vii
TABLE OF CONTENTS
CHAPTER
TITLE
Title page
i
Declaration of originality
ii
Dedication
iii
Acknowledgements
iv
Abstract
v
Abstrak
vi
Table of Contents
vii
List of Tables
x
List of Figures
xi
List of Symbols
xvi
List of Appendices
1
2
PAGE
xviii
INTRODUCTION
1
1.1
Overview
1
1.2
Problem Statement
2
1.3
Research Objective
3
1.4
Research Scope
4
1.5
Thesis Outline
4
THEORY
6
2.1
Introduction
6
2.2
Laser Beam Focusing
7
2.3
Photodisruption
9
viii
2.3.1
Optical Breakdown
11
2.3.2
Plasma
14
2.3.2.1 Plasma Formation
14
2.3.2.2 Plasma Temperature
15
Acoustic Shockwave Generation
18
2.3.3
3
2.4 Laser Interaction with Transparent Material
20
2.5
22
Conclusion
METHODOLOGY
23
3.1
Introduction
23
3.2
Samples
24
3.2.1
Saline Solution
24
3.2.2
Polymethylmethacrylate (PMMA)
25
3.3 Nd:YAG Laser System
3.3.1
Pockels Cell
27
3.3.2
External Triggering Circuit
28
3.4 Measurement Equipment
4
25
30
3.4.1
Power Meter
30
3.4.2
Photodetector
31
3.4.3
Langmuir Probe
31
3.4.4
Pressure Sensor
33
3.5 Imaging Equipment
33
3.6
Image Calibration
36
3.7
Experimental Setup
37
3.7.1
Observation of Plasma Formation
37
3.7.2
Plasma Temperature Measurement
39
3.7.3
Detection of Pressure Waves
40
3.7.4
Photodisruption Effects on PMMA
41
PLASMA FORMATION
43
4.1
Introduction
43
4.2
Plasma Formation Induced by Single Lens Technique
44
4.3
Plasma Formation Induced by Combination Lenses
48
ix
Technique
4.4
5
6
7
8
Measurement of the Plasma Length
50
PLASMA TEMPERATURE
54
5.1
Introduction
54
5.2
Plasma Temperature
55
GENERATION OF PRESSURE WAVES
62
6.1
Introduction
62
6.2
Pressure Measurement
63
6.3
Pressure Profile
67
PHOTODISRUPTION EFFECTS ON PMMA
70
7.1
Introduction
70
7.2
Photodisruption Effects
71
CONCLUSION
79
8.1
Introduction
79
8.2
Conclusion
80
8.3
Recommendations
81
REFERENCES
Appendices A - G
83
89 - 98
x
LIST OF TABLES
TABLE NO.
3.1
TITLE
PAGE
Values of laser beam parameters for different focusing
techniques.
38
4.1
Plasma length measured for both techniques.
52
5.1
Data obtained from the Langmuir probe signal detected
by oscilloscope.
6.1
Amplitude of the signals detected for different oscillator
voltages.
6.2
67
Damaged area measured for different laser energy for 1,
5 and 10 pulses.
7.2
67
Pressure amplitude as a function of laser energy at
various distances.
7.1
59
77
Damaged area measured for various number of laser
pulses.
77
xi
LIST OF FIGURES
FIGURE NO.
TITLE
2.1
The depth of focus of the laser light [11].
2.2
Beam diameter of a Gaussian beam as fundamental mode
TEM00 and function of z [11].
2.3
7
8
Mechanism of photodisruption induced by Q-switched
Nd:YAG laser [21].
2.4
PAGE
11
(a) Initiation, (b) electron avalanche growth and (c) plasma
formation by optical breakdown. The dominant mechanism
of initiation of ionization by a Q-switched pulse is
thermionic emission [21].
13
2.5
Current-voltage (I-V) characteristic curve of plasma [40].
16
2.6
Schematic diagram of breakdown due to Q-switched laser
pulse in PMMA. f denotes the position of the focus [57].
3.1
22
Samples used in the experiment:
(a) Saline solution
(b) PMMA
24
xii
3.2
Photograph of HY200 Nd:YAG laser.
26
3.3
HY200 Nd:YAG laser component layout [61].
26
3.4
Simplified four level system for solid-state Nd:YAG
laser [11].
27
3.5
Schematic diagram of the external trigger circuit.
29
3.6
Output pulse of the external trigger circuit.
29
3.7
Time delay between the external trigger and the laser.
30
3.8
The Langmuir probe
32
3.9
The Langmuir probe and its detection circuit:
(a) The detection circuit of the Langmuir probe
(b) Schematic diagram of Langmuir probe detection
circuit [68].
3.10
The voltage mode pressure sensor used to detect the
pressure waves signals.
3.11
32
33
Photographs of imaging equipments:
(a) CCD Camera
(b) Photomicroscope
34
3.12
Interface of the Matrox Inspector software.
35
3.13
VideoTest 5.0 software used to analyze the laser beam on
3.14
burn paper.
35
Image of wire taken using CCD camera.
36
xiii
3.15
Single lens focusing technique.
