|
|
|
Laser principle
|
|
| |
|
Light is a form of electromagnetic radiation with wavelengths less than several hundred m m. The
emission and absorption of electromagnetic radiation is caused by the acceleration of electrically
charged particles. If the frequencies are sufficiently small, corresponding to large
wavelengths, this occurs, for instance, via alternating current in antennas. In the visible spectrum, atoms or
molecules act as these antennas which can emit or absorb light on transition between the
various
internal states.
On absorption, a quantum (a unit of energy) of
electromagnetic field is captured by an atom or molecule and the energy in the light quantum is
converted into atomic excitation energy. The energy in a quantum of light is equal to the product of Planck’s
constant, a natural constant, and the frequency of the
light. The excited atom can re-emit this excitation energy in the form of a quantum of
light, either spontaneously or through stimulation by the external
radiation field. During spontaneous emission, the
quantum is emitted in a random direction at a random
phase. This occurs, for instance, in ordinary lamps, and the resultant radiation field consists of overlapping wave
trains at various phases and in various directions. In
contrast, the atoms emitted during stimulated emission are forced into phase by the radiation field. When a
number of these in-phase wave trains overlap each other, the resultant radiation field propagates in the one
direction with a very stable amplitude. The spectral bandwidth of the radiation is also much smaller than that
of an ordinary lamp.
|
|
|
|
|
|
Two conditions must be met in order to synchronise this stimulated atomic emission: firstly, there
must be more atoms present in their higher, excited states than in the lower energy
levels, i.e. there must be an inversion. This is necessary otherwise the stimulated emissions of quanta will be
directly re-absorbed by the atoms which are present in lower energy states. The
inverted
condition does not prevail in nature: the lower energy levels are normally more densely populated
than the higher levels. Some means of ‘pumping’ the atoms is therefore needed.
The second important condition is that the radiation field is sufficiently large in order that there are
more stimulated emissions than spontaneous ones. Some laser media are reinforced to such an
extent that the radiation field is almost large enough as it is. An
optical resonator must normally be
used to feed the radiation field back into the medium. The two most important components in a
laser are thus
|
| |
|
|
