternative approach to obtain a femtosecond scale pulse duration from the presented amplifier is studied. In one of the first implementations of a multipass nonlinear broadening stage, pulses of about 18 mJ are spectrally broadened with a repetition rate of 5 kHz, and compressibility down to 41 fs is demonstrated. Further pulse energy scaling to 40 mJ and 75 mJ is shown. To the best of the authors knowledge these energies represent the highest energies at which a pulse was spectrally broadened in the multi-kilohertz regime. The proposed approach features unprecedented throughputs over 95% and excellent average power scalability. In contrast to the OPA scheme most of the input pulse energy is transferred into the femtosecond scale pulse and hence less complex source lasers are necessary for a given output pulse energy. The record-breaking results of this thesis constitute an important step for next generation femtosecond laser sources. A simulated extension of the system allows for the generation of a pulse energy of 100 mJ, a pulse duration of tens of femtosecond and a repetition rate of 5 kHz close to the envisioned parameters.