A laser is a device that emits coherent light through stimulated emission of photons. It works by pumping a gain medium with an external energy source to produce a population inversion between quantum energy states. Photons are emitted through stimulated emission, resulting in a collimated beam of light that is monochromatic, coherent, and highly directional. Common laser types include solid-state, liquid, gas, excimer, and semiconductor lasers, which use different gain media and pumping mechanisms but operate on similar principles of stimulated emission and optical feedback in a resonant cavity. Lasers have applications in science, industry, medicine, communications, and defense.
2. What is Laser?
Light Amplification by Stimulated Emission of
Radiation
• A device produces a coherent beam of optical radiation
by stimulating electronic, ionic, or molecular transitions
to higher energy levels
• Mainly used in Single Mode Systems
• Light Emission range: 5 to 10 degrees
• Require Higher complex driver circuitry than LEDs
• Laser action occurs from three main processes: photon
absorption, spontaneous emission, and stimulated
emission.
3. Properties of Laser
• Monochromatic
Concentrate in a narrow range of wavelengths
(one specific colour).
• Coherent
All the emitted photons bear a constant phase
relationship with each other in both time and
phase
• Directional
A very tight beam which is very strong and
concentrated.
4. Basic concepts for a laser
• Absorption
• Spontaneous Emission
• Stimulated Emission
• Population inversion
5. Absorption
• Energy is absorbed by an atom, the electrons
are excited into vacant energy shells.
6. Spontaneous Emission
• The atom decays from level 2 to level 1 through
the emission of a photon with the energy hv. It is
a completely random process.
7. Stimulated Emission
atoms in an upper energy level can be triggered
or stimulated in phase by an incoming photon of
a specific energy.
8. Stimulated Emission
The stimulated photons have unique properties:
– In phase with the incident photon
– Same wavelength as the incident photon
– Travel in same direction as incident photon
10. Laser Diode Optical Cavity
• One reflecting mirror is at one end while the other end
has a partially reflecting mirror for partial emission
• Remaining power reflects through cavity for amplification
of certain wavelengths, a process known as optical
feedback.
• Construction very similar to the ELEDs.
21. Condition for the laser operation
If n1 > n2
• radiation is mostly absorbed absorbowane
• spontaneous radiation dominates.
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
if n2 >> n1 - population inversion
Necessary condition:
population inversion
E1
E2
22. Population Inversion
• A state in which a substance has been
energized, or excited to specific energy levels.
• More atoms or molecules are in a higher excited
state.
• The process of producing a population inversion
is called pumping.
• Examples:
→by lamps of appropriate intensity
→by electrical discharge
23. E1
E2
• n1 - the number of electrons of
energy E1
• n2 - the number of electrons of
energy E2
2 2 1
1
( )
exp
n E E
n kT
− −
=
Boltzmann’s equation
example: T=3000 K E2-E1=2.0 eV
4
2
1
4.4 10
n
n
−
=
24. Einstein’s relation
Probability of stimulated absorption R1-2
R1-2 = r (n)n1 B1-2 where spectral density is r (n)
& Einstein coeff. of absorbtion is B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) n2B2-1 + n2A2-1
Einstein coeff. of stimulated and spontaneous emission are B2-1& A2-1
Assumption : For a system in thermal equilibrium, the upword and downword
transition rate must be equal :
R1-2 = R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)
2 1 2 1
1 1 2
2 2 1
/
=
1
A B
n B
n B
r n − −
−
−
( )
−
E1
E2
25. B1-2/B2-1 = 1
According to Boltzman statistics:
r (n) = =
1
2 1
2
exp( ) / exp( / )
n
E E kT h kT
n
n
= − =
1
)
exp(
/
1
2
2
1
1
2
1
2
−
−
−
−
−
kT
h
B
B
B
A
n 1
)
/
exp(
/
8 3
3
−
kT
h
c
h
n
n
3
3
1
2
1
2 8
c
h
B
A n
=
−
−
Planck’s law
26. The probability of spontaneous emission A2-1 /the probability of stimulated
emission B2-1r(n ):
1. Visible photons, energy: 1.6eV – 3.1eV.
2. kT at 300K ~ 0.025eV.
3. stimulated emission dominates solely when hn /kT <<1!
(for microwaves: hn <0.0015eV)
The frequency of emission acts to the absorption:
if hn /kT <<1.
1
)
/
exp(
)
(
1
2
1
2 −
=
−
− kT
h
B
A
n
n
r
1
2
1
2
1
2
1
2
2
1
1
1
2
2
1
2
2 ]
)
(
1
[
)
(
)
(
n
n
n
n
B
A
B
n
B
n
A
n
x
+
=
+
=
−
−
−
−
−
n
r
n
r
n
r
x~ n2/n1
27. Two-level Laser System
• Unimaginable
as absorption and stimulated processes
neutralize one another.
• The material becomes transparent.
29. Three-level Laser System
• Initially excited to a
short-lived high-energy
state .
• Then quickly decay to
the intermediate
metastable level.
