Vortex fiber nulling for exoplanet observations I Experimental demonstration in monochromatic light

Vortex fiber nulling (VFN) is a method for spectroscopically characterizing exoplanets at small angular separations, $\lesssim\lambda/D$, from their host star. The starlight is suppressed by creating an optical vortex in the system point spread function, which prevents the stellar field (on-axis) from coupling into the fundamental mode of a single-mode optical fiber. Light from the planet, on the other hand (slightly off-axis), couples into the fiber and is routed to a spectrograph. Using a prototype VFN designed for monochromatic light, we demonstrate coupling fractions of $6\times10^{-5}$ and $>0.1$ for the star and planet, respectively.