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

Vortex fiber nulling 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 from coupling into the fundamental mode of a single-mode optical fiber. Light from the planet, on the other hand, couples into the fiber and is routed to a spectrograph. Using a prototype vortex fiber nuller (VFN) designed for monochromatic light, we demonstrate coupling fractions of $6\times10^{-5}$ and $>0.1$ for the star and planet, respectively.