Media Briefing Paper Photonics: The Future of Communications This briefing note has been produced for journalists from a seminar and discussion on Photonics held at the Institute of Physics on Tuesday 22 May 2001. The seminar is part of an initiative to raise the profile of physics. Photonics: The future of communications This briefing note has been produced for journalists by Wendy Barnaby from a seminar and discussion on Photonics held at the Institute of Physics on Tuesday, 22 May 2001. The past few decades have brought exciting new developments and potential from all the sciences. However, the physical sciences – and physics in particular – remain the bedrock of our understanding of the natural world, the development of techniques and the source of applications for wealth creation. Responding to a concern that inadequate attention was being paid to retaining the strengths of our physical sciences research, the Institute of Physics has initiated a programme to draw out examples of exciting and important new areas of research in physics and their application. This takes the form of a series of Vision Papers and seminars. To date, these have covered High Intensity Lasers, Physics in Finance, Exotic Nuclear Beams, Quantum Information, Life Physics, New Plastics, Spintronics, Climate Change, The Large Hadron Collider and New Colliders. Copies of all the vision papers as well as media briefings from previous seminars are available from the Public Affairs Department, see address at the end. Building the glass ether Everyone using computers knows the annoyance of waiting for data to be downloaded. Images take minutes to dribble down copper wires. Sound files take hours. Optical fibres are quicker, but even they are forecast to fall behind the amount of data we want to exchange. Medical imaging and diagnosis, transmitting high- resolution X-ray pictures; video conferencing; exchanging high quality astronomical images - all need bigger files with better resolutions and the transmission of larger data sets. Unless optical fibres can be radically improved, our frustrations are set to grow. This seminar discussed the increasing demands for telecommunications capacity and new developments that will be important in satisfying them. According to Professor Stewart, chief scientist at Marconi, there is still 100% per year growth rate in demand for internet services. The rate of increase of capacity that has been achieved on a single fibre is 72% per year - remarkably high. The latest fibres can transmit 10 terabits (10 million million bits) of information in two minutes: an amount equal to all the 40,000 movies that have ever been produced. Stewart forecasts that strong growth in demand will continue. To make video conferencing possible with several participants, he says, we need one thousand times more than we have at the moment. The ideal world of data transmission is to minimise distance by being able to send limitless amounts of data extremely quickly. The radically new fibres being developed by Physics Professor Philip Russell and his team at the University of Bath are coming closer to that ideal than anyone could have dreamed a mere ten years ago. Says Russell: “There are a lot of fundamental things you can’t overturn in conventional fibre optics. We have the means of overturning many of those and doing a lot of new things as well.” Conventional optical fibres are made entirely from silica glass. The cladding surrounds a central core slightly different in composition so that light pulses sent down the core are reflected off the interface between it and the cladding. This process is known as total internal reflection, and it means that light can travel the length of the fibre without escaping through the cladding. Russell’s fibre is also made of silica glass, but there the comparison stops. All the glass is of the same composition, and the fibre contains tiny air channels in a geometric pattern around its centre, and which run down its entire length. And tiny means tiny: the largest has a diameter of a few microns (one-millionth of a metre), while the smallest are a mere 25 nanometres across (one nanometre is a thousand millionth of a metre). Because the holes are regularly spaced, they look like atoms in a crystal; hence the name of these totally new tubes: photonic crystal fibres (PCF). If the smallest holes were scaled up to represent the diameter of the Channel Tunnel, a few kilometres of the fibre would stretch from England to Jupiter. It is the way PCFs transmit light that heralds their huge potential. Unlike conventional fibres they do not use normal total internal reflection, which transmits different patterns of light simultaneously and limits the ultimate amount of data that can be carried. A PCF acts instead as a guiding sieve. One wavelength of light - the fundamental guided resonance - goes straight down the core and is too big to squeeze between the holes into the outer part of fibre. It is trapped behind bars - or, rather, the spaces between the bars - of its confining sieve. The shorter waves produced by higher guided resonances of light are however sieved out between the airholes and escape into the outer part of the fibre. Irrespective of the wavelength, the fibre guides the fundamental down the centre, sieving out all other modes, under all circumstances. This is a real breakthrough in fibre optics, as it allows the fibre to carry much more power than conventional fibres, and therefore to transmit much more data. According to Stewart, the communications industry can see its way to coping with the vastly increased amounts of information that will be transmitted over the next five years or so. The real problem, however, is not in the transmission of data but in the routing of data from one point to another in the communications network. At the moment, routers can only cope with about one terabit - and some fibres can already transmit ten times this amount. This means that the system needs many routers for each fibre, which is the opposite of the desirable configuration (one router for many fibres). At the moment, routing technology is based on silicon; but it will have to change to optical switching in order to increase its capacity. Stewart sees PCF as a technology which could be part of the solution here, although not immediately. The reason PCFs would be useful is that - in one of their other forms - they can transmit very high light intensities confined in a very small core. This stresses the glass and changes its characteristics. The electrons in the glass are pushed around very strongly by the light’s electromagnetic field, resulting in what is technically known as an optical nonlinearity. This is a disadvantage in conventional fibres because it mixes up the data being sent on different channels. Given