The achromatic chessboard, a new concept of a phase shifter for nulling interferometry III. Experimental performances

Context. Dark fringe interferometry in the thermal infrared is one way to detect directly a planet orbiting a star, and so to characterize the planet's atmosphere through spectroscopy. This method demands a phase shift of pi(1) in one arm of the interferometer. In order to detect various bio-tracers gases, a broad wavelength range (6-18 mu m)(2-3) is necessary, therefore an achromatic phase shift of p is required. The achromatic device presented here is a phase shifter made of two cellular mirrors, in which each cell induces a specific phase shift. Aims. We wish to demonstrate that this theoretical concept is experimentally valid. We present in this paper the setup and the very first results. Methods. In a first step, we have consolidated the theoretical ground and in a second step we developed an optical bench in the visible domain to test the concept and measure the performances of this device. Results. The preliminary experimental tests show evidences that such a device is working as expected in terms of nulling and achromatism: in spite of an error on one cell of the prototype, it provides a nulling of 2.10(-3) at one wavelength, and this value is close to the expected value. Besides, a nulling of 1.10(-2) in a 450 to 750 nm bandwidth: a hint that a perfect device should be achromatic.