Silicon Photonics: Fundamentals and Devices

7.7 Optical Structures

Si-based modulators can be classified in different ways. The optical structures may differ from device to device. In fact, starting from 1987, many different optical structures have been employed to date. The electrical structures have also evolved since the first practical realization of the modulator. The classification may be made in terms of the method of introducing the RI variation. Mainly two different methods are employed: EO and TO methods. In the EO method also, different ways exist. Finally, the modulators may differ in terms of materials. As noted already, silicon-based devices used silica on Si or Si–SiGe heterostructures initially; in recent years, the emphasis is on using silicon-on-insulator (SOI) structures. The electrical structures will be considered in Section 7.8. In this section, we shall consider the basic optical structures used to construct the modulators.

Broadly there are two categories. First is the Mach–Zehnder interferometer (MZI) configuration. Second is the resonant structure which may be either a Fabry–Perot or a ring resonator. Again the mirrors in a Fabry–Perot structure may be formed in different ways; presently the mirrors are Bragg reflectors. In all the configurations the basic idea is to introduce a change in phase of the lightwave by changing the RI. Either the EO effect or TO effect is introduced to bring about the RI changes

7.7.1 MZI

The basic operating principle of the Mach–Zehnder interferometer (MZI) will be ...