PHYA4-LASERS.ppt, for first year B.E./BTech

Prof. J.K. Goswamy
UIET, Panjab University
Chandigarh
LASERS

Light
Amplification
by
Stimulated
Emission
of
Radiation
Basics of

Light Emission: Atomic Basis


Time-Energy Uncertainty Principle

This principle states that uncertainty in energy of emitted radiation
depends upon the time interval available for its measurement.

PROPERTIES OF LASER
 Intensity
 Monochromaticit
y
 Directionality
 Coherence

Intensity
Laser sources are
brighter than any
other light source

Monochromaticity
Lasers have very small spectral width and high
monochromaticity.

Directionality
Lasers are highly directional beams with small
divergence.
Ordinary light spreads in all
directions

Coherence
Lasers are highly coherent
beams.
Two sources are said to be coherent if they emit light waves Continuously
having same frequency or wavelength and bearing a constant phase
difference.

Laser light can’t be perfectly
 Monochromatic
 Directional
 Coherent
However they are far more
coherent than light from any other
source.

Output of Laser
• Continuous Wave Laser (CW)
has output power which
remains constant with time.
They are powerful and versatile.
• Pulsed Laser has power
distribution varying with time.
Total power of such a laser is
given as the product of pulse
repetition frequency and
average power carried by each
pulse.

History of Lasers
Name Year Contribution
Albert Einstein 1917 Quantum Theory of Lasers
C Townes 1954 Basic idea of MASER
C. Townes and A. Schalow 1958 Basic idea of LASER
T H Maimann 1960 First RUBY Laser
E Snitzer 1961 Nd3+ Glass Laser
A Jawan 1961 First He-Ne Laser
Several Authors 1962 GeAs Diode Laser
C K N Patel 1964 CO2 Laser
W B Bridges 1964 Ar+ Laser
G S Jeusic 1964 Nd: YAG Laser
J V V Kasper & J C Pimmental 1965 Chemical Laser (HCl)
P P Sorokin & J R Lankard 1966 Dye Laser
N G Bosov 1971 Xe2+ Laser
Gordon Gould 1977 Awarded Patent for Laser
P F Moulton 1984 Ti: Sapphire Laser
 Invention of MASER in 1954 by
C Townes.
 Invention of LASER in 1958 by
C. Townes & A. Schalow. They
got Nobel prize for this work in
1964.
 It was based on prediction of
stimulated emission by
Einstein in 1917.

Atomic transitions are of three types:
 Absorption
 Spontaneous Emission
 Stimulated Emission
Atomic Transitions

• The absorption occurs through supply of
appropriate energy quantum to the atom.
• The rate of this process depends upon the
population density of atoms in the state and
radiation field density affecting this process.
Absorption

Stimulated Emission
• The stimulated emission occurs when
excited atom gets de-excited by
external stimulus such as a photon of
same energy as difference between
two levels involved in de-excitation.
• The rate of stimulated emission
depends upon population density of
atoms in the excited state and radiation
density of the stimulating photons.
• Photons emitted in this process are
identical to the stimulating photon in
energy, field distribution and phase.

Hence stimulated emissions can cause
photon multiplication or light amplification.
Stimulated Emission

Consider an atomic system with two levels.
The population densities for ground and
excited states are N1 and N2 respectively.
Due to excitation mechanism, at any room
temperature, we must have:
The light amplification requires stimulated
emission to dominate spontaneous emission
process. Hence
The situation where excited state is densely
populated relative to its lower state is called
population inversion.
Light Amplification :Population Inversion
 
kT
E
E
e
N
N 1
2
2
1


1
2 N
N 

The population inversion requires an
Active Medium which can exist in the state
of population inversion under appropriate
excitation conditions.
 Such a medium has one or more
long lived or metastable states.
 The excited electrons get trapped in
such states thereby causing
population inversion relative to a
lower state.
Light Amplification :Active Medium

Light Amplification : Cavity Resonator
 When N2>N1, we require to boost the intensity of laser beam through
stimulated emissions occurring repeatedly. This is achieved using cavity
resonator.
 If medium has many excited atoms, this set-up will multiply photon intensity
indefinitely like a chain reaction through repeated reflections, which are
achieved using reflector (mirror 1) and output coupler (mirror 2).
 The laser is to be emitted at some juncture which is made possible through
output coupler.

