Transmission and routing of optical signals in on-chip waveguides for silicon microphotonics

Abstract

In this thesis, guiding and routing of optical signals in high index difference ([delta]m) waveguide systems are studied for silicon microphotonic applications. High [delta]n waveguide systems offer compact device sizes that enable highly dense integrated optics suitable for silicon microphotonics. Scattering loss due to the roughness at the core/cladding interfaces is identified as a major source of loss in a high M system. Using both experimental and theoretical approaches, the interdependence of scattering loss, waveguide dimension, and roughness is investigated. We developed a 3 dimensional model that successfully explains the scattering loss dependence on the waveguide dimension. Using this model, a loss contour map is constructed to better understand the scattering loss from interface roughness. This map provides an effective methodology to reduce roughness scattering, which we used to develop two fabrication technologies. Loss reduction from 32 dB/cm to 0.8 dB/cm is achieved for [delta]n =2.0. This is the lowest loss ever achieved for a single-mode, high An system. PolySi/Si02 waveguide systems are investigated due to the compatibility of multi-level processing. Our best PolySi/Si02 waveguide shows additional 10 dB/cm loss, coming mainly from the top surface roughness due to grain boundary grooving. compared to a Si/Si02 waveguide. Compact high An routing devices such as round bends, Y-splitters, and Multi-Mode Interference (MMI) splitters are fabricated and tested. We show that single-mode waveguide bends exhibit μm size bending with low loss and single-mode splitters show splitting with good uniformity. MMis show advantages over equivalent Y-splitter based structures in terms of size and loss. Our MMI design led to the fabrication of the smallest optical 1x16 fanout ever built. High Transmission Cavity (HTC) based bends, splitters, and resonators, that are compatible with an anisotropic etching technique, are demonstrated. An index engineering map, which shows competing trends of minimum bending radius and scattering loss as tin is changed. is constructed. From this map, the optimal M can be found for a given fabrication technology. Improvement in the fabrication technology allows for higher tin and provides a scaling law in optical devices. This point is proven by our 0.8 dB/cm Si/Si02 waveguides, which lifts the upper limit of the usable [delta]n.