Highly efficient dielectric gratings for high-power ultrafast femtosecond fiber laser systems

Abstract

Micromachining applications require high pulse energy (>1μJ) short pulse ( 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.