The present invention relates to an assembly and method for measuring the optical activity of a stimulus of interest. The assembly comprises a source of incident electromagnetic radiation (1), a lens arrangement (2), an input polariser (3), a planar waveguide structure (4) (or an optical fiber), an output polariser (5), a lens arrangement (6) for focussing the output electromagnetic radiation and a detector. The polarisation of the incident electromagnetic radiation relative to the planar waveguide structure (4) is determined by the input polariser (3). The planar waveguide structure in this embodiment comprises a silicon substrate layer (4c) and an absorbent layer (4e). The silicon oxynitride layer (4c) acts as the reference waveguide and the absorbent layer (4e) acts as the sensing waveguide. The sensing waveguide (4e) is exposed in the localised environment (8) to a sample containing the stimulus of interest which is optically active. The adjusted attitude of the output polariser (5) is directly proportional to the optical activity of the stimulus of interest.