INTRODUCTION OF LASER

L – LIGHT

A – AMPLIFICATION

S – STIMULATED

E – EMISSION

R - RADIATION

BASIC IDEA

Consider a group of atoms exposed stream

of photons, each with energy h. Let us

assume two energy levels E1 and E2 of an

atom.During transition from one energy state to another, the

light is absorbed (or) emitted by particles. Under this

action, 3 processes can occur.

They are,

Stimulated absorption

Spontaneous emission

Stimulated emission

MECHANISMS OF LIGHT EMISSION

1. Absorption

2. Spontaneous Emission

3. Stimulated Emission

For atomic systems in thermal equilibrium with their surrounding, the

emission of light is the result of:

Absorption

And subsequently, spontaneous emission of energy

There is another process whereby the atom in an upper energy level can

be triggered or stimulated in phase with the an incoming photon. This

process is:

Stimulated emission

It is an important process for laser action

Therefore 3 process

of light emission:

ATOMS AND MOLECULES CAN ABSORB PHOTONS,MAKING A TRANSITION FROM A LOWER LEVEL TOA MORE EXCITED ONE.

This is, of course,

absorption.

Energ

y

Ground level

Excited level

INDUCED ABSORPTION

Let us consider two energylevel having energy E1 & E2resp.

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

When an EM wave i.e photonof particular freq fall on it ,there is finite probability thatatom will jump form energystate E1 to E2.

photon

E1

E2

EXCITED ATOMS EMIT PHOTONSSPONTANEOUSLY.

When an atom in an excited state falls to a lower energy level, it emits a

photon of light.

Molecules typically remain excited for no longer than a few nanoseconds.

This is often also called fluorescence or, when it takes longer,

phosphorescence.

Energ

y

Ground level

Excited level

SPONTANEOUS EMISSION

Consider an atom in higher

state (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.

Photon

hv=E2-E1

E2

E1

STIMULATED EMISSION

There are meta-stable state i.e.transition from this state is notallowed acc to selection rule.

There life time is 10-3 sec.

Atom in this state can’t jump tolower state at there own.

When an photon of suitablefreq arrive it make the atom inmeta-stable unstable.

The emitted photon is incoherence with incidentphoton.

Incident photon

Emitted

Photon

coherent

Metastable state(10-3sec)

LASER FUNDAMENTALS

The light emitted from a laser is monochromatic, that is, it is of

one color/wavelength. In contrast, ordinary white light is a

combination of many colors (or wavelengths) of light.

Lasers emit light that is highly directional, that is, laser light is

emitted as a relatively narrow beam in a specific direction. Ordinary

light, such as from a light bulb, is emitted in many directions away

from the source.

The light from a laser is said to be coherent, which means that the

wavelengths of the laser light are in phase in space and time.

Ordinary light can be a mixture of many wavelengths.

These three properties of laser light are what can make it more

hazardous than ordinary light. Laser light can deposit a lot of

energy within a small area.

INCANDESCENT VS. LASER LIGHT

1. Many wavelengths

2. Multidirectional

3. Incoherent

1. Monochromatic

2. Directional

3. Coherent

POPULATION INVERSION

The process by which the population of a particular higher energy state is

made more than that of a specified lower energy state is called population

inversion.

N2 > N1

PRINCIPLE OF LASER ACTION

Due to stimulated emission the photons multiply in eachstep giving rise to an intense beam of photons that arecoherent and moving in the same direction . Hence theLight Is Amplified by Stimulated Emission of Radiation

High ReflectanceMirror (HR)

Output CouplerMirror (OC)

ActiveMedium

Output

Beam

Excitation Mechanism

Optical Resonator

LASER COMPONENTS

LASING ACTION

1. Energy is applied to a medium raising electrons to an unstableenergy level.

2. These atoms spontaneously decay to a relatively long-lived, lowerenergy, meta-stable state.

3. A population inversion is achieved when the majority of atoms havereached this meta-stable state.

4. Lasing action occurs when an electron spontaneously returns to itsground state and produces a photon.

5. If the energy from this photon is of the precise wavelength, it willstimulate the production of another photon of the same wavelengthand resulting in a cascading effect.

6. The highly reflective mirror and partially reflective mirror continuethe reaction by directing photons back through the medium alongthe long axis of the laser.

7. The partially reflective mirror allows the transmission of a smallamount of coherent radiation that we observe as the “beam”.

8. Laser radiation will continue as long as energy is applied to thelasing medium.

LASING ACTION DIAGRAMEne

rgy

Int

roduc

tion

Ground State

Excited State

Metastable State

Spontaneous Energy Emission

Stimulated Emission of Radiation

THREE-LEVEL LASER SYSTEM

Initially excited to a short-lived high-energy state .

Then quickly decay to theintermediate meta-stablelevel.

Population inversion iscreated between lowerground state and a higher-energy metastable state.

Two-level

system

Laser

Transition

Pump

Transition

At best, you get

equal populations.

No lasing.

It took laser physicists a while to realize that four-level systems are best.

Four-level

system

Lasing is easy!

Laser

Transition

Pump

Transition

Fast decay

Fast decay

Three-level

system

If you hit it hard, you

get lasing.

Laser

TransitionPump

Transition

Fast decay

TWO, THREE & FOUR LEVEL SYSTEM

HELIUM-NEON LASER

•Laser medium is mixture of Helium and Neon gases in the ratio 10:1

•Medium excited by large electric discharge, flash pump or continuous high power

pump

•In gas, atoms characterized by sharp energy levels compared to solids

•Actual lasing atoms are the Neon atoms

Pumping action

•Electric discharge is passed through the gas

•Electrons are accelerated, collide withs He atoms and excite them to higher

energy levels

He-Ne lasers are normally small, with cavity lengths of around 15 cm up to 0.5

m.

The optical cavity of the laser typically consists of a plane, high-reflecting

mirror at one end of the laser tube, and a concave output coupler mirror of

approximately 1% transmission at the other end.

Electric discharge pumping is used.

Optical output powers ranging from 1 mW to 100 mW.

ENERGY LEVEL DIAGRAM

WORKING OF HE-NE LASER

APPLICATIONS OF HE-NE LASER

•It is used in laboratories to perform experiments.

•It is used in optical communication without fibre for moderate distance.

•It is used to produce holograms.

ADVANTAGES OF HE-NE LASER

•Operates in a continuous-wave mode.

•It has stability of frequency.

•No cooling is required.

•Less expensive.