Composite optical waveguide structures: technology and features

Abstract

ABSTRACT The paper presents complex waveguide structures which have been fabricated on a basis of the gradient waveguide on the top of which a uniform waveguide film of high refractive index is deposited. The gradient waveguides were fabricated using th e ion exchange method, whereas the uniform film with the application of the sol -gel method. The application of such a waveguide structure in the system of difference interferometer makes it possible to obtain high sensitivities of the effective refractive index against the changes of sensor layer parameters , in particular when the interference of the basic mode TM 0 with the first order mode TM 1 is applied. The paper presents the theoretical analysis of complex waveguide structures, their fabrication technol ogy and the r esults of experimental studies. Keywords: composite optical waveguide, sol -gel, ion -exchange, evanescent wave sensor, grating coupler 1. Introduction The application of optical transducers in the systems of chemical and biochemical systems off ers a potential possibility to obtain high sensitivities [1,2], in particular in planar sensors working in the techn ique of Evanescent Field Spectroscopy. Due to the properties of planar technologies, the structures fabricated using them can be modified wi thin a wide range. Hence , the planar sensor structures are relatively easy to optimize. The main physical effects used in planar chemical and biochemical sensors involve the following quantities: a change of a refractive index of the waveguide cover and a change of sensitive film thickness [1]. The presence of these effects brings about the change of effective refractive indexes of modes propagating in the structure. The se changes can be measured with the use of grating couplers [1 -4] or planar interferomet ers. The latter ones can be fabricated on a basis of slab waveguides [1,2] or channel waveguides [5,6 ,7] and can be of different types: Mach -Zehnder, Young, Michelson interferometers [ 8,9,10 ] as well as the difference interferometer s [1 -3,11 ,12 ]. In the planar differe nce interferometer , a single optical path is applied, made up by a planar or channel waveguide [1 ,2]. A different impact of the changes of waveguide parameters on the modes propagating in the waveguide are applied. The signal obtained from th e interference of the applied modes can be indicative of the change of the parameters of waveguide cover. In the difference interfer ometer, single - or multimode (two -mode) planar or channel waveguides can be applied. In the single -mode systems, the fundame ntal modes are used [1,2]. T heoretical investigations of the influence of selected waveguides geometric al parameters and wavelength on their homogeneous sensitivity can be found in [ 13 ,14 ]. In the two -mode systems, we apply the interference of different or der modes having the same polarization states [3]. The interference of modes of the same polarization can be realized in gradient waveguides, homogeneous waveguides [1,2] or in complex waveguide structures [11 ,12 ]. The complex waveguide structures are comp osed of the ion -exchange gradient waveguide and the homogeneous film of high refractive index deposited on it . The highest sensitivities can be obtained in complex waveguide structures in which the gradient waveguide is characterized by a low refractive in dex gradient, and the homogeneous waveguide layer h as high refractive index [11 ,12 ]. The paper concentrates on complex waveguide structures. Gradient waveguides were fabricated using the method of ion exchange in glass, and the homogeneous waveguide layers were fabricated with the use of sol -gel method. The paper presents the results of theoretical analysis of complex waveguide structures and the r esults of experimental studies.