Wirth, C.C.WirthSchreiber, T.T.SchreiberRekas, M.M.RekasTsybin, I.I.TsybinPeschel, T.T.PeschelEberhardt, R.R.EberhardtTünnermann, A.A.Tünnermann2022-03-112022-03-112010https://publica.fraunhofer.de/handle/publica/36671710.1117/12.841650We report on the high power amplification of narrow linewidth laser radiation with close to diffraction limited beam quality using a large mode area photonic crystal fiber amplifier. The observation of threshold-like higher order mode amplification by transverse spatial-hole burning at the highest power level is reported. The measured M² stays below 1.3 but increases at the critical power level, where the fundamental mode turns into the next higher order mode. At the maximum power of 1.2 kW a linewidth of <80 pm limited by self-phase modulation is obtained.enfiber laserfiber amplifier620conference paper