## 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  
In order to reach the high fluences necessary to carry out this study, we used a high-energy SESAM modelocked Yb:YAG TDL seeding a high-precision nonlinear reflectivity measurement setup to characterize nonlinear reflectivity, IA, and damage of our SESAMs. The experimental setup is presented in Figure 6, and in more detail in reference [46]. The SESAM modelocked Yb:YAG TDL delivers 15 W of average power at a repetition rate of 10.7 MHz, corresponding to a pulse energy of 1.4 \(\upmu\)J in 1-ps pulses, which allowed testing SESAMs up to an unprecedentedly high fluence of 0.21 J/cm\({}^{2}\). #### 2.3.2 Experimental Setup and Measurement Procedure  
In order to reach the high fluences necessary to carry out this study, we used a high-energy SESAM modelocked Yb:YAG TDL seeding a high-precision nonlinear reflectivity measurement setup to characterize nonlinear reflectivity, IA, and damage of our SESAMs. The experimental setup is presented in Figure 6, and in more detail in reference [46]. The SESAM modelocked Yb:YAG TDL delivers 15 W of average power at a repetition rate of 10.7 MHz, corresponding to a pulse energy of 1.4 \(\upmu\)J in 1-ps pulses, which allowed testing SESAMs up to an unprecedentedly high fluence of 0.21 J/cm\({}^{2}\). \begin{table}
\begin{tabular}{l l l l} \hline
**Parameter** & **Yb:YAG [17]** & **Yb:YAG [18]** & **Yb:LuO [26]** \\ \hline Average output power & 275 W & 145 W & 141 W \\ Repetition rate & 16.3 MHz & 3.5 MHz & 60 MHz \\ Pulse duration & 583 fs & 1100 fs & 740 fs \\ Output pulse energy & 16.9 \(\upmu\)J & 41 \(\upmu\)J & 2.35 \(\upmu\)J \\ Output peak power & 25.6 MW & 33 MW & 2.8 MW \\ \hline Intracavity average power & 2.5 kW & 0.2 kW & 1.5 kW \\ Intracavity pulse energy & 154 \(\upmu\)J & 57 \(\upmu\)J & 25 \(\upmu\)J \\ Intracavity peak power & 236 MW & 46 MW & 30 MW \\ \hline Spot radius on SESAM (1/e\({}^{2}\)) & 1.2 mm & 0.55 mm & 0.63 mm \\ Fluence on SESAM & 3.4 mJ/cm\({}^{2}\) & 6 mJ/cm\({}^{2}\) & 2 mJ/cm\({}^{2}\) \\ Average intensity on SESAM & 55 kW/cm\({}^{2}\) & 21 kW/cm\({}^{2}\) & 12 kW/cm\({}^{2}\) \\ Peak intensity on SESAM & 5.2 GW/cm\({}^{2}\) & 4.8 GW/cm\({}^{2}\) & 5 GW/cm\({}^{2}\) \\ \hline Saturation fluence of SESAM & 140 \(\upmu\)J/cm\({}^{2}\) & 61 \(\upmu\)J/cm\({}^{2}\) & 60 \(\upmu\)J/cm\({}^{2}\) \\ Saturation parameter (\(S\)=\(F\)/\(F_{\text{sat}}\)) & 24 & 100 & 33 \\ \hline \end{tabular}
\end{table} 
Table 2: Typical operation parameters of SESAMs in state-of-the-art high-power TDLs. Figure 6: **(a)** Experimental setup used for the nonlinear reflectivity characterization and damage measurements; **(b)** Typical measurement of nonlinear reflectivity measured using this setup, and important macroscopic parameters extracted using a least-squares fitting procedure [47]. The QWs of this sample were grown at \(T\approx 400\)\({}^{\circ}\)C, resulting in a recovery time at 1/e of \(\tau_{\mathrm{i}\omega}\approx 200\) ps and low nonsaturable losses \(<\)0.1%. Three different topcoatings were grown on this structure. In the first case, a semiconductor topcoating was chosen with four pairs of quarter-wave GaAs/AlAs layers (semiconductor topcoating (SCTC), see Figure 7b) grown by molecular beam epitaxy (MBE). In this design, the field enhancement in the absorber is reduced by a factor of \(\approx\)4 compared to the uncoated sample (see Figure 7d). Therefore the same increase of the saturation fluence is expected. For the other cases, a dielectric SiO\({}_{2}\)/Si\({}_{3}\)N\({}_{4}\) topcoating deposited by plasma-enhanced chemical vapor deposition (PECVD) was chosen. Two sets of samples were coated with two and three pairs of quarter-wave layers (DTC2 and DTC3, see Figure 7c). In this case, we expect an increase of the saturation fluence by a factor of 3 and 5, respectively Figure 7d. The dielectric topcoating can be applied after MBE growth, which allows for additional flexibility in terms of finesse change of the structure.