In this Letter we report on the generation of 830 W compressed average power from a femtosecond fiber chirped pulse amplification (CPA) system. In the high-power operation we achieved a compressor throughput of about 90% by using high-efficiency dielectric gratings. The output pulse duration of 640 fs at 78 MHz repetition rate results in a peak power of 12 MW. Additionally, we discuss further a scaling potential toward and beyond the kilowatt level by overcoming the current scaling limitations imposed by the transversal spatial hole burning.

We report on the observation and experimental characterization of a threshold-like onset of mode instabilities, i.e. an apparently random relative power content change of different transverse modes, occurring in originally single-mode high-power fiber amplifiers. Although the physical origin of this effect is not yet fully understood, we discuss possible explanations. Accordingly, several solutions are proposed in this paper to raise the threshold of this effect.

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We report on the incoherent beam combination of the four narrow-linewidth fiber amplifier chains running at different wavelengths. Each main amplifier stage consists of a large-mode-area photonic crystal fiber delivering more than 2 kW of optical power. The four output beams are spectrally combined to a single beam with an output power of 8.2 kW using a polarization-independent dielectric reflective diffraction grating mainly preserving the beam quality of the individual fiber amplifiers.

We report on beam combining of four narrow-linewidth fiber amplifier chains, running at different wavelengths and each delivering 500 W optical output power. The main amplifier stage consists of a large mode area photonic crystal fiber. The four output beams of the amplifier chains are spectrally (incoherent) combined using a polarization-independent dielectric reflective diffraction grating to form an output beam of 2 kW continuous-wave optical power with good beam quality (M(2)x = 2.0, M(2)y = 1.8).