The present invention is directed to an evanescent field induced optical sensor of a transparent polymer substrate coated with a thin transparent polymer waveguide for use as biosensor for detecting biological molecules, wherein the transparent polymer waveguide has a thickness of .gtoreq.0.10 .mu.m and .ltoreq.0.50 .mu.m and a refractive index n.sub.D of 1.39 to 1.79, said transparent polymer waveguide has a coupling grating recess structure for enhancing the coupling of the light wave into said transparent polymer waveguide, the lower surface of said transparent polymer waveguide contacts the transparent polymer substrate, said transparent polymer substrate has a refractive index n of 1.29 to 1.69, wherein the material of said transparent polymer waveguide and said transparent polymer substrate is selected such that the difference of the refractive index .DELTA.n.sub.D of said transparent polymer waveguide and said transparent polymer substrate is at least .DELTA. n 0.1, the outer upper surface of said polymer waveguide possess specific binding compounds to detect at least one specific chemical and/or biochemical substance or the outer upper surface of said polymer waveguide is covered with a thin noble metal layer.