optical based optical waveguide grating The optical grating coupler sensor responds to the change of optical refractive index of the liquid or gas cover medium and to the adsorption or binding of molecules on the surface. The optical grating coupler sensor chip is based on a fine optical grating prepared on a thin waveguide layer carried by a glass substrate. The optical grating couples the light of a He-Ne laser at a given resonance angle into the waveguide layer. This incoupling angle is very sensitive to the presence of adsorbed molecules and to any change of refractive index of the medium covering the surface of the chip. By precise measurement of the incoupling angle, the amount of the adsorbed material can be determined with ultra high sensitivity.