Download Lasers-An Overview

LASER SCIENCE & TECHNOLOGY
An Overview
Dr. BC Choudhary,
Professor, Applied Physics
NITTTR, Chandigarh-160019
Content Outlines

Historical Developments

Laser Types and Output

Laser Beam Characteristics

Major Application areas

Laser Hazards and Safety Measures.
LASER

An Acronym for
“ Light Amplification by Stimulated Emission of Radiations”
 One of the outstanding inventions of 20th century.

A light source – but, very much different from traditional
light sources.

Not used for illumination purposes

Widely used as a high power EM beam rather than a
light beam.
Common Light Source Vs Laser
•
Many wavelengths

Monochromatic
•
Multidirectional

Directional
•
Incoherent

Coherent

High Power
IMPORTANCE
 Next to computers it is the laser that is bringing
changes in our lives.
 Directly or indirectly it is helping us in living a
better life.

LASER: A generator of light – Store Energy
 It is a high technology device, used profitably in almost
every field.
 Entertainment electronics, Industrial electronics, Consumer
market, Communication, Mechanical industry, Metrology,
Surveying, Surgery and related medical fields, Computers,
Information processing, Sensing, Defense, Warfare etc.
 A HIGH TECHNOLOGY TOOL
 Drill bit: To drill holes in hard/soft materials
 A saw: To cut thick metal/non-metal sheets
 A phonograph needle: For compact discs
 A knife: During surgical operations
 A Target Designator: For military weapons
Lasers in daily Life
Military and Space
aircraft are equipped
with laser guns
Airplanes are
equipped with
laser radar
Bad eyesight can be
corrected by optical
surgery using lasers
CD-Rom discs
are read by lasers
Dentists use
laser drills
Tattoo removal is
done using lasers
Laser tech. is used in printers,
copiers, and scanners
DVD players read
DVD’s using lasers
CD-Audio is
read by a laser
Laser pointers can
enhance
presentations
Bar codes in
grocery stores are
scanned by lasers
Video game systems such as
PlayStation 2 utilize lasers
Brief History of Laser
1917 - Einstein predicted the possibility of Stimulated radiations.
1952 - Charles H Townes, J. Gorden & H. Zeiger in USA and N. Basov &
A. Prokhorov in USSR – independently suggested the principle of
generating and amplifying microwave oscillations based on stimulated
radiations.
1954 - Invention of MASER (Microwave Amplification by Stimulated
Emission of Radiations).
1958 - Townes & Schawlow and Basov & Prokholov – independently
extended the maser concept to optical frequencies i.e. LASER
 Townes, Basov and Prokhorov awarded Nobel Prizes for their
work in this field.
1960 - Theodore Maimann – developed first laser using a Ruby crystal as
amplifier and flash lamp as energy source.
LASER HISTORY

Sir Albert Einstein
 In 1917, the first foundation of laser was set in by
Sir Albert Einstein with the concept of photons and
stimulated emission of radiations.
•


In 1954, Charles Townes (Left) from US, Bosov (M) and
Prokorov (R) from USSR put forwarded the details for
the experimental set up for amplification of microwaves
and the first MASER was discovered.

Charles H. Townes (1915- 2015 )
Born in Greenville, South Carolina,
Arthur L. Schawlow (1921-99)
Born in Mount Vernon, N.Y.

In 1958, Dr. Charles Townes (L) and Prof. Schawlow
calculated the conditions for visible Laser light and
theory of Stimulated Emission of radiations.

