laser course

8/2/2019 Laser Course

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211jan 2009 Made by Mrs MANDEEP KAUR,NWIET, DHUDIKE


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LASERA laser is an amplifier of light. When thelaser is suitably excited by optical or

electrical energy, the light of the properfrequency entering the laser cavity isamplified in such a manner that laser

output wave is in phase with input.Practical utility of a laser is as anOSCILLATOR-- a generator of light. Thuslaser is also known as GENERATOR oflight.

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LASER ACTION Laser action is based on amplification of EM waves by meansof forced or induced atoms or molecules.

A laser radiation uses three fundamental phenomena when EM

waves interacts with the matter namely

Laser interaction

Spontaneous emission Stimulated emission Spontaneous absorption



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Excited atoms emit photonsspontaneously.

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

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

Energy

Ground level

Excited level

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Atoms and molecules can also absorb photons,making a transition from a lower level to a

more excited one.

This is, ofcourse,

absorption.Energy

Ground level

Excited level

Absorption lines in anotherwise continuous

light spectrum due to acold atomic gas in front

of a hot source.

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Spontaneous absorption Let us consider two energylevel having energy E1 &E2 resp.

The atom will remain inground state unless someexternal stimulant is appliedto it.

When an EM wave i.e

photon of particular freq fallon it , there is finiteprobability that atom will

jump form energy state E1to E2.

photon

E1

E2



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Spontaneous emission Consider an atom in higherstate (E2).

It can decay to lower energy

level by emitting photon.

Emitted photon have energy

hv=E2-E1.

Life time of excited state is

10-9sec.

Photonhv=E2-E1

E2

E1



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Stimulated emission There are metastable statei.e. transition from this stateis not allowed acc toselection rule.

There life time is 10-3 sec.

Atom in this state cantjump to lower state at thereown.

When an photon of suitablefreq arrive it make the atomin metastable unstable.

The emitted photon is incoherence with incidentphoton.

Incident photon

EmittedPhotoncoherent

Metastable state(10-3sec)



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Stimulated Emission

The stimulated photons have uniqueproperties:

In phase with the incident photon

Same wavelength as the incident photon

Travel in same direction as incidentphoton



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Stimulated vs Spontaneous EmissionStimulated emission requires the presence of a photon. An

incoming photon stimulates a molecule in an excited state todecay to the ground state by emitting a photon. Thestimulated photons travel in the same direction as theincoming photon.

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



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another process, stimulatedemission, can occur.

BeforeAfter

Absorption

Stimulatedemission

Spontaneousemission



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The processes that account for absorption and emission ofradiation and the attainment of thermal equilibrium. The

excited state can return to the lower state spontaneously as

well as by a process stimulated by radiation already present

at the transition frequency.



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Before

Absorption

Stimulatedemission

Spontaneousemission

After

In 1916, Einstein showed that anotherprocess, stimulated emission, can

occur.



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EINSTEINS THEORY OF

RADIATIONS

Incidentphoton

Stimulatedemission

Spontaneous emission

E2

E1



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EINSTEINS THEORY OFRADIATIONS

Ra=rate of absorption per unit volume It depends upon:

1.N1: no. of atom in ground state.2.(v): energy density per unit freq ofincident wave.



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Rsp=rate of emission per unitvolume.

It depends upon:1.N2: no. of atom in exicited state.



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Rst= rate of stimulated emission per unitvolume

It depends upon:1.N2: no. of atom in exicited state.2.(v): energy density per unit freq of

incident wave.



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Properties of Laser

MonochromaticThe light emitted from a laser ismonochromatic, that is, it is of one wavelength(color). In contrast, ordinary white light is a

combination of many different wavelengths(colors).



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Properties of Laser

Directional: Lasers emit light that is

highly directional. Laser

light is emitted as arelatively narrow beam in aspecific direction. Ordinarylight, such as coming fromthe sun, a light bulb, or a

candle, is emitted in manydirections away from thesource.



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Properties of Laser

Coherent The light from a

laser is said to becoherent,whichmeans thewavelengths of the

laser light are inphase in space and

time.



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Population Inversion

A state in which a substance has been energized, orexcited to specific energy levels.

More atoms or molecules are in a higher excited state.

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Population Inversion The process of producing a population

inversion is called pumping.

