LASERLASER

LASERLASER• A laser is an A laser is an amplifier of light. amplifier of light. When the When the

laser is suitably laser is suitably excitedexcited by optical or by optical or electrical electrical energyenergy, the light of the proper , the light of the proper frequency entering the laser cavity is frequency entering the laser cavity is amplified in such a manner that laser amplified in such a manner that laser output wave is in phase with inputoutput wave is in phase with input. . Practical utility of a laser is as an Practical utility of a laser is as an OSCILLATOROSCILLATOR –-- a generator of light. Thus –-- a generator of light. Thus laser is also known as laser is also known as GENERATOR of lightGENERATOR of light..

LASER ACTIONLASER ACTION• Laser action is based on amplification of EM waves by means Laser action is based on amplification of EM waves by means

of forced or induced atoms or molecules.of forced or induced atoms or molecules.

• A laser radiation uses three fundamental phenomena when EM A laser radiation uses three fundamental phenomena when EM waves interacts with the matter namelywaves interacts with the matter namely

Laser interaction

Spontaneous emission Stimulated emission Spontaneous absorption

Excited atoms emit photons Excited atoms emit photons spontaneously.spontaneously.

When an atom in an excited state falls to a lower energy level, it emits a photon of light.

Molecules typically remain excited for no longer than a few nanoseconds. This is often also called fluorescence or, when it takes longer, phosphorescence.

Energ

y

Ground level

Excited level

Atoms and molecules can also Atoms and molecules can also absorbabsorb photons, making a transition from a lower photons, making a transition from a lower level to a more excited one.level to a more excited one.

This is, of course, absorption.

Energ

y

Ground level

Excited level

Absorption lines in an otherwise continuous light spectrum due to

a cold atomic gas in front of a hot source.

Spontaneous absorptionSpontaneous absorption• Let us consider two energy Let us consider two energy

level having energy E1 & E2 level having energy E1 & E2 resp.resp.

• The atom will remain in The atom will remain in ground state unless some ground state unless some external stimulant is applied external stimulant is applied to it.to it.

• When an EM wave i.e photon When an EM wave i.e photon of particular freq fall on it , of particular freq fall on it , there is finite probability that there is finite probability that atom will jump form energy atom will jump form energy state E1 to E2.state E1 to E2.

photon

E1

E2

Spontaneous emissionSpontaneous emission• Consider an atom in higher Consider an atom in higher

state (E2).state (E2).

• It can decay to lower energy It can decay to lower energy level by emitting photon.level by emitting photon.

• Emitted photon have energy Emitted photon have energy hv=E2-E1.hv=E2-E1.

• Life time of excited state is Life time of excited state is 1010-9-9sec.sec.

Photonhv=E2-E1

E2

E1

Stimulated emissionStimulated emission• There are metastable state There are metastable state

i.e. transition from this state i.e. transition from this state is not allowed acc to is not allowed acc to selection rule.selection rule.

• There life time is 10There life time is 10-3-3 sec. sec.• Atom in this state can’t jump Atom in this state can’t jump

to lower state at there own.to lower state at there own.• When an photon of suitable When an photon of suitable

freq arrive it make the atom freq arrive it make the atom in metastable unstable.in metastable unstable.

• The emitted photon is in The emitted photon is in coherence with incident coherence with incident photon.photon.

Incident photon

EmittedPhoton coherent

Metastable state(10-3sec)

Stimulated Emission

The stimulated photons have unique properties:

– In phase with the incident photon

– Same wavelength as the incident photon

– Travel in same direction as incident photon

Stimulated vs Spontaneous Emission

Stimulated emission requires the presence of a photon. An “incoming” photon stimulates a molecule in an excited state to decay to the ground state by emitting a photon. The stimulated photons travel in the same direction as the incoming photon.

Spontaneous emission does not require the presence of a photon. Instead a molecule in the excited state can relax to the ground state by spontaneously emitting a photon. Spontaneously emitted photons are emitted in all directions.

In 1916, Einstein showed In 1916, Einstein showed that another process, that another process, stimulated emission, can stimulated emission, can occur. occur.

