Design of spatial and modal filters for nulling interferometers

Spatial or modal filters are essential parts of highly rejecting nulling interferometers. We review the principle of operation of both types of filters and explain the fundamental physical difference. We point out the filter’s individual properties and potentials, and analyze practical limitations. For modal filters we discuss implementation alternatives, also with regard to their suitability for mid-infrared operation. For a single-mode fibre filter we analyze the broadband performance and the minimum length ensuring a prescribed filter action. We further present simulation results of a DARWIN-representative nulling interferometer breadboard which confirm the distinct improvement in rejection ratio due to spatial or modal filtering.