  We study analytically the polarization behaviour of directional couplers
composed of birefringent waveguides, showing that they can induce polarization
transformations that depend on the specific input-output path considered. On
the basis of this study, we propose and demonstrate experimentally, by
femtosecond laser writing, directional couplers that are free from this problem
and also yield a polarization independent power-splitting ratio. More in
detail, we devise two different approaches to realize such devices: the first
one is based on local birefringence engineering, while the second one exploits
ultra-low birefringence waveguides obtained by thermal annealing.
