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!