Unit 6

UNIT INDEX

S. No. Module Lectur PPT
e No. Slide
No.
1 Introduction, characteristics L 1 4--5
of lasers.
2 Spontaneous & Stimulated L 2-3 6-14
emission of Radiation
Population Inversion.
3. Types of Lasers L 4-5 15-34

4. Applications of lasers L6 35-40

2

APPLIED PHYSICS
CODE : 07A1BS05
I B.TECH
CSE, IT, ECE & EEE
UNIT-6
NO. OF SLIDES :40

3

Lecture-1
INTRODUCTION
 The word “LASER” is an acronym for Light
Amplification by Stimulated Emission of
Radiation.

4

Characteristics
 Monochromacity
 High Intensity
 Coherence
 Directionality

5

Lecture-2
Types of coherence
 Temporal coherence
 Spatial coherence
 Temporal coherence measures the continuity of
a wave along its length.
 Spatial coherence measures the maximum
seperation between any two points on the cross
section of the wavefront which maintain
correlation between them.

6

Stimulated Absorption
 Excitation of atoms from lower energy
state to higher energy state due to
interaction of radiation with matter is
known as Stimulated absorption.

7

Spontaneous emission
 When an electron in the excited level E2
falls spontaneously to lower energy level
E1 after its lifetime a photon is emitted.
The energy of the emitted photon is given
by E2-E1=h

8

Stimulated emission
 When an electron in the excited level E2 is
induced (stimulated) by a photon of
energy (E2-E1), the electron moves to
lower energy level E1 emitting another
photon of energy E2-E1. This process is
called stimulated emission.

9

 Both stimulated and stimulating
photons are in phase with each other.
 Stimulated emission of radiation

(light) results in amplification of light

10

Lecture-3
Population inversion
 For light amplification by stimulated
emission of radiation the population of
excited state must be greater than the
population of lower energy state. This
condition is called population inversion.

11

Pumping mechanisms
 The process of sending atoms from lower
energy state to higher energy state is
called Pumping.
 Optical pumping
 Electric discharge
 Chemical reaction
 Injection current through p-n junction

12

Optical Feed back
 To direct the amplified light to travel back
and forth through the active medium many
times two end mirrors are kept at both the
ends of the laser. These mirrors provide
necessary optical feed back.

13

Threshold inversion density
 Only if the population inversion density is
sufficiently large so that the loss is
compensated by the gain, lasing action
starts. The inversion density for which the
gain is just sufficient to compensate for the
loss is called threshold inversion density.

14

Lecture-4

Conditions for Lasing
For laser action to take place, the three
requisites are
 Suitable active medium

 Creation of population inversion

 Proper optical feed back

15

L
L
e
e
c
RUBY LASER
c
t
t

u A ruby laser is a solid-state laser
u
r
r
e
e
-
-
2
 It uses a synthetic ruby crystal as its
2

gain medium.
 It was the first type of laser invented,
and was first operated by
Theodore H. "Ted" Maiman at
Hughes Research Laboratories on
1960.
16

 The ruby laser produces pulses of visible
light at a wavelength of 694.3 nm, which
appears as deep red to human eyes.
 Typical ruby laser pulse lengths are on
the order of a millisecond. These short
pulses of red light are visible to the human
eye, if the viewer carefully watches the
target area where the pulse will fire.

17

Applications

 Ruby lasers have declined in use with the
discovery of better lasing media. They are
still used in a number of applications
where short pulses of red light are
required.
 Holographers around the world produce
holographic portraits with ruby lasers, in
sizes up to a metre squared.
 The red 694 nm laser light is preferred to
the 532 nm green light of
frequency-doubled Nd:YAG. 19

 Many non-destructive testing labs use
ruby lasers to create holograms of large
objects such as aircraft tires to look for
weaknesses in the lining. Ruby lasers
were used extensively in tattoo and
hair removal, but are being replaced by
alexandrite lasers and Nd:YAG lasers in
this application.

20

Helium-neon laser

21

He –Ne Laser
 A helium-neon laser , usually called a HeNe
laser , is a type of small gas laser.
 HeNe lasers have many industrial and scientific
uses, and are often used in laboratory
demonstrations of optics.
 Its usual operation wavelength is 632.8 nm, in
the red portion of the visible spect
 The gain medium of the laser, as suggested by
its name, is a mixture of helium and neon gases,
in a 5:1 to 20:1 ratio, contained at low pressure
(an average 50 Pa per cm of cavity length ) in a
glass envelope.
22

He-Ne Laser

 The energy or pump source of the laser is
provided by an electrical discharge of around
1000 volts through an anode and cathode at
each end of the glass tube.
 A current of 5 to 100 mA is typical for CW
operation.
 The optical cavity of the laser typically consists
of a plane, high-reflecting mirror at one end of
the laser tube, and a concave output coupler
mirror of approximately 1% transmission at the
other end.
23

He-Ne Laser
 HeNe lasers are typically small, with
cavity lengths of around 15 cm up to
0.5 m, and optical output powers
ranging from 1 mW to 100 mW.
 The red HeNe laser wavelength is
usually reported as 632nm. However

24

 The true wavelength in air is 632.816 nm, so
633nm is actually closer to the true value.
 For the purposes of calculating the photon
energy, the vacuum wavelength of 632.991 nm
should be used. The precise operating
wavelength lies within about 0.002 nm of this
value, and fluctuates within this range due to
thermal expansion of the cavithy.