37
3.16
Combination of two lenses to focus the laser beam.
37
3.17
Experimental setup to study the generation of plasma in
saline using combination of two lenses.
39
3.18
Schematic diagram of experimental setup
40
3.19
Experimental arrangement for pressure wave detection
41
3.20
Schematic diagram of experimental setup to study the
damage on PMMA.
4.1
42
Plasma produced when single lens technique used.
Magnification of 6x. The direction of laser beam is
toward the right.
4.2
46
Growth of plasma anterior to the predicted focal point
[21]:
(a) a threshold pulse with spherical breakdown at the
beam waist;
(b) a greatly suprathreshold pulse attains breakdown
threshold anterior to the minimal spot size
(c) a moderately suprathreshold pulse extends toward the
laser source in a multilobed configuration
47
4.3
Multiple breakdown due to longer focal region [56].
47
4.4
Plasma formed in saline solution. Magnification factor is
8x. The laser is incident from the left.
49
xiv
4.5
Observation
of
plasma
using
different
focusing
51
techniques:
(a) Single lens focusing technique
(b) Combination lenses focusing technique
4.6
The distribution of plasma beam along the x-axis [11]:
(a) Gaussian beam profile
(b) Plasma configuration
52
4.7
Plasma length with respect to laser energy.
53
5.1
Typical signals collected by Langmuir probe as a
function of positive bias voltage.
5.2
56
Typical signals collected by Langmuir probe as a
function of negative bias voltage.
57
5.3
I-V characteristic curve of Langmuir probe.
60
5.4
Linear part of the I-V characteristic curve.
61
6.1
Typical acoustic shockwave signal detected at different
voltage at a distance of 1.87 mm.
6.2
Typical acoustic shockwave signal detected at different
voltage at a distance of 2.56 mm.
6.3
65
Typical acoustic shockwave signal detected at different
voltage at a distance of 5.76 mm.
6.4
64
66
Acoustic shockwave pressure as a function of laser
energy at three different distances.
69
xv
6.5
Acoustic shockwave pressure plotted against various
distances.
7.1
Damage induced by a single laser pulse on PMMA
(Magnification of 10x).
7.2
75
Damaged area as a function of laser energy for different
number of pulses.
7.6
74
Target irradiated at different number of pulses at laser
energy of 93.0 mJ. (Magnification of 10x).
7.5
73
Effects on PMMA which has been exposed to 10 pulses
of Q-switched Nd:YAG laser (Magnification of 10x).
7.4
72
Damage induced by 5 pulses of Q-Switched laser on
PMMA (Magnification of 10x).
7.3
69
78
Damaged area versus number of laser pulses taken at
laser energy of 93.0 mJ.
78
xvi
LIST OF SYMBOLS
a
-
Radius of the aperture
Cp
-
Specific heat
d,D
-
Distance
E
-
Laser energy
Ea
-
Absorbed laser energy
Eo
-
Electric field strength
f
-
Focal length
I
-
Current
Is
-
Electron saturation current
L
-
Lens
M
-
Magnification factor
ne
-
Electron density
P
-
Pressure
Pd
-
Power density
Rb
-
Radius of the optical beam
RL
-
Resistor
Rt
-
Acoustic source radius
r
-
Radius of the beam spot
Te
-
Electron temperature
V
-
Voltage amplitude
V
-
Optical absorbed volume
Vf
-
Floating potential
Vs
-
Plasma potential
Vpp
-
Probe potential
W
-
Laser power
xvii
w
-
Beam radius
w0
-
Beam waist
z
-
Depth of focus
z0
-
Focal point
zR
-
Rayleigh region

-
Absorption coefficient of the liquid
â
-
Thermal expansion coefficient
∆T
-
Temperature rise

-
Wavelength
eff
-
Penetration coefficient
v
-
Speed of sound

-
Density of the liquid
xviii
LIST OF APPENDICES
APPENDIX
TITLE
A
Measurement of laser beam parameters
B
Refractive index of natrium chloride solution as a
PAGE
89
function of its concentration expressed in percentage
[58].
93
C
Main properties of PMMA [84].
94
D
Table 1: Q-switched Nd:YAG laser energy upon
oscillator voltage.
E
Dimension of 2013V High Sensitivity Microphone
[65].
F
96
Calculation of the pressure of the acoustic shockwave
(Chapter 6, Section 6.2)
G
95
97
Calculation of damage threshold of PMMA (Chapter 7,
Section 7.2)
98