• Laser system behaves like an oscillator. The
active medium, enclosed by highly reflecting
mirror and output coupler, forms optical
cavity.
• The photon produced in stimulated emission
oscillate in this cavity leading to photon
multiplication or light amplification.
• Back and forth oscillations of photons form
standing waves characterized by different
frequency, phase and field distribution. Each
standing wave forms a mode of oscillations.
• Certain modes gain energy in subsequent
oscillations till achieving saturation while
Cavity Oscillations

Let’s consider an atom having a set of two levels i.e.
ground and excited state of energy E1 and E2
respectively. The population density of ground and
excited states are N1 and N2 respectively.
 Stimulated Absorption : If an atom in the ground state is
supplied energy ΔE=E2-E1=h, it gets excited to higher
state. The rate of stimulated absorption is given as:
I(ω) is the radiation field density supplying energy for
excitation. It is also called pumping intensity.
Einstein’s Theory of Lasers
)
(
1
12 
I
N
B
Rab 

The de-excitation of the atom can proceed through two
competing processes:
 Spontaneous emission in which atom de-excites on its
own without being influenced by any external stimulus.
The rate of spontaneous emission of photon is:
 Stimulated emission in which atom de-excites under the
stimulus of external radiation field. The rate of emitted
photons is given as:
2
21N
A
Rsp 
)
(
2
21 
I
N
B
Rst 

 In thermal equilibrium, we have:
A21, B12 and B21 are called Einstein’s coefficients.
 The population density of atoms in the ground and excited
states obey the Maxwell-Boltzmann statistics and these
are related as:
kT
h
kT
E
E
e
e
N
N







 



1
2
1
2
)
1
(
)
(
)
(
)
(
21
2
1
12
21
1
12
2
21
2
21
B
N
N
B
A
I
I
N
B
I
N
B
N
A








In thermal equilibrium of collection of atoms of active
medium considered, the emitted radiation field density
obeys the black body radiation distribution given by
Planck’s radiation law as:
Comparing equations (1A) and (2), we can determine the
Einstein’s coefficients as:
)
2
(
1
1
.
8
)
( 3
3










kT
h
e
c
h
I 



3
3
21
12
21
8
c
h
B
A
A
A
B
B
B







)
1
(
)
(
21
12
21
A
B
e
B
A
I
kT
h

 


Initiation of Lasing Action: The ratio of rate of
stimulated emission to that of spontaneous
emission is:
)
3
(
)
(
8
)
( 3
3



 I
h
c
I
A
B
R
R
sp
st


Rate of stimulated emission can exceed that of spontaneous
emission by pumping intense radiation flux over active medium.

Interpreting Population Inversion: The ratio of stimulated
emission to stimulated absorption is given as:
)
4
(
)
(
)
(
1
2
1
2 kT
E
ab
st
e
N
N
N
BI
N
BI
R
R







 For stimulated emission to exceed photon
absorption, population inversion (i.e. N2 > N1) is
needed.
 Population inversion (N2 > N1) implies departure
from thermal equilibrium i.e. it is phenomenon
involving non-thermal equilibrium.
 Population inversion, according to Maxwell-
Boltzmann Statistics, implies negative absolute
temperatures.

Population Inversion in Two Level System
 
.
2
1
2
1
2
2
0
:
,
positive
always
is
N
B
A
I
where
I
I
N
I
A
B
N
N
AN
N
BI
A
N
A
AN
N
BI
have
we
state
steady
In
sat
sat























It is impossible to achieve
population inversion in a
two level system.

