  We propose to explore a cascade extreme Adaptive optics (ExAO) approach with
a second stage based on a Zernike wavefront sensor (ZWFS) for exoplanet imaging
and spectroscopy. Most exoplanet imagers currently use a single-stage ExAO to
correct for the effects of atmospheric turbulence and produce high-Strehl
images of observed stars in the near-infrared. While such systems enable the
observation of warm gaseous companions around nearby stars, adding a
second-stage AO enables to push the wavefront correction further and possibly
observe colder or smaller planets. This approach is currently investigated in
different exoplanet imagers (VLT/SPHERE, Mag-AOX, Subaru/SCExAO) by considering
a Pyramid wavefront sensor (PWFS) in the second arm to measure the residual
atmospheric turbulence left from the first stage. Since these aberrations are
expected to be very small (a few tens of nm in the near-infrared domain), we
propose to investigate an alternative approach based on the ZWFS. This sensor
is a promising concept with a small capture range to estimate residual
wavefront errors thanks to its large sensitivity, simple phase reconstruction
and easiness of implementation. In this contribution, we perform preliminary
tests on the GHOST testbed at ESO to validate this approach experimentally.
Additional experiments with petalling effects are also showed, giving promising
wavefront correction results. Finally, we briefly discuss a first comparison
between PWFS-based and ZWFS-based second-stage AO to draw preliminary
conclusions on the interests of both schemes for exoplanet imaging and
spectroscopy with the upgrade of the current exoplanet imagers and the
envisioned ExAO instruments for ELTs.
