The Thin-Disk Laser for the 2S – 2P Measurement in Muonic Helium

Abstract

In the project presented in this thesis, a laser system for the spectroscopy of muonic helium ions has been developed. Five 2S-2P transitions were successfully measured in 2013 and 2014 using this laser system. Eventually the alpha particle and the helion charge radii will be deduced from these measurements with accuracies of few parts per thousand. These values will be benchmarks for nuclear structure theories, and bear information contributing to the solution of the so-called proton radius puzzle. The laser system developed is composed of a thin-disk laser operated at a wavelength of 1030 nm pumped with a high-power diode laser. The pulses of the thin-disk laser are frequency-doubled and used to pump a Ti:sapphire laser. The pulsed Ti:sapphire laser is injection-seeded by a frequency-stabilized cw Ti:sapphire laser with a wavelength tunable between 800 nm and 970 nm. The pulses of the Ti:sapphire laser are then transported and coupled into the multi-pass cavity surrounding the volume the muons are stopped at. The most challenging building block of the laser system is the thin-disk laser. A thin-disk laser was developed based on a Q-switched oscillator followed by a multi-pass amplifier. The thin-disk laser has to deliver pulses with 100 mJ of energy, at average repetition rates of at least 200 Hz with stochastically distributed (in time) triggers having a minimal delay time between pulses down to 1.2 ms. In addition, pulse-to-pulse fluctuations smaller than a few % are required, as well as a latency time between trigger and emission of the pulse of < 500 ns, and good transverse beam mode quality of M2 < 1.1 for efficient frequency doubling. Special emphasis was devoted to the design of resonators and multi-pass amplifiers that minimizes the sensitivity to thermal lens effects. In addition, aperture effects that naturally occur in the pumped active medium have been discussed in detail in this thesis because they are usually neglected in the thin-disk laser community. Yet, they may play an important role in laser design. The laser development that has been motivated by the muonic helium spectroscopy has also led to several additional results published in papers reproduced in the second part of this thesis. As the first additional result, a novel multi-pass architecture is proposed that solves present energy scaling limitations of mode-locked multi-pass laser oscillators. Contrarily to the state-of-the-art layouts based on 4f-imaging, the stability region of our multi-pass resonator does not shrink with the number of passes at the active medium. Hence, our design sustains thermal lens variations that are by at least an order of magnitude larger compared to state-of-the-art multi-pass designs. This implies an order of magnitude larger output powers, and laser output pulses with mJ energy at MHz repetition rates directly from an oscillator. As second additional result, we expose a novel limitation for the power scaling of thin-disk lasers. This limitation is related to misalignment induced by thermal lens effects. From its modeling a parameter has been obtained that can be used to design laser resonators circumventing this limitation. The third additional result is related to novel pump optic schemes having an increased number of passes at the thin disk as compared to standard designs while maintaining the same requirement for the pump beam quality and size of the pump optics.