basic concept of laser,population inversion,absorption,stimulated emission,spontaneous emission,laser drawbacks

1. LASER
SUBJECT HANDLER
V.REVATHIAMBIKA
LECTURER IN PHYSICS

2. INTRODUCTION OF LASER
 L – LIGHT
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 A – AMPLIFICATION
 S – STIMULATED
 E – EMISSION
 R - REDIATION
 A. L. SCHAWLOW and C. H. TOWNES IN 1958
 RUBY LASER by T. H. MAIMANN IN 1960 2

3. DEFINITION OF
LASER
A laser is a device that generates light by a
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process called STIMULATED EMISSION.
 The acronym LASER stands for Light
Amplification by Stimulated Emission of
Radiation
 Semiconducting lasers are multilayer
semiconductor devices that generates a coherent
beam of monochromatic light by laser action. A
coherent beam resulted which all of the photons
are in phase.
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4. THE OPERATION OF THE
LASER
 In1958, Charles Townes and Arthur Schawlow
theorized about a visible laser, an invention that
would use infrared and/or visible spectrum light.
 Light
Amplification by Stimulated Emission of
Radiation- (LASER).

5. BASIC IDEA
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 Consider a group of atoms exposed stream of photons,
each with energy hυ. Let us assume two energy levels E1
and E2 of an atom.
 During transition from one energy state to another, the
light is absorbed (or) emitted by particles. Under this
action, 3 processes can occur.
 They are,
 Stimulated absorption
 Spontaneous emission
 Stimulated emission
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6. ABSORPTION
 Spontaneous event in
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which an atom or molecule
absorbs a photon from an
incident optical field
 The asborption of the
photon causes the atom or
molecule to transition to
an excited state
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7. ABSORPTION
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E1
E2
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8. SPONTANEOUS EMISSION
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 Statistical process (random phase) – emission by an
isolated atom or molecule
 Emission into 4π steradians 8

9. SPONTANEOUS
EMISSION
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10. STIMULATED EMISSION
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 Same phase as “stimulating” optical field
 Same polarization
 Same direction of propagation 10

11. STIMULATED EMISSION
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12. STIMULATED EMISSION
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13. ABSORPTION, SPONTANEOUS
EMISSION & STIMULATED EMISSION
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E2 E2 E2
hυ hυ
hυ h υ In
Out
hυ
E1 E1 E1
(a) Absorption (b) Spontaneous emission (c) Stimulated emission
Absorption, spontaneous (random photon) emission and stimulated
emission.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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14. ABSORPTION
Light that falls on a piece of material will decrease
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
exponentially.
 α = (N -N )B (hf) n/c
1 2 21
 N is often more than N (N < N
1 2 1 2)
 Example for tungsten
α is typically 106m-1 (+ve)
 If we want implication, α must be –ve
 i.e. N > N
2 1
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15. PUTTING IT ALL TOGETHER…
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 Assume that we have a two state system in equilibrium
with a blackbody radiation field.
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16. Two level system
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E2 E2
hν hν
hν
hν =E2-E1
E1 E1
absorption Spontaneous Stimulated
emission
emission
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17. Boltzmann’s equation
E2
n2  −( E2 − E1 )  E1
= exp  ÷
n1  kT 
example: T=3000 K E2-E1=2.0 eV
• n1 - the number of electrons of energy E1
n2
• n2 - the number of electrons of energy E2 = 4.4 × 10−4
n1

18. Einstein’s coefficients
E2
Probability of stimulated absorption R1-2
R1-2 = ρ (ν) B1-2 E1
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Probability of stimulated and spontaneous emission :
R2-1 = ρ (ν) B2-1 + A2-1
assumption: n1 atoms of energy ε 1 and n2 atoms of energy ε 2 are in thermal equilibrium at
temperature T with the radiation of spectral density ρ (ν):
n1 R1-2 = n2 R2-1 n1ρ (ν) B1-2 = n2 (ρ (ν) B2-1 + A2-1)
⇒
A2−1 / B2 −1
ρ (ν ) = 18
n1 B1− 2
−1
n2 B2−1

