Transcript Document

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
P
n
hr 2
Spontaneous Emission Stimulated emission
•
•
•
•
•
Incoherent
Less intensity
Poly chromatic
Less directionality
More angular spread
•
•
•
•
•
Coherent
High intensity
Mono chromatic
High directionality
Less angular spread
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.
N2
Spontaneous
emission
E2
Stimulated
emission
absorption
N1
E1
 ( ) Supplied energy
(1) Absorption:
Stimulatedabsorptionrateis proportion
al to N1 &  (v)
 B12 N1 (v)
Wheretheconstantof proportion
ality B12 is theEinstiencoefficient
of stimulatedabsorption, &  (v) energydensityof interacting radiation.
(2) Spontaneous Emission:
Spontaneous emission rateis proport ion
al to N 2
 A 21 N 2      (2)
Wheretheconstantof proport ion
ality A 21is theEinstiencoefficient
of spont aneou
s emission.
(3) StimulatedEmission:
Stimulatedemission rateis proport ion
al to N 2 &  (v)
 B21 N 2  (v)............(3)
Where theconstantof proport ion
ality
B21is theEinstiencoefficient of st imulatedemission.
For system in a equilibrium
N1  (v ) B12  N 2  (v ) B21  N 2 A21
N1  (v ) B12  N 2  (v ) B21  N 2 A21
N 2 A21
 (v ) 
N1 B12  N 2 B21
 (v ) 
A21
N1
B12  B21
N2
A21
B21
 (v ) 
N1 B12
1
N 2 B21
Accordingt o Bolt sman dist ribut ion law
N1 g1
h

exp( )
N2 g2
kT
Whereg1 & g 2 are degeneracy' s of energy levelsE1 & E 2 .
 (v ) 
A21
B21
B12 g1
h
exp( )  1
B21 g 2
kT
...............(5)
from planck's law of black body radiat ion,
t he radat iondensit yis given by
8h 3
1
 (v) 
{
}..............(6)
3
h
c
exp( )  1
kT
Comparing equations (5) & (6)
B12 g1
1
B21 g 2
g1 B12  g 2 B21 ........(7 )
A21 8h 3

........(8)
3
B21
c
The equation shows ratio of spontaneous
Emission Rate to stimulated emission rate.
N 2 A21
R 
N 2  (v ) B21
R 
A21
 (v ) B21
fro m
eq n 8
8h 3
8h 3
(
)
(
)
3
3
c
c
R 

3
8h
1
 (v )
{
}
3
h

c
ex p(
) 1
kT
h
R  ex p(
) 1
kT
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.
N2
E2
N 2  N1
N1
E1
Three level Laser system
E
E2
E1
Fast decay
Meta stable level
pumping
Laser transition
E0
N
Four level laser system
E
E3
E2
pumping
Fast decay
Meta stable state
Laser transition
E1
E0
N
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
Completely
Reflecting
surface
Xenon flash lamp
Partially reflecting
surface
Laser
output
Ruby material
cooling
Energy level diagram of Ruby laser
E
E2
E1
Short lived state
Fast decay
Meta stable level
pumping
Laser transition
E0
N
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.
Fully reflecting
mirror
Partially reflecting
mirror
Discharge tube
He + Ne (10:1)
0.1mm of Hg
Laser out put
Discharge
electrodes
Energy level diagram of He-Ne laser
E
E3
E2
Fast decay
Meta stable state
pumping
Laser transition
E1
E0
He
N
Ne
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.
Cooling
Partially reflected
surface
Fully reflected
surface
Co2+N2+He
1:4:5
Out put
laser
Discharge electrodes
Energy level diagram of co2 laser
E
Fast decay
Laser
transition
collisions
collisions
pumping
He
N2
co2
Vibrational modes of the CO2 molecule
Oxygen
Carbon
Oxygen
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…
The Energy band gap of a material
E g  h
Eg 
hc

hc
 
Eg
Where c is the velocity of light & h is Planck's constant.
Positive
Metal contact
P
Active
region
Negative
Metal contact
N
Forward
biasing