Q-SWITCHED Nd:YAG LASER INDUCED PHOTODISRUPTION IN
AN EYE MODEL
WAN RASHIDAH BINTI WAN MAJID
A thesis submitted in fulfilment of the
requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
JANUARY 2010
iii
Dedicated to:
My Parent: Wan Majid Wan Idris & Hasimah Awang,
Siblings: Redhuan, Rodzli, Rodhiah, Ridzaudin, Robiatul Adawiyah
Husband and son: Abd Rahman Tamuri and Abdullah Uwais Abd Rahman
Thanks for the endless love, advices and supports
iv
ACKNOWLEDGEMENTS
In the name of Allah, Most Gracious, Most Merciful
Alhamdulillah, all the praise to Allah the Almighty, for giving me the
strength, patience and time for completing this study.
A million of thanks to my supervisors; Prof. Dr.Noriah Bidin and Dr. Jasman
Zainal for their help and guidance throughout the work. Their encouragements really
help me to stand up and continue the research to the end.
A special thanks also go to the lab assistants, En. Ab. Rasid Isnin and Pn.
Ruzilah Omar for helping me in preparing for the experimental work and also in
finishing my writing process. To all my labmates, especially Aishah, Nazariah, Aizi
and Fairuz, thanks for the helps during the hard and joyful times in the laser
technology lab.
Last but not least, thanks to Universiti Teknologi Malaysia (UTM) and
MOSTI for the scholarship award and financial help which is really supporting me in
the 2 years of study in UTM.
v
ABSTRACT
This study attempts to characterize the photodisruption in simulated eye
model induced by Nd:YAG laser. To simulate the eye environment, saline solution
was chosen as vitreous filled pyrex cuvette which acted like eye ball.
Polymethylmethacrylate (PMMA) plate later on was placed in the pyrex cuvette to
be as an intraocular lens. The laser beam was focused into saline solution using two
techniques. The first technique uses single camera lens and the second technique uses
combination of negative and positive lenses. Activities at the focal region were
visualized by means of CCD video camera and interfaced with image processing
system via Matrox Inspector sofware. The pressure wave induced at the focal region
was detected using hydrophone and the plasma temperature was measured and
estimated using Langmuir probe. The damage induced after exposure of laser on
PMMA was observed using optical microscope. By focusing light pulses lasting in
nanoseconds to a spot size, this laser can create an optical breakdown associated with
plasma formation. Multiple breakdowns were observed when the laser was focused
using single lens. A single ellipsoidal plasma configuration was generated with a
combination of lenses. A series of acoustic-shockwave signals representing the
pressure waves produced at the focal region had also been recorded. From the
measurement, a maximum pressure of 0.0254 bar was obtained. The temperature
raised at the plasma region was estimated to be 12,064 K or 1.04 eV. The damage
threshold was obtained at a fluence of 6.86 x 102 Jcm-2 on the PMMA with various
damage formations. Severe damage was observed as the number of laser pulses
increases. In short, all the mechanisms involved have been successfully
characterized. These information can be very useful in recognizing the opportunities
and limitations of the Nd:YAG laser in medical applications.
vi
ABSTRAK
Kajian ini bertujuan untuk mencirikan fotopenghancuran dalam sampel
simulasi mata yang dijana oleh laser Nd:YAG. Untuk menyediakan sampel mata,
larutan garam dipilih untuk menggantikan cecair dalam mata yang diisi di dalam
bekas pyrex yang bertindak sebagai bebola mata. Kepingan perspeks (PMMA)
kemudiannya diletakkan sebagai kanta intraokular. Alur laser difokuskan ke dalam
larutan garam dengan menggunakan dua teknik. Teknik pertama adalah
menggunakan satu kanta kamera dan yang kedua menggunakan kombinasi kanta
negatif dan kanta positif. Aktiviti pada kawasan pemfokusan diperhatikan
menggunakan kamera video CCD yang diantaramuka dengan sistem pemprosesan
imej melalui perisisian Matrox Inspector. Gelombang tekanan yang dijana pada
kawasan pemfokusan dikesan menggunakan hidrofon dan suhu plasma diukur dan
dianggarkan menggunakan penduga Langmuir. Kerosakan yang dijana selepas
dedahan laser ke atas PMMA diperhatikan menggunakan mikroskop optik. Dengan
memfokuskan denyut cahaya nanosaat kepada satu saiz titik, laser ini boleh
menghasilkan keruntuhan optik diikuti dengan pembentukan plasma. Keruntuhan
berganda dapat diperhatikan semasa laser difokuskan menggunakan satu kanta. Satu
plasma berbentuk elipsoid dijana dengan kombinasi kanta. Beberapa siri isyarat
gelombang akustik-kejutan yang mewakili tekanan gelombang yang dijana pada
kawasan pemfokusan juga dirakamkan. Berdasarkan pengukuran, tekanan
maksimum sebanyak 0.0254 bar diperolehi. Peningkatan suhu pada kawasan plasma
dianggarkan sebanyak 12,064 K atau 1.04 eV. Kerosakan ambang berlaku pada 6.86
x 102 Jcm-2 di atas permukaan PMMA dengan beberapa bentuk kerosakan.
Kerosakan yang berlaku didapati meningkat dengan peningkatan kuantiti denyut
laser yang digunakan. Secara ringkas, semua mekanisma ini telah berjaya dicirikan.
Semua maklumat ini boleh menjadi sangat berguna dalam mengenalpasti peluang
dan had dalam mengaplikasikan laser Nd:YAG dalam perubatan.
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
CHAPTER 1
INTRODUCTION
1.1
Overview
The remarkable properties of laser radiation make it such a useful tool to be
applied for medical applications. The laser beam can be controlled, focused and
manipulated to give precise, specific and localized effects in tissues [1]. The
applications of lasers and other optical technology in biomedicine is a rapidly
growing field. These applications can be classified as diagnostic or therapeutic. In a
diagnostic application, the goal is to learn something about the physiology or
pathology of the tissue through its interaction with light. On the other hand, for
therapeutic use, it is involved with permanent modification of tissue. This can range
from simple cutting associated with surgery to the initiation of cytotoxic chemical
reactions in photodynamic therapy [2].
The most widespread medical application for laser technology in medicine
has occurred in ophthalmology. Ophthalmic laser applications have experienced
rapid growth with the use of argon, krypton, argon pumped dye, Nd:YAG and most
recently, near-IR diode lasers [3] since the introduction of ruby laser in 1960s.
2
In 1961, Zaret [4] employed a ruby laser for iris and retinal photocoagulation
in rabbits. Delivery systems for retinal photocoagulation employing ruby laser had
been developed by Campbell and Koester as well as Zweng and his associates in
1963 [5, 6]. The ruby laser was a valuable tool, but it is quickly supplanted with the
introduction of the argon laser photocoagulator. It was because the output of the
argon laser was a steady continuous wave instead of a short pulse and it could be
moved by existing fiber optic technology into slit lamp. The argon laser is the most
widely used to treat extrafoveal chorioretinal diseases such as age-related macular
degeneration and diabetic retinophathy, and also been successfully used to treat
glaucoma by iridectomy or trabeculoplasty [7].
Ophthalmology offers wide application of lasers since eye is one of the most
accessible human organs, and its media (cornea, aqueous humor, lens and vitreous)
are transparent to visible light, allowing direct inspection of its internal structures for
diagnosis and treatment [3].
1.2
Problem Statement
Photodisruptor laser applications are very useful for cutting, incising or
vaporizing intraocular tissue [8]. When laser is deposited on a tissue as thermal
energy, there are several mechanisms that may occur such as optical breakdown
associated with plasma and acoustic-shockwave generation. Effects generated by this
laser-tissue interaction depend on the target material (gas, liquid or solid). Biological
tissues are more complex and variable.
In this study, saline solution and polymethylmethacrylate (PMMA) are used
to simulate the eye condition. Some experimental work has been setup to observe the
photodisruption mechanism induced by Q-switched Nd:YAG laser. The mechanism
is studied based on laser parameters (energy, number of pulses and distance of
observation). It is very crucial to study plasma formation and acoustic-shockwave
3
generation as they are the main processes of the photodisruption. The investigation
on damages induced by photodisruption on the target is crucial as it can be very
useful or can be a very destructive. These observations are required to ensure a safety
use of laser as a photodisruptor in ophthalmology.
Therefore, the characterization of the photodisruption induced by Q-switched
Nd:YAG laser would provide some useful information on how the mechanism of
photodisruption depends on the laser parameters. This information also can be very
useful indications for clinician and for the system designer to recognize the
opportunities and limitations of lasers in applying these devices in medicines.
1.3
Research Objective
The main objective of the research is to characterize the mechanism of
photodisruption induced by Q-Switched Nd:YAG laser. This goal can be achieved as
the following:
a)
Observation of plasma formation in saline water
b)
Measurement of plasma temperature using Langmuir probe
c)
Measurement of acoustic-shockwave generation in saline water using
piezoelectric transducer
d)
Investigation of photodisruption effects on transparent material
(PMMA) using image analysis.
4
1.4
Research Scope
In this study, a Q-switched Nd:YAG laser with a fundamental wavelength of
1064 nm and 10 ns pulse duration has been employed as a source to generate
photodisruption. The laser beam has been focused using two focusing techniques.
One is a single lens technique and the other is combination of two lenses technique.
The plasma formation and the generation of acoustic-shockwave were being studied
in saline solution. PMMA was utilized as a target material to observe the effects of
photodisruption. The dynamic expansion of plasma was observed using CCD camera
which was interfaced to a personal computer. The plasma temperature was measured
using Langmuir probe. Pressure generated by acoustic-shockwave was detected
using piezoelectric transducer which was linked to an oscilloscope. The effects of
photodisruption mechanism were then observed using photomicroscope and analyzed
using image processing software.
1.5
Thesis Outline
This thesis is divided into eight chapters. Chapter 1 describes the general
overview of the research project. The history of laser use in medicine and laser as a
photodisruptor are also reviewed. The theory of photodisruption mechanism induced
by Q-switched laser will be detailed in Chapter 2. The discussions will include
optical focusing technique and laser induced damage on transparent material. The
samples, instruments and the experimental setup used to study the photodisruption
are presented in Chapter 3. The results and findings of this project are being
discussed in Chapter 4 to Chapter 7. The plasma formation and plasma temperature
measurement are discussed in Chapter 4 and Chapter 5, respectively while acousticshockwave generation is described in Chapter 6. In Chapter 7, damage effects
produced by the photodisruption mechanisms on transparent material are discussed.
5
Finally, Chapter 8 comprises the conclusion of the study and recommendations for
future work.
CHAPTER 2
THEORY
2.1
Introduction
In photodisruption, plasma is created inside the transparent media of the eye
at the laser focal site as a result of optical breakdown. The photodisruption will
produce not only the desired tissue destruction but also leads to acoustic shockwave
phenomena. Since the optical breakdown in transparent media is always localized at
the laser focus area, the treated site can be selected freely by moving the laser focus
[9]. Therefore it is possible to perform treatment on any intraocular regions without
the need to open the eyeball [10]. All these mechanisms and the laser interaction with
transparent material will be reviewed in this chapter. As the precision of the
photodisruption depends on the quality of the optical focusing, the laser beam
focusing will also be discussed.
