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

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

Where F Pi is

Where F Pi is the i-th free parameter and N is the total number of free parameters. The majority of parameters can be taken directly from the two calibrations. The uncertaintiy of the distance from the sample surface to the focal plane though is dominated by the uncertainty introduced by the required exchange of beam profiling camera and sample. We assume it to be ±1.5mm. The uncertainty of the angle of incidence is approximated as 0.5 ◦ To get the final uncertainty of the calculated peak fluences we have to take the systematical error from the power meter into account. As the measured power simply goes linear with the peak fluence we can write ∆ ˆ Jtotal = (∆ ˆ Jstatistical) 2 + (∆ ˆ Jsystematical) 2 (3.10) Gentec states an accuracy of 2% for their power detection head in combination with their powermeter[71], but only at the calibration wavelength of 1064nm and at the calibration power wich is unknown in our case. We assume for our measurements an accuracy of 5% for the power meter. This means, that the actual power lies within a rectangluar distribution around the measured value, leading to an uncertainty of 5 · 1 √ 3 %. Figure 3.12: Calculated relative error of the fluence on the sample as a function of the fluence. Figure 3.12 shows the relative error calculated for each point of a measurement data set. The strong increase of the relative error for fluences lower then 0.02 J cm 2 comes from the uncertainty of the additive constant from equation 3.2. With a calibration power meter with a higher dynamic range this increase could be shifted towards lower fluences. In our range of interest from 0.1 J cm 2 to 10 J cm 2 a relative error of lower than 5% is achieved. 30

Chapter 4 Presentation of Results 4.1 Comparative measurement of different mirror types Figure 4.1 shows some of the measurements conducted so far. The measurements on the silver mirror, the gold mirror and the fused silica substrate were done in order to check the measured damage thresholds for plausibility, all of them will be discussed in more detail in the next section. We were quite surprised to find the damage threshold of metallic mirrors in the same order as the threshold fluence for dielectric high reflectors, as it was common in our laboratory to use dielectric mirrors for their apparently higher damage threshold. The measurements on the Ta2O5 and TiO2 high refelctors (HR) were the first measurements by us that were actually conducted on a dielectric multilayer stack. We measured about 0.11 J cm 2 for the TiO2 mirror and 0.32 J cm 2 for the Ta2O5 mirror. For dielectric mirrors the measured damage thresholds are quite low, in a recent study on dielectric mirrors for fs Ti:Sa applications the average damage threshold was about 0.6 J cm2 .[72] Furthermore we expected the damage thresold of the TiO2 mirror to be lower based on the different band gap energies,[5] but only by some ten percent. The reason for the way larger difference of the measured damage threshold lies most probably in the different layer structure of both mirrors. Further investigations will be done on this. Measurements on the same silver mirror, at different days and with different sets of calibration data yielded the same results for the damage theshold of that mirror. 31

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