Breadboarding an achromatic phase shifter for mid-infrared nulling interferometry

Direct detections of Earth-like extrasolar planets are extremely challenging and require to overcome the huge brightness contrast between two sources that have a very small angular separation. One possible solution to this problem is nulling interferometry at mid-infrared wavelengths where the flux ratio between host star and planet is more favorable than in the visible. The beams of an array of telescopes are combined so that the light from the on-axis direction (the star) is canceled by destructive interference, while the light from an off-axis direction (the planet) is kept. The global performance of such a system depends strongly on the accuracy and stability of the achromatic phase shift and the beam combination. To assess the technological feasibility of this technique, the European Space Agency (ESA) and IAS Paris have initiated a study of different physical concepts and technical realizations of achromatic phase shifters (APS) that fulfill the following requirements: allowing a >10(-6) rejection rate or better over a wavelength range 6-20 mu m and providing a transmission better than 95%. MPIA, in collaboration with the Kayser-Threde GmbH in Munich and the IOF Fraunhofer institute for applied optics in Jena has breadboarded and studied a phase shifter that is based on the geometric reversal of the electric field vectors (pupil flip) at two successive antisymmetric 90 degree reflections. In this paper we describe the bread-boarded phase-shifter device and the results of our characterization measurements in the Lab.