HELIUM-NEON

LASER

BY-ARPIT SHUKLA

EC ,SRMCEM LUCKNOW

INTRODUCTION

A laser is a device that emits light through a

process of optical amplification based on the

stimulated emission of electromagnetic radiation.

It can be classified in four basic types-

* SOLID ( ruby laser )

* LIQUID ( dye laser )

* GAS (helium- neon laser)

*PLASMA

DISADVANTAGE OF 3 LEVEL LASER

Occur in non continuous pulses

High power pumping source req.

Difficult to maintaie pop. Inversion

Less efficient

These drawbacks led to dev. of four level laser

EMERGENCEIt was invented by Iranian Physicist Ali Javan &

William R Bennet in 1960.

They worked on the principle of converting electric

energy to a laser light output.

CONSTRUCTION

1-Glass envelop

It consist of CAPILARY TUBE

(its main use is to pass electric discharge)

2- Cathode and anode

use to generate electric discharge

3- ReflectorsCombination of mirrors

One is perfectly reflecting and the other is partially

4-Active medium

Mixture of he ne gas which is responsible for

atom excitation

WORKING OF THREE LEVEL LASER

o Energy level diagram

Three energy levels required (ie Ground state, Metastable state and Ordinary excited state).

Pumping of atom is done from Ground state E1 to ordinary excitated state E3.

Energy of photon hv=E3-E1.

Spontaneous emission take place ie some atoms falls to ground state E1 and some on metastable state E2.

Further atoms falls from metastable state to ground state by stimulated emmision which produces the lasing action.

A problem arised when the photon responsible for lasing action also cause indused absorption which severely effect the lasing action.

This was 1 of its major drawback!!!

WORKING OF HELIUM-NEON LASER

Energy level diagrams of four level larser

(2 metastable state M1 & M2)

Mixture of He & Ne in ratio 10:1 at 1 torr is kept in a glass tube.

Due to electric discharge He atom collide with the electrons and

get excited to reach the ordinary excited state(20.61eV).

These excited He atom transfer their energy through collision to

Ne atoms (unexcited) to reach at an energy level at 20.66eV.

Due to spontanious emission the Ne atoms falls on metastable

state M2.

At metastable state 2 stumilated emission takes place and the

neon atoms falls on metastable state 1 (M1).

This transition results in lasing action of He-Ne laser.

Lasing energy is same as the energy difference of the two

metastable states.

Further a radiation less transiton takes place due to which

atom at metastable state 1 eaches he ground state.

Thus the laser light produced in He-Ne laser is Dense,

Non-pulsating & unidirectional.

POINTS TO REMEMBER

Four energy states are required.

The metastable state should near to ordinary excited state

and ground state.

The lifetime of metastable state 2(near oes) should be longer

to facilitate population inversion.

The lifetime of metastable state 1(near gs) should be very

short so as to rapidly get depopulated to avoid re-absoption.

Purpose of having He atoms in He-Ne laser is to help the

population inversion of Ne atoms.

COMPARISION OF THREE & FOUR

LEVEL LASER’S ENERGY DIAGRAM

CHARACTERISTIC

The light is extremly monochromatic with

wavelength 632.8nm.

The light has high temporal coherence.

The light is unidirectional and aligned.

The light is spatially coherent.

application of he –ne laser

1- He-Ne lasers are produced in large quantities from

many years.

2- Many schools / colleges / universities use this type

of laser in their science programs and experiments.

3- He-Ne lasers also used in super market checkout

counters to read bar codes and QR codes.

4- He-Ne lasers can be use as an alignment tool

5-It is also used in Guns for targeting.

ADVANTAGES OF HE –NE LASER

1- It is used in Guns for targeting.

2- He-Ne laser provide inherent safety due

to low power output

3- He-Ne laser tube has very small length

approximately from 10 to 100cm and best

life time of 20.000 hours.

DISADVANTAGE OF HE-NE LASER

1- It is relatively low power device means

its output power is low.

2- He-Ne laser is low gain system/ device

3- High voltage requirement can be

considered its disadvantage.

4- Escaping of gas from laser plasma

tube is also its disadvantage