I. Introduction
The aim of integrated optics technology is to realize a variety of different optical components on a single chip. As a consequence of the mature and widespread state of silicon-based technology, Si substrate has served as a base for many different waveguide material systems used in integrated optics. These include, e.g., silica [1], [2], germanium [3], silicon oxynitride [4], polymers [5], and silicon-on-insulator (SOI) [6]–[11]. Also III–V semiconductors have been structured on silicon [12]. One of the main advantages of SOI waveguides is the possibility for a size reduction of integrated optical structures. This results from the high refractive index of silicon, setting a high index contrast between the silicon core and the surrounding oxide cladding. Another strong motivation to study such structures has risen from the desire to monolithically integrate SOI-based optical components and control electronics.