Notes. The test samples are cables made of a fiber from the Fiber Coupler, a FDDL, and a FPR. Roughly 2 m of fibers at ≈ 290 K were connected to the polarization bench outside the cool vessel. The FDDLs and FPRs are paired (labels A to L) as used on the instrument and labeled accordingly as either Fringe Tracker or Science Channel. Telescope is the telescope number in the GRAVITY instrument. Telescope 1 is fed by either AT4 or UT4, Telescope 2 by AT3 or UT3, etc. The basic measurement is the differential birefringence phase δφ. No error bars are given but the method allows to detect phases as low as 0.05 rad, giving an idea of the accuracy of the measurements. The beating length L_B and the minimum and maximum contrasts are deduced from this quantity. The ranges of fringe contrasts are obtained by pairing a fiber with the fibers of closest or most different measured birefringence in a given channel, the lowest contrast being obtained by adding the birefringent phases, and the highest contrasts by subtracting them.