Achromatic phase shifters for nulling interferometry

In the search for extra-solar Earth-like planets, nulling interferometry is required. This technique provides the extinction of the light from the parent star so as to "see" objects in its immediate vicinity. This nulling process is based on appropriate phase shifts applied to the collected waves made to destructively interfere. These phase shifts must be performed in the thermal infrared domain and must be achromatic so as to enable working on a large bandwidth. After briefly recalling the principle of nulling interferometry, we state what should mean "achromatic" in the context and we present various techniques potentially able to meet the stated requirements. Critical points and interesting features of the presented techniques are outlined.