1. The document discusses the key concepts behind how lasers work, including definitions of laser, absorption and emission, spontaneous and stimulated emission, population inversion, and Einstein coefficients. 2. It explains that lasers generate coherent light through stimulated emission, which requires achieving a population inversion where more atoms are in an excited state than a lower state. This can be achieved through three-level or four-level pumping schemes. 3. The principles of laser operation are described as light amplification through repeated stimulated emissions of photons with the same frequency, direction, and phase as they pass through excited atoms in the gain medium.

1. OPTOMETRY – Part VII
LASER
ER. FARUK BIN POYEN
DEPT. OF AEIE, UIT, BU, BURDWAN, WB, INDIA
FARUK.POYEN@GMAIL.COM

2. Contents:
1. Definition of LASER
2. Absorption & Emission
3. Spontaneous & Stimulated Emission
4. Population Inversion
5. Einstein Coefficient
6. Principle of LASER Operation
7. Types of LASER
8. Applications of LASER
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3. Definition of LASER:
 The term “LASER" originated as an acronym for "light amplification by stimulated
emission of radiation".
 A device that generates an intense beam of coherent monochromatic light (or other
electromagnetic radiation) by stimulated emission of photons from excited atoms or
molecules through a process of optical amplification.
 A MASER (an acronym for "microwave amplification by stimulated emission of
radiation") is a device that produces coherent electromagnetic waves through
amplification by stimulated emission.
 The laser is an optical maser.
 Einstein showed in 1917 how atoms, ions or molecules can emit radiation in the form of
energy quanta (photons) through spontaneous (disordered photon emission) or photon
emission stimulated through a signal.
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4. Characteristics of LASER:
 Four unique characteristics:
1. Coherence
2. Directionality
3. Monochromatic
4. High intensity
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5. Absorption Spectrum:
 Electrons exist in energy levels within an atom.
 These levels have well defined energies and electrons moving between them must
absorb or emit energy equal to the difference between them.
 Energy levels associated with molecules, atoms and nuclei are in general discrete,
quantized energy levels and transitions between those levels typically involve the
absorption or emission of photons.
 Absorption of a photon will occur only when the quantum energy of the photon
precisely matches the energy gap between the initial and final states.
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6. Absorption Spectrum:
 Planck Hypothesis:
To explain the frequency distribution of radiation from a hot cavity (blackbody
radiation) Planck proposed the ad hoc assumption that the radiant energy could exist
only in discrete quanta which were proportional to the frequency.
 The equation that defines Planck's constant is called the Planck-Einstein relation.
 Absorption is the process where the electrons of a substance absorb or take up the
energy wavelengths incident on them. The atomic and molecular structure of the
material governs its level of absorption, along with the amount of electromagnetic
radiation, temperature, solid crystal structure, and intermolecular interactions.
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7. Spontaneous Emission:
 Emission is the process where a substance gives off or emits radiation when it is heated
or treated chemically.
 Definition: A quantum effect, causing the spontaneous decay of excited states of atoms
or ions is called Spontaneous Emission.
 Spontaneous emission is a quantum effect.
 The level of emission of a substance depends on its spectroscopic composition and
temperature, properties of the atom and by the mode structure of the surrounding
medium.
 Light produced by spontaneous emission is called luminescence.
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8. Spontaneous Emission:
 When an atom (or a laser ion in a gain medium) is excited into a higher-lying energy
level, e.g. by absorption of a photon, it may after some time spontaneously return to its
ground state, or to some intermediate energy level, by releasing the energy in the form
of a photon, which carries the energy in some random direction.
 More precisely, the photon can correspond to any propagation mode of the medium
surrounding the atom or ion.
 This process is called spontaneous emission.
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9. Stimulated Emission:
 Stimulated emission is the process by which an incoming photon of a specific frequency
can interact with an excited atomic electron (or other excited molecular state), causing it
to drop to a lower energy level.
