LASER

Light Amplification by Stimulated Emission of Radiation

Objectives… Introduction and understand the principle of

LASER

• Light Amplification by Stimulated Emission of Radiation

• Absorption• Spontaneous Emission• Stimulated Emission• Population Inversion• Optical Pumping

Objectives… Characteristics or Properties of Laser Light

• Coherence• High Intensity• High directionality• High monochromaticity

Laser light is highly powerful and it is capable of propagating over long distances and it is not easily absorbed by water.

Introduction• LASER

“Light Amplification by Stimulated Emission of Radiation”

• MASER (1939 Towner)“Microwave Amplification by Stimulated

Emission of Radiation” • Stimulated Emission - Einstein in 1917.• Ruby Crystal LASER - Maiman, California in 1960.• He-Ne LASER - Ali Javan in 1961.• Diode LASER- Hall in 1962.

Light having following Properties

Wavelength Frequency Amplitude Phase Coherence/Incoherence Velocity Direction

Absorption

• E1 = Ground state

• E2 = Excited State• E = hν (Photon Energy)

• According to Bohr’s law atomic system is characterized by discrete energy level.

• When atoms absorb or release energy it transit upward or downward.

• Lower level E1 & Excited level E2

• So, h ƒ = E2 – E1 • The rate of absorption depends on no. of

atoms N1 present in E1 & spectral energy density u(ƒ) of radiation

• So, P12 α N1 u(ƒ)

• P12= B12N1 u(ƒ)

Spontaneous Emission

• E1 = Ground State

• E2 = Excited State

• E = E2 – E1

= ΔE = hν

• System having atoms in excited state.• Goes to downward transition with emitting

photons, hƒ = E1 – E2.• Emission is random, so if not in same phase

becomes incoherent.• The transition depends on atoms in excited state

N2.

P12(spont) α N2 = A21 N2

• Where, A21 = Einstein coefficient for spontaneous Emission. we get Incoherent radiation forms heat by light amplification of radiation by spontaneous emission..

Stimulated Emission

• System having atoms in excited state.• Goes to downward transition with emitting

photons.• 2hƒ = E1 – E2. After applying photon energy hƒ.• Emission is depends on energy density u(ƒ) & No. of

atoms in excited state N2

• P12(stimul) α u(ƒ) N2 = B21 N2 u(ƒ)

• Where, B21 = Einstein coefficient for Stimulated Emission.

• Thus one photon of energy hƒ stimulates two photons of energy hƒ in same phase & directions. So, we get coherent light amplification of radiation by stimulated emission.

Population Inversion• It is the process of increasing exited electrons in

higher energy levels.

• Due to this process the production of laser is possible.

• The energy level between the ground state E1 (1st

level) and exited state E3 (3rd level) is known as metastable state E2 (2nd level).

• By optical pumping electrons from ground state jumps to exited state by absorbing photons.

• The electrons remain only for 10-8 sec in exited state E3, so most of them jumps back to the ground state E1 by emitting photons. But some of them jumps to the metastable state E2.

• They (electron) stay in metastable state for more then 10-3 sec.

• So electron density increases in metastable state.

• Thus the transitions are possible it takes more no. of electrons together and ν – (knew) 12 photon beam is produced which constitute laser beam.

Optical PumpingThere are no of techniques for pumping a

collection of atoms to an inverted state. • Optical pumping• Electrical discharge • Direct conversion

When photon of blue green light incident on Ruby crystal, electrons from ground state absorbs and exited and jumps on higher energy state levels and comes back to metastable state. They increase population of electrons in metastable state. This process is called optical pumping which is done by flash tube.

Relation between Einstein’s ‘A’ and ‘B’ coefficients• Einstein obtained a mathematical expression for the

existence of two different kinds of processes, (1) Spontaneous emission (2) Stimulated emission• Consider all atoms r in thermal equilibrium at T.• Radiation of freq. ƒ & energy density u(ƒ).• N1 & N2 r atoms in E1 & E2 respectively.• In equilibrium absorption rates & emission rates must be

same.• i.e. B12 N1 u(ƒ) = A21 N2+ B21 N2 u(ƒ)

A21 N2= u(ƒ) [B12N1 – B21N2]

So, u(f) = [A21 N2 / (B12 N1 – B21 N2)] ---------(1)

