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LASER

LightAmplification by Stimulated Emission

of Radiation (Yjis name was coined byGordon Gould in the 1950)

http://upload.wikimedia.org/wikipedia/commons/a/a0/Military_laser_experiment.jpg

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Agenda

Revision of light properties Types of emissions (Qualitative & Quantitative) Population inversion

Amplification Process LASER equipment LASER properties Types of LASER & uses

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Properties of light (Revision) Wave length Frequency

Velocity Monochromatic (Single frequency) Coherent

Polarized

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Properties of wave

Frequency = number of oscillations per second

Amplitude Wave length

Wave length is distance traveled by wave in one

oscillation

Speed of wave = Frequency X wave length m/sec

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Polarization is possible only for transverse waves

Polarizer plateallows only single

plane vibrations to

go thru

Polarized light wave having

only single plane of vibrations

Transverse

vibrations in alldirections

Rotation of this polarizerplate allows full light to

pass thru in one orientation

& zero light at 900 position

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Frequency :indicates type

of wave

In light, waves of different frequency give different

colours

Wave Characteristics :

A singlefrequency lightwave is called

Monochromatic

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Start point ofoscillations in bothwaves marked

is same

Start point of bothwaves marked is

not same.

Monochromatic &Coherent waves

Monochromatic &Non coherent waves

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Agenda

Revision of light properties Types of Emissions (Qualitative & Quantitative) Population Inversion

Amplification Process LASER equipment LASER Properties Types of LASER & uses

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Normal Spontaneous

emissions

Photons are emitted

randomly

No phase relationshipexist between them.

Hence light emitted is

non coherent & unpolarized.

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Recap

Until now we have studied how a photon

interacts with electron at ground state However a photon can interacts with a electron

which is already in excited state

This interaction is totally different Photon can not give energy to electron as the

electron is already at higher energy state

Hence photon will not disappears

This interaction produces LASSER

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Interaction of photon with excited state electron

Photon stimulates the excited electron to jumps

to ground state producing a second photon

identical to itself

The intruding photon continues to exist Thus two identical photons come into

existences. They are coherent, monochromatic

& polarized

This process is known as stimulated emission.

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stimulated emission

http://normal%20%26%20simulated%20emission.swf/

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Until now we have done qualitative analysis of aphoton with electron at ground or excited state

Now we will do quantitative analysis same. ie

If N photons are available under a givencondition, then how many will interact with Electron at ground state

Electron in excited state

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As number of electron & photons is very high

& many random interactions are possible our

study is based on statistical analysis

Scientist Boltzmann found formula. Using it we

can calculate that, in a given sample how

many atoms have electrons in ground state

and in excited state

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Having N number atoms having one electron

This electron can have two energy states

The ground state, with energy E1;

The excited state, with energy E2, withE2 >E1.Let N1 = Atoms have electrons in ground state

Let N2 = Atoms have electron in excited state

Consider a gas sample

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Using Boltzmann formula N2 / N1 ratio can be

calculated if there is no external excitation.

As per the law

At room temperature most atoms are in the

ground state. As Tincreases, N2 (excited electrons)

increases, but N2 can never exceeds N1

even at infinite temperature,

In other words, N2/N1 < 1 without external

excitation

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Agenda

Revision of light properties Types of Emissions (Qualitative & Quantitative) Population Inversion

Amplification Process Laser equipment Laser Properties Types of Laser & uses

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The probability of stimulation emission rate is

proportional to Intensity of external light photons

Number of atoms N2

in the excited state,

Albert Einstein proved that for a photon

Probability of Stimulation emission and

absorption by a ground state atom are same.

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In normal condition (N1 > N2), hence absorption

dominates and there can not be accumulation

of photons.

ForN1 = N2, the rate of absorption of light

exactly balances the rate of emission; themedium is then said to be optically transparent.

ForN1 < N2, stimulation emission process

dominates, photon accumulate and light in the

system undergoes a net increase in intensity.

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Thus to produce a high intensity light stimulated

emissions must dominate absorptions,

For this we must have a state where N2/N1 > 1

In other words more atoms should have exited

electrons than atoms with ground state

This is called population inversion & is essential

to produce LASER

H hi l i i i

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How to achieve population inversion

Theoretically ,in a atom electrons can not jump

in some orbits (called forbidden orbits). This

depends on spin momentum of electron

But in reality due to other effects, electron dojump into forbidden orbit. From this orbits

Jump to lower orbit happens at slower rate

But Stimulated emission can occur .

