Schematic diagram of a dual field optical interferometer using two telescopes. A beam splitter creates two images of the focal plane, where two fibres are positioned – one on the star to feed Interferometer 1 (serving as the phase reference) and the other on the exoplanet, feeding Interferometer 2. The exoplanet is detected as coherent flux in Interferometer 2, with a phase offset relative to the stellar coherent flux due to an optical path delay δOPD, corresponding to the scalar product of the narrow angle baseline vector B and the angular separation between the star and exoplanet α: δOPD = α ⋅ B.