Study Of Three-Dimensional Multimode-Interference Optical Power Splitters

Conventional integrated optical devices are made on a two-dimensional (2D) plane. It means that these devices can only deal with a one-dimensional spatial light beam. In fact, many applications, such as mass data flow, image processing, artificial vision, nerve networks, and optical parallel computing systems need 2D spatial optical signal processing. To get over the limitation of the 1D input and output, three-dimensional (3D) integrated optics with 2D planes for input and output interconnects are one of the potential directions. Adding the third dimension to conventional planer integrated optical devices would greatly increase the integration density and would overcome some problems of the input and output fibers or electrical connections while preserving the reliability and structural stability of 2D devices.Multimode-interference (MMI) devices based on the self-imaging effect are characterized by their compact size, low loss, large bandwidth and good fabrication tolerance. For these reasons, they are widely used in planar lightwave circuits (PLC). However, most of MMI devices are based on 1D confinement. Thus, the minimum horizontal spacing limits the number of channels on a single chip. In recent years, there has been a growing interest in 2D confinement MMI devices. The waveguides of 2D confinement MMI devices are multimode waveguides in two transverse directions. The possibility of adding the third dimension to the traditional one-dimensional confinement MMI devices would greatly increase the integration density while preserving the advantages of 1D MMI couplers.In this dissertation, general and overlapping self-imaging properties of 1D MMI couplers were analyzed. Simple expressions for positions were derived, and some rules of self-imaging were also concluded. These formulae and conclusions can be used to indicate the positions of the images with the arbitrary input field and positional number without complex calculations, which provide a useful tool for the design of 2D MMI couplers.Self-imaging properties of general 2D MMI couplers were derived. The following rule was concluded that the positions of the self-images in 2D MMI couplers is a combination of two 1D MMI couplers, and the phases of self-images in 2D MMI couplers equals to the sum of the phases of self-images in two directions.The overlapping-imaging properties in multimode interference couplers with two-dimensional confinement were analyzed in detail. The positions, intensities, numbers and phases of the overlapping-images are directly related to the position of the input field. Overlapping-image two dimensional multimode interference couplers permit nonuniform power splitting in one or two directions at the end of multimode interference section. Formulas for the positions, intensities and phases of theoverlapping-images were derived. And overlapping-imaging properties in multimode interference couplers with two-dimensional confinement were also concluded. The analytical results were verified with the guided-mode propagation analysis method and BeamPROP software.2D MMI optical power splitters based on SOI were experimentally demonstrated for the first time. The near-field outputs were obtained. Results showed this kind of three-dimensional integrated optical component can be used for optical power splitting in both the horizontal and vertical directions. The new device will find its application in multi-level integrated optical circuits and optical synchronization.Based on the weakly coupled mode theory, the coupled-mode equations of the spatial multiwaveguide system are presented in general. As an application example, the coupled-mode equation is applied to analyze the coupling properties in spatial three- and four-waveguide systems. The analytical intensity distributions in the two specific cases are given, and the three-dimensional full-vector beam propagation method is used to verify the analytical results.Three-dimensional optical beam splitters are basic components in 3D integrated optical circuits. In this dissertation, we study three-dimen...