“I hereby declare that I have read this thesis and in my opinion this thesis is sufficient in
terms of the scope and quality for the award of the degree of
Master of Science (Physics).”
Signature
:……….………………..
Name of Supervisor
: Assoc. Prof. Dr. Noriah Bidin
Date
:…21 / 11 / 2005………………
DEVELOPMENT OF A WATER COOLING SYSTEM FOR
Nd:YAG LASER CHAMBER
NOR AZIAWATI BINTI AZAHARI
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
NOVEMBER 2005
I declare that this thesis entitled “Development of A Water Cooling System for Nd:YAG
Laser Chamber” is the result of my own research except as cited in the references. The
thesis has not been accepted for any degree and is not concurrently submitted in
candidature of any other degree.
Signature
:………………………
Author’s name
: NOR AZIAWATI BINTI AZAHARI
Date
:…21 / 11 / 2005…………………….
Dedication to my beloved father and mother
(Azahari Yusoff and Azizah Yaman),sisters,
brother and all my friends.
Thanks for everything …………
ACKNOWLEDGEMENT
First and foremost I wish to give all the praise to Almighty God for giving me the
strength and time to complete this research. With His blessings may this work be
beneficial for the whole of humanity.
I’m deeply indebted to my supervisor, Associate. Professor Dr. Noriah Bidin,
for her help, guidance and encouragement throughout this work. She has thought me
her professionalism and the profound art of research which inevitably is reflected in this
thesis. For all these, and for innumerable friendly discussions we have had, I am very
grateful. Thanks also go to my co-supervisor, Dr Johari Adnan for his advice, guidance
and motivation. Without their continue support and interest, this thesis would not have
been the same as presented here.
My sincere appreciations extend to all my colleagues and friends at Laser
Technology Laboratory for their co-operation and assistance. I would also like to
acknowledge the support of my parents, sisters and brother for their kindly good
encouragement in the course of my study.
Finally, I am also indebted to Universiti Teknologi Malaysia for the financial
support in my Master study and Government of Malaysia, Ministry of Science,
Technology and Innovation for granting this project through IRPA vote, 74531.
Without this financial support, this project would not be possible.
ABSTRACT
In solid state lasers, only a small fraction of electrical input power is converted
to laser radiation. The remainder of the input power is converted to heat. Therefore,
solid state lasers require cooling for the pump source and active medium. In the case of
flashlamp pumping usage, a cooling system in the chamber is desirable. Without
adequate cooling, the laser seals, pumping cavity, lamps and the rod itself would be
damaged by overheating. Thus, the aim of this project is to develop a water cooling
system such that the lowest practical operating temperature is produced, and to monitor
temperatures of the laser chamber during the pumping process. In order to achieve
these objectives, a refrigerated water cooling system was developed which included an
internal and external water cooling system. Measurements of various parameters of this
water cooling system were made in order to determine its appropriateness in solid state
laser chamber. A laser chamber was set-up, which comprised of a Nd:YAG laser rod,
flashlamp, chamber heat sink and stainless steel blocks. An aluminium laser house was
designed inclusive with electrical and water piping system. After assembling the whole
system, the circulation of water in the cooling system was tested. This is to ensure no
leakage occurred during the pumping process. The flow rate of water during circulation
is 9.83 ± 0.01 liter / min. The minimum temperature of the cooling system that could be
achieved was 18.00 ± 0.05 oC. The temperature distribution during pumping process
was monitored at different points on the laser chamber. The information obtained leads
to the calculation of heat dissipation from the laser chamber which operated with and
without chilled distilled water. The comparison results shows that 20% improvement in
heat liberated from flashlamp, whereas, 90% and 86% improvement in heat absorption
in chamber heat sink and stainless steel blocks respectively. This indicated that the
cooling system provided in the laser chamber was very effective in carrying out the
excess heat from pumping process.
