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"Free-space" optics requires no fibre. That may be an advantage

FIBRE optics revolutionised communication by abolishing the law that light can
travel only in a straight line. From that point on, light signals could be
treated in the same way as electrical ones, and bent round corners. Some
people, however, are never satisfied. And these dissatisfied engineers are
trying to turn the clock back by developing systems that use "free-space"
optics-in other words sending information from place to place by shining laser
beams through the air.

Free-space optics has three advantages. It is easy to install. It can handle a
technology known as wavelength division multiplexing (WDM) without, as it
were, blinking. And it seems suited to a new-and allegedly
uncrackable-encryption technique called quantum key distribution.

Speed of installation comes from not having to dig up the road to lay
conduits. Free-space optics may thus be an answer to the difficulty of
providing broadband connections to customers' homes and offices-the so-called
"last mile". Free-space links that operate at speeds of up to 20 gigabits a
second-as good as fibre-have now been demonstrated. They can be installed in
hours rather than the weeks or months normally needed for broadband access.
And if they can be put into place quickly, they can be upgraded quickly, too.

That matters in the context of WDM, a technique that allows a single optical
path to carry thousands of parallel channels, as long as each is encoded in a
slightly different colour. Upgrading a fibre network for WDM is hard. First,
individual fibres are each compatible with only a few WDM schemes. The exact
chemical composition of a fibre's glass determines how transparent it is to
different frequencies, and also its tendency to disperse those frequencies
even when it is transparent. Both restrictions reduce the number of channels
that can be carried. Moreover, even if a particular fibre can be used with a
particular scheme, the light sources, amplifiers, switches and associated
paraphernalia usually cannot. Amplifiers, for instance, will not boost all
colours equally, so special devices are necessary to compensate.

Free-space optics suffers from none of these problems. Air is transparent to a
wide range of frequencies and has few dispersive tendencies (at least, when
the weather is good). And with the associated kit clustered together in base
stations, upgrades are easy to carry out.

The third advantage-for quantum key distribution-is more speculative. The
technique exploits the arcana of quantum mechanics to let two computers swap a
cryptographic key (and thus the means to decode a message) with perfect
security.

Quantum key distribution has been demonstrated successfully in fibres, but it
suffers from one major drawback: it requires a dedicated link, and so cannot
be implemented in a network. However, two experiments carried out in the past
few weeks have shown that it works with free-space optics. First, researchers
at QinetiQ, a   British-government-owned company, and Ludwig Maximilian
University, in Munich, Germany, exchanged keys between two alpine
mountain-tops more than 23km apart, though they did so at night, when sunlight
could not confuse the signal. Then, another group of researchers, from Los
Alamos National Laboratory in New Mexico, announced that they had performed a
10km key exchange in broad daylight.

These two groups are working towards military applications in which the key is
exchanged from the ground to a satellite. But both recognise that the
technology might be exploited commercially, and are part of a European Union
collaboration called QuComm that is encouraging this.

Free-space optics would have the odd drawback, such as flocks of birds,
showers of snowflakes or banks of fog interrupting the beams. But
message-encoding systems are already set up to cope with lost data. Many
customers might be willing to put up with a 99.999% available service that
could be installed straight away, rather than waiting indefinitely for the
100% availability of fibre.


Copyright © 2002 The Economist Newspaper and The Economist Group. All
rights reserved.


-- 
-----------------
R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
44 Farquhar Street, Boston, MA 02131 USA
"... however it may deserve respect for its usefulness and antiquity,
[predicting the end of the world] has not been found agreeable to
experience." -- Edward Gibbon, 'Decline and Fall of the Roman Empire'

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