  Coronagraphs allow for faint off-axis exoplanets to be observed, but are
limited to angular separations greater than a few beam widths. Accessing
closer-in separations would greatly increase the expected number of detectable
planets, which scales inversely with the inner working angle. The Vortex Fiber
Nuller (VFN) is an instrument concept designed to characterize exoplanets
within a single beam-width. It requires few optical elements and is compatible
with many coronagraph designs as a complementary characterization tool.
However, the peak throughput for planet light is limited to about 20%, and the
measurement places poor constraints on the planet location and flux ratio. We
propose to augment the VFN design by replacing its single-mode fiber with a
six-port mode-selective photonic lantern, retaining the original functionality
while providing several additional ports that reject starlight but couple
planet light. We show that the photonic lantern can also be used as a nuller
without a vortex. We present monochromatic simulations characterizing the
response of the Photonic Lantern Nuller (PLN) to astrophysical signals and
wavefront errors, and show that combining exoplanet flux from the nulled ports
significantly increases the overall throughput of the instrument. We show using
synthetically generated data that the PLN detects exoplanets more effectively
than the VFN. Furthermore, with the PLN, the exoplanet can be partially
localized, and its flux ratio constrained. The PLN has the potential to be a
powerful characterization tool complementary to traditional coronagraphs in
future high-contrast instruments.
