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

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

Figure 3.6: Example

Figure 3.6: Example focus characterization measurement. The two curves indicate two perpendicular axes of the beam showing a clearly different focus position and a different minimal beamwidth. The position is given relative to the translation stage, the beam profiling camera is mounted on. The error bars show the standard deviation of the average. The beam was found to be astigmatic, as two perpendicular focal lines with a different focus position and beam width could be observed. An example measurement showing this astigmatism is shown in figure 3.6. Repeated focus characterization measurements show that the positions and sizes of the foci are stable if the laser system is running long enough with a given alignment to reach thermal equilibrium. Due to the experimental character of the system a realignment of the multi pass amplifier is done at least once a day, requiring every time a re-characterization of the focus. Under measurement conditions we try to position the sample at the position where both main axes show a similar beam width, leading to an approximately round intensity distribution at that position. Figure 3.7 shows a two dimensional plot of the focus indicating the two main axes and a three dimensional plot of the focus. In order to verify the Gaussian shape of the intensity distribution we took two slices along the main axes of the focus and tried to fit a Gaussian distribution on it, both slices matched well. Figure 3.8 shows one of the two fit curves. Evaluating the pulse energy in the focus leaves only a maximum of approximately 1.2 mJ from the initial 1.6 mJ from the laser. This is because the beam travels through four glass surfaces each time loosing 4 % of its energy and a minor clipping issue at the focussing lens, which is slightly smaller then the beam itself. 24

(a) ←→ 100µm (b) ←→ 100µm Figure 3.7: a) Two-dimensional picture of the approximately round focus, the two lines are indicating the two main axes. b) Three-dimensional plot of the same focus. Both pictures were taken with a WinCamD-UCD23 beam profiling camera Figure 3.8: Fit of a Gaussian distribution to a slice of the intensity distribution along one of the two main axes. 25

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