ABSTRAK
Dalam laser pepejal, hanya pecahan kecil kuasa masukan elektrik ditukarkan
kepada pancaran laser. Lebihan daripada kuasa masukan ditukar kepada haba. Oleh
itu, laser pepejal perlu penyejukan pada sumber pengepaman dan medium aktif. Dalam
kes yang melibatkan penggunaan lampu kilat, sistem penyejukan dalam kebuk
diperlukan. Tanpa penyejukan secukupnya, pelekat-pelekat dalam laser, rongga
pengepaman, lampu dan rod laser sendiri akan rosak disebabkan oleh pemanasan
berlebihan. Oleh itu, tujuan projek ini adalah untuk membina sistem penyejukan air
supaya suhu proses terendah yang praktikal dihasilkan, dan juga untuk memantau suhu
kebuk laser semasa proses pengepaman. Untuk mencapai objektif ini, sistem
penyejukan air telah dibangunkan yang terdiri daripada sistem penyejukan dalaman dan
luaran. Pengukuran pelbagai parameter sistem penyejukan air ini telah dilakukan
dengan tujuan untuk menentukan kesesuaiannya dalam kebuk laser pepejal. Kebuk
laser yang terdiri daripada rod laser Nd:YAG, lampu kilat, kebuk penebat haba dan blok
keluli tahan karat telah dibina. Rumah laser aluminium juga dibangunkan yang lengkap
dengan sistem saluran elektrik dan air. Selepas menggabungkan seluruh sistem, edaran
air dalam sistem penyejukan diuji. Ini dilakukan untuk memastikan tiada kebocoran
semasa proses pengepaman. Kadar aliran air semasa edaran diperolehi sebagai 9.83 ±
0.01 liter/ min. Suhu minimum sistem penyejukan yang dapat dicapai adalah 18.00 ±
0.05 oC. Taburan suhu semasa proses pengepaman dipantau di titik berlainan pada
kebuk laser. Maklumat yang diperoleh digunakan untuk pengiraan haba dikeluarkan
daripada kebuk laser dimana dikendalikan dengan dan tanpa penyejukan air suling.
Keputusan yang diperolehi hasil perbandingan menunjukkan lebihan haba yang
dibebaskan oleh lampu kilat dalam sistem yang disejukkan adalah 20%, sementara haba
yang diserap oleh kebuk penebat haba dan keluli tahan karat masing-masing didapati
bertambah sebanyak 90% dan 86% . Ini menunjukkan sistem penyejukan yang
dibekalkan pada kebuk laser amat efektif untuk membawa keluar lebihan haba semasa
proses pengepaman.
TABLE OF CONTENTS
CHAPTER
1
TITLE
PAGE
DECLARATION
ii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
CONTENT
vii
LIST OF TABLES
xi
LIST OF FIGURES
xii
LIST OF SYMBOLS
xv
INTRODUCTION
1.1
Overview
1
1.2
Thermal Loading of Lamp-pumped
3
Nd:YAG Lasers
2
1.3
Previous Research
4
1.4
Problem Statement
5
1.5
Research Objective
5
1.6
Research Scope
6
1.7
Thesis Outline
6
LITERATURE REVIEW
2.1
Introduction
8
2.2
Solid State Laser
8
2.3
Basic Construction of Solid State Lasers
10
2.4
The Nd:YAG Laser
12
2.4.1
Principle of Operation
13
2.5
Energy Transfer in Solid State Lasers
15
2.6
Thermo-optics Effects
16
2.7
Fundamental of Heat Transfer
17
2.8
Modes of Heat Transfer
18
2.8.1
Conduction
18
2.8.2
Convection
19
2.8.3
Radiation
19
2.9
Cooling Techniques in Solid State Lasers
20
2.9.1
Liquid Cooling
20
2.9.1.1 Water Cooler with a Liquid to Air
22
Heat Exchanger
2.9.1.2 Water Cooler with a
23
Liquid to Liquid Heat Exchanger
2.9.1.3 Water Cooler with a Refrigeration
24
Unit
2.10
3
2.9.2
Air or Gas Cooling
25
2.9.3
Conductive Cooling
26
Summary
27
DEVELOPMENT OF A WATER COOLING SYSTEM
3.1
Introduction
28
3.2
Water Cooling System
28
3.2.1
External Coolant
29
3.2.2
Internal Coolant
34
4
3.3
Laser Chamber
36
3.4
Lab Recorder
36
3.5
Monitoring Laser Chamber
38
CHARACTERIZATION OF THE WATER COOLING SYSTEM
4.1
Introduction
40
4.2
Water Temperature
40
4.2.1
Temperature of External Coolant
41
4.2.2
Temperature of Internal Coolant
43
4.3
4.4
5
Water Quality
46
4.3.1
The pH Level
47
4.3.2
Conductivity and Resistivity of Water
49
Summary
53
DEVELOPMENT OF A LASER CHAMBER
5.1
Introduction
54
5.2
Laser Chamber
55
5.2.1
Nd:YAG Laser rod
56
5.2.2
Flash lamp
56
5.2.3
Flow Tube
57
5.2.4
Stainless Steel Block
