FOUR LEVEL LASER, physics ppt on laser, four level

Four-level lasers
A fast transition from E4 to Eu populates the upper state of the laser transition. A fast transition from El to E3 then depopulates the lower level of the laser transition, maintaining a population inversion between Eu and El until E4 is no longer able to populate Eu.

1. FOURLEVELLASER
SOUSAN
A274153121006
HG&MM

2. INTRODUCTION
➢Light amplification by stimulated emission of radiation
➢Adevice that generates an intense beam of coherent
monochromatic light (or other electromagnetic radiation) by
stimulated emission of photons from excited atoms or
molecules.

3. POPULATIONINVERSION
➢Population inversion is the state of a system in which the population
of a particular higher energy state is more than that of a specified
lower energy state.
➢A system in which population inversion is achieved is called an active
system. The state of the system in which population inversion is
achieved is called the negative temperature state.

4. ➢Based on the mechanism in which population
inversion is achieved:
1.Three-level laser
2. four-level laser

5. FOUR LEVEL LASER
➢Four energylevelenergies
➢E1, E2, E3, E4
➢Populations
➢N1,N2, N3, N4
➢Theenergiesare:
➢E1 < E2 < E3 < E4
➢ThelevelE3andE2 are intermediate levels
➢E3= Upper metastable, E2 = Lower metastable

7. ➢TheatomsarepumpedfromgroundlevelE1toexcitedstateE4
➢Theatomsdecaybyafast,non-radiatetransition(Ra)tolevelE3
➢Atthisstage,levelE3isvirtuallyvacant
➢ThelifetimeoflasertransitionfromE3toE2islongincomparison
toRa
➢PopulationinversionbetweenlevelE3andlevelE2isestablishedas
soonasthepumpenergyisappliedtothesystem
➢Lasergainisrealizedasthespontaneousphotonofenergyinitiate
chainofstimulatedemission
➢AtomsofE2losetheirexcessenergybymostlynon-radiative
transition(Rb)tolevelE1

9. Differences between 3 Level laser and Four
level laser

10. EXAMPLESOFFOURLEVELLASER
➢Helium Neon gas laser
➢Electric pumping excites helium atoms to an excited state
whose energy is roughly same as upper short lived state in the
neon atom
➢The solepurposeoftheheliumatomsistoexchangeenergy
withneonatomsviacollisionalexcitation

11. ADVANTAGES OF FOUR LEVEL LASERS
➢The lasing threshold of a four-level laser is lower
➢The efficiencyishigher
➢Required pumping rate is lower
➢Continuous operation ispossible

12. THANK YOU