STUDY OF SATURABLE ABSORBER MATERIALS FOR Q-SWITCHING DYE
LASER
NUR FARIZAN BINTI MUNAJAT
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
AUGUST 2005
PSZ 19:16 (Pind. 1/97)
UNIVERSITI TEKNOLOGI MALAYSIA
BORANG PENGESAHAN STATUS TESIS
JUDUL: STUDY OF SATURABLE ABSORBER MATERIALS FOR QSWITCHING LASER
SESI PENGAJIAN: 2004 / 2005
NUR FARIZAN BINTI MUNAJAT _____ _____
Saya
(HURUF BESAR)
mengaku membenarkan tesis ini disimpan di Perpustakaan Universiti Teknologi Malaysia dengan
syarat-syarat kegunaan seperti berikut :
1.
Hakmilik tesis adalah dibawah nama penulis melainkan penulisan sebagai projek bersama dan
dibiayai oleh UTM, hakmiliknya adalah kepunyaan UTM.
Naskah salinan di dalam bentuk kertas atau mikro hanya boleh dibuat dengan kebenaran bertulis
daripada penulis.
Perpustakaan Universiti Teknologi Malaysia dibenarkan membuat salinan untuk tujuan pengajian
sahaja.
Tesis hanya boleh diterbitkan dengan kebenaran penulis. Bayaran royalti adalah mengikut kadar
yang dipersetujui kelak.
*Saya membenarkan/tidak membenarkan Perpustakaan membuat salinan tesis ini sebagai bahan
pertukaran antara institusi pengajian tinggi.
**Sila tandakan (√)
2.
3.
4.
5.
6.
√
SULIT
(Mengandungi maklumat yang berdarjah keselamatan atau
kepentingan Malaysia seperti yang termaktub di dalam AKTA
RAHSIA RASMI 1972)
TERHAD
(Mengandungi maklumat TERHAD yang telah ditentukan oleh
organisasi/badan di mana penyelidikan dijalankan)
TIDAK TERHAD
Disahkan oleh
_____________________________
(TANDATANGAN PENULIS)
Alamat Tetap: 1526, JALAN TEMPINIS,
FELDA MEDOI, 85050 SEGAMAT,
JOHOR DARUL TAKZIM
_____________________________
(TANDATANGAN PENYELIA)
P. M. DR NORIAH BT BIDIN
(NAMA PENYELIA)
Tarikh: AUGUST 2005
Tarikh: AUGUST 2005
CATATAN: * Potong yang tidak berkenaan.
** Jika Tesis ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/
organisasi berkenaan dengan menyatakan sekali tempoh tesis ini perlu dikelaskan
sebagai SULIT atau TERHAD.
“I hereby declare that I have read this thesis and in my
opinion this thesis is sufficient in terms of scope and quality for the
award of the degree of Master of Science (Physics)”
Signature
:
.............................................
Name of Supervisor :
P.M. Dr. Noriah Binti Bidin
Date
............................................
:
“I declare that this thesis entitled “Study of Saturable Absorber Materials for
Q-Switching Dye Laser” is the result of my own research work except as cited in
references. The thesis has not been accepted for any degree and is not concurrently
submitted in candidature of any degree”.
Signature
: ……….………………….
Author’s name
:
Nur Farizan Binti Munajat
Date
:
…………………………..
Dedication to my beloved father, mother, family, dear and friends…..
iv
ACKNOWLEGMENT
First at all, in humble way I wish to give all the praise to Allah, the Almighty
God for with His mercy has given me the strength, keredhaanNya and time to complete
this work.
Now, I would like to express my sincere gratitude and appreciation to my
supervisor, Associate Professor Dr. Noriah Bidin for her supervision, ideas, guidance
and enjoyable discussion throughout this study. I hope all this valuable time and
experience will keep in continue.
I would like to acknowledgement the help and kindly assistance of the following
persons; Allahyarham En. Nyan Abu Bakar for assisting in carrying out experimental
works and colleagues from laser laboratory for their continuing corporation,
encouragement and useful comment to complete the work.
Here, I would like to take this opportunity to thanks to UTM-PTP and Universiti
Teknologi Malaysia for granting this project through vote, 75341. Without this financial
support, this project would not be possible.
Thanks also to all my friends and course mates for their views, concerns and
encouragement. Last, but not least, I am grateful to my beloved family for their praying,
continuing support, patience, valuable advices and ideas throughout the duration of this
research.
v
ABSTRACT
Q-switching is a technology widely used in lasers to generate short pulses with
high peak powers. In practice, Q-switching can be realized with various methods
including mechanically by rotating mirror, actively either by acousto-optic or electrooptic method, or passively using a saturable absorber. The first two techniques have their
own problems especially the spinning machine and the driver to get a shorter pulse
duration. Therefore, passive Q-switch was chosen in this study because it requires less
optical element inside the laser cavity and no outside driving circuitry and makes this
technique simple and relatively cheaper compared to the other two techniques. Passive
Q-switching is a better choice for those applications where compactness of the laser is a
prime requirement. The objective of this project is to study and characterize the suitable
material to be saturable absorber for passive Q-switching laser. The dye laser was
utilized as a source of Q-switching laser. As a preliminary, the laser was calibrated to
determine the best performance of laser beam. Various materials including 3, 3’Diethyloxadicarbocyanine Iodide (DODCI), 1,3'-Diethyl-4, 2’-quinolyloxacarbocyanine
Iodide (DQOCI) and 1,1'-Diethyl-4, 4’-carbocyanine Iodide (Cryptocyannine) and
Chromium-doped Yttrium Aluminium Garnet (Cr4+: YAG) crystal are employed as a
saturable absorber material. The pulse width, the single pulse energy and the peak power
of the Q-switched laser output are measured. Two of the tested materials namely 1,3'Diethyl-4, 2’-quinolyloxacarbocyanine Iodide (DQOCI) and Chromium-doped Yttrium
Aluminium Garnet (Cr4+: YAG) crystal demonstrate a good performance to be a
saturable absorber. The output characteristics of the passive Q-switch laser possess a
uniphase of TEM00 mode.
vi
ABSTRAK
Pensuisan-Q merupakan satu teknologi yang digunakan secara meluas dalam
teknologi laser untuk menjana denyut pendek yang berkuasa tinggi. Secara praktis,
pensuisan-Q boleh dibina dengan pelbagai cara termasuk secara mekanikal dengan
kaedah putaran cermin, secara aktif sama ada dengan kaedah akusto-optik atau elektrooptik, atau secara pasif menggunakan penyerap tepu. Dua teknik pertama mempunyai
masalah tersendiri terutamanya mesin putaran dan pemacu untuk mendapatkan tempoh
denyut yang pendek. Oleh yang demikian, dalam penyelidikan ini pensuisan-Q pasif
dipilih kerana ia kurang memerlukan elemen optik yang banyak dalam rongga laser dan
tidak memerlukan litar memacu luaran menjadikan teknik ini ringkas dan lebih murah
berbanding dua teknik yang lain. Teknik pensuisan-Q merupakan pilihan yang tepat
untuk penggunaan yang memerlukan satu system laser yang padat. Objektif
penyelidikan ini adalah untuk mengkaji dan melakukan pencirian terhadap bahan yang
sesuai untuk dijadikan sebagai bahan penyerap tepu. Laser pencelup telah digunakan
sebagai sumber laser pensuisan-Q. Sebagai kajian awal, laser tersebut telah ditentu ukur
terlebih dahulu untuk menentukan prestasi terbaik cahaya laser. Pelbagai bahan
termasuk 3, 3’-Diethyloxadicarbocyanine Iodide (DODCI), 1,3'-Diethyl-4, 2’quinolyloxacarbocyanine Iodide (DQOCI) and 1,1'-Diethyl-4, 4’-carbocyanine Iodide
(Cryptocyannine) dan Chromium-doped Yttrium Aluminium Garnet (Cr4+: YAG) kristal
digunakan sebagai bahan penyerap tepu. Tempoh denyut, tenaga keluaran dan kuasa
keluaran laser pensuisan-Q diukur. Dua daripada bahan yang telah diuji, iaitu 3'-Diethyl4, 2’-quinolyloxacarbocyanine Iodide (DQOCI) dan Chromium-doped Yttrium
Aluminium Garnet (Cr4+: YAG) kristal dikenalpasti sebagai bahan yang baik untuk
dijadikan sebagai penyerap tepu. Keluaran laser pensuisan-Q adalah sefasa dalam mod
TEM00.
vii
TABLE OF CONTENTS
CHAPTER
1
2
TITLE
PAGE
DECLARATION
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
LIST OF CONTENTS
vii
LIST OF TABLES
x
LIST OF FIGURES
xii
LIST OF SYMBOLS
xvi
INTRODUCTION
1.1 Literature Review
1
1.2 Q-Switching Laser
2
1.3 Passive Q-Switching
5
1.4 Research Objective
6
1.5 Research Scope
6
1.6 Thesis Outline
7
THEORY
2.1 Introduction
9
2.2 Pulsed Dye Laser
9
2.2.1
Active Medium
10
2.2.2
Wavelength Selection in Pulsed Dye Lasers
10
2.2.3
Excitation Method
11
viii
2.3
2.4
2.5
3
2.3.1
15
Pumping Mechanism of Q-Switching
Q-Switching Methods
16
2.4.1
Mechanical Q-Switches
17
2.4.2
Electro-Optic Q-Switches
17
2.4.3
Acousto-Optic Q-Switches
18
Passive Q-Switches
19
2.5.1
20
Mechanism Of Bleaching
3.1
Introduction
24
3.2
Sample Preparation
25
3.2.1
Organic Dyes
25
3.2.2
Cr4+: YAG Crystal
26
3.4
Dye Laser
26
3.3.1
Pulse Detection
28
3.3.2
Mode of Triggering
29
Image Processing System
31
3.4.1
31
Processing Software
3.5
Calibration of Optimum Laser Performance
33
3.6
Experimental Setup
34
DYE LASER
4.1
Introduction
36
4.2
Externally Triggrering Dye Laser
37
4.3
Diagnose The High Performances of Dye Laser
40
4.3.1
Wavelength
40
4.3.2
Cavity Length
44
4.4
5
14
METHODOLOGY AND MATERIAL
3.3
4
Q-Switching
Summary
50
PASSIVELY Q-SWITCHED DYE LASER
5.1
Introduction
51
5.2
Passive Q-Switch With Different Saturable Absorber
52
ix
5.2.1
Organic Dyes Saturable Absorber
52
5.2.1.1
DODCI
53
5.2.1.2
DQOCI
60
5.2.1.3
Cryptocyannine
65
5.2.1.4
Comparison of Organic Dyes
70
Saturable Absorber
5.2.2
5.3
6
Summary
75
76
DIAGNOSE OF PASSIVE Q-SWITCH LASER BEAMS
6.1
Introduction
78
6.2
Analyzing The Beam
79
6.2.1
Gaussian Fit Analysis
79
6.2.2
Beam Spot
86
6.3
7
Cr4+: YAG Crystal Saturable Absorber
Summary
89
CONCLUSIONS AND SUGGESTION
7.1
Conclusions
90
7.2
Problems And Suggestion
93
REFERENCES
95
APPENDIX 1
100
x
LIST OF TABLES
TABLE NO.
4.1
TITLE
Power depending on wavelength of dye laser with
PAGE
41
Coumarin 500 as a lasing medium
4.2
Power depending on wavelength of dye laser with
Rhodamine 590 as a lasing medium
41
4.3
Pulse duration of dye laser with Coumarin 500 as lasing
medium produced at different cavity length
45
4.4
Pulse duration of dye laser with Rhodamine as lasing
medium produced at different cavity length
45
4.5
Pulse Energy of dye laser with Coumarin 500 as lasing
47
medium produced at different cavity length
4.6
Pulse Energy of dye laser with Rhodamine 590 as lasing
medium produced at different cavity length
47
5.1
Pulse duration of passive Q-switch laser upon concentration
of DODCI
53
5.2
Pulse energy of passive Q-switch laser upon concentration
of DODCI
55
5.3
Peak Power of passive Q-switch laser upon concentration of
58
DODCI.
xi
5.4
Pulse duration of passive Q-switch laser upon concentration
of DQOCI.
61
5.5
Pulse energy of passive Q-switch laser upon concentration
of DQOCI
63
5.6
Peak power of passive Q-switch laser upon concentration of
DQOCI
64
5.7
Pulse duration of passive Q-switch laser upon concentration
66
of Cryptocyannine.
5.8
Pulse energy of passive Q-switch laser upon concentration
of Cryptocyannine
67
5.9
Peak power of passive Q-switch laser upon concentration of
DQOCI
69
5.10
Pulse duration, pulse energy and peak power for different
position of saturable absorber
75
6.1
Gaussian width in horizontal and vertical profiles for
83
DQOCI saturable absorber due to working distance.
6.2
Gaussian width in horizontal and vertical profiles for Cr4+:
YAG saturable absorber due to working distance.
83
6.3
Beam spot perimeter and area due to working distance for
passively Q-switched dye laser with Coumarin 500 as a
lasing medium
87
6.4
Beam spot perimeter and area due to working distance for
passively Q-switched dye laser with Rhodamine 590 as a
lasing medium
88
xii
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
Energy level diagram of a dye laser
2.2
Absorption and fluorescence emission spectrum of a typical
organic molecule
13
2.3
Mechanism for dye laser action
2.4
Development of a Q-switched laser pulse. (a). The pumping
source output, (b). Cavity loss, (c).Population inversion,
and (d). Q-switch output.
2.5
Mechanical Q-switch (rotating chopper).
2.6
Electro-optic Q-switch
2.7
Acousto-optic Q-switch.
2.8
Saturable absorber Q-Switch
2.9
Absorption as a function of incident light intensities for a
saturable absorber
12
14
16
17
18
19
20
21
xiii
2.10
Energy-level diagram for most saturable absorber dye
molecules in solution
21
2.11
Schematic illustration of the three processes, absorption,
spontaneous emission and stimulated emission.