Before After

Absorption

Stimulated emission

Spontaneous emission

The processes that account for absorption and emission ofradiation and the attainment of thermal equilibrium. Theexcited state can return to the lower state spontaneously aswell as by a process stimulated by radiation already presentat the transition frequency.

Before

Absorption

Stimulated emission

Spontaneous emission

After

In 1916, Einstein showed that another process, stimulated emission, can occur.

EINSTEIN’S THEORY OF EINSTEIN’S THEORY OF RADIATIONSRADIATIONS

Incident photon

Stimulatedemission

Spontaneous emission

E2

E1

EINSTEIN’S THEORY OF EINSTEIN’S THEORY OF RADIATIONSRADIATIONS

• RRaa=rate of absorption per unit =rate of absorption per unit volumevolume

• It depends upon:It depends upon:

1.N1.N11: no. of atom in ground state. : no. of atom in ground state. 2.2.ρρ(v): energy density per unit freq (v): energy density per unit freq of incident wave. of incident wave.

EINSTEIN’S THEORY OF EINSTEIN’S THEORY OF RADIATIONSRADIATIONS

• RRspsp=rate of emission per unit =rate of emission per unit

volume.volume.

• It depends upon:It depends upon:

1.N1.N22: no. of atom in exicited state. : no. of atom in exicited state.

EINSTEIN’S THEORY OF EINSTEIN’S THEORY OF RADIATIONSRADIATIONS

• RRstst= rate of stimulated emission per = rate of stimulated emission per unit volumeunit volume

• It depends upon:It depends upon:

1.N1.N22: no. of atom in exicited state. : no. of atom in exicited state. 2.2.ρρ(v): energy density per unit freq (v): energy density per unit freq of incident wave. of incident wave.

Properties of Laser• MonochromaticThe light emitted from a laser is

monochromatic, that is, it is of one wavelength (color).  In contrast, ordinary white light is a combination of many different wavelengths (colors).

Properties of LaserProperties of Laser

• Directional:Directional:• Lasers emit light that is Lasers emit light that is

highly highly directional.  directional.  Laser Laser light is emitted as a light is emitted as a relatively narrow beam relatively narrow beam in a specific direction.  in a specific direction.  Ordinary light, such as Ordinary light, such as coming from the sun, a coming from the sun, a light bulb, or a candle, is light bulb, or a candle, is emitted in many emitted in many directions away from the directions away from the source.source.

Properties of LaserProperties of Laser

• CoherentCoherent• The light from a The light from a

laser is said to laser is said to be be coherent, coherent, which means the which means the wavelengths of wavelengths of the laser light the laser light are in phase in are in phase in space and time.space and time.  

Population InversionPopulation Inversion

• A state in which a substance has been energized, A state in which a substance has been energized, or excited to specific energy levels.or excited to specific energy levels.

• More atoms or molecules are in a higher excited More atoms or molecules are in a higher excited state. state.

Population InversionPopulation Inversion• The process of producing a The process of producing a

population inversion is called population inversion is called pumpingpumping..

• Examples: Examples: → →by lamps of appropriate intensityby lamps of appropriate intensity → →by electrical dischargeby electrical discharge

Achieving inversion: Achieving inversion: Pumping the laser Pumping the laser mediummediumNow let I be the intensity of (flash lamp) light used to pump energy into the laser medium:

R = 100% R < 100%

I0 I1

I2I3 Laser medium

I

Will this intensity be sufficient to achieve inversion, N2 > N1?

It’ll depend on the laser medium’s energy level system.

In what energy levels do molecules In what energy levels do molecules reside? Boltzmann population factorsreside? Boltzmann population factors

Ni is the number density of molecules in state i (i.e., the number of molecules per cm3).

T is the temperature, and kB is Boltzmann’s constant.

exp /i i BN E k T

Energ

y

Population density

N1

N3

N2

E3

E1

E2

Boltzmann Population FactorsBoltzmann Population Factors

In equilibrium, the ratio of the populations of two states is: In equilibrium, the ratio of the populations of two states is:

NN22 / N / N11 = = exp(– exp(–E/kE/kBBT T ), where ), where E = EE = E22 – E – E11 = h = h

As a result, higher-energy states are always less populated than theAs a result, higher-energy states are always less populated than the ground state, and absorption is stronger than stimulated emission. ground state, and absorption is stronger than stimulated emission.

