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

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

3.2 The

3.2 The damage threshold setup 3.2.1 Damage detection principle We use a change of the scattering behavior of the sample as indicator for damage. This method has been used in various previous experiments by other groups[23, 30, 31, 32, 33] and is recommended by ISO11254-2. The laser is focused on the to be investigated sample, at the same time a photo detector is positioned close to the beam above the sample to monitors the backscattered light. In the undamaged case a linear increase of the backscattered light with the incident light can be observed. As soon as the surface topology is changed due to damage a strong increase of the scattered light occurs, see figure 2.1 and figure 3.4. (a) undamaged (b) damaged Figure 3.4: Schematic diagram of the undamaged (a) and damaged (b) case. The change of the surface topology in the second case leads to a strong increase in the backscattered light. The thick solid black line indicates the incoming and reflected laser beam. This measurement principle has one major drawback, namely the insensitivity to damage that doesn’t change the scattering properties of the sample. For instance the generation of permanent colorcenters which is by definition damage. 3.2.2 Setup design A schematic layout of the setup is displayed in figure 3.5. The beam coming from the PFS front end goes over two alignment mirrors and then directly through a Thorlabs NDC-100C-4 continuously variable neutral density filter wheel, this wheel changes the optical density (OD) linearly with the rotation angle from OD zero to four. We can control the wheel with an attached DC motor which is connected to the 5V output of a 22

Meilhaus RedLab 1208LS USB analog-digital converter, the filter wheel can thus be rotated with an attached computer. After passing the ND-Filter approximately 0.1% of the beam is reflected on the surface of an antireflective (AR) coated glass plate which is approximately under 45 ◦ AOI, this part of the beam is used to do a online power measurement with a Thorlabs DET36A photodiode. A Tectronix TDS3054B oscilloscope is used to read out the signal from the photo diode. It is connected via ethernet to the control computer. Figure 3.5: Schematic layout of the damage threshold setup. BS: beam splitter, F: variable neutral density filter wheel, L: focusing lens, PD1: power diode, PD2: scattered light detection diode, TS: translation stage with sample mount. The rest of the beam is focused by a 700-900nm achromatic lens with a focal length of 1.5 m. To reduce the required space on the optical table, the beam path is folded twice before the beam hits the sample mount on the motorized x-y-z-stage consisting of three PI M505-PD translation stages. The photo diode to measure the scattered light from the sample surface is also a Thorlabs TDS3054B and it is read out the same way as the power measurement diode. 3.2.3 Focus characterization In order to state the peak fluence as a function of the incident power it is inevitable to fully characterize the focus. We attached therefore a WinCamD-UCD23 beam profiling camera to the translation stage and moved it in defined intervals through the focus. One problem of this method is, that the focus has an approximate diameter of 85 µm FWHM and the pixel size of the camera CCD is only 6.45 µm[69], one beam axis was thus only represented by 13 pixels. In order to improve the quality of measurement we averaged several measurements. 23

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