------------(2) • Boltzmann distribution law,

------------(3)

• So, -----------(4) • But, E2 – E1 = hf -----------(5) • So, -----------(6)

21

21

12 1

21 2

( )[ ]

ƒ1

A

Bu

B N

B N

1

2

/1 0

/2 0

E kT

E kT

N N e

N N e

2 1( )/1

2

E E kTNe

N

h /1

2

ƒ kTNe

N

---------- (7)

• According to plank’s radiation formula, ----------- (8)• Where, B12 = B21 & A21 / B21 = ------------ (9) • So, Ratio of spontaneous to stimulated emission:

--------- (10)

21

21

ƒ12

21

h /

ƒ1

( )[ ]kTe

A

Bu

B

B

3

3 ƒh /

8 1( ) ( )

[ ]

ƒƒ

1kTu

c

h

e

3

3

8 ƒh

c

2 21 21

2 21 21

3

3

8

( ) ( ) ( )

ƒ

ƒ ƒ ƒ

N A A hR

B u B u ucN

• So, --------- (11)

--------- (12)

• So, R = ---------- (13) If hƒ << kT, in thermal equilibrium,

then R = << 1• hƒ<<kT – Stimulated emission–Valid in microwave region (MASER)

• hƒ>>kT – Spontaneous emission–Valid in visible region, incoherent

3

3 /

3

3

ƒh

8( )

8

ƒƒ

&

ƒƒ

1

1( ) ( )

[ ]kT

h

uc

uR

h

e

c

ƒh / 1[ ]kTe

ƒh / 1[ ]kTe

Types of LASER

There are three types of lasers

1. Solid Laser (Ruby Laser)2. Liquid Laser3. Gas Laser ( He – Ne Laser, CO2 Laser)

Ruby Laser…To produce laser from solid, Ruby crystal is used.Ruby is an aluminum oxide crystal (Al2O3) in which

some of the aluminum atoms have been replaced with Cr+3 chromium atoms (0.05% by weight).

It was the first type of laser invented, and was first operated by Maiman in Research Laboratories on 1960.

Chromium gives ruby its characteristic pink or red color by absorbing green and blue light.

For a ruby laser, a crystal of ruby is formed into a cylinder. The ruby laser is used as a pulsed laser, producing red light at 6943 Å.

Ruby crystal is surrounded by xenon tube. Ruby crystal is fully silvered at one side and partially silvered at the other end.

A strong beam of blue green light is made to fall up on crystal from xenon tube and this light is absorbed by the crystal.

Because of this, many electrons from ground state or normal state are raised to the excited state or higher state and electron falls to metastable state.

During this transition photon is not emitted but excess energy of the electrons absorbed in crystal lattice.

As electron drops to metastable state they remain there for certain time ~ 10-6 sec.

Thus the incident blue green light from tube increases the number of electron in metastable state and then the population inversion can be achieved.

If a light of different frequency is allowed to fall on this material, the electrons move back and forth between silvered ends of the crystal.

While moving through they get stimulated and exiced electrons radiate energy.

Thus readia photon has the same frequency as that of incident photon and is also in exactly same phase.

When the intensity of light beam is increased the same process is repeated.

Finally extremely intensified beam of light energies from the semi silvered side of the crystal.

This way it is possible to get extremely intensified and coherent beam of light from the crystal. This beam is nothing but higher energetic beam – ie. LASER beam.

Applications of Ruby Laser…Ruby lasers have declined in use with the

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

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

Ruby lasers were used extensively in tattoo and hair removal.

Drawbacks of Ruby Laser…• The laser requires high pumping power because

the laser transition terminates at the ground state and more than half of ground state atoms must be pumped to higher state to achieve population inversion.

• The efficiency of ruby laser is very low because only green component of the pumping light is used while the rest of components are left unused.

• The laser output is not continues but occurs in the form of pulses of microseconds duration.

• The defects due to crystalline imperfections are also present in this laser.

Gaseous Laser (He – Ne Laser)A helium - neon laser, usually called a He-Ne laser,

is a type of small gas laser. He-Ne lasers have many industrial and scientific uses, and are often used in laboratory demonstrations of optics.

He-Ne laser is an atomic laser which employs a four-level pumping scheme.

The active medium is a mixture of 10 parts of helium to 1 part of neon.

