A planar optical waveguide interferometer includes a substrate, a bimodal waveguide having at least one layer deposited on the substrate, and a sensor plate located in a selected area of the upper side of the bimodal waveguide. The bimodal waveguide supports a zero-order and a first-order transverse propagating modes where the transverse propagating modes has different dispersion. The bimodal waveguide is designed for confining light in lateral direction and thus is designed for supporting one lateral mode. The sensor plate is designed for receiving a chemical, biological or physical stimulus. The stimulus changes the effective refractive index of said bimodal waveguide. Chip, sensor and sensing method utilize a planar optical waveguide interferometer are provided.