22
3.1
Schematic dimension of Cr4+: YAG crystal
26
3.2
LN120C top view
27
Dye module Optical Layout
27
3.4
BPX 65 Photo detector Circuit
29
3.5
Block diagram of trigger unit
30
3.6
Experimental set-up for measuring the delay time of the dye 30
laser
3.7
CCD Profiler Window
31
3.8
Calibration screen option for Video Test 5.0 Software
32
3.9
Dye laser alignment set-up for calibration
34
3.10
Laser cavity of passively Q-switched dye laser
34
3.11
Schematic diagram of experimental set up for passively QSwitched dye laser
35
4.1
Pulse of input voltage from trigger unit
37
4.2
(a). Channel 1: Pulse from trigger unit.
38
3.3
(b). Channel 2: Pulse from BPX 65 photo detector
4.3(a)
Pulse shape of the dye laser with Coumarin 500 as a lasing
medium.
39
4.3(b)
Pulse shape of the dye laser with Rhodamine 590 as a
lasing medium
39
xiv
4.4
Accumulated power of dye laser with Coumarin 500 upon
on wavelengths
42
4.5
Accumulated power of dye laser with Rhodamine 590 upon
on wavelength
43
4.6
The pulse duration and output power versus cavity length of 46
passively Q-switched Rhodamine 590 dye laser.
4.7
The output energy of dye laser upon cavity length with
different lasing medium
4.8
The pulse duration and output power versus cavity length of 49
passively Q-switched Rhodamine 590 dye laser.
5.1
Pulse Duration versus concentration of DODCI on different
lasing medium
54
5.2
Pulse Energy versus concentration of DODCI on different
lasing medium
55
5.3
Graph natural logarithm of pulse energy versus
concentration
57
5.4
Peak Power versus concentration of DODCI on different
lasing medium
59
5.5
Graph natural logarithm of peak power versus concentration 60
5.6
Pulse Duration versus concentration of DQOCI on different
lasing medium
61
5.7
Graph natural logarithm of pulse duration versus
concentration
62
5.8
Pulse Energy versus concentration of DQOCI on different
lasing medium
63
5.9
Peak Power versus concentration of DQOCI on different
lasing medium
65
5.10
Pulse Duration versus concentration of Cryptocyannine on
different lasing medium
66
48
xv
5.11
Pulse Energy versus concentration of Cryptocyannine on
different lasing medium
68
5.12
Peak Power versus concentration of Cryptocyannine on
different lasing medium
69
5.13(a)
Figure 5.13(a): Pulse duration versus concentration of
different saturable absorber materials
70
5.13(b)
Pulse duration versus concentration of different saturable
absorber materials
71
5.14(a)
Pulse energy versus concentration of different saturable
absorber materials
72
5.14(b)
Pulse energy versus concentration of different saturable
absorber materials
73
5.15(a)
Peak power versus concentration of different saturable
absorber materials
74
5.15(b)
Peak power versus concentration of different saturable
absorber materials
74
6.1
Gaussian profile of passive Q-switch laser beam. (a)
Vertical profile, (b). Horizontal profile
80
6.2
Beam Profile of Q-switching laser; (a). Three-dimensional
image shows the distribution of Gaussian beam profile (b).
Two-dimensional image.
81
6.3
Correlation upon working distances for DQOCI saturable
absorber.
85
6.4
Correlation upon working distances for Cr4+: YAG
saturable absorber.
85
6.5
Two-dimensional image of passively Q-switched dye laser
using DQOCI upon different working distance
86
6.6
Two-dimensional image of passively Q-switched dye laser
using Cr4+: YAG crystal upon different working distance
87
6.7
Beam spot area against working distance
88
xvi
LIST OF SYMBOLS
Q
-
Quality factor
F
-
Photon flux of the incident light
σ12
-
Absorption cross section
W12
-
Absorption probability
σ 21
-
Stimulated emission cross section
m
-
Grating order
λ
-
Wavelength
d
-
Grating constant
θ
-
Angle between the grating and the laser gain axis
I
-
The intensity of a pixel at location x
i(h, v)
-
The intensity at location (h, v)
V
-
The maximum intensity of the fitted Gaussian curve (Peak
Intensity)
C
-
The centre of the Gaussian fit peak (Centroid)
σ
-
The radius of the Gaussian fit curve at the 1/e2 intensity
level (diameter)
CHAPTER 1
INTRODUCTION
1.1
Literature Review
Human beings are really clever in making use of different kinds and forms of
energy. Laser material processing relies on laser systems of desired properties. An
inspiring thing in laser processing is the application of ultra-short pulsed lasers. Ultrafast
lasers can give scientist opportunities to probe the behavior of matter when exposed to
intense radiation and do studies in fields such as astrophysics, general relativity and
quantum mechanics.
This ultrafast laser currently enable scientist to observe the occurrence of the
fastest chemical reaction (Kodymova et al., 2004). This is an advanced technology used
for ionizing all material within a small area without any heat or mass flow affecting the
surrounding area (Charschan, 1972) and carry out precise micromachining (Liu et al.,
1997). This technology is also used to design high density, high-speed communications
networks, which an ultrafast laser’s bandwidth is equivalent to millions of telephone
calls. Another application is to design compact particle accelerators and generate fusion
energy (Lerner, 1998).
2
The ultrafast lasers can also imitate the conditions at the center of stars allowing
astrophysicists to experiment with possible ways in which stars form and explode in
supernovas. These high powered lasers can focus the power of all sunlight falling on
Earth onto a spot a tenth of a millimeter on a side, accelerate electrons close to speed of
light and generate pressures hundreds of time those of light and create magnetic fields a
billion times of Earth (Lerner, 1998). One way of achieving these high power pulses
with short duration is by Q-switching technique.
1.2
Q-switching Laser
The power output may be increased by Q-switching, which is achieved by
exciting the laser medium so that a population inversion occurs but delaying the
application of feedback from the axial mirrors (Hellwarth, 1961). On the simplest way,
we can define the Q-switching as a method of using an optical switch inside the laser
cavity. This optical switch has two states; it’s open when the radiation pass through the
switch undisturbed and closed when the radiation cannot pass through the switch.
Switching the laser system means transferring the system from on to off or the other
way.
The possibility of Q-switching laser was first proposed by Hellwarth in 1961. In
practice, Q-switching can be achieved by deflecting the beam at the high reflecting
mirror mechanically (Collins and Kisliuk, 1962) or by acousto-optic (Koechner, 1976)
or electro-optic (Hellwarth and McClung, 1962) devices or by using an opaque saturable
absorber (Soffer, 1964) that bleaches transparent when the fluorescent light output
reaches a given level. McClung and Hellwarth made the first experimental observation
of Q-switched pulse behavior in 1962 using an electro-optical Q-switch in a ruby laser.
3
Electro-optic Q-switches employs materials that exhibit birefringence under an
applied electric field (Kelin, 1998). The advantages of this pure electronic control of Qswitching are many. Such as, the fast switching times, the precise control over, and
flexibility of Q-switching and ease to synchronization of the modulation with electronics
and measuring apparatus. However, the existence of a Kerr cell (or other electronically
controlled switching) inside the laser cavity has presented many problems. They are
usually fabricated from crystal such as KDP or LiNbO3, which are hygroscopic and
prone to damage, by the laser beam. The polarization requirements mean that the laser
beam must be polarized. These Q-switches also require some cleverness in the design of
the electrical signal without transients. This combination of factors means that the
electro-optic Q-switches are typically used in high peak power pulsed lasers, as well as
in high gains CW lasers.
Acousto-optic Q-switches, which employ materials such as quartz that exhibit a
change in the index of refraction when the material is acoustically excited have the
advantages of being low-loss elements when not Q-switching. In contrast with the
electro-optic crystals, acousto-optic crystals have high damage thresholds and are
typically not hygroscopic. However, the Q-switches generally require high-power RF
power supplies at 20 to 60 W, 50 to 150 MHz. This combination of factors means that
acousto-optic Q-switches are typically used in low-gain cw lasers. Their most common
application is in continuous wave Nd: YAG Q-switched and mode-locked laser systems
(Koechner, 1976).
Another method to Q-switch a laser cavity is incorporating a mechanical device
within the cavity that blocks the laser beam. The first mechanical Q-switch used a
rotating chopper (Collins and Kisliuk, 1962). However, the choppers are slow and
vibration-prone, and such techniques were soon abandoned in favor of rotating mirrors
and prisms (Benson and Mirachi, 1964). With rotating mirrors, the approach is to spin
the mirror using high-speed motor. Such designs usually incorporate a multisided mirror
or multiplicative optical geometries so that several reflections are possible for each
rotation (Daly and Sims, 1964). However, rotating mirror Q-switches are prone to
4
alignment difficulties because each face of the mirror must be aligned to within a
fraction of miliradian. Although the mechanical Q-switches are the simplest and less
expensive, the high rotational speed means that the devices are noisy and process
relatively short lifetimes. Furthermore, mechanical components are not robust in harsh
environment.
The first three methods of Q-switching are active types, where the switching of
the laser light occurs externally. Besides the active type, which is difficult to implement,
complex for installation, alignment and operation, laser also can be switching passively.
Passive Q-switching received its name from the action of generated radiation itself
(Smith and Sorokin, 1966). This technique potentially offers an advantage of low cost,
reliability and emission of pulses with a relatively narrow linewidth (Koechner, 1976). It
is also simple in fabrication and operation since it requires no high voltages or fast
electro-optic devices. Passive Q-switches can be used with pulse pumped systems only
because a CW pumped laser never produces sufficient fluorescence to bleach the dye
(Kuhn, 1998).
As summary, the reading of all the papers and articles on Q-switching
applications and techniques has driven us even stronger to study, diagnose and
characterize the fundamental of Q-switching laser. Although it can be achieved by
various techniques, this study on passive Q-switching laser and the focus of this research
work will be on the materials used as saturable absorber and how to improve the laser
outputs.
5
1.3
Passive Q-switching
Passive Q-switching laser exploits the bleaching of saturable materials. The
rising flux within the laser is capable of decreasing the absorptivity of certain saturable
absorber placed in the laser cavity. The sudden decrease of absorption has the same
effect as the removal of an obstacle in the path of the beam. When properly adjusted,
these lasers containing saturable materials trigger themselves to emit a giant pulse.
In the earliest experiments with saturable absorber, Master and Murray (1965)
who used an absorbing dye smeared on a microscope slide, and Grant (1963), who used
an aluminized Mylar film, produced light pulses of quality and efficiency comparable to
that achieved with Kerr cell switches (Geller et al., 1963). However, the absorber was
always damaged. The optical saturation in these instances was presumably caused by the
evaporation of the thin absorber so as to render the absorber transparent. Subsequently,
saturable absorbers have been found which show little or no damage after producing a
good quality giant pulse (Soffer, 1964; Sorokin et al.; 1964, Kafalas et al., 1964; Bret
and Gires, 1964). These employ the saturation of some transition which, because it has a
high absorption cross section per absorbing molecule at the laser frequency, requires
relatively for photons absorbed, rendering their normal value.
Sorokin et al. (1964) found that metalphthalocyanine dyes, dissolved in either
nitrobenze or chlornaphthalene (the latter showed some deterioration after several
pulses) produced good giant pulses when placed inside the laser cavity. Apparently, the
threshold pump energy was not appreciably changed (the actual value was not given).
Soffer (1964) has achieved giant pulses of exceptional spectral purity and normal energy
content using a saturable absorber of dilute Cryptocyannine. To produce these high
quality pulses, 3000 J pump energy was required, as compared to 900 J for normal
operation. Kafalas et al. (1964) have also used an absorber of Cryptocyannine (dissolved
in methanol) to achieve giant pulse outputs. No deterioration of the Cryptocyannine was
observed.
6
All of works explained above are passively Q-switched solid-state laser.
Braveman (1975) have demonstrated the first and only passive Q-switching for
Nitrogen-laser-pumped-dye-laser. He used a DODCI as a saturable absorber inside the
dye laser cavity (the actual solvent type and concentration value was not given). This
experiment produces single high-repetition-rate high peak power tunable subnanosecond
pulses. However, this experimental set-up makes used a wide space in the laser cavity.
The Avco model C950 nitrogen laser source used in this study showed a thermal
distortion dominated the mode structure after 50 pulses per second (pps).
1.4
Research Objective
The main objective of this research is to study the saturable absorber material for
passively Q-switched nitrogen-laser-pumped-dye-laser. This includes diagnosing the dye
laser in order to utilize the system at its optimum performance. Then, characterize the
output of Q-switching laser by altering some laser parameters.
1.5
Research Scope
Several materials are determined as saturable absorbers. The dye laser pumped
by nitrogen laser is utilizing as a source to be switched. The dye laser cavity is aligning
to get its best performance. An external trigger unit builds for the dye laser in order to
get a single shot. The photodetector also builds to detect the laser beam.
7
1.6
Thesis Outline
This thesis is divided into 7 chapters. The first chapter is the review of some
applications of Q-switching laser. Previous research related to miscellaneous Qswitching methods and passive Q-switching also presented. This chapter also
emphasizes the aim of the research.
Chapter II reviews the background or the theory related to the research. This will
cover the basic theory of Q-switching such as quality factor, Q and pumping mechanism.
This chapter also explains the various methods of Q-switching and briefly describes the
mechanism of passive Q-switching.
Chapter III describes the sample preparation and methodology for passively Qswitched dye laser. This would include image processing software and experimental
setup.
Chapter IV discusses the diagnosed results of dye laser. Various laser parameters
are tested such as wavelength, cavity length, working distance and repetition rate in
order to determine the current performances of dye laser as a source to be switched.
In chapter V, the pulse width and output energy of passively Q-switched dye
laser in various manners are presented. These experimental results were compared with
the current standard dye laser performances.
Chapter VI presents the diagnostic analysis of passive Q-switches beam.
BeamStar CCD Laser Beam Profiler was utilized as diagnostic system.
8
Finally, the conclusions of the project are made in chapter VII. includes the
summarization of the whole project, the problems involved and experience during
performances of the project and some works to be carried out in the near future are
suggested.
