An optical waveguide to fiber coupler comprises a substrate, a first waveguide and a second waveguide. The first and second waveguides are formed on the substrate and intersect at a right angle. A diffraction grating structure is formed at the intersection of the first and second waveguides, such that, when the coupler is physically abutted with a single mode optical fiber, in operation, a polarization split is obtained that couples orthogonal modes from the single-mode optical fiber into single identical modes in the first and second waveguides. Also, employing the coupler in optical polarization diverse applications provides for implementing a polarization insensitive photonic integrated circuit using such diffraction grating structures, such as, for example, photonic crystals.