Introduction to Laser Principles
Stimulated Emission
The
word "laser" is an acronym for Light Amplification by Stimulated Emission
of Radiation. The easiest laser model to understand is the two level system.
In a two level system, the particles have only two availible energy levels,
separated by some energy difference which is typically referred to in terms
of the photon energy, hv0. These two levels are generally referred
to as the upper and lower laser states. When a particle in the upper state
interacts with a photon matching the energy separation of the levels, the
particle may decay, emitting another photon with the same phase and
frequency as the incident photon. Thus we have gotten two photons for
the price of one. This process is known as stimulated emission.
Population Inversion
A
fundamental concept in lasers is the idea of a "population inversion".
A normal thermal population in any material will have most of the particles
in the ground state. However, we would prefer to have most of the particles
in the excited state so we can get free photons through stimulated emission.
Thus in a laser we strive to create a "population inversion" where most
or all of the particles are in the excited state. This is achieved by adding
energy to the laser medium (usually from an electrical discharge or an
optical source such as another laser or a flashlamp); this process is called
pumping .
Gain
Another fundamental concept in lasers is the idea of gain, which is basically
a short way of referring to the "free" photons described earlier. Suppose
we have just pumped our laser medium so that all of the particles are in
their excited state. One of those particles now spontaneously decays back
down to its ground state, emitting a photon (hv0). This photon
is of the right frequency to stimulate emission from another excited state
particle, which emits another photon which can stimulate another excited
state particle, and so on. (see the figure below).
Loss
In addition to stimulated emission processes there are also stimulated
absorption processes in which a ground state particle absorbs a photon
matching the energy gap and jumps to the excited state. (represented by
the gray arrow in the above figure). Thus we lose one photon to each stimulated
absorption process. Since the probabilities for stimulated absorption and
emission processes are equal (relative to population of the ground and
excited states -- Einstein's famous result), it is clearly detrimental
to the laser to have any particles in the ground state. For this reason,
two level lasers are not practical -- it is not in general possible to
pump more than half of the molecules into the excited state.
