Simulation and experimental investigation of beam distortions in end-pumped laser rod amplifiers. Power scaling of end-pumped laser rod amplifiers is challenging due to beam distortions at high powers. It is difficult to understand these distortions since several physical effects contribute at the same time. We present a detailed investigation of the beam distortions by numerical simulation and experiments in different gain regimes (passive regime with no gain, small signal gain regime,

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... d gain regime) in order to decouple effects caused by thermal lensing from aberrations induced by the transversal gain profile. Simulation and experiment are in good agreement and show the influence of the gain regime in details and reveal a significant impact of the transversal gain profile on the output beam quality.