OVERVIEW

Upgrading telecommunication networks to increase their capacity is becoming increasingly important due to the rapid increase in network traffic caused by multimedia communications. Although optical technologies are replacing most transmission lines, the nodes of the networks, such as switching and cross-connect nodes, are still depend on relatively slow electrical technologies. This will be a serious problem because nodes in the networks will limit the throughput all over the networks, due to the limitations of the electrical circuits. Making the nodes optical, therefore, is important for solving these issues. It requires multiplexing and demultiplexing (mux/demux) via optical technologies. The time-division multiplexing, or TDM, systems that are widely used in existing optical communications systems, are inherently depend on electrical circuits for multiplexing and demultiplexing. The nodes in TDM systems use optical electrical conversion, electrical demulti- and multiplexing, and electrical optical conversion. This means the throughput of the node is limited by the processing speed in the electrical circuits. Wavelength division multiplexing, or WDM, technologies, on the other hand, enable optical multi- and demultiplexing because individual signals have different light wavelengths and can be separated easily by wavelength-selective optical elements. This may enable us to construct WDM networks in which node functionality is supported by optical technologies without electrical mux/demux.

The most prominent feature of the silica waveguides is their simple and well-defined waveguide structures [1]. This allows us to fabricate multibeam or multistage interference devices, such as arrayed-waveguide gratings and lattice-form programmable dispersion equalizers. A variety of passive PLCs, such as N