Physical foundations of achromatic nulling interferometry for stellar coronagraphy

We propose an achromatic interferometer for the observation of a faint off-axis light source against the background of a bright on-axis light source. The on-axis source and its copy acquire an achromatic phase shift by 180° and interfere with a phase difference of π. The achromatic phase shift is attributable to the geometric phase in the three-dimensional interferometer scheme. Interference spatially separates the nulled and bright fields of the on-axis source, redirecting them on opposite sides of the beamsplitter. Interference does not attenuate the field of the off-axis source and redirects it with an equal intensity on both sides of the beamsplitter. We consider the principle of operation of the nulling interferometer and constraints on the attenuation of an extended source due to the decrease in coherence. The laboratory breadboard and experiment are briefly described.