Evolution of optical modulation in silicon-on-insulator devices

Abstract

One of the main goals of silicon photonics is to enable the fabrication and integration of electronic and photonic components on the same chip using the existing complementary metal -oxide -semiconductor (CMOS) processing platform. Due to its intrinsic structural properties, silicon does not exhibit a useful electrooptic effect that could enable light modulation. However, what makes silicon an attractive optical material is its transparency to infrared communication wavelengths and its high refractive index which facilitates the miniaturization of photonic devices. This also enables a high level of light confinement in nanometersized waveguides and provides an excellent basis to fabricate micro-optical devices. In silicon, light modulation is usually achieved via the free-carrier plasma dispersion effect, in which a change in carrier concentration (holes and electrons) is used to change the refractive index of the semiconductor, which, in turn, modifies the propagation velocity of light and the absorption coefficient. In resonators or interferometers, this effect enables the fabrication of modulators based on silicon-on-insulator (SOI) technology, which uses a layered silicon-insulator-silicon substrate in place of conventional silicon substrates. Design rules for optical modulators in silicon should take into account single-mode waveguide design, large bandwidth operation, high modulation speed, high extinction ratio, small device size and low power consumption. Many different approaches have been used to satisfy these design goals on silicon platforms. Driven by these requirements, researchers have recently significantly reduced the waveguide core size from the micrometer range, producing silicon nanowires with a height and width of about 500nm and less. During the past four years, the scaling of waveguides using the plasma dispersion effect has enabled optical modulation frequencies to evolve in silicon from the MHz range to approximately 30GHz. This substantial leap in operating frequency now allows silicon-based optical devices Figure 1. Positive-intrinsic-negative (PIN) junction of an injectiontype modulator.