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 PAGE

Title page

Declaration of originality

Dedication

Acknowledgements

Abstract

Abstrak

Table of Contents

List of Tables

List of Figures

List of Symbols

List of Appendices

i

ii

iii

iv

v

vi

vii

x

xi

xvi

xviii

1 INTRODUCTION 1

1.1 Overview

1.2 Problem Statement

1.3 Research Objective

1.4 Research Scope

1.5 Thesis Outline

1

2

3

4

4

2 THEORY 6

2.1 Introduction

2.2 Laser Beam Focusing

2.3 Photodisruption

6

7

9

viii

2.3.1 Optical Breakdown

2.3.2 Plasma

2.3.2.1 Plasma Formation

2.3.2.2 Plasma Temperature

2.3.3 Acoustic Shockwave Generation

2.4 Laser Interaction with Transparent Material

2.5 Conclusion

11

14

14

15

18

20

22

3 METHODOLOGY 23

3.1 Introduction

3.2 Samples

3.2.1 Saline Solution

3.2.2 Polymethylmethacrylate (PMMA)

3.3 Nd:YAG Laser System

3.3.1 Pockels Cell

3.3.2 External Triggering Circuit

3.4 Measurement Equipment

3.4.1 Power Meter

3.4.2 Photodetector

3.4.3 Langmuir Probe

3.4.4 Pressure Sensor

3.5 Imaging Equipment

3.6 Image Calibration

3.7 Experimental Setup

3.7.1 Observation of Plasma Formation

3.7.2 Plasma Temperature Measurement

3.7.3 Detection of Pressure Waves

3.7.4 Photodisruption Effects on PMMA

23

24

24

25

25

27

28

30

30

31

31

33

33

36

37

37

39

40

41

4 PLASMA FORMATION 43

4.1 Introduction

4.2 Plasma Formation Induced by Single Lens Technique

4.3 Plasma Formation Induced by Combination Lenses

43

44

48

ix

Technique

4.4 Measurement of the Plasma Length

50

5 PLASMA TEMPERATURE 54

5.1 Introduction

5.2 Plasma Temperature

54

55

6 GENERATION OF PRESSURE WAVES 62

6.1 Introduction

6.2 Pressure Measurement

6.3 Pressure Profile

62

63

67

7 PHOTODISRUPTION EFFECTS ON PMMA 70

7.1 Introduction

7.2 Photodisruption Effects

70

71

8 CONCLUSION 79

8.1 Introduction

8.2 Conclusion

8.3 Recommendations

79

80

81

REFERENCES

Appendices A - G

83

89 - 98

x

LIST OF TABLES

TABLE NO.

TITLE

PAGE

3.1 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.

59

6.1 Amplitude of the signals detected for different oscillator

voltages.

67

6.2 Pressure amplitude as a function of laser energy at

various distances.

67

7.1 Damaged area measured for different laser energy for 1,

5 and 10 pulses.

77

7.2 Damaged area measured for various number of laser

pulses.

77

xi

LIST OF FIGURES

FIGURE NO.

TITLE

PAGE

2.1 The depth of focus of the laser light [11].

7

2.2 Beam diameter of a Gaussian beam as fundamental mode

TEM00 and function of z [11].

8

2.3 Mechanism of photodisruption induced by Q-switched

Nd:YAG laser [21].

11

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].

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].

22

3.1 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].

32

3.10 The voltage mode pressure sensor used to detect the

pressure waves signals.

33

3.11 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

burn paper.

35

3.14 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.

42

4.1 Plasma produced when single lens technique used.

Magnification of 6x. The direction of laser beam is

toward the right.

46

4.2 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

techniques:

(a) Single lens focusing technique

(b) Combination lenses focusing technique

51

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.

56

5.2 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.

64

6.2 Typical acoustic shockwave signal detected at different

voltage at a distance of 2.56 mm.

65

6.3 Typical acoustic shockwave signal detected at different

voltage at a distance of 5.76 mm.

66

6.4 Acoustic shockwave pressure as a function of laser

energy at three different distances.

69

xv

6.5 Acoustic shockwave pressure plotted against various

distances.

69

7.1 Damage induced by a single laser pulse on PMMA

(Magnification of 10x).

72

7.2 Damage induced by 5 pulses of Q-Switched laser on

PMMA (Magnification of 10x).

73

7.3 Effects on PMMA which has been exposed to 10 pulses

of Q-switched Nd:YAG laser (Magnification of 10x).

74

7.4 Target irradiated at different number of pulses at laser

energy of 93.0 mJ. (Magnification of 10x).

75

7.5 Damaged area as a function of laser energy for different

number of pulses.

78

7.6 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 PAGE

A Measurement of laser beam parameters

89

B Refractive index of natrium chloride solution as a

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.

95

E Dimension of 2013V High Sensitivity Microphone

[65].

96

F Calculation of the pressure of the acoustic shockwave

(Chapter 6, Section 6.2)

97

G 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 acoustic-

shockwave 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.