Examples:by lamps of appropriate intensityby electrical discharge


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A hi i i i


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Achieving inversion:Pumping the laser mediumNow letIbe the intensity of (flash lamp) light used to pump energyinto the laser medium:

R = 100% R < 100%

I0 I1

I2I3 Laser medium

I

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

Itll depend on the laser mediums energy level system.

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In what energy levels do molecules reside?Boltzmann population factors

Ni is thenumberdensity ofmolecules instate i (i.e.,

the numberof moleculesper cm3).

Tis the

temperature,and kB

isBoltzmannsconstant.

exp / i i BN E k T

Energy

Population density

N1

N3

N2

E3

E1

E2


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Boltzmann Population Factors

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

N2 / N1 = exp(DE/kBT), where DE =

E2

E1 = hn

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.

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1 1

exp / exp /

B

B

E k TN

N E k T

Low T High T

Energ

y

Molecules

Energy

Molecules

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1

2

1

3



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Components of LASER


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

energy to various atoms of laser medium & excitesthem . 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 uponnature of medium .Examples: electric discharges,flashlamps, arc lamps and chemical reactions.

The type of pump source used depends on the gain

medium.A helium-neon (HeNe) laser uses an

electrical discharge in the helium-neon gasmixture.Excimer lasers use a chemical reaction.


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Gain Medium When energy is given to laser medium a

small fraction of medium shows lasing action.This part of laser medium is called Activecenters. For examples in ruby laser Cr+++isactive center, in He-Ne laser Ne are activecenters.

It is the Major determining factor of thewavelength of operation of the laser.

Excited by the pump source to produce apopulation inversion.

Where spontaneous and stimulated emissionof photons takes place.

Example:solid, liquid, gas and semiconductor.


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Optical Resonator 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.


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Optical Resonator Two parallel mirrors placed around the gain

medium. Light is reflected by the mirrors back into the

medium and is amplified . The design and alignment of the mirrors withrespect to the medium is crucial.

Spinning mirrors, modulators, filters and

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


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to a chain reaction and laser


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to a chain reaction and laseremission.

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 isamplified by a medium is called the gain and is represented by G.


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efficientpu

mping

slow

relax

ation

Metastable state

fast

slow

Population

inversion

Fast relaxation

Requirements for Laser Action


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Four-level Laser System

Laser transition takes

place between thethird and secondexcited states.

Rapid depopulation ofthe lower laser level.


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FOUR LEVEL LASER: STEP 1- PUMPING: atoms are excited to

higher energy level by providing energyfrom ext. source.

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

metastable state and hence population

inversion take place.

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

metastable state decays to lower state bystimulated emission and hence laseraction take place.

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

state by spontaneous emission.

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FOUR LEVEL LASER:

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Three-level Laser System Initially excited to a

short-lived high-energystate .

Then quickly decay tothe intermediatemetastable level.

Population inversion is

created between lowerground state and ahigher-energymetastable state.

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Three-level Laser System

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Two-level Laser System

Unimaginable

as absorption and stimulated processesneutralize one another.

The material becomes transparent.

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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

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Th L


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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 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

I2I

3 Laser mediumwith gain, G

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Laser TypesAccording to the active material:

solid-state, liquid, gas, excimer or

semiconductor lasers.

According to the wavelength:

infra-red, visible, ultra-violet (UV) or x-raylasers.

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Laser Types

According to the nature of pumping.flash type, chemical pumping & electric

discharge lasersAccording to the nature of output:

pulsed & continuous wave lasers.

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s.n Name of wave- Active Nature Spectral


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s.no

Name oflaser

wavelenght

Activemedium

Natureof o/p

Spectralregion

1 Ruby laser 6943 solid pulsed visible

2 CO2 laser 10.6 m gas Contin-ous

Infra red

3 He-Ne

laser

6328 gas pulsed visible

4 Nd : YAGlaser

1.06 m solid pulsed Infra red

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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 thelaser intensity is concentrated in a very short time duration.

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Solid-state Laser Example: Ruby Laser Operation wavelength: 694.3 nm (IR)

3 level system: absorbs green/blueGain 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.

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How a laser works?

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RUBY LASER

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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.

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3. Mirrors at each end reflect the

photons back and forth, continuing

this process of stimulated emissionand amplification.

