Planar optical waveguide interferometer (15, 25, 35, 45) comprising: a substrate (8, 28, 38, 48); a bimodal waveguide (10, 20, 20′, 30, 40) comprising at least one layer (1, 2, 3) deposited on said substrate (8, 28, 38, 48), said bimodal waveguide (10, 20, 20′, 30, 40) being designed for supporting a zero-order and a first-order transverse propagating modes, said transverse propagating modes having different dispersion; a sensor plate (21, 31, 41, 51) located in a selected area of the upper side of said bimodal waveguide (10, 20, 20′, 30, 40), said sensor plate (21, 31, 41, 51) being designed for receiving a chemical, biological or physical stimulus, said stimulus being capable of changing the effective refractive index of said bimodal waveguide (10, 20, 20′, 30, 40). The bimodal waveguide (10, 20, 20′, 30, 40) comprises confining means (9) designed for confining light in lateral direction, the bimodal waveguide (10, 20, 20′, 30, 40) being thus designed for supporting one lateral mode. Chip, sensor and sensing method comprising a planar optical waveguide interferometer are provided.