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Laser (Light Amplification by
Stimulated Emission of Radiation)
BY SUMITESH MAJUMDER
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.
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.
Basic concepts for a laser
• Absorption
• Spontaneous Emission
• Stimulated Emission
• Population inversion
Absorption
• Energy is absorbed by an atom, the electrons
are excited into vacant energy shells.
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.
Stimulated Emission
atoms in an upper energy level can be triggered
or stimulated in phase by an incoming photon of
a specific energy.
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
Stimulated Emission
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.
Mirror Reflections
The operation of the Laser
The operation of the Laser
E4
E3
E2
E1
The operation of the Laser
E4
E3
E2
E1
absorption
The operation of the Laser
E4
E3
E2
E1
Spontaneous emission
The operation of the Laser
Spontaneous emission
1. Incoherent light
2. Accidental direction
The operation of the Laser
E4
E3
E2
E1
The operation of the Laser
E4
E3
E2
E1
Stimulated emission
The operation of the Laser
Light: Coherent, polarized
The stimulating and emitted
photons have the same:
frequency
phase
direction
How a Laser Works
Condition for the laser operation
E2
E1
If n1 > n2
• radiation is mostly absorbed absorbowane
• spontaneous radiation dominates.
if n2 >> n1 - population inversion
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
Necessary condition:
population inversion
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
Boltzmann’s equation
E2
E1
n2
 ( E2  E1 ) 
 exp 

n1
kT


• n1 - the number of electrons of
energy E1
• n2 - the number of electrons of
energy E2
example: T=3000 K
E2-E1=2.0 eV
n2
4
 4.4 10
n1
Einstein’s relation
E1
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)

E2
A21 / B21
r n  =
n1 B1 2
1
n2 B21
According to Boltzman statistics:
r (n) =
n1
 exp( E2  E1 ) / kT  exp(hn / kT )
n2
A21 / B21
B1 2
hn
exp( )  1
B21
kT
=
8hn 3 / c 3
exp(hn / kT )  1
Planck’s law
B1-2/B2-1 = 1
A21 8hn

B21
c3
3
The probability of spontaneous emission A2-1 /the probability of stimulated
emission B2-1r(n :
A21
 exp(hn / kT )  1
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:
x
if hn /kT <<1.
n2 A21  n2 B21r (n )
A21 n2 n2
 [1 
] 
n1B1 2 r (n )
B21r (n ) n1 n1
x~ n2/n1
Two-level Laser System
• Unimaginable
as absorption and stimulated processes
neutralize one another.
• The material becomes transparent.
Two level system
E2
hn
hn=E2-E1
E2
hn
hn
E1
absorption
E1
Spontaneous
emission
Stimulated
emission
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.
Three-level Laser System
 3   2 1   2
Nd:YAG laser
  1.06  m
τ2  2.3 104 s
He-Ne laser
1  3.39  m 2  0.6328  m
3  1.15  m
τ  100ns τ1  10ns
2
Four-level Laser System
• Laser transition takes
place between the
third and second
excited states.
• Rapid depopulation of
the lower laser level.
Four-level Laser System
 3   2
Ruby laser
1  0.6943m
2  0.6928m
τ  10 s τ  3  10 s
3
7
3
2
Multimode Laser Output
Spectrum
Mode
Separation
(Center Wavelength)
g(λ)
Longitudinal
Modes
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
Fabry-Perot Laser
(resonator) cavity
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
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
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
Laser Construction
• A pump source
• A gain medium or laser medium.
• Mirrors forming an optical resonator.
Laser Construction
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.
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.
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.
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.
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.
Ruby laser
Energy
Al2O3
Cr+
rapid decay
LASING
How a laser works?
Ruby laser
First laser: Ted Maiman
Hughes Research Labs
1960
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.
dye laser
Schematic diagram of a 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.
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
He-Ne laser
1  3.39 μm 2  0.6328 μm 3  1.15 μm
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.
Applications of laser
• 1. Scientific
a. Spectroscopy
b. Lunar laser ranging
c. Photochemistry
d. Laser cooling
e. Nuclear fusion
Applications of laser
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
• 3. Medical
a. eye surgery
b. cosmetic surgery
Applications of laser
• 4. Industry & Commercial
a. cutting, welding, marking
b. CD player, DVD player
c. Laser printers, laser pointers
d. Photolithography
e. Laser light display