OPTICAL WAVEGUIDES

Abstract

This thesis concerns the analysis of rib optical waveguides and their applications. A general Discrete Spectral Index (DSI) formulation for single, double and triple rib waveguides has been derived by introducing general Fourier cosine and sine series for the E-field solutions in the regions of the ribs and the base respectively. Extremely accurate modal indices for the rib waveguides have been calculated by the new DSI formulation which are within 0.001% from those computed by the Semi¬ vectorial Finite Difference (SVFD) method. Therefore, the validity and accuracy of the DSI formulation developed in this thesis is verified. With the accurate analysis of the DSI formulation, the parameters of coupling length and crosstalk for double rib waveguides are examined. The DSI formulation is also used to explore the design of 3-guide directional couplers, power dividers and filters using rib waveguides. In the 3-guide directional coupler, it has been reported that maximum power transfer from one outermost waveguide to the other outermost waveguide requires that the difference of the propagation constants of the 3 lowest order modes must be equal, i.e. ?TE0-?TE1 = ?TE1-?TE2· The accurate analysis of the DSI formulation shows that the height of the middle rib of a triple rib waveguide vitally controls the propagation constants of the modes; hence the middle rib height contributes to an additional degree of freedom in attaining the maximum power transfer condition. Thus, it is affirmed in this thesis that there is greater flexibility in designing 3-guide directional couplers using rib waveguides.

The interaction of the 2 symmetric (TEO and TE2) modes of the triple rib waveguide allows it to act as a power divider where the power input at the central waveguide is split into the 2 outer waveguides after a coupling length. With the analysis of the DSI formulation, it is shown that maximum power division is attained when the 3 ribs of the waveguide are identical. Such a triple rib waveguide power divider is able to avoid the power loss at discontinuities inherent in Y-junction power dividers.

Double and triple rib waveguide filters have been compared in this thesis. The sidelobes in the wavelength response of power transfer in the triple rib waveguide filter are higher and also nearer the mainlobe as compared to that of the double rib waveguide filter of similar composition. Although triple rib waveguide filters have narrower mainlobes (smaller 3d.B bandwidth), their coupling lengths can be as much as 44% longer than those of the double rib waveguide filter. It is also necessary to introduce asymmetric separations between the ribs in the triple rib waveguide filter in order to tune the central filtering wavelength to the designated value. The simplicity in design and better features of the double rib waveguide filter has hence rendered it superior in performance to the triple rib waveguide filter.