In the absence of collisions,molecules tend to remainin the lowest energy stateavailable.

Collisions can knock a mole-cule into a higher-energy state.The higher the temperature, the more this happens.

22

1 1

exp /

exp /B

B

E k TN

N E k T

Low T High T

Energ

y

Molecules

Energ

y

Molecules

32

1

2

1

3

Components of LASERComponents of LASER

Pump SourcePump Source• A pump is basic energy source for a laser. It gives A pump is basic energy source for a laser. It gives

energy to various atoms of laser medium & energy to various atoms of laser medium & excites them . So that population inversion can excites them . So that population inversion can take place & it is maintained with time. The take place & it is maintained with time. The excitation of atom occur directly or through atom excitation of atom occur directly or through atom or atom collision.or atom collision.

• There is various type of pump depending upon There is various type of pump depending upon nature of medium .nature of medium .Examples: electric discharges, Examples: electric discharges, flashlamps, arc lamps and chemical reactions. flashlamps, arc lamps and chemical reactions.

• The type of pump source used depends on the The type of pump source used depends on the gain medium. gain medium.

→ →A helium-neon (HeNe) laser uses an A helium-neon (HeNe) laser uses an electrical discharge in the helium-neon electrical discharge in the helium-neon gas gas mixture.mixture.

→ →Excimer lasers use a chemical reaction. Excimer lasers use a chemical reaction.

Gain MediumGain Medium• When energy is given to laser medium When energy is given to laser medium

a small fraction of medium shows lasing a small fraction of medium shows lasing action. This part of laser medium is action. This part of laser medium is called Active centers. For examples in called Active centers. For examples in ruby laser ruby laser CrCr++++++ is active center, in He-is active center, in He-Ne laser Ne laser Ne Ne are active centers.are active centers.

• It is the Major determining factor of the It is the Major determining factor of the wavelengthwavelength of operation of the laser. of operation of the laser.

• Excited by the pump source to produce Excited by the pump source to produce a population inversion.a population inversion.

• Where spontaneous and stimulated Where spontaneous and stimulated emission of photons takes place.emission of photons takes place.

• Example:Example:solid, liquid, gas and semiconductor.solid, liquid, gas and semiconductor.

Optical ResonatorOptical Resonator• It is an set up used to obtain amplification It is an set up used to obtain amplification

of stimulated photons, by oscillating them of stimulated photons, by oscillating them back & forth between two extreme limits. back & forth between two extreme limits. Consist of:Consist of:

1.1.Two plane or concave mirrors placed co-Two plane or concave mirrors placed co-axially.axially.

2.2.One mirror is reflecting & other is partially One mirror is reflecting & other is partially reflecting.reflecting.

Optical ResonatorOptical Resonator• Two parallel mirrors placed around the Two parallel mirrors placed around the

gain medium.gain medium.• Light is reflected by the mirrors back into Light is reflected by the mirrors back into

the medium and is amplified .the medium and is amplified .• The design and alignment of the mirrors The design and alignment of the mirrors

with respect to the medium is with respect to the medium is crucialcrucial..• Spinning mirrors, modulators, filters and Spinning mirrors, modulators, filters and

absorbers may be added to produce a absorbers may be added to produce a variety of effects on the laser output.variety of effects on the laser output.

Stimulated emission can Stimulated emission can lead to a chain reaction lead to a chain reaction and laser emission.and laser emission.

Excited medium

If a medium has many excited molecules, one photon can become many.

This is the essence of the laser. The factor by which an input beam is amplified by a medium is called the gain and is represented by G.

effi

cient

pum

pin

g

slow

rela

xati

on

Metastable state

fast

slow Population inversion

Fast relaxation

Requirements for Laser Action

Four-level Laser SystemFour-level Laser System

• Laser transition takes Laser transition takes place between the place between the third and second third and second excited states.excited states.

• Rapid depopulation of Rapid depopulation of the lower laser level.the lower laser level.

FOUR LEVEL LASER:FOUR LEVEL LASER:• STEP 1- PUMPINGSTEP 1- PUMPING: atoms are excited : atoms are excited

to higher energy level by providing to higher energy level by providing energy from ext. source.energy from ext. source.

