Photonics The Future of Communications by rraul

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									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,
will.stewart@marconi.com                                   p.s.j.russell@bath.ac.uk
w.stewart@ieee.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: philip.diamond@iop.org, 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: dianne.stilwell@iop.org

Alice Bows, Press Officer, The Institute of Physics, 76 Portland Place, London, W1N 3DH. Tel: 020 7470
4800 Fax: 020 7470 4848 email : alice.bows@iop.org


The Institute of Physics is a leading international professional body and learned society with over 30,000
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• scientific publishing and electronic dissemination of physics;
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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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