Page 349 - Electrical Properties of Materials

Applications 331

fibres and semiconductor lasers has made it fashionable to convert all kinds of
input variables into light signals. The reason is low attenuation, flexibility (in
both senses of the word), high information capacity, compactness, light weight
and, last but not least, the potentially low price of the device.

12.13.16 Communications
I have already mentioned communications several times. There is no doubt
that lasers, combined with the advent of optical fibres, are responsible for the
enormous increase in volume, and for the drastic decline in the cost of transat-
lantic and transcontinental calls. In the second edition of this book, published
in 1979 the following prediction was made about optical communications: ‘It
is bound to come, and bound to be followed by cheap intercontinental com-
munications. In 10 years’ time you will probably be able to call Uncle Billy in
New York for ten pence.’ Well, ten years was not quite the right prediction but
20 or 25 years would be just about correct. And it is actually not ten pence but
two pennies or less. The future? There is some overcapacity at present but, we
strongly believe that, barring a major catastrophe, all kinds of communications
will increase. The sky is the limit.
While on communications I must mention a new development that may very
well come. The problem to solve is known as that of the Last Mile, concerned
with transmitting information from the fibre terminal to each home. Since in-
stalling fibres into every house is rather expensive the present solution is to
change to coaxial cable at the fibre terminal. The new solution envisaged is
to put the information on infrared lasers at the terminal and radiate it directly,
without any cables, into one’s sitting room.

12.13.17 Nuclear applications

Let us turn now to some potential applications which may acquire high import-
ance in the future. Take laser fusion for example. The chances of success seem
fairly small, but the possible rewards are so high that we just cannot afford
to ignore the subject. The principles are simple. As I have already mentioned,
a plasma may be heated by absorbing energy supplied by a number of high-
power pulsed lasers. The fusion fuel (deuterium and tritium) is injected into
the reactor in the form of a solid pellet, evaporated, ionized, and heated in-
stantly by a laser pulse, and the energy of the liberated neutrons is converted
into heat by (in one of the preferred solutions) a lithium blanket, which also
provides the much needed tritium.
Next in importance is another nuclear application, namely isotope separa-
tion. With the change from fossil to fissile energy sources, we shall need more
and more enriched uranium. The cost of uranium enrichment in the USA for
the next 20 years has been estimated at over 100 000 million dollars. Thus, the
motivation for cheaper methods of separation is strong.
The laser-driven process, estimated to be cheaper by a factor of 20, is based
on the fact that there is an optical isotope shift in atomic and molecular spectra.
Hence, the atoms or molecules containing the desired isotope can be selectively
excited by laser radiation. The separation of excited atoms may, for example,