A high-power Laser Beam with a single optical Frequency can be fed through an optical cavity (resonator) in order to improve its beam quality. When the laser frequency is locked to a Fundamental Mode resonance, the higher-order Modes of the Resonator are not in resonance and are therefore strongly attenuated in transmission. The transmitted beam can thus have a strongly improved beam quality. At the SAME time, there is some cleanup of the optical spectrum. For good power efficiency, the Mode matching of the input beam to the fundamental mode of the resonator has to be optimized.