Fundamentals and Design Guides for Optical Waveguides

Abstract

Next-generation high-end data processing systems such as Internet switches or servers approach aggregate bandwidth in excess of 1 Tb/s. The task of providing hundreds of individual links as speeds in excess of 10 Gb/s over the link distances becomes increasingly difficult for conventional copper-based interconnect technology. Optical interconnects are foreseen as a potential solution to improve the performance of data transmission on chip, PCB, and system levels. They carry data signals as modulation of optical intensity, through an optical waveguide, thus replacing traditional electrical interconnects. Optical devices can overcome the bottleneck imposed by the limited bandwidth of electronic circuits in areas such as computing, data storage, or telecommunication networks. The basic element of any optical circuit is the optical waveguide, which permits to connect optically different devices. To build integrated optical circuits that substitute micro-electronic circuits, integrated optical waveguides with light confinement in a size of the order of the wavelength are mandatory. Optical waveguides can be classified according to their geometry (planar, strip, or fiber waveguides), mode structure (single-mode, multimode), refractive index distribution (step or gradient index) and material (glass, polymer, or semiconductor). They are designed as energy flow only along the waveguiding structure but not perpendicular to it, so radiation losses can be avoided. Usually, optical integrated waveguides rely on the principle of total internal reflection, using materials with low absorption loss. The waveguide cross section should be as small as possible to permit high-density integration, functionally linking devices or systems or implementation of complex functionalities, such as splitters/combiners, couplers, AWGs, and modulators. A wide range of materials can be used, with their corresponding advantages and drawbacks. Current commercial devices are mostly based on silicon/silica waveguides, III–V compounds, and lithium niobate waveguides. Silicon waveguides offers the possibility of mass-manufacturing and a high level of integration, which would result in cheaper chips. Novel materials such as photonic crystals can provide advantages to fulfill the requirements for high-density photonic integration. This chapter will review fundamentals and design guides of optical waveguides, including state-of-the-art and challenges, fundamental theory and design methodology, fabrication techniques, as well as materials selection for different level waveguide components and integration structures.