Continuously Tunable Optical Buffer

Modern society is based to a large extent on the fast and reliable exchange and processing of information. This has led to an explosive growth of the internet, and every year new applications and services are added with ever increasing demands on information transfer capacity - music downloads are replacing high street CD purchases, video downloads have reached 3bn per day on YouTube alone, TV is streamed live on the internet, HD video-on-demand is just round the corner, and cloud computing may mean that data is increasingly stored and processed remotely. The vast majority of these data are transmitted in form of small data packets over a worldwide network of optical fibres. The bottleneck in the capacity is currently formed by the routers, the distribution centres of the internet where packets are switched between optical fibres depending on their destination. This process is done by electronics, and thus at much lower speeds than the capacity of the optical transmission fibres. Moreover, as usage nears the network capacity limits, data congestion at the routers is a serious issue which requires storage of data packets electronically until they can be re-transmitted. Finally, the conversion from optical to electronic to optical is also inefficient and thus consumes significant amounts of energy. One of the most attractive solutions to this problem is storing data in its optical form until it can be re-transmitted. Such an optical buffer should be fast, allow for arbitrary storage times, and should be broadband, that is, it should work over the whole range of optical wavelengths used for data transmission in fibres. Several optical buffers have been suggested and partially demonstrated so far, but none of them fulfils all these requirements. Here, we propose a novel type of optical buffer to meet these specifications. It is based on integrated photonics, which will ultimately allow the buffer to be scaled and mass fabricated for the market. In its simplest form, the chip contains two parallel optical waveguides whose separation can be controlled electronically. Light propagating simultaneously through the two waveguides is coupled optically through the separating air gap and the propagation velocity depends on the exact size of that gap. In other words, the speed of light and hence the time the pulse spends on the chip can be controlled by moving the waveguides. We have already shown through simulations that the delay time can be changed by a factor of three using this method. Using our optical buffer in a ring configuration can therefore create any arbitrary time delay. Moreover, the buffer is predicted to work at all wavelengths relevant for optical telecommunications. In practice, the controllable separation of the two waveguides will be achieved using the latest micro-electromechanical technology on a III-V semiconductor platform. In this project, we will first design and optimise the optical buffer by theoretical analysis and simulations. We will then fabricate the device using III-V deposition, e-beam lithography, and a combination of plasma and wet etching techniques. We will characterise and evaluate the device, and finally demonstrate the optical buffer in an optical telecommunication system. The project is a collaboration between the University of Southampton and University College London and will bring together their expertise in photonics (UoS) and III-V nanofabrication (UCL) to investigate and fabricate a device which has the potential to become an enabling technology for further acceleration of packet-switched networks and thus for future growth of the internet.