Chinese Academy of Sciences, Beijing 100083

Abstract: It is compared with each other that MMIs made of SOI wafers with different thickness of upper Si layer have different properties. MMI coupler made of SOI wafers with thinner Si layer has better self-image effect and shorter coupler length. So SOI wafer with thinner up Si layer is more adapted for MMIs. Besides, a modification that substitute nr in equation(1) by the real refraction index of the core layer will lead to more accurate parameters of MMIs. Multimode interferences(MMI) based on Silicon-on-Insulator(SOI) are widely used in passive and active waveguide-based devices and planar lightwave circuits, such as switches, couplers, splitters and so on[1]. It is not paid more attention that MMIs made of SOI wafers with different thickness of upper silicon(Si) layer have different performance. In this paper, it is compared with each other that MMIs made of SOI wafers with upper Si layer 5μm and 8μm thick. Generally, SOI waveguide is designed as rib structure, as shown in Fig.1, in order to satisfy single-mode propagation[2], due to its large refraction index difference. By conventional method, a three dimensional SOI rib waveguide is simplified as a two dimentional structure. It is because that the horizontal size of a rib waveguide is much larger than the vertical dimension. Fig.1. The cross section of a rib waveguide The effective width of a multimode waveguide can be expressed as follows:)2 1(222)())( ( −−+ = c