Laser,Optical Fibres and Ultrasonics

1. 1. DEFINITION OF LASER
A laser is a device that generates light by a
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.

2. 3 MECHANISMS OF LIGHT EMISSION
1. Absorption
2. Spontaneous Emission
3. Stimulated Emission
Therefore 3 process of
light emission:

3. BEFORE AFTER
(i) Stimulated absorption
) Spontaneous emission
Stimulated emission

4. )II) SPONTANEOUS EMISSION
Consider an atom (or molecule) of the material is existed
initially in an excited state E2 No external radiation is
required to initiate the emission. Since E2>E1, the atom will
tend to spontaneously decay to the ground state E1, a
photon of energy h =E2-E1 is released in a random direction
as shown in (Fig. 1-ii). This process is called “spontaneous
emission ”
Note that; when the release energy difference (E2-E1) is
delivered in the form of an e.m wave, the process called
"radiative emission" which is one of the two possible ways
“non-radiative” decay is occurred when the energy
difference (E2-E1) is delivered in some form other than e.m
radiation (e.g. it may transfer to kinetic energy of the
surrounding)

5. (III) STIMULATED EMISSION
Quite by contrast “stimulated emission” (Fig. 1-iii)
requires the presence of external radiation when an
incident photon of energy h =E2-E1 passes by an atom in
an excited state E2, it stimulates the atom to drop or
decay to the lower state E1. In this process, the atom
releases a photon of the same energy, direction, phase
and polarization as that of the photon passing by, the net
effect is two identical photons (2h) in the place of one,
or an increase in the intensity of the incident beam. It is
precisely this processes of stimulated emission that
makes possible the amplification of light in lasers.

6. ND (NEODYMIUM) – YAG (YTTRIUM ALUMINIUM GARNET)
LASER
PRINCIPLE CHARACTERISTICS
Doped Insulator laser
refers to yttrium
aluminium garnet doped
with neodymium. The
Nd ion has many
energy levels and due
to optical pumping
these ions are raised to
excited levels. During
the transition from the
metastable state to E1,
the laser beam of
wavelength 1.064μm is
emitted
Type : Doped Insulator Laser
Active Medium : Yttrium Aluminium Garnet
Active Centre : Neodymium
Pumping
Method
: Optical Pumping
Pumping
Source
: Xenon Flash Pump
Optical
Resonator
: Ends of rods silver coated
Two mirrors partially and
totally reflecting
Power Output : 20 kWatts
Nature of
Output
: Pulsed
Wavelength
Emitted
: 1.064 μm

7. ND (NEODYMIUM) – YAG (YTTRIUM ALUMINIUM
GARNET) LASER
Capacitor
Power Supply
Resistor
Laser Rod
Flash Tube
M1– 100%
reflector mirror
M2 – partial
reflector mirror

8. Energy Level Diagram of Nd– YAG LASER
E1, E2, E3 – ENERGY LEVELS OF ND
E4 – META STABLE STATE
E0 – GROUND STATE ENERGY LEVEL
Non radiative decay
E4
E1
APPLICATIONS
TRANSMISSION OF SIGNALS OVER LARGE DISTANCES
LONG HAUL COMMUNICATION SYSTEM
ENDOSCOPIC APPLICATIONS
REMAOTE SENSING
Laser
1.064μm
Non radiative decay
E3
E2
E0

9. CARBON DI OXIDE LASER
PRINCIPLE
THE TRANSITION BETWEEN THE ROTATIONAL AND VIBRATIONAL ENERGY LEVELS LENDS TO
THE CONSTRUCTION OF A MOLECULAR GAS LASER. NITROGEN ATOMS ARE RAISED TO
THE EXCITED STATE WHICH IN TURN DELIVER ENERGY TO THE CO2 ATOMS WHOSE
ENERGY LEVELS ARE CLOSE TO IT. TRANSITION TAKES PLACE BETWEEN THE ENERGY
LEVELS OF CO2 ATOMS AND THE LASER BEAM IS EMITTED.
Type : Molecular gas laser
Active Medium : Mixture of CO2, N2, He or H2O vapour
Active Centre : CO2
Pumping Method : Electric Discharge Method
Optical Resonator : Gold mirror or Si mirror coated with Al
Power Output : 10 kW
Nature of Output : Continuous or pulsed
Wavelength Emitted : 9.6 μm or 10.6 μm

