Birefringent achromatic phase shifters for nulling interferometry and phase coronagraphy

Achromatic Phase Shifters are mandatory components in the field of nulling interferometry and phase coronagraphy. Indeed, in order to detect faint objects (e.g. exo-planets) around a central bright star, one must attenuate the latter by all means. To do this, the most efficient way is to "create" a destructive interference on the blinding light. This technique is called nulling and is common to the two fields mentioned above; the first with several telescopes, the second with only a single aperture. How does it work? The incoming light from the central object is divided into several beams on which one applies a differential pi phase shift by means of these so-called phase shifters. Then, when the beams are properly recombined together, they simply subtract from each other. "Achromatic" means that we can apply this process over a large spectral bandwidth. This quality will not only be required for the detection and characterization of exo-planets but also for several other astrophysical objects involving huge contrasts like dust shells around voting bright stars, AGN,... The proposed birefringent phase shifters and the original implementation schemes we suggest should provide flexible and powerful solutions for these purposes.