At the same time, Basov and Prokhorov independently
expressed their idea about extending the maser concept
to optical frequencies i.e. Laser.
Development of First Laser
Theodore Maiman (1927-2007)
Los Angeles, California

In 1960, Dr.T. H. Maiman for the First time demonstrated
the phenomenon of Laser Action using Ruby Crystal and
the First Optical Laser was invented.
Nobel Prize in Physics
 In 1964, Townes, along with two
Russian laser Pioneers, Aleksander
Prokhorov and Nikolai Basov, were
awarded with The Nobel Prize in
Physics.
Major Landmarks in Development of Lasers
Year
Discoverer
Type of Laser/Principle
1917
Albert Einstein
Stimulated Emission
1952
N.G. Basov, A.M. Prokhorov
and Townes
Townes, Gorden, Zeiger
Townes, Schawlow, Basov
and Prokhorov
Theodore Maiman
A. Javan, W. Bennett and
D. Harriott
L.F. Johnson & K. Nassau
R. Hall
Maser Principle
1954
1958
1960
1961
1961
1962
Maser
Laser Principle
Ruby Laser
Helium-Neon Laser
Neodymium Laser
Semiconductor Laser
1963
C.K.N. Patel
Carbon Dioxide Laser
1964
W. Bridges
Argon Ion Laser
1966
W. Silfvast, G.R. Fowles,
and B.D. Hopkins
He-Cd Laser
1966
P.P. Sorokin & J.R. Lankard
Tunable Dye Laser
1975
J.J. Ewing & C. Brau
Excimer Laser
1976
J.M.J. Madey & coworkers
Free- electron Laser
1979
Walling & coworkers
Alexandrite Laser
1985
D. Mathews & coworkers
X-ray Laser
Types of Lasers
 Solid State (Ruby, Nd:YAG, Ti:Sapphire, Diode)
 Powered by light or electricity
 Gas (He-Ne, CO2, Argon, Krypton)
 Powered by electricity
 Liquid (Dye)
 Powered by light
 Chemical (HF)
 Powered by chemical energy
 Semiconductor or Diode Lasers
 Direct e-h transfer/injection currents
Visible Light Wave Region

More than 150 lasers have been developed over
whole range of the optical spectrum (IR-Visible-UV).
WAVELENGTHS OF MOST COMMON LASERS
Laser Type
Argon fluoride (Excimer-UV)
Krypton chloride (Excimer-UV)
Krypton fluoride (Excimer-UV)
Xenon chloride (Excimer-UV)
Xenon fluoride (Excimer-UV)
Helium cadmium (UV)
Nitrogen (UV)
Helium cadmium (violet)
Krypton (blue)
Argon (blue)
Copper vapor (green)
Argon (green)
Krypton (green)
Frequency doubled Nd -YAG
(green)
Helium Neon (green)
Krypton (yellow)
Copper vapor (yellow)
Key:
Wavelength (mm)
0.193
0.222
0.248
0.308
0.351
0.325
0.337
0.441
0.476
0.488
0.510
0.514
0.528
0.532
0.543
0.568
0.570
UV = ultraviolet (0.200-0.400 µm)
VIS = visible (0.400-0.700 µm)
NIR = near infrared (0.700-1.400 µm)
Helium Neon (yellow)
Helium Neon (orange)
Gold vapor (red)
Helium Neon (red)
Krypton (red)
Rohodamine 6G dye (tunable)
Ruby (CrAlO3) (red)
Gallium arsenide (diode-NIR)
Nd:YAG (NIR)
Helium Neon (NIR)
Erbium (NIR)
Holmium (NIR)
Helium Neon (NIR)
Hydrogen fluoride (NIR)
Carbon dioxide (FIR)
Carbon dioxide (FIR)
0.594
0.610
0.627
0.633
0.647
0.570-0.650
0.694
0.840
1.064
1.15
1.504
2.10
3.39
2.70
9.6
10.6
Various Types of Lasers
Laser Output
Pulsed Output (P)
Energy (Watts)
Energy (Joules)
Continuous Output (CW)
Time
Watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second).
Time
Joule (J) - A unit of energy
Energy (Q) - Energy content is commonly used to characterize the output from pulsed lasers and is
generally expressed in Joules (J).
Irradiance (E) - Power per unit area, expressed in watts per square centimeter.
Laser Beam Characteristics

Laser light differs from the light emitted by
conventional light sources.
 Most striking features are;
 Directionality
 High Coherence
 High Intensity
 Mono-Chromaticity
 Laser light can be produced as Polarized light
 Can be generated as very short pulses, at High power
Directionality
 Conventional light sources emit light in all directions.
 Lasers emit light only in one direction (along cavity axis).