58
5.2.5
Chamber Heat Sink
60
5.2.6
Base Plate
62
5.2.7
Technical accessories
64
5.3
Assembly of the Laser Chamber
64
5.4
The Laser House
68
6
5.5
Piping
71
5.6
Testing the Circulation System
72
5.7
Water Flow
74
5.8
Summary
76
TEMPERATURE MONITORING DURING PUMPING
PROCESS
6.1
Introduction
77
6.2
Heat Loss
78
6.3
The Temperature Distribution of Laser Chamber
80
6.3.1
80
The Temperature Distribution of the
Laser Chamber with chilled water
6.3.2
The Temperature Distribution of the
84
Laser Chamber without water cooling
7
6.4
Heat at Different Part of the Laser Chamber
87
6.5
Summary
89
CONCLUSION AND SUGGESTIONS
7.1
Conclusion
91
7.2
Problems and Suggestions
93
REFERENCES
96
APPENDIX A
101
PUBLICATIONS
103
LIST OF TABLES
TABLE NO.
TITLE
PAGE
4.1
Temperature measurement of external coolant upon times
42
4.2
Temperature measurement of internal coolant times domain
44
4.3
pH of distilled water at various temperatures
47
4.4
Conductivity of distilled water at various temperatures
50
4.5
Resistivity of distilled water as the reciprocal of conductivity
51
at various temperatures
5.1
Physical properties of pump used by internal cooling system
75
6.1
Physical properties of laser chamber
79
6.3
Temperature at different parts of laser chamber with
81
chilled water operation
6.4
Temperature of laser chamber at different part without
85
chilled water
6.5
The results of energy absorbed at different parts in
laser chamber during pumping process
89
LIST OF FIGURES
FIGURE NO.
TITLE
PAGE
2.1
A generic solid state laser
10
2.2
Typical Pumping Cavities ;(a) Single ellipse,
11
(b) Double ellipse, (c) Circular Cylinder, (d) Close-warp
2.3
Energy level system of Nd:YAG laser
13
2.4
The flowchart of energy transfer in solid state lasers
16
2.5
Schematic of a water cooler containing a water to water
23
heat exchanger
2.6
Schematic of a water cooler employing a refrigeration unit
24
2.7
Laser head of a small air-cooled Nd:YAG laser
25
2.8
Typical geometry of a conductively cooled laser rod
26
3.1
The schematic diagram of whole cooling system
29
3.2
The circulation in external coolant
30
3.3
The schematic diagram of external coolant
32
3.4
Photograph of compressor based refrigeration system
33
components
3.5
Photograph of whole system of external coolant
33
3.6
The schematic diagram of internal coolant
35
3.7
The schematic diagram for Lab Recorder calibration
37
3.8
Experimental set-up for temperature measurement of
39
pumping system
4.1
The temperature of external coolant with respect to time
43
4.2
The temperature of internal coolant at various times
45
4.3
Graph pH versus temperature for distilled water
48
4.4
Graph resistivity versus temperature of distilled water
52
5.1
The photograph of Nd:YAG laser rod
56
5.2
The photograph of flashlamp
57
5.3
The photograph of flow tube
57
5.4
Engineering drawing of block 1. (Dimensions are in
58
millimeters (mm))
5.5
Engineering drawing of block 2. (Dimensions are in
59
millimeters (mm))
5.6
The photograph of stainless steel blocks 1 and 2
60
5.7
Dimension of top view of the chamber heat sink. (Dimensions
60
are in millimeters (mm))
5.8
The photograph of chamber heat sink
61
5.9
Dimension of side view of the chamber heat sink. (Dimensions
61
are in millimeters (mm))
5.10
Dimension of front view of the chamber heat sink. (Dimensions
62
are in millimeters (mm))
5.11
Dimension of top view of base plate. (Dimensions are
63
in millimeters (mm))
5.12
Dimension of side view of base plate. (Dimensions are
63
in millimeters (mm))
5.13
The photograph of base plate
63
5.14
The photograph of O ring, block clamps and other screw
64
used to ensemble laser chamber
5.15
The schematic diagram of complete pumping chamber
66
from side view
5.16
The schematic diagram of pumping chamber without
66
chamber heat sink from side view
5.17
The photograph of pumping chamber (From side view)
