The increasing interest in 2 µm laser systems can be attributed to their large number of applications e.g. in medicine, remote sensing, and gas detection. Thulium doped solid-state lasers offer excellent properties for the generation of 2 µm laser radiation, since they can be pumped at around 800 nm, where high performance laser diodes are available, while due to a cross relaxation process a quantum efficiency of up to two can be achieved. Sesquioxides offer great properties as host crystals for thulium doping, such as strong Stark splitting, high thermal conductivities and low phonon energies. Recently efficient high power laser operation could be achieved in Tm:Lu2O3 and Tm:Sc2O3 [1,2] and preliminary results on mode-locking of Tm:Lu2O3 with carbon nanotube saturable absorbers were obtained . Especially for mode-locking broad and smooth emission spectra are essential for obtaining ultra short pulse lengths.
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