7
2.2
Laser Beam Focusing
Theoretically, laser beams are propagated in the form of Gaussian beams
which behave differently from geometrical beams. Therefore, the laser beam will not
be focused at only one focal spot, but it has a range of distance along the focal
region. The distance is known as depth of focus, ± ∆z as shown in Figure 2.1.
a
± ∆z
Figure 2.1: The depth of focus of the laser light [11].
The depth of focus, ± ∆z can be defined as the distance over which the focused
beam has about the same intensity, or distance over which the focal spot size changes
from -5 % ~ +5 % [12]. The electrical field delivered to the depth of focus is strong
enough to ionize the atom or molecule in that region.
Propagation of Gaussian beams through an optical system can be treated
simply by geometrical optics. If a collimated beam of light passing through a convex
lens, it will be focused to a sharp point, at the focal length of the lens. In contrast, a
Gaussian beam cannot be merged into a sharp point. Therefore, it will always have a
beam waist such as shown in Figure 2.2.
8
}
Figure 2.2: Beam diameter of a Gaussian beam as fundamental mode TEM00 and
function of z [11].
Figure 2.2 shows the beam diameter as a function of z. The beam propagates
within the direction of z. At the position of z = zo, the beam has the smallest radius.
The beam radius increases linearly with increasing distance. At a focal point, zo, the
beam radius is wo and at Rayleigh region, zR, the beam radius is extend to wo 2 . In
general, the beam radius, w could be defined as [13];
 z
w( z )  wo 1  
 zR
2


(2.1)
where wo is the smallest beam radius at the waist. It is measured from the optical axis
to the smallest point of the beam edge. The intensity and the temperature of the
focused laser beam at the smallest beam waist are very high [11]. zR is the Rayleigh
range and given as in Equation (2.2).
z R  wo
2


(2.2)
Based on Equation (2.1), the beam waist is dependent on Rayleigh range [14].
The Rayleigh range is a region where the beam waist is still considered small. It is
also known as damaging region where any material placed in this region will suffer
damage caused by the laser. In practice, the beam waist can be estimated from the
9
optical alignment [15] as stated in Equation (2.3) where r is the radius of the beam
spot, f is the focal length of the lens, and a is the radius of the aperture.
r
 f
 2a
(2.3)
If the typical value of 5 % is chosen or w(z) = 1.05wo, and z = ∆z, we can get
[16];
z  
0.32wo2

(2.4)
by using Equation (2.1) and Equation (2.2).
In this study, the laser beam spots have been taken at different position within
the depth of focus and the beam diameters have been measured using image analysis
software. The beam waist and Rayleigh Range has been calculated using ISO
Standardized Method as discussed in Appendix A.
2.3
Photodisruption
In the late 1970s, short pulsed 1064 nm Nd:YAG lasers were introduced to
create optical breakdown and photodisruption of ocular tissue. The laser beam is
brought to focus to generate optical breakdown. The irradiated target disintegrates
into plasma associated with acoustic-shockwave which can cause mechanical
disruption at the focal site. Photodisruption has become well-established tool of
minimally invasive surgery and now are a common clinical photodisruptor [8].
Krasnov [17] was the first to demonstrate that high peak power pulses could
be used to produce clinically desirable disruption of ocular structures. He had used a
10
Q-switched ruby laser to treat the trabecular meshwork of eyes with open angle
glaucoma in 1972. In 1978, Aron-Rosa recognized the utility of the Nd:YAG laser
photodisruption for posterior capsulotomy which is necessary after cataract
extraction [18]. Fankhauser et al [19] reported success with the Nd:YAG laser in
performing iridectomies on a series of 35 patients. They also suggested the use of
Nd:YAG laser in the thermal mode for other pathologies traditionally tackled by the
visible beam lasers, employing techniques such as laser gonioplasty, laser
trabeculoplasty and irradiation of the retina and choroids [20]. This discovery
developed commercial interest in the development and utilization of Nd:YAG laser
in ophthalmology field.
Photodisruption can be defined as the use of high peak power ionizing laser
pulses to disrupt tissue [21]. In photodisruption, high power laser pulses are focused
to a small spot size to produce the irradiance needed for optical breakdown. The
optical breakdown can occur when the irradiance of a pulsed laser on a tissue
exceeds about 1000 Wm-2 (or 107 Wcm-2) [9]. The physical effects associated with
optical breakdown are plasma and acoustic shockwave generation [22]. This complex
process involves the ionization of molecules as electrons are stripped by the
extremely high electrical fields of the laser light. The rapid expansion of the plasma
elicits a rapidly expanding bubble that then collapses to produce extreme pressure
[9]. These will create mechanical forces which will rupture tissue during
photodisruption. This process can occur even in a transparent, non absorbing tissue,
because it depends not on certain wavelengths but on the total energy absorbed per
unit time. The Nd:YAG laser uses this photodisruptive mechanism in lasing
secondary cataract membranes and vitreous membranes [23]. Figure 2.3 shows the
dominant mechanisms of photodisruption induced by a Q-switched Nd:YAG laser in
ophthalmic application.
11
Protein
denaturation
Focal thermal
effects
Vaporization
Plasma
expansion
High irradiance
laser pulse
(> 1010 W/cm2)
Optical breakdown
and plasma formation
Stimulated
Brillouin
scattering
Acoustic and
shock wave
Figure 2.3: Mechanism of photodisruption induced by Q-switched Nd:YAG laser
[21].
2.3.1
Optical breakdown
Optical breakdown can be produced when Q-switched and mode-locked
Nd:YAG lasers is focused to a small spot less than 50 microns in diameter [21]. This
nonlinear effect is achieved when laser light is sufficiently condensed in time and
space to achieve high irradiance or density of power. Optical breakdown can also be
defined as a sudden event associated with plasma formation that is visibly manifested
by a spark and accompanied by an audible snap [17, 21, 24]. Figure 2.4 summarizes
the three distinct stages in optical breakdown named as initiation, growth and plasma
formation which can lead to photodisruption phenomenon.
In the initiation stage, Q-switched pulses of several nanoseconds’ duration
cause ionization, mainly by focal heating of the target in a process called thermionic
emission [25-26]. Temperatures greater than several thousand degrees Celcius were
achieved at the focal spot and this process are greatly enhanced by the presence of
impurities in the target [21]. At near-threshold levels, Q-switched breakdown is
perceived as being more explosive, because for nanosecond-long pulses the
irradiance necessary for thermionic initiation is greater than the irradiance necessary
for plasma growth.
12
Q-switched avalanche ionization is therefore precipitous once initiation
occurs. The longer Q-switched pulse does not have adequate electrical field strength
to initiate ionization by multiphoton absorption and depends on heating enhanced by
focal impurities for the initiation ionization [26-27].
Once the starting free electrons have been generated, plasma grows through
the mechanism of electron avalanche or cascade. A free electron absorbs a photon
and accelerates. The accelerated electron strikes another atom and ionizes it,
resulting in two free electrons each with less individual energy. These two free
electrons, in turn, absorb more photons, accelerate, strike other atoms, and release
two more electrons, and so forth, as shown in Figure 2.4 (c) [26-28]. The process of
photon absorption and electron acceleration in the presence of an atom or ion is
technically known as inverse bremsstrahlung [29-31].
13
Figure 2.4: (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].
14
2.3.2
Plasma
Plasma is essentially a gas consisting of charged particles, electrons and ions,
rather than neutral atoms or molecules [32-33]. In plasma, electrons have freely
dissociated from atoms, which then become positive ions in a process that occurs in
the presence of photons. Thus plasma can conduct electricity, like a metal, but in
most other properties plasma behaves like a gas. Therefore, plasma is considered as a
fourth state of matter, along with solids, liquids and gaseous [34].
Plasmas can be created by heat, electricity, or radiant energy, such as laser
light [21]. The light energy is able to create plasma when focused to high irradiance,
commonly between 1010 and 1012 W/cm2 [21]. In addition, the electric field strength
E0 of a focused laser beam of power density Pd is given by Equation (2.5) [29],
E 0  2.74 x10 3 Pd1 / 2
(2.5)
For a laser pulse with power density of Pd = 1010-1012 W/cm2, the calculated electric
strength is 106 to 107 V/cm. Therefore, the level of Q-switched laser irradiance
necessary for initiating optical breakdown should has an electrical field strength in
excess of 107 V/cm [17, 21].
2.3.2.1 Plasma formation
The formation of plasma can take in any shape. Experimentally the common
shape is bead-like, also known as an ellipsoidal shape. It can also be described as a
space fireballs, as it is the term commonly used in nuclear weapon explosion
experiments [11].
15
The ions in the plasma expand in the longitudinal direction because the
electric charge is not compensated inside. Size information of the plasma is gained
by varying laser power, W: the creation of the first electron is of statistical nature,
and consequently its probability depends on the number of ionizable electrons in the
focal region. More precisely, this depends on the number of electrons in a valence
band or, if the bond character of constituents within the particle does not permit
delocalization of electrons, this depends on the number of outer shell electrons in the
individual atoms or molecules involved [35].
2.3.2.2 Plasma Temperature
Plasma can be diagnosed using various methods such as optical diagnostics,
neutron diagnostics, spectroscopic methods, microwave systems, magnetic probe and
electric probe [36-37]. Among these methods, the electric or Langmuir probes [38]
have been most widely used in the measurements of basic plasma parameters. The
Langmuir probes are known for their ability to provide local measurements of such
basic plasma parameters as electron density ne, electron temperature Te and plasma
potential Vp and electron and ion beam energy [36, 38-39]. Good temporal
resolutions make electric probes a useful tool in plasma fluctuation studies [38].
This method involves the measurements of electron and ion current to a probe
as different voltages are applied to the probe. This yields a curve called the currentvoltage (I-V) characteristics of the plasma as shown in Figure 2.5.
16
Figure 2.5: Current-voltage (I-V) characteristic curve of plasma [40].
The I-V characteristic curve consists of three regions: the electron saturation
region, electron retardation region and ion saturation region [36, 41]. The behavior of
the probe characteristic may be explained as follows.
In the electron saturation region, the probe is biased positively with respect to
the plasma potential Vp. When the probe has high positive potential, negative ions
and electrons are attracted to it, and electron saturation occurs. Above the plasma
potential, the low energy ions are repelled and only electrons are collected. An
electron sheath formed around the probe tip. The current is limited by the flux of
electrons arriving at the boundary of the sheath as a result of their random thermal
motion. This current, almost independent of the potential, is the electron saturation
current.
If the probe potential V is decreased below the plasma potential Vs, only
electrons with sufficient kinetic energy can reach it and the current decreases. If the
electron distribution is in thermal equilibrium, the electron current is exponential
upon the applied probe potential and the slope of the exponential region yields the
17
electron temperature. The electron temperature can be calculated by using the
following equation [40]:
 e 
dI
 Is 

dV
 2kTe 
(2.6)
where,
dI
dV
Is
kTe
e
= slope of the linear part of I-V curve
= electron saturation current
= electron temperature in electron volts (eV)
Equation 2.6 will be used to measure and calculate the plasma temperature in
Chapter 5, Section 5.2.
