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.