Population Inversion in Three Level System
0
1
1




















N
I
I
for
Now
B
A
I
where
I
I
I
I
N
N
sat
sat
sat
sat
Three level system can generate
laser only if the pumping intensity
exceeds beyond the saturation
value.

Population Inversion in Four Level System
.
0
1
1
situations
all
in
holds
N
Now
B
A
I
where
I
I
I
I
N
I
A
B
I
A
B
N
N sat
sat
sat




























 Population inversion is most
easily achieved in 4 level
system.

saturation intensity
If you hit hard,
you get lasing
Comparison: 2-, 3- and 4-level Systems
Laser
Transition
Pump
Transition
Fast Decay
Three-Level
System

• Optical pumping is used for excitation of
dye lasers and solid lasers.
• Flash lamp is the device used for optical
pumping of energy into active medium.
• It consists of a cylindrical quartz tube
filled with some gas (Xenon or Krypton)
and is fitted with electrodes at its end.
• The high potential difference applied
between two electrodes of lamp causes
electric discharge in enclosed gas and
emission of high flux of photons which
are used to excite active medium.
Excitation Mechanism :Optical Pumping
Linear Flash Lamp
Helical Flash Lamp

 The electrical discharge, due to high voltage
applied across the electrodes, through the gas,
produces free electrons which get accelerated to
high speeds before colliding with gas molecules (or
atoms).
 The gas molecules/atoms get excited and
subsequently de-excite through spontaneous
emission.
 These photons are made to cause lasing action in
the active medium.
Excitation Mechanism : Electrical Pumping

Active Medium
 Atoms: He-Ne laser , He-Cd laser,
Copper Vapor laser.
 Molecules: CO2 laser, Excimer
(KrF, ArF) laser, N2 lasers.
 Liquids: Organic dye molecules
diluted in solvents.
 Dielectric Solids: Nd atoms
doped in YAG or glass, Ruby
laser.
 Semiconductors: GaAs or InP
crystals.

Classification of Lasers
Part of EM Spectrum
Ultraviolet
Visible
Infrared
Output Duration
Pulsed Laser
Continuous Wave
Laser
Power of Laser
Class I
(<1W)
Class II
(<1mW)
Class IIIA
(1-5mW)
Class IIIB
(5-500mW)
Class IV
(>500mW)

Laser Types
Laser Type Active Medium Pumping Mode Lasers
Solid Lasers Lasing material
distributed in solid
Optical Pumping Nd-YAG
Ruby
Ti-Sapphire
Semiconductor Lasers PN-junction Electric discharge
through forward bias
GaAs
InP
Dye Lasers Organic dyes as liquid
solution
Optical pumping or
powered by laser
Rhodamine-6G
Gas Lasers Gas or their mixture. Electric discharge He-Ne
Ar
CO2
Excimer Lasers Mixture of reactive (Cl,
F) and inert gases
Electric discharge XeF
KrF

• The active media are a group of optically clear
crystals to which impurity atoms are doped in
trace amounts.
• These lasers operate in pulsed as well as
continuous mode.
• The input energy for initiating stimulated
emission usually lies in the visible region of EM
spectrum.
• Light is absorbed by the doped ion (or atom)
and lasing action initiates and returns it to the
Solid Lasers

Ruby Laser: Components
Ruby Crystal
• It consists of single cylindrical ruby crystal whose
ends are flat, with one end completely silvered and
other end partially silvered.
• Ruby crystal is composed of Al2O3 with some of its
Al3+ ions replaced by Cr3+ ions by 0.058% in weight.
• The energy level structure of Cr3+ ions have two
energy bands labeled E1 and E2 with lifetime of
~10ns while there exists meta-stable state M with
lifetime of 3ms.
Xenon Flash Lamp
• It is in the form of glass tube spiral (filled with xenon
gas) surrounding the ruby crystal and two
electrodes at ends are connected to capacitor
charged to few kilovolts.
• The energy stored in the capacitor (~few kJ) is
discharged through the xenon lamp in few
It was the first laser invented in
1960 by T.H. Maiman at Hughes
Research laboratory.