The current recorded in the electron retardation region is a mixed contribution
of ion and electron currents. At a certain potential, the fluxes of electrons and ions
are equal which means the total current is equal to zero. The potential where the
current goes zero is called the floating potential, Vf.
When the potential becomes sufficiently negative which means a negligible
number of electrons can reach it, a sheath of positive charge is set around it and the
current level off. The value of current at which this occurs is the ion saturation
current. Its value depends on the potential required to repel the bulk of the electrons.
This ion saturation region yields the ion number density in the plasma [42].
18
2.3.3
Acoustic Shockwave Generation
The generation of high-pressure transients with laser was demonstrated
shortly after the invention of the Q-switched ruby laser [43]. The generation of
acoustic shock wave in liquids by focused laser beam has been reported by many
investigators. The generation of sound by the absorption of laser light in liquid was
first reported by Askar’yan [44]. Felix and Elis [45] have clarified the exact sequence
of events that occurs during liquid breakdown and the wave propagation caused by a
focused Q-switched neodymium laser.
Generally, there are several important mechanisms that contribute to the
generation of acoustic shockwave which are optical breakdown, material ablation,
thermoelastic process, electrostriction and the radiation pressure [21, 46]. Their
contributions depend on the parameters of the laser beam as well as on the optical
thermal parameters of the target [47].
As described in Section 2.3.1, the optical breakdown induced by laser leads to
the formation of plasma associated by the generation of acoustic wave. When laser
energy is delivered to the medium in a very short time, thermal expansion of the
medium due to the laser heating will be produced. The rapid thermal expansion will
generate a very large reaction force because of the inertia of the medium. Under this
large reaction force, the medium may be slightly compressed. This compressionextension action will generate acoustic shockwave [14].
The optical breakdown produces the highest amplitude of acoustic shockwave
compared to other mechanisms. Normally, this occurs in the focal region of the
focused laser beams [48]. The acoustic shockwave tends to move spherically
outwards from the center of the laser breakdown [46]. The acoustic shockwave is
initially propagated at hypersonic speed and slows down to the speed of sound [22].
This is the most efficient process of converting optical energy to acoustic energy as
its conversion efficiency may reach 30 % in liquids [47].
19
In practice, spherical waves may be considered plane waves after traveling a
very short distance. A small section of a spherical surface is very close
approximation to a plane [14]. If all of the absorbed energy E a is converted into
thermal energy to heat the liquid, it causes a temperature rise ∆T and a pressure
increase, P in the illuminated region [49]. By applying the laws of thermodynamics,
∆T can be estimated as:
T 
Ea
C p V
(2.6)
where V  Rb2  eff is the optical absorbed volume and, ñ and Cp are the density of
the liquid and the specific heat respectively. The pressure increase can be expressed
as:
P  v 2 T
(2.7)
By substituting Equation (2.6) and V  Rb2  eff into Equation (2.7), the
pressure amplitude becomes
P
E o v 2
Cp
(2.8)
where the energy fluence of the laser pulse, Eo = E/ðRb2. E is the laser pulse energy
and it can be assumed to equal Ea in the case of strong absorbing media [49].
In Equation (2.8), Eo is proportional to the laser pulse energy, E and the
absorption coefficient of the liquid, á which is equal to the inverse of the penetration
coefficient, ìeff-1, Rb is the radius of the optical beam. â and v are thermal expansion
coefficient and the speed of sound respectively.
As the acoustic waves propagated spherically in shape, the absorbed laser
pulse energy, Ea can be assumed to be distributed homogenously. Therefore, the
20
pressure amplitude can be derived as for the plane wave [50]. For a short light pulse
source (ôp<<ôac) or a big source, the pressure amplitude, Pt of the acoustic
shockwaves is expressed as
Pt ( r ) 
E a v 2
3
2  2
eC p Rt2 r
(2.9)
and for a long light pulse (ôp>>ôac) or a point source[50], the pressure amplitude is
given by,
P (r ) 
Ea 
3
2  2 C p r 2p
(2.10)
In Equation (2.9) and (2.10), Ea is proportional to the laser pulse energy, E
and the absorption coefficient of the liquid. Rt is the acoustic source radius, ôp is laser
pulse width and r is the distance between the source and the point of observation. ôac
is given by ratio of light penetration depth, ìeff and acoustic velocity, í. Equation
(2.9) and (2.10) will be used for discussion in Chapter 6, Section 6.3.
2.4
Laser Interaction with Transparent Material
An understanding of mechanism of damage in transparent material induced
by Q-switched Nd:YAG laser are particularly important as the Q-switched Nd:YAG
laser has been widely used for posterior capsulotomy [21]. Recently, photodisruption
with a Q-switched Nd:YAG laser is routinely used with a pulse duration of a few
nanoseconds for iridotomy in acute narrow-angle glaucoma [51] and for cutting
secondary cataract membranes [52]. In this study, polymethylmethacrylate (PMMA)
was used to simulate an eye in order to investigate the damage effect produced by
Nd:YAG laser photodisruption on transparent material.
21
Generally, the effects of interaction of laser beam with transparent material
are highly power dependent. The laser beam passes through transparent material
with no apparent effect below certain threshold value of laser power. Absorption
effects such as material removal from the target surface, internal voids production,
melting and vaporization begin above the threshold value of laser power [31].
Damage produced by high-power lasers in solids can take a wide variety of
forms. The damages can appear as microcracks or large pulverized regions, melted
voids, shattered surface and holes [31]. Several mechanisms were proposed to
explain the effects of laser on transparent material. The main suggested mechanisms
are stimulated Brillouin scaterring, thermal shock and microplasma production, and
localized heating and vaporization [31, 53-54].
Production of sufficient intensity of phonons (hypersound) in a stimulated
Brillouin scattering process leads to material fracture by acoustic wave. Brillouin
scaterring involves an interaction between an optical field and an acoustic field. The
electric field associated with the light beam produces electrostriction, which in turns
exerts a pressure in the material and drives an acoustic wave [31]. In addition,
absorption of light by original material defects or structural inhomogeneties present
may serve as primary source for breakdown [21, 31] with resultant thermal shock
and microplasma production [31]. Meanwhile, heating and vaporization near the
focal point of the laser can be expected to cause melting and cracking [53]. In
electron avalanche phenomenon, free electrons absorb the laser radiation and
accelerate in the electric field which result in ionizing collisions and more free
electrons production. These processes lead to absorption of the laser light and
therefore cause an intense localized heating [31].
In particular, damage in transparent material may begin with a small fracture
produced by a hypersonic wave. Once the first damage has been produced, the
incident light can be absorbed and intense heating can occurred at the laser focal
point [31]. For polymers such as polymethylmethacrylate (PMMA), the breakdown
region produced by a Q-switched laser consists of very fine cracks contained within a
cone whose diameter decreases as one approaches the focal point as shown in Figure
2.6. The orientation of the cracks is random. When the polymer is stressed, the
22
orientation of cracks changes so that the cracks tend to be oriented in planes
perpendicular to the stress [55-56].
f
Figure 2.6: Schematic diagram of breakdown due to Q-switched laser pulse in
PMMA. f denotes the position of the focus [57].
2.5
Conclusion
As a conclusion, laser photodisruption basically consists of three main
mechanisms which are optical breakdown, plasma formation and acousticshockwave formation. These mechanisms may cause damage to target material
which might be very useful or destructive. Several studies have been carried out to
investigate how the mechanism of photodisruption depends on laser parameters and
the focusing techniques in the following chapters. The effects of the photodisruption
on target material will also be discussed.
CHAPTER 3
METHODOLOGY
3.1
Introduction
This chapter will cover the material or sample used as a target, the
equipments used in the experiment and measurements, software utilized for analysis
including the experimental setup and arrangement.
The Nd:YAG laser with fundamental wavelength of 1064 nm was employed
as a source of energy and operated with the application of Q-Switched system.
Samples were comprised of saline solution and polymethylmethacrylate (PMMA).
The interaction between laser and target was visualized and recorded using CCD
camera. Langmuir probe was used to measure the plasma temperature and a pressure
sensor was utilized to investigate qualitatively the generation of pressure wave in
saline solution. A trigger unit circuit was developed to operate the laser externally.
Video Test 5.0 and Matrox Inspector 2.1 software were used to precisely measure
and analyzed the physical quantity.
24
3.2
Samples
Shown in Figure 3.1 (a) and (b) is saline solution and PMMA which were
used as a target material to simulate the eye model in this experiment.
(a) Saline solution
(b) PMMA
Figure 3.1: Samples used in the experiment.
3.2.1
Saline Solution
Saline solution is generally water that contains a significant concentration of
dissolved salts (NaCl). The concentration is usually expressed in parts per million
(ppm) of salt. In this study, saline solution (Normal Saline, Pharmasafe Laboratories,
Kuala Lumpur) contains 0.9 % Natrium Chloride was confined in a Pyrex cuvette
with dimension of 3.5 x 3.5 x 3.5 cm3. As the concentration of the solution is less
than 2.5 %, the refractive index of this solution is nearly the same as that of pure
water which is 1.331 [58]. Refractive index for common salt solution as a function of
concentration is given in Appendix B.
In this study, saline has been used as a model for intraocular fluids to provide
reproducible experimental conditions. This is justified by the fact that the threshold
25
for plasma formation either in saline or distilled water and ocular media are similar
[59-60].
3.2.3
Polymethylmethacrylate (PMMA)
PMMA has been used to simulate as intraocular lens which usually implanted
to replace the eye lens after cataract extraction. The material has a distinctive optical
clarity and stability which is very useful for medical field. PMMA is a transparent,
colourless and thermoplastic polymer. Main properties of PMMA are shown in
Appendix C.
3.3
Nd:YAG Laser System
In this work, a Nd:YAG laser (HY200, Lumonics, Warwickshire) with a
fundamental wavelength of 1064 nm was employed as a source of energy. It is
operated in the transverse monomode of TEM00. It is a Q-switched laser which
delivers a maximum of 200 mJ per pulse with pulse duration of 10 ns. The laser is
operated in variable voltage in the range of 500 V to 740 V. The measurement of the
laser energy upon operating voltage is shown in Appendix D. The repetition rate of
the laser can be selected within 1 Hz to 50 Hz. A single mode was operated using a
developed external trigger. He-Ne laser was coaxial with the Nd:YAG laser beam to
ease the alignment of optical component such as mirror, lens or prism. The He-Ne is
illuminated from the back of the rear mirror and passes through the output coupler. In
this way, the He-Ne beam was aligned in axis with the Nd:YAG laser. Figure 3.2
shows the photograph of HY200 Nd:YAG laser and the component layout of
Nd:YAG laser is shown in Figure 3.3.
26
Figure 3.2: Photograph of HY200 Nd:YAG laser.
45o Mirror
Output
beam
He-Ne
Laser
45o
Mirror
Output
mirror
Shutter
Laser
Stabilized
rod
Beam
Pockels
resonator
expander structure
cell
telescope
Rear
mirror
Figure 3.3: HY200 Nd:YAG laser component layout [61].
