## 2 Average Power Scaling of Modelocked Thin-Disk Lasers: Challenges and Milestones
### SESAMs for High-Power Femtosecond Modelocking
#### 2.3.2 Experimental Setup and Measurement Procedure
From a material point of view, SiO\({}_{2}\) and Si\({}_{3}\)N\({}_{4}\) are dielectric materials, and therefore exhibit negligible TPA compared to GaAs [50], which results in reduced IA. Figure 7: Different SESAM structures used for the damage studies (**a**) SESAM design with 3 QWs and no topcoating (NTC); (**b**) same SESAM but with a 4-quarter-wave-pair GaAs/AlAs semiconductor topcoating (SCTC); (**c**) same SESAM but with a 3-quarter-wave pair SiO\({}_{2}\)/Si\({}_{3}\)N\({}_{4}\) dielectric topcoating (DTC3); (**d**) field enhancement in the absorber section. The nonlinear reflectivity measurements and corresponding extracted parameters are presented together with the measured damage thresholds in Figure 8a, and Table 3. We can clearly see the effect of the different topcoatings on the saturation parameters of the samples: as the saturation fluence increases, the modulation depth decreases by a similar factor. The saturation fluences of all the topcoated samples are larger than 150 \(\upmu\)J/cm\({}^{2}\) and their modulation depths are between 0.4% and 0.8%. The 3-QW non-topcoated sample with 2% modulation depth is ideally suited for topcoatings designed for a saturation fluence increase of 3-5. All SESAMs have negligible nonsaturable losses \(<\)0.1% and therefore minimal thermal load. In terms of IA, all topcoated SESAMs have increased \(F_{2}\) coefficients compared to that of the non-topcoated sample. However, the semiconductor topcoated SESAM (SCTC) shows only a small increase compared to the dielectric topcoated samples (DTC2 and DTC3). Although the field in the DBR structure and the absorber section is reduced in all samples, the GaAs/AlAs topsection experiences a strong electric field, and GaAs has a strong TPA coefficient. In the case of the dielectric topcoating, both materials have a negligible TPA coefficient compared to GaAs. Therefore, SESAMs with similar saturation parameters (similar electric field distributions in the DBR and the absorber region) but different topcoatings have different IA responses. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multirow{2}{*}{**Sample**} & \(\mathbf{F_{\mathrm{sat}}}\) & \multirow{2}{*}{\(\Delta R\) **(\%)**} & \multirow{2}{*}{\(\Delta R_{\mathrm{as}}\) **(\%)**} & \(\mathbf{F_{2}}\) & \multirow{2}{*}{\(\mathbf{F_{\mathrm{d}}}\) **(mJ/cm\({}^{2}\))**} & \multirow{2}{*}{\(\mathbf{S_{\mathrm{d}}}\) **= \(\mathbf{F_{\mathrm{d}}}\)/\(\mathbf{F_{\mathrm{sat}}}\)**} \\  & **(μJ/cm\({}^{2}\))** & & & & & \\ \hline NTC & 72 & 2.05 & \(<\)0.1 & 3200 & 32.6 & 450 \\ SCTC & 279 & 0.52 & \(<\)0.1 & 5500 & 44.1 & 158 \\ DTC2 & 168 & 0.71 & \(<\)0.1 & 31700 & 122 & 726 \\ DTC3 & 247 & 0.43 & \(<\)0.1 & 346000 & \(>\)210 & \(>\)850 \\ \hline \end{tabular}
\end{table} 
Table 3: Damage thresholds of the different representative SESAMS together with their saturation parameters. The samples with a 3-pair dielectric topcoating did not show damage up to the maximum available fluence in our setup of 0.21 J/cm\({}^{2}\). Figure 8: **(a)** Damage threshold (indicated with a star) of the different tested SESAMs together with their nonlinear reflectivity measurement; **(b)** Lifetime curves of different representative SESAMs, where one can see a large shift of the lifetime of the sample with a dielectric topcoating to higher fluences. The sample with a 3-pair dielectric topcoating (DTC3) could not be damaged at the maximum available fluence in our setup of 0.21 J/cm\({}^{2}\). The topcoated SESAMs show, in all cases, higher damage fluences than without a topcoating. The damage fluences for the dielectric topcoated SESAMs (DTC2 and DTC3) are much higher than for the semiconductor topcoated SESAM (SCTC) with similar saturation parameters. It is interesting to note that regardless of the sample, instantaneous damage occurs at fluences deep in the rollover regime, where SESAM modelocked lasers would not operate in a stable regime (e.g., saturation parameters larger than 150).