We report on a high-power ytterbium doped photonic crystal fiber amplifier using a single-frequency Nd:YAG non-planar ring oscillator seed source. With a large-mode-area photonic crystal fiber, operation below the threshold of stimulated Brillouin scattering is demonstrated with up to 148 W of continuous-wave output power and a slope efficiency of 75%. At maximum output power the amplified spontaneous emission was suppressed by more than 40 dB and the polarization extinction ratio was better than 22 dB. In order to investigate the overlap of the photonic crystal fiber transverse-mode with a Gaussian fundamental mode, sensitive beam quality measurements with a Fabry-Perot ring-cavity are presented.
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