Characterization, Stabilization and Coherent Combination of High Power Laser Beams for Gravitational Wave Detectors

Abstract

Current and future gravitational wave detectors (GWDs) require high power and low noise laser systems at a wavelength of 1064nm in the continuous wave regime, with excellent spatial beam quality. These systems are highly complex and not commercially available. Hence, this thesis is dedicated to the development of a laser system, to be used in current GWDs, and of different concepts for laser systems, suitable to provide even laser powers of ∼400W or larger for future detectors. It also presents a promising solution to transport the generated high power laser beams via a hollow-core fiber from the laser table into the GWD’s isolated in-vacuum environment. First, investigations on a sequential installation of solid-state laser amplifiers are presented. They confirm the suitability of these amplifiers for the generation of laser powers up to 195W, and uncover limitations of them. These results built the basis for the laser system that the advanced Laser Interferometer Gravitational- Wave Observatorys (aLIGOs) will use in their fourth science run, a prototype of which was tested successfully at the aLIGO Livingston site within this thesis. In sequence, three different configurations for the coherent combination of two laser beams are reported as possibility to increase the laser power above the level available from sequential amplifier chains. They were investigated under the different aspects important for GWD laser systems. The first configuration is a pre-stabilized laser system (PSL) based on the coherent combination of two laser beams from the same seed laser source amplified by solid-state laser amplifiers, that allowed for the generation of a 100W laser beam, with beam quality and noise characteristics similar or better as for current GWD systems. The second configuration is a coherent combination of two laser beams from the same seed source amplified by fiber laser amplifiers, which generated a total output power of 398W with beam quality and free-running noise within the requirements of current GWD systems. This power level exceeds the laser power of ∼200W so far available for GWD PSLs. Finally, the benefit of a variable beam splitter for the coherent combination of two independent laser beams with different power levels, is demonstrated. As final investigations, novel technologies for the generation and transportation of laser beams at the power levels required for future GWDs are presented. The coherent combination of three laser beams with a bow-tie resonator as combining element is a promising possibility for further power scaling. The first results of a proof-of-experiment are promising and further plans for an extension of this concept are described. The transportation of high power laser beams from the laser table to the GWD main vacuum system is especially challenging. Here, an analysis of the beam quality and laser noise of a high power beam transported by a hollow core fiber, encourages for further research in this direction. The results presented in this thesis built a foundation for the challenging development of high power laser systems at a wavelength of 1064nm and the transportation of the generated laser beams for current and future GWDs.