The achromatic chessboard, a new concept of a phase shifter for nulling interferometry V. Experimental demonstration and performance

Context. To find evidence one day of life on extra solar planets, one will have to directly detect photons of the exoplanet to obtain spectra and to look for specific spectroscopic features. One possible technique is dark fringe interferometry with several telescopes in space. This type of interferometry requires an achromatic p phase shift in one arm of an interferometer. We have already presented a concept of a quasi-achromatic phase shifter that is made of two cellular mirrors in which each cell position and phase shift is specific, so that the behavior of the null depth as a function of the wavelength is flat within a broad range. Aims. We want to experimentally validate this concept of an achromatic phase shifter and measure its performance in two different cases: a transmissive mask, which is made in bulk optics that are machined with ion etching and a reflective one, which is made by using a commercial segmented deformable mirror that is properly controlled. Methods. We assembled a dedicated optical bench, nicknamed DAMNED, to assess the concept and characterize its performance in the visible and to determine the limitations of this phase shifter. We analyze its performance by comparing the experimental results with a numerical instrument model. Results. We tested several transmissive masks and a reflective one. We reached an attenuation of about 2 x 10(-3) with a white source (Delta lambda = 430 to 830 nm) that proved to be the actual achromatic behavior of the phase shifter, despite its modest value. Extrapolated to mid-IR, its performance would be within typical specifications of a space mission as Darwin.