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LASERS
LASER is an acronym for light amplification by Stimulated Emission of radiation.
When radiation interacts with matter we have three processes to generate laser light.
(1) Optical Absorption(2) Spontaneous Emission(3) Stimulated Emission
Characteristics of Lasers
The most important characteristics of lasers are
1.Directionality → Angular spread 10 micro radians
2.Monochromacity
3.Coherence → (a). Spatial coherence
(b). Temporal coherence
4.Intensity:
The number of photons coming out from a laser per second per unit area is given by
2rh
Pn
Spontaneous Emission
• Incoherent • Less intensity• Poly chromatic• Less directionality• More angular spread
Stimulated emission
• Coherent • High intensity• Mono chromatic• High directionality• Less angular spread
absorption
Spontaneous
emission
Stimulated emission
E1
E2
N1
N2
)(Supplied energy
Einstein Co-efficient• Consider two energy levels E1 and E2.
• N1 and N2 be the number of atoms per unit volume present at the levels E1 and E2 respectively.
radiation. ginteractin ofdensity energy (v) & ,absorption stimulated of
t coefficienEinstien theis Bality proportion ofconstant theWhere
)(B
(v) & N toalproportion is rate absorption Stimulated
:Absorption (1)
12
112
1
vN
emission. sspontaneou of
tcoefficienEinstien theisAality proportion ofconstant the Where
)2(A
N toalproportion is rateemission sSpontaneou
:Emission sSpontaneou (2)
21
221
2
N
emission. stimulated oft coefficienEinstien theisB
ality proportion ofconstant thee Wher
)3...().........(B
)(&N toalproportion is rateemission Stimulated
:Emission Stimulated (3)
21
221
2
vN
v
For system in a equilibrium
1)(
)(
)(
)()(
)()(
21
12
2
1
21
21
21122
1
21
212121
212
212212121
212212121
BB
NNBA
v
BBNN
Av
BNBN
ANv
ANBvNBvN
ANBvNBvN
)6.....(}.........1)
kTh
exp(
1{
8(v)
bygiven isdensity radation the
radiation,body black of law splanck' from
)5.....(..........1)exp(
)(
.E &E levelsenergy of s'degeneracy are g & g Where
)exp(
lawon distributiman Bolts toAccording
3
3
2
1
21
12
21
21
2121
2
1
2
1
c
h
kTh
gg
BB
BA
v
kT
h
g
g
N
N
Comparing equations (5) & (6)
)8........(8
)7........(
1
3
3
21
21
212121
2
1
21
12
c
h
B
A
BgBg
g
g
B
B
1)kT
hexp(R
}1)
kTh
exp(
1{
8
)8
(
)(
)8
(R
8
)(
)(
3
3
3
3
3
3
21
21
212
212
ch
ch
vch
eqfrom
Bv
AR
BvN
ANR
n
The equation shows ratio of spontaneous
Emission Rate to stimulated emission rate.
Population inversion To achieve more stimulated emission population of the
excited state N2 should be made larger than the lower state N1 and this condition is called population inversion.
E1
E2
N1
N2
12 NN
Three level Laser system
N
E
E0
E1
E2 Fast decay
Laser transition
pumping
Meta stable level
Four level laser system
N
E
E0
E1
E2
Fast decay
Laser transitionpumping
E3
Meta stable state
Ruby LASER
1. Maiman in 1960.2. Solid State Laser.3. Active Medium: Al2O3 doped with 0.05% Cr3+
ions(10cm long & 0.5cm diameter).4. Resonant Cavity: Fully reflecting surface &
partially reflecting surface.5. Pumping System: Helical Xenon flash lamp.6. Three level laser system.7. Wave Length: 694.3nm.8. Pulsed Laser.9. Widely used in Echo technique & Pulsed
Holography
Partially reflecting
surface
Completely
Reflecting
surface
cooling
Xenon flash lamp
Ruby material
Partially reflecting
surface
Completely
Reflecting
surface
cooling
Xenon flash lamp
Ruby material
Laser
output
N
E
E0
E1
E2 Fast decay
Laser transition
pumping
Meta stable level
Energy level diagram of Ruby laser
Short lived state
He-Ne LASER
1.Ali Javan in 1961.2.Gas Laser..3.Active Medium: Helium & Neon Mixture
10:1 ratio...at 0.1mm of Hg.4.Resonat Cavity: Fully & partially reflected
surfaces…5.Pumping System: Discharge electrodes…6.Four level Laser System.7.Wave Length:632.8nm.8.Red color Continuous Laser.9.Widely used in Interferometer Experiments &
Holography.
Discharge
electrodes
He + Ne (10:1)
0.1mm of Hg
Discharge tube
Fully reflecting
mirrorPartially reflecting
mirror
Laser out put
N
E
E0
E1
E2
Fast decay
Laser transitionpumping
E3
Meta stable state
NeHe
Energy level diagram of He-Ne laser
CO2 LASER
1.CKN Patel in 1963.2.Gas Laser..3.Active Medium: CO2 , N2 & helium mixture
1:4:5 ratio...at 0.1mm of Hg.4.Resonat Cavity: Fully & partially
reflected surfaces…5.Pumping system: Discharge electrodes…6.Large no of energy levels are contributes
for out put laser radiation..7.Wave Length:10.6micro meters.8.Pulsed & Continuous Laser.9.Widely used in Material processing such as
Cutting , Drilling, Welding.
Fully reflected
surface
Partially reflected
surface
Cooling
Discharge electrodes
Co2+N2+He
1:4:5
Out put
laser
Energy level diagram of co2 laser
He N2
E
pumping
Fast decay
Laser
transition
collisions collisions
co2
Vibrational modes of the CO2 molecule
Oxygen OxygenCarbon
Symmetric
mode
Bending
modes
Asymmetric
modes
Semiconductor Laser
1.Semiconductor Laser is also called as Diode Laser.
2.The wave length of the emitted light depends upon the Energy band gap of the material.
3.Diode Lasers are always operated in forward bias..4.Working Principle: When we apply forward bias
to a PN-Diode, charge carrier recombination takes place.. Then in such a process the energy is emitted in the form of light radiation..
5.Active Medium: GaAlAs diode or GaAsP diode..6.Out put Wave length: GaAlAs:750-900nm,.GaAsP:1100-1600nm..7.Pulsed & Continuous Laser…
g
g
g
E
hc
hcE
hE
The Energy band gap of a material
Where c is the velocity of light & h is Planck's constant.
P
NActive
region
Positive
Metal contact
Negative
Metal contact
Forward
biasing