Tuesday, March 16, 2010 | David R. Selviah, Dept. of Electronic and Electrical Engineering, UCL
Digital information, encoded onto light signals, is regularly sent along optical fibres over distances varying from a few metres to thousands of kilometres. Fibres have largely replaced traditional copper cables for high performance broadband communication over distances exceeding a metre, as they offer advantages such as lower cost, immunity to electrical interference and weight savings. Printed circuit board (PCB) backplanes are widely used in the electronic cabinets, or racks, that form the heart of a variety of IT systems and incorporate connectors to allow other PCBs to be attached and detached at right angles (Figure 1). In the highest speed computers, for communication between the central processor arrays, hard disc storage arrays, and through data routing switches, there is now considerable interest in incorporating high speed "optical wiring," by means of plastic light-guides, within large, metre-scale, electrical PCBs combining optical and electrical interconnections (OPCBs).
Figure 1: Schematic view of system backplane architecture.
Optical interconnections were investigated in this IeMRC "Integrated Optical and Electronic Interconnect PCB Manufacturing" flagship project for short distance, high-speed, data communication applications on PCBs to replace high data rate copper tracks which suffer severe cross-talk and increased loss and increased cost at data rates above 10 Gb/s. This three-year research project explored methods for the manufacture of optical waveguides within an optical layer laminated into the board and investigated their compatibility with techniques already in use in commercial PCB manufacturers.
Four different polymer waveguide manufacturing techniques were investigated and compared: Photolithography, direct laser writing, laser ablation and inkjet printing. Some of these techniques were very new, while others were more established, although in all cases their extension to larger areas and adaptation for use by PCB manufacturers remained a significant challenge. As the photolithographic fabrication technique was better established, so waveguides fabricated by this method at two companies in two polymers, were measured and used as a benchmark to compare with those fabricated by the other methods as they were developed during this project.
Another aim of the research was to take the findings and use them to adapt existing computer programmes, used for laying out patterns of copper tracks, so that they incorporated new rules suitable for designing optical waveguide layouts in OPCBs. By working with materials suppliers within the consortium, new polymer formulations were developed suitable for rapid production of waveguides on large area boards and characterised. In addition, research considered low cost methods for polishing the waveguide end facets to increase light throughput and waveguides were tested under severe conditions of high temperature and humidity to determine these effects on their losses.