10. FIBER OPTICS TECHNOLOGY

11. OPTICAL FIBER: ADVANTAGES
 Capacity: much wider bandwidth
(10 GHz)
 Crosstalk immunity
 Immunity to static interference
 Lightening
 Electric motor
 Florescent light
 Higher environment immunity
 Weather, temperature, etc.

12. OPTICAL FIBER: ADVANTAGES
 Safety: Fiber is non-metalic
 No explosion, no chock
 Longer lasting
 Security: tapping is difficult
 Economics: Fewer repeaters
 Low transmission loss (dB/km)
 Fewer repeaters
 Less cable
Remember: Fiber is non-conductive
Hence, change of magnetic field has
No impact!

13. DISADVANTAGES
 Higher initial cost in installation
 Interfacing cost
 Strength
 Lower tensile strength
 Remote electric power
 More expensive to repair/maintain
 Tools: Specialized and sophisticated

14. OPTICAL FIBER ARCHITECTURE
TX, RX, and Fiber Link
Transmitter
Input
Signal
Coder or
Converter
Light
Source
Source-to-Fiber
Interface
Fiber-to-light
Interface
Light
Detector
Amplifier/Shaper
Decoder
Output
Fiber-optic Cable
Receiver

15. OPTICAL FIBER ARCHITECTURE –
COMPONENTS
 Light source:
 Amount of light emitted is
proportional to the drive
current
 Two common types:
 LED (Light Emitting
Diode)
 ILD (Injection Laser
Diode)
 Source–to-fiber-coupler
(similar to a lens):
 A mechanical interface to
couple the light emitted by
the source into the optical
fiber
Input
Signal
Coder or
Converter
Light
Source
Source-to-Fiber
Interface
Fiber-to-light
Interface
Light
Detector
Amplifier/Shaper
Decoder
Output
Fiber-optic Cable
Receiver
 Light detector:
 PIN (p-type-intrinsic-n-type)
 APD (avalanche photo diode)
 Both convert light energy into
current

16. OPTICAL FIBER CONSTRUCTION
 Core – thin glass center of the
fiber where light travels.
 Cladding – outer optical
material surrounding the core
 Buffer Coating – plastic
coating that protects
the fiber.

17. FIBER TYPES
 Plastic core and cladding
 Glass core with plastic cladding PCS (Plastic-
Clad Silicon)
 Glass core and glass cladding SCS: Silica-clad
silica
 Under research: non silicate: Zinc-chloride
 1000 time as efficient as glass
Core Cladding

18. PLASTIC FIBER
 Used for short distances
 Higher attenuation, but easy to install
 Better withstand stress
 Less expensive
 60% less weight

19. A LITTLE ABOUT LIGHT
 When electrons are excited and
moved to a higher energy state they
absorb energy
 When electrons are moved to a
lower energy state  loose energy
 emit light
 photon of light is generated
 Energy (joule) = h.f
 Planck’s constant: h=6.625E-23
Joule.sec
 f is the frequency
http://www.student.nada.kth.se/~f93-jhu/phys_sim/compton/Compton.htm
DE=h.f

20. REFRACTION
 Refraction is the change in direction of a
wave due to a change in its speed
 Refraction of light is the most commonly seen
example
 Any type of wave can refract when it
interacts with a medium
 Refraction is described by Snell's law, which
states that the angle of incidence is related to
the angle of refraction by :
 The index of refraction is defined as the
speed of light in vacuum divided by the speed
of light in the medium: n=c/v

