The POLARIS-laser system aims at generation of high peak power femtosecond pulses in the I PW range at a repetition rate of about 0.1 Hz. We choose a direct pumping scheme and an Yb3doped fluoride phosphate glass as active material. The choice was based on its long fluorescence life time of about 1.4 ms, the availability with high quality at sizes of several tens of cm3, its broad band width, that allows generation of pulse widths below 100 fs and its low nonlinear refractive index. Antireflective coatings on the glass surface allow to use both polarizations for pumping and amplifying as well. The pump system consists of stacked laser diode bars at 940 nm wavelength focused tightly to the glass. Joule-level amplification of broad band width chirped pulses was demonstrated. The experiments yield important results for the maximum allowed energy density and the thermal issues that have to be considered in the last two POLARIS-amplifier stages.
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