25

 The laser process in a HeNe laser starts with
collision of electrons from the electrical
discharge with the helium atoms in the gas.
 This excites helium from the ground state to the
23S1 and 21S0 long-lived, metastable excited
states. Collision of the excited helium atoms with
the ground-state neon atoms results in transfer
of energy to the neon atoms, exciting neon
electrons into the 3s2 level. This is due to a
coincidence of energy levels between the helium
and neon atoms.
26

 This process is given by the reaction
equation:
 He(21S)* + Ne + ΔE → He(11S) + Ne3s2*
 ΔE is the small energy difference between
the energy states of the two atoms, of the
order of 0.05 eV or 387 cm-1, which is
supplied by kinetic energy.

27

.

 The number of neon atoms entering the
excited states builds up as further
collisions between helium and neon atoms
occur, causing a population inversion.
Spontaneous and stimulated emission
between the 3s2 and 2p4 states results in
emission of 632.82 nm wavelength light,
the typical operating wavelength of a
HeNe laser.
28

.

 After this, fast radiative decay occurs from the 2p to the
1s ground state. Because the neon upper level saturates
with higher current and the lower level varies linearly with
current, the HeNe laser is restricted to low power
operation to maintain population inversion.
 Spectrum of a helium neon laser showing
 With the correct selection of cavity mirrors, other
wavelengths of laser emission of the HeNe laser are
possible. There are infrared transitions at 3.39 μm and
1.15 μm wavelengths, and a variety of visible transitions,
including a green (543.5 nm, the so-called GreeNe
laser), a yellow (594 nm) and an orange (612 nm)
transition.

29

SEMICONDUCTOR LASER
Lecture-5
L
e
c A semiconductor laser converts electrical energy

t
u into light. This is made possible by using a
r semiconductor material, whose ability to conduct
e
- electricity is between that of conductors and
2 insulators.
insulators.
 By doping a semiconductor with specific
amounts of impurities, the number of negatively
charged electrons or positively charged holes
can be changed.
 Compared to other laser types, semiconductor
lasers are compact, reliable and last a long time.

30

SEMICONDUCTOR LASER

 Such lasers consist of two basic components, an
optical amplifier and a resonator. The amplifier is
made from a direct-bandgap semiconductor
material based on either gallium arsenide
(GaAs) or InP substrates.

 These are compounds based on the Group III
and Group V elements in the periodic table.
Alloys of these materials are formed onto the
substrates as layered structures containing
precise amounts of other materials.
31

L
e
Semiconductor laser
c
t
u
r
e
-
2

32

HEAVILY DOPED p-n JUNCTION DIODE
(a)In equilibrium. (b)With forward bias.

33

DOUBLE HETEROJUNCTION SC LASER

34

L Raw Materials
e
c
t
u
 The conventional semiconductor laser consists of
r
e
a- compound semiconductor, gallium arsenide.
This material comes in the form of ingots that are
2
then further processed into substrates to which
layers of other materials are added. The materials
used to form these layers are precisely weighed
according to a specific formula.
 Other materials that are used to make this type of
laser include certain metals (zinc, gold, and
copper) as additives (dopants) or electrodes, and
silicon dioxide as an insulator.
35

Lecture-6

APPLICATIONS OF LASERS
 Lasers are uused in local area network to
transfer the data from the memory storage
of one computer to other computer.
 These are used to store large amount of
data in CD-ROM

36

 Lasers can be used to blast holes in
diamonds and hard steel.
 They are used as a source of intense
heat.
 They are used to cut, drill, weld, and to
remove metal from surfaces.

37

 These are used in spacecrafts and
submarines.
 They are also used in high speed
photocopiers and printers.
 They are used in the field of 3-d
photography.

38

 Lasers can serve as a war weapon.
 High energy lasers are used to destroy
enemy aircrafts and missiles.
 These are used to produce certain
chemical reactions.

39

 Lasers are used in controlling
haemorrhage.
 Lasers are used for elimination of

moles and tumors.
 Lasers are used in the treatment of

glaucoma.

40

 Co2 laser is used in spinal and brain
tumor and kidney stone extrusion.
 Lasers are used to correct a condition

called retina detachments by eye
specialist.
 Argon and Co lasers are used in the
2
treatment of liver and lungs.
41

Unit 6

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