  Recent experiments demonstrated strongly directional coupling of light into
waveguide modes. We identify here the mechanisms behind this effect. We
consider emitters near a waveguide, either centered on the median plane of the
waveguide, or displaced from such plane. We show that, independently of the
displacement, the directionality is mostly due to a mirror symmetry breaking
caused by the axial character of the angular momentum of the emitted light. The
sign of the angular momentum along an axis transverse to the waveguide
determines the preferential coupling direction. The degree of directionality
grows exponentially as the magnitude of such transverse angular momentum
increases linearly. We trace this exponential dependence back to a property of
the evanescent angular spectrum of the emissions. A binary and less pronounced
directional coupling effect due to the chiral character of the handedness of
the emission is possible when the displacement of the emitter breaks another of
the mirror symmetries of the waveguide. We find a selection rule that allows or
prevents the coupling of centered electric(magnetic) multipolar emissions onto
the waveguide modes. We also show that the selection of a different angular
momentum axis made in some experiments causes significant differences in the
way in which directionality depends on angular momentum. We then use these
differences to propose an experiment featuring a transverse magnetic bias that
allows to aggregate the directional emissions from quantum dots on top of
waveguides. Our symmetry-based results apply to any emitted multipolar order,
clarify the spin-momentum locking concept, and generalize it to an
exponentially-strong locking between the transverse angular momentumand the
preferential coupling direction.