19. n1
According to Boltzman statistics: = exp( E2 − E1 ) / kT = exp( hν / kT )
n2
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A2 −1 / B2 −1 8πhν 3 / c 3
ρ (ν) = B1− 2 hν =
exp( ) − 1 exp(hν / kT ) − 1
B2 −1 kT
Planck’s law
B1-2/B2-1 = 1 A2 −1 8πhν 3
=
B2−1 c3 19

20. The probability of spontaneous emission A 2-1 /the probability of stimulated
emission B2-1ρ(ν ):
A2 −1
= exp(hν / kT ) − 1
B2 −1ρ (ν )
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1. Visible photons, energy: 1.6eV – 3.1eV.
2. kT at 300K ~ 0.025eV.
3. stimulated emission dominates solely when hν /kT <<1!
(for microwaves: hν <0.0015eV)
The frequency of emission acts to the absorption:
n A + n B ρ (ν ) A2 −1 n2 n2
x = 2 2 −1 2 2 −1 = [1 + ] ≈
n1B1− 2 ρ (ν ) B2 −1ρ (ν ) n1 n1
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if hν /kT <<1. x~ n2/n1

21. DIFF B/W SPONTANEOUS & STIMULATED EMISSION
S.NO SPONTANEOUS EMISSION STIMULATED EMISSION
1. The atom in the excited state returns to An atom in the excited state is induced
ground state thereby emitting a photon to return to ground state thereby
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,without any external inducement . resulting in two photons of same
frequency and energy
2. The emitted photons can move randomly The emitted photon move in same
direction and is highly directional
3. The photons are not in phase The photons are in phase
4. The rate of transition is given by The rate of transition is given by
R sp = A21 N2 R st = B21 N2 ρ
5. Incoherent radiation Coherent radiation
6. Having more angular spread during Having less angular spread during
propagation propagation 21
Ex: light from sodium (or) mercury vapour Ex: light from laser source
lamp

22. POPULATION INVERSION
 Therefore we must have a mechanism where N2 > N1
 This is called POPULATION INVERSION
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 Population inversion can be created by introducing a so call metastable centre
where electrons can piled up to achieve a situation where more N 2 than N1
 The process of attaining a population inversion is called pumping and the
objective is to obtain a non-thermal equilibrium.
 It is not possible to achieve population inversion with a 2-state system.
 If the radiation flux is made very large the probability of stimulated emission
and absorption can be made far exceed the rate of spontaneous emission.
 But in 2-state system, the best we can get is N = N .
1 2
 To create population inversion, a 3-state system is required.
 The system is pumped with radiation of energy E then atoms in state 3 relax
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to state 2 non radiatively.
 The electrons from E will now jump to E to give out radiation. 22
2 1

23. Condition for the laser operation E2
E1
If n1 > n2
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• radiation is mostly absorbed absorbowane
• spontaneous radiation dominates.
if n2 >> n1 - population inversion
• most atoms occupy level E2, weak absorption
• stimulated emission prevails
• light is amplified
Necessary condition:
population inversion
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24. How to realize the population inversion?
Thermal excitation: E2
n2  −∆E 
= exp  ÷
n1  kT  E1
impossible.
The system has to be „pumped”
Optically,
electrically.

25. The laser operation
Three level laser
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E3
Fast transition
E2
Laser action
E1
• 1→3 pumping
• spontaneous emission 3 →2.
• state 2 is a metastable state
• population inversion between states 2 and 1.
• stimulated emission between 2 i 1.
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26. E3
szybkie przejścia
The laser operation
E2
akcja laserowa
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E1
- optical pumping - occupation of E3 of a short life time,
10-8s. It is a band, the metastable and ground states are narrow :
∆ε∆t ≥ 
- electrons are collected on E2: population inversion
- stimulated emission (one photon emitted spontaneously starts the
stimulated radiation )
- Beam of photons moves normally to the mirrors – standing wave.
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27. POPULATION INVERSION
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When a sizable population of electrons resides in upper levels, this
condition is called a "population inversion", and it sets the stage for
stimulated emission of multiple photons. This is the precondition for
the light amplification which occurs in a LASER and since the emitted
photons have a definite time and phase relation to each other, the light
has a high degree of coherence.
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