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High-power lasers are characterized by high gains and large apertures. This leads to multimode operations of most high-power lasers (longitudinal and transverse), which in turn results in the complicated spatiotemporal modal behavior and the partial coherence of high-power laser beams. In section 2 we present some experimental results which show that the characterization of high-power laser beams using the beam quality such as K* and M2 is not sufficient and the application results are dependent on the intensity profile, its propagation and intensity fluctuations. According to the spatial coherence of the laser beam the average intensity distribution along the propagation can change distinctly. This is especially true in the case of laser beams from unstable resonators. To characterize high-power laser beams the propagation of the intensity profile has to be described. As shown in section 3 the propagation of the intensity profile is determined if the mode coherence coefficients (MCCs) are known. Furthermore, temporal local fluctuations notorious with high power lasers impair all applications, notably those with high processing speed and high process quality. Therefore, to correlate the application results to the laser beam performances the information about the intensity fluctuation is necessary. In section 4 we discuss a method to characterize the local intensity fluctuation. Finally, the main results are summarized in section 5.
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