Darwin Infrared Nulling Interferometer Demonstrator

Darwin is a space based interferometry mission 1 of the European Space Agency (ESA) with the aim to detect and characterise earth-like planets outside our solar system. The current Darwin baseline consists of four spacecrafts (3 telescopes). Destructive interference of the starlight is required to allow detection of much fainter planet signals. The nulling ratio required is 10 5 . For Darwin high requirements are set upon the wavefront quality of the beams. In order to be able to have destructive interference with a contrast factor of 10 5 , a wavefront quality of λ/1400 (λ=6 micrometer) is needed. With current and/or foreseen technology, it is not possible to produce the optical elements with sufficient quality to meet this requirement. This means it is vital to develop wavefront filter devices for Darwin. Most promising for this purpose are single mode fibres. For visible and near-infrared light commercially available single mode fibres are available, however they do not extend yet to wavelengths above 4 micrometer. To overcome this shortcoming new single mode fibres are developed (i.e. by Astrium and TNO/ University of Rennes) for the Darwin wavelength range (6-20 μm). To characterize and test these fibres a system is designed allowing to determine the possible star light suppression with the fibre. This system is called "Darwin Infrared Nulling Interferometer Demonstrator" (DINID). The system is designed using the in-house knowledge from previous nulling set-ups in the visible and near-infrared wavelength range. It will permit to test fibres around 4 and 9 micrometer and includes an optical path difference control in order to compensate drifts. This paper describes the basis on which the set-up is designed.