Laser

BASICS OF
LASER AND LASER LIGHT
L ight
A mplification by
S timulated
E mission of
R adiation

PROPERTIES OF LASER BEAM:
 High degree of coherence:
The wave trains which are identical in phase and
direction are called coherent waves.
Since all the constituent photons of laser beam
possess the same energy, momentum and propagate
in same direction, the laser beam is said to be highly
coherent.
 High Intensity:
Due to coherent nature of laser, it has the ability to
focus over a small area of 10–6 cm2

Highly Directional:
An ordinary light source emits light in all possible
directions. But, since laser travels as a parallel beam
it can travel over a long distance without spreading.
The angular spread of a laser beam is 1 mm/meter
 High Monochromaticity:
The light from a normal monochromatic source
spreads over a range of wavelength of the order of
100 nm. But the spread is of the order of 1 nm for
laser.
Hence laser is highly monochromatic, that is, it can
emit light of single wavelength.

Coherent light wavesIncoherent light waves
Conventional Source Laser Source

ATOMIC TRANSITION
Almost all electronics transitions occur in atoms that involve photons fall into
one of three categories:
1) Absorption : If a photon of energy hv is incident
on the atom In the lower state, the atom absorbs the
incident photon and gets excited, to jump the higher
energy state.This process is called absorption.
Atom + photon = Atom*
E1 + hv = E2
hv = E2 - E1
The rate of absorption R12 is proportional to the population of lower energy level N1 and to the
density of incident radiation ρ. Hence
R12 α N1ρ or R12 = B12N1ρ
where B12 is the proportionality const. known as prob. of absorption of radiation per unit time.

2) Spontaneous Emission : It is a process in which there is an emission
of a photon whenever an atom transits from a higher to lower energy state without
the aid of any external agency.
 For this process to take place, the atom has to be in the excited state.
 Since the higher energy level is an unstable one, the excited atom in the higher
level E2 spontaneously returns to the lower energy level E1with the emission of
photon of energy
hν = E2 - E1
Atom* = Atom + photon
 The rate of spontaneous emission of radiation R21(sp)
is proportional to the population N2 at the higher energy
level E2. Hence
R21(sp) α N2 or R21(sp) = A21N2
where A21 is the proportionality constant known as prob. of spontaneous emission per unit time.

3) Stimulated Emission : It is a emission of photon whenever an atom
transits from a higher to lower energy state under the influence of an external agency
i.e., an external photon
 For this atom should be already in excited state.
 Let a photon having an energy hν interact with
the atom in the excited state. This incident photon
triggers the excited atom in the higher level E2 to
transit to lower level E1, resulting in the emission of
another photon of energy hν.
 Both the inducing (incident) and the emitted photon
have the same phase, energy and direction of movement.
 This kind of emission is responsible for laser action.
 The rate of stimulated emission of radiation R21(st) is proportional to the population
N2 at the higher energy level E2 and to the density ρ of the inducing photon.
R21(st) α N2ρ or R21(st) = B21N2ρ
where B21 is the proportionality constant known as prob. of stimulated emission per
unit time.

BASICS CONCEPTS :
Population Inversion: It is a state of achieving more number of atoms in
excited state compared to ground state. It is an essential condition for producing
laser beam.
Life Time: The limited time for which a atom remains in the excited is known as
life time. It is about a nano second.
Metastable state: It is an energy level in an atomic system where the life time
of atoms is very large (of the order 10-3 to 10-2 seconds).It helps in achieving the
population inversion.
Lasing : The process which leads to emission of stimulated photons after
establishing the population inversion is referred as lasing.

LASING ACTION DIAGRAM :
9
Energy
Introduction
Ground State
Excited State
Metastable State
Spontaneous
Energy
Emission
Stimulated
Emission of
Radiation

BASICS COMPONENTS :
Active Medium: A medium in which population inversion is achieved for laser action
is called active medium. The medium can be solid, liquid and gas.
Pumping mechanism: It is the mechanism of exciting atoms from the lower
energy state to a higher energy state by supplying energy from an external source.
The most commonly used pumping mechanism are: optical, electrical and direct.
Optical Pumping: In this type of pumping atoms are excited by means of an external
optical source. This type is adopted in solid state lasers such as ruby and Nd: YAG
laser.
Electrical Pumping: In this type of pumping the electrons are accelerated to a high
velocity by a strong electric field. These moving electrons collide with the neutral
gas atoms and ionize the medium. Thus, due to ionization they get raised to a
higher energy level. This technique is adopted in gas lasers such as CO2 laser.
Direct Conversion: In this type a direct conversion of electric energy into light takes
place. This technique is adopted in semiconductor laser.
In addition to above three, the other types of pumping are inelastic collision between atoms
and chemical methods which are respectively adopted in He-Ne gas laser and in dye and
chemical lasers.

