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Characterization of the laser induced damage threshold of mirrors in ...

Characterization of the laser induced damage threshold of mirrors in ...

modern science. The

modern science. The potential of ever shorter and more powerful pulses is limited only by the extremely expensive laser systems and the technical ability to manufacture ever bigger or higher damage threshold optics. Figure 1.1: A 40 cm x 40 cm wedged lens used at NIF.[1] At the Max-Planck-Institute for Quantum Optics (MPQ) a laser is built on a similar principle as ELI, the petawatt field synthesizer (PFS). When completed, it will be able to deliver pulses of 5 fs, pulse energies of ≥ 3 J and thus create pulses with a peak power of ≥ 0.6 ·10 15 W.[19] Some coatings and optics for PFS will be supplied by a cooperating group from the Ludwig-Maximilian-Universität München (LMU). For laser design considerations it is important to know the approximate damage threshold of all these optics, most prominent the damage threshold of the unique broadband chirped mirrors manufactured for the various pulse compressors of PFS. A chirped mirror is a dielectric multilayer structure with an arbitrarily changing layer thickness introducing a well defined change in the spectral phase [20, 21]. Once it is possible to compare several optics with respect to the breakdown threshold the LMU group can start optimizing their mirror designs on damage threshold and investigate the underlying damage mechanisms. The development of higher damage threshold optics is an important task, as it would result in less expensive laser components for high power lasers and overall smaller systems. 1 It would allow more research groups to acquire such a system and thus directly benefit the scientific progress in fields like laser-matter interaction or high field physics. 1 NIF is about 150m times 90m large 2

1.2 Goal of this Thesis The goal of this thesis was to build and characterize a setup for the evaluation of femtosecond laser induced damage threshold of dielectric mirrors using the PFS front end as light source. When completed, the setup should be able to do comparative and absolute damage threshold measurements of dielectric mirrors under arbitrary angles of incidence. These comparative studies should not only be used to optimize current mirror designs on their damage threshold, but also to get a deeper insight into the nature of laser damage of complex multilayer structures. 1.3 Thesis outline Chapter 2 summarizes the literature about femtosecond laser pulse induced damage threshold of dielectric materials. We present there the required basic definitions and a theoretical and mathematical description of single and multi pulse laser induced damage threshold. The last two sections of Chapter 2 are dealing with the important dependencies of the damage threshold on various parameters and their influence on the reliability of literature values. Chapter 3 represents the materials and methods part of this thesis. An overview of the laser system and the characterization of the laser output is given. This section is followed by the description of the damage threshold setup including a discussion of the detection principle, a typical set of calibration data and the mathematical evaluation of measurement data including the calculation of statistical and systematical errors. An overview of the so far conducted measurements is given in Chapter 4, including the comparison of experimental data with literature for SiO2 bulk and a thin gold film. 3

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