## 4 Self-Referenced DPSSLs in Thin Disk Geometry  
TDLs constitute a particular DPSSL technology where the gain medium is made of a very thin disk-shaped crystal (typical thickness of a few hundred micrometers) instead of the mm-size bulk gain medium commonly used in conventional DPSSLs previously discussed in Section 3. The disk thickness is much smaller than the laser beam diameter, so that the heat generated by pump absorption is principally extracted in the longitudinal direction through the backside of the disk rather than transversely. This geometry first introduced in the early 1990s provides an efficient cooling of the disk from the back side that leads to straightforward power scalability [77]. As for DPSSLs with a bulk gain medium, SESAM-modelocking and KLM are the main modelocking mechanisms used in TDLs. SESAM-modelocked TDLs enable direct power scaling by increasing the beam size on the thin disk gain medium and on the SESAM, without detrimental increase of the nonlinearities. As a result, TDLsare able to reach high power operation and Yb:YAG SESAM-modelocked lasers have demonstrated the highest average power (275 W in 583 fs pulses) and pulse energy (80 \(\upmu\)l in 1.1 ps pulses) of any type of modelocked oscillators [36,78]. Therefore, TDLs are well-suited as driving sources for experiments requiring high intensities at megahertz repetition rates, e.g., to generate vacuum ultra-violet (VUV) or extreme ultra-violet (XUV) radiation by high harmonic generation (HHG). Today, TDLs are developed mainly in the 1 \(\upmu\)m spectral range based on Yb-doped disks, such as Yb:YAG. The small disk thickness results in an inefficient pump absorption in the gain medium. Therefore, a multi-pass pump arrangement is generally used, where a parabolic mirror combined with prism retroreflectors enables for typically 8, 16 or 24 double passes of the pump radiation through the disk without stringent requirements on the pump beam quality. For this purpose, the back side of the disk has a highly reflective coating for both pump and laser wavelengths, whereas the front side is anti-reflection coated. TDLs typically use high-power laser diode bars coupled into a highly multi-transverse-mode optical fiber (with a typical \(M^{2}\) value \(>\)100) and require current drivers operating at several tens of amperes. For a long time, it was not clear if this pumping scheme introduces too high noise for comb self-referencing. The first CEO beat detection in a TDL was achieved by [PERSON] _et al._, in 2011 [60]. The SESAM-modelocked Yb:Lu2O3 TDL emitted 7 W of average output power in pulses of 142 fs duration at 64 MHz repetition rate. The laser was pumped with 47 W at 976 nm delivered by a VHG-stabilized diode with 24 passes on the disk. The very short pulses emitted by this TDL enabled SC generation in a PCF using less than 1% of the laser output power without any pulse amplification or compression. The low soliton order \(N_{\mathrm{sol}}\) = 5 of the pulses launched into the PCF led to a coherent SC according to the argumentation presented in Section 2.2. A CEO beat with an SNR \(>\) 30 dB in a RBW of 1 kHz was detected in an _f_-to-_2\(f\) interferometer. Tuning of _f_CEO with the current of the pump diode was observed in this TDL, but no self-referencing was performed due to the damage of the only crystal available at the time of the experiment. The first self-referenced TDL was demonstrated by [PERSON] _et al._[61]. The KLM Yb:YAG laser delivered an average output power of 17 W at a center wavelength of 1030 nm in 250 fs pulses and 38 MHz repetition rate. It was pumped by 110 W from a fiber-coupled diode at 940 nm with 24 passes on the disk. The output pulses were compressed to 30 fs using chirped mirrors before being launched into an _f_-to-_2\(f\) interferometer that included a 8 cm long PCF for SC generation. Two identical interferometers were built to provide in-loop and out-of-loop measurements. The free-running CEO beat was detected with an SNR of about 35 dB in a RBW of 300 kHz.