21. FIBER TYPES
 Modes of operation (the path which the light is
traveling on)
 Index profile
 Step
 Graded

22. TYPES OF OPTICAL FIBER
Single-mode step-index Fiber
Multimode step-index Fiber
Multimode graded-index Fiber
n1 core
n2 cladding
no air
n1 core
n2 cladding
Variable
n
no air
Light
ray
Index profile

23. SINGLE-MODE STEP-INDEX FIBER
Advantages:
 Minimum dispersion: all rays take same path, same time to travel
down the cable. A pulse can be reproduced at the receiver very
accurately.
 Less attenuation, can run over longer distance without repeaters.
 Larger bandwidth and higher information rate
Disadvantages:
 Difficult to couple light in and out of the tiny core
 Highly directive light source (laser) is required
 Interfacing modules are more expensive

24. MULTI MODE
 Multimode step-index Fibers:
 inexpensive
 easy to couple light into Fiber
 result in higher signal distortion
 lower TX rate
 Multimode graded-index Fiber:
 intermediate between the other two types of Fibers

25. ACCEPTANCE CONE & NUMERICAL APERTURE
n2 cladding
n1 core
n2 cladding
Acceptance
Cone
qC
Acceptance angle, qc, is the maximum angle in which
external light rays may strike the air/Fiber interface
and still propagate down the Fiber with <10 dB loss.
Note: n1 belongs to core and n2 refers to cladding)
2
2
2
1
sin 1 n n C    q

26. Introduction to Ultrasonics
 The word ultrasonic combines the Latin roots ultra,
meaning ‘beyond’ and sonic, or sound.
 The sound waves having frequencies above the audible
range i.e. above 20000Hz are called ultrasonic waves.
 Generally these waves are called as high frequency
waves.
 The field of ultrasonics have applications for imaging,
detection and navigation.
 The broad sectors of society that regularly apply ultrasonic
technology are the medical community, industry, the
military and private citizens.
PH0101 UNIT 1 LECTURE 6 26

27. PROPERTIES OF ULTRASONIC WAVES
(1) They have a high energy content.
(2) Just like ordinary sound waves, ultrasonic waves
get reflected, refracted and absorbed.
(3) They can be transmitted over large distances
with no appreciable loss of energy.
(4) If an arrangement is made to form stationary waves of
ultrasonics in a liquid, it serves as a diffraction grating. It is called
an acoustic grating.
(5) They produce intense heating effect when passed through a
PH0101 UNIT 1 LECTURE 6 27
substance.

28. ULTRASONICS PRODUCTIONS
Ultrasonic waves are produced by the
following methods.
(1) Magneto-striction generator or oscillator
(2) Piezo-electric generator or oscillator
PH0101 UNIT 1 LECTURE 6 28

29. MAGNETOAGNETOSTRICTION GENERATOR
Principle: Magnetostriction effect
When a ferromagnetic rod like iron or
nickel is placed in a magnetic field
parallel to its length, the rod
experiences a small change in its
length.This is called magnetostricion
effect.
PH0101 UNIT 1 LECTURE 6 29

30. The change in length (increase or decrease) produced in the
rod depends upon the strength of the magnetic field, the
nature of the materials and is independent of the direction of
the magnetic field applied.
PH0101 UNIT 1 LECTURE 6 30

31. PH0101 UNIT 1 LECTURE 6
31
CONSTRUCTION
The experimental arrangement is shown in Figure
Magnetostriction oscillator

32.  XY is a rod of ferromagnetic materials like iron or nickel.
The rod is clamped in the middle.
 The alternating magnetic field is generated by electronic
oscillator.
 The coil L1 wound on the right hand portion of the rod
along with a variable capacitor C.
 This forms the resonant circuit of the collector tuned
oscillator. The frequency of oscillator is controlled by the
variable capacitor.
 The coil L2 wound on the left hand portion of the rod is
connected to the base circuit. The coil L2 acts as feed –
back loop.
PH0101 UNIT 1 LECTURE 6 32

33. 2  L C
1
1
PH0101 UNIT 1 LECTURE 6 33
WORKING
 When High Tension (H.T) battery is switched on, the
collector circuit oscillates with a frequency,
f =
 This alternating current flowing through the coil L1
produces an alternating magnetic field along the
length of the rod. The result is that the rod starts
vibrating due to magnetostrictive effect.

