  The direct imaging of an Earth-like exoplanet will require sub-nanometric
wavefront control across large light-collecting apertures, to reject host
starlight and detect the faint planetary signal. Current adaptive optics (AO)
systems, which use wavefront sensors that reimage the telescope pupil, face two
challenges that prevent this level of control: non-common-path aberrations
(NCPAs), caused by differences between the sensing and science arms of the
instrument; and petaling modes: discontinuous phase aberrations caused by pupil
fragmentation, especially relevant for the upcoming 30-m class telescopes. Such
aberrations drastically impact the capabilities of high-contrast instruments.
To address these issues, we can add a second-stage wavefront sensor to the
science focal plane. One promising architecture uses the photonic lantern (PL):
a waveguide that efficiently couples aberrated light into single-mode fibers
(SMFs). In turn, SMF-confined light can be stably injected into high-resolution
spectrographs, enabling direct exoplanet characterization and precision radial
velocity measurements; simultaneously, the PL can be used for focal-plane
wavefront sensing. We present a real-time experimental demonstration of the PL
wavefront sensor on the Subaru/SCExAO testbed. Our system is stable out to
around ~400 nm of low-order Zernike wavefront error, and can correct petaling
modes. When injecting ~30 nm RMS of low order time-varying error, we achieve
~10x rejection at 1 s timescales; further refinements to the control law and
lantern fabrication process should make sub-nanometric wavefront control
possible. In the future, novel sensors like the PLWFS may prove to be critical
in resolving the wavefront control challenges posed by exoplanet direct
imaging.
