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

New Trends in Physics Teaching IV
chromium. This laser is operated by irradiating the ruby with a high intensity flash from a xenon
flash tube. The ruby itself is in the form of a cylindrical rod with polished ends. Some of the
atoms achieve a metastable level and their decay in the confines of the ‘cavity’ produced by the
polished parallel end faces leads to the avalanche effect. The energy required to operate the flash
tube rapidly enough to produce continuous radiation is enormous and so ruby lasers are usually
used only in a pulsed mode.
Gas phase lasers using, at first, mixtures of helium and neon, and later many other gases, are
now well established and continuously operating lasers developing 5 to 10 watts entirely at one
frequency in the visible region are now made commercially.
Perhaps the most exciting of recent laser developments is tlie dye laser. Organic molecules
have a great many possible excited states when one considers all the possible vibrational and
rotational modes. In fact it turns out that the energy levels become almost continuous distributions.
In order to act effectively as a laser, the molecules used must absorb very strongly; dye molecules
have this property to a marked degree. A solution of a dye such as rhodamine wil lase over a
considerable band of frequencies. If then the beam, after traversing the dye cell, is either diffracted
by a reflection grating, or dispersed by a prism and a mirror, it is possible to ‘tune’ the laser so
that it operates at a specific frequency within the band determined by the geometry of the grating
or prism.
Before leaving the topic of laser sources, it is important to mention semiconductor lasers
which, as we shall see in the next section, play a very important role in communication. Imagine
a p-n junction in which the n-side of the junction is made negative and the p-side positive. As a
result the free electrons in the n-type material wil be driven towards the junction and, similarly,
the holes in the p-type material move towards the p-n layer. Under certain conditions, holes and
electrons can combine in the layer to give a photon of energy corresponding to the energy gap.
A typical semi-conductor laser might be made of gallium arsenide. The faces of the materials in
contact might be about I mm square and the actual p-n layer may be only of the order of a
micron thick. A pair of faces of the junction perpendicular to the junction layer is highly polished
and the laser action takes place in the layer. A very high current (for the given dimensions, it
might be as much as 100 A) is passed through the junction and, although continuous action is
possible, it is usually more convenient to use pulsed operation. For communication purposes -
especially as nowadays signals are conveyed in pulse-coded form - these tiny devices can be of
enormous value.
The fact that laser light is so highly coherent means that many of the techniques that were
previously possible only at radio or microwave frequencies can now be used at the very much
higher frequencies of infra-red and visible light. For example the heterodyne principle is widely
used in radio, and with laser sources it becomes possible with light. For example, if a laser beam
is reflected back from a mirror on a moving object a doppler shift, occurs in the frequency of the
returned beam. This ‘beats’ with the original beam to produce a heterodyne signal which can be
used as a measure of the velocity.
The non-linear properties of some solid materials can be used to produce interactions or
modulation between two laser beams. In particular, if a powerful laser beam is passed through
non-linear material, the output wil contain components of twice the frequency. Hence a frequency
doubler at visible frequencies is possible. Since laser action becomes increasingly difficult as the
frequency rises, this provides a valuable source of higher frequency laser light.
The enormous intensity achievable with continuously operating lasers - quite apart from all
the other useful properties - has in itself led to a revolution. For example, in my own work on
optical transforms in the 195Os, we used a high pressure compact mercury arc as source - it was
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