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12 September 2012 Birefringence compensation in PIONIER
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In an optical interferometer, the phase of astronomical signals is corrupted not only by the Earth’s atmosphere, but also by instrumental artifacts, among which birefringence, that affects the relative phase of two orthogonal polarizations. Instrumental birefringence occurs at each mirror reflection, but can also occur in the instrument where the signals are recombined., especially when optical fibers are used on the signal path. We present here a new technique to cancel instrumental birefringence, in the context of the PIONIER instrument at the VLTI (ESO, Paranal observatory). This is achieved by introducing in each of the beams a birefringent plate with an adjustable inclination, that allows to cancel the differential birefringence between the beams before recombination. We present our derivation of the amount of birefringence introduced by such an inclined plate. We show the actual performance achieved with the Pionier instrument. The initial alignment takes of order of one hour, and is stable for several days. Tweaking is performed at the beginning of each observing run and takes approximately fifteen minutes. We demonstrate a simple and effective technique to cancel differential birefringence in interferometric instruments. This technique should be applicable to second generation VLTI instruments.
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Bernard Lazareff, Jean-Baptiste Le Bouquin, Jean-Philippe Berger, Laurent Jocou, Pierre Kern, and Gérard Zins "Birefringence compensation in PIONIER", Proc. SPIE 8445, Optical and Infrared Interferometry III, 84452W (12 September 2012);

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