Conventional adaptive optics systems correct the wavefront by
adjusting a deformable mirror based on measurements of the phase
aberration taken in a pupil plane. The ability of this technique,
known as phase conjugation, to correct aberrations is normally
limited by the maximum spatial frequency of the DM. In this paper
we show that conventional phase conjugation is not able to achieve
the dark nulls needed for high-contrast imaging. Linear
combinations of high frequencies in the aberration at the pupil
plane "fold" and appear as low frequency aberrations at the
image plane. After describing the frequency folding phenomenon,
we present an alternative optimized solution for the shape of the
deformable mirror based on the Fourier decomposition of the
effective phase and amplitude aberrations.
Conventional adaptive optics methods use phase conjugation based
on measurements of the phase aberrations at the pupil plane. The
measurements are typically done using a Shack-Hartmann sensor
sampling at spatial frequencies determined by the spatial
frequency limitations of the deformable mirror. The work presented
here shows that the nulling needed for high contrast imaging
cannot be achieved using such a methodology. Linear combinations
of high frequencies in the aberration at the pupil plane "fold"
and appear as low frequency aberrations at the image plane. We
present an optimized solution for the shape of the deformable
mirror based on the Fourier decomposition of the effective phase
aberration.
We present a numerical investigation of the parameters characterizing
the recombination gain in the LiIII 2→1 transition
(13.5nm). The numerical model includes the initial Optical Field
Ionization (OFI) of the plasma by an intense 100fs laser pulse, taking
into account ATI heating, particle collisions and spatial
effects. Gain is then calculated during the process of recombination
as the plasma expands and cools down. We characterize the behavior of
the gain under different pumping parameters and initial plasma
conditions. In addition we present the latest experimental results on
Raman amplification of ultrashort pulses in the pursuit for a better
and more compact scheme to enable a ``table-top'' x-ray laser.
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