Abstract A new design of a highly sensitive optical biosensor is described. It consists of a planar waveguide with a corrugated surface, tuned to be a resonant Bragg reflector. Only the heights of the corrugation are covered by an opto-chemical transducer layer that selectively adsorbs target biomolecules. Upon adsorption, the corrugation depth rises and reflection coefficient changes. This is used to monitor surface reactions. Theoretical analysis and mathematical modeling proves that the device sensitivity compares favorably with current known designs. The proposed biosensor is shown to possess a unique capability of unambiguous discrimination between surface reactions and refraction index changes in the bulk solution.