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The thesis focuses the design and fabrication of silica-on-silicon multimode interference (MMI) devices for optical communications. Firstly, the relationship between different kinds of multimode interference was established for the first time. This gives a clearer understanding of the multimode interference and helps to design better performance optical MMI devices With the consideration of weak lateral light confinement, different kinds of novel approaches to designing high performance MMI devices are developed. The first new approach is for optimization of silica-on-silicon MMI couplers. It is shown that the length of the multimode section can be varied in a well-defined range to find optimal device performance. The range is linked to the propagation constant spacing of fundamental and higher order modes of the multimode waveguide. The second approach is to introduce a new criterion for designing a MMI coupler with central input. According to overlapping interference analysis, one image space should be left for the adjacent output waveguides because of the lateral distribution of alternatively vanishing and non-vanishing images. This consideration is neglected in previous work and is shown to be important for achieving good uniformity MMI power splitters. Thirdly, a new design of silica-on-silicon multimode interference (MMI) device is proposed. Deeply etched air trenches define the boundaries of the multimode section to achieve strong lateral confinement, resulting in lower loss and imbalance. The access waveguides, however, are buried channel and square core, giving low fibre insertion loss and low polarization dependence. The novel design balanced requirement of the strong lateral confinement of the field in the multimode waveguides and the matching between the fiber and the access waveguides. Then a new approach of designing silica-on-silicon optical switches based on cascaded MMI couplers is presented. The approach combines the transfer matrix method, optimisation of the MMI dimensions, and mode propagation analysis (MPA) for calculation of phase shifts. The feasibility of the large port count switches is also discussed. It is shown that the good performance devices can be realized with a large port count of 32. Finally MMI couplers based on silica-on-silicon optical waveguides were fabricated. The Ge-doped silica waveguides were fabricated by HC-PECVD and RIE. Fabrication processes such as thin film deposition and etching are discussed. Good performance devices have been realized.

The thesis focuses the design and fabrication of silica-on-silicon multimode interference (MMI) devices for optical communications. Firstly, the relationship between different kinds of multimode interference was established for the first time. This gives a clearer understanding of the multimode interference and helps to design better performance optical MMI devices With the consideration of weak lateral light confinement, different kinds of novel approaches to designing high performance MMI devices are developed. The first new approach is for optimization of silica-on-silicon MMI couplers. It is shown that the length of the multimode section can be varied in a well-defined range to find optimal device performance. The range is linked to the propagation constant spacing of fundamental and higher order modes of the multimode waveguide. The second approach is to introduce a new criterion for designing a MMI coupler with central input. According to overlapping interference analysis, one image space should be left for the adjacent output waveguides because of the lateral distribution of alternatively vanishing and non-vanishing images. This consideration is neglected in previous work and is shown to be important for achieving good uniformity MMI power splitters. Thirdly, a new design of silica-on-silicon multimode interference (MMI) device is proposed. Deeply etched air trenches define the boundaries of the multimode section to achieve strong lateral confinement, resulting in lower loss and imbalance. The access waveguides, however, are buried channel and square core, giving low fibre insertion loss and low polarization dependence. The novel design balanced requirement of the strong lateral confinement of the field in the multimode waveguides and the matching between the fiber and the access waveguides. Then a new approach of designing silica-on-silicon optical switches based on cascaded MMI couplers is presented. The approach combines the transfer matrix method, optimisation of the MMI dimensions, and mode propagation analysis (MPA) for calculation of phase shifts. The feasibility of the large port count switches is also discussed. It is shown that the good performance devices can be realized with a large port count of 32. Finally MMI couplers based on silica-on-silicon optical waveguides were fabricated. The Ge-doped silica waveguides were fabricated by HC-PECVD and RIE. Fabrication processes such as thin film deposition and etching are discussed. Good performance devices have been realized.

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