Neon atoms are centers and have energy levels suitable for laser transitions while helium atoms help efficient excitation of neon atoms.

The most common wavelength is 6328 Å. These lasers produced powers in the range 0.5 to 50 mW in the red portion of the visible spectrum.

They have long operating life of the order of 50,000 hrs.

Construction…It consists of a glass discharge tube of about

typically 30 cm long and 1.5 cm diameter.The tube is filled with a mixture of helium and

neon gases in the 10:1.Electrodes are provided in the tube to produce a

discharge in the gas.They are connected to a high voltage power

supply. The tube is hermetically sealed with glass windows oriented at Brewster angle to the tube. The cavity mirrors are arranged externally.

Working…When the power is switched on , a high voltage of

about 10 kV is applied across the gas. It is sufficient to ionize the gas.The electrons and ions are produced in the process

of discharge are accelerated toward the anode and cathode respectively.

The electron have a smaller mass, they acquire a higher velocity. They transfer their kinetic energy to helium atoms through inelastic collisions.

The initial excitation effects only the helium atoms. They are in metastable state and cannot return in ground state by the spontaneous emission.

The excited helium atoms can return to the ground state by transforming their energy to neon atoms through collision. This transformation take place when two colliding atoms have initial energy state. It is called resonant transfer of energy.

So, the pumping mechanism of He-Ne Laser is when the helium atom in the metastable state collides with neon atom in the ground state the neon atom is excited and the helium atom drops back to the ground state.

The role of helium atom is thus to excite neon atom and cause, population inversion. The probability of energy transfer from helium atoms to neon atoms is more as there are 10 atoms of helium per 1 neon atom in gas mixture.

Without the Brewster windows, the light output is unpolarized, because of it laser output to be linearly polarized.

When the excited Ne atom passes from metastable state (3s) to lower level (2p), it emits photon of wavelength 632 nm.

This photon travels through the gas mixture parallel to the axis of tube, it is reflected back and forth by the mirror ends until it stimulates an excited Ne atom and causes it to emit a photon of 632nm with the stimulating photon.

The stimulated transition from (3s) level to (2p) level is laser transition.

Although 6328 Å is standard wavelength of He-Ne Laser, other visible wavelengths 5430 Å (Green) 5940 Å (yellow-orange), 6120 Å (red-orange) can also produced.

Overall gain is very low and is typically about 0.010 % to 0.1 %.

The laser is simple practical and less expensive.

The Laser beam is highly collimated, coherent and monochromatic.

Applications of He-Ne Laser…

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

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

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

Semiconductor Laser (Diode Laser)• A semiconductor laser is a laser in which a

semiconductor serves as a photon source.• The most common semiconductor material that

has been used in lasers is gallium arsenide.• Einstein’s Photoelectric theory states that light

should be understood as discrete lumps of energy (photons) and it takes only a single photon with high enough energy to knock an electron loose from the atom it's bound to.

• Stimulated, organized photon emission occurs when two electrons with the same energy and phase meet. The two photons leave with the same frequency and direction.

P type Semiconductors

• In the compound GaAs, each Ga atom has three electrons in its outermost shell of electrons and each As atom has five.

• When a trace of an impurity element with two outer electrons, such as Zn (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 is a “hole” for an electron but there is no electron available.

• This forms a p-type semiconductor.

N type Semiconductors• When a trace of an impurity element with six

outer electrons, such as Se (selenium), is added to a crystal of GaAs, it provides on additional electron which is not needed for the bonding.

• This electron can be free to move through the crystal.

• Thus, it provides a mechanism for electrical conductivity.

• This type is called an n-type semiconductor.

• 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.

Application of Lasers… Laser beam is used to measure distances of sun,

moon, stars and satellites very accurately. It can be used for measuring velocity of light, to

study spectrum of matters, to study Raman effect.

It can be is used for increasing speed and efficiency of computer.

It is used for welding. It is used in biomedical science. It is used in 3D photography.

Application of Lasers…

It is used for communication, T. V. transmission, to search the objects under sea.

It can be used to predict earthquake. Laser tools are used in surgery. It is used for detection and treatment of cancer. It is used to aline straight line for construction of

dam, tunnels etc. It is used in holography. It is used in fiber optic communication. It is also used in military, like LIDAR. It is used to accelerate some chemical reactions.