the power that PCFs can transmit, however, the nonlinear effects can be used for optical switching in which one signal is directed from one channel to another. “We have made fibres where the effective non-linearity was a hundred times greater than anything possible in previous fibres”, says Russell. Until now, optical devices such as switches and amplifiers have needed huge power - and room-size lasers. The beauty of PCFs is that they need relatively little power to produce the effect. Nonlinear effects also work for PCFs in making a new kind of small, powerful laser. Russell and his colleagues have taken a conventional laser producing invisible light in very short pulses and squirted the pulses into a PCF. Again, because of the tiny space which confines the light and therefore its hugely-increased intensity, the result is strong nonlinearity. This broadens the invisible spectrum of the pulses into what Russell calls a supercontinuum: a spectrum which is like sunlight, going all the way from the infra red to the ultra violet. It arrives so quickly (one picosecond - a thousand millionth of a second) and in such a small area that its intensity is brighter than 10,000 Suns. This makes a very remarkable light source - a “sunlight laser” - which can be used in various ways. One is in spectroscopy: determining the chemical composition of some unknown material by analysing how it absorbs or reflects light. It may be used on a future Mars probe to look for life, because it’s so light and compact. It could also be used for optical coherence tomography - medical imaging of the body - which gives clear images of for example tumours in a breast by sending in light at different angles. More importantly, the sunlight laser is an exciting new source for telecommunications. It can increase the amount of information transmitted in a fibre carrying multiple wavelengths at the same time: a technique currently used to enable conventional fibres to carry more data. In order to make it work, wavelengths have to be maintained very accurately. If they start changing, they drift and start interfering with each other and also distort the fibre. The sunlight laser produces closely-spaced, very precise wavelengths very accurately, and these can be injected into the fibre. Having them so accurately defined reduces the cross- talk between the various channels. This offers a way of boosting the capacity of existing optical fibres. Yet another type of PCF turns all these advantages on their head for even more benefit. Whereas the sunlight laser uses nonlinearity to good effect, the hollow fibre transmits light with hardly any nonlinearity at all. A conventional hollow core fibre - a solid glass fibre with a hollow channel in it - cannot transmit light because total internal reflection does not work with this combination of materials. PCF’s, however, can be made with hollow cores. The light is trapped in the core by the pattern made by the holes in the fibre. This blocks the movement of light outward into the glass, so it is transmitted down the hollow core. “For the first time”, says Russell, “this is a true waveguide with a hollow core using a new guidance mechanism. Because the waveguide is travelling in the tube and not the glass, we get almost complete suppression of nonlinearities.” This means that a very high-power pulse can be injected into the fibre: impossible to do currently because it would distort conventional fibres. At the moment, signals sent down conventional fibres travel less than 100 km before they need to be amplified. Using the new fibres, it may be possible to send signals under the Atlantic in one go, without needing to boost them at all. This would really bring down the cost of trans-Atlantic data exchange. Russell and his colleagues have formed a spin-out company called Blazephotonics, which has recently received $9 million investment to exploit this technology. According to Russell: “The hollow core photonic crystal fibre will allow very high-power delivery - perhaps up to one thousand times what is possible in conventional fibres - and could be the ultimate communications fibre.” Wendy Barnaby Information and links The two speakers at the Institute’s seminar were: Prof Will J Stewart FREng Professor Philip Russell Chief Scientist Optoelectronics Group Marconi Department of Physics, University of Bath 1, Bruton St Claverton Down London, W1J 6AQ Bath BA2 7AY, firstname.lastname@example.org email@example.com firstname.lastname@example.org Tel +44 (0)207 3061432 Tel +44 (0)1225 826946 Fax +44 870 0560255 Further Reading: Frequency chains get short shift, Professor P S J Russell, Physics World, pp3, October 2000 Fundamentals of Photonics, Bahaa E A Saleh Photonics Rules of Thumb: Optics, Electro-Optics, Fibre Optics, and Lasers, John Lester Milller et al. Optics and Photonics: An Introduction, Francis Graham-Smith, Terry A King http://www.bath.ac.uk/Departments/Physics/groups/opto/ http://www.bath.ac.uk/Departments/Physics/groups/opto/publications.html For further information on the Institute’s initiatives to raise the profile of physics research please contact: Philip Diamond, Manager, Higher Education and Research, The Institute of Physics, 76 Portland Place, London, W1N 3DH. Tel: 020 7470 4800 Fax: 020 7470 4848 email: email@example.com, Web: www.iop.org/Policy/profile.html Dianne Stilwell, Manager, Public Affairs, The Institute of Physics, 76 Portland Place, London, W1N 3DH. Tel: 020 7470 4800 Fax: 020 7470 4848 email: firstname.lastname@example.org Alice Bows, Press Officer, The Institute of Physics, 76 Portland Place, London, W1N 3DH. Tel: 020 7470 4800 Fax: 020 7470 4848 email : email@example.com The Institute of Physics is a leading international professional body and learned society with over 30,000 members, which promotes the advancement and dissemination of a knowledge of and education in the science of physics, pure and applied. It has a world-wide membership and is a major international player in: • scientific publishing and electronic dissemination of physics; • setting professional standards for physicists and awarding professional qualifications; • promoting physics through scientific conferences, education and science policy advice. The Institute works in collaboration with national physical societies, plays an important role in transnational societies such as the European Physical Society and represents British and Irish physicists in international organisations. In Great Britain and Ireland the Institute is active in providing support for physicists in all professions and careers, encouraging physics research and its applications, providing support for physics in schools, colleges and universities, influencing government and informing public debate.
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