Beier, F.F.BeierHupel, C.C.HupelNold, J.J.NoldKuhn, S.S.KuhnHein, S.S.HeinIhring, J.J.IhringSattler, B.B.SattlerHaarlammert, N.N.HaarlammertSchreiber, T.T.SchreiberEberhardt, R.R.EberhardtTünnermann, A.A.Tünnermann2022-03-052022-03-052016https://publica.fraunhofer.de/handle/publica/24459510.1364/OE.24.006011We report on a newly designed and fabricated ytterbium-doped large mode area fiber with an extremely low NA (~0.04) and related systematic investigations on fiber parameters that crucially influence the mode instability threshold. The fiber is used to demonstrate a narrow linewidth, continuous wave, single mode fiber laser amplifier emitting a maximum output power of 3 kW at a wavelength of 1070 nm without reaching the mode-instability threshold. A high slope efficiency of 90 %, excellent beam quality, high temporal stability, and an ASE suppression of 70 dB could be reached with a signal linewidth of only 170 pm.en621journal article