CHAPTER 2
THEORY
2.1
Introduction
Q-switching is a method of modulating the Q factor of a laser cavity to obtain
high peak power with short duration laser pulses. These may be accomplished by
inserting an optical switch inside the laser cavity. Therefore, it is better to understand
the phenomena related to Q-switching including the source to be switched, quality factor
Q, Q-switching mechanism and various techniques of Q-switching.
2.2
Pulsed Dye Laser
In this study, dye laser was utilized as a source to be switched. The organic dye
laser is widely used in scientific research because of its unusual flexibility; it can
provide sources of coherent light easily tunable over considerable bands of the visible
spectrum.
10
Sorokin and Lankard demonstrated the first dye laser in 1965 at IBM Laboratories in the
US. They discovered the dye laser action during a fluorescence research of organic dye
molecules, which were excited by Ruby laser. In 1967, these scientists discovered the
possibility to tune the emitted wavelength using a grating at the end of the laser cavity.
2.2.1
Active Medium
The active medium of dye lasers consists of organic dye molecules dissolved in a
fluid solvent, such as methanol, ethanol, dimethylformamide or just water (Duarte and
Hillman, 1990). In this study, 7-Ethylamino-4-trifluormethylcoumarin (Coumarin 500)
and Benzoic Acid, 2-[6-(ethylamino)-3-(ethylimino)-2, 7-dimethyl-3H-xanthen-9-yl]ethyl ester, monohydrochloride (Rhodamine 590) dissolved in ethanol were utilized as a
lasing medium. These solutions were placed in a square cell.
2.2.2
Wavelength Selection in Pulsed Dye Lasers
The dye laser system used in this study utilized a diffraction grating as a
wavelength selecting device. A light beam incident upon a reflection grating plane will
be reflected back along the axis of incidence if the following grating equation is satisfied
(Soffer and McFarland, 1967),
m λ = 2 d sin θ
(2.1)
where; m is diffraction order, λ is wavelength, d is a grating spacing, and θ is an angle of
incidence measured from the normal to the grating.
11
A diffraction grating used in this manner is said to be mounted in the Littrow
configuration. Rotating the grating about an axis parallel to its grooves since this
configuration acts as a tunable reflector can thus vary the lasing wavelength. It has high
reflectivity at one wavelength at a time, depending upon its angular setting.
The metallic coatings of gratings for commercial dye lasers are usually made of
aluminum, gold, or silver for operation between 400-1100 nm. Such coatings are very
fragile. Thus, a beam-expanding telescope is sometimes used between the dye cell and
the grating to illuminate most of the surface of the grating to prevent the formation of hot
spots.
2.2.3
Excitation Method
Organic dye molecules are complicated structures and composed of a large
number of several species. The energy level structure of an organic dye molecule is
correspondingly complex. A highly simplified energy level diagram for a typical dye is
shown in Figure 2.1.
12
Excited state (singlet)
Intersystem crossing
Triplet state
Fluorescence
Phosphorescence
Ground state (singlet)
Figure 2.1: Energy level diagram of a dye laser (Schafer, 1990)
The ground states of most molecules are called singlet state. In the process of
pumping the laser, the molecule in ground state is first excited by absorbing a pump
photon into an excited state. Spontaneous emission then results in fluorescence.
However, it is possible through a process call intersystem crossing for dye molecules to
switch into their excited triplet state. This transition can only occur through collisions
between molecules. Spontaneous emission from a triplet state occurs very slowly, by
comparison with fluorescence transition, and is called phosphorescence. Molecular
fluorescence is responsible for dye laser emission. The wavelength of laser emission is
limited by the range of fluorescence wavelengths. Figure 2.2 shows that the wavelength
of light emitted by the dye when it fluorescence is longer than that of the absorbed pump
light.
13
Figure 2.2: Absorption and fluorescence emission spectrum of a typical organic
molecule (Kagan et al., 1968)
The mechanism for dye laser is shown in Figure 2.3. The dye molecules absorb
the pump light. They then release this energy by three processes. Some of the energy
goes into thermal heating of the dye and solvent. The rest molecules then release the
energy in the form of light. There are two ways that the dye molecule may emit the light.
The first process is spontaneous emission. In the spontaneous emission process, the dye
molecule lowers its energy by spontaneously emitting a photon or quantum of light
energy. The direction and phase of the emitted photon is random or incoherent. Its
energy is lost from the system. The second optical process is stimulated emission.
Stimulated emission occurs when there are other photons present that interact with the
dye molecule. Under stimulated emission, the dye molecule again loses its energy by
emitting a photon. This photon has the same phase and direction as the other photons
present. This photon is coherent since its energy coherently adds to the energy of the
other photons. Stimulated emission is the gain process that increases the optical
coherently.
14
Figure 2.3: Mechanism for dye laser action (Thyagarajan and Ghatak, 1981)
2.3 Q-switching
The idea of Q-switching is to delay the stimulated emission until a large number
of atoms are excited and a lot of photons are released during spontaneous emission and
the great number of photons starts the stimulated emission together. Then, a strong beam
of laser can be produced.
The term “Q” or “quality factor” describes the ability of the laser cavity to store
light energy in the form of standing wave. The quality factor, Q is defined as the ratio of
the energy contained in the cavity divided by the energy lost during each round trip in the
cavity (Luxon and Parker, 1992);
Q = 2π
energy stored in the cavity
energy lost in a cycle
(2.2)
When the Q-switch of a laser is off, there is no feedback and thus no standing wave. The
loss is very high and thus the quality factor is zero. When the Q-switch is on, a strong
standing wave is formed, causing the loss to be reduced. The Q-switch receives its name
15
from the fact that it allows the "Q" of the cavity to be "switched" from (feedback
blocked) a low value to a high value (feedback restored).
2.3.1
Pumping Mechanism of Q-switching
In the technique of Q-switching, energy is stored in the amplifying medium by
optical pumping while the cavity Q is lowered to prevent the onset of laser emission.
Although the energy stored and the gain in the laser medium also high, lasing action is
prohibited, and the population inversion reaches a level far above the threshold for
normal lasing action. When a high cavity Q is restored, the stored energy is suddenly
released in the form of a very short pulse of light, because of the light gain created by the
stored energy in an extremely short time. The peak power of the resulting pulse exceeds
that obtainable from an ordinary long pulse by several orders of magnitude. Because of
its extremely high power, the pulse so produced is called a giant pulse.
Figure 2.4 shows a typical time sequence of the generation of a Q-switched pulse.
Lasing action are disabled in the cavity by a low Q of the cavity. Toward the end of the
pumping pulse, when the inversion has reached its peak value, the Q of the resonator is
switched to some high value. At this point a photon flux starts to build up in the cavity,
and shortly afterwards a Q switch pulse is emitted.
16
Figure 2.4: Development of a Q-switched laser pulse. (a). The pumping source output,
(b). Cavity loss, (c). Population inversion and (d) Q-switch output (Koechner and Bass,
2003)
2.4
Q-switching Methods
Q-switching may be accomplished by changing the reflectivity of one of the
mirrors by inserting or removing a diaphragm, by changing the paths of the rays between
the mirrors, and also by changing the transparency of the material within the laser cavity.
There are four major technologies used for Q-switches: mechanical, electro-optic,
acousto-optic, and saturable absorber. The first three methods are an active type and will
17
be described in this section. Another method is a passive type and will be briefly describe
in next section.
2.4.1
Mechanical Q-switches
Conceptually the simplest method of Q-switching is to make one of the choppers
rotates rapidly as shown in Figure 2.5. For most of the time, the alignment of two mirrors
will be such that the loss will be highly and hence the Q low. This will allow a large
population inversion to develop. At the instant that the two mirrors are aligned, the Q
will be highly and a large output pulse will be developed. Although this method was the
first developed for Q-switching, it does not give the performance in terms of peak power
of some of the newer methods of Q-switching.
Rotating copper
M2
M1
Laser rod
Figure 2.5: Mechanical Q-switch (rotating chopper)
2.4.2
Electro-Optic Q-switches
The electro-optic Q-switch (see Figure 2.6), is usually required the placing of two
elements into the reflecting cavity between the laser rod and the maximum reflecting
mirror. These elements are a polarization filter (passive) and a polarization rotator
18
(active). Producing a low cavity feedback with these devices involves rotating the
polarization vector of the laser beam inside the cavity so that it cannot pass through the
polarization filter. When this polarization rotation is removed, the cavity reflectivity is
relatively high and the system will produce a giant pulse. Two of the electro-optic
devices used in this application are Kerr cells and Pockel’s cells. Electro-optical Qswitches have high dynamic loss (99%) and relatively high insertion losses (15%)
because of the losses in the optical elements. Switching time is fast; typically less than a
nanosecond, and synchronization is good. Electro-optical Q-switches are well suited for
pulsed systems but cannot be used with CW pumped lasers as their high insertion loss
prevents lasing.
M1
Polarizer
Laser rod
During pumping
During lasing
Pockels
cell
M2
V applied
V removed
Figure 2.6: Electro-optic Q-switch (Kuhn, 1998)
2.4.3
Acousto-Optic Q-switches
Acousto-optic Q-switches employ transparent materials such as quartz, that
exhibit a change in the refraction when the material is acoustically excited. The material
was placed in the cavity between the laser rod and the high reflection mirror as shown in
Figure 2.7. The acoustic standing wave will generate a corresponding standing wave in
the index of refraction, exhibits a diffraction effect on the intracavity laser beam and
diffracts part of the beam out of the cavity alignment. This results in a relatively low
19
feedback. When the acoustic wave is removed, the diffraction effect disappears, the
cavity is again aligned, and the system emits a giant pulse. Acousto-optic devices have
low insertion loss (typically less than 1%) and low dynamic loss (50% maximum).
Switching time is slow at 100 ns or greater, and the synchronization is good. These
devices are ideally suited for use with CW pumped systems or low-gain pulsed lasers.
They cannot be used with most pulse pumped systems because their low dynamic loss
will not prevent lasing.
M1
M2
Laser rod
Acousto-optic cell
Figure 2.7: Acousto-optic Q-switch (Kuhn, 1998)
2.5
Passive Q-switches
Passive Q-switches received its name due to the action of the generated radiation
itself when in on condition. When generation begins, the absorbing molecules are
transferred to an excited state, and the material becomes transparent (i.e., the switch is
opened). This bleaching process will be briefly described in this section.
20
2.5.1
Mechanism of Bleaching
Passive Q-switching consists of a saturable absorber placed inside the optical
cavity, preferably between the laser medium and the high reflection mirror as shown in
Figure 2.8. Saturable absorber is a material whose absorption decreases with increasing
light intensity, as shown in Figure 2.9.
M1
Saturable absorber
M2
Lasing medium
Figure 2.8: Saturable absorber Q-Switch (Kuhn, 1998)
The saturable absorber solution in the cell strongly absorbs light of the active
material frequency, and this absorption prevents net amplification of light from occurring
until a much larger proportion of saturable absorber molecules have been pumped to the
excited state. The pumping energy input increases until amplification in the active
material overcomes the loss due to absorption in the cell, and the laser begins to emit
coherent light weakly. A very small amount of this weak laser light ‘bleaches’ the
saturable absorber solution, which then becomes almost perfectly transparent to the active
material light. At this instant, there is suddenly a large net amplification and a narrow
pulse that containing all the stored energy in the lasing medium, develops rapidly. After
the pulse, the molecules of the solution returns quickly to its absorbing state, ready for
formation of the next narrow pulse.
Absorption
21
Intensity
Figure 2.9: Absorption as a function of incident light intensities for a saturable absorber
(Wilson and Hawkes, 1983)
Bleaching of the saturable absorber dye is brought about by saturation of the
absorption at the laser medium frequency. The solution is in the bleached condition
when the population densities in the upper and lower states of the saturable absorber
molecule (S’,S in Figure 2.10) are roughly equal. Because of their large cross sections
for capturing laser medium light, only a relatively small number of saturable absorber
molecules are required, and for this reason, saturation occurs very rapidly, with the
extraction of only a relatively few quanta from the laser beam being required to cause
bleaching.
Excited triplet
state T’
First Excited
singlet state S’
Triplet state T
Ground state S
Figure 2.10: Energy-level diagram for most saturable absorber dye molecules in solution
(Velarde et al., 1989)
22
To understand this non-linear optical effect we can consider bleachable absorber
to be representable by a simple two-level with population densities N2 and N1 for energy
level E1 and E2. The existence of the triplet state T (Figure 2.10) is ignored. When a
beam of light interacts with a material, three fundamental phenomena will occur, namely,
the process of spontaneous and stimulated emission and the process of absorption (Figure
2.11).
N2
E2
Spontaneous
emission
Absorption
Stimulated
emission
N1
E1
Figure 2.11: Schematic illustration of the three processes, absorption, spontaneous
emission and stimulated emission (Smith and Sorokin, 1966)
For the level E1, the change with time of the population density due to the absorption is
given by (Svelto, 1976),
 dN 1 
= −σ 12 F = −W12 N 1


 dt  absorption
(2.3)
where, F is a photon flux of the incident light, σ12 is a absorption cross section and W12 is
absorption probability
The absorption process populates level 2
 dN 2 
= −W12 N 2


 dt  absorption
(2.4)
Processes of spontaneous and stimulated emission are depopulating level 2 by the rate
23
 dN 2 
= − A21 N 2


 dt  spon tan eous
(2.5)
 dN 2 
= −σ 21 F = −W21 N 2


 dt  stimulated
(2.6)
where σ21 is called the stimulated emission cross section.
In the stationary case where dN2/dt = 0, the solution is
N2
W12
=
N 1 W12 + A21
(2.7)
For high values of the pump rate (W12 >> A21), N2/N1 approximately 1. This means that
the number of absorbed photon becomes the same as the emitted ones. In this case the
medium is transparent.
CHAPTER 3
METHODOLOGY AND MATERIAL
3.1
Introduction
In this chapter, the requirements for Q-switching laser will be discussed. Starting from
the preparation of saturable absorber solutions, develop a photodetector and develop an
external trigger unit. Pulse duration and pulse energy of Q-switching laser beam are
measured.
