Evanescently coupled dewpoint sensor based on a silicon waveguide

A dewpoint sensor is described, based on the evanescent field generated at the surface of a silicon wafer. The wafer, polished on both surfaces, is used as a slab waveguide for light of wavelength 1330 nm. The light is coupled into high-order modes and undergoes multiple total internal reflections at the silicon/air interface. The critical angle shifts in response to the change in refractive index associated with condensation of water vapour on the exposed surface. Consequently, the modes are no longer confined and the resultant reduction in transmitted intensity provides a sensitive measure of the onset of condensation. This alternative to the well-established chilled mirror hygrometer may prove more amenable to the production of miniaturised, integrated sensor systems.