FORESIGHT Exploiting the electromagnetic spectrum: State of the science overviews OFFICE OF SCIENCE AND TECHNOLOGY The DTI drives our ambition of ‘prosperity for all’ by working to create the best environment for business success in the UK. We help people and companies become more productive by promoting enterprise, innovation and creativity. We champion UK business at home and abroad. We invest heavily in world-class science and technology. We protect the rights of working people and consumers. And we stand up for fair and open markets in the UK, Europe and the world. Contents Introduction 1 Switching to light: 3 all-optical data handling Manufacturing with light: 7 photonics at the molecular level Inside the wavelength: 11 electromagnetics in the near field Picturing people: 15 non-intrusive imaging State of the Science Overviews Introduction Many technologies already make good use of some part of the electromagnetic spectrum: telecommunications networks carry signals as light waves down optical fibres; mobile phones send microwave signals through the atmosphere; and x-rays have been used for more than a century in medical imaging. Now, however, new methods of manipulating electromagnetic waves are on the brink of revolutionising many established technologies and making new ones possible. The aims of the Foresight ‘Exploiting the Written by experts in the field, these reviews are electromagnetic spectrum’ (EEMS) project were technical documents that have been used to help to identify key areas of long-term opportunity inform the drawing up of technology timelines across the spectrum, assess these against UK and plans for action. The reviews look at new capabilities and agree a plan of action to help the technological advances, assess their likely UK exploit these areas. Four topic areas were impacts in 10–20 years’ time and consider the selected through a rigorous scoping process, UK’s relative strengths in these areas. This involving the academic, business, and user report provides short and accessible overviews communities, along with representatives from of the state of the science reviews that have other government departments and funding been written for the project by Judy Redfern, bodies. As part of the detailed study of these an experienced science journalist. four topics, state of the science reviews were commissioned for each. 1 Switching to light: all-optical data handling Introduction Bottlenecks occur in communications networks because optical signals must be converted to electrical signals for routing and processing. This review looks at the prospects of speeding up networks by developing all-optical methods of signal processing. Current networks get around this problem by converting the optical signal to an electrical one for routing, and then back to an optical signal for onward transmission. However, the conversion process introduces a bottleneck because it is slow and limits the information transfer rate. At current volumes of telecommunications traffic, such bottlenecks are not an issue, but they will become significant as we move into a broadband future. The problem would be solved if the signal could be kept in optical form throughout the switching and routing process. To achieve this, there are major science and engineering challenges to be tackled. For example, a ‘memory’ is a key feature of any router. Electronic memory is easily available, but an optical memory is very hard to make – light signals are just too fast and Over the past 20 years, optical fibre has taken slippery to be held in one place for more than a over from copper cable as the medium by which moment. Nonlinear optical techniques, made data is transmitted over the telecommunications accessible by confining light in tiny spaces, offer network. Only the final link to your home remains some prospects, as do fast-tuning lasers. But via copper cable. Instead of travelling as pulses of the ‘Holy Grail’ of an ultra-fast all-optical network electrons down a wire, the data is encoded onto a may have to wait for new approaches to reach light wave that travels along an optical fibre. greater maturity. The most promising appear to However, some of the properties that make light be photonic bandgap structures, new materials so superior for carrying data make it difficult to that can manipulate light in novel ways. 3 route to ensure the data reaches the right In the meantime, hybrid systems are under destination. Light is good for transmission, but development that route and switch an optical electricity is easier to manipulate and so is better signal electronically, without converting it. for switching and routing. Research is also going on into network architecture since the structure of a network can Nonlinear impairments result from weak have a considerable effect on the amount of interactions between wavelengths in neighbouring routing and switching needed. channels and cause signal distortion. The following summarises research on routing Minimising these limitations depends on an and switching, networks, emerging technologies optimum choice among a number of factors: the and finally, as a postscript, the synergy between wavelength of light and the wavelength range; research into optical communications and power per wavelength; channel spacing; the quantum computing and cryptography. First, composition of the optical fibre and its cladding; however, we take a look at the basics of the diameter of the core; and how the light optical transmission and the state of the art signal is packaged and introduced into the fibre. in fibre development. Attenuation, however, can never be entirely eliminated, so amplifiers are required to boost Optical transmission signals over more than about 80 km. The erbium-doped fibre amplifier was the first device The increase in the transmission capacity of capable of doing this entirely optically for many optical fibre has more than kept up with the different wavelengths simultaneously. growth in data traffic. The current state-of-the-art fibre, a so-called An optical fibre consists of a glass core 'standard' single-mode fibre, operates over a surrounded by cladding made from a slightly broad spectral range from 1,260–1,625 different type of glass. The careful choice of nanometres (10-9 m), is impervious to water materials ensures that light is confined to travel (which if allowed to penetrate a fibre will absorb along the core by a process known as total signals in the 1,300–1,400-nanometre range) and internal reflection: whenever the light beam hits has a very narrow core diameter (9 micrometres) the interface with the cladding it is reflected to eliminate dispersion. The fibre's spectral back towards the core. range gives it a frequency bandwidth of 35 THz, Optical fibres can carry far more data than which is more than enough to carry all the traffic copper wires. As each fibre is no more than a on the US internet. At present, fibre capacity hair's breadth thick, many can be bundled into outstrips demand and hence the drive for further one cable. Light also has another advantage: improvement is minimal. because different wavelengths do not interact with each other, data can be transmitted on Routing and switching closely spaced wavelengths without risk of All-optical routers and switches require cross-talk. This means that data can be carried optical processing which is at the cutting on several hundred different channels down edge of scientific research. Hybrid systems one fibre, a process called wavelength that process electronically tagged optical division multiplexing. signals could offer an interim solution. In designing the optimum optical fibre, 4 Research in optical processing has centred on researchers need to address three main developing a method based on a loop of optical limitations: attenuation, dispersion and nonlinear fibre. Light entering the loop can leave by one of impairments. Attenuation causes the strength of two exits depending on whether a laser beam is the signal to drop off rapidly after travelling a also flashed into the loop. The decision 'on' or few kilometres. Dispersion causes the different 'off' is very fast, but the device is physically too wavelengths of light in the signal to spread out. Switching to light: all-optical data handling big for use in most networks and there is little routed through the network individually via scope for miniaturisation because of the limit on nodes that decide on the next leg of the route, how far optical fibre can be bent. given the density of network traffic. When the packets are near their destination, they are re- Hybrid devices, in which a signal remains optical assembled into the original message for delivery. throughout but has an electronic label for Packets travel between nodes as optical signals, switching and routing, show more medium- to but they have to be converted to electronic long-term promise. Several have been signals for processing at nodes. developed. One of the most promising is a switch based on microelectromechanical Just as it can be quicker to take a long route via systems (MEMS) technology. It consists of tiny backstreets to avoid a hold-up at traffic lights, so mirrors etched onto silicon in much the same conventional packet-switched data networks way that transistors are etched onto an route packets to avoid bottlenecks at nodes. integrated circuit. The mirrors move in response However, they under-use the bandwidth to an electrical signal and can be directed to available in the optical cable between nodes. send an incoming wavelength to a specific Network architectures are under development output port. MEMS switches with thousands of that could reduce the need for processing at tiny mirrors, offering more than 10 terabits per nodes in order to concentrate on making the second of switching capacity, have been best use of this bandwidth. The simplest is a demonstrated, but their practical use is limited wavelength-routed optical network (WRON) in because they allow the signal to degrade.