Usually, the pulse energy of Q-switched laser is lower compared to the normal
operation laser. This phenomenon is common because of the losses and the absorption of
the photons along the instance of the Q-switching process. However, the peak power of
this Q-switched laser is much higher because of the short duration of the laser pulse. So,
in this study the peak power of Q-switched laser was obtained using relationship of
(Zhang et al., 1994),
Peak Power ,W ( watt ) =
Pulse Energy, E ( Joule)
Pulse Duration, t (sec ond )
(3.1)
25
The peak power of Q-switched laser is the ratio of pulse energy to pulse
duration. In Q-switching laser, the total energy is reduces and the pulse duration is
compress thus produces high peak power.
3.2
Sample Preparation
Basically, four types of sample that have different optical characteristics were
employed as saturable absorber materials. The first three samples were comprised of
different types of organic dyes, which include 3, 3’-Diethyloxadicarbocyanine Iodide
(DODCI), 1,3'-Diethyl-4, 2’-quinolyloxacarbocyanine Iodide (DQOCI) and 1,1'-Diethyl4, 4’-carbocyanine Iodide (Cryptocyannine). And the fourth sample is a solid state type
and made of Chromium-doped Yttrium Aluminum Garnet (Cr4+: YAG) crystal.
3.2.1
Organic Dyes
Organic dyes were prepared by weighing out the amount of dye powder using
analytical balance and transferring it to a volumetric flask. The solvent was added to fill
the flask which was shaking to dissolve the powder completely. When the solid is
dissolved completely, more solvent was added to bring the level of the meniscus to the
mark on the neck of the flask. The solution then transferred to a quartz Cuvette with
dimension of 1 mm x 1 mm. The dye concentration may be varied to achieve various
transmissions, thus changing the laser output power and pulse duration.
26
3.2.2
Cr4+: YAG crystal
The Chromium-doped Yttrium Aluminum Garnet (Cr4+: YAG) crystal used in
this work was manufactured by Red Optronics with dimension of 7x1.4 mm. The
schematic dimension of the sample is depicted in Figure 3.1.
1.4 mm
1.4 mm
7 mm
Figure 3.1: Schematic dimension of Cr4+: YAG crystal
Due to its specific properties such as broad band at the vicinity of 1 mm, high
damage threshold (> 500 MW/cm2), long lifetime, large absorption cross section at the
near IR, good thermal stability and easy operation, Cr4+: YAG crystal offers a good
potential to be used as saturable absorber (Ter-Mikirtychev et al., 1997).
3.3
Dye Laser
In this study, the dye laser model LN120C manufactured by Laser Photonics
utilize as the source to be switched. This LN120C NitroDye laser consists of a LN120C
UV Nitrogen Laser and Optional Dye Laser Module Combination as shown in Figure
3.2. The modules use the Nitrogen beam as the pumped source.
27
Nitrogen Laser Chassis
HV
Bulkhead
Laser Head
Dye Module
Safety Interlock
Normal operation of dye laser
Nitrogen laser
Figure 3.2: LN120C top view
The optical layout of a dye module is shown in Figure 3.3. This laser system was
operated at nitrogen pressure of 25 psi. The Plano-convex lens the 337.1 nm nitrogen
beam. A reflectivity flip-flop mirror turns the beam 90o to pump a dye module.
Output Coupler
Mirror
Dye Cell
High Reflection
Mirror
Output
Plano Cylindrical
Lens
Plano-convex
Lens
High reflectivity
Flip-flop Mirror
Figure 3.3: Dye module Optical Layout
Nitrogen
beam
28
The Nitrogen laser wavelength is fixed, while the dye modules offer variable
wavelengths. The mechanism for this tunability feature is based on grating mirror. In
this system, a Littrow grating configuration is used such that the output coupler and the
grating mirror forms the cavity. The wavelength output characteristics are then governed
by the relationship (LN120C Manual, 1991),
mλ = 2dsinθ
(3.2)
where m is the grating order, λ the wavelength, d is the grating constant and θ is the
angle between the grating normal and the laser gain axis. This system uses a micrometer
driven mechanism in order to tune the grating and thus the wavelength. Output
wavelength is indicated by the inch micrometer setting and the wavelength, in
nanometers, corresponds to the number of thousands of an inch displayed on the
micrometer.
3.3.1
Pulse Detection
The output beam of the dye laser can be detected by a high-speed BPX 65
Photodetector (Noriah et al., 1992). The rise time of the Photodetector is typically 1.5 ns
with surface area of the pin photodiode of about 1 mm2. The BPX 65 silicon photodiode
was placed in a series circuit comprising a 9 V DC source and a 50-ohm load resistor as
shown in Figure 3.4. Most fast photodetectors use the 50 ohm load impedence in order
to maintain their linearity of output (Wilson and Hawkes, 1983)
The laser pulse signal was detected by the photodetector, which couple to the
Textronix digital oscilloscope model TDS540. The time delay of the laser pulse after
being triggered is noted from the oscilloscope display panel.
29
BPX 65
photodiode
50 ohm
Oscilloscope
9 volt
Figure 3.4: BPX 65 Photo detector Circuit
The pulse duration is determined when a small portion of the laser pulse is directed onto
a fast photodiode. The photodiode converts part of the laser power to an electrical signal,
which is displayed on an oscilloscope. The pulse duration is measured at full wave half
maximum (FWHM). The peak power then is calculated from Equation (3.1).
3.3.2
Mode of Triggering
The dye laser can be operated either by internal mode or external mode. For
internal mode, the laser can be operated with repetition rate up to 20 Hz. This mode is
used when aligning the optical system and also for the purpose of doing calibration.
For the purpose to produce single pulse per trigger, an external mode is used. An
external trigger control unit was employed in order to trigger a single pulse laser. In this
work, a single pulse laser obtained by using a simple trigger circuit which was designed
using single pulse generator, the 74121-monostable multivibrators as shown in Figure
3.5.
30
5V
Switch
74121
7667
Nitrodye
Monostable
Dual Power Mosfet driver
Figure 3.5: Block diagram of trigger unit
The pulse shape of the dye laser system was obtained by using the combination
of the BPX 65 photodetector and trigger pulse circuit as shown in Fig. 3.5. The beam
from the dye laser was absorbed in the photodetector and the results were displayed on
the Textronix TDS 3054B of 500 MHz frequencies and sampling rate of 5 GS/s.
Dye
Laser
Oscilloscope
BPX 65
Trigger
unit
Figure 3.6: Experimental set-up for measuring the delay time of the
dye laser
Using the same arrangement of Figure 3.6, the delay time from external trigger input to
the evolution of the laser light was obtained.
31
3.4
Image Processing System
In order to observe the two and three-dimensional analysis distribution on laser
beam, a BeamStar CCD laser Beam Profiler was used as a beam diagnostic system. It
comprised a video camera and PC card to image, capture, and store the laser beam
profile. Then, all two-dimensional images were processed, manipulated and analyzed
using Video Test 5.0 Software.
3.4.1
Processing Software
The BeamStar CCD Laser Beam Profiler is a beam diagnostic measurement
system for real time measurement of continuous or pulsed laser system. Some
applications for the BeamStar CCD Profiler include laser beam optimization, quality
control, Gaussian fit analysis, and beam alignment (BeamStar user manual). The option
screen is depicted in Figure 3.7.
T
Figure 3.7: CCD Profiler Window
32
The main technologies available for laser beam diagnostic are using spatial
cameras as the beam characterization system and using moving mechanical slit or knife
edges to scan across the incoming beam. The BeamStar CCD Profiler uses a video
camera and PC card to image, capture, store and perform two-dimensional intensity
distribution analysis on laser beams. The advantages of a CCD based laser beam profiler
is fully utilized by powerful software that displays any structure larger than one pixel in
vivid colors, calculates the beam distribution and profile as well as total beam intensity
distribution, in order to allow full analysis of the laser beam’s characteristics.
Another software that was utilized in this work is Video Test 5.0. The function
of this software is to determine an area of beam spot. Before any measurement is
performed, calibration should be made to identify the magnification factor and it was
very important to represent the image in a real field measurement system. Furthermore,
an accurate distance or calculated area measurement can be made via a marker when
grid option was active. The calibration screen of Video Test 5.0 is depicted in Figure
3.8.
Figure 3.8: Calibration screen option for Video Test 5.0 Software
33
The result of the calibration shows that, the ratio between the measurement taken
in real field and from computer unit was 1 mm is equal to 157 pixel or 0.006369 mm per
pixel. This calibration condition was saving into active mode. It can be applied to
measure the real distance and area of the beam spot.
3.5
Calibration of Optimum Laser Performance
In order to operate the dye laser, the most important step is calibration. This
calibration was performed to diagnose and verify the performance of the laser beam.
Prior to the Q-switching process, it is worth while to calibrate the optimum performance
of the dye laser without the saturable absorber inside the laser cavity. In this case, the
dye laser system was operated internally. This repetitive condition is for the alignment
and calibration purpose. In this study, the wavelength and working distance was varied.
The optimum value out of these tests will be selected and compared with Q-switching
output.
The calibration was carried out by operating the dye laser system repetitively and
the laser beam was measured by Ophir pulsed energy/power meter. The whole
experimental setup for the calibration is shown in Figure 3.9.
34
Dye Laser
Detector
Beam splitter
Ophir
Pulsed
Power
Meter
BPX 65
Photodetector
Textronix Digital
Oscilloscope (TDS 540)
Figure 3.9: Dye laser alignment set-up for calibration
3.6
Experimental Setup
During the experiment, dye laser was utilized as a source to be switched. The
saturable absorber material was inserted inside the laser cavity between lasing medium
and highly reflecting mirror as shown in Figure 3.10.
Output Coupler
Mirror
Lasing Medium
Highly Reflecting
Mirror
Saturable
Absorber
Figure 3.10: Laser cavity of passively Q-switched dye laser
Then the parameter such as cavity length and working distance will vary
according to passive Q-switching. The pulse durations are obtained with combination of
photodetector and digital oscilloscope. The pulse energy of Q-switching laser was
measured using Ophir pulse energy meter model PE10-SH-V2. This energy meter
35
comprised a Pyroelectric absorber and digital meter. The experimental arrangement of
passively Q-switched dye laser is shown in Figure 3.11.
Personal Computer (Controlled
BeamStar CCD)
CCD Camera
NitroDye Laser
Detector
Beam Splitter
Ophir
Pulsed
Energy
Meter
BPX 65 Photodetector
Textronix Digital Oscilloscope
(TDS 540)
Optical line
Electronic line
Figure 3.11: Schematic diagram of experimental set up for passively Q-Switched
dye laser
In order to observe the two and three-dimensional distributions of the laser beam, a
BeamStar CCD laser Beam Profiler was used as a beam diagnostic system.
CHAPTER 4
DYE LASER
4.1
Introduction
In order to utilize a dye laser as a source to be switched in the process of Qswitching, the most important thing is to diagnose its current performances. The most
useful feature of dye lasers is their tunability, therefore in this experiment, the
wavelength for a given lasing medium were tuned by diffraction grating. Two organic
dyes have been used as a lasing medium; they are 7-Ethylamino-4trifluormethylcoumarin (Coumarin 500) and Benzoic Acid, 2-[6-(ethylamino)-3(ethylimino)-2, 7-dimethyl-3H-xanthen-9-yl]-ethyl ester, monohydrochloride
(Rhodamine 590). Some measurements also made with various cavity lengths in order
to study the length dependence on the dye laser output.
37
4.2
Externally Triggering Dye Laser
In this study, the dye laser system that was utilized as a source to be switched can
be triggered either by internal mode or external mode. For the purpose to produce one
pulse per trigger, an external mode is used. An external trigger control unit was
employed in order to trigger a single pulse laser.
When the system was triggered externally, the nitrogen laser received a pulse
having amplitude of 4.16 volts with pulse width of 384.2 ms as shown in Figure 4.1.
After a delay of 298.9 µs from input to trigger signal, the dye laser beam was produced
and detected by the high-speed photodetector BPX 65. The time delay from external
trigger input to evolution of laser light is shown in Figure 4.2.
4.16 V
384.2 ms
Figure 4.1: Pulse of input voltage from trigger unit
38
Figure 4.2: Channel 1: Pulse from trigger unit.
Channel 2: Pulse from BPX 65 photo detector
The typical pulse shape for both dye laser with Coumarin 500 and Rhodamine
590 as a lasing medium is shown in Figure 4.3. The horizontal time scale is 40 µs per
division, and the vertical amplitude is arbitrarily scaled in order to obtain the best pulse
shape. The pulse duration of dye laser at full wave half maximum is up to 52 µs and 53
µs for Coumarin 500 and Rhodamine 590 respectively.
39
4.2 V
52 us
Figure 4.3 (a): Pulse shape of the dye laser with Coumarin as a lasing
medium
4.2 V
53 us
Figure 4.3 (b): Pulse shape of the dye laser with Rhodamine as a lasing
medium
40
4.3
Diagnosing of The High Performances of Dye Laser
In order to operate the dye laser, the most important step is calibration. The
calibration was performed to diagnose and verify the performance of the laser beam.
Prior to the switching process, it is worth it to calibrate the optimum performance of the
dye laser. In this case, several parameters will be measured including power of the laser
beam, output energy, and wavelength and cavity length. The optimum value out of these
tests will be selected and utilized for the Q-switching process.
4.3.1
Wavelength
The most crucial feature of dye laser is its tunability, that is, the lasing
wavelength for a given dye may be varied over a wide range. In this study, the
diffraction grating is used in the laser cavity to perform selective tuning. This grating
was mounted in Littrow arrangement so that the first order reflection of the desired
wavelength was reflected back upon itself along the axis of the laser. This wavelength
depends on the orientation of the grating. Tuning is thus accomplished by rotating the
grating. Such tuning can yield extremely narrow pulse duration.
In this study, the dye laser system was triggered internally. The grating is set to
reflect the wavelength of back and forward through the dye cell. Two dyes, Coumarin
and Rhodamine, were utilized as a lasing medium of the dye laser. The data collected for
both Coumarin and Rhodamine are listed in Table 4.1 and Table 4.2 respectively.
