  In this contribution we investigate the transversal mode instability behavior
of an ytterbium doped commercial 20/400 fiber and obtain 2.9 kW of output power
after optimizing the influencing parameters. In this context, we evaluate the
influence of the bend diameter and the pump wavelength within the scope of the
absorption length and the length of the fiber. Furthermore with a newly
developed fiber we report on 4.4 kW of single-mode output power at 40 cm bend
diameter.
