We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband XUV high harmonic radiation generated by the non-linear interaction of a fs, 10 Hz laser with neutral gas. The experiment was carried out at Lund University in collaboration with Laboratoire d’Optique Appliquée, ELI-ALPS and Imagine Optic. Optimization of the focusing optics alignment is carried out with the aid of an XUV Hartmann wavefront sensor commercialized by Imagine Optic.
Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 * 4.0 µm2 full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 *3.2 µm2.
We will show also how the optimization of the high harmonic beam by the use of an infrared adaptive optic may help for compensating the residual aberrations of the Wolter, leading to a clear improvement of the focal spot.
A compact, high-repetition rate optical parametric chirped pulse amplifier system emitting CEP-stable, few-cycle pulses
with 10 μJ of pulse energy is reported for the purpose of high-order harmonic generation. The system is seeded from a
commercially available, CEP-stabilized Ti:sapphire oscillator, delivering an octave-spanning spectrum from 600-1200 nm. The oscillator output serves on the one hand as broadband signal for the parametric amplification process and
on the other hand as narrowband seed for an Ytterbium-based fiber preamplifier with subsequent main amplifiers and
frequency doubling. Broadband parametric amplification up to 17 μJ at 200 kHz repetition rate was achieved in two
5 mm BBO crystals using non-collinear phase matching in the Poynting-vector-walk-off geometry. Efficient pulse
compression down to 6.3 fs is achieved with chirped mirrors leading to a peak power exceeding 800 MW. We observed
after warm-up time a stability of < 0.5 % rms over 100 min. Drifts of the CE-phase in the parametric amplifier part could
be compensated by a slow feedback to the set point of the oscillator phase lock. The CEP stability was measured to be
better than 80 mrad over 15 min (3 ms integration time).
The experimentally observed output spectra and energies could be well reproduced by simulations of the parametric
amplification process based on a (2+1)-dimensional nonlinear propagation code, providing important insight for future
repetition rate scaling of OPCPA systems. The system is well-suited for attosecond science experiments which benefit
from the high repetition rate. First results for high-order harmonic generation in argon will be presented.