41
Table 4.1: Power dependence on wavelength of dye laser with Coumarin 500 as a
lasing medium
Wavelength, λ
Power, P + 1 (mW)
(nm)
Experiment 1
Experiment 2
Experiment 3
Average
480
58
57
58
58
490
62
62
61
62
500
64
65
65
65
510
64
64
65
64
520
62
62
61
62
530
60
61
61
61
540
59
59
58
59
Table 4.2: Power dependence on wavelength of dye laser with Rhodamine 590 as a
lasing medium
Wavelength, λ
Power, P + 1 (mW)
(nm)
Experiment 1
Experiment 2
Experiment 3
Average
570
28
28
27
28
575
37
36
37
37
580
42
43
43
43
585
57
57
57
57
590
62
63
62
62
595
59
58
58
58
600
55
56
56
56
605
47
48
48
48
The data then used to plot a graph of power versus wavelength. The graphs are
shown in Figure 4.4 and Figure 4.5 respectively. Nonlinear curves are obtained in both
graphs.
42
66
Output Power (mW)
65
64
63
62
61
60
59
58
57
470
480
490
500
510
520
530
540
550
Wavelength (nm)
Figure 4.4: Accumulative power of dye laser with Coumarin 500 upon on wavelengths
Initially, the curve obtained from Figure 4.4 shows that the accumulative power
measurement is increased with respect to wavelength setting until it reach 500 nm.
Then, from 510 nm to 540 nm there was a downward trend of decreasing power
measurement. The optimum accumulative power from Coumarin 500 dye laser was
obtained as 65 mW at wavelength of 500 nm. This means; at wavelength of 500 nm, the
degree of absorbance of Coumarin is higher compared to other wavelengths. It can be
explained as follow; the wavelength of laser emission is corresponding to the maximum
of the gain curve. The gain is the ratio of photons leaving the dye cell divided by the
photons entering the dye cell. Stimulated emission then causes the gain to be greater
than one. At the same time, the absorption decreases the gain. The absorbance can be
express as the logarithm of the ratio of the incident power to the power of the
transmitted radiation. Thus, the peak value maximum corresponds to the maximum
value of the molar absorptivity of the dye.
43
65
Output Power (mW)
60
55
50
45
40
35
30
25
20
565
570
575
580
585
590
595
600
605
610
Wavelength (nm)
Figure 4.5: Accumulative power of dye laser with Rhodamine upon on wavelength
The same trend of Figure 4.4 is obtained when Rhodamine was used as a lasing
medium. Initially, Figure 4.5 curve shows that the accumulative power measurement is
almost linearly increased upon to the wavelength setting until it reached 590 nm with the
optimum power of 62 mW obtained. This is the wavelength of maximum laser
emission. Then, from 595 nm to 605 nm the measured power was decreased gradually.
The wavelength of laser emission is limited by the range of fluorescence wavelengths.
The wavelength ranges of Rhodamine are wider than Coumarin because different
molecules will absorb radiation of different wavelengths. The molecules absorb and
emit light only at wavelengths that fit the energy differences between their orbital. The
spectrum of molecule depends on the separation of its orbital energies. The separations
are different for every molecule, so the molecule’s spectrum is like a fingerprint and can
be used to identify it.
In general, the emission wavelength produced from dye laser either by Coumarin
or Rhodamine lasers is longer than wavelength emits by pumping laser. This is can be
explained from Stoke’s Law that some of the energies have been used to absorb the
molecule at lower level. Thus the fluorescence beam has less energy, or longer
wavelength.
44
4.3.2
Cavity Length
The most important part of any laser is the feedback system in which the light
bounces back and forward. In this study, the dye laser consists of two mirrors with the
active medium located between them. The space separating the mirrors called the cavity
length. The elements of the cavity define a self-repeating path that rays of light can
follow. The light intensity within the cavity can be hundreds to million times higher
than the incident light intensity.
It is the purpose of this section to determine the dependence of output dye laser
on cavity length, in which it offers the best performance for Q-switching process. The
cavity length was varied in the range of 5 cm to 9 cm by moving the high-reflectivity
mirror from the lasing medium. However, these experimental works were limited on
standard configuration of the dye laser system. Some parameters of laser were set at
constant value. The dye laser was trigger externally, whereas the frequency was set to
be 1 Hz.
The pulse duration of dye laser, which taken of full wave at half maximum was
measured at various cavity lengths. The data collection for both Coumarin 500 and
Rhodamine 590 as a lasing medium are listed in Table 4.3 and Table 4.4 respectively.
45
Table 4.3: Pulse duration on wavelength of dye laser with Rhodamine 590 as a lasing
medium
Cavity Length,
L + 1 (cm)
Pulse Duration,
t + 1 (µs)
5
29.0
6
34.5
7
34.5
8
39.6
9
52.0
Table 4.4: Pulse duration of dye laser with Rhodamine as lasing medium produced at
different cavity length
Cavity Length,
L + 1 (cm)
Pulse Duration,
t + 1 (µs)
5
31.2
6
37.4
7
38.4
8
40.5
9
53
The graphs of pulse duration of dye laser for both Coumarin 500 and Rhodamine are
depicted at Figure 4.6.
46
Pulse Duration (us)
60
50
40
30
20
10
0
5
6
7
8
9
Cavity Length (cm)
Coumarin 500
Rhodamine 590
Figure 4.6: The pulse duration of dye laser upon cavity length with different
lasing medium
From Figure 4.6, it can be seen that the results show a pulse duration dependence
on the dye laser cavity. The wider the cavity length, the broader the pulse duration
obtained.
In this experiment, the energy of the dye laser beam was measured as the cavity
length changes. The data collected are listed in Table 4.5 and Table 4.6 for dye laser
operated with Coumarin 500 and Rhodamine 590 as a lasing medium respectively. The
obtainable data then used to plot graph of energy versus cavity length. The graphs are
shown in Figure 4.7.
47
Table 4.5: Pulse energy of dye laser with Coumarin 500 as lasing medium produced at
different cavity length
Cavity Length,
L + 1 (cm)
Output Energy,
5
1.28
6
1.14
7
1.06
8
1.00
9
0.83
E0 + 0.02 (mJ)
Table 4.6: Pulse energy of dye laser with Rhodamine 590 as lasing medium produced at
different cavity length
Cavity Length,
L + 1 (cm)
Output Energy,
5
0.73
6
0.62
7
0.52
8
0.53
9
0.45
E0 + 0.02 (mJ)
48
Output Energy (uJ)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
5
6
7
8
9
Cavity Length (cm)
Coumarin 500
Rhodamine 590
Figure 4.7: The output energy of dye laser upon cavity length with different lasing medium
As shown in Figure 4.7, dye laser with both lasing mediums exhibit a general
decrease in output energy with increasing the cavity length. It can be explained as
follow; as a cavity length longer, the energy loss as light travels around the cavity.
Overall the dye laser with Coumarin 500 as a lasing medium showed the higher energy
compared to dye laser with Rhodamine 590 as a lasing medium.
The peak power of dye laser was obtained using Equation (3.1). The graph of
peak power on different cavity length was depicted at Figure 4.8.
49
50
Output Power (mW)
45
40
35
30
25
20
15
10
5
0
5
6
7
8
9
Cavity Length (cm)
Coumarin 500
Rhodamine 590
Figure 4.8: The peak power of dye laser upon cavity length with different lasing medium
Figure 4.8 shows that the longer the cavity length, the lower the peak power.
The decreasing of peak power due to the decreased beam path for amplification. Hence,
the dye laser operation was suggested to be performed at cavity length of 5 cm. This is
because at longer cavity length the peak power was found lower.
As a summary, the longer the cavity length, the wider the pulse duration and the
lower the output energy thus result the lower the peak power.
50
4.4.
Summary
In this diagnosis, pulse duration, peak power and output energy of dye laser were
measured in order to compare with passively Q-switching laser. Dye laser was operated
internally and externally.
In order to produce one pulse per trigger, the dye laser was triggered externally.
The input trigger pulse has amplitude of 4.16 V and pulse width of 384.2 ms. The delay
time of the laser signal was obtained as 298.9 µs.
The power of output laser was measured under different wavelength. The
wavelength of the dye laser has been studied in the range of 470 to 550 nm for Coumarin
500 as a lasing medium and 565 to 610 nm for Rhodamine 590. The result obtained from
this investigation shows that the power of the laser was found optimum at 500 nm for
Coumarin 500 and 590 nm for Rhodamine 590.
Another parameter also considered under this observation is cavity length. The
result obtained showed that the energy produced was inversely proportional to cavity
length in the tested range of 5.0 to 9.0 cm. The Q-switching was suggested to be
performed at cavity length of 5 cm for the longer working distance.
CHAPTER 5
PASSIVELY Q-SWITCHED DYE LASER
5.1
Introduction
The pronounced nonlinear characteristics of Q-switching can cause peak outputs
many orders of magnitude larger than the average output occurring under normal,
unmodulated conditions. One way of achieved this laser is by passive Q-switch
technique.
In this study, the peak power of passively Q-switched dye laser was obtained by
measure their output energy and pulse duration. Dye laser system that was utilized as a
source to be switched was operated under single shot condition. Four samples have been
tested as a saturable absorber inside the dye laser cavity. The first three materials are
organic dyes dissolved in suitable solvent; they are DODCI, DQOCI and
Cryptocyannine. Another sample is Cr4+: YAG crystal. The concentrations of organic
dye solutions were varied in order to obtain the suitable concentration to be a saturable
absorber. For Cr4+: YAG crystal, the position inside the laser cavity was varied to obtain
the shortest pulse duration.
52
5.2
Passive Q-switch with Different Saturable Absorber
In passive Q-switching, the laser cavity contains a lasing medium and an
absorbing medium, therefore, nonlinear in response. In this section, the results obtained
due to the passively Q-switched dye laser using different saturable absorber materials are
described. The materials employed in this study comprised of two types, there are
organic dye and inorganic Cr4+: YAG crystal. These saturable absorber materials were
inserted inside the dye laser cavity, which employed Coumarin 500 and Rhodamine 590
dissolved in ethanol as a lasing medium.
5.2.1
Organic Dyes Saturable Absorber
In this study, three organic dyes dissolved in methanol have been tested as a
saturable absorber. There are 3, 3’-Diethyloxadicarbocyanine Iodide (DODCI), 1, 3’Diethyl-4, 2’-quinolyloxacarbocyanine Iodide (DQOCI) and 1, 1’-Diethyl-4, 4’carbocyanine Iodide (Cryptocyannine). In order to obtain the suitable concentration of
organic dyes to use as a saturable absorber material, the concentration was varied. The
quartz cell contains the saturable absorber solution is placing inside dye laser cavity
between lasing medium and high reflecting mirror. Pulse duration and output energy of
passively Q-switched dye laser was measured upon the various concentration of saturable
absorber solution.
53
5.2.1.1 DODCI
The concentration of DODCI solution was varied in the range of 0.0001 M to 0.05
M. Further increases of the DODCI concentration are not allowed since it ceases lasing.
Laser output was detected by photodetector and coupled to the oscilloscope. Pulse
duration from a gain spectrum was measured and collected data are listed in Table 5.1.
Figure 5.1 shows the relationship between pulse duration and concentration of saturable
absorber in different lasing medium.
Table 5.1: Pulse duration of passive Q-switch laser upon concentration of DODCI
Concentration, C
(M)
Pulse Duration, t + 0.1 (µs)
Coumarin 500
Rhodamine 590
1X10-5
30.8
29.8
5X10-5
32.8
28.1
1X10-4
22.8
27.5
5X10-4
21.2
26.7
1X10-3
19.5
26.1
5X10-3
17.4
18.3
1X10-2
16
16.4
5X10-2
14.1
11.9
54
Pulse Duration (us)
40
35
30
25
20
15
10
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.1: Pulse Duration versus concentration of DODCI on different lasing medium
Figure 5.1 shows the pulse duration obtained when passively Q-switched dye
laser with different lasing medium upon concentration of saturable absorber. In general,
the pulse duration of Q-switched laser gradually decreased with increasing of the
concentration. The shortest pulse duration obtained from passively Q-switched dye laser
with Coumarin 500, as a lasing medium is 14.1 µs corresponds to 0.05 M. While, the
shortest pulse duration of 11. 9 µs was obtained at concentration of 0.05 M when Qswitching dye laser with Rhodamine 590 as a lasing medium. These pulse durations were
smaller than those obtained with dye laser under normal operation.
It is true because in laser operation with Q-switching, the saturable absorber is
used to store or delay the laser oscillation. During this storage, more molecules can be
excited to the upper level. Once the bleaching occurred, the photons can pass through and
strike with many excited molecules, hence produced many stimulated emission. This
condition results in the production of giant pulse. The pulse will emit within a very short
time.
55
The pulse energy of DODCI Q-switching laser takes ten times for each concentration
of the solution. Average values are listed in Table 5.2. The graph of the pulse energy
versus concentration of saturable absorber is depicted in Figure 5.2.
Table 5.2: Pulse energy of passive Q-switch laser upon concentration of DODCI
Concentration, C
(M)
Pulse Energy, E0 + 0.01 (µJ)
Coumarin 500
Rhodamine 590
-5
1.06
1.18
5X10-5
0.85
0.83
1X10-4
0.62
0.55
5X10-4
0.51
0.43
1X10-3
0.27
0.28
5X10-3
0.24
0.24
1X10-2
0.12
0.18
5X10-2
0.05
0.08
1X10
Pulse Energy (uJ)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Expon. (Rhodamine 590)
Poly. (Coumarin 500)
Figure 5.2: Output Energy versus concentration of DODCI in different lasing medium
56
The curves in Figure 5.2 show that the pulse energies of Q-switched laser for both
Coumarin and Rhodamine as a lasing medium almost decrease exponentially with respect
to the concentration of DODCI. Generally, the pulse energy from passively Q-switched
dye laser is high when the concentration of the saturable absorber is low and decreased at
higher concentration. This is true because the photon will experience less resistant in low
concentration rather than higher once. As a result, there is a higher amplification in the
cavity and this produces higher energy of the laser output. The maximum output energy
was obtained about 1.06 µJ for Coumarin 500 dye laser and 1.18 µJ for Rhodamine 590
dye laser.
