Otto, Hans-JürgenHans-JürgenOttoJauregui, CesarCesarJaureguiEidam, TinoTinoEidamStutzki, FabianFabianStutzkiJansen, FlorianFlorianJansenLimpert, JensJensLimpertTünnermann, AndreasAndreasTünnermann2022-03-042022-03-042012https://publica.fraunhofer.de/handle/publica/22915710.1364/CLEO_SI.2012.CF2N.72-s2.0-848703547742-s2.0-84890772755Mode instabilities occurring in fiber laser systems during high power operation are currently the most limiting effect for average power scaling. We experimentally and theoretically investigate this effect. A high-resolution high-speed camera has been used to temporally resolve the dynamics of mode-instabilities. We show that periodic fluctuations of the beam are observable right above the mode instability threshold. Additionally, the temporal fluctuations become slower with larger mode-field diameters. These results support the thesis of a thermally induced long-period grating as the origin of mode-instabilities.en620journal article