Nulling interferometry for spectroscopic investigation of exoplanets - A statistical analysis of imperfections

In ESA's Infrared Space Interferometry mission, a multi-aperture interferometer fed by telescopes will serve to analyse exoplanets orbiting bright stars. Spectroscopy of the planet's radiation could give hints on the possibility of the existence of life. However, for a Sun/Earth-like constellation, a star light rejection ratio of some 80 dB is required. This is the factor by which the star light is suppressed, when comparing the interferometer with a standard, wide-field-of-view telescope. We investigate the nulling capability of space-based interferometers, realized either in fiber or bulk optics, in the presence of imperfections of the structure and of optical components. Mismatch of amplitude, optical path length, and polarization among the interferometer arms is taken into account, as well as multiple reflections and telescope imperfections. The parameters describing the interferometer's receive characteristic, which are actively controlled or influenced by environmental disturbances, are modeled stochastically. We analyse Sun/Earth-like constellations by numerical simulation for a wavelength range of 6 to 18 microns. The expected value of the star light rejection ratio is calculated for several interferometer configurations. The exemplary numerical results confirm the extreme requirements for interferometer uniformity and give a quantitative insight into the dependence of the attainable rejection ratio on individual and/or combined interferometer imperfections.