1

UNIT-6UNIT-6LasersLasers

2

S. No.S. No. ModuleModule LectuLecture re No.No.

PPT PPT Slide Slide No.No.

11 Introduction, Introduction, characteristics of characteristics of lasers.lasers.

L 1L 1 4--54--5

22 Spontaneous & Spontaneous & Stimulated emission Stimulated emission of Radiationof Radiation

Population InversionPopulation Inversion..

L 2-3L 2-3 6-146-14

3.3. Types of LasersTypes of Lasers L 4-5L 4-5 15-3415-34

4.4. Applications of lasers Applications of lasers L 6L 6 35-4035-40

UNIT INDEXUNIT INDEX

3

APPLIED PHYSICSAPPLIED PHYSICS

CODE : 07A1BS05CODE : 07A1BS05

I B.TECHI B.TECH

CSE, IT, ECE & EEECSE, IT, ECE & EEE

UNIT-6UNIT-6

NO. OF SLIDES :40NO. OF SLIDES :40

4

INTRODUCTIONINTRODUCTION

The word “LASER” is an acronym for The word “LASER” is an acronym for Light Amplification by Stimulated Light Amplification by Stimulated Emission of Radiation.Emission of Radiation.

Lecture-1

5

CharacteristicsCharacteristics

MonochromacityMonochromacity High IntensityHigh Intensity CoherenceCoherence DirectionalityDirectionality

6

Types of coherenceTypes of coherence

Temporal coherenceTemporal coherence Spatial coherenceSpatial coherence Temporal coherence measures the Temporal coherence measures the

continuity of a wave along its length.continuity of a wave along its length. Spatial coherence measures the maximum Spatial coherence measures the maximum

seperation between any two points on the seperation between any two points on the cross section of the wavefront which cross section of the wavefront which maintain correlation between them.maintain correlation between them.

Lecture-2

7

Stimulated AbsorptionStimulated Absorption

Excitation of atoms from lower Excitation of atoms from lower energy state to higher energy state energy state to higher energy state due to interaction of radiation with due to interaction of radiation with matter is known as Stimulated matter is known as Stimulated absorption.absorption.

8

Spontaneous emissionSpontaneous emission

When an electron in the excited level When an electron in the excited level E2 falls spontaneously to lower E2 falls spontaneously to lower energy level E1 after its lifetime a energy level E1 after its lifetime a photon is emitted. The energy of the photon is emitted. The energy of the emitted photon is given by E2-E1=h emitted photon is given by E2-E1=h

9

Stimulated emissionStimulated emission

When an electron in the excited level When an electron in the excited level E2 is induced (stimulated) by a E2 is induced (stimulated) by a photon of energy (E2-E1), the photon of energy (E2-E1), the electron moves to lower energy level electron moves to lower energy level E1 emitting another photon of E1 emitting another photon of energy E2-E1. This process is called energy E2-E1. This process is called stimulated emission.stimulated emission.

10

Both stimulated and stimulating Both stimulated and stimulating photons are in phase with each photons are in phase with each other.other.

Stimulated emission of radiation Stimulated emission of radiation (light) results in amplification of (light) results in amplification of lightlight

11

Population inversionPopulation inversion

For light amplification by stimulated For light amplification by stimulated emission of radiation the population emission of radiation the population of excited state must be greater than of excited state must be greater than the population of lower energy state. the population of lower energy state. This condition is called population This condition is called population inversion.inversion.

Lecture-3

12

Pumping mechanismsPumping mechanisms

The process of sending atoms from The process of sending atoms from lower energy state to higher energy lower energy state to higher energy state is called Pumping.state is called Pumping.

Optical pumpingOptical pumping Electric dischargeElectric discharge Chemical reactionChemical reaction Injection current through p-n junctionInjection current through p-n junction

13

Optical Feed backOptical Feed back

To direct the amplified light to travel To direct the amplified light to travel back and forth through the active back and forth through the active medium many times two end mirrors medium many times two end mirrors are kept at both the ends of the are kept at both the ends of the laser. These mirrors provide laser. These mirrors provide necessary optical feed back.necessary optical feed back.