• STEP 2- POPULATION INVERSION:STEP 2- POPULATION INVERSION: atom via radiation less decay, atom via radiation less decay,

decays to metastable state and decays to metastable state and hence population inversion take hence population inversion take place.place.

FOUR LEVEL LASER:FOUR LEVEL LASER:• STEP 3- LASER ACTION:STEP 3- LASER ACTION: atom from atom from

metastable state decays to lower metastable state decays to lower state by stimulated emission and state by stimulated emission and hence laser action take place.hence laser action take place.

• STEP 4- BACK TO GROUND STATE:STEP 4- BACK TO GROUND STATE: atom from excited state decays to atom from excited state decays to

lower state by spontaneous emission.lower state by spontaneous emission.

FOUR LEVEL LASER:FOUR LEVEL LASER:

Three-level Laser SystemThree-level Laser System• Initially excited to a Initially excited to a

short-lived high-short-lived high-energy state .energy state .

• Then quickly decay Then quickly decay to the intermediate to the intermediate metastable level. metastable level.

• Population inversion Population inversion is created between is created between lower lower ground stateground state and a higher-energy and a higher-energy metastable statemetastable state. .

Three-level Laser SystemThree-level Laser System

Two-level Laser SystemTwo-level Laser System

• UnimaginableUnimaginable

as absorption and stimulated as absorption and stimulated processes neutralize one another. processes neutralize one another.

• The material becomes transparentThe material becomes transparent..

Two-Level System

En, Nn

Em, Nm

En, Nn

Em, Nm

Even with very a intense pump source, the best one can achieve with a two-level system is

excited state population = ground state population

Usually, additional losses in intensity occur, such as absorption, scat-tering, and reflections. In general, the laser will lase if, in a round trip:

Gain > Loss This called achieving Threshold.

The LaserThe LaserA laser is a medium that stores energy, surrounded by two mirrors.A partially reflecting output mirror lets some light out.

A laser will lase if the beam increases in intensity during a round trip:that is, if

3 0I I

R = 100% R < 100%

I0 I1

I2I3 Laser medium with gain, G

Laser TypesLaser Types• According to the According to the active materialactive material: :

solid-state, liquid, gas or solid-state, liquid, gas or semiconductor lasers.semiconductor lasers.

• According to the According to the wavelengthwavelength: : infra-red, visible, ultra-violet (UV) or infra-red, visible, ultra-violet (UV) or x-ray lasers.x-ray lasers.

Laser TypesLaser Types

• According to the According to the nature of pumping.nature of pumping.

flash type, chemical pumping & flash type, chemical pumping & electric discharge laserselectric discharge lasers

• According to the According to the nature of outputnature of output: :

pulsed & continuous wave lasers.pulsed & continuous wave lasers.

s.ns.noo

Name of Name of laserlaser

wave-wave-lenghtlenght

Active Active mediumediumm

Nature Nature of o/pof o/p

Spectral Spectral regionregion

11 Ruby Ruby laserlaser

6943 Å6943 Å solidsolid pulsedpulsed visiblevisible

22 CO2 CO2 laserlaser

10.6 10.6 μμmm

gasgas Contin-Contin-ousous

Infra redInfra red

33 He-Ne He-Ne laserlaser

6328 Å6328 Å gasgas pulsedpulsed visiblevisible

44 Nd : YAG Nd : YAG laserlaser

1.06 1.06 μμmm

solidsolid pulsedpulsed Infra redInfra red

Continuous vs Pulsed Lasers

Pump Source : Excitation of the lasing atoms or molecules by an external source of light (such as a lamp) or another laser

The output of the laser light can be a continuous wave (cw) if the pumping is continuous or pulsed if the pumping is pulsed.

Pulsed lasers have very high peak intensities because the laser intensity is concentrated in a very short time duration.

Solid-state LaserSolid-state Laser• Example: Ruby LaserExample: Ruby Laser

• Operation wavelength: 694.3 nm (IR)Operation wavelength: 694.3 nm (IR)

• 3 level system: absorbs green/blue3 level system: absorbs green/blue

•Gain Medium: crystal of aluminum oxide (Al2O3)

with small part of atoms of aluminum is replaced

with Cr3+ ions.•Pump source: flash lamp •The ends of ruby rod serve as laser mirrors.