The Nd:YAG laser is the most commonly solid-state laser used in medical
applications other than ruby laser [21]. The host medium is Yttrium Aluminium
Garnet (Y3Al5O12) with Neodymium ion, Nd3+ present as impurity providing the
laser transitions and pumping. The YAG host is hard and has a high thermal
conductivity with good optical quality [62].
The Nd:YAG laser has four main levels system. The terminal level, 4I11/2 is
far from the ground level and has zero population at room temperature. When the
flashlamp is being triggered, it will emit large amounts of spectral energy in short
duration pulses. Thus this energy is optically pumping the Nd atoms at the ground
level to pump band at 4S3/2. The excited atoms then undergo rapid nonradiative
transition to the metastable 4F3/2 level. The laser transition takes place from the 4F3/2
level and terminates at the terminal 4I11/2 level. The relaxation time from the 4F3/2
27
level to 4I11/2 is longer (10-5 to 10-3 s) compared to the rapid nonradiative transition.
This lasing process emits light at wavelength of 1064 nm [11]. This simplified
energy level for this four level system is depicted in Figure 3.4.
Figure 3.4: Simplified four level system for solid-state Nd:YAG laser [11].
3.3.1
Pockels Cell
Pockels cell is one of the major component in this Nd:YAG laser system. It is
used to operate the laser in a mode of Q-switching which generates a high pulse
power in a short time. The Pockels cell consists of potassium dihydrogen phosphate
(KD*P). The state of polarization of a light beam which passes through the crystal
will change when a voltage is applied [61]. As the electro-optic coefficient of the
crystal is strongly temperature-dependent, it is important to make sure the laser is
always connected to the main power supply to maintain the crystal temperature at
36oC [63].
28
Q-Switching occurs when the avalanche transistor chain is triggered to
remove high voltage across the Pockels cell. The cell remains at zero voltage after Qswitching until the flashtube is again fired [63]. The quality factor Q is defined as the
ratio of the energy stored in the cavity to the energy loss per cycle. Consequently, the
higher the quality factor, the lower the losses [62].
3.3.2
External Triggering Circuit
The Nd:YAG laser can be operated either by internal or external mode of
triggering. For internal mode of triggering, the laser can be operated with repetition
rate in a range of 1 to 50 Hz while the external trigger control unit is utilized in order
to trigger a single pulse laser. The operation of the laser can be triggered externally
by connecting a positive pulse of 3 to 30 V with pulse duration greater than 20 µs
[63]. A simple trigger circuit was designed and constructed using integrated circuit of
LM 555 and potentiometer as shown in Figure 3.5.
In this work, a power supply of 12 V was connected to the trigger circuit and
disconnected from the ground by using a push button, S1. The RC circuit (R1, R2
and C1) is used to prevent the short circuit when S1 is pressed. When the push button
is pressed, the RC circuit will discharge and the PIN2 of the LM 555 is triggered. An
output pulse of 30 µs will be generated at PIN3 as shown in Figure 3.6. The output
pulse of the trigger unit was then connected to the external trigger connector of the
Nd:YAG laser via a 50 Ù coaxial cable. Figure 3.7 shows the laser light signal
emitted with a delay time of 212 µs after being triggered by external pulse. This is
determined by detecting the light using a photodiode, which was coupled to an
oscilloscope.
29
Figure 3.5: Schematic diagram of the external trigger circuit.
30 µs
Figure 3.6: Output pulse of the external trigger circuit.
30
212 µs
Figure 3.7: Time delay between the external trigger and the laser.
3.4
Measurement Equipment
3.4.1
Power Meter
One of the most fundamental measurements for a laser is the output power
and energy. In this study, the laser energy is measured using a Broadband
Energy/Power Meter (13 PEM 001/J, Melles Griot, Colorado). This Integrated 2-watt
Broadband Power and Energy Meter System is a high sensitivity instrument for
measuring optical radiation from the ultraviolet to the far infrared (200 nm to 20
ìm). The instrument features a sensitive, but low drift, thermopile sensor head with a
very high damage threshold. The Power Meter can be used to measure the output
power of cw lasers and the energy of a laser pulse in a range of 10 ìJ to 2J or 10 ìW
to 2W [16]. The energy calibration of the HY 200 Nd:YAG laser system as shown in
Table 1 in Appendix D.
31
3.4.2
Photodetector
The pulse of the laser beam can be detected by using a photodetector. The
photodetector was chosen because it can provide a direct measurement of the laser
pulse signal. The laser pulse signal detected by the photodetector is displayed on an
oscilloscope (HP54522A, Agilent HP, California) with 2 GSa/s and bandwidth of
500 MHz. The photodetector has an active area of 10 mm2 with response wavelength
in the range of 350 nm to 1100 nm. In this work, the photodetector was used to
determine the optical time delay between the laser and the external trigger of the
Nd:YAG laser system.
3.4.3
Langmuir Probe
The plasma temperature can be measured using an electric probe such as
Langmuir probe. A Langmuir probe consists of tungsten wire of 3 cm length and
0.127 mm diameter, protruding from cylindrical Teflon insulator of 16.40 mm
diameter. The probe is shown in Figure 3.8 and its associated detection circuit is
shown in Figure 3.9. The detection circuit consists of three parts which are the
Langmuir Probe, power supply and current sensor circuit. When a voltage is supplied
to the probe, the electrons and negative ions are collected at the probe. Therefore,
current will be produced. The current produced is depends on the number of
electrons and negative ions gathered at the probe. Then, only pulse current from the
probe can pass C1. The current then can be calculated using Ohm’s Law, V=IRL
where V is the voltage amplitude of the signal recorded by the oscilloscope and RL
(0.1 Ù) is the resistor of the current sensor circuit.
32
Figure 3.8: The Langmuir probe
(a) The detection circuit of the Langmuir probe
(RL)
(b) Schematic diagram of Langmuir probe detection circuit [65].
Figure 3.9: The Langmuir probe and its detection circuit.
33
3.4.4
Pressure Sensor
In this study, a High Sensitivity Microphone (2013V, Dytran Instruments,
Chatsworth) with surface diameter of 0.618 inches and sensitivity of 1.96 V/Psi was
used to pick up the noise created during optical breakdown process. This voltage
mode pressure sensor will convert the sound signal into electrical signal and sent out
the signal to the oscilloscope. The whole sensor components are shown in Figure
3.10. The dimension of the instrument [65] is shown in Appendix E.
Figure 3.10: The voltage mode pressure sensor used to detect the pressure waves
signals.
3.5
Imaging Equipment
The photodisruption mechanism was visualized and recorded using a CCD
camera (TMC – 7DSP, JAI Pulnix, Copenhagen) which operating via the aid of
Maxtrox Meteor II Standard frame grabber card. The Matrox Meteor II Standard
board is supported by Matrox Imaging software including Matrox Imaging Library
(MIL) and Matrox Inspector 4.1 software for its operation [66]. The CCD camera
was being used to capture the image of plasma during photodisruption process and to
observe the damage mechanisms on PMMA material.
34
Other imaging equipment used was optical microscope (REICHERT
POLYVAR 2 MET) which consists of objective lenses with magnification factor
range from 5x to 150x. The optical microscope was used to analyze the damage
patterns induced by Nd:YAG laser after interaction with laser plasma. The
photomicroscope was connected to computer for easy image grabbing and
measurement. Figure 3.11 (a) and (b) show the photograph of photomicroscope and
CCD camera respectively.
(a) Optical microscope
(b) CCD camera
Figure 3.11: Photographs of imaging equipments
All the images captured using the CCD camera and photomicroscope can be
analyzed using Matrox Inspector 2.1 and Video Test 5.0 as shown in Figure 3.12 and
Figure 3.13. The physical properties of the images such as absorption value, area,
diameter or length can be measured using these softwares.
35
Figure 3.12: Interface of the Matrox Inspector software.
Figure 3.13: VideoTest 5.0 software used to analyze the laser beam on burn paper.
36
3.6
Image Calibration
Image calibration is essential in order to measure the actual size of an object.
The size of the real object might be different compared to the size of the recorded
image. Therefore, a copper wire as shown in Figure 3.14 was employed as an object
and was placed at the focal point in this work. The image of the wire was captured
using CCD camera and measured using Matrox Inspector 2.1 software.
Magnification factor M, is the ratio of the image and the size of the object.
M 
size of image
size of object
Figure 3.14: Image of wire taken using CCD camera.
(3.1)
37
3.7
Experimental Setup
3.7.1
Observation of Plasma Formation
In this experiment, Q-switched Nd:YAG laser (HY 200, Lumonics,
Warwickshire) with fundamental wavelength of 1064 nm and 10 ns pulse duration
was employed to create an optical breakdown associated with plasma formation. This
experiment has been carried out using two focusing technique which are single lens
and combination of lens technique as shown in Figure 3.15 and Figure 3.16
respectively.
Figure 3.15: Single lens focusing technique.
Figure 3.16: Combination of two lenses to focus the laser beam.
In a single lens technique, the laser beam is focused by a biconvex lens at the
focal region to provide the irradiance needed for optical breakdown. In combination
lenses techniques, f1 represents the focal length of the first lens, L1 and f2 represents
38
the focal length of the second lens, L2. The focal region depends on the focal length
of the second lens, L2. The shorter the focal length of the second lens, the smaller the
focal region.
In the second technique, two different types of lens are used. One is negative
lens or divergence lens with focal length of -25 mm. The other one is positive or
camera lens of 28 mm focal length. The diameter of the laser beam is normally small
around 2 mm. Hence, the laser beam was first diverged by negative lens. The
expanded beam was then converged by positive lens. The laser beam was focused
into a saline solution using a camera lens (28 mm) and divergence lens (-25 mm).
The solution was confined in a Pyrex cuvette with dimension of 3.5 x 3.5 x 3.5 cm3.
Different focusing techniques have been used in this experiment to
investigate how the focusing geometry affects the formation of plasma in target
material. The beam waist and the Rayleigh range of the single lens and combination
lenses focusing techniques have been calculated as shown in Appendix A. The value
of the laser beam parameters are listed in Table 3.1.
Table 3.1: Values of laser beam parameters for different focusing techniques.
Laser beam parameters
Single lens focusing
Combination lenses
technique
focusing technique
Beam waist, w0 (mm)
0.81
0.47
Rayleigh range, zR (mm)
5.57
3.60
Depth of focus, ± ∆z (mm)
11.14
7.20
The formation of the plasma in the breakdown region was visualized by using
CCD video camera which was interfaced to personal computer. The image of the
plasma was grabbed for different laser energy range between 30 to 200 mJ and then
stored in the personal computer for analysis. The analysis of the plasma length can be
measured precisely with the aid of Matrox Inspector version 2.1 image processing
software. This measurement will be discussed in Chapter 4, Section 4.4. Figure 3.17
shows the experimental setup to study the plasma generation in saline solution.
39
Figure 3.17: Experimental setup to study the generation of plasma in saline using
combination of two lenses.
3.7.2
Plasma Temperature Measurement
The Q-switched Nd:YAG laser with fundamental wavelength of 1064 nm and
10 ns pulse duration was employed as a source of plasma generation. Copper was
employed as a target material in this experiment. The laser beam was focused using a
160 mm focal length to a 3 x 3 cm target at normal incidence. The probe is put near
to target and plasma to make sure the maximum charge gathered around the probe.
