Lasers-An Overview.pdf
Transcript of Lasers-An Overview.pdf
LASER SCIENCE & TECHNOLOGYAn 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.
L A S E R
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.
• Many wavelengths
• Multidirectional
• Incoherent
Monochromatic
Directional
Coherent
High Power
Common Light Source Vs Laser
IMPORTANCE
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.
LASER: A generator of light – Store Energy
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.
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
Dentists use
laser drills
Bad eyesight can be
corrected by optical
surgery using lasers
CD-Audio is
read by a laser
Tattoo removal is
done using lasers
CD-Rom discs
are read by lasers
Laser pointers can
enhance
presentations Bar codes in
grocery stores are
scanned by lasers
Video game systems such as
PlayStation 2 utilize lasers
DVD players read
DVD’s using lasers
Airplanes are
equipped with
laser radar
Military and Space
aircraft are equipped
with laser guns
Laser tech. is used in printers,
copiers, and scanners
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
In 1917, the first foundation of laser was set in by
Sir Albert Einstein with the concept of photons and
stimulated emission of radiations.
Sir Albert Einstein
•
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.
In 1958, Dr. Charles Townes (L) and Prof. Schawlow
calculated the conditions for visible Laser light and
theory of Stimulated Emission of radiations.
Charles H. Townes (1915- 2015 )
Born in Greenville, South Carolina,
Arthur L. Schawlow (1921-99)
Born in Mount Vernon, N.Y.
At the same time, Basov and Prokhorov independently
expressed their idea about extending the maser concept
to optical frequencies i.e. Laser.
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.
Theodore Maiman (1927-2007)
Los Angeles, California
Development of First Laser
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 Maser Principle
and Townes
1954 Townes, Gorden, Zeiger Maser
1958 Townes, Schawlow, Basov Laser Principle
and Prokhorov
1960 Theodore Maiman Ruby Laser
1961 A. Javan, W. Bennett and Helium-Neon Laser
D. Harriott
1961 L.F. Johnson & K. Nassau Neodymium Laser
1962 R. Hall Semiconductor Laser
1963 C.K.N. Patel Carbon Dioxide Laser
1964 W. Bridges Argon Ion Laser
1966 W. Silfvast, G.R. Fowles, He-Cd Laser
and B.D. Hopkins
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).
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)
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
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
Key: UV = ultraviolet (0.200-0.400 µm)
VIS = visible (0.400-0.700 µm)
NIR = near infrared (0.700-1.400 µm)
WAVELENGTHS OF MOST COMMON LASERS
Wavelength (mm)Laser Type
Various Types of Lasers
Laser Output
Watt (W) - Unit of power or radiant flux (1 watt = 1 joule per second). 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.
Continuous Output (CW) Pulsed Output (P)E
nerg
y (
Watt
s)
TimeE
nerg
y (
Jo
ule
s)
Time
Laser Beam Characteristics
Laser light differs from the light emitted by
conventional light sources.
Laser light can be produced as Polarized light
Can be generated as very short pulses, at High power
Most striking features are;
Directionality
High Coherence
High Intensity
Mono-Chromaticity
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
0d
42
where d0 = 2W0 is the diameter of the
beam waist
Divergence is inversely
proportional to „d0‟
Large for a beam of
small waist.
Beam waist and divergence of laser beam
Beam divergence due to diffraction is determined from
Rayleigh’s criterion;
D22.1
; 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 cross-
section High intensity
2
2
WmP10
I
where P is the power radiated by the laser.
Intensity of a laser beam approximately given by
In case of 1mW He-Ne laser of wavelength, = 632810-10 m
211
210
3
Wm105.2)106328(
10100I
To obtain same intensity from a Tungsten bulb, temperature have
to be raised to 4.6106 K (normal operating temp. of bulb ~2000K)
Brightness: Power per unit area per unit solid angle
Due to high emittance laser beams
are not allowed to see directly
Brightness of Sun
Bsun = = 1000 W. cm-2. Sr
2
T4
1mW He-Ne laser, = 632810-10 m
B He-Ne =300,000 W.cm-2. Sr = 300 Bsun
Coherence
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.
Light waves are coherent if they are in phase with each other.
maintain crest-to-crest and trough-to-trough correspondence.
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.
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.
Two classes of Coherence
Temporal Coherence: Same phase for any time interval of same
duration.
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.
• Fluorescent tubes,
lcoh = 5040 Ao
• Sodium lamp,
lcoh = 0.29 mm
• He-Ne laser,
lcoh = 100 m
c
2L
2
coh
Monochromaticity - a measure of temporal coherence.
For, (t2-t1) = (t4-t3) ; if 2 = 1
Temporally coherent waves
• Characteristic of a single beam.
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
Conventional light sources: Unpolarized light
Laser output: Unpolarized or Polarized
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.
Applications of Lasers
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.
Profitably used in almost every field.
Broadly divided into two groups
involving laser beams of high power
involving laser beams of low power.
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 DEFENCE
Ranging
Weapon Guide
Weapon itself
LASERS IN MEDICINES
Diagnostics, Alignments
Surgery, Therapy
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 Holograms
Viewing of Holograms
CONSUMER ELECTRONICS INDUSTRY
Super Market Scanners,
Compact Discs
Optical Data Storage
Optical Communication
Optical Computer
ENVIRONMENT STUDIES
For measurement of concentrations of
various atmospheric pollutants: gases
& particulate matter.
Laser HazardsLasers 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 ClassCriterion 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
As we advance towards the mid century, it is inevitable that
laser technology will play an increasingly important role in
the society. . .
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.
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!