We report on the high power amplification of narrow linewidth laser radiation with close to diffraction limited beam quality using a large mode area photonic crystal fiber amplifier. The observation of threshold-like higher order mode amplification by transverse spatial-hole burning at the highest power level is reported. The measured MÃÂ² stays below 1.3 but increases at the critical power level, where the fundamental mode turns into the next higher order mode. At the maximum power of 1.2 kW a linewidth of <80 pm limited by self-phase modulation is obtained.© (2010) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.