14

Threshold inversion densityThreshold inversion density

Only if the population inversion Only if the population inversion density is sufficiently large so that density is sufficiently large so that the loss is compensated by the gain, the loss is compensated by the gain, lasing action starts. The inversion lasing action starts. The inversion density for which the gain is just density for which the gain is just sufficient to compensate for the loss sufficient to compensate for the loss is called threshold inversion density.is called threshold inversion density.

15

Conditions for Lasing Conditions for Lasing

For laser action to take place, the For laser action to take place, the three requisites arethree requisites are

Suitable active mediumSuitable active medium Creation of population inversionCreation of population inversion Proper optical feed backProper optical feed back

Lecture-4

16

RUBY LASERRUBY LASER A A ruby laserruby laser is a is a solid-state lasersolid-state laser

It uses a synthetic It uses a synthetic rubyruby crystal as crystal as its its gain mediumgain medium. .

It was the first type of It was the first type of laserlaser invented, and was first operated invented, and was first operated by by Theodore H. "Ted" Theodore H. "Ted" MaimanMaiman at at Hughes Research LaboratoriesHughes Research Laboratories on on 1960.1960.

Lecture-2

Lecture-2

17

The ruby laser produces pulses of visible The ruby laser produces pulses of visible light at a wavelength of 694.3 nm, which light at a wavelength of 694.3 nm, which appears as deep red to human eyes.appears as deep red to human eyes.

Typical ruby laser pulse lengths are on Typical ruby laser pulse lengths are on the order of a millisecond. These short the order of a millisecond. These short pulses of red light are visible to the pulses of red light are visible to the human eye, if the viewer carefully human eye, if the viewer carefully watches the target area where the pulse watches the target area where the pulse will fire.will fire.

18

19

ApplicationsApplications

Ruby lasers have declined in use with the Ruby lasers have declined in use with the discovery of better lasing media. They discovery of better lasing media. They are still used in a number of applications are still used in a number of applications where short pulses of red light are where short pulses of red light are required. required.

Holographers around the world produce Holographers around the world produce holographicholographic portraits with ruby lasers, in portraits with ruby lasers, in sizes up to a metre squared. sizes up to a metre squared.

The red 694 nm laser light is preferred to The red 694 nm laser light is preferred to the 532 nm green light of the 532 nm green light of frequency-doubledfrequency-doubled Nd:YAGNd:YAG..

20

Many Many non-destructive testingnon-destructive testing labs labs use ruby lasers to create holograms use ruby lasers to create holograms of large objects such as aircraft tires of large objects such as aircraft tires to look for weaknesses in the lining. to look for weaknesses in the lining. Ruby lasers were used extensively in Ruby lasers were used extensively in tattootattoo and and hair removalhair removal, but are , but are being replaced by being replaced by alexandritealexandrite lasers lasers and and Nd:YAGNd:YAG lasers lasers in this application. in this application.

21

Helium-neon laserHelium-neon laser

22

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 lasergas laser..

HeNe HeNe laserslasers have many industrial and have many industrial and scientific uses, and are often used in scientific uses, and are often used in laboratorylaboratory demonstrations of demonstrations of opticsoptics. .

Its usual operation Its usual operation wavelengthwavelength is 632.8 is 632.8 nmnm, , in the in the redred portion of the visible spect portion of the visible spect

The The gain mediumgain medium of the laser, as suggested of the laser, as suggested by its name, is a mixture of by its name, is a mixture of heliumhelium and and neonneon gases, in a 5:1 to 20:1 ratio, contained gases, in a 5:1 to 20:1 ratio, contained at low pressure (an average 50 at low pressure (an average 50 PaPa per cm of per cm of cavity length ) in a glass envelope. cavity length ) in a glass envelope.

23

He-Ne LaserHe-Ne Laser

The energy or pump source of the laser is The energy or pump source of the laser is provided by an provided by an electricalelectrical discharge of discharge of around 1000 around 1000 voltsvolts through an through an anodeanode and and cathodecathode at each end of the glass tube. at each end of the glass tube.

A current of 5 to 100 mA is typical for A current of 5 to 100 mA is typical for CWCW operation.operation.

The The optical cavityoptical cavity of the laser typically of the laser typically consists of a plane, high-reflecting consists of a plane, high-reflecting mirrormirror at one end of the laser tube, and a concave at one end of the laser tube, and a concave output coupleroutput coupler mirror of approximately 1% mirror of approximately 1% transmission at the other end.transmission at the other end.

