Robinson, Henry; (1992) Waveguide electro-optic modulators using the parallel-plate configuration. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Conventional waveguide electro-optic modulators use the fringing fields below a gap between coplanar electrodes. These fields are applied to a waveguide which is fabricated in an electro-optic substrate. The aim of the present work was to a develop a device with an inherently more efficient geometry, viz. the parallel-plate configuration. A computer model was developed and tested, to calculate optical propagation coefficients and losses in multilayer waveguides incorporating optically absorbing materials such as metals. Both electro-optic and electrical characteristics were modelled, for a variety of parallel-plate structures. It was found that metallic electrodes engendered unfeasibly high losses via surface plasmons. A novel structure was devised, using silicon electrodes above and below a thin film of lithium niobate. High losses were avoided by operating the structure above the band gap wavelength. Remaining losses were due to free carrier absorption. Based on this scheme, a family of efficient modulator structures was designed. RF sputtering was used to grow thin films of lithium niobate for the electrooptic layer. The growth characteristics were studied, and a regime was established for growing lithium niobate with a preferred z-orientation as indicated by X-ray crystallography. A fabrication sequence and a mask set were designed, with a view to fabricating test modulators. As well as the logistical problems, structural difficulties arose from the combination of silicon and lithium niobate: these were identified and overcome, and test devices were fabricated. These test structures were electrically and optically assessed, and the electrooptic effect was measured. In the final chapter, the project is reviewed. There is a discussion of the feasibility and implications of combining these structures with silicon integrated circuits. An assessment is made of the contribution to integrated optics.

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