| Summary: | 博士 === 國立臺灣大學 === 光電工程學研究所 === 103 === Silicon nanowires on the silicon-on-insulator (SOI) platform have great potential for ultrasmall photonic integrated circuits (PICs) because of their compatibility with mature complementary metal-oxide-semiconductor (CMOS) technologies. However, the polarization-dependence issues such as polarization dependent dispersion and polarization-dependent loss of SOI PICs highly restrict their integration with modern fiber-optics communication system. In this thesis, horizontal slot waveguide-based directional couplers (DCs) are successfully employed to overcome the problem of polarization-dependence in designing fundamental SOI waveguide-based components. Through tailoring the ratio of the coupling lengths for quasi-TE and quasi-TM modes, a highly efficient polarization beam splitter (PBS) and a polarization-independent DC (PIDC) are proposed. Owing to structural birefringence, the coupling effects of the quasi-TE and quasi-TM modes in the DC may vary with the waveguide geometry. Therefore, numerical simulations based on finite-element method (FEM) are conducted to obtain the optimal design parameters for high efficiency and compact device size by varying the aspect ratios and waveguide spacing.
Furthermore, the relation between the slot thickness and the propagation losses of optical mode power leaked into the silicon substrate of practical SOI wafer is investigated in detail. The simulation shows that there exists a trade-off between the attenuation constant and the coupling lengths of both the quasi-TE and quasi-TM modes. In the thesis, the coupling length of the proposed PIDC is only 6.93 μm, and its extinction ratio is kept at around 15 dB with a 1-dB bandwidth larger than 100 nm. For the proposed PBS, the coupling length is 65.87μm while delivering the good performance with extinction ratios of more than 20 dB and 1-dB bandwidth of larger than 30 nm. In view of future device fabrication, the fabrication-error tolerances on device length and waveguide width are also discussed.
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