CHAPTER 3: LASER SOURCE

CHAPTER 3: LASER SOURCE
Light Amplification by Stimulated Emission of Radiation, commonly referred to as "Laser"
describes a wide range of devices. The lasers can function as oscillators (sources of light) and as
amplifiers. Lasers have revolutionized various fields of science and technology, and are being
used in a wide range of applications in medicine, communications, defense, measurement, and as
a precise light source in many scientific investigations. Commercially available lasers can be
categorized based on the characteristics:
Wavelength: Lasers span the entire light spectrum from infrared to ultraviolet.
Power: The power output from a laser ranges from a milliwatt to millions of watts.
Output beam: The laser output may be a continuous wave, where the lasers emit light in a
continuous manner or it might be pulsed, where the lasers emit light in short bursts.
3.1 Principle of operation
The principle of operation remains the same though there is a wide range of lasers. Laser action
occurs in three stages: photon absorption, spontaneous emission, and stimulated emission. The
above three processes are represented in the Figure 3.1, where E1 is the ground-state or lower
energy level and E2 is the excited-state or higher energy level. The particle of the material,
which undergoes the process of excitation, might be an atom, molecule, or ion depending on the
laser material.
Photon absorption: In any material, during thermal equilibrium the number of particles in the
excited state is very small and is negligible. When the number of particles in the excited state is
greater than the number of particles in the ground state, the material is in a state of "Population
Inversion". Population inversion is a prerequisite for laser action. Energy can be transferred into
a laser medium to achieve population inversion by several mechanisms including absorption of
photon, collision between electrons (or sometimes ions) and species in the active medium,
collisions among atoms and molecules in the active medium, recombination of free electrons
with ionized atoms, recombination of current carriers in a semiconductor, chemical reactions
producing excited species, and acceleration of electrons [14].
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In photon absorption, the laser material is optically excited to achieve population inversion based
on Planck’s law. According to Planck’s law, the change of energy level from E1 to E2 or vice
versa, results in the absorption or emission of photon respectively.
Figure 3.1 Laser Action
Impinging
photon
E2
E1
Absorption
Output
photon
E1
E2 Spontaneous Emission
Stimulating Photon
E1
E2
Stimulated
Emission
Stimulated Emission
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Spontaneous emission: The excited particles resulting from population inversion are unstable.
They release their excess energy by non-radiative process, such as collisions with other excited
particles or by photon emission (Planck’s law), and return to the stable ground state. The
emission of a photon can be spontaneous or stimulated. Spontaneous emissions occur without
any external stimulus, when the laser material drops to its ground state after a characteristic delay
time. Spontaneous emissions are random and isotropic in nature.
Stimulated emission: The excited particles can be made to return to the ground-state through an
external stimulation. When an external photon having the same energy as the energy difference
between the ground state and excited state, impinges on the excited laser material, the particles
will drop to the ground-state and emit a photon. A photon having the exact energy necessary to
cause stimulated emission is made available by the spontaneous emission. These photons from
spontaneous emission trigger stimulated emission of other photons resulting in a cascade of
stimulated emission. The photons due to stimulated emission are
1. highly monochromatic (single wavelength),
2. coherent (all the waves have the same phase), and
3. collimated (parallel rays) or appear to originate from a point source
If during the process of stimulated emission, the population inversion is maintained by
continuous pumping of energy, the laser action continues indefinitely and the result is a
continuous wave laser. On the other hand, if the pumping cannot be maintained the output is a
pulsed laser.
3.2 Construction of a laser
A laser consists of an active laser material, a source of excitation energy