  The state of polarization (SOP) is an inherent property of the vectorial
nature of light and a crucial parameter in a wide range of remote sensing
applications. Nevertheless, the SOP is rather cumbersome to probe
experimentally, as conventional detectors only respond to the intensity of the
light, hence loosing the phase information between orthogonal vector
components. In this work, we propose a new type of polarimeter that is compact
and well-suited for in-plane optical circuitry, while allowing for immediate
determination of the SOP through simultaneous retrieval of the associated
Stokes parameters. The polarimeter is based on plasmonic phase-gradient
birefringent metasurfaces that facilitate normal incident light to launch
in-plane photonic waveguide modes propagating in six predefined directions with
the coupling efficiencies providing a direct measure of the incident SOP. The
functionality and accuracy of the polarimeter, which essentially is an
all-polarization sensitive waveguide metacoupler, is confirmed through
full-wave simulations at the operation wavelength of 1.55$\mu$m.
