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Dynamics and origin of mode instabilities in high power fiber laser amplifiers

Mode 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.