In order to prove exponential relationship obtained in Figure 5.2, a graph of
logarithm of energies are plotted against concentration as shown in Figure 5.3(a) and
Figure 5.3(b).
Assume that the exponential equation, y = ceax which ‘y’ represent as pulse
energy, ‘x’ is the concentration of the saturable absorber and ‘c’ is the constant value
while x = 0. By changing the exponential equation y = ceax into natural logarithm, the
equation becomes linear equation of ln y = ax + ln c, where ‘a’ is the graph gradient. The
graphs ln (pulse energy) versus concentration need to be in linear pattern to prove that the
graph in Figure 5.2 is exponential decreased.
57
ln (pulse energy)
-3
ln y = -0.4125x + 0.7177
R2 = 0.9449
-2.5
-2
-1.5
-1
-0.5
0.00001 0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
0
0.5
Concentration (M)
Figure 5.3 (a): Graph natural logarithm of pulse energy versus concentration for
Coumarin dye laser
-3
ln (Pulse Duration)
-2.5
y = -0.35x + 0.5245
2
R = 0.9739
-2
-1.5
-1
-0.5
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
0
0.5
Concentration (M)
Figure 5.3 (b): Graph natural logarithm of pulse energy versus concentration for
Rhodamine dye laser
58
Figure 5.3(a) and Figure 5.3(b) show linear graphs. These indicate that the graph
in Figure 5.2 is exponential decreased of the pulse energy with respect to the DODCI
concentration. The output energy from a Q-switched laser is only a fraction of the output
energy in normal operation. This is because more energies of the cavity are loss in
saturable absorber.
Peak power of passive Q-switch with DODCI was measured using Equation (3.1).
The measured data are listed in Table 5.3.
Table 5.3: Peak power of passive Q-switch laser upon concentration of DODCI
Concentration, C
(M)
Peak Power, P + 0.01 (mW)
Coumarin 500
Rhodamine 590
1X10-5
34.42
39.6
5X10-5
25.91
29.54
1X10-4
27.19
20
5X10-4
24.06
16.1
1X10-3
13.85
10.73
5X10-3
13.79
13.11
1X10-2
7.5
10.98
5X10-2
3.55
6.72
The output power data in Table 5.3 was used to plot a graph. Figure 5.4 shows the
relationship between output power of Q-switched laser and concentration of DODCI for
different lasing medium.
Peak Power (mW)
59
50
45
40
35
30
25
20
15
10
5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.4: Peak power versus concentration of DODCI on different lasing medium
Figure 5.4 shows the relationship of peak power of passively Q-switched dye
laser with different concentration of DODCI for two different lasing medium. In general,
the peak power curves decreased exponentially with concentration of saturable absorber
solution. The profile of the peak power for Rhodamine laser is more consistent.
Coumarin laser indicates that the productions of peak powers are fluctuated upon
concentration of the solution. This result also means that the laser output is unstable if
DODCI is used as a saturable absorber. Hence DODCI material is more appropriate to be
used with Rhodamine laser compared to Coumarin.
A graph logarithm of peak power versus concentration is plotted to prove that the
Rhodamine laser curve in Figure 5.4 is exponentially decreased. The graph obtained is
shown in Figure 5.5.
60
4
3.5
ln(Peak Power)
3
2.5
2
y = -0.2266x + 3.7807
2
R = 0.9221
1.5
1
0.5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Figure 5.5: Graph natural logarithm of pulse energy versus concentration for Rhodamine
dye laser
The graph in Figure 5.5 shows linear relationship. This indicates that the
Rhodamine curve in Figure 5.4 decreases exponentially with respect to the DODCI
concentration.
5.2.1.2 DQOCI
In this study, the concentration of DQOCI solution was varied in the range of
0.00001 M to 0.005 M. The laser is cease as the concentration is further increases. The
laser output is diagnosed by measuring the pulse duration and the pulse energy. The pulse
duration of passively Q-switched dye laser is listed in Table 5.4. The plotted graph of
output energy on different lasing medium according to concentration of saturable
absorber is represented in Figure 5.6.
61
Table 5.4: Pulse duration of passive Q-switch laser upon concentration of DQOCI
Concentration, C
(M)
Pulse Duration, t + 1 (ns)
Coumarin 500
Rhodamine 590
1X10-5
2000
1160
5X10-5
1140
1100
1X10-4
456
824
5X10-4
400
400
1X10-3
212
216
5X10-3
512
448
1X10-2
Nil
Nil
5X10-2
Nil
Nil
Pulse Duration (ns)
2500
2000
1500
1000
500
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.6: Pulse duration versus concentration of DQOCI on different lasing medium
The results obtained when DQOCI was applied as a saturable absorber inside the
dye laser cavity indicated that Coumarin dye laser is more stable than Rhodamine. Pulse
duration obtained in Coumarin dye laser is decreased nonlinearly with respect to the
concentration. However, for Rhodamine laser, the pulse duration was found to be
62
fluctuated and much lower compared to the result of Coumarin dye laser. The shortest
pulse duration of Q-switched Coumarin dye laser is 212 ns at concentration of 0.001 M.
while, for Rhodamine dye laser, the shortest pulse duration of 216 ns was found at same
concentration. These pulses were slightly smaller than those produced with DODCI
saturable absorber.
The graphs ln (pulse duration) versus concentration is also plotted to show that
the Coumarin dye laser curve in Figure 5.6 is exponentially decreasing.
8
ln (Pulse Duration)
7
6
5
y = -0.341x + 7.588
2
R = 0.6418
4
3
2
1
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
Concentration (M)
Figure 5.7: Graph of natural logarithm of pulse energy versus concentration for
Coumarin dye laser
The graph in Figure 5.7 shows a linear relationship. This indicates that the
Coumarin curve of pulse duration versus DQOCI concentration in Figure 5.6
exponentially increases.
The output energy of the both laser using DQOCI as saturable absorber was
measured and the collected data are listed in Table 5.5. Figure 5.5 shows the relationship
63
between output energy of passive Q-switched laser and concentration of DQOCI
concentration using data of Table 5.5.
Table 5.5: Pulse energy of passive Q-switch laser upon concentration of DQOCI
Concentration, C
Pulse Energy (us)
(M)
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.00001
Pulse Energy, E0 + 0.001 (µJ)
Coumarin 500
Rhodamine 590
1X10-5
1.582
1.057
5X10-5
1.392
0.731
1X10-4
1.339
0.616
5X10-4
1.141
0.528
1X10-3
0.828
0.524
5X10-3
0.418
0.446
1X10-2
Nil
Nil
5X10-2
Nil
Nil
0.00005
0.0001
0.0005
0.001
0.005
Concentration (M)
Coumarin 500
Rhodamine 590
Poly. (Coumarin 500)
Poly. (Rhodamine 590)
Figure 5.8: Pulse energy versus concentration of DQOCI on different lasing medium
64
The output energy produced by passively Q-switched Coumarin dye laser is found
almost inversely proportional to the concentration of DQOCI. However, the output
energy produced by Rhodamine 590 was found relatively smaller compared to the output
energy obtained by Coumarin 500 lasing medium. The relationship for Coumarin is
found to be inversely proportional with the concentration. These output energies were
found smaller than obtained in pure dye laser.
The attenuation of output energy of Q-switching laser from pulse energy of
normal operation has been mention in previous section. Another factor contributing to
this is that some of the energy is usually left in the active medium. In addition, the
fluorescence begins with pumping and considerable energy is lost through spontaneous
emission before the Q-switching develops.
Peak power of passive Q-switch with DODCI was calculated using Equation 3.1.
The measurement data are listed in Table 5.6. The obtainable peak powers in Table 5.6
are used to plot a graph such as shown Figure 5.9.
Table 5.6: Peak power of passive Q-switch laser upon concentration of DQOCI
Concentration, C
(M)
Peak Power (P) + 0.01 (W)
Coumarin 500
Rhodamine 590
1X10-5
0.79
0.91
5X10-5
1.22
0.66
1X10-4
2.94
0.75
5X10-4
2.85
1.32
1X10-3
3.91
2.43
5X10-3
0.82
1.00
65
4.5
Peak Power (mW)
4
3.5
3
2.5
2
1.5
1
0.5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.9: Peak power versus concentration of DQOCI on different lasing medium
Initially, the peak power of Coumarin laser is almost linear upon the DQOCI
concentration. After reaching the optimum peak power of 3.91 W, the peak power
drastically drops. Overall the peak power of Coumarin laser is higher in comparison to
the Rhodamine dye laser. Although, the peak power for Rhodamine laser is relatively
lower, the curve configuration is almost similar. The Rhodamine dye laser achieved the
maximum point at the same concentration of 0.001 M. It means at this point; the ability
of DQOCI to absorb dye laser is higher than others.
5.2.1.3 Cryptocyannine
In this study, the concentration of Cryptocyannine solution was varied in the
range of 1x10-5 M to 5x10-2 M. The results of pulse duration and pulse energy
measurements of passively Q-switched dye laser are listed in Table 5.7 and Table 5.8
respectively. The data are used to plot graphs of pulse duration versus concentration and
output energy versus concentration.
66
Table 5.7: Pulse duration of passive Q-switch laser using Cryptocyannine
Concentration, C
Pulse Duration, t + 0.01 (µs)
(M)
Coumarin 500
Rhodamine 590
1X10
-5
31.00
31.32
5X10
-5
30.32
25.24
1X10
-4
29.76
23.80
5X10
-4
27.72
20.42
1X10-3
21.04
21.20
5X10
-3
20.52
21.84
1X10
-2
21.64
16.16
5X10-2
24.84
17.56
As we can see in Table 5.7, the temporal behavior of the laser emission is very
similar to the one obtained in normal operation when low concentration solution is
inserted. The data are used to plot graphs. Figure 5.10 shows the relationship between
Pulse Duration (us)
pulse duration of passive Q-switch laser and concentration of Cryptocyannine.
35
33
31
29
27
25
23
21
19
17
15
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.10: Pulse duration versus concentration of Cryptocyanine on different lasing
medium
67
Overall, the pulse duration of both lasers are fluctuating when the concentration
of Cryptocyannine is varied. The minimum pulse duration for Rhodamine laser is 16.16
µs corresponding to 0.01 M concentration of Cryptocyannine. Whereas, the minimum
pulse duration for Coumarin laser is obtained as 20.52 µs corresponding to the
concentration of 0.001 M. Generally, Coumarin laser has longer pulse duration
compared to Rhodamine laser.
The output laser using Cryptocyannine modulator is measured and the collected
data are listed in Table 5.8. Graph of the pulse energy versus concentration for the
particular saturable absorber is shown in Figure 5.11.
Table 5.8: Pulse energy of passive Q-switch laser using Cryptocyannine
Concentration, C
(M)
Pulse Energy, E0 + 0.001 (µJ)
Coumarin 500
Rhodamine 590
1X10-5
1.290
1.283
5X10-5
1.040
1.125
1X10-4
0.840
0.860
5X10-4
0.705
0.728
1X10-3
0.630
0.540
5X10-3
0.542
0.442
1X10-2
0.410
0.300
5X10-2
0.523
0.150
68
1.6
Pulse Energy (uJ)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.11: Pulse energy versus concentration of Cryptocyannine on different lasing
medium
In general, the output energy produced by passively Q-switched dye laser for both
Coumarin and Rhodamine lasing medium were found almost proportional inversely to
the concentration of Cryptocyannine. This is means that the energy of the laser beam
decreases as the concentration increased. It is true because as mentioned in theory, the
bleaching condition of saturable absorber occurred when population at lower level almost
balances with upper level. If there is medium with large population, the possibility
achieve this saturation condition is more easily compared to the material with low
concentration. Thus, when medium with higher concentration was used, more energy will
be used to excite the population in lower level to higher level in order to balance each
other. This will cause reduction of output energy from Q-switched laser.
Peak power of passive Q-switch with Cryptocyannine was calculated using
Equation (3.1). The calculation data are listed in Table 5.9.
69
Table 5.9: Peak power of passive Q-switch laser upon concentration of Cryptocyannine
Concentration, C
Peak Power, P + 0.01 (mW)
(M)
Coumarin 500
Rhodamine 590
1X10-5
41.61
40.96
5X10-5
34.30
44.57
1X10-4
28.23
36.13
5X10-4
25.43
35.65
1X10-3
29.94
25.47
5X10-3
26.41
20.24
1X10-2
18.95
18.56
5X10-2
21.05
8.50
The data of peak power in Table 5.9 are used to plot a graph. Figure 5.12 shows
the relationship of output power of Q-switched laser and concentration of saturable
absorber for different lasing medium.
50
Peak Power (mW)
45
40
35
30
25
20
15
10
5
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
Coumarin 500
Rhodamine 590
Figure 5.12: Peak power versus concentration of Cryptocyanine on different lasing
medium
70
Figure 5.12 shows the relationship of peak power of passively Q-switched dye
laser with different concentration of Cryptocyannine for different lasing medium. In
general, the output power inversely proportional to the concentration of saturable
absorber solution. The peak power produced from Coumarin dye laser is relatively
smaller compared to Rhodamine dye laser. Hence, Cryptocyannine saturable absorber is
preferable to Rhodamine dye laser.
5.2.1.4 Comparison of organic dyes saturable absorber
Initially, the passively Q-switched dye laser was performed using three organic
dyes saturable absorber. The quartz cell contains dye solution was placed within the
cavity between active medium and high reflecting mirror. The concentration of DODCI,
DQOCI and Cryptocyannine solution were varied in order to determine the suitable
concentration to be used as a saturable absorber.
40.5
Pulse Duration (us)
35.5
30.5
25.5
20.5
15.5
10.5
5.5
0.5
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyaninne
Figure 5.13(a): Pulse duration versus concentration of different saturable absorber
materials
71
35
Pulse Duration (us)
30
25
20
15
10
5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyaninne
Figure 5.13(b): Pulse duration versus concentration of different saturable absorber
materials
Figure 5.13(a) and Figure 5.13(b) show the pulse duration of the three different
saturable absorber materials upon their concentration, for Coumarin and Rhodamine dye
laser. In general, the pulse duration depend on the concentration of the solution. The
pulse duration of Q-switched laser for Coumarin and Rhodamine dye laser shows
decreased with increasing the concentration of saturable absorber. The DQOCI present a
best performance as a saturable absorber for both dye lasers with shortest pulse duration.
