  We experimentally analyze the average-power-scaling capabilities of
ultrafast, thulium-doped fiber amplifiers. It has been theoretically predicted
that thulium-doped fiber laser systems, with an emission wavelength around 2
um, should be able to withstand much higher heat-loads than their Yb-doped
counterparts before the onset of transverse mode instability (TMI) is observed.
In this work we experimentally verify this theoretical prediction by operating
thulium doped fibers at very high heat-load. In separate experiments we analyze
the performance of two different large-core, thulium-doped fiber amplifiers.
The first experiment aims at operating a short, very-large core, thulium-doped
fiber amplifier at extreme heat-load levels of more than 300 W/m. Even at this
extreme heat-load level, the onset of TMI is not observed. The second
experiment maximizes the extractable average-output power from a large-core,
thulium-doped, fiber amplifier. We have achieved a pump-limited average output
power of 1.15 kW without the onset of TMI. However, during a longer period of
operation at this power level the amplifier performance steadily degraded and
TMI could be observed for average powers in excess of 847 W thereafter. This is
the first time, to the best of our knowledge, that TMI has been reported in a
thulium-doped fiber amplifier.
