title: The design of ring-resonators for integrated optics using silver ion-exchanged waveguides
Creators: name: Walker, Robert Graham
Publisher: name: University of Glasgow
Year: 1981
Subjects: subject: Electrical engineering, optics.

description: This thesis is concerned with the design and ultimately, the fabrication of a ring-resonator filter for integrated optics. The resonant structure of such a filter consists of a closed circular loop of stripe (2-D) optical waveguide which resonates whenever the circumference is an integer number of guide-wavelengths. It thus possesses a "comb" frequency response. In order to construct the required conceptual framework, a fairly brief treatment of dielectric waveguide theory is included. This is followed by a more detailed consideration of the elements which constitute a ring-resonator; the directional coupler (used for coupling light into the ring), the curved dielectric waveguide and the resonator itself. The .optimum; input coupling is derived in terms of the ring losses and various theoretical frequency-response curves are given. A major part of the thesis, and almost all of the experimental work, is devoted to the study of stripe-waveguides made by silver/sodium ion- exchange in soda-lime glass, made by immersing the glass in a bath of molten silver nitrate for a period of time. This is the type of guide which was chosen as a basis for the ring-resonator design work; its lateral refractive index difference is such as to offer a suitable compromise to the conflicting requirements of (i) low bending loss and (ii) strong directional coupling. In order that some non-empirical design work might be possible (of the directional coupler in particular) an accurate mathematical model of the ion-exchanged waveguide is required. Accordingly, some space is given to the theory of ion-exchange and the resulting model. This takes the form of two computer programs; one to solve the non-linear partial differential equation which describes the ion-exchange process, in two dimensions, using a finite difference technique; the other to solve the wave equation in a two-dimensional region of arbitrary, varying refractive index, using a variational technique. The substantial accuracy of these numerical solutions is demonstrated. As essential background, and also to determine the physical parameters of the ion-exchange system, a study of slab (1-D) ion-exchanged waveguides was undertaken, comparing computed and measured guided modes. It is concluded that the computed diffusion profile,, differs from the real profile in certain respects; this is caused by factors for which the ion- exchange theory does not account. Stripe waveguides are defined by coating the glass with a diffusion mask in which slit-like apertures are opened prior to diffusion. The aperture width is accurately measured by utilising fraunhofer diffraction, and the waveguide effective refractive indices by prism-to-guide coupling. When the diffusion mask is of aluminium, metallic silver is reduced along each edge, causing high attenuation of the guided power and other strong deviations from the computed theory. When the aluminium diffusion mask is converted to aluminium oxide, by anodisation, these effects do not occur; the anodised mask is capable of producing waveguides which behave as predicted by the mathematical model, and have low loss. Experimental directional couplers have been made, with transfer lengths which are generally in good agreement with the computed values. Low-loss waveguide bends with radii less than 300 microns are reported, and 2 micron wide stripe waveguides with a linear attenuation of less than 3 dB/cm have been made. Finally, design parameters for a ring-resonator are suggested, but none has yet been made.