• The blue and green wavelengths in emission
spectrum of xenon lamp are strongly absorbed by
ruby crystal.
• This results in excitation of Cr3+ ions to either the
band E1 (660nm) or the band E2 (400nm).
• Subsequent to both the excitations, the Cr3+ ion
makes an immediate transition to the meta-stable
state M (694.3nm) having lifetime of 3ms.
• As the state M has relatively longer life (3ms), its
population increases relative to ground state G and
a population inversion is achieved between M and
G.
• First few de-excitations from the meta-stable state
are spontaneous emission and photons emitted in
this process initiate lasing action and subsequent
400nm
660nm 694.3nm
M (3ms)
G
E2
E1
Cr3+
Ruby Laser: Lasing Action

• The flash operation of xenon lamp results in pulsed or
spiked output of the ruby laser.
• The high power supplied by the lamp leads to very rapid
population inversion and subsequent lasing action. The
light amplification crosses steady state within small
time.
• The rate of depletion of meta-stable state is much
higher than the pumping rate. As a result, the
population inversion reaches below threshold in short
time and laser action ceases.
• Laser action stops for few microseconds during which
flash lamp again pumps atoms in the ground state to
upper levels and population inversion crosses the
threshold for subsequent lasing.
Ruby Laser: Pulsed Output

• Thermal Scattering Losses: The ruby laser,
involving a solid active medium, suffers heat
dissipation due to thermal scattering of photons
caused by vibrating ions or vacancies in the
crystal.
• Efficiency of Lasing: Very small portion (<3%) of
pumped energy is used in generation of laser
and remaining energy gets dissipated as heat in
the active medium.
• Cooling Systems: Consequently these laser
systems have bulky cooling system to remove
the large amount of heat generated within the
active medium.
Drawbacks of Ruby Laser

• The intrinsic semiconductor crystals have
valence and conduction bands separated by a
narrow forbidden region of ~1eV energy. At
room temperatures, the thermal excitations of
electrons from valence to conduction band
occurs.
• If the semiconductor crystal is highly doped,
then there are abundance of majority carriers
(either electrons or holes) in conduction band
which leads to the state of population inversion.
• A spontaneously produced photon, resulting
from de-excitation of electron from conduction to
valence band, stimulates the lasing action.
Lasing Action in Semiconductors

Diode Laser
 A semiconductor junction diode has p and n
sections with intermittent depletion region
formed by diffusion of majority charge carriers
across the barrier.
 The side surfaces of the diode, except the
region of depletion, are polished so as to form
cleaved surface mirrors.
 If diode is forward biased, then electrons
flows from n to p and holes flow in opposite
direction.
 If forward current is sufficiently large, the ion
recombination takes place in the depletion
region and leads to spontaneous emission of
photon which further stimulates lasing action.

 The refractive index of semiconductor is
sufficiently large which causes reflection of
photons at semiconductor-air interface. This helps
in sustaining cavity oscillations.
 The laser photons are emitted from the depletion
region leading to wide angle of emergence.
 As de-excitations of electrons occurs between two
bands, the monochromaticity is poor than other
lasers.
 Owing to small size, they are used in portable
radars, modulated output for communication
purpose.
 Most commonly used diode lasers are:
 GaAlAs (750-950nm)

• The He-Ne system forms 4 level lasing system.
• The active medium consists of a gas mixture of He and
Ne in the ratio 10:1 maintained at a pressure of 10mm
Helium-Neon Laser
Discharge tube
Perfectly reflecting mirror
Output Coupler
Power Supply
Anode
Cathode