24

He-Ne LaserHe-Ne Laser

HeNe lasers are typically small, HeNe lasers are typically small, with cavity lengths of around 15 with cavity lengths of around 15 cm up to 0.5 m, and optical cm up to 0.5 m, and optical output output powerspowers ranging from 1 m ranging from 1 mWW to 100 mW. to 100 mW.

The red HeNe laser wavelength The red HeNe laser wavelength is usually reported as 632nm. is usually reported as 632nm. HoweverHowever

25

The true wavelength in air is 632.816 nm, The true wavelength in air is 632.816 nm, so 633nm is actually closer to the true so 633nm is actually closer to the true value. value.

For the purposes of calculating the photon For the purposes of calculating the photon energy, the vacuum wavelength of energy, the vacuum wavelength of 632.991 nm should be used. The precise 632.991 nm should be used. The precise operating wavelength lies within about operating wavelength lies within about 0.002 nm of this value, and fluctuates 0.002 nm of this value, and fluctuates within this range due to thermal expansion within this range due to thermal expansion of the cavithy.of the cavithy.

26

The laser process in a HeNe laser starts with The laser process in a HeNe laser starts with collision of collision of electronselectrons from the electrical from the electrical discharge with the helium atoms in the gas. discharge with the helium atoms in the gas.

This excites helium from the This excites helium from the ground stateground state to to the 23S1 and 21S0 long-lived, the 23S1 and 21S0 long-lived, metastablemetastable excited states. Collision of the excited helium excited states. Collision of the excited helium atoms with the ground-state neon atoms atoms with the ground-state neon atoms results in transfer of energy to the neon results in transfer of energy to the neon atoms, exciting neon electrons into the 3s2 atoms, exciting neon electrons into the 3s2 level. This is due to a coincidence of level. This is due to a coincidence of energy levelsenergy levels between the helium and neon between the helium and neon atoms.atoms.

27

This process is given by the reaction This process is given by the reaction equation:equation: He(21S)* + Ne + ΔE → He(11S) + He(21S)* + Ne + ΔE → He(11S) +

Ne3s2* Ne3s2* ΔE is the small energy difference ΔE is the small energy difference

between the energy states of the two between the energy states of the two atoms, of the order of 0.05 eV or 387 atoms, of the order of 0.05 eV or 387 cm-1, which is supplied by kinetic cm-1, which is supplied by kinetic energy.energy.

28

..

The number of neon atoms entering the The number of neon atoms entering the excited states builds up as further excited states builds up as further collisions between helium and neon collisions between helium and neon atoms occur, causing a atoms occur, causing a population inversionpopulation inversion. . SpontaneousSpontaneous and and stimulated emissionstimulated emission between the 3s2 between the 3s2 and 2p4 states results in emission of and 2p4 states results in emission of 632.82 nm wavelength light, the typical 632.82 nm wavelength light, the typical operating wavelength of a HeNe laser. operating wavelength of a HeNe laser.

29

..

After this, fast radiative decay occurs from the 2p After this, fast radiative decay occurs from the 2p to the 1s ground state. Because the neon upper to the 1s ground state. Because the neon upper level saturates with higher current and the lower level saturates with higher current and the lower level varies linearly with current, the HeNe laser level varies linearly with current, the HeNe laser is restricted to low power operation to maintain is restricted to low power operation to maintain population inversion.population inversion.

Spectrum of a helium neon laser showingSpectrum of a helium neon laser showing With the correct selection of cavity mirrors, other With the correct selection of cavity mirrors, other

wavelengths of laser emission of the HeNe laser wavelengths of laser emission of the HeNe laser are possible. There are infrared transitions at are possible. There are infrared transitions at 3.39 μm and 1.15 μm wavelengths, and a variety 3.39 μm and 1.15 μm wavelengths, and a variety of visible transitions, including a green (543.5 nm, of visible transitions, including a green (543.5 nm, the so-called GreeNe laser), a yellow (594 nm) the so-called GreeNe laser), a yellow (594 nm) and an orange (612 nm) transition.and an orange (612 nm) transition.

30

SEMICONDUCTOR LASERSEMICONDUCTOR LASER A semiconductor laser converts electrical A semiconductor laser converts electrical

energy into light. This is made possible by energy into light. This is made possible by using a semiconductor material, whose using a semiconductor material, whose ability to conduct electricity is between that ability to conduct electricity is between that of conductors and insulators.of conductors and insulators.

