Introduction

Lasers

Recommended Books

• Lasers: Peter Milonni and Joseph H. Eberly

• Principle of Lasers: Orazio Svelto and David C Hanna

• Laser Fundamentals : William Tb Silfast

• Lasers and Non-Linear Optics: B B Laud

• Laser Electronics: Joseph T Verdeyen

• Lasers: Principles, Types and Applications: K R Nambiar

• Understanding Lasers : Jeff Hecht

• Lasers – Theory and Applications: Ghatak and Thyagarajan

• Wikipedia – Free Online Encyclopedia

• Acronym for Light Amplification by the Stimulated Emission of Radiation

• Spontaneous Emission , Stimulated Emission (A. Einstein)

• Highly directional, intense, monochromatic and coherent

• Dimensions of lasers – Vary small to very large in size

• The output power: 10-9 Watts to 1020 Watts

• Wavelength Range: microwave to the soft x ray region

• Frequency Range: 1011 Hz to 1017 Hz

• Pulse energy can go up to 1017 Joules

• Pulse Duration can be as short as 6x10-15 secs

LASERLASER

IntroductionIntroduction

HistoryHistory

• MaserMaser : Microwave amplification by the stimulated Emission of Radiation – Charles H Townes in 1951

• Ruby LaserRuby Laser : (694 nm) Theodore Maiman, 1960

• Nobel PrizeNobel Prize: Townes & Two Soviet Maser pioneers, Nikolai Basov and Aleksander Prokhorov shared 1964 in Physics for ‘Maser/Laser principle”

ApplicationsApplications

• Reading and WritingReading and Writing -Playing audio compact discs-Playing video disc-Reading universal product codes

• Measurement & InspectionMeasurement & Inspection-Measuring the range to distant objects-Measuring small distances very precisely-Detecting flaws in aircraft tires

• MedicineMedicine-Laser Surgery-Shattering of kidney stones

• Materials WorkingMaterials Working -Cutting, drilling and welding plastics / Hard Materials-Engraving wood-Marking identification codes

• Military ApplicationsMilitary Applications-Ant sensor Weapons/Anti-satellite weapons-pinpointing targets for bombs and missiles

• Other ApplicationsOther Applications-Making holograms-Laser Pointers-Controlling chemical reactions-Producing nuclear fusion-Basic Research, Spectroscopy

How Lasers Work:

The Fundamental Processes

Contents

• Spontaneous Emission• Stimulated Emission• Absorption• Rate Equations• Einstein Coefficients• Population Inversion• Two Level System• Three Level Laser• Four Level Laser

Spontaneous Emission• It is the process by which an

atom or a molecule in an excited state drops to ground state resulting in Photon.

• The photon will have frequency ν and energy hν, given by: E2 − E1 = hν,

• The phase of the photon in spontaneous emission is random as is the direction the photon propagates in. This is not true for stimulated emission

Consider number of atoms with energy level E1 as N1,with E2 as N2.

Stimulated Emission• Process by which, when perturbed by photon, matter may

lose energy resulting in the creation of another photon

• The perturbing photon is not destroyed in the process (cf. absorption), and the second photon is created with the same phase, frequency, polarisation and direction of travel as the original

Absorption

• If a photon, whose energy is exactly equal to E2 - E1 is incident on an atom in the ground level, the atom will absorb this energy and get excited to an energy level E2 from

E1

Rate Equations: Einstein A and B Coefficients

If the number of atoms in the excited state 2 is N2, and state

1 is N1

• Rate at which spontaneous spontaneous emissionemission occurs, that is rate of transition from E2 to E1 is given by the following exp.

• The rate of stimulated emissionstimulated emission is given by following expression where u(ν) is the radiation density of photons of frequency ν

• The rate of absorption absorption (stimulated(stimulated))

• Rate: No of absorption/time/vol

221 2

tanSpon eous

dNN

dtA

212 1( )

Absorbed

dNu N

dtB

221 2( )

Stimulated

dNu N

dtB

Detailed Balancing

• In thermal equilibrium (const T) or in a steady state condition, Upward Transition must balance all downward transition

12 1 21 2 21 2( ) ( )u N u N A NB B

• If the group of atoms are in thermal equilibrium, it can be shown from thermodynamics that the ratio of the number of atoms in each state is given by a Boltzman distribution:

2 12 2

1 1

( )B

E Ek TN g

eN g

2 1E E h

Maxwell- Boltzmann Distribution

21

2112

12

1.( , )

Bh

k T

ABBB

eu T

Solving

Planck's law of black body radiation

A black body is an idealized body that is a perfect radiator and perfect absorber of electromagnetic radiation. A black body not only absorbs all wavelengths of energy and radiates at all wavelengths, but it does this at the maximum possible intensity for any given temperature.

Equilibrium distribution of the photons, as stated in Planck's law of black body radiation. (Here u is spectral Energy Density which has units of energy per unit volume per unit frequency (joule per cubic meter per hertz)

3

3

8 1.

1( , )

Bh

k T

hc e

u T

Relation between the Einstein Coefficients

• Comparing the equations ( Why ?Why ? )

321 1

321 2

8A g hB g c

21 1

12 2

B gB g

and

gi is the degeneracy of the state i

Significance of Einstein Coefficients

• All the three fundamental processes are interrelated by the A and B coefficients. Given one coefficient , one can find out other processes

• Further at thermal equilibrium

23B

h

k T

•For Optical source, T~103K with λ=600 nm

Thus at optical frequencies the emission is predominantly due to spontaneous transitions : Hence the emission from Usual light sources is incoherent

21

21

( ) 1( ) B

A hexp

B u k T

1021

21

10( )

A

B u

Population Inversion

• In a gas of atoms in thermal equilibrium, the population of the lower level will always be greater than the population of the upper level. Therefore, if a light beam is incident on the medium,there will always be more upward transitions due to absorption than downward transitions due to stimulated emission. Hence there will be net absorption, and the intensity of the beam will diminish on progressing through the medium.

