Mode locking of an active multipass geometry (colored edition)Ultrashort laser pulses in the microjoule regime are of prime importance for many applications. Previous record pulse energies obtained directly from an oscillator in ambient atmosphere were below 2 _J. This thesis describes a newly developed thin disk laser oscillator with an increased round trip gain by employing a self imaging active multipass geometry. The generation of high energy subpicosecond pulses with a pulse energy of up to 25. 9 _J at a