We use this property to get population inversion

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There are many other ways of creating

population inversion The frequency of out coming laser depend

on medium

This phenomenon can occur over full electromagnetic spectrum

We call LASER if this frequency is in visiblerange

l ti i i

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1.Electron are excitedfrom L1 to L3

Emission involves L3 to L1

Spontaneous & fast L3 to L2 Radiation

less jump L2 to L1 Slower

jump

Forbidden orbit

L1 Ground state, E1, N1

Laser transmission

In a continuous excitation process electronaccumulate at level L2 due to slower rate .Aftersome time N2 >N1, creating required population

inversion between L2-L1 level

population inversion

How to achieve amplification

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1.Now if we have aexiting electro ofenergy = E2E1

2.It will stimulate L2 toL1 jump, creating

accumulation ofphoton with energyE=E2-E1

Forbidden orbit

L1 Ground state, E1, N1

Laser transmission

These accumulated photon come out ofmedium as a strong coherent ,monochromaticLASER beam of frequency = ( E2-E1) / h

How to achieve amplification

Lesser energy

level

http://lesser.mov/

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Agenda

Revision of light properties Types of Emissions (Qualitative & Quantitative) Population Inversion

Amplification Process & Laser properties Laser equipment Types of Laser & uses

H lifi ti t k l

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If the number of photons being stimulated per

unit time is greater than the number of photons

being absorbed, then the net result is a

continuous accumulation of identical photons in

the medium .

Thus intensity of light in the media increases.Thus amplification takes place

How amplification takes place

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Stimulated photons are reflected back into

system using two mirrors called optical

resonator

These photon add up to new stimulated photon

Thus density of identical photon increases

After reaching a level theses photons come out

of one end thru partially reflective mirror

The optical resonator or optical ca it

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The optical resonator, or optical cavity

Mirrors with optical coating are placed around

the gain medium to pump back photons intosystem. Coating decides reflective properties

One - A high reflector,

Other-A partial reflector (output coupler)

laser beam comes thru this mirror.Out putcoupler

All ti l t h t t t l

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All stimulate photon come out spontaneously as

highly energy intense light pulses

Pluses are generated very fast hence we see acontinuous LASER beam

The energy of beam & its frequency (color) can

be decided by suitably selecting Laser medium

Type of mirrors

Pumping arrangement

Energy can be as high that it can cut 50mm steel

sheet or trigger a nuclear reaction

LASER b i

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LASER beam properties

It is monochromatic, coherent, & polarized

It is parallel (all photons having exactly thesame direction. It travels hundreds of kilo

meters without loosing intensity

Hence energy get transmitted efficiently

Used to focus over long distance

Have high intensity - Since the emission ofall photons in a pulse is synchronized

energy concentration is very high

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Agenda

Revision of light properties Types of Emissions (Qualitative & Quantitative) Population Inversion

Amplification Process Functioning of Laser equipment Laser Properties Types of Laser & uses

F ti i f t l l

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Functioning of a actual lesser

( Energy source or )

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Pump source: It provides energy laser system.

Examples of pump sources

Electrical discharges (Helium-Nion Laser)

Flash lamps ( Nd:YAG laser- xenon lamp )

Diode lasers,

Arc lamps,

Light from another laser, Chemical reactions, (Excimer lasers)

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Gain medium; Laser is produced in this medium)

It determining wavelength, intensity of laser Atoms of gain medium are excited by the pump

source to produce a population inversion

Examples of different gain media include:

Semiconductors

Metal doped crystals Certain gases

Dye solutions

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Agenda

Revision of light properties Types of Emissions (Qualitative & Quantitative) Population Inversion

Amplification Process Laser equipement Laser Properties Types of Laser & uses

Types of LASER

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Diode lasers:( typical red laser pointers)

Lightweight, cheap, durable,

Used in CD player

Mediumsemiconductor, Pumping-Electricity

Have an inferior coherence

Solid state lasers (Ex. Green laser pointers)

Expensive, easy to maintain, Small

Used for missile or bomb guidance systems.

Medium: metal ions doped in solid crystals

Pumping flash lamps or diode

Types of LASER

Gas lasers;( First developed Ammonium maser 1954)

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Gas lasers;( First developed Ammonium maser 1954) Easy to maintain.CO2-lasers are most

powerful

Used in industrial, medical applications. Mediums Helium-Neon (Red), Krypton( blue, green,red)

He-Cd - (Cadmium - blue) CO2-Laser (infrared).

Dye lasers:

Narrow frequency, highly coherent & intense. Difficult to maintain, expensive, Medium- a dye solution ( ex Rhodamine ) Pumping - other lasers)..

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Uses of Laser

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Uses of Laser Diode (red color) or infrared laser

To read CDs, As pointers

As a switch in alarm systems In computer laser printers In telephonic communication

To measure distances ,time In medicine for surgery (CO2-laser, infrared)

NdYAG, infrared lasers : For Navigation of

planes and missiles CO2-lasers: In welding, cutting and drillingmaterials

Gas lasers : Used in holography

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Chargesdrum + ve

Sharp lesser move onrotating drum & drawletters in form of dotsby discharging thepoints on the drum asper (called electricimage)

Further ink is depositedon drum with staticcharge attraction &printing is done

Laser role in printer

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Safety precautions

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Lasers are dangerous above a certain intensity.

Maximum risk is for eyes. Damages Retina ) Laser-pointers and show-lasers (very low

power) are relatively safe

But even these laser should not be focused

in eye, Eye lens focus them further on retina

Generally lasers are harmless to the skin, buthigh powered focused lasers can cause severe

burns.

Safety precautions

Singapore Laser show in sky

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Singapore Laser show in sky

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Laser show in sky

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End

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