Laboratory characterization of the chessboard achromatic phase shifter

We recently presented a new concept for designing an achromatic phase shifter. An APS is required in nulling interferometry, a technique that aims at directly detecting and characterizing planets around a star in the thermal infrared. Our solution is based on two cellular mirrors (alternatively, transparent plates can be used) where cells have thickness which introduce OPD that are respectively odd and even multiples of half the central wavelength, on the fraction of the wave it reflects. A destructive interference is thus produced on axis for the central wavelength when recombining the two beams. We have shown that if the thicknesses are distributed according to the Pascal triangle, a fair quasi-achromatism is also reached on typically one octave in wavelength, provided there is a sufficient number of cells. The major interest of this solution is that it allows a compact, simple and fully symmetric design, without complex sub-systems to adjust. In this paper, after reminding the basic concept, we first present the theoretical estimations for the expected performances in the two possible regimes of recombination: on axis and multi-axial (Fizeau). We then describe the laboratory setup of the demonstration bench we are developing, as well as the first results obtained.