The potential at which the probe was held was referred to the grounded target and
plasma flowed past the probe. Meanwhile, the current to the probe is measured by
the resulting potential drop across 0.1 Ω resistor as depicted in Figure 3.9 (b). The
potential is swept by varying the power supply V. The potential drop signal detected
by the Langmuir probe is displayed on the oscilloscope with 2 GSa/s and bandwidth
of 500 MHz. A low resistance value was chosen to ensure that the potential drop
across the resistor (RL) was as small as possible, thus resulting a constant probe
potential during current collection. The signals were taken through several ablation
events, for a range of probe potential from -18 to +18 V. The laser induced plasma
interaction was visualized using a CCD video camera. The experimental setup of
plasma diagnostic using Langmuir probe is shown in Figure 3.18.
40
CCD camera
Target
Computer
Langmuir probe
Oscilloscope
Probe circuit
Power supply
Convex
lens
Nd:YAG laser
Figure 3.18: Schematic diagram of experimental setup.
3.7.3
Detection of Pressure Waves
The Q-switched Nd:YAG laser with pulse duration of 10 ns and repetition
rate of 1 Hz was used as a source of pressure wave generation. The energy of the
laser was varied between 60 to 200 mJ. The laser was focused using combination
lens technique into a saline solution which simulates the ocular media. The solution
was confined in a cuvette with dimension of 3.5 x 3.5 x 3.5 cm3. A 2013V High
Sensitivity Microphone was used to detect the pressure wave which generated during
the optical breakdown. The transducer was fixed on a vertical and horizontal
translation stage. In this way, the movement of the transducer can be precisely
controlled. The transducer is immersed in the saline solution. The signal detected by
the transducer is displayed on the oscilloscope with 2GSa/s and bandwidth of 500
MHz. The formation of the breakdown was visualized by a CCD video camera which
was interfaced to personal computer. Matrox Inspector software was used to
precisely measure the distance between the transducer and the optical breakdown.
41
Figure 3.19 shows the experimental arrangement used for the detection of the laser
generated pressure waves in saline solution.
Pressure
sensor
Nd:YAG laser
Concave
lens
Internal
trigger unit
Convex
lens
Saline
solution
Oscilloscope
CCD Camera
Computer
Figure 3.19: Experimental arrangement for pressure wave detection.
3.7.4 Photodisruption Effects on PMMA
In this work, transparent solid material is employed to study the damage
induced by photodisruption mechanism. PMMA material has been used as a sample
which simulates as an eye lens throughout the experiment. The PMMA was put
inside the cuvette filled with saline solution and was placed at the focal point of the
optical breakdown. The HY200 Nd:YAG laser was externally triggered and was
being focused using combination lenses on the sample. The laser was operated in the
range of energy between 30 mJ to 115 mJ with different number of laser pulses. The
activities were visualized using CCD camera and the damage on the exposed sample
was observed under optical microscope. The damage size was then measured using
VideoTest 5.0 software. The experimental setup is shown in Figure 3.20.
42
Concave
lens
Camera
lens
PMMA
Nd:YAG Laser
Saline
CCD
Camera
External
trigger unit
Computer
Figure 3.20: Schematic diagram of experimental setup to study the photodisruption
effects on PMMA.
CHAPTER 4
PLASMA FORMATION IN LIQUID
4.1
Introduction
When a Q-switched ruby laser pulse is passed through a lens in air, a spark
explodes at the focal point, exactly as in the electrical breakdown across a discharge
gap. The first report of laser-induced spark formation was made in 1963 by Maker et
al [35] in Paris. The discovery of the laser-induced spark was the focus for the
development of new experimental and theoretical studies in plasma physics for
decade. Krasnov was the first to demonstrate that high peak power pulses could be
used to produce clinically desirable disruption of ocular structures. In 1972, he
reported use of a Q-switched ruby laser to treat the trabecular meshwork of eyes with
open angle glaucoma [17].
Since the introduction of the ruby laser, ophthalmic laser applications have
experienced rapid growth with the use of argon, krypton, argon pumped dye,
Nd:YAG and near-IR diode laser [3]. However, most investigations were done at
wavelength of 1064 nm. This wavelength is optimally suited for intraocular surgery
due to the high transmission of the ocular media, the low absorption on the retina,
and the invisibility of the radiation avoiding dazzling of the patient [67].
44
In recent years, laser-induced plasma formation has been used in various
fields of laser medicine [67-70] for photodisruption, ablation, or lithotripsy [69-70],
and it has become especially important in intraocular microsurgery [21, 32]. This has
raised an interest in gaining better understanding of plasma formation in liquids
which for many years received less attention than plasma formation in solids and
gases. Owing to its importance for medical application and laser safety, this study
has been carried out to investigate how the plasma formation in liquid depends on the
laser energy and focusing technique.
4.2
Plasma Formation Induced by Single Lens Technique
In this experiment, a camera lens with 28 mm focal length was used to focus
the laser beam into saline solution. Results obtained from this experiment are shown
in Figure 4.1. The frames in Figure 4.1 are arranged in the increasing order of laser
energy delivered to the focal point. The plasma is represents as a white spot in each
frame. As the experiment is done in a dark room, the plasma image is bright enough
to be captured without the aid of the flash light of the camera.
Generally, the plasma tends to spread in the direction of the incoming light,
as illustrated in Figure 4.2, in a multilobed configuration. The growth fills the
angular cone defined by the converging laser beam. The anterior growth of the
plasma may be understood as the absorption of incoming light by the plasma.
Absorption and further growth of the plasma thus occur at the anterior boundary of
plasma first encountered by the incoming light. This result is similar as obtained in
literature [21, 71-72].
The formation of the plasma in saline solution was first detected when the
laser voltage was set at 580 V as shown in Figure 4.1. Therefore the minimum
energy required to initiate plasma formation in saline solution is 39.3 mJ. At the
threshold of the laser pulse, the plasma can be seen as a nearly round spark. The
45
plasma was then expanded as the laser energy increased above the threshold.
Multilobed pattern can be seen at double of the threshold energy which is 80.9 mJ
corresponding to capacitor voltage of 620 V. At higher energies, the multilobed
plasmas are greatly elongate up to seven times longer than the plasma seen at
threshold energy.
Similar studies have been reported by Hunkeler [21], using a Q-switched
ophthalmic Nd:YAG laser. They had confirmed that increasing the energy level
above the breakdown threshold causes the breakdown region to elongate along the
beam path in a multilobed configuration. For this work, the Nd:YAG laser used has
higher energy than the ophthalmic Nd:YAG laser used by Hunkeler.
It is also noticed that there is only a single spot of plasma seen in the cuvette
at capacitor voltage of 580 V equivalents with the laser energy of 39.3 mJ. As the
energy gets higher, the plasma is found to be increased. There are about five plasmas
appeared at 640 V and nine plasmas are detected at 720 V. Therefore, multiple
plasmas are observed within the focal region when the laser was focused using a
single lens.
At threshold voltage of 580 V, only a single spot was observed. This means
that the laser can be focused at a focal point. However, as the pumped power
increases, the configuration of the plasma changed. A possible reason for such
plasma formation is due to the existence of longer focal depth as shown in Figure
4.3. The focal depth value of single lens focusing technique is 11.14 mm as shown in
Appendix A.
In addition, such particular regions encourage suprathreshold to occur.
Suprathreshold means the breakdown occurs earlier than the focal point. As a result,
the incoming laser beam will be block and formed multilobe plasma. Therefore,
longer focal depth introduced suprathreshold phenomenon and multilobe plasma
formation. The length of focal depth also contribute multibreakdown as indicated by
the formation of multiplasma. The suprathreshold getting longer and longer as the
pumped power increases. The increment of pumped power can be noticed by the
increasing of capacitor voltage.
46
(a) 580 V
(b) 600 V
(c) 620 V
(d) 640 V
(e) 660 V
(f) 680 V
(g) 700 V
(h) 720 V
Figure 4.1: Plasma produced when single lens technique used in saline solution. Magnification of 6x. The direction of laser beam is toward the
right. These images are raw images captured during the experimental work.
47
(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
Figure 4.2: Growth of plasma anterior to the predicted focal point [21].
Laser
pulse
Normal beam
contour
Normal beam waist
at focal point
Figure 4.3: Multiple breakdown due to longer focal region [56].
48
4.3
Plasma Formation Induced by Combination Lenses Technique
Typical results obtained from this experiment are shown in Figure 4.4.
Entirely different configuration of plasma was observed. The threshold energy was
detected when the capacitor voltage was 600 V equivalent with laser energy of 60.2
mJ.
In the first three frames (Figure 4.4 (a), (b), and (c)), the shape of the plasmas
are almost circular, or disk-like shape. At higher laser energy, the plasma shape
changed to broad ellipse. Generally, the plasma also expanded as the laser energy
increases. By using this combination lenses technique, the image of plasma was
found to be more concentrated and sharper compared to single lens technique. Only
single breakdown have been observed throughout the experiment.
49
(a) 600 V
(b) 620 V
(c) 640 V
(d) 660 V
(e) 680 V
(f) 700 V
(g) 720 V
(h) 740 V
Figure 4.4: Plasma formed in saline solution. Magnification factor is 8x. The laser
is incident from the left. These images are raw images captured during the
experimental work.
50
4.4
Measurement of the Plasma Length
From the results, the white image area is expanding when higher laser energy
were applied for both cases. Therefore, the plasma expanded as the laser energy was
increased. The lateral measurement represents the plasma length. The plasma was
observed from the horizontal view of the laser beam as shown in Figure 4.5. The
distribution of plasma beam along the x-axis is shown in Figure 4.6. When the laser
beam is brought to focus by lens, the beam propagated in the form of Gaussian beam
which is shown in Figure 4.6 (a). The plasma was formed within the range of –x to x
or twice of the Rayleigh range which is also referred as confocal parameter [11] as
shown in Figure 4.6 (b). The Rayleigh range for single lens technique is 5.57 mm
and for combination lenses is 3.60 mm. Therefore, the confocal parameter for single
lens technique is 11.14 mm while for combination lenses is 7.20 mm.
The measurement of the plasma length is listed in Table 4.1. The relationship
between the plasma length and the laser energy is presented in Figure 4.7. The upper
curve represents the plasma length of single lens technique while the bottom curve
represents plasma length of combination lenses technique. The graph shows that the
length of plasma produced by a single lens focusing technique is always longer than
the plasma produced by combination lenses at equal energy, probably due to
different focal region. It was found that the focal region of the single lens technique
is longer than the combination lenses technique as shown in Appendix A. The
focusing geometry may also cause the different in the threshold energy for both
techniques. From the graph shown in Figure 4.7, the threshold energy of the single
lens technique is 39.3 mJ while the threshold energy of combination lenses technique
is 60.2 mJ.
For single lens technique, the plasma length increased with laser energy with
slope of 0.0309. In contrast, the plasma length produced by combination lens
technique increased gradually with slope of 0.0075 when the laser energy got higher.
Maximum length of plasma produced by single lens technique is 5.97 mm whereas
1.91 mm for combination lenses. The focal depth value of single lens technique is
11.14 mm while 7.20 mm for combination lenses technique. Therefore, the formation
51
of plasma is still in the range of the focal depth value for both techniques. It is
believed that the plasma length can reach the maximum value of the focal depth if
the laser energy is further increased. In this study, the maximum operating voltage of
the laser used is 740 V or at maximum energy of 180.3 mJ.
