  A new wavefront sensing approach, derived from the successful curvature
wavefront sensing concept but using a non-linear phase retrieval wavefront
reconstruction scheme, is described. The non-linear curvature wavefront sensor
(nlCWFS) approaches the theoretical sensitivity limit imposed by fundamental
physics by taking full advantage of wavefront spatial coherence in the pupil
plane. Interference speckles formed by natural starlight encode wavefront
aberrations with the sensitivity set by the telescope's diffraction limit
lambda/D rather than the seeing limit of more conventional linear WFSs.
Closed-loop adaptive optics simulations show that with a nlCWFS, a 100 nm RMS
wavefront error can be reached on a 8-m telescope on a mV = 13 natural guide
star. The nlCWFS technique is best suited for high precision adaptive optics on
bright natural guide stars. It is therefore an attractive technique to consider
for direct imaging of exoplanets and disks around nearby stars, where achieved
performance is set by wavefront control accuracy, and exquisite control of low
order aberrations is essential for high contrast coronagraphic imaging.
Performance gains derived from simulations are shown, and approaches for high
speed reconstruction algorithms are briefly discussed.
