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Light Amplification by Stimulated Emission of Radiation
(LASER)
Is a mechanism for emitting light within the electromagnetic radiation
region of the spectrum, via the process of stimulated emission.
light broadly denotes electromagnetic radiation of any frequency, not
only the visible spectrum; hence infrared laser, ultraviolet laser, X-ray
laser, and microwave laser (maser).
Properties of laser light:
1. The laser light is (usually) narrow low-divergence beam, that can
be manipulated with lenses.
2. coherent light waves of identical frequency and phase.
3. the laser light is a narrow-wavelength electromagnetic spectrum
monochromatic light.
Basic Principles of Lasers
To explain the process of light amplification in a laser requires an
understanding of the energy transition phenomena in the atoms of its
active medium. They include: spontaneous emission, stimulated
emission/absorption and non-radiative decay.
The theory of quantum mechanics states that the electrons of atoms can
take different energy states, E1, E2, E3, for example, with E1<E2<E3.
Spontaneous Emission
By quantum mechanics the lower energy level is more stable than higher
energy levels, so electrons tend to occupy the lower level. Those
electrons in higher energy levels decay into lower levels, with the
emission of EM radiation. This process is called spontaneous emission.
The radiation emitted is equal to the energy difference between the two
levels.
E2 - E1 = hn0
Where E2 is the upper energy level
E1 is the lower energy level
h is Plank’s constant
n0 is frequency of the radiated EM wave.
Stimulated Emission
Suppose the atoms of the active medium are initially in E2. If external
EM waves with frequency n0 that is near the transition frequency
between E2 and E1 is incident on the medium, then there is a finite
probability that the incident waves will force the atoms to undergo a
transition E2 to E1. Every E2-E1 transition gives out an EM wave in the
form of a photon. We call this stimulated emission since the process is
caused by an external excitation. The emitted photon is in phase with the
incident photon, has the same wavelength as it and travels in the same
direction as the incident photon.
Stimulated Absorption
If the atom is initially in the ground level E1, the atom will remain in this
level until it gets excited. When an EM wave of frequency n0 is incident
on the material, there is a finite probability that the atom will absorb the
incident energy and jump to energy level E2. This process is called
Stimulated Absorption.
Non-Radiative Decay
Note that the energy difference between the two levels can decay by nonradiative decay. The energy difference can change into kinetic energy or
internal energy through collisions with surrounding atoms, molecules or
walls.
Population Inversion
Normally the population of the lower energy levels is larger than that of
the higher levels. The processes of stimulated radiation/absorption and
spontaneous emission are going on in the same time, yet even if we
ignore the decay factors, stimulated absorption still dominates over
stimulated radiation. This means that the incident EM wave cannot be
amplified in this case.
Amplification of incident wave is only possible when the population of
the upper level is greater than that of the lower level. This case is called
Population Inversion. This is a mechanism by which we can add more
atoms to the metastable level and hold them there long enough for them
to store energy, thereby allowing the production of great numbers of
stimulated photons.
To do this, we pump atoms into the metastable level at a rate that exceeds
the rate at which they leave. A large number of atoms are therefore
excited to and held in this level, leaving an almost empty level below it.
The atoms stay in this metastable level without de-exciting while the
population builds up, giving rise to a population inversion.
In practice laser action cannot be achieved for only two levels, as
described above. Three and four level systems work however. An
analysis of these systems follows, followed by a description of the
pumping schemes for each system.
(Note: A metastable level is one that has a long lifetime and the for which
the probability of spontaneous emission is low. This favors conditions for
stimulated emission. If an atom is excited to a metastable state it can
remain there long enough for a photon of the correct frequency to arrive.
This photon will then stimulate the emission of a second photon.)
Principal components of laser system:
1. Gain medium(active medium)
2. Laser pumping energy
3. High reflector
4. Output coupler