(a)
(b)
Single lens focusing technique
Combination lenses focusing technique
Figure 4.5: Observation of plasma using different focusing techniques.
52
Intensity
(a)
Gaussian beam profile
High intensity
Confocal parameter
-x
(b)
x=0
x
Plasma configuration
Figure 4.6: The distribution of plasma beam along the x-axis [11]
Table 4.1: Plasma length measured for both techniques.
Voltage (V)
Energy (mJ)
Plasma length (mm)
Single lens
Combination lenses
580
39.3
0.82
Nil
600
60.2
1.75
1.05
620
80.7
2.47
1.27
640
99.0
3.47
1.36
660
115.3
3.86
1.41
680
133.0
3.43
1.59
700
149.7
3.97
1.86
720
165.0
4.72
1.95
740
180.3
5.97
1.91
53
7.00
Plasma Length (mm)
6.00
y1 = 0.0309x - 0.1308
5.00
4.00
3.00
y2 = 0.0075x + 0.7794
2.00
1.00
Single lens
Combination lens
0.00
0
50
100
150
Laser energy (mJ)
Figure 4.7: Plasma length with respect to laser energy.
200
CHAPTER 5
PLASMA TEMPERATURE
5.1
Introduction
Langmuir probes have been used routinely in plasma diagnostics because of
its good measurement ability and simple structure [36-39, 41, 73]. They are generally
easy to use and robust enough to withstand considerable heat fluxes. Besides, they
have excellent spatial resolution, limited only by the probe size and by the accuracy
of the positioning mechanisms [36, 38, 64, 74]. Furthermore, they are the least
expensive and still the fastest and most reliable diagnostic tools allowing one to
obtain the values of very important plasma parameters. Although probes generally
perturb their local surrounding to some extent and interpretation of the data is not
straightforward, the above mentioned advantages usually justify the use of probe
techniques where possible [38].
Langmuir probe data analyses are carried out manually or more frequently
with computer assistance to obtain the plasma parameters. When analyzing the
Langmuir probe data, the conditions under which the plasma is generated and the
probe’s interaction with the local plasma need to be considered. Otherwise, it is easy
to misinterpret the results pertaining to the plasma potential, temperature or plasma
density [41].
55
5.2
Plasma Temperature
The plasma temperature depends on the laser power, target material, position
of probe inserted in plasma, oscilloscope resolution and environment under which
the plasma is generated [41, 64]. In practice, Langmuir probe data contain noise due
to the background noise contributed by the flashlamp driver, cooling system and
other electronic devices. Therefore, in order to get a good Langmuir probe data, a
Faraday cage has been utilized to minimize the noise throughout the experiment.
Although those noises are avoidable using Faraday cage, some unwanted signals are
still recorded. However, since the current drawn from the interaction of local plasma
and the probe is high, the signal-to-noise ratio is still good enough [41].
The typical current signals in term of voltage amplitude collected by
Langmuir probe are shown in Figure 5.1 and 5.2. From Figure 5.1, it can be seen that
the higher the potential supplied to the probe, the higher the amplitude of the signal
produced by the probe. The same condition goes when the probe potential is
decreased as shown in Figure 5.2. The negative signal amplitude increases with the
increase of the negative bias voltage of the probe.
56
(a) 1.5 V
(b) 4.0 V
(c) 6.5 V
(d) 10.0 V
(e) 12.0 V
(f) 18.0 V
Figure 5.1: Typical signals collected by Langmuir probe as a function of positive
bias voltage. (y-axis: 500 mV/division, x-axis:200 ns/division)
57
(a) 0.5 V
(b) -2.0 V
(c) -6.5 V
(d) -9.5 V
(e) -12.5 V
(f) -18.0 V
Figure 5.2: Typical signals collected by Langmuir probe as a function of negative
bias voltage. (y-axis: 500 mV/division, x-axis:200 ns/division)
58
The current values are calculated using Ohm’s law, V=IRL where V is the
voltage amplitude of the signal and RL (0.1 Ù) is the resistor value of the Langmuir
probe detection circuit as shown in Figure 3.9 (b). The data analyzed from the signals
are tabulated in Table 5.1. The collected data were plotted in a graph of Langmuir
probe current versus probe voltage. The I-V characteristic curve obtained from the
Langmuir probe experiment is shown in Figure 5.3. The trend of the graph obtained
is in a good agreement with the theoretical I-V characteristic as shown in Figure 2.5.
The electron temperature can be calculated by taking the slope of the I-V
curve at the origin according to Equation (2.6). The value of Is was found to be 9.00
A from the I-V characteristic graph. The linear part of the graph shown in Figure 5.4
has a slope of 4.3218. Therefore, the electron temperature of copper plasma was
found to be 1.04 ± 0.31 eV. A temperature of 1eV corresponds to 11600 K. Thus the
plasma temperature was 12064 ± 3619 K or 11791 ± 3537 oC.
The obtainable plasma temperature is almost similar to the literature [31, 75].
Thus, such high temperature has a great opportunity in damaging the target material
through the mechanism of vaporization and melting. Since the size of plasma is in
micro scale, thus the damage size involves on the target material will be quite a small
volume. Consequently, the interaction of the focused laser beam with material will
induced a high potential of damage.
59
Table 5.1: Data obtained from the Langmuir probe signal detected by oscilloscope.
Probe
potential
(± 0.01 V)
-18.00
-17.50
-17.00
-16.50
-16.00
-15.50
-15.00
-14.50
-14.00
-13.50
-13.00
-12.50
-12.00
-11.50
-11.00
-10.50
-10.00
-9.50
-9.00
-8.50
-8.00
-7.50
-7.00
-6.50
-6.00
-5.50
-5.00
-4.50
-4.00
-3.50
-3.00
-2.50
-2.00
-1.50
-1.00
-0.50
-0.30
-0.10
0.00
0.20
Signal
Amplitude
(± 9% mV)
-587.500
-570.833
-570.833
-575.000
-597.917
-577.083
-512.500
-487.500
-464.583
-475.000
-454.167
-433.333
-391.667
-360.417
-335.417
-337.500
-333.333
-291.667
-304.167
-281.250
-250.000
-235.417
-225.000
-197.917
-195.833
-200.000
-195.833
-181.250
-183.333
-175.000
-164.583
-164.583
-156.250
-150.000
-125.000
-122.917
-117.000
-109.250
-104.167
95.000
Probe
current
(± 14% A)
-5.88
-5.71
-5.71
-5.75
-5.98
-5.77
-5.13
-4.88
-4.65
-4.75
-4.54
-4.33
-3.92
-3.60
-3.35
-3.38
-3.33
-2.92
-3.04
-2.81
-2.50
-2.35
-2.25
-1.98
-1.96
-2.00
-1.96
-1.81
-1.83
-1.75
-1.65
-1.65
-1.56
-1.50
-1.25
-1.23
-1.17
-1.09
-1.04
0.95
Probe
potential
(± 0.01 V)
0.30
0.40
0.50
0.70
0.90
1.00
1.50
2.00
2.50
3.00
3.50
4.00
4.50
5.00
5.50
6.00
6.50
7.00
7.50
8.00
8.50
9.00
9.50
10.00
10.50
11.00
11.50
12.00
12.50
13.00
13.50
14.00
14.50
15.00
15.50
16.00
16.50
17.00
17.50
18.00
Signal
amplitude
(± 9% mV)
148.000
177.125
218.750
375.000
421.875
468.750
500.000
515.625
531.250
546.875
567.708
572.917
593.750
604.167
630.208
666.667
703.125
708.333
723.958
729.167
739.583
760.417
765.625
791.667
812.500
808.875
817.708
817.708
822.917
859.375
854.167
890.625
890.558
901.042
880.208
890.625
875.000
885.417
218.750
375.000
Probe
current
(± 14% A)
1.48
1.77
2.19
2.67
3.44
3.75
4.22
4.69
5.00
5.16
5.31
5.47
5.68
5.73
5.94
6.04
6.30
6.67
7.03
7.08
7.24
7.29
7.40
7.60
7.66
7.92
8.13
8.09
8.18
8.18
8.23
8.59
8.54
8.91
8.91
9.01
8.80
8.91
8.75
8.85
60
Probe current, I (A)
12.00
11.00
Is
Electron saturation
10.00
9.00
8.00
Electron retardation
7.00
6.00
5.00
4.00
3.00
2.00
1.00
-20.00
-15.00
-10.00
-5.00
0.00
-1.000.00
Linear part
5.00
10.00
-2.00
15.00
20.00
Probe potential, Vpp (V)
-3.00
-4.00
-5.00
-6.00
-7.00
-8.00
Figure 5.3: I-V characteristic curve of Langmuir probe.
650 V
61
Probe current, I (A)
5
y = 4.3218x - 0.2611
4
3
2
1
-0.20
0
0.00
0.20
0.40
0.60
0.80
1.00
1.20
Probe potential, Vp (V)
-1
-2
Figure 5.4: Linear part of the I-V characteristic curve.
CHAPTER 6
GENERATION OF PRESSURE WAVES
6.1
Introduction
Disruption of ocular tissue by high-powered laser pulses occurs as a
consequence of optical breakdown, plasma formation and related mechanical events
[76]. The rapid expansion of plasma leads to acoustic shockwave generation [43]
which produces extreme pressure changes [2]. This experiment was carried out to
estimate the pressure produced by focusing a Q-switched Nd:YAG laser in liquid.
Various methods have been used to measure the generation of acoustic shockwave in
liquids. Direct measurement of the acoustic shockwave signals or pressure pulse
generated by lasers in various liquids have been made using a piezoelectric
transducer [43, 77-79]. Different magnitudes have been recorded, for example, 1-10
kbar [77] and 13 bar was reported for measurement 14 mm below the water surface
where the CO2 laser radiation was strongly absorbed [61]. Lower magnitudes of 0.1
bar and 0.2 bar for 3.21 and 3.78 ìm irradiation of ArF Excimer laser have also been
reported by Lee and Doukas [43]. Vodop’yanov et al [80] have generated pressure of
20 kbar in liquids using Er:YAG mode-locked laser.
63
6.2
Pressure Measurement
In this study, the acoustic shockwave was detected by 2013V High
Sensitivity Microphone with sensitivity of 1.96 V/psi which was coupled to an
oscilloscope. The acoustic signal detected by the transducer was converted to an
electrical signal which is shown as a voltage-time curve. The distance was measured
between the probe surface and the point of the acoustic shockwave source.
The signal of the acoustic shockwave was taken at various laser voltages and
at different distances as shown in Figure 6.1, Figure 6.2 and Figure 6.3. Generally,
only one signal was picked up at low energy. As the energy increased, two signals
were detected. The first peak corresponds to the acoustic shockwave from the initial
laser breakdown. The second signal which has lower amplitude might result from the
cavitations process as it appeared about 300 µs after the initial laser breakdown.
These results are in good agreement with literature [61].