By doping a semiconductor with specific By doping a semiconductor with specific amounts of impurities, the number of amounts of impurities, the number of negatively charged electrons or positively negatively charged electrons or positively charged holes can be changed.charged holes can be changed.

Compared to other laser types, Compared to other laser types, semiconductor lasers are compact, reliable semiconductor lasers are compact, reliable and last a long time. and last a long time.

Lecture-2

Lecture-5

31

SEMICONDUCTOR LASERSEMICONDUCTOR LASER

Such lasers consist of two basic components, Such lasers consist of two basic components, an optical amplifier and a resonator. The an optical amplifier and a resonator. The amplifier is made from a direct-bandgap amplifier is made from a direct-bandgap semiconductor material based on either semiconductor material based on either gallium arsenide (GaAs) or InP substrates. gallium arsenide (GaAs) or InP substrates.

These are compounds based on the Group III These are compounds based on the Group III and Group V elements in the periodic table. and Group V elements in the periodic table. Alloys of these materials are formed onto Alloys of these materials are formed onto the substrates as layered structures the substrates as layered structures containing precise amounts of other containing precise amounts of other materials.materials.

32

Semiconductor laserSemiconductor laserLecture-2

33

HEAVILY DOPED p-n JUNCTION DIODEHEAVILY DOPED p-n JUNCTION DIODE

(a)In equilibrium. (b)With (a)In equilibrium. (b)With forward bias.forward bias.

34

DOUBLE HETEROJUNCTION SC LASERDOUBLE HETEROJUNCTION SC LASER

35

Raw MaterialsRaw Materials

The conventional semiconductor laser The conventional semiconductor laser consists of a compound semiconductor, consists of a compound semiconductor, gallium arsenide. This material comes in the gallium arsenide. This material comes in the form of ingots that are then further processed form of ingots that are then further processed into substrates to which layers of other into substrates to which layers of other materials are added. The materials used to materials are added. The materials used to form these layers are precisely weighed form these layers are precisely weighed according to a specific formula.according to a specific formula.

Other materials that are used to make this Other materials that are used to make this type of laser include certain metals (zinc, type of laser include certain metals (zinc, gold, and copper) as additives (dopants) or gold, and copper) as additives (dopants) or electrodes, and silicon dioxide as an insulator. electrodes, and silicon dioxide as an insulator.

Lecture-2

36

APPLICATIONS OF LASERSAPPLICATIONS OF LASERS

Lasers are uused in local area Lasers are uused in local area network to transfer the data from the network to transfer the data from the memory storage of one computer to memory storage of one computer to other computer.other computer.

These are used to store large amount These are used to store large amount of data in CD-ROMof data in CD-ROM

Lecture-6

37

Lasers can be used to blast holes in Lasers can be used to blast holes in diamonds and hard steel.diamonds and hard steel.

They are used as a source of intense They are used as a source of intense heat.heat.

They are used to cut, drill, weld, and They are used to cut, drill, weld, and to remove metal from surfaces.to remove metal from surfaces.

38

These are used in spacecrafts and These are used in spacecrafts and submarines.submarines.

They are also used in high speed They are also used in high speed photocopiers and printers.photocopiers and printers.

They are used in the field of 3-d They are used in the field of 3-d photography.photography.

39

Lasers can serve as a war weapon.Lasers can serve as a war weapon. High energy lasers are used to High energy lasers are used to

destroy enemy aircrafts and missiles.destroy enemy aircrafts and missiles. These are used to produce certain These are used to produce certain

chemical reactions.chemical reactions.

40

Lasers are used in controlling Lasers are used in controlling haemorrhage.haemorrhage.

Lasers are used for elimination of Lasers are used for elimination of moles and tumors.moles and tumors.

Lasers are used in the treatment Lasers are used in the treatment of glaucoma.of glaucoma.

41

CoCo2 2 laser is used in spinal and laser is used in spinal and brain tumor and kidney stone brain tumor and kidney stone extrusion. extrusion.

Lasers are used to correct a Lasers are used to correct a condition called retina condition called retina detachments by eye specialist.detachments by eye specialist.

Argon and CoArgon and Co2 2 lasers are used in lasers are used in the treatment of liver and lungs.the treatment of liver and lungs.