full working of laser is explained here

1. AA
presentation onpresentation on
LASERLASER
Presented By
lk
B.Sc-II(2015-2016)
Guided By
?
(Dept. of Physics)

2. ContentsContents
 Definition of lasersDefinition of lasers
 Terminology -Coherence and photonTerminology -Coherence and photon
 Absorption of radiationAbsorption of radiation
 Self or spontaneous emissionSelf or spontaneous emission
 Stimulated emissionStimulated emission
 Condition required for Laser actionCondition required for Laser action
 3 state system3 state system
 Video clipVideo clip

3. Definition of laserDefinition of laser
 A laser is a device that generates light by a processA laser is a device that generates light by a process
calledcalled STIMULATED EMISSIONSTIMULATED EMISSION..
 The acronym LASER stands forThe acronym LASER stands for LLightight
AAmplification bymplification by SStimulatedtimulated EEmission ofmission of RRadiationadiation

4. Terminology -Coherence and photonTerminology -Coherence and photon
 Coherence:-Coherence:- Two waves of same frequencyTwo waves of same frequency
with a phase difference not changing withwith a phase difference not changing with
time is called coherence.time is called coherence.

5. Terminology continue..Terminology continue..
PHOTON:-PHOTON:-
 These are the form of Energy and having zeroThese are the form of Energy and having zero
rest mass.rest mass.
 In our topic photon is a light energy.In our topic photon is a light energy.

6. NOTE:-
Ground state is the minimum energy state.
Staying time limit for the atom is approx 10-10
second in E1
Atoms absorb photon of corresponding energy(E1-E0)
Any atom
Any photon of
energy(E1-E0)

7. AbsorptionAbsorption
E1
E0
(Excited state)
(Ground state)

8. Mathematical form of absorptionMathematical form of absorption
Rate of absorption, rc α Ν1 and
Rate of absorption, rc α u
=> rc = BΝ1u
Here B=Einstein’s absorption coefficient
Ν1= no. of atoms present in ground state
u= density of radiation

9. Self EmissionSelf Emission
Note:- After the end of 10 -10
second,atom come
back to E0 via emiting a photon whose energy is
equal to E1-E0 =∆E =hν = 10 J (say)
here h is Planck’s constant (6.6 X 10-34
J s)

10. Self EmissionSelf Emission
E1
E0

11. Self emission continue….Self emission continue….
Conclusion of Selfof Self emission
 Irregular nature and occurs at different
instants from different atoms.
 Light obtained is incoherent and in
different direction,means not in phase,
so it is not useful for LASER.

12. Mathematical form of self emissionMathematical form of self emission
Rate of self emissionself emission, ra α Ν2
=> ra = AΝ2
Here A=Einstein’s emission coefficient
Ν2= no. of atoms present in excited state E1

13. Recalling last topic , we found that energy difference b/w 2 states
is 10J,so if we incident photon of energy 10J then atom comes to
E0 very soon and emits exactly the same photon as we incident ,
means ,
frequency of photon incident (10J) = frequency of photon emitted(10J)

14. Stimulated EmissionStimulated Emission
E1
E0

15. Mathematical form of stimulated emissionMathematical form of stimulated emission
Rate of stimulated emissionstimulated emission, rb α Ν2 and
Rate of stimulated emissionstimulated emission, rb α u
=> rb = BΝ2u
Here B=Einstein’s coefficient of stimulatedstimulated
emissionemission
Ν2= no. of atoms present in excited energy stateE1
u= density of radiation

16. Stimulated emission continue…Stimulated emission continue…
Below figure shows a chain process for LASER
E1
E1

17. Comparisons of all 3 processComparisons of all 3 process
E1
E2
hυ
(a) Absorption
hυ
(b) Spontaneous emission
hυ
(c) Stimulated emission
In
hυ
Out
hυ
E2
E2
E1 E1
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

18. Condition required for Laser actionCondition required for Laser action
1.1. For laser action NFor laser action N22 > > N> > N1.1.
This is calledThis is called POPULATION INVERSIONPOPULATION INVERSION
2. Population inversion can be achieved by introducing2. Population inversion can be achieved by introducing metastablemetastable
state where atoms can gathered to achieve the situationstate where atoms can gathered to achieve the situation
where Nwhere N22 > > N> > N1.1.
3. The process of attaining a population inversion is called3. The process of attaining a population inversion is called opticaloptical
pumping.pumping. Means external energy source is required.(battery)
4. Radiation density should be high,for this a enclosed cavity made of
two parallel reflectors are used.
NOTE:-NOTE:-
 In metastable state self emission is forbidden.In metastable state self emission is forbidden.
 To create population inversion, at least 3-state system is required.To create population inversion, at least 3-state system is required.

19. 3 states system3 states system
E2
E1 (metastable)
E0
absorption
Self emission
Stimulated emission
Population inversion
Non radiative

20. Video clipVideo clip

21. Video clipVideo clip

22. THANK YOUTHANK YOU