The amplitudes of the acoustic shockwave as a function of laser voltage at
various distances are tabulated in Table 6.1. The pressure of the acoustic shockwave
is indicated by the peak-to-peak amplitude of the first signal. Calculation of the
pressure generated is shown in Appendix F. The calculated pressures for different
voltage and distance are listed in Table 6.2.
At a short distance of 1.87 mm, the acoustic shockwave pressure detected is
0.0127 bar corresponding to laser energy of 60.2 mJ. The pressure was found to
increase to 0.0199 bar and 0.0254 bar corresponding to laser energy of 115.3 mJ and
180.3 mJ respectively. Thus, the pressure of the acoustic shockwave is linearly
increased with respect to the increase in laser energy. This trend is similar at further
distances but with smaller pressure as stated in Table 6.2. Based on the listed data,
the maximum pressure detected is 0.0254 bar.
Lee and Doukas [43] have reported a pressure value of 0.1 bar generated at
1.6 mm by a free electron laser. According to them, these values are much lower than
64
what is needed to produce a biological response [43]. In addition, 95 % of biological
tissue remained intact at pressure value of 700 bar [81].
(a) 600 V
(b) 620 V
(c) 640 V
(d) 680 V
(e) 700 V
(f) 740 V
Figure 6.1: Typical acoustic shockwave signals detected at different voltage for
constant distance of 1.87 mm. (y-axis: 200 mV/division, x-axis: 500 ns/division)
65
(a) 600 V
(b) 620 V
(c) 640 V
(d) 680 V
(e) 700 V
(f) 740 V
Figure 6.2: Typical acoustic shockwave signals detected at different voltages for
constant distance of 2.56 mm. (y-axis: 200 mV/division, x-axis: 500 ns/division)
66
(a) 600 V
(b) 620 V
(c) 640 V
(d) 680 V
(e) 700 V
(f) 740 V
Figure 6.3: Typical acoustic shockwave signals detected at various voltages for
constant distance of 5.76 mm. (y-axis: 200 mV/division, x-axis: 500 ns/division)
67
Table 6.1: Amplitude of the signals detected for different oscillator voltages.
Signal amplitude (± 7% mV)
Laser voltage
Laser energy
(± 1 V)
(± 0.1 mJ)
D1 (1.87 mm)
D2 (2.56 mm)
D3 (5.76 mm)
600
60.2
360.417
314.583
266.667
620
80.7
375.000
356.167
308.333
640
99.0
495.833
431.250
375.000
660
115.3
566.333
554.167
491.500
680
133.0
608.083
593.750
562.333
700
149.7
647.917
620.833
600.000
720
165.0
689.500
647.917
616.667
740
180.3
722.917
691.667
660.617
Table 6.2: Pressure amplitude as a function of laser energy at various distances.
Pressure (± 7% bar)
Laser energy
6.3
(± 0.1 mJ)
D1 (1.87 mm)
D2 (2.56 mm)
D3 (5.76 mm)
60.2
0.0127
0.0111
0.0094
80.7
0.0132
0.0125
0.0108
99.0
0.0174
0.0152
0.0132
115.3
0.0199
0.0195
0.0173
133.0
0.0214
0.0209
0.0198
149.7
0.0228
0.0218
0.0211
165.0
0.0243
0.0228
0.0217
180.3
0.0254
0.0243
0.0232
Pressure Profile
A graph of pressure versus laser energy was plotted as shown in Figure 6.4.
In general, the pressure induced by shockwave is linearly increased with respect to
68
the laser energy with a slope of 0.0001. This indicates that the higher the laser
energy, the stronger the pressure induced by the acoustic shockwave. The top curve
indicates the profile of acoustic shockwave pressure at the shortest distance. This is
followed by the shorter distance and the lowest graph shows the pressure profile of
the longest distance.
This result is in a good agreement with theory based on the Equation (2.9)
and (2.10) which has been discussed previously in Section 2.3.3 of Chapter 2.
According to Equation (2.9) and (2.10), the pressure increases as the laser pulse
energy increases. It is understood as the plasma formation also increases as the laser
energy increases. In particular, when the target is irradiated by the laser, the laser
energy will be absorbed and free electrons will be generated. Once the starting free
electrons have been generated, plasma grows through the mechanism of electron
avalanche. The plasma then explodes and drives an acoustic shockwave at the focal
region. As more laser energy is absorbed, more free electrons will be generated.
Thus, larger plasma will grow and higher pressure of acoustic shockwave will be
generated.
Graph of pressure amplitude as a function of distance was then plotted as
shown in Figure 6.5. From the graph, it can be seen that the differences of pressure
amplitude between the tested distances are very small. Therefore, the pressure
amplitude remains constant upon the distance of observation. The result obtained is
against the literature [50]. According to Equation (2.9) and (2.10), the pressure
decreases when the distance increases. This unexpected finding could be due to the
small range of the tested distances. The range of the tested distance should be
expanded to get a better result but in this case, it is limited by the size of the cuvette.
In addition, the pressure value detected in this experiment is too low. This
may be because the transducer used to detect the pressure was placed very far from
the point of optical breakdown. For future work, measurement of pressure at
micrometer range of distance from the point of breakdown need to be done for
further understanding.
69
0.0300
0.0250
Pressure (bar)
0.0200
0.0150
0.0100
D1 (1.87 mm): y 1 = 0.0001x + 0.0057
D2 (2.56 mm): y 2 = 0.0001x + 0.0042
0.0050
D3 (5.76 mm): y 3 = 0.0001x + 0.0018
0.0000
50.0
75.0
100.0
125.0
150.0
175.0
200.0
Laser energy (mJ)
Figure 6.4: Acoustic shockwave pressure as a function of laser energy at three
different distances.
0.0250
Pressure (bar)
0.0230
0.0210
0.0190
0.0170
0.0150
0
0.5
1
1.5
2
2.5
3
3.5
Distance, D (mm)
Figure 6.5: Acoustic shockwave pressure plotted against various distances for laser
energy of 149.7 mJ.
CHAPTER 7
PHOTODISRUPTION EFFECTS ON PMMA
7.1
Introduction
In this study, clear PMMA was used as a target material. The Nd:YAG laser
was focused by using combination lenses. The combination lenses focusing
technique was chosen as the plasma produced is more concentrated and sharp. The
target surface was placed in a pyrex cuvette filled with saline solution. The aim of
the study is to investigate the photodisruption effects. The PMMA was used to
simulate intraocular lenses (IOLs) which normally made of plastic. The target was
irradiated at different energy and number of laser pulses. The exposed material was
then examined using image analysis method.
71
7.2
Photodisruption Effects
The typical results obtained from photodisruption experiment are shown in
Figure 7.1. The images are arranged in the increasing order of laser energy, which
was verified by the capacitor voltage. The effect or damage on PMMA has first been
detected when the laser voltage was at 600 V. The damage threshold for PMMA is
found to be at energy fluence of 6.87 x 102 J/cm2 or about 1010 W/cm2. The damage
threshold is given by threshold power per unit area as shown in Appendix G. The
laser energy was increased by operating the laser beyond the threshold voltage.
In this study, the results obtained show that the damage takes the form of
melted voids, holes, cracks and large pulverized regions. Absorption of energy plays
a major role and produced different patterns of damage. In our opinion, the
responsible mechanisms that induce these damages are thermal shock and plasma
production, and localized heating and vaporization.
The forms of damage like cracks and large pulverized region need to be
avoided in ophthalmology applications as it can be very destructive to the eye
structure. Therefore, it is suggested that the laser beam need to be carefully focused
on or slightly behind the treated target. Otherwise, undesirable pitting or cracking of
the target may occur.
Further experiment was carried out by increasing number of pulses to 5 and
10 pulses such as shown in Figure 7.2 and Figure 7.3 respectively. In addition, the
experiment was also done for different number of pulses at constant laser energy.
Typical results obtained for different number of pulses at constant energy operated at
capacitor voltage of 700 V are shown in Figure 7.4. The images were analyzed using
VideoTest 5.0 software. In this particular case, the area of damage for each image
was measured.
72
(a) 30.9 mJ
(b) 43.4 mJ
(c) 69.9 mJ
(d) 93.0 mJ
(e) 101.8 mJ
(f) 112.6 mJ
Figure 7.1: Damage induced by a single laser pulse on PMMA (Magnification of
10x). The resolution of the actual image is 640 x 512 pixels.
73
(a) 30.9 mJ
(b) 56.9 mJ
(c) 69.9 mJ
(d) 82.3 mJ
(e) 101.8 mJ
(f) 112.6 mJ
Figure 7.2: Damage induced by 5 pulses of Q-Switched Nd:YAG laser on PMMA
(Magnification of 10x). The resolution of the actual image is 640 x 512 pixels.
74
(a) 30.9 mJ
(b) 56.9 mJ
(c) 69.9 mJ
(d) 82.3 mJ
(e) 101.8 mJ
(f)112.6 mJ
Figure 7.3: Effects on PMMA which has been exposed to 10 pulses of Q-switched
Nd:YAG laser (Magnification of 10x). The resolution of the actual image is 640 x
512 pixels.
75
(a) 1 pulse
(b) 2 pulses
(c) 4 pulses
(d) 6 pulses
(e) 7 pulses
(f) 8 pulses
Figure 7.4: Target irradiated at different number of pulses at laser energy of 93.0 mJ
(Magnification of 10x). The resolution of the actual image is 640 x 512 pixels.
76
The data measured from Figure 7.1 to Figure 7.3 are tabulated in Table 7.1
whereas the measurements taken from Figure 7.4 are listed in Table 7.2. A graph of
damaged area is plotted with respect to laser energy. The three curves representing
the damage profile due to single pulses, five pulses and ten pulses shown in Figure
7.5. The top curve represents the profile of damage obtained after 10 pulses.
Optimum damage occurred at laser energy of 40 mJ. As the laser energy was further
increased, the damage areas drop and remain almost constant at higher laser energy.
In contrast, after 5 pulses of exposure, the damaged area obtained at threshold
voltage is considerably highest among the tested pulses which is 0.0385 mm2. In
comparison to the single and ten pulses, the damage area is minimum which is
0.0045 mm2. However, beyond threshold voltage, the photodisruption effect on the
material is slightly increased. In fact, photodisruption effects due to the single pulse
have almost the same profile with 5 pulses. Therefore, it can be stated that the
damage profiles are almost overlaps.
As a result, the damage occurs is almost constant. In other word, the results
obtained show that the damage or effect of photodisruption is independent with
respect to the energy of the laser. These unexpected results may be due to the
presence of internal defects or structural inhomogeneities such as microcracks or
voids in the target. These structural inhomogeneities may serve as starting points for
optical breakdown. It is difficult to understand unless further investigation on the
PMMA is being done.
From the result of measurement obtained in Figure 7.6, the damaged area was
plotted against number of pulses, while the energy of laser is kept constant. The
photodisruption effect is found to be constant after being exposed at single pulse up
to five pulses. The damaged area is drastically increased after being exposed to more
than five pulses. As a result, the damage profile increases nonlinearly with respect to
the number of pulses. Overall, the photodisruption effects induced by focusing
Nd:YAG laser is found dependent on the number of pulses or exposure. This could
be understood as a cumulative effect. In particular, when the target is exposed to a
number of laser pulses, damage may begin with a small fracture. Once the first
damage has been produced, the laser light will be absorbed and heating effects can
77
occur in the interior of the material. Heating and vaporization near the focal point of
the laser can be expected to cause melting and cracking in the material. Thus, the
effects will be more extensive when larger number of laser pulses being focused to
the target.
