Möller, F.F.MöllerDistler, V.V.DistlerWalbaum, T.T.WalbaumHaarlammert, N.N.HaarlammertNold, J.J.NoldSchreiber, T.T.SchreiberUnger, S.S.UngerReichel, V.V.ReichelKobelke, J.J.KobelkeBierlich, J.J.BierlichSchwuchow, A.A.SchwuchowEschrich, T.T.EschrichLeich, M.M.LeichWondraczek, K.K.WondraczekTünnermann, A.A.Tünnermann2022-03-062022-03-062020https://publica.fraunhofer.de/handle/publica/26488810.1364/OE.403460In this paper we present numerical and experimental results revealing that the mode instability threshold of highly Yb-doped, Ce/Al co-doped pedestal fibers is affected by the size of the index-increased pedestal structure surrounding the core. An alternative preparation technology for the realization of large mode area fibers with very large Al-doped silica pedestals is introduced. Three different pedestal fiber design iterations characterized by low photo darkening were manufactured and tested in counter-pumped amplifier setups. Up to 1.9 kW continuous wave output power of near-diffraction-limited beam quality (M2 = 1.26) was achieved with an 18/200/420 mm fiber of very low NA = 0.042, limited only by the occurrence of mode instabilities.encontinuous wavecounter-pumped amplifiersdiffraction-limited beamsfiber opticmode instabilities620621journal article