Compared to dye laser under normal operation, the passive Q-switching lasers have
shorter pulse duration.
The reduction in pulse duration observed in the passive Q-switching can be
explained as follows. In a dye laser under normal operation, the laser pulse starts to form
as soon as population inversion is created. However, the resulting laser pulse In a Qswitched laser, energy is stored in the saturable absorber while the cavity Q is lowered to
prevent the onset of laser emission. Although the energy stored and the gain in the lasing
medium, the cavity losses are also high, lasing action is prohibited and the population
inversion reaches a level far above the threshold for normal lasing action. When a high
72
cavity Q is restored, the stored energy is suddenly released in the form of very short
pulses light.
The output energy of Q-switched laser is found increased with regard to the
concentration of saturable absorber materials as illustrated in Figure 5.14. At higher
concentration, the possibility to get saturation condition is more easily. The ability of a
substance to selectively absorb certain wavelengths of light while transmitting others is
determined by its molecular and atomic structure. The concentration of the saturable
absorber material is concentration of the colored substance in the saturable absorber
solution increases, absorbance increases and the amount of light passing through
decreases.
1.8
Pulse Energy (uJ)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyaninne
Figure 5.14(a): Pulse energy versus concentration of different saturable absorber
materials
73
1.6
Pulse Energy (uJ)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyannine
Figure 5.14(b): Pulse energy versus concentration of different saturable absorber
materials
Although the pulse energy from the Q-switching is smaller than that from purely
dye laser, the pulse peak power was not significantly reduced. The reason for the small
reduction in peak power is because the loss of the pulse energy occurs mainly in the
falling edge of the laser. Figure 5.15 shows the peak power of Q-switching laser for
Coumarin and Rhodamine dye laser with different saturable absorber.
The peak power of Q-switching laser was measured using Equation (3.0). As
shown in Figure 5.15, DQOCI was produced a highest peak power compared with
DODCI and Cryptocyannine. The peak maximum correspond to the maximum value of
the molar absorptivity and hence the sensitivity of the analysis. The strength of the molar
absorbance is related to the concentration by Beer’s Law, the more concentrate molecule
that the light comes in to contact with, the more light will be absorbed.
74
4.5
Peak Power (mW)
4
3.5
3
2.5
2
1.5
1
0.5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyaninne
Figure 5.15(a): Peak power versus concentration of different saturable absorber
materials
3
Peak Power (mW)
2.5
2
1.5
1
0.5
0
0.00001
0.00005
0.0001
0.0005
0.001
0.005
0.01
0.05
Concentration (M)
DODCI
DQOCI
Cryptocyannine
Figure 5.15(b): Peak power versus concentration of different saturable absorber
materials
75
5.2.2
Cr4+: YAG crystal Saturable Absorber
In this study, a Cr4+: YAG crystal has been tested as a saturable absorber inside
the dye laser cavity. The position of Cr4+: YAG crystal inside the laser cavity was
adjusted in order to obtain the shortest pulse duration of passively Q-switched dye laser.
The Cr4+: YAG crystal was placed in two positions. The first position is between
lasing medium and reflecting mirror, which is, referred front position. The second
position is between lasing medium and output coupler mirror, which is referred as back
position. Pulse duration, output energy and peak power of Q-switched laser for different
lasing medium are listed in Table 5.10.
Table 5.10: Pulse duration, pulse energy and peak power for different position of
saturable absorber
Pulse duration,
Pulse energy,
Peak Power,
t + 10 (ns)
E0 + 0.001 (µJ)
P + 0.01 (W)
Dye Laser
Front
Back
Front
Back
Front
Back
Coumarin
740
208
1.057
1.283
1.43
6.17
Rhodamine
744
220
0.528
0.733
0.71
3.33
In Table 5.10, obviously shows that pulse duration obtained when saturable absorber
was placed at back position is smaller than front position. The shortest pulse duration of
208 ns was produced by passively Q-switched Coumarin dye laser with placing saturable
absorber between active medium and reflection mirror. This is mainly due to the
nonlinear absorption of the saturable absorber, which leads to a much faster falling edge
in the pulse profile.
76
In general, output energy obtained from passively Q-switched laser using Cr4+:
YAG crystal placed in back position was found higher than front position. The result
with inorganic saturable absorber was lower than pure dye laser, similar with organic
dyes. This value mainly resulted from the large insertion loss of the Cr4+: YAG crystal
inside the laser cavity.
The optimum peak power of passively Q-switched dye laser was obtained when
placing the Cr4+: YAG crystal at back position of laser cavity. Table 5.10 shows the
highest output power of passively Q-switched Coumarin laser was 6.17 W. While for
Rhodamine laser, the optimum output power was obtained as 3.33 W.
5.3 Summary
Passively Q-switched dye laser was performed for Coumarin 500 and Rhodamine
590 as a lasing medium. The pulse duration and output energy of Q-switching laser then
were compared with dye laser under normal operation. The pulse duration and output
energy then have been used to calculate peak power of Q-switched laser.
Two types of material were employed as a saturable absorber. The organic type
consists of three dyes dissolved in ethanol; they are 3, 3’-Diethyloxadicarbocyanine
Iodide (DODCI), 1, 3’-Diethyl-4, 2’-quinolyloxacarbocyanine Iodide (DQOCI) and 1, 1’Diethyl-4, 4’-carbocyanine Iodide (Cryptocyannine). Another type is inorganic material,
which consists of Chromium-doped Yttrium Aluminum Garnet (Cr4+: YAG) crystal. The
saturable absorber material was inserted inside the dye laser cavity. As the conclusion,
the DQOCI with concentration of 0.001 M was suggested to perform a passive Qswitching for both Coumarin and Rhodamine laser.
77
Finally, the passively Q-switched dye laser was carried out using Cr4+: YAG
crystal as a saturable absorber. In this study, the shortest pulse duration and higher output
energy obtained when placing the saturable absorber material between lasing medium
and high reflection mirror. The optimum output power achieved when passively Qswitched dye laser with Coumarin 500 as a lasing medium.
In summary, with the optimized selections of concentration of organic dyes and
the best position of Cr4+: YAG crystal saturable absorber for each lasing medium of dye
laser, the shorter the pulse duration, the lower of the single pulse energy and the higher of
peak power of the Q-switched pulses could be obtained. The shortest pulse duration and
optimum output power obtained using DQOCI and by placing Cr4+: YAG crystal between
lasing medium and high reflection mirror when passively Q-switched Coumarin 500 dye
laser. This is may be due to the by short wavelength of Coumarin compared with
Rhodamine. The shorter the wavelength, the more energetic the photons are. Photons
with shorter wavelengths are easier to be absorbed by the materials than photons with
longer wavelengths.
CHAPTER 6
DIAGNOSES OF PASSIVE Q-SWITCH LASER BEAMS
6.1
Introduction
In many applications, the laser beam needs to be focused to a very small spot
size, or else the overall brightness of the beam is crucial parameter. In this case, the
focusability or beam quality is the key parameter to be considered. Laser beams are
characterized by how good they are compared to the ultimate limit of a perfect beam.
There are limited only by the inherent diffraction of the light wave, hence diffraction
limited or lowest order mode beam. This beam is characterized by a pure Gaussian
cross-section intensity profile, and its propagation is governed by Gaussian beam
equations, which differ markedly from geometrical optical calculations.
In this chapter, the passive Q-switch laser beam images in two- and threedimensional will be investigated using BeamStar CCD camera profiler. CCD camera
laser beam profiles are based on a mosaic of two-dimensional detectors called pixels.
The two-dimensional mosaic like detector instantly records the amount of energy
impending on its surface, thus recording the optical pattern of the laser beam.
79
The intensity distribution of the laser beam is recorded pixel by pixel and
displayed as a two-dimensional topographic map or a three-dimensional isometric
view. CCD camera profiler also gives an analysis of Gaussian fit.
The spot area produced by passively Q-switched dye laser is considered
small. To precisely measure the spot area, a metallurgical method was employed.
The images of spot area were analyzed via the Video Test 5.0 Software. Two
materials were used as a saturable absorber inside the laser cavity, which are DQOCI
and Cr4+: YAG. Dye laser used in this experiment utilized the Coumarin 500 as a
lasing medium.
6.2
Analyzing the beam
In this study, the BeamStar CCD Laser Beam Profiler was used as a
diagnostics measurement system of passive Q-switched laser beams. It provides an
extensive range of graphical presentations and analysis capabilities of laser beam
parameters, such as beam width, shape, position and intensity profile. The BeamStar
CCD Profiler uses a video camera and PC card to image, capture, store, and perform
two-dimensional intensity distribution analysis on laser beams. Laser parameters
such as beam width, Gaussian Width and correlation were observed at various
working distances.
6.2.1 Gaussian Fit Analysis
Vertical profile and horizontal profile displays the profiles from two
orthogonal axis, horizontal and vertical. Each image is a digital representation of the
spatial power distribution across the beam. The angle at which the profile is cut is
controlled by profile.
80
The Gaussian fit profile shows how closely the measured beam profile
matches a Gaussian profile. The typical results obtained from this experiment are
depicted in Figure 6.1.
Beam
Profile
Gaussian
Profile
(a)
(b)
Figure 6.1: Gaussian profile of passive Q-switched laser beam. (a). Vertical profile,
(b). Horizontal profile
The Gaussian fit profile shows how closely the measured beam profile matches a
Gaussian profile. The Gaussian fit profile is displayed on top of both the vertical and
horizontal profiles in green.
The Gaussian fit is a least-square fit of a Gaussian equation to the cross
section beam profiles. The correlation coefficient is the normalized sum of the fit
residuals. The Gaussian fit was calculated from following equation:
81
 − (x − c )
I = V exp 

 σ
2
(6.1)
Where I is the intensity of a pixel at location x, V is the maximum intensity of the
fitted Gaussian curve (Peak Intensity), c is the center of the Gaussian fit peak
(Centroid), and σ is the radius of the Gaussian fit curve at the 1/e2 intensity level
(diameter).
Figure 6.2 shows the typical two- and three-dimensional images of passive Qswitch laser beam. Three dimensional far field transverse beam profile of the Qswitching laser output beam is illustrated in Figure 6.2(a). The beams are distributed
in the form of Gaussian beam profile. The gain spectrum is relatively quite broad.
Two-dimensional image of the Q-switch laser beam is shown in Figure 6.2(b). A
single spot obtained, indicated that the laser was operated at a uniphase mode. The
beam spot is accompanied with beam noise and diffraction effect.
(a)
(b)
Figure 6.2: Beam Profile of Q-switching laser; (a). Three-dimensional image shows
the distribution of Gaussian beam profile (b). Two-dimensional image
82
The symmetry and uniformity of the laser beam are both indicated that the
laser operates in the TEM00 mode. This mode is also called the mono-mode. The
mode pattern is stable with time and constitutes a spatially coherent output. Thus,
TEM00 has the lowest beam to be focused to the smallest possible spot.
BeamStar CCD Profiler has graphical analysis capabilities of Q-switch laser
beam parameters such as centroid, beam width, Gaussian width and correlation. The
numerical value of these parameters upon working distance for passively Q-switched
dye laser using DQOCI and Cr4+: YAG saturable absorbers are listed in Table 6.1
and Table 6.2 respectively. The beam column displays the Q-switch laser beam
reading while the Gaussian column displays the data of the ideal Gaussian profile.
These results are calculated and displayed for both the horizontal and vertical profile.
83
Table 6.1: Gaussian width in horizontal and verticals profiles for DQOCI saturable absorber due to working distance
Working
Distance
+ 1 (cm)
10
15
20
25
30
35
40
Centroid + 0.01 (µm)
Horizontal
3421.38
3212.92
3296.97
3329.05
3001.28
2950.44
3175.09
Vertical
2747.73
2906.90
3010.83
3024.73
2778.71
2863.91
3148.26
Beam + 0.01 (µm)
Horizontal
789.83
1379.80
1700.42
1907.86
1853.32
1841.61
2427.42
Vertical
759.45
1319.28
1614.47
1674.57
2092.04
2169.21
2351.95
Gaussian + 0.01 (µm)
Horizontal
766.24
1456.31
1776.57
1947.62
1880.15
1845.97
2370.83
Vertical
746.31
1301.49
1594.92
1648.06
2031.33
2087.79
2298.88
Correlation (%)
Horizontal
94.22
91.78
91.61
90.39
92.48
93.53
91.37
Vertical
95.53
91.36
90.55
89.35
90.05
88.96
90.38
Table 6.2: Gaussian width in horizontal and vertical profiles for Cr4+: YAG saturable absorber due to working
distance
Working
Distance
+ 1 (cm)
10
15
20
25
30
35
40
Centroid + 0.01 (µm)
Horizontal
3439.55
3450.07
3386.96
3460.59
3386.96
3576.29
3365.92
Vertical
2970.57
2982.65
5699.64
2994.72
2970.57
6665.68
5554.73
Beam + 0.01 (µm)
Horizontal
983.77
1028.69
896.52
1104.85
1062.54
680.02
908.10
Vertical
922.66
856.68
1938.74
874.81
950.45
1735.31
2029.82
Gaussian + 0.01 (µm)
Horizontal
985.21
1002.75
968.09
1067.87
1044.31
758.88
1010.87
Vertical
924.84
862.98
1953.00
891.69
962.98
1719.31
2046.33
Correlation (%)
Horizontal
88.61
87.43
80.36
88.13
87.09
81.69
82.15
Vertical
97.08
96.59
95.56
95.51
93.86
92.07
91.79
84
The BeamStar CCD Profiler determines the location of the beam Centroid by
summing the intensities of all image pixels in both horizontal and vertical axes, and
computing the center of gravity of the beam intensity. The pixel coordinates at this
location define the Centroid. The horizontal (H) and vertical (V) coordinates of the
Centroid are computed using the following formula;
H =Σ
h × i (h, v )
I
V =Σ
v × i ( h, v )
I
(6.2)
(6.3)
Where i (h, v) is the intensity at location (h, v) and I is the total intensity taken over
the total area
The beam width of Q-switched laser was measured at lowest clip level, which
is 13.5 % of the profile peak. The 13.5 % level corresponds to the 1/e2 point of a
Gaussian profile. The percentage deformation calculation from the ideal Gaussian
beam then presented as a correlation.
