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

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

3.2.4 Measurement

3.2.4 Measurement procedure First we move the mirror with the x-y-z-stage so that the focus is positioned on an undamaged site on the mirror, then we measure the scattered light for a defined timespan and increase the fluence by rotating the neutral density filter wheel. When this is done for a defined range of fluences, we plot the signal from the scattering diode as a function of incident fluence. A strong change in the scattering behavior indicates damage. (a) (b) Figure 3.9: Flow diagram of a) the ISO procedure and b) our modification. To do a ISO11254-2 conform measurement, we would need to change the focus position on the sample whenever we increase the fluence in order to avoid preconditioning of the irradiation site. In our opinion this is not necessary, as under realistic operation conditions various incident fluences may occur. In order to get a representative value for the long term damage threshold we would even consider our method better. Additionally for a rough determination of the damage threshold one single damage site is required, for better statistics some tens of damage sites are sufficient compared to hundreds of damage sites required for a ISO measurement. A flow diagram of the ISO procedure and our procedure is shown in figure 3.9. 3.2.5 Calibration and measurement As shown in the appendix the peak fluence ˆ J is related to the average power ¯ P by the following expression ˆJ = 2 · EP · 1 Aσ 26 = 2 ¯ P frep · 1 Aσ (3.1)

Aσ denotes here the area in the beam, where the intensity is higher or equal to 1 e 2 times the peak intensity. EP stands for the pulse energy. With this equation we can estimate the reachable peakfluence in the focus. As stated before a typical maximal available pulse energy is about 1.2 mJ and a typical value for the focus radius is rσ = 70 µm, leading to a maximal reachable peak fluence of 16 J cm 2 . The average laser power on the sample Not only the integral of the photo diode output but also the peak value UP is proportional to the average laser power. We used the latter one as the Tektronix TDS3054B oscilloscope, which we used to measure the output voltage of the photo diode, has a standard data transfer rate over Ethernet of only one complete data set (10000 data points) per second. For the evaluation of the peak value way less data points were needed, so we were able to switch to another mode transferring only 500 consecutive data points from the complete data set.[70] We could thereby increase the transfer rate almost by a factor of 10. In order to calibrate the the photo diode a commercial Gentec TPM-300CE laser power meter was used together with a Gentec PS-310WB thermal power detector. An example calibration measurement is shown in figure 3.10. The calibration curve shows a linear dependence of the average power on UP and indicates the absence of nonlinear absorption in our measurement regime. The two fit parameters a and b defining the linear fit, allow us to calculate ¯ P . ¯P = a + b · UP (3.2) Figure 3.10: Example measurement for the calibration of the power determination diode. 27

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