  Optical nanofiber waveguides are widely used for near-field delivery and
measurement of light. Despite their versatility and efficiency, nanofibers have
a critical drawback - their inability to maintain light's polarization state on
propagation. Here, we design a directional coupler consisting of two crossed
nanofibers to probe the polarization state at the waist region. Directionality
of coupling occurs due to asymmetric dipolar emission or spin-locking when the
evanescent field pattern breaks the mirror symmetry of the crossed-nanofiber
system. We demonstrate that, by monitoring the outputs from the directional
coupler, two non-orthogonal polarization states can be prepared at the
nanofiber waist with a fidelity higher than 99%. Based on these states, we
devise a simple and reliable method for complete control of the polarization
along a nanofiber waveguide.
