Laser PPT

Made by Mr. Mohit Dadheech,RIET,Mechanical

1

TODAYS CHAPTER:TODAYS CHAPTER:TODAYS CHAPTER:TODAYS CHAPTER:

LASERLASER

2

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..

3

4

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

55

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.

66

7

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

8

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

9

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)

10

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.

1111

Before

Absorption

Stimulated emission

Spontaneous emission

After

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

1212

13

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.

14

EINSTEIN’S THEORY OF EINSTEIN’S THEORY OF RADIATIONSRADIATIONS

Incident photon

Stimulatedemission

Spontaneous emission

E2

E1

15


• 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.

16


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

volume.volume.


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

17


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


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.

18

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.

19

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.

20

21

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 population The process of producing a population

inversion is called inversion is called pumpingpumping..• Examples: Examples: → →by lamps of appropriate intensityby lamps of appropriate intensity → →by electrical dischargeby electrical discharge

22

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.2323

24

25

26

27

28

29

30

31

Components of LASERComponents of LASER

32

33

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.

34

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.

35

36

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.

3737

38

effi

cient

pum

pin

g

slow

rela

xati

on

Metastable state

fast

slow Population inversion

Fast relaxation

Requirements for Laser Action

3939

40

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.

41

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.

42

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.

43

FOUR LEVEL LASER:FOUR LEVEL LASER:

44

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. .

45

Three-level Laser SystemThree-level Laser System

46

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

4747

48

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

solid-state, liquid, gas, excimer or solid-state, liquid, gas, excimer 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.

49

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.

50

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.

5151

52

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.

53

How a laser works?How a laser works?

54

RUBY LASERRUBY LASER

55

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.

56

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.

57

58

• As the flash lamp stop operting, the As the flash lamp stop operting, the population of the upper level population of the upper level decreases very rapidally & lasing decreases very rapidally & lasing action stops till the further operation action stops till the further operation of next flash. As the production of of next flash. As the production of laser beam depends upon the laser beam depends upon the operation of flash lamp the ruby laser operation of flash lamp the ruby laser is pulsed type laser.is pulsed type laser.

59

• During the period of operation of two During the period of operation of two flash laser output is oscillating & flash laser output is oscillating & output is highly irregular function of output is highly irregular function of time, shows random fluctuations in time, shows random fluctuations in the amplitude. This type of output is the amplitude. This type of output is called as laser SPIKING.called as laser SPIKING.

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 , 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

60

http://en.wikipedia.org/wiki/Gas_laser

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.

61

62

Ele

ctro

n im

pact

Sp

onta

neou

s em

issi

on

Radiation less decay

CARBON CARBON DIOXIDEDIOXIDECarbon dioxide lasers Carbon dioxide lasers are the highest-power are the highest-power continuous wave lasers continuous wave lasers that are currently that are currently available. They are also available. They are also quite efficient: the ratio quite efficient: the ratio of output power to of output power to pump power can be as pump power can be as large as 20%.large as 20%.

The COThe CO22 laser produces laser produces a beam of infrared light a beam of infrared light with the principal with the principal wavelength bands wavelength bands centering around 9.4 centering around 9.4 and 10.6 micrometers.and 10.6 micrometers.

63

64

SemiconductSemiconductor laseror laserLasers which use Lasers which use semiconductor as active semiconductor as active medium. The majority of medium. The majority of semiconductor materials are semiconductor materials are based on a combination of based on a combination of elements in the third group of elements in the third group of the Periodic Table (such as Al, the Periodic Table (such as Al, Ga, In) and the fifth group Ga, In) and the fifth group (such as N, P, As, Sb) hence (such as N, P, As, Sb) hence referred to as the III-V referred to as the III-V compounds. compounds.

65

P- and N-type P- and N-type SemiconductorsSemiconductors

• In the compound GaAs, each gallium atom has three electrons in In the compound GaAs, each gallium atom has three electrons in its outermost shell of electrons and each arsenic atom has five. its outermost shell of electrons and each arsenic atom has five. When a trace of an impurity element with two outer electrons, such When a trace of an impurity element with two outer electrons, such as zinc, is added to the crystal. The result is the shortage of one as zinc, is added to the crystal. The result is the shortage of one electron from one of the pairs, causing an imbalance in which there electron from one of the pairs, causing an imbalance in which there is a “hole” for an electron but there is no electron available. is a “hole” for an electron but there is no electron available. This This forms a p-type semiconductor. forms a p-type semiconductor.

• When a trace of an impurity element with six outer electrons, such When a trace of an impurity element with six outer electrons, such as selenium, is added to a crystal of GaAs, it provides on additional as selenium, is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding. This electron can be electron which is not needed for the bonding. This electron can be free to move through the crystal. Thus, it provides a mechanism for free to move through the crystal. Thus, it provides a mechanism for electrical conductivity. electrical conductivity. This type is called an n-type semiconductor.This type is called an n-type semiconductor.

6666

Under forward bias (the p-type side is made positive) the majority carriers, electrons in the n-side, holes in the p-side, are injected across the depletion region in both directions to create a population inversion in a narrow active region.The light produced by radioactive recombination across the band gap is confined in this active region 6767

68

69

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.

7070

Uses Of LASER

7171

7272

Thank You

Documents

Laser PPT