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Micromachining applications require high pulse energy (>1μJ) short pulse (<1ps) laser systems at high repetition rates. Rare-earth doped fibers are attractive to generate these target values by the amplification of ultrafast femtosecond seed sources. Two favored techniques have been used: the chirped pulse amplification (CPA) scheme where the pulses are stretched in the time domain to reduce nonlinearity in the amplifier stage and the parabolic pulse amplification scheme where the combined effect of nonlinearity, normal dispersion and gain in the fiber generate linearly chirped parabolic shaped pulses. Both approaches can be scaled to higher power by reducing the nonlinearity in the amplifiers. To achieve this, we discuss novel photonic crystal fiber designs which allow for larger single-mode core diameter and reduced absorption length and therefore reduced nonlinearity. The so generated high average power of >100 W at repetition rate up to several tens of MHz cannot be compressed by gold gratings to femtosecond pulse duration due to thermal heating. We focus on the development of dielectric gratings in fused silica which can handle this power levels due to their high damage threshold. Two kinds of gratings are discussed. Firstly, the transmission gratings with a period of 800 nm were designed to possess 96% diffraction efficiency over a spectral range from 1.03μm to 1.09μm. The fabrication of the rectangular groove profile was done using electron beam lithography and reactive ion beam etching into the fused silica substrate. The measured diffraction efficiency was 96.5% @ 1060nm. Secondly, dielectric reflection gratings, which consist of a dielectric grating on top of a high-reflective layerstack, can theoretically exhibit a diffraction efficiency of even higher than 99%. To achieve this we chose a period of 1060nm. The fabrication was done similar to the transmission gratings, though a HR-coated substrate had to be used instead of the simple fused substrate. The fabricated gratings show a diffraction efficiency of 99.6%. Both are applied to the discussed high power fiber amplifier stages to generate linearly polarized femtosecond pulses at ~100 W average power with a repetition rate of 80 MHz.
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