Young's fringes from vertically integrated slab waveguides: Applications to humidity sensing

Using a multiple layer optical waveguide system consisting of two verticall y slab waveguides, classical Young's fringes may be obtained in the far-fie ld diffraction plane. In agreement with the simple theory of diffraction in terference the spacing of the far-field fringes is easily observed on mm to cm dimensions without further transformation of the output light. The simp le methods of fabrication and means of optical coupling should provide a re adily adaptable method for examining the principles of interferometry in an integrated optical format. The structure acts to transform polarized incid ent plane wave input light into separate slab modes of the device which eme rge as two closely spaced and coherent sources at the output. The elements required for a classical Young's fringe demonstration are therefore all emb odied in this approach. The basic concept can be applied to an optical meth od for sensing. In one example of this we demonstrate measurement of the ph ase difference induced between the upper and lower propagating modes in str uctures due to water vapor diffusion into the layers which are formed from hydrophilic polymers. The Young's fringe patterns exhibit a spatial intensi ty distribution which is sensitive to water vapor introduced over the surfa ce of the structure. Differences in the effective index between the modes o f the two waveguides during the diffusion of the vapor causes phase shifts which result in redistribution in the fringe pattern. The anticipated limit of detection of these devices is lower than 1 ppm for water vapor. (C) 199 9 American Institute of Physics. [S0021-8979(99)03823-2].