• To amplify the beam, we require that the rate of stimulated emission transitions exceeds the rate of absorption. This implies that N2 must exceed N1. This is a highly non-equilibrium situation, and is called population inversion. Inspection of Maxwell Bolzman distribution implies that population inversion corresponds to negative temperatures ! This is not as ridiculous as it sounds, because the atoms are not in thermal equilibrium.

• Once we have population inversion, we have a mechanism for generating gain in the laser medium. The art of making a laser operate is to work out how to get population inversion on the transition you want to get lasing.

Population Inversion –Cont…• So what is the way out ?

Population Inversion

2

1

1N

N

“Negative Temperature”

TheoreticallyTheoretically two basic ways of producing Population Inversion

1. Put excess of atoms or molecules in the higher state

2. Depopulate the lower level

Population Inversion-Cont• What is our Aim ?• What is essential for

Laser ?

• To Build a Laser• Stimulated Emission

– A difficult Job

Why ?

172 1

2

1

( )

1.5 10B

E Ek TN

eN

In Thermodynamic Equilibrium

2 1

5

1

8.6 10 /

300B

E E eV

k eV K

T K

Probability for Stimulated Emission is extremely low: Why ?

Excitation Mechanism

How to produce population Inversion ExperimentallyExperimentally ??

Is it possible by merely heating it or by increasing its temperature ?

2

1

2 1( )

B

E Ek TN

eN

Heating the material increases the average energy but does not make N2 greater than N1

To produce Population Inversion , the atoms has to be selectively selectively excitedexcited to particular levels: Different mechanism are possible

Masers and Two Level System

• Charles Townes made the first maser at columbia university . He used a molecular beam technique to separate excited ammonia molecules from ground state molecules

• In general Population Inversion is not possible in two level system. At equilibrium both the populations become same – Rate of absorption is equal to the rate of stimulated emission that is N1=N2 = N/2 known as saturation state or optically transparent. But we can’t achieve Population Inversion or optical gain

Is Population Inversion possible in a two level system ?

Three Level Laser

Population Inversion is achieved in level 2

τ21 >> τ32

Four Level Laser

Population inversion is achieved between Level 3 and level 2

τ32 >> τ43, τ21

The energy of the pumping transition is greater than laser transition.

Four Level laser is more Efficient than three level Laser (Why, 2 reasons)

Laser Pumping

• Laser pumping is the act of energy transfer from an external source into the laser gain medium. The energy is absorbed in the medium, producing excited states in its atoms. When the number of particles in one excited state exceeds the number of particles in ground state or a less-excited state, population inversion is achieved. In this condition, the mechanism of stimulated emission can take place and the medium can act as a laser or an optical amplifier. The pump power must be higher than the lasing threshold of the laser.

• Lasers can be pumped in several ways:

Laser Pumping/Excitation techniques

• Optical pumping– Flash lamps are the oldest energy source for lasers.

They are used for lower energies in both solid state and dye lasers. They produce a broad spectrum, causing most of the energy to be wasted as heat in the gain medium. Flash lamps also tend to have short lifetime.

– Lasers of a suitable type can be used to pump another laser. Their narrow spectrum makes them more efficient way of energy transfer than flash lamps. Diode lasers are used to pump DPSS lasers.

– Microwaves are used to excite gas lasers.

Pumping Mechanisms -Cont

• Electrical pumping Electric glow discharge is common in gas lasers. Eg. in the helium-

neon laser the electrons from the discharge collide with the helium atoms, the helium atoms get excited, they transfer their energy by collision with neon atoms, and inverse population of neon atoms builds up.

Electric current is typically used to pump semiconductor lasers. Electron beams are used in special applications, eg. the free

electron lasers or excimer laser

Other Mechanism• Chemical reaction is used as a power source in chemical lasers.

This allows for very high output powers difficult to reach by other means.

• Nuclear fission is used in exotic nuclear pumped lasers (NPL), directly employing the energy of the fast neutrons released in a nuclear reactor. [1] [2]

Flash Lamp

Xenon Flash Lamp

• A xenon flash lamp is a electric glow discharge lamp designed to produce extremely intense, incoherent, full-spectrum white light for very short durations

U shaped Xenon Flash Lamp

Flash Lamp-Operation• The lamp is comprised of a sealed tube, often made of fused quartz,

which is filled with a mixture of gases, primarily xenon, and electrodes to carry electrical current to the gas mixture.

• A flash is initiated by first ionizing the gas mixture, then sending a very large pulse of current through the ionized gas. Ionization is necessary to decrease the electrical resistance of the gas so that a pulse measuring as much as thousands of amperes can travel through the tube. The initial ionization pulse, or trigger pulse, may be applied to one of the internal electrodes, or to a metal band or wire that is wrapped around the glass tube. When the trigger pulse is applied, the gas becomes ionized, and the capacitor immediately discharges through the tube. When this current pulse travels through the tube, it excites electrons surrounding the xenon atoms causing them to jump to higher energy levels. The atoms' electrons immediately drop back to a lower orbit, producing photons in the process. Depending on the size and application of the flashlamp, xenon fill pressures may range from a few kilopascals to tens of kilopascals (0.01–0.1 atmosphere or tens to hundreds of torr).

• krypton flashlamps are more suitable than xenon flashlamps for pumping Nd:YAG lasers, as krypton emission in near infrared is better matching to the absorption spectrum of Nd:YAG.