Use of OPD modulation techniques in nulling interferometry

Future nulling space interferometers, such as Darwin and TPF, under study by the European Space Agency and NASA respectively, will rely on fast internal modulation techniques in order to extract the planet signal from the much larger background noise. In this modulation scheme, the outputs of a number of sub-arrays are combined with a variable, achromatic phase shift. In this paper, we discuss the use of well-known OPD modulation techniques in nulling interferometry. The main attractiveness of this approach is that a small OPD modulation at frequency f will modulate the stellar leakage at frequency 2f, since leakage does not depend from the sign of the OPD. In turn, a planet transiting a quasi-linear portion of the transmission map will induce a signal at frequency f at the nulled output, which can be extracted by coherent detection techniques. The properties of this modulation scheme are analyzed, using the Bracewell configuration as a test case. The significance of this technique for ESA's Darwin mission, and its ground-based technology precursor GENIE, are discussed.