The optical properties of silicon, and the index and absorption modulation mechanisms available in Si are described. We discuss the characteristics of SOI waveguides, and highlight the design and fabrication challenges in view of the demand for downscaling of waveguide circuits. Scaling laws governing the mode number and size, propagation loss, bend loss, effective index and the birefringence of planar waveguides are discussed, and practical techniques for dealing with the challenges are presented. In particular, we describe the local stress engineering as an effective method for eliminating birefringence in SOI waveguides, as well as opening other design possibilities for Si components. A waveguide coupler scheme based on a graded index layer on top of the waveguide is described. This coupler design is a simple solution to the coupling problem, with only modest fabrication requirements. Implementations and advantages of Si based modulators and switches, spectrometers, light sources and photodetectors are also discussed.
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