Influence of optical path delay control on a ground-based nulling interferometer

Atmospheric turbulence is a major source of noise in any fiber-linked interferometer through the piston effect between the two arms of the interferometer which induces an erratic movement of the fringes. Because the stellar light has to be permanently cancelled by the central dark fringe, a ground-based nulling interferometer is not possible without stringent optical path delay (OPD) control. In this paper, we investigate the influence of the residual OPD error of a fringe tracking unit on the performances of a nulling interferometer. The accuracy required for this control system strongly depends on the observation wavelength. If we want to detect exozodiacal clouds ten times as dense as our zodiacal dust cloud in the near-infrared (L' band), the performances of the fringe tracker are crucial: the residual OPD should be about 20 nanometers RMS. This specification can only be reached if the fringe sensor is optimized for bright sources. In the mid-infrared (N band), the requirements are strongly relaxed: OPD control with an accuracy of 400 nm RMS is sufficient to be background-limited, but a lower residual OPD (about 50 nm RMS) is strongly recommended to reduce the stellar leakage.