Inherent modulation: a fast chopping method for nulling interferometry

The reduction of the thermal background emission from the local and exozodiacal dust clouds is a critical element for the success of ESA's space mission, DARWIN. Internal modulation, a technique using fast signal chopping, isolating the planetary signal from these noise sources, was proposed by Mennesson and Léger. In this paper, a short review of internal modulation is given, and new configurations with internal modulation are proposed to reduce the complexity of the beam-combining optics. A modification to the implementation of internal modulation is then investigated. It provides similar performance with a single detector and a greatly simplified optical layout: the number of beam-combiners is reduced by a factor of about two. The principle of inherent modulation is different from internal modulation in that no sub-interferometers are used: different phase shifts are applied to the input beams before recombination such that an asymmetric transmission map is obtained directly, without plus or minuspi/2 modulation as used in internal modulation. By combining the phase shifts and the input beams differently a transposed transmission map is obtained, allowing the signal to be chopped. During operations, multiplexing between the two interferometers is performed, such that at any time only one interferometer is being used.