Directionality of a laser beam expressed
in terms of  “ Beam Divergence”
 Beam Divergence
 Light from a laser diverges very little.

Upto certain distance, beam remains a bundle of parallel light
rays; distance from the laser over which the light rays remain
parallel is called  “Rayleigh range”.

The laser beam diverges beyond Rayleigh range
Divergence of a laser beam
 Divergence angle is measured from the center of the beam to the
edge of the beam,

Edge: location in the beam where intensity decreases to 1/e2 of that at the
center.
 Twice the angle of divergence is known as full angle beam
divergence  Spot size
 Measure of how much the beam will spread as it travels
through the space.

Two parameters, which cause beam divergence
1. Size of the beam waist
2. Diffraction
 Full angle divergence is given by
2 
4
d 0
where d0 = 2W0 is the diameter of the
beam waist
 Divergence is inversely
proportional to „d0‟

Beam waist and divergence of laser beam
Large for a beam of
small waist.
 Beam divergence due to diffraction is determined from
Rayleigh’s criterion;

  1.22
D
; D is the diameter of laser’s aperture
 In case of gas lasers, the diffraction divergence is about twice
as large as beam-waist divergence.
 A typical value of divergence for a He-Ne laser is;   10-3 rad.

implies that the laser beam diameter increases by about 1 mm for every
metre it travels.
 Beam divergence of large lasers is  micro-degree (10-6).
 A laser beam of 5 cm diameter (divergence  10-6 degree) when focused
from earth spread to a diameter of only about 10m on reaching the surface
of the moon  An Extreme Collimation
Laser beam Targeting The Moon
 APOLLO 11 Expedition
Intensity

Power output of laser may vary from a few mWs to few kWs.

This energy is concentrated in a beam of very small crosssection  High intensity
 Intensity of a laser beam approximately given by
2
 10 
I    P Wm 2


where P is the power radiated by the laser.
In case of 1mW He-Ne laser of wavelength,  = 632810-10 m
100 10 3
11
2
I

2
.
5

10
Wm
(6328 10 10 ) 2
To obtain same intensity from a Tungsten bulb, temperature have
to be raised to 4.6106 K (normal operating temp. of bulb ~2000K)
Brightness: Power per unit area per unit solid angle
 Brightness of Sun
 T 4 
 

 2 
Bsun = = 1000 W. cm-2. Sr
 1mW He-Ne laser,  = 632810-10 m
B He-Ne =300,000 W.cm-2. Sr = 300 Bsun
 Due to high emittance laser beams
are not allowed to see directly
Coherence
 Light waves are coherent if they are in phase with each other.


maintain crest-to-crest and trough-to-trough correspondence.
Two conditions Necessary for Coherence

They must start with same phase at the same position.

Wavelengths must be same otherwise they will drift out of
phase  crests of higher frequency wave will arrive ahead of
the crests of lower frequency wave.
 Conventional light sources : Incoherent- light that emerges
is a combination of photons in random manner
 Lasers: Coherent – output that emerges is a resultant of large
number of identical photons, which are in phase.
 Coherence requires - a connection between the amplitude and
phase of the light at one point and time, and the amplitude and
phase of the light at another point and time.
 Two
classes of Coherence
 Temporal Coherence (Longitudinal): The constancy and
predictability of phase as a function of time when the waves travel along
the same path at slightly different times.
 Spatial Coherence (Transverse): The phase relationship between
waves traveling side by side at the same time but at some distance from
one another.
Temporal Coherence: Same phase for any time interval of same
duration.
For, (t2-t1) = (t4-t3) ; if 2 = 1
 Temporally coherent waves
• Characteristic of a single beam.
 T.C. characterised by two parameters
• Coherence length, lcoh
• Coherence time, tcoh
 Both
measure how long light waves
remain in phase as they travel in space.
2
c
L coh 

