## 3 Results
### Laser-Induced Damage Threshold  
To investigate the influence of the applied cleaning strategy on the LIDT of the as-cleaned substrates, a set of samples without an AR coating were laser damage tested at 1064 nm with 10 ns pulses subsequent to the cleaning. Due to the high surface quality, the estimated 0% LIDT values were between 95 and 230 J/cm\({}^{2}\), which is in the range of the bulk LIDT. We assume that the observed damage is not limited to the surface. Figure 4 shows a picture of an uncoated sample after laser damage testing. Due to the high fluences, the damage events produce large cracks and many particles that affect the subsequent measuring sites. Therefore, no analyzable damage probability curve, and no clear correlation between cleaning and LIDT were observed.  
Subsequent to coating, the simultaneously AR-coated samples were laser irradiated using the same parameters (1064 nm, 10 ns pulses). Figure 5 shows exemplary results of the laser damage testing. From the data points (black crosses) in Figure 5a, the damage probabilities (blue dots) were calculated using the cumulative data reduction technique [16]. In accordance with ISO 21254, a linear fit was applied (dashed line). The corresponding characteristic damage curve is shown in Figure 5b. As can be seen, the LIDT saturates between 100 and 1000 pulses. Furthermore, no significant mitigation effects were observed.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **As-Polished** & **As-Cleaned** & **As-Coated** \\ \hline  & & Set A & Set B & Set A \\ RMS roughness [Å] & 2.7 +/\(-\) 0.5 & 3.2 +/\(-\) 0.1 & 2.7 +/\(-\) 0.5 & 3.1 +/\(-\) 0.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Root-mean-square (RMS) roughness of the as-polished, as-cleaned, and as-coated samples measured by means of a white light interferometer at WZW Optic AG.  
Figure 4: Typical picture of an as-cleaned uncoated sample after laser damage testing.  
Figure 3: Plot of the white light interferometer measurements of (**a**) a polished sample, (**b**) a cleaned sample (CP 1; RMS roughness = 3.2 Å), and (**c**) a coated sample (CP 2; RMS roughness = 3.2 Å).  
Taking this into account when calculating the 0% and 50% LIDT values, the results after 100 pulses were used and depicted for the different sets of cleaning processes in Figure 6. For better comparison, the LIDT values were normalized to the LIDT of the standard cleaning process A-CP1 in Figure 6a,b. The results from set A indicated that the standard process, including an alcoholic pre-clean, gave the best LIDT results. Without pre-clean (A-CP2), the LIDT was reduced by about 35%. In case of ultrasonic frequency variation without power adjustment in A-CP3 the LIDT was reduced by almost 50% compared to the standard cleaning. In contrast, applying the automatic cleaning procedure combined with the same variation of the ultrasonic frequencies but with power adjustment in bath 2 (B-CP2) increased approximately 20% compared to cleaning B-CP1 using only 40 kHz. The reduced LIDT values observed for B-CP3 indicated that an adequate composition of ultrasonic frequencies must be chosen. The best results for LIDT and automatic cleaning were observed with an additional 500 kHz step (B-CP4), similar to the standard manual cleaning A-CP1.