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
pulse duration of 928 fs, while operated in ambient atmosphere, could be demonstrated.
Stable single-pulse operation has been obtained with an average output
power in excess of 76W at a repetition rate of 2.93MHz. Self starting passive modelocking
was accomplished using a semiconductor saturable absorber mirror (SESAM)
with a modulation depth of 3.5%%. The laser was operated at output-coupling rates of
up to 78%%.