Development of Tunable Optical Filters for WDM Optical Communication Using High Coupling SH-Type Surface Acoustic Waves

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The purpose of this research project is to develop high efficiency tunable optical filters for wavelength-division-multiplexing (WDM) optical communication by utilizing interaction between high coupling SH (shear-horizontal)-type surface acoustic waves and optical TE- and TM-mode waves. The results obtained in this project are summarized as follows : 1. It has been shown that there exists the BGS wave, which is a pure shear surface acoustic wave, on the metalized surface of Z-axis cylinder cuts of KNbOィイD23ィエD2, and the electromechanical coupling factor KィイD12ィエD1 is very high, about 53%. 2. The change of refractive index induced by the BGS wave on Z-axis cylinder cuts of KNbOィイD23ィエD2 has been analyzed and the results revealed the index change to be 18 times larger than that of the conventional Rayleigh waves. 3. By depositing a ZnO film with a low shear wave velocity on the Y-X LiNbOィイD23ィエD2 substrate the leaky SH-type surface wave is turned into a non-leaky SH-type surface wave with no propagation loss and the KィイD12ィエD1 is as high as 21.4% for the ZnO film thickness about 0.1λ. This has been experimentally proved. 4. The refractive index change induced by the SH surface wave on ZnO/Y-X LiNbOィイD23ィエD2 has been shown theoretically to be large near the substrate surface compared with those of the Rayleigh waves. The coupling length needed for 100%TE-TM mode conversion has been calculated using the coupled-mode theory to be about 1/2.5 of that of the Rayleigh wave. 5. ZnO/Y-X LiNbOィイD23ィエD2 acousto-optic tunable filters have been designed and fabricated. The experimental evaluation of TE-TM mode conversion characteristics are now under investigation.