2

• Fluorescent tubes,
lcoh = 5040 Ao
• Sodium lamp,
lcoh = 0.29 mm
• He-Ne laser,
lcoh = 100 m
 Monochromaticity - a measure of temporal coherence.
Spatial Coherence: Phase difference
of waves remains same all times.
• Phase difference between E1 and E2
remains same (zero) at t1 and t2.
• Spatial coherence measures the area
over which light is coherent.
 Spatial
incoherence arises due to size
of the light source.
 Interference – a manifestation of
coherence.

More number of fringes – longer T.C.

Degree of contrast – measure of S.C.
Laser is both Temporally & Spatially Coherent to a high degree
Monochromaticity
 Light coming for a source has only one frequency of oscillation.
 Monochromatic
light from a monochromatic source
 IN PRACTICE, NOT POSSIBLE TO PRODUCE LIGHT
WITH ONLY ONE FREQUENCY
 Light form any source consists of a band of frequencies ‘’
closely spaced around the central frequency, 0
 - linewidth or bandwidth.
 Conventional sources :
  1010 Hz or more.
 Light from Lasers :
  100 Hz
Polarization
 Light Waves:
Electric & Magnetic fields vibrating perpendicular
to each other and to the direction of propagation.

Light as an
electromagnetic wave
 Polarization (P): Measure of alignment of electric and magnetic
fields in a light wave.
• Types: Linear, Circular & Elliptical
Simplest is
Linear
or Plane polarization
 Linearly polarized light beam: Orientation of electric field
remains in one plane while its magnitude changes with time.
 Any other type of polarized light: A result of superposition of
two linearly polarized waves having electric fields perpendicular
to each other.

Unpolarized light can be divided into two components with
linear polarization, one with a vertical field and other with a
horizontal field.
 Conventional light sources: Unpolarized light
 Laser output: Unpolarized or Polarized
Applications of Lasers
Profitably used in almost every field.
 Broadly divided into two groups
 involving laser
beams of high power
 involving laser
beams of low power.
 High power Gas and Solid State lasers are used in: material
processing, nuclear fusion, medical field, defence etc.
 Low power (semiconductor lasers) are used in: CD players,
laser printers, optical floppy discs, optical memory cards, data
processing and information processing devices, range finders,
holograms, optical communication etc.
Some Important and Well Established
Applications of Lasers
 LASERS IN MECHANICAL INDUSTRY
 Drilling
 Cutting
 Welding
 Heat Treatment
 LASERS IN ELECTRONICS INDUSTRY

Scribing

Soldering

Trimming
 LASERS IN NUCLEAR ENERGY

Isotope Separation

Nuclear Fusion
 LASERS IN MEDICINES

Diagnostics, Alignments

Surgery, Therapy
 LASERS IN DEFENCE

Ranging

Weapon Guide

Weapon itself
 MEASUREMENT OF DISTANCE

Interferometric Methods

Laser Rangers

Optical Radar or LIDAR

Surveying
 VELOCITY MEASUREMENTS

Doppler Velocimeters: measuring fluid flow rates

Portable velocity measuring meters
• Used by traffic police
 HOLOGRAPHY
 Generation of
 Viewing
Holograms
of Holograms
 ENVIRONMENT STUDIES
 For
measurement of concentrations of
various atmospheric pollutants: gases
& particulate matter.
 CONSUMER ELECTRONICS INDUSTRY

Super Market Scanners,

Compact Discs

Optical Data Storage

Optical Communication

Optical Computer
Laser Hazards
 Lasers can be hazardous if necessary control measures
are not followed.
Types of Laser Hazards
 Eye : Acute exposure of the eye to lasers of certain wavelengths and
power can cause corneal or retinal burns (or both).

