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Prototype for the European Southern Observatory VLTI fringe sensor

The operation of stellar interferometers suffers from turbulence- induced random fluctuations of optical pathlength difference between collected fields. Active compensation needs an error signal which is provided by a fringe sensor. A phase A study for a fringe sensor for the ESO Very Large Telescope Interferometer (VLTI) has been conducted at OCA, leading to a proposition for the completion of a prototype. In this article, the goals and the principle of the sensor are recalled (see Gay and Rabbia, preceding paper, in this symposium). Its optimal working wavelength is discussed. Results of a numerical simulation of the sensor operation are reported, comprising sensitivity estimates. The proposed setup is then described in the details, emphasis being put on monomode optical fiber related items. Finally, current plans for the testing and the future use of the prototype are outlined.