The collected data in Table 6.1 and 6.2 are used to plot graph of correlation
against working distances. The graphs are illustrated in Figure 6.3 and Figure 6.4 for
both DQOCI and Cr4+: YAG saturable absorbers respectively.
85
Correlation (%)
97
95
93
91
89
87
10
15
20
25
30
35
40
Working Distance (cm)
Horizontal
Vertical
Figure 6.3: Correlation upon working distances for DQOCI saturable absorber
As depicted in Figure 6.3, the curves show that the correlation percentage for
both horizontal and vertical axis of passively Q-switched dye laser beam using
DQOCI is nonlinearly decreased with respect to working distance. However, the
correlation percentage was found highest at short working distance for both
horizontal and vertical axis. At far working distance, the correlation percentage
fluctuates.
Correlation (%)
100
95
90
85
80
75
10
15
20
25
30
35
40
45
Working Distance (cm)
Horizontal
Vertical
Figure 6.4: Correlation upon working distances for Cr4+: YAG saturable absorber
86
Figure 6.4 shows the correlation percentage graph of the passively Qswitched dye laser using Cr4+: YAG crystal upon different working distance. For
vertical axis, the percentage inversely proportional with working distance. However,
for horizontal axis the percentage seem to be fluctuated. The highest correlation
percentages for both horizontal and vertical axis were found at short working
distance.
6.2.2
Beam Spot
Another parameter, which may be important to be considered is the laser spot
size. Some applications require a small spot for high-resolution measurement while
others require a larger diameter spot for averaging rough surfaces or for eye safety
concerns.
In this study, two-dimensional images of passively Q-switched Coumarin 500
dye laser using DQOCI and Cr4+: YAG crystal were analyzed using Video Test 5.0
software in order to precisely measure the spot area. The typical results of spot area
for both DQOCI and Cr4+: YAG crystal as a saturable absorber are shown in Figure
6.5 and Figure 6.6 respectively.
10 cm
15 cm
20 cm
25 cm
30 cm
35 cm
40 cm
Figure 6.5: Two-dimensional image of passively Q-switched dye laser using
DQOCI upon different working distance
Qualitatively, the beam spot of passive Q-switched dye laser using DQOCI in
Figure 6.5 was found bigger for working distance in the range of 15 to 25 cm. The
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colours of beam spot (red) become lesser and lesser from left to right, as to the target
distances increases.
10 cm
15 cm
20 cm
25 cm
30 cm
35 cm
40 cm
Figure 6.6: Two-dimensional image of passively Q-switched dye laser using
Cr4+: YAG crystal upon different working distance
Similar with the previous procedure, the beam spot of passively Q-switched
Coumarin 500 dye laser using Cr4+: YAG crystal are arranged in the increasing order
of the working distance. In the range from 15 to 20 cm, the beam spot seem to be
bigger than others. From 25 to 40 cm, the beam spot almost in the same size. The
smallest beam spot was found at working distance of 10 cm.
The beam spot area was measured in millimeter square based on Figure 6.5
and Figure 6.6. The collected data are listed in Table 6.3 and Table 6.4 for DQOCI
and Cr4+: YAG crystal saturable absorber respectively.
Table 6.3: Beam spot perimeter and area used DQOCI as a saturable absorber
Working
Distance + 1 (cm)
10
Beam Spot
Perimeter +
Area + 0.0001
0.0001 (mm)
(mm2)
7.5859
4.2375
15
21.2872
33.7942
20
28.4227
60.4961
25
25.3697
49.2869
30
34.8406
21.1930
35
21.1625
33.6505
40
22.0161
35.5014
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Table 6.4: Beam spot perimeter and area used Cr4+: YAG crystal as saturable absorber
Beam Spot
Perimeter +
Area + 0.0001
0.0001 (mm)
(mm2)
9.00195
6.54420
Working
Distance + 1 (cm)
10
15
14.4673
16.46684
20
14.5993
16.6879
25
13.7484
14.6770
30
16.1815
18.6124
35
17.25803
15.61651
40
18.41149
15.28642
The collected data in Table 6.3 and Table 6.4 are plotted into a same graph.
The results were compared between DQOCI and Cr4+: YAG crystal as a saturable
absorber. The plotted graph is depicted in Figure 6.7.
Area (mm x mm)
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
40
45
Worrking Distance (cm)
DQOCI solution
Cr4+: YAG crystal
Figure 6.7: Beam spot area against working distance
Spot area of passively Q-switched dye laser using Cr4+: YAG crystal
saturable absorber was found smaller than spot area produced by using DQOCI
saturable absorber. As shown in Figure 6.7, initially, the spot area of Q-switch laser
using DQOCI saturable absorber drastically increased with respect to working
distance. Nevertheless, the spot area drops suddenly and become fluctuate, as the
89
distance gets longer. Different with The spot area of Q-switch laser using Cr4+: YAG
crystal saturable absorber where initially it gradually increased and remains almost
constant at greater working distance.
6.3
Summary
The Gaussian fit analysis was performed via BeamStar CCD Laser Beam
Profiler. Besides providing a laser beam parameters such as position, beam width,
Gaussian beam and correlation percentage, this analysis also provide three- and twodiemansional images of passively Q-switched dye laser. The beam spot of passive Qswitch laser was measured regarding it two-dimensional images.
Two saturable absorber materials are utilized to passively Q-switched the dye
laser with Coumarin 500 as a lasing medium. They are DQOCI and Cr4+: YAG
crystal. The concentration of DQOCI was chosen as 0.001 M while, the position of
Cr4+: YAG crystal was chosen between lasing medium and high reflection mirror due
to previous experiment.
Initially, the passive Q-switch was analyzed using BeamStar CCD Laser
Beam Profiler. The output characteristics of the Q-switch laser posses of a uniphase
of TEM00 mode. The centroid, beamwidth and Gaussian beam were obtained at
various working distances. The beam width then compared with the Gaussian beam
and measured as correlation percentage. From this analysis, the highest percentage
was found at lower working distance for DQOCI and Cr4+: YAG saturable absorber
of both horizontal and vertical axis.
In general, the beam spot of passive Q-switch laser are independent with
working distances. At lower working distance, the beam spot was found to be
smallest. At the middle, the beam spot becomes wider before remain smaller and
constant at higher working distances. The colours of beam spot become less as the
target distance increases.
CHAPTER 7
CONCLUSIONS AND SUGGESTIONS
7.1
Conclusion
The objectives of this research were successfully achieved, which hopefully will
contribute to a better understanding of passive Q-switching laser. Passive Q-switches
technique using saturable absorber material such as DODCI, DQOCI, Cryptocyannine
and Cr4+: YAG crystal have been studied. The passively Q-switching nitrogen-laserpumped-dye-laser has been successfully developed. Generally, dye laser with Coumarin
500 and Rhodamine 590 were utilized as a source to be switched during the Q-switching
works.
As a preliminary step to Q-switched the dye laser, the laser itself had to be
calibrated in order to determine the current performance of laser beam. Firstly, the dye
laser was externally triggered in order to produce a single shot pulses. Because the
interaction of the color molecules and the solvent, there is a broadening of the
vibrational energy levels, thus a wide spectrum bands are formed.
91
In this experiment, the lasing wavelength for Coumarin and Rhodamine was
varied over 470 to 550 nm and 565 to 610 nm respectively. The optimum accumulation
power was found at 500 nm for Coumarin and 590 nm for Rhodamine. The wavelength
of laser emission is limited by the range of fluorescence wavelengths. The fluency pulse
energy of laser beam was observed at various cavity lengths. The pulse duration and
output energy obtained for dye lasers are linearly increased with respect to the cavity
length. By determining the pulse duration and peak power of laser beam, the
performance trend of dye laser can be traced before utilized the beam as source to be
switched. The best way to Q-switched the dye laser is in the wavelength with highest
power and shortest cavity length, which concerning the best performance of the laser.
The passive Q-switching was chosen in this study because its offer an advantages
of economy, simplicity of operation, and the emission of the output pulse in a narrow
linewidths. Passive Q-switching relies on the action of so-called saturable absorber
which is materials whose absorption decrease with increasing irradiance. This material is
placed inside the laser cavity. At the beginning, the saturable absorber absorbs the dye
laser fluorescent emission to the degree that the high reflection mirror is optically
isolated from the remainder of the laser cavity. When the dye suddenly bleaches, the
laser radiation can reach the high reflection mirror and laser oscillation occurs.
There are four materials, such as DODCI, DQOCI, Cryptocyannine and Cr4+:
YAG have been employed as a saturable absorber material. The first three materials are
organic dyes dissolved in methanol and the last one is crystal. The concentration of the
dyes saturable absorption was varied to produce any desired absorption. While, the
position of Cr4+: YAG crystal inside the dye laser cavity was varied in order to study the
length dependence of the Q-switching output. During Q-switching operation, the cavity
length of dye laser was set at 5 cm.
The concentrations of organic dyes were varied over 0.00005 M to 0.05 M. the
position of Cr4+: YAG crystal was adjusted. Front position was refers to the placing of
92
the crystal between lasing medium and output coupler mirror, while back position means
the placing of the crystal between lasing medium and high reflection mirror. The
dependence of pulse duration and output energy on the concentration saturable absorber
was determined. Then, these values were utilized to measure the peak power.
In general, the pulse duration of Q-switching was found decreases with the
concentration of dye saturable absorber. The temporal behavior of the Q-switch laser
emission is very similar to the one obtained in normal operation when low
concentrations are inserted. The pulse was slightly smaller than those produced with dye
laser in normal operation. The shortest pulse duration was found when passively Qswitched Coumarin dye laser with 0.001 M of DQOCI. Similar result obtained whereby,
the shortest pulse duration belongs to back position of Cr4+: YAG crystal.
All the tested saturable absorber of organic dye shows similar result, in which the
output energy was found dependence on concentration of the solution. The output
energy from a Q-switched laser is only fraction of the output energy in normal operation
at the same condition because scattering and absorption losses are always present in the
cavity.
The peak power of passive Q-switching laser was calculated as output energy
divided by pulse duration obtained. The small reduction of peak power is because the
falling edge of the laser. The peak maximum corresponds to the maximum value of the
molar absorbance of the saturable absorber solution. The passively Q-switched dye laser
using DQOCI with concentration of 0.001 M stated the optimum peak power for both
Coumarin and Rhodamine lasing medium.
Cr4+: YAG crystal is potential material to be used as saturable absorber material
in this study. Due to its specific properties such as broad absorption band, high damage
threshold, long lifetime, large absorption cross-section and easy operation. In this study,
the Cr4+: YAG crystal was placed inside the dye laser at two positions in order to obtain
93
the possibility of shortest pulse duration. From this experiment we found that with
placed the Cr4+: YAG saturable absorber between lasing medium and high reflector
mirror, the shortest pulse duration could be obtained.
Three- and two- dimensional images of Q-switching lasers were observed via
BeamStar CCD Laser Beam Profiler. The output characteristics of Q-switching laser
posses of a uniphase of TEM00 mode. This software also performed the Gaussian fit
analysis for both vertical and horizontal axis. The beam width then compared with
Gaussian beam and measured as correlation percentage upon various working distance.
The highest percentage was found at lower working distance.
The beam spot of passive Q-switch laser was measured regarding it twodimensional, which then analyzed by Video Test 5.0 software. The perimeter and area of
passive Q-switching lasers were observed upon the working distance. In general, the
passive Q-switch lasers are independent with working distances.
7.2
Problems And Suggestions
The main problem in this study is actually contributed from the degradation of
the dye. Damage to a dye Q-switch can happen in two ways. First, UV light from the
pumping source of the dye laser and light from outside, particularly below 3500 nm will
breakdown the long-chain dye molecules. These will cause the dye cannot be used as
saturable absorber material for a long time. A suggestion solution is to place a UV filter
with sufficient absorption to reduce this kind of degradation, in front of the dye cell. The
dye cell also should kept in the dark area and minimize to expose it to the light from
outside. Preferably, the experiment should perform at dark room. Second, the dye
solvents change their refractive index of refraction with an increase in light intensity,
and the core of the dye through which the laser light passes has a higher index of
refraction than the outside. Hence, a lens effect, called self-trapping, occurs which can
94
greatly increase the light power density in the dye. This effect can minimize by working
with a fairly thin dye cell.
As mentioned earlier, this research is an initial stage of gaining the knowledge of
Q-switching using saturable absorber. Dilute dye saturable absorber are not only
effective media for initiation of passive Q-switching, but also have the advantages that
their strength can be accurately controlled and varied continuously. Actually, the
saturable absorber can also been use in mode-locking mode, which is another technique
to produce shorter pulse. Therefore, further studies can be carried out in order to get
more information about the saturable absorber material, growing interest in the shortest
pulse. Hopefully, all the efforts and experimental works in this studied will be a good
reference for future work and come out with new bright ideas.
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PRESENTATION AND CONFERENCES
1. Nur Farizan Munajat and Noriah Bidin., Q-Switched by Saturable Absorber,
Annual Fundamental Science Seminar 2004 (AFSS 2004), 14 June – 15 June
2004, Skudai, Johor
2. Nur Farizan Munajat.and Noriah Bidin., Q-Switching Dye Laser by Saturable
Absorber, Malaysian Science and Technology Congress 2004 (MSTC 2004),
5 – 7 October 2004, Kuala Lumpur
3. Nur Farizan Munajat and Noriah Bidin., Diagnose of Q-Switching NitrogenLaser-Pumped-Dye-Laser, The XXI Regional Conference and Workshop on
Solid State Science & Technology (RCWSST 2004), 10th – 13th October 2004,
Kota Kinabalu, Sabah