 Energetic electrons in glow discharge
collide with He atoms and excite them to
1s12s1 meta-stable state with excitation
energy of 20.61eV.
 The excitation energy acquired by He
atoms is transferred to Ne atom through
collision, which are excited to 2p54s1 and
2p55s1 meta-stable states with nearly
same excitation.
 The excited neon atoms relax to two
meta-stable states. As Ne atoms excite
and subsequent relax at rapid rate,
population inversion is achieved rapidly.
 3 intense transitions 1.15µm (IR),
3.39µm (IR) and 632.8nm (Red) form
Working of Helium-Neon Laser
He
Ne
1s22s22p6
3p
4p
543nm
1s2
T 1s12s1
S 1s12s1
3s
4s
5s
3390nm
(IR)
632.8nm
1152nm
(IR)
1118nm
(IR)
594.5nm

• Communication
• Data Storage
• Machining
• Surgery
• Laser Spectroscopy
• Metrology
• Entertainment
• Weaponary
Applications of Lasers

Non-Beam Hazards
Lasers systems are classified according to their level of
hazard. The classification of lasers is based on:
 Electrical Hazards
 Smoke & Fumes
 Mechanical Hazards
 Process Radiation
 Flash lamp Light
 Chemical Hazards

CLASS 1
• Safe during normal use
• Incapable of causing injury
• Low power or enclosed beam
Label not required.
 Class 1 laser does not cause
injury during normal use as
either they have very low power
or the beam is fully enclosed.
 The operators of class 1 lasers
do not need to take any
precautions or protection from
laser hazards.
 Visible lasers with :
 >500 nm, 0.4mW
  < 450 nm, 40mW
lie in class 1 limits.

CLASS 2
Laser Scanners
 Class 2 lasers are visible in light.
The natural aversion response to
bright light will cause a person to
blink before class 2 laser can
produce an eye injury.
 Continuous wave of power ~ 1mW.
 The average time for human
aversion response to bright light is
190ms and is always less than
0.25s.
 Only protection needed from class 2
laser is not to overcome aversion
response and stare directly into the
beam.

CLASS 3a
Laser Pointers • Class 3a lasers are Marginally Unsafe
implying that the aversion response is
not adequate protection for direct
exposure of the eye to the laser beam.
• The class 3a lasers are visible with
power limited to 5 mW.
• Lasers with small beam and 1 mW
power can enter the pupil of the eye, it
carries a DANGER label.
• If the beam is expanded to be area
large enough that only 1 mW can pass
through pupil, the laser carries a
CAUTION label.
• Class 3a laser user must recognize the
level of hazard and avoid direct eye
exposure.

CLASS 3b
• Class 3b lasers are hazardous for direct eye
exposure to the laser beam, but diffuse reflections
are not usually hazardous.
• The maximum average power for a CW or
repetitive pulse class 3b laser is 0.5W.
• The maximum pulse energy for a single pulse class
3b laser in visible and near IR varies with the
wavelength.
Visible lasers Max. pulse energy=30mJ.
IR(1050-1400nm) Max. pulse energy=150mJ
UV and Far IR Max. pulse energy=125mJ.
• Class 3b lasers operating near upper power or
energy limit of the class may produce minor skin
hazards.
• Most class 3b lasers do not produce diffuse
reflection hazards. However, single pulse visible or
near IR class 3b lasers with ultra-short pulses can
produce diffuse reflection hazards of more than a

CLASS 4
Laser-Professionals.com
• Class 4 lasers are powerful enough that
even diffuse reflection becomes hazard.
• The lower power limit for CW and
repetitive pulsed class 4 lasers is about
0.5W.
• The lower limit for single pulse class 4
lasers varies from 0.03J for visible
wavelengths to 0.15 J for some near
infrared wavelengths.
• Class 4 lasers require the most
stringent control measures.

PHYA4-LASERS.ppt, for first year B.E./BTech

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