Optical Resonator: It is a pair of reflecting surfaces (mirrors) of which one is a
perfect reflector and the other is a partial reflector. It is used for amplification of
photons thereby producing an intense and highly coherent output.
High Reflectance Mirror: A mirror which reflects essentially 100% of the laser
light.
Partially Transmissive Mirror: A mirror which reflects less than100% of the
laser light and transmits the remainder.
Principle of LASER: Laser is based on the principle of stimulated emission of
radiation with light amplification. For stimulated emission of radiation to take place,
the population of atoms in higher energy level should be greater than the lower
energy level, i.e., N2 > N1. This can be achieved by pumping. Light amplification is
achieved by photon multiplication within an optical resonator cavity.
Initially, the state of population inversion has to be achieved in the active medium
which is within a resonator cavity. Then, a spontaneously emitted photon by one of
the excited atom simulates another atom it encounters in its path to release a
second photon.
Thus these two photons which are coherent in nature stimulates other two atoms to
produce another two photons. Hence there will be 4 coherent photons. Thus photon
number gets multiplied just like chain reaction thereby producing an amplified light
by stimulated emission of radiation in an highly intense beam called laser.

• Circular and elliptical cylinder mirrors are to focus light from linear flash lamps to
linear laser rods.
• The pumping must be strong enough to create population inversion and above
threshold.
• Pumping = Excitation = Up Transition
12

Examples of Electrical and Optical pumping
13

14
The following figures illustrate the initial phases of laser operation.

Electrons (or the atomic system) make a transition from a higher energy state
to a lower energy state and emit photons.
16

17
(Emitting photons spontaneously = Down Transition )

(Stimulated Emission is essential in producing laser light )
18

The following 6 figures illustrate the initial phases of laser operation, similar to the 5
figures shown earlier, except that the energy level diagram is used in this
explanation.
Note that energy level
3 is unstable, the
lifetime of this level is
very short. Little
population can be
accumulated there.
However, the lifetime
of level 2 for a good
laser material is
relatively long to allow
significant population
accumulation there.
The condition of
population inversion
(N2>N1) can thereby
be created.
be
19

Nd: YAG LASER
Active Medium:
This is a four-level solid state laser system. Yttrium Aluminum
Garnet (Y3Al5O12), commonly known as YAG, doped with
neodymium ions Nd3+ is the active medium. The active medium
is taken in the form of a crystal and drawn into a rod.
Resonator Cavity:
The end faces of the Nd: YAG rod are ground polished and
silvered to act as the optical resonator mirrors, or the optical
cavity can be formed by using two external reflecting mirrors.
Optical Pumping:
A Xenon flash lamp or a krypton flash lamp is used as a
pumping source.

Nd: YAG LASER contd……
Construction:
A Nd: YAG rod and a krypton flash lamp are enclosed inside
and ellipsoidal reflector. In order to make the entire flash
radiations to focus on the laser rod, the Nd: YAG rod is placed
at one focal axis and the flash lamp at the other focal axis of
the ellipsoidal reflector.

Working: E4
E3
E0
E2
E1
0.73m
0.80m
Metastable state
Non-radiative transition
Non-radiative transition
LASER
 The flash lamp is
switched on.
 Optical pumping excites
Nd3+ from g.s. E0 to E3
& E4 by absorbing
wavelength 0.80 m &
0.73 m resply.
 Excited Nd3+ ions then make a transition from these energy levels.
 Transition E4  E2 is a non – radiative transition. The E2 state is metastable state.
 Upon continuous excitation, population inversion is achieved at metastable state E2.
 Any of the spontaneously emitted photon will make the excited Nd3+ ions to undergo
a transition between E2  E1 state. Thus stimulated photos are generated.
 The photons travelling parallel to the resonator axis experience multiple reflections at
the mirrors. As a result, the transition E2  E1 yields an intense and coherent laser
beam of wavelength 1.604 m. These lasers give beam continuously.
 The ions Nd3+ then make transition E1  E0 which is non – radiative.
 Only part of energy emitted by flash lamp is used to excite Nd3+ while the rest heats
up the crystal. Thus system is cooled by air or water circulation.

