Efficient spectral broadening of multi-mJ pulses in long hollow fibers

In this work, the capability of the new capillary design for spectral broadening of multi-mJ pulses is explored. It si demonstrated that the conservative approach (linearly polarized pulses and constant gas pressure) can work well even in the multi-mJ regime if the dimensions of the waveguide are scaled up properly. In the experiment, 2.1 mJ, 36 fs pulses from a kHz Ti:sapphire CPA system were launched into a 3 meter long stretched flexible hollow fiber assembly of 450 μm inner diameter. In case of evacuated fiber the measured transmission is 67.5%. Results show that the core contains 1.38 mJ while the full beam has 1.52 mJ energy. To the authors´ knowledge this is the highest throughput ever achieved for the spectral broadening of multi-mJ pulses in a hollow fiber facilitating few-cycle pulse generation. In the experiment, the output energy is limited exclusively by the laser and not by the fiber stage. Further experiments on higher-energy lasers are in progress, whose results will also be presented.