A growing number of applications in science and industry are currently pushing the development of reliable ultrafast laser technologies that enable high average powers. SESAM modelocked thin disk lasers (TDLs) currently achieve higher pulse energies and average powers than any other ultrafast oscillator technology, making them excellent candidates in this goal. During this thesis, the frontiers of this technology both in terms of average power and pulse duration were pushed to new records. On the one hand, 275 W of average power were demonstrated with a pulse duration of 583 fs, which represents the highest average power so far demonstrated from an ultrafast oscillator. In addition, another major milestone was achieved, with the demonstration of a TDL with nearly bandwidth-limited 96-fs long pulses. The progress achieved in terms of pulse duration of such sources enabled the first measurement of the carrier-envelope offset frequency of a modelocked TDL, which is the first key step towards full stabilization of such a source. The cutting-edge sources developed during the timeframe of this thesis will enable exciting new applications, and open the door to further extending the current performance milestones.weiterlesen