 If an electron is already in an excited state (an upper energy level, in contrast to its
lowest possible level or "ground state"), then an incoming photon for which the
quantum energy is equal to the energy difference between its present level and a lower
level can "stimulate" a transition to that lower level, producing a second photon of the
same energy.
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10. Stimulated Emission:
 The liberated energy transfers to the electromagnetic field, creating a new photon with a
phase, frequency, polarization, and direction of travel that are all identical to the photons
of the incident wave.
 This is in contrast to spontaneous emission, which occurs at random intervals without
regard to the ambient electromagnetic field.
 Stimulated emission requires (like absorption and spontaneous emission) that the photon
energy given by the Planck relationship be equal to the energy separation of the
participating pair of quantum energy states.
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11. Population Inversion:
 Definition: A state of a medium where a higher-lying electronic level has a higher
population than a lower-lying level.
 More laser-active ions are in the upper state than in the lower state. This condition of the
laser medium is called population inversion.
 Population Inversion occurs while a system (such as a group of atoms or molecules)
exists in a state in which more members of the system are in higher, excited states than
in lower, unexcited energy states.
 It is called an "inversion" because in many familiar and commonly encountered physical
systems, this is not possible.
 The achievement of a significant population inversion in atomic or molecular energy
states is a precondition for laser action.
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12. Population Inversion:
 A population inversion cannot be achieved with just two levels because the probability
for absorption and for spontaneous emission is exactly the same when they are in
thermal equilibrium.
 To achieve non-equilibrium conditions, an indirect method of populating the excited
state must be used and they are
1. Three Level Laser
2. Four Level Laser
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13. Population Inversion – Three Level Laser:
 We consider a group of N atoms, this time with each atom able to exist in any of three
energy states, levels 1, 2 and 3, with energies E1, E2, and E3, and populations N1, N2,
and N3, respectively.
 We assume that E1 < E2 < E3; that is, the energy of level 2 lies between that of the
ground state and level 3.
 Initially, the system of atoms is at thermal equilibrium, and the majority of the atoms
will be in the ground state, i.e., N1 ≈ N, N2 ≈ N3 ≈ 0.
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14. Population Inversion – Three Level Laser:
 The atoms are subject to light of a frequency ν13 =
1
ℎ
(𝐸1 − 𝐸3), the electrons will be
excited to L3 from ground.
 This process is called “pumping” which can be via optical absorption, electrical
discharge or chemical reactions.
 The level 3 is sometimes referred to as the pump level or pump band, and the energy
transition E1 → E3 as the pump transition, which is shown as the arrow marked P in the
diagram on the right.
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15. Population Inversion – Three Level Laser:
 If we continuously pump electrons, we will excite an appreciable number of them into
level 3, such that N3 > 0.
 To have a medium suitable for laser operation, it is necessary that these excited atoms
quickly decay to level 2.
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16. Population Inversion – Four Level Laser :
 Here, there are four energy levels, energies E1, E2, E3, E4, and
populations N1, N2, N3, N4, respectively. The energies of each level are such
that E1 < E2 < E3 < E4.
 In this system, the pumping transition P excites the atoms in the ground state (level 1)
into the pump band (level 4).
 From level 4, the atoms again decay by a fast, non-radiative transition Ra into the level
3.
 Since the lifetime of the laser transition L is long compared to that of Ra (τ32 ≫ τ43), a
population accumulates in level 3 (the upper laser level), which may relax by
spontaneous or stimulated emission into level 2 (the lower laser level).
 This level likewise has a fast, non-radiative decay Rb into the ground state.
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17. Population Inversion – Four Level Laser : 17
In a 4-level laser, only a few
electrons are excited to achieve
population inversion. Therefore, a
4-level laser produces light
efficiently than a 3-level laser.

18. Einstein’s Coefficients:
 Einstein coefficients are mathematical quantities which are a measure of the probability
of absorption or emission of light by an atom or molecule.
 The Einstein A coefficient is related to the rate of spontaneous emission of light and the
Einstein B coefficients are related to the absorption and stimulated emission of light.
