This paper presents a label-free optical biosensor based on a Young's interferometer configuration that uses a vertically integrated dual-slab waveguide interferometer as sensing element. In this element, linearly polarized light is coupled into a dual-slab waveguide chip from the input end-face, and the in-coupled zeroth order mode propagates in separated upper and lower waveguides. At the output end-face, the two closely spaced coherent beams diffract out and produce an interference fringe pattern. An evanescent wave field, generated on the surface of the upper waveguide, probes changes in the refractive index of the studied sample, causing a phase shift in the fringe pattern. Compared to a conventional integrated Young's interferometer utilizing a Y-junction as the beam splitter, the dual-slab waveguide Young's interferometer has the advantage of easy fabrication and large tolerance to the input-coupling beam. This paper builds a measurement system to investigate sensor performance using glucose solutions with various concentrations. These glucose concentration measurements are performed within the physiological range of 30mg/dl ~ 500mg/dl. The results indicate that a dual-slab waveguide interferometer yields an average phase resolution of 0.002 rad, which corresponds to an effective refractive index change of 4×10-8 with an interaction path length of 15 mm.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.