New trends in physics teaching, v.4; The ... - unesdoc - Unesco

Optics regained
the brightest source available at the time - with multi-layer dielectric interference filters to
isolate one particular spectral line - usually the green. Typical exposures were measured in
minutes (between half a minute and twenty minutes was the common range). With even a 1 mW
laser, the exposures can now be measured in thousandth’s of a second!
The energy density in a high-intensity laser beam can be so high that the electrical breakdown
strength of the gas or liquid through which it is travelling may be exceeded and, under the right
circumstances, this can be used to provide highly condensed plasma regions. Whether the containment
problem can be solved so that such concentrations could reach the temperature levels
required for fusion to occur remains to be seen. But the possibility is an exciting one.
Optical communication
Fibre optics has made incredible strides since the basic idea was used as a parlour trick a hundred
years ago. A beam of light was passed horizontally through an empty flask which had a hole in
the side. When the flask was filled with water the beam of light was trapped in the jet of water
emerging from the hole and followed a curved path to the floor. An optical fibre is a core of glass
of high refractive index coated with a sheath of glass of lower refractive index. A typical fibre
may be 10 microns (10 X low6 m) in diameter, is very flexible and is capable of transmitting light
over great distances without appreciable loss. An unsheathed fibre wil of course transmit by total
internal reflection just as effectively, but moisture condensed on the surface, or contact between
two fibres, etc. will lead to ‘leakage’; the lower refractive index coating minimizes this problem.
Coherent bundles of fibres - i.e. those in which the relative position of any fibre is the same at
both ends - may be used for transmitting images and are finding wide use in medical and other
applications. They are, however, expensive and incoherent bundles in which the emphasis is
simply on transmitting a quantity of light from one end to the other are very much cheaper.
The combination of fibre optics transmission and modulated laser sources perhaps provides
the most exciting of all the many prospects for the future. Even when the necessary protective
armour is added, optical cables are extremely light compared with copper conductors. But they
have further advantages that they are not subject to earthing or cross-talk problems; they are
immune to electromagnetic fields; they are very difficult to ‘tap’ and so lead to improved security
for the data transmitted; and finally, of course, they have the supreme advantage of involving
very high frequencies indeed (at 600 nanometres the frequency is 500 million megahertz) and
hence the bandwidths that can be used are vey much larger than for radio waves.
The technology of these communications is developing rapidly. For example, a wide range
of materials other than glass may be used. A single-mode fibre in current use, for example, has
germanium doped silica (n = 1.471) for the core which is 2.5pm in diameter. The cladding is
pure silica (n = 1.457) and a thickness of 40pm and hence the overall diameter is 82.5pm. A
typical 4800-pair copper telephone cable might be about 5 cm in diameter; an optical cable with
the same equivalent number of pairs would be only about 6 mm in diameter and would also have
the other features mentioned above - increased bandwidth, greater security, etc.
Some of the key problems are those of making and breaking connections with minimum light
loss, the development of suitable light-sources and the design of amplifier systems to act as
‘repeater’ stations on long links. Tiny semi-conductor lasers may well prove to be the most useful
for both purposes, especially as many signals nowadays are transmitted in digital or pulsecoded
form.
Because fibre optics started from the notion of multiple total internal reflection, the fact that
optical fibres are really wave guides is sometimes missed. But thinking of them in this way is
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