34. PH0101 UNIT 1 LECTURE 6 34
ADVANTAGES
1. The design of this oscillator is very simple and its
production cost is low
2. At low ultrasonic frequencies, the large power output can
be produced without the risk of damage of the oscillatory
circuit.
Disadvantages
1. It has low upper frequency limit and cannot generate
ultrasonic frequency above 3000 kHz (ie. 3MHz).
2. The frequency of oscillations depends on temperature.
3. There will be losses of energy due to hysteresis and eddy
current.

35. PIEZO ELECTRIC GENERATOR OR OSCILLATOR
Principle : Inverse piezo electric effect
 If mechanical pressure is applied to one pair of opposite
faces of certain crystals like quartz, equal and opposite
electrical charges appear across its other faces.This is
called as piezo-electric effect.
 The converse of piezo electric effect is also true.
 If an electric field is applied to one pair of faces, the
corresponding changes in the dimensions of the other
pair of faces of the crystal are produced.This is known as
inverse piezo electric effect or electrostriction.
PH0101 UNIT 1 LECTURE 6 35

36. PH0101 UNIT 1 LECTURE 6
36
CONSTRUCTION
The circuit diagram is shown in Figure
Piezo electric oscillator

37.  The quartz crystal is placed between two metal plates A
and B.
 The plates are connected to the primary (L3) of a
transformer which is inductively coupled to the electronics
oscillator.
 The electronic oscillator circuit is a base tuned oscillator
circuit.
 The coils L1 and L2 of oscillator circuit are taken from
the secondary of a transformer T.
 The collector coil L2 is inductively coupled to base coil
L1.
 The coil L1 and variable capacitor C1 form the tank
circuit of the oscillator.
PH0101 UNIT 1 LECTURE 6 37

38. Advantages
 Ultrasonic frequencies as high as 5 x 108Hz or 500 MHz
can be obtained with this arrangement.
 The output of this oscillator is very high.
 It is not affected by temperature and humidity.
Disadvantages
 The cost of piezo electric quartz is very high
 The cutting and shaping of quartz crystal are very
complex.
PH0101 UNIT 1 LECTURE 6 38

39. Applications of Ultrasonic Waves in Engineering
(1)DETECTION OF FLAWS IN METALS (NON
DESTRUCTIVE TESTING –NDT)
Principle
 Ultrasonic waves are used to detect the presence
of flaws or defects in the form of cracks, blowholes
porosity etc., in the internal structure of a material
 By sending out ultrasonic beam and by measuring
the time interval of the reflected beam, flaws in the
metal block can be determined.
PH0101 UNIT 1 LECTURE 6 39

40. PH0101 UNIT 1 LECTURE 6
40
EXPERIMENTAL SETUP
It consists of an ultrasonic frequency generator and a cathode
ray oscilloscope (CRO),transmitting transducer(A), receiving
transducer(B) and an amplifier.

41. PH0101 UNIT 1 LECTURE 6 41
WORKING
 In flaws, there is a change of medium and this
produces reflection of ultrasonic at the cavities or
cracks.
 The reflected beam (echoes) is recorded by using
cathode ray oscilloscope.
 The time interval between initial and flaw echoes
depends on the range of flaw.
 By examining echoes on CRO, flaws can be detected
and their sizes can be estimated.

42. THANK YOU
THANK YOU
PH0101 UNIT 1 LECTURE 6 42