Chronic exposure to excessive levels may cause corneal or lenticular
opacities (cataracts) or retinal injury.
 Skin : Acute exposure to high levels of optical radiation may cause skin
burns; while carcinogenesis may occur for UV wavelengths (290-320 nm)
 Chemical : Some lasers require hazardous or toxic substances to
operate (i.e., chemical dye, Excimer lasers).
 Electrical : Most lasers utilize high voltages that can be lethal.
 Fire : Solvents used in dye lasers are flammable. High voltage pulse or
flash lamps may cause ignition.

Flammable materials may be ignited by direct beams or specular reflections
from high power continuous wave (CW) infrared lasers.
Common Laser Signs and Labels
Laser Safety Standards and Hazard
Classification
 Lasers are classified by hazard potential based upon their
optical emission.
 Necessary control measures are determined by these
classifications.
 In this manner, unnecessary restrictions are not placed on
the use of many lasers which are engineered to assure
safety.
 Laser classifications are based on American National
Standards Institute’s (ANSI) Z136.1-Safe Use of Lasers.
Laser Class
Criterion used to classify lasers:
1. Wavelength. If the laser is designed to emit multiple wavelengths
the classification is based on the most hazardous wavelength.
2. For continuous wave (CW) or repetitively pulsed lasers the
average power output (Watts) and limiting exposure
time inherent in the design are considered.
3. For pulsed lasers the total energy per pulse (Joule), pulse
duration, pulse repetition frequency and emergent
beam radiant exposure are considered.
ANSI Classifications
 Class 1 : Laser or laser systems that do not, under normal operating
conditions, pose a hazard.
 Class 2 : Low-power visible lasers or laser systems which, because of
the normal human aversion response (i.e., blinking, eye movement,
etc.), do not normally present a hazard, but may present some
potential for hazard if viewed directly for extended periods of time.
 Class 3a : Lasers or laser systems having a CAUTION label that
normally would not injure the eye if viewed for only momentary periods
with the unaided eye, but may present a greater hazard if viewed using
collecting optics.

Class 3a lasers have DANGER labels and are capable of exceeding
permissible exposure levels. If operated with care Class 3a lasers
pose a low risk of injury.
 Class 3b : Lasers or laser systems that can produce a hazard if
viewed directly. This includes intrabeam viewing of specular
reflections.
 Normally, Class 3b lasers will not produce a hazardous diffuse
reflection.
 Class 4 : Lasers and laser systems that produce a hazard not only
from direct or specular reflections, but may also produce significant
skin hazards as well as fire hazards.
CONTROL MEASURES
Engineering Controls
 Interlocks
 Enclosed beam
Administrative Controls
 Standard Operating Procedures (SOPs)
 Training
Personnel Protective Equipment (PPE)
 Eye protection
Concluding Thoughts
Laser technology has already
contributed to furthering the
goals of humanistic advancements
New ideas and applications
are changing our every-day
Life as we know it…
 The key to managing today‟s rapidly evolving technology is
to constantly analyze how each advance affects us as
individuals and as a society as a whole.
 As we advance towards the mid century, it is inevitable that
laser technology will play an increasingly important role in
the society. . .
References:
1. LASERS: Theory and Applications; MN Avadhanulu, S. Chand
& Company Ltd.
2. Lasers & Optical Instrumentation; S.Nagabhushana and N.
Sathyanarayana, IK International Publishing House (P) Ltd.
3. Experiments with He-Ne Laser, RS Sirohi, 2nd Ed. New Age
International Publishers
4. http://www.colorado.edu/physics/lasers/
5. www.Google.co.in/Search engine
CAUTION:
Do not look a laser with remaining eye!