Alternative fringe sensor for DARWIN mission

Large stellar telescope is indispensable for astronomy. Aperture synthesis is a well-known technique to simulate a large space telescope by an array of small telescopes. Condition for aperture synthesis is that the light of the telescopes have to be combined coherently. Therefore, an interferometric Fringe Sensor (FS) to detect and stabilize the Optical Path Difference (OPD) between light from the different telescopes is required. Conventional Fringe Sensor for Space Interferometer utilizes either Quadrature Stabilization or Double Synchronous Detection to find and control OPD=0. OPD demodulation based on Quadrature Stabilization is sensitive to change in the visibility V of the interferometric signal, while Double Synchronous Detection requires an active modulation of the OPD to generate the required carrier signal. To overcome these problems, TNO develops a Fringe Sensor based on a 3x3 Fiber Optic (FO) coupler. A breadboard demonstrator operating around 830 nm is built. A piezo stretcher and a translation stage are used to generate the OPD. High-speed OPD measurement down to 0.15 nm is demonstrated. The influence of the visibility V of the interferometric signal is also investigated. Even for V=0.2, an OPD modulation of 0.4 nm can still be detected.