4. The photons leave through the

partially silvered mirror at oneend. This is laser light.

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As the flash lamp stop operting, thepopulation of the upper level decreasesvery rapidally & lasing action stops till thefurther operation of next flash. As theproduction of laser beam depends uponthe operation of flash lamp the ruby laser

is pulsed type laser.

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During the period of operation of twoflash laser output is oscillating & output is

highly irregular function of time, showsrandom fluctuations in the amplitude. Thistype of output is called as laser SPIKING.

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He-NE LASER A helium-neon laser, usually called a HeNe

laser, is a type of small gas laser. HeNe lasershave many industrial and scientific uses, and are

often used in laboratory demonstrations ofoptics. Its usual operation wavelength is 632.8nm, in the red portion of the visible spectrum

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http://en.wikipedia.org/wiki/Gas_laserhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Optical_spectrumhttp://en.wikipedia.org/wiki/Redhttp://en.wikipedia.org/wiki/Nanometrehttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Opticshttp://en.wikipedia.org/wiki/Laboratoryhttp://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Gas_laser


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He-Ne laser

He-Ne lasers arenormally small, with cavitylengths of around 15 cm upto 0.5 m.The optical cavity of thelaser typically consists of a

plane, high-reflectingmirror at one end of thelaser tube, and a concaveoutput coupler mirror ofapproximately 1%transmission at the other

end.Electric dischargepumping is used.Optical output powersranging from 1 mW to 100

mW.65

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http://en.wikipedia.org/wiki/Optical_cavityhttp://en.wikipedia.org/wiki/Mirrorhttp://en.wikipedia.org/wiki/Output_couplerhttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Watthttp://en.wikipedia.org/wiki/Power_(physics)http://en.wikipedia.org/wiki/Output_couplerhttp://en.wikipedia.org/wiki/Mirrorhttp://en.wikipedia.org/wiki/Optical_cavity


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E

lectronimpact

Radiation less decay

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CARBON


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CARBONDIOXIDE

Carbon dioxide lasers arethe highest-powercontinuous wave lasers thatare currently available.They are also quiteefficient: the ratio of output

power to pump power canbe as large as 20%.The CO2 laser produces abeam ofinfraredlight withthe principal wavelengthbands centering around 9.4

and 10.6 micrometers.

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http://en.wikipedia.org/wiki/Laserhttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Micrometrehttp://en.wikipedia.org/wiki/Micrometrehttp://en.wikipedia.org/wiki/Wavelengthhttp://en.wikipedia.org/wiki/Lighthttp://en.wikipedia.org/wiki/Infraredhttp://en.wikipedia.org/wiki/Laser


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SemiconductorlaserLasers which use semiconductor asactive medium. The majority ofsemiconductor materials are basedon a combination of elements inthe third group of the PeriodicTable (such as Al, Ga, In) and thefifth group (such as N, P, As, Sb)hence referred to as the III-V

compounds.

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P- and N-type Semiconductors

In the compound GaAs, each gallium atom has three electrons in itsoutermost shell of electrons and each arsenic atom has five. When a traceof an impurity element with two outer electrons, such as zinc, is added to

the crystal. The result is the shortage of one electron from one of thepairs, causing an imbalance in which there is a hole for an electron butthere is no electron available. This forms a p-type semiconductor.

When a trace of an impurity element with six outer electrons, such asselenium, is added to a crystal of GaAs, it provides on additional electronwhich is not needed for the bonding. This electron can be free to movethrough the crystal. Thus, it provides a mechanism for electricalconductivity. This type is called an n-type semiconductor.

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Under forward bias (the p-type side is made positive)

the majority carriers,electrons in the n-side,holes in the p-side, areinjected across thedepletion region in both

directions to create apopulation inversion ina narrow active region.The light produced by

radioactive recombinationacross the band gap isconfined in this activeregion

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Components of LASER

1. PUMP.2. ACTIVE MEDIUM.

3. OPTICAL RESONATOR.

A pump is basic energy source for a laser. It givesenergy to various atoms of laser medium & excitesthem . So that population inversion can take place & itis maintained with time. The excitation of atomoccur

directly or through atom or atom collision.There is various type of pump depending upon natureof medium

When energy is given to laser medium a smallfraction of medium shows lasing action. Thispart 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.

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