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Mode Locking of an Active Multipass Geometry

Ultrashort laser pulses in the microjoule regime are of prime importance for manyapplications. Previous record pulse energiesobtained directly from an oscillator in ambient atmosphere were below 2 Î1⁄4J. This thesis describes a newly developed thin-disk laser oscillator with an increasedround-trip gain by employing a self-imaging active multipass geometry. The generationof high energy subpicosecond pulses with a pulse energy of up to 25.9 Î1⁄4J at apulse duration of 928 fs, while operated in ambient atmosphere, could be demonstrated.Stable single-pulse operation has been obtained with an average outputpower in excess of 76W at a repetition rate of 2.93MHz. Self starting passive modelockingwas accomplished using a semiconductor saturable absorber mirror (SESAM)with a modulation depth of 3.5%. The laser was operated at output-coupling rates ofup to 78%.

However, especially for an AMC laser, this approach is only applicable within
certain limits: as is shown in Chap. 4, the stability width of an AMC, in terms of
dioptric power of the TD, strongly decreases with increasing spot size on the TD.