How a laser works?How a laser works?

RUBY LASERRUBY LASER

Three Level Laser System: Ruby Three Level Laser System: Ruby LaserLaser

Ruby Laser Ruby Laser IntroductionIntroduction

• A ruby laser is a solid-state laser that uses a A ruby laser is a solid-state laser that uses a synthetic ruby crystal as its gain medium. synthetic ruby crystal as its gain medium.

• It was the first type of laser invented, and It was the first type of laser invented, and was first operated by Theodore H. "Ted" was first operated by Theodore H. "Ted" Maiman at Hughes Research Laboratories on Maiman at Hughes Research Laboratories on 16 -05-1960.16 -05-1960.

• The ruby mineral (corundum) is aluminum The ruby mineral (corundum) is aluminum oxide oxide AlAl22OO33 with a small amount(about 0.05%) with a small amount(about 0.05%) of of CrCr which gives it its characteristic which gives it its characteristic pink or pink or redred color by absorbing color by absorbing greengreen and and blueblue light. light. The ruby laser is The ruby laser is used as a The ruby laser is The ruby laser is used as a pulsed laser, producing red light at pulsed laser, producing red light at 694.3 nm. 694.3 nm. After receiving a pumping flash from the flash After receiving a pumping flash from the flash tube, the laser light emerges for as long as tube, the laser light emerges for as long as the excited atoms persist in the ruby rod, the excited atoms persist in the ruby rod, which is typically about a which is typically about a millisecomillisecond. nd.

1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of Cr3+ in the ruby crystal to higher energy levels.

2. At a specific energy level, some Cr3+ emit photons. At first the photons are emitted in all directions. Photons from one Cr3+ stimulate emission of photons from other Cr3+ and the light intensity is rapidly amplified.

3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification.

4. The photons leave through the partially silvered mirror at one end. This is laser light.

Three Level Laser System: Three Level Laser System: Ruby LaserRuby Laser

Three Level Laser System: Three Level Laser System: Ruby LaserRuby Laser

Three Level Laser System: Three Level Laser System: Ruby LaserRuby Laser

ApplicationApplication• Ruby lasers have declined in use with the Ruby lasers have declined in use with the

discovery of better lasing media. They are discovery of better lasing media. They are still used in a number of applications where still used in a number of applications where short pulses of red light are required. short pulses of red light are required. Holographers around the world produce Holographers around the world produce holographic portraits with ruby lasers, in sizes holographic portraits with ruby lasers, in sizes up to a metre squared.up to a metre squared.

• Many non-destructive testing labs use ruby Many non-destructive testing labs use ruby lasers to create holograms of large objects lasers to create holograms of large objects such as aircraft tires to look for weaknesses such as aircraft tires to look for weaknesses in the lining.in the lining.

• Ruby lasers were used extensively in tattoo Ruby lasers were used extensively in tattoo and hair removaland hair removal

He-NE LASERHe-NE LASER• A A helium-neon laserhelium-neon laser, usually called a , usually called a

HeNe laserHeNe laser, is a type of small gas laser. , is a type of small gas laser. HeNe lasers have many industrial and HeNe lasers have many industrial and scientific uses, and are often used in scientific uses, and are often used in laboratory demonstrations of optics. Its laboratory demonstrations of optics. Its usual operation wavelength is 632.8 nm, usual operation wavelength is 632.8 nm, in the red portion of the visible spectrumin the red portion of the visible spectrum

He-Ne laserHe-Ne laserHe-Ne lasers are He-Ne lasers are normally small, with normally small, with cavity lengths of around cavity lengths of around 15 cm up to 0.5 m.15 cm up to 0.5 m.The optical cavity of the The optical cavity of the laser typically consists of laser typically consists of a plane, high-reflecting a plane, high-reflecting mirror at one end of the mirror at one end of the laser tube, and a concave laser tube, and a concave output coupler mirror of output coupler mirror of approximately 1% approximately 1% transmission at the other transmission at the other end.end.Electric discharge Electric discharge pumping is used.pumping is used.Optical output powers Optical output powers ranging from 1 mW to 100 ranging from 1 mW to 100 mW.mW.

