In the examples provided herein, a system includes a loop waveguide; and a grating coupler formed on the loop waveguide to couple light impinging on the grating coupler having a first polarization into the loop waveguide in a first direction, and to couple light having a second polarization, orthogonal to the first polarization, into the loop waveguide in a second direction. The system also includes a ring resonator positioned near the loop waveguide tuned to have a resonant wavelength at a first wavelength to couple light at the first wavelength out of the loop waveguide into the ring resonator. An output waveguide positioned near the ring resonator couples light out of the ring resonator into the output waveguide; and a photodetector detects light propagating out of a first end and a second end of the output waveguide.