• Population inversion is
created between lower
ground state and a
higher-energy
metastable state.
30.
31. 2
3
2
1
1.06 m
=
4
2
τ 2.3 10 s
−
1 3.39 m
= 2 0.6328 m
=
3 1.15 m
=
100ns
τ2
10ns
τ1
Three-level Laser System
Nd:YAG laser
He-Ne laser
32. Four-level Laser System
• Laser transition takes
place between the
third and second
excited states.
• Rapid depopulation of
the lower laser level.
33. Four-level Laser System
2
3
m
6943
.
0
1 =
m
6928
.
0
2 =
s
7
3
10
τ −
s
3
2
10
3
τ −
Ruby laser
35. Lasing Characteristics
• Lasing threshold is
minimum current that must
occur for stimulated
emission
• Any current produced below
threshold will result in
spontaneous emission only
• At currents below threshold
LDs operate as ELEDs
• LDs need more current to
operate and more current
means more complex drive
circuitry with higher heat
dissipation
• Laser diodes are much
more temperature sensitive
than LEDs
37. Modulation of Optical Sources
• Optical sources can be modulated either
directly or externally.
• Direct modulation is done by modulating
the driving current according to the
message signal (digital or analog)
• In external modulation, the laser is emits
continuous wave (CW) light and the
modulation is done in the fiber
38. Types of Optical Modulation
• Direct modulation is done by
superimposing the modulating (message)
signal on the driving current
• External modulation, is done after the light
is generated; the laser is driven by a dc
current and the modulation is done after
that separately
• Both these schemes can be done with
either digital or analog modulating signals
39. Direct Modulation
• The message signal (ac) is superimposed on
the bias current (dc) which modulates the laser
• Robust and simple, hence widely used
• Issues: laser resonance frequency, chirp, turn
on delay, clipping and laser nonlinearity
40. Laser Construction
• A pump source
• A gain medium or laser medium.
• Mirrors forming an optical resonator.
42. Pump Source
• Provides energy to the laser system
• Examples: electrical discharges, flashlamps,
arc lamps and chemical reactions.
• The type of pump source used depends on
the gain medium.
→A helium-neon (HeNe) laser uses an
electrical discharge in the helium-neon
gas mixture.
→Excimer lasers use a chemical reaction.
43. gain medium
• Major determining factor of the wavelength of
operation of the laser.
• Excited by the pump source to produce a
population inversion.
• Where spontaneous and stimulated emission
of photons takes place.
• Example:
solid, liquid, gas and semiconductor.
44. Optical Resonator
• Two parallel mirrors placed around the
gain medium.
• Light is reflected by the mirrors back into
the medium and is amplified .
• The design and alignment of the mirrors
with respect to the medium is crucial.
• Spinning mirrors, modulators, filters and
absorbers may be added to produce a
variety of effects on the laser output.
45. Laser Types
• According to the active material:
solid-state, liquid, gas, excimer or
semiconductor lasers.
• According to the wavelength:
infra-red, visible, ultra-violet (UV) or x-ray
lasers.
46. Solid-state Laser
• Example: Ruby Laser
• Operation wavelength: 694.3 nm (IR)
• 3 level system: absorbs green/blue
•Gain Medium: crystal of aluminum oxide (Al2O3)
with small part of atoms of aluminum is replaced
with Cr3+ ions.
•Pump source: flash lamp
•The ends of ruby rod serve as laser mirrors.
50. Liquid Laser
• Example: dye laser
• Gain medium: complex organic dyes, such
as rhodamine 6G, in liquid solution or
suspension.
• Pump source: other lasers or flashlamp.
• Can be used for a wide range of
wavelengths as the tuning range of the
laser depends on the exact dye used.
• Suitable for tunable lasers.
51. Schematic diagram of a dye laser
dye laser
A dye laser can be considered to be basically a four-level system.
The energy absorbed by the dye creates a population inversion, moving the
electrons into an excited state.
52. Gas Laser
• Example: Helium-neon laser (He-Ne laser)
• Operation wavelength: 632.8 nm
• Pump source: electrical discharge
• Gain medium : ratio 5:1 mixture of helium and neon
gases
54. Semiconductor laser
Semiconductor laser is a laser in which semiconductor serves as
photon source.
Semiconductors (typically direct band-gap semiconductors) can be
used as small, highly efficient photon sources.
55. Applications of laser
• 1. Scientific
a. Spectroscopy
b. Lunar laser ranging
c. Photochemistry
d. Laser cooling
e. Nuclear fusion
56. 2 Military
a. Death ray
b. Defensive applications
c. Strategic defense initiative
d. Laser sight
e. Illuminator
f. Rangefinder
g. Target designator
Applications of laser
57. • 3. Medical
a. eye surgery
b. cosmetic surgery
Applications of laser
58. • 4. Industry & Commercial
a. cutting, welding, marking
b. CD player, DVD player
c. Laser printers, laser pointers
d. Photolithography
e. Laser light display
Applications of laser