Applications:
1)These lasers are widely used for cutting, drilling,
welding and surface hardening of the industrial
products.
2)These lasers are used in military as range finder and
as target designations.
3)These lasers are used in medical field for cataract
surgery, to treat gastrointestinal bleeding and gall
bladder surgery.
4)It is used in long – distance communication.
5)It is used in the study of inertial confinement fusion.

There are many different types of lasers. The laser medium can be a gas, liquid, solid
or semiconductor. Lasers are commonly designated by the type of lasing material
employed:
Solid-state lasers have lasing material distributed in a solid matrix (such as the ruby
or neodymium:yttrium-aluminum-garnet "Yag" lasers). Ruby lasers emit red
light.Neodymium-Yag lasers emit infrared light. They are powerful lasers.
Semiconductor lasers, sometimes called diode lasers, are not solid-state lasers.
These optoelectronic devices are generally very small, consume low power and
have been built into transmitters for optical telecom. They may also be built into
larger arrays, such as the writing source in some laser printers or CD players.
Dye lasers use complex organic dyes, such as rhodamine 6G, in liquid solution or
suspension as lasing media. They are tunable over a broad range of wavelengths.
25
TYPES OF LASER:

Gas lasers (helium-neon, Argon and Krypton are the most common gas
lasers) have a primary output of visible light. CO2 lasers emit energy in the
far-infrared, and are powerful enough to be used for cutting hard materials.
Excimer lasers (the name is derived from the terms excited dimers) use
reactive gases X, such as chlorine and fluorine (F, Cl), mixed with inert
gases A such as argon, krypton or xenon (Ar, Kr, Xe) . When electrically
stimulated, a pseudo molecule (dimer) is produced. When lased, the dimer
produces light in the ultraviolet range that is very useful in the lithography
step in electronic manufacturing.
A* + X  (A+X-)* + 6eV
26

APPLICATIONS:
In Industry
For cutting,welding,melting and drilling
To test the quality of the material
For the heat treatment
In medicine
Treatment of detached retinas.
Micro and bloodless surgery
Treatment of cancers and skin tumours
In Science
Fiber optical communication
Under water communication
Measurement of long distance

Emission wavelengths of common lasers:
A nanometer is 1x10-9 meters. 28

ELECTROMAGNETIC WAVES
Light is a small segment of the electromagnetic spectrum
that extends from radio waves to g-rays. The only difference
among the different waves is the wavelength (frequency).

WIDELY-USED LASERS
WAVELENGTH OF OPERATION
Source: Shore Laser Center

LASERS VS. CONVENTIONAL LIGHT SOURCES
LED (Light Emitting Diode)
Nd:YA
G
Laser
Source: Google

CHARACTERISTICS OF LASER LIGHT
MONOCHROMATIC
DIRECTIONAL
COHERENT
The combination of these three properties makes laser
light focus 100 times better than ordinary light
Laser-Professionals.com

LASER COMPONENTS
High Reflectance
Mirror (HR)
Output Coupler
Mirror (OC)
Active
Medium
Output
Beam
Excitation
Mechanism
Optical Resonator

Radio
Long WavelengthShort Wavelength
Gamma Ray X-ray Ultraviolet Infrared Microwaves
Visible
ELECTROMAGNETIC SPECTRUM
Lasers operate in the ultraviolet, visible, and infrared.
Radio
RedBlue YellowGreen

LASER SPECTRUM
10-13 10-12 10-11 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 102
LASERS
200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 10600
Ultraviolet Visible Near Infrared Far Infrared
Gamma Rays X-Rays Ultra- Visible Infrared Micro- Radar TV Radio
violet waves waves waves waves
Wavelength (m)
Wavelength (nm)
Nd:YAG
1064
GaAs
905
HeNe
633
Ar
488/515
CO2
10600
XeCl
308
KrF
248
2w
Nd:YAG
532
Retinal Hazard Region
ArF
193
Communication
Diode
1550
Ruby
694
Alexandrite
755

Rear Mirror
Adjustment Knobs
Safety Shutter Polarizer Assembly (optional)
Coolant
Beam
Tube
Adjustment
Knob
Output
Mirror
Beam
Beam Tube
Harmonic
Generator (optional)
Laser Cavity
Pump
Cavity
Flashlamps
Nd:YAG
Laser Rod
Q-switch
(optional)
Courtesy of Los Alamos National LaboratoryCourtesy of Los Alamos National Laboratory
NEODYMIUM YAG LASER

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