𝐴
𝐵
= 8𝜋ℎν3/𝑐3
 Now according to Planck’s radiation law, the energy density of the black body radiation
of frequency v at temperature T is given by
𝜌(ν) = 8𝜋ℎν3/𝑐3
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19. Principle of LASER Operation:
 Lasing: The process which leads the emission of stimulated photons due to the transition
of atoms from the metastable state to the ground state after achieving population
inversion.
 The process is based on phenomenon of stimulated emission and spontaneous emission
 Active medium should have one metastable state besides excited state and ground state.
 The lifetime of atoms in excited state is 10−8 sec but it is longer in metastable state.
 When atoms are excited with light of suitable wavelength, they jump from lower energy
state to excited state by absorbing photons.
 But atoms can remain in excited state only for a small amount of time and they drop
back by spontaneous emission.
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20. Principle of LASER Operation:
 Many of them are trapped in the metastable state where its lifetime is greater and
population inversion is obtained.
 After getting population inversion, a photon got from spontaneous emission is made to
strike an atom of the metastable state.
 The excited atom of metastable state is stimulated to emit a photon of the same energy
as that of the stimulating photon.
 The stimulating and stimulated photons yield a large number of coherent photons by
repeated stimulated emissions as they pass through the atom.
 Hence light amplification occurs due to multiplication of photons all of which have
same frequency, direction and phase.
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21. Types of LASER:
 Solid state lasers have lasing material distributed in a solid matrix.
E.g. the ruby or neodymium-YAG (Nd-YAG) lasers.
 Gas lasers have a primary output of a visible red light.
E.g. He, He-Ne, CO2 laser.
 Excimer lasers (the name is derived from the terms excited and dimers) use reactive gases
such as chlorine and fluorine mixed with inert gases such as argon, krypton, or xenon. When
electrically stimulated, a pseudo molecule or dimer is produced and when lased, produces
light in the ultraviolet range.
 Dye lasers use complex organic dyes like rhodamine 6G in liquid solution or suspension as
lasing media. They are tunable over a broad range of wavelengths.
 Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers. These
electronic devices are generally very small and use low power. They may be built into larger
arrays, e.g., the writing source in some laser printers or compact disk players.
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22. Homojunction & Heterojunction:
 A homojunction is a semiconductor interface that occurs between layers of similar
semiconductor material, these materials have equal band gaps but typically have
different doping. In most practical cases a homojunction occurs at the interface between
an n-type (donor doped) and p-type (acceptor doped) semiconductor such as silicon, this
is called a p-n junction.
 A heterojunction is the interface that occurs between two layers or regions of dissimilar
crystalline semiconductors and has unequal band gaps.
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23. Comparison of Few LASER Types: 23

24. Applications of LASER:
1. Communications: besides fiber-optic communication, lasers are used for free-space optical
communication, including laser communication in space.
2. Medicine: see below.
3. Industry: cutting, welding, material heat treatment, marking parts, non-contact measurement of
parts.
4. Military: marking targets, guiding munitions, missile defense, electro-optical countermeasures
(EOCM), lidar, blinding troops. See below
5. Law enforcement: LIDAR traffic enforcement. Lasers are used for latent fingerprint detection in
the forensic identification field[65][66]
6. Research: spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry,
lidar, laser capture micro-dissection, fluorescence microscopy, metrology.
7. Commercial products: laser printers, barcode scanners, thermometers, laser pointers, holograms,
bubblegrams.
8. Entertainment: optical discs, laser lighting displays.
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25. Reference:
1. https://www.rp-photonics.com/spontaneous_emission.html
2. http://hyperphysics.phy-astr.gsu.edu/hbase/mod5.html
3. https://en.wikipedia.org/wiki/Population_inversion
4. https://en.wikipedia.org/wiki/Einstein_coefficients
5. https://en.wikipedia.org/wiki/Laser
6. http://www.physics-and-radio-electronics.com/physics/laser/laser-
populationinversion.html
7. http://www.physics-and-radio-electronics.com/physics/laser/characteristics-of-
laser.html
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