Analysis Of Single-mode Condition For SOI Waveguide At 1550 Nm Wavelength

With the development of silicon photonics, high density integrated optical devices is becoming a hot topic of research in recent years. For a single photonic device, meeting the requirements of single mode transmission is mostly fundamental to avoid coupling between modes, dispersion and other negative effects; in addition, the input and output ports of photonic devices also should enable better coupling with a standard single-mode fiber. For integrated optics, Silicon on insulator(SOI) material is widely used, due to large refractive index difference between wave-guiding layer(top Si) and cladding layer(i.e. Si O2), and a rib waveguide fabricated on SOI exhibits a strong light confinement in the vertical direction but weak in the horizontal direction, since the higher-order modes will be rapidly attenuated. In the end there is only fundamental mode propagating in the waveguide, leading to a single-mode transmission feature. Additionally, increasing the rib width as large as possible will be helpful for a better coupling efficiency under a condition of the single-mode transmission.This thesis starts from Maxwell equations of electromagnetic wave. Three different mode solvers including film-mode-matching method(FMM), finite-elements method(FEM), and finite-difference method(FDM) were used to analyze propagation characteristics of SOI rib waveguides at 1550 nm wavelength. Here a commercial available FIMMWAVE software package was used to analyze various waveguide geometries in the thesis. For instance, the rib height with 2 μm, 3μm and 5μm were studied, respectively. These results were compared with classic single-mode conditions of a rib waveguide originating from Soref’s expression and Pogossian’s effective index method. The study shows that there is not a constant α to meet all rib waveguides with different geometries. Thus α was modified in our study for the three rib waveguides, which can provide larger width range for a better coupling with a single-mode fiber. For TE polarization, if the height exceeds 5 μm, α is fitted by 0.3, which is similar to the Soref’s single-mode formula; if the height is 2 μm, 3 μm, and 5 μm, respectively, the corresponding α is 0.45, 0.4 and 0.35, respectively. For TM polarization, a constant α was found, and it is 0.25. Using this single-mode rib waveguide design procedure, a Silicon-based Mach-Zender thermo-optic switch device is successfully realized.In this thesis, four groups of rib waveguides with different etching depth were fabricated using a standard photolithography method. Each group contains couple of waveguides with different widths including both single-mode and multi-mode waveguides. Optical modes from these waveguide were measured via an end-fire coupling setup. The testing process was introduced with addressing how to correctly determine whether an optical fiber coupling with a waveguide in the section of measurement part. Lastly, some problems existing in the measurements such as not observing multi-modes were discussed.