In summary, the photodisruption due to Nd:YAG laser exposure is
independent upon laser energy, but dependent to the number of pulses.
Table 7.1: Damaged area measured for different laser energy for 1, 5 and 10 pulses.
Damaged Area (mm2)
Laser energy (mJ)
1 pulse
5 pulses
10 pulses
30.9
0.0045
0.0385
0.0045
43.4
0.0045
Nil
0.1110
56.9
0.0005
0.0015
0.0300
69.9
0.0025
0.0080
0.0200
82.3
0.0035
0.0080
0.0260
93.0
0.0160
0.0130
0.0055
101.8
0.0065
0.0200
0.0185
112.6
0.0075
0.0205
0.0175
Table 7.2: Damaged area measured for various number of laser pulses.
Number of laser pulses
Damaged Area (mm2)
1
0.0280
2
0.0250
3
0.0950
4
0.0175
5
0.0240
6
0.0670
7
0.1125
8
0.2800
78
0.12
1 pulse
5 pulses
0.1
2
Damaged area (mm )
10 pulses
0.08
0.06
0.04
0.02
0
20
30
40
50
60
70
80
90
100
110
120
Laser energy (mJ)
Figure 7.5: Damaged area as a function of laser energy for different number of
pulses.
0.3000
2
Damaged area (mm )
0.2500
0.2000
0.1500
0.1000
0.0500
0.0000
0
1
2
3
4
5
6
7
8
9
Number of pulses
Figure 7.6: Damaged area versus number of laser pulses taken at laser energy of 93.0
mJ.
CHAPTER 8
CONCLUSION
8.1
Introduction
In this work, the elements of photodisruption mechanism were studied. A Qswitched Nd:YAG laser was used as a photodisruptor. A simulation of an eye model
was conducted as specimen. The laser was focused in the eye medium that is saline
solution and PMMA material as intraocular lens. The plasma formation and acoustic
shockwave generation were identified as photodisruption mechanism. The dynamic
expansion of plasma and the plasma temperature were measured using CCD camera
and Langmuir probe respectively. The pressures were measured using piezoelectric
transducer and the effect of photodisruption was studied using image analysis
method.
80
8.2
Conclusion
The 1064 nm and 10 ns Q-switched Nd:YAG laser has been utilized to induce
the photodisruption in saline solution. The main elements of photodisruption that
have been characterized are plasma formation and acoustic-shockwave generation.
The effects of laser photodisruption are tested on PMMA target which was placed in
saline solution to simulate eye condition.
Firstly, the plasma formation has been observed using a CCD camera which
was interfaced with personal computer. The plasma is generated using two different
focusing techniques. The first one is single lens technique while the latter is
combination of lenses technique. Multiple plasma formations have been observed for
the first technique. By applying combination of lenses technique, the plasma is more
concentrated and stable compared to the formation of plasma by single lens focusing.
The length of plasma was then measured and the result shows that the plasma
formation depends on the laser energy and the geometrical distribution of the beam.
As a conclusion, the plasma induced in the saline solution increases in length as the
pulse energy increased. In addition, the overall length of the plasma depends on the
beam geometry, being higher for larger spot size and lower for smaller one.
Langmuir Probe was employed to measure the plasma temperature. The
temperature of plasma produced in this study is measured to be 1.04 eV or 12,064 K.
This high temperature will cause a focal heating which can lead to a phase change
and thermal expansion. Both mechanisms may combine to generate acousticshockwaves radiating from the zone of optical breakdown.
The experimental work was then carried out to observe the pressure of
acoustic-shockwave associated with plasma formation during optical breakdown.
The pressure amplitude of acoustic-shockwave increased with laser energy. The
maximum pressure amplitude detected was 0.0254 bar. This value was too low to
produce biological response [43] and it can be neglected as it is safe to be applied for
medical. Mulholland et al [83] have reported 95 % of biological tissue remained
intact at pressure value of 700 bar.
81
PMMA has been used as a target to demonstrate the effects of
photodisruption. The damage threshold was estimated to be 6.86 x 102 J/cm2. The
damages are expected to be dependent on the energy but the damaged area was found
to fluctuate with the increment of the laser energy. In contrast, damages produced
increase nonlinearly with respect to the number of laser pulses. The damage patterns
produced are random. Based on the results, the damages are in the form of cracks,
melted voids, holes and large pulverized regions. These results may be affected by
the internal defects of the target material but somehow still not to be understood.
As a conclusion, the characterization of photodisruption induced by Qswitched Nd:YAG laser has been successfully carried out. The damages induced by
Q-switched Nd:YAG laser photodisruption can be very useful for medical
application but it also produce some undesired damages which should be avoided.
Thus, the photodisruption mechanisms need to be carefully characterized to ensure
the safety and efficient use of Q-switched Nd:YAG laser for medical applications.
8.3
Recommendations
For future works, it is suggested that the imaging equipment is synchronized
with the laser source. An ultra high-speed of photographic system can be used in
order to precisely capture the formation of plasma. Furthermore, instead of using the
piezoelectric transducer, interferometer method could also be used to study the
generation of acoustic-shockwave. The technique would provide more information
which could be used to estimate the physical parameters changes in the focal region.
The conventional PMMA used can be replaced with PMMA IOLs. The IOL
manufacturing techniques and materials had minimized in inhomogeneities,
impurities and inclusions which result in a highly polished surface. This could reduce
the tendency of IOL damage during laser photodisruption.
82
In addition, the laser beam was brought to the target through air using some
optical lenses alignment. In future study, fiber optic can be employed to deliver the
laser beam to a target site as the current fiber optic technology has been welldeveloped.
83
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89
APPENDIX A
Measurement of laser beam parameters
The laser beam spots have been taken at different position along the focal region and
the beam diameters have been measured using VideoTest 5.0 software. Based on ISO
Standardized Method [82, 83], the measured beam diameters are fitted with a
hyperbola using the independent fit parameters A, B, C:
d 2  z   A  Bz  Cz 2
(1)
The laser beam parameters are given by:
Beam waist, w0 =
A
Rayleigh Range, z R 
a)
B2
4C
1
C
(2)
AC 
B2
4
Single lens technique
a  2mm
28 mm
Figure 3.15: Single lens focusing technique.
(3)
90
Table 1: Measured beam diameters at different position along the laser beam waist
Position
Beam diameter
Position
Beam diameter
(± 0.1 mm)
(mm)
(± 0.1 mm)
(mm)
33.0
1.82
42.0
0.72
34.0
1.77
43.0
0.80
35.0
1.56
44.0
0.93
36.0
1.17
45.0
1.18
37.0
0.91
46.0
1.39
38.0
0.78
47.0
1.57
41.0
0.68
48.0
1.70
2.50
Beam diameter (mm)
y = 0.0209x 2 - 1.7048x + 35.415
2.00
1.50
1.00
0.50
0.00
30.0
35.0
40.0
45.0
50.0
Position (mm)
Figure 1: Measured beam diameters taken at different positions for single
lens focusing technique.
91
b)
Combination lenses technique
a  2mm
28 mm
-25 mm
Figure 3.16: Combination of two lenses to focus the laser beam.
Table 2: Measured beam diameters at different position along the laser beam waist
Position
Beam diameter
Position
Beam diameter
(± 0.1 mm)
(mm)
(± 0.1 mm)
(mm)
1.0
1.18
10.0
0.21
3.0
0.91
11.0
0.35
4.0
0.66
12.0
0.51
5.0
0.48
13.0
0.60
6.0
0.25
14.0
0.72
7.0
0.21
15.0
0.94
9.0
0.19
16.0
1.10
92
Beam diameter (mm)
1.40
y = 0.0173x 2 - 0.3007x + 1.5303
1.20
1.00
0.80
0.60
0.40
0.20
0.00
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
16.0
18.0
Position (mm)
Figure 2:
Measured beam diameters taken at different positions for
combination lenses focusing technique.
Value of laser beam parameters
By using Equation (2) and (3), the laser beam waist, w0 and the Rayleigh range,
zR has been calculated. The depth of focus (also referred as focal region) is
indicated by twice of Rayleigh range where ± ∆z = 2 x zR The laser beam
parameters values are listed in Table 3.
Table 3: Laser beam parameters values of single lens and combination lenses
focusing techniques.
Laser beam parameters
Single lens focusing
Combination lenses
technique
focusing technique
Beam waist, w0 (mm)
0.81
0.47
Rayleigh range, zR (mm)
5.57
3.60
Depth of focus, ± ∆z (mm)
11.14
7.20
93
APPENDIX B
Refractive index of natrium chloride solution as a function of its concentration
expressed in percentage [58].
94
APPENDIX C
Main properties of PMMA [84].
Property
Thermal expansion coefficient (0oC)
Compressibility
Density
Dielectric constant (1 kHz) (25oC)
Elongation at break
Glass transition
Impact strength (notched Izod)
Value
7 x 10-5 K-1
245 x 106 MPa
1.195 g/cm3
3.0
2 % to 10 %
105 oC
2 MPa to 3.4 MPa
Melting point (isotactic)
138 oC
Refractive index
1.492
Tensile strength
48.3 MPa to 75.8 MPa
Tensile modulus
2619 MPa
95
APPENDIX D
Table 1: Q-switched Nd:YAG laser energy upon laser voltage.
Energy (mJ)
Voltage (V)
Internal trigger
External trigger
500
0.0
0.0
520
0.0
0.0
540
3.5
1.3
560
20.1
10.3
580
39.3
18.7
600
60.2
30.9
620
80.7
43.4
640
99.0
56.9
660
115.3
69.9
680
133.0
82.3
700
149.7
93.0
720
165.0
101.8
740
180.3
112.6
96
APPENDIX E
Dimension of 2013V High Sensitivity Microphone [65].
97
APPENDIX F
Calculation of the pressure of the acoustic shockwave (Chapter 6, Section 6.2)
Vp-p = 647.917 mV
Signal detected when the laser operated at 700 V.
Peak-to-peak amplitude of the first signal, Vp-p = 647.917 mV
Sensitivity of the transducer, S = 1.96 V/psi = 1960 mV/psi
1 psi = 0.069 bar
Pressure, P =
=
VP  P
S
647.917 mV
(1960 mV / psi )
= 0.33057 psi
= 0.0228 bar
98
APPENDIX G
Calculation of damage threshold of PMMA (Chapter 7, Section 7.2)
Minimum laser energy required to induce damage on PMMA, Emin =30.9 mJ
Area of damage measured at 30.9 mJ, A = 0.0045 mm2
Energy fluence, E
=
E min
A
0.309 J
=
2
0.0045  10  cm
= 6.87 x 102 J/cm2
Threshold power, Pmin =
=
E min
t
0.309 J
10 ns
= 3.09 x 106 J
Damage threshold, Pth =
=
Pmin
A
3.09  10 6 J
0.0045  10  2 cm 2
= 6.87 x 1010 W/cm2
≈
1010 W/cm2
2