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4 November 1994 Time-resolved diagnostics of energy coupling during material processing with excimer lasers
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Proceedings Volume 2246, Laser Materials Processing and Machining; (1994)
Event: Optics for Productivity in Manufacturing, 1994, Frankfurt, Germany
Ablation of materials in air at ambient pressure of 1 bar with a KrF excimer laser (3 J/cm2 - 47 J/cm2, FWHM 30 ns) leads to gasdynamic discontinuities in the surrounding atmosphere. A plasma plume above the target surface arises caused by its vaporization and by interaction between the compressed air and the laser beam directly. These phenomena affect the exactness of the achievable geometry as well as the efficiency of the ablation process and the debris formation in the environment of the irradiated area. In this paper time resolved measurements of the transmitted laser radiation through the absorbing plasma plume are presented. From these results conclusions can be drawn about the efficiency of the ablation process which is dependent on the energy density as well as on the amount of the energy value. The experiments have further shown that immediately after reaching the laser pulse plateau the energy coupling to the target drastically decreases. As an example the energy loss by the absorption mechanisms in the plasma plume can be up to 80% at an energy density of about 30 J/cm2. The different behavior between a polymer sample and a metal sample is discussed. The propagation mechanism of the laser induced shock wave front has been studied as well and an attempt was made to explain it by the Sedov theory. A good agreement results from the measured and the theoretically calculated shock wave distance. Thus, an evaluation of the energy content of the laser induced shock waves using the distance- time law appears reasonable and the results are presented. They confirmed the directly measured high absorption in the plasma plume. The presented results allow a monitoring of the energy coupling.
© (1994) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Gert Callies, Henrik Schittenhelm, Peter Berger, Friedrich Dausinger, and Helmut Huegel "Time-resolved diagnostics of energy coupling during material processing with excimer lasers", Proc. SPIE 2246, Laser Materials Processing and Machining, (4 November 1994);

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