Ele

ctro

n im

pact

Sp

onta

neou

s em

issi

on

Radiation less decay

Applications of He-Ne laserApplications of He-Ne laser

•The Narrow red beam of He-Ne laser is The Narrow red beam of He-Ne laser is used in supermarkets to read bar codes.used in supermarkets to read bar codes.

•The He- Ne Laser is used in Holography in The He- Ne Laser is used in Holography in producing the 3D images of objects.producing the 3D images of objects.

•He-Ne lasers have many industrial and He-Ne lasers have many industrial and scientific uses, and are often used in scientific uses, and are often used in laboratory demonstrations of optics. laboratory demonstrations of optics.

HELIUM-NEON GAS LASER

Laser-Professionals.com

Courtesy of Metrologic, Inc.

Four Level Laser System: He-Four Level Laser System: He-Ne LaserNe Laser

Components of LASERComponents of LASER

1.1. PUMP.PUMP.

2.2. ACTIVE MEDIUM.ACTIVE MEDIUM.

3.3. OPTICAL RESONATOR.OPTICAL RESONATOR.

A pump is basic energy source for a laser. It gives energy to various atoms of laser medium & excites them . So that population inversion can take place & it is maintained with time. The excitation of atomoccur directly or through atom or atom collision.

There is various type of pump depending upon nature of medium

When energy is given to laser medium a small fraction of medium shows lasing action. This part of laser medium is called Active centers. For examples in ruby laser Cr+++ is active center, in He-Ne laser Ne are active centers.

It is an set up used to obtain amplification of stimulated photons, by oscillating them back & forth between two extreme limits. Consist of:

1. Two plane or concave mirrors placed co-axially.

2. One mirror is reflecting & other is partially reflecting.

Laser Applications - Laser Applications - EngineeringEngineering

Barcode Scanners

Precise Welding- Metals- Microelectronics

Precise Cutting- Metal sheets- Fiber cutting

Precise Drilling - Micro dimensional PCB- Electronic components

Field Surveys

Arial Surveys Arial Surveys

Applications of laserApplications of laser 1 Military 1 Military

2. Death ray2. Death ray

3. Defensive applications3. Defensive applications

4. Strategic defense 4. Strategic defense initiativeinitiative

5. Laser sight5. Laser sight

6. Illuminator6. Illuminator

7. Rangefinder7. Rangefinder

8. Target designator8. Target designator

Laser Applications - MedicineLaser Applications - Medicine

• Photodynamic therapy --Treatment with drugs that become Photodynamic therapy --Treatment with drugs that become active when exposed to light. These drugs kill cancer cellsactive when exposed to light. These drugs kill cancer cells

• Hyperthermia of Tumors--Body temperature that has been Hyperthermia of Tumors--Body temperature that has been raised abnormally high to kill tumor cells or make them more raised abnormally high to kill tumor cells or make them more sensitive to the effects of radiation and certain chemotherapy sensitive to the effects of radiation and certain chemotherapy drugsdrugs

• Endoscopies : A diagnostic test that allows a physician to view Endoscopies : A diagnostic test that allows a physician to view the upper gastrointestinal tract via a flexible tube inserted the upper gastrointestinal tract via a flexible tube inserted down the patient’s throatdown the patient’s throat

• Painless Surgery --Lasik, removing kidney stones, The hot Painless Surgery --Lasik, removing kidney stones, The hot beam cauterizes (seals off) open blood vessels making the beam cauterizes (seals off) open blood vessels making the surgery bloodless. surgery bloodless.

• Soft tissue surgery Clean, neat and easy cut through human Soft tissue surgery Clean, neat and easy cut through human tissue (CO2 laser) tissue (CO2 laser)

• Laser Angioplasty Ability to remove the plaque from blocked Laser Angioplasty Ability to remove the plaque from blocked arteries arteries

• Dermatologists For cosmetic surgery Dermatologists For cosmetic surgery • Dentistry --To remove the cavities a laser that is strong enough Dentistry --To remove the cavities a laser that is strong enough

to destroy the cavity yet too weak to hurt the enamel is used. to destroy the cavity yet too weak to hurt the enamel is used. This procedure is painless and bloodless. This procedure is painless and bloodless.

• Chromosome manipulation Chromosome manipulation