67
5.18
The photograph of pumping chamber without
67
chamber heat sink (From top view)
5.19
Technical drawing of front cover of laser house from
(a) front view , (b) side view. (Dimensions are in
68
millimeters (mm))
5.20
Technical drawing of housing from (a) front view (b) top view
69
(c) side view. (Dimensions are in millimeters (mm))
5.21
The photograph of laser (a) from top view without laser trail,
70
(b) from top view with laser trail and (c) from side view
including the laser chamber
5.22
Dimension of side view of L pipe. (Dimensions are in
71
millimeters (mm))
5.23
The photograph of L pipe
71
5.24
Schematic diagram showing circulation of cooling system
72
from coolant to the laser head
5.25
The flow path of distilled water in laser chamber
73
during circulation (top view)
5.26
The detail of circulation system in laser chamber
74
(focused in ellipsoid space)
6.1
Temperature profile at laser chamber when operated
83
with chilled water
6.2
Temperature profile without chilled water
87
LIST OF SYMBOLS
Qo
-
Heat before
Q
-
Heat after
t2
-
Time maximum
t1
-
Time minimum
∆θ
-
Temperature change
cp
-
Specific heat
m
-
Mass
θ
-
Temperature
R
-
Water flow rate
v
-
Volume
t
-
Time
Pabsorbed
-
Power absorbed
Pi
-
Power input
%Pabsorbed
-
Percentage of power absorbed
CHAPTER 1
INTRODUCTION
1.1
Overview
The first decade of solid-state laser technology has seen the development of
an enormous number of lasing materials and a large variety of interesting design
concepts. However, in recent years the technology has matured to a point where solid
state lasers have reached a plateau in their development. To a major extent, the growth
in importance of solid state lasers for industrial and military applications and as a
general research tool are due to the improvement in reliability and maintainability of
these systems. A wealth of applications for solid state lasers has emerged in materials
processing, holography, range finding, target illumination and designation, satellite and
lunar ranging, thermonuclear fusion, plasma experiments, and in general for scientific
work requiring high power densities (Koechner, 1976).
A solid state laser system contains, for its lasing element, a ruby, Nd-YAG,
Nd-glass or the like. Solid state lasing elements are fabricated into solid cylinders of
various lengths and diameters. The rods are optically transparent and the ends are cut
flat and parallel to each other. The end surfaces are polished very highly and coated
with a reflective material. These laser elements are optically pumped (illuminated) by a
high intensity flashlamp or krypton-arc or tungsten halogen lamp. Some of these lasers
operate in the pulsed mode and others operate in both pulsed and continuous wave
modes. They are cooled either by air or tap water circulating through the laser head,
which includes flashlamp (Muncheryan, 1983).
In all solid state laser elements, the excitation to emission occurs in the dopant,
for example the dopant is neodymium ions in the YAG lasers. The energy of radiation
from the flashlamp is at least equal or greater than the energy of the photons produced
in the respective dopant. The excited atoms are raised to a higher than normal quantum
state (energy state) from which they return to the ground state in steps, emitting photons
of wavelengths characteristics of the dopant. The greater the energy applied to the
dopant from the optical pump the greater is the intensity of the emitted radiation; this
stimulating energy does not alter the frequency of the radiation from the particular
dopant. Because the photons in the lasing cavity are produced by equal-energy photons,
any two photons in the cavity are of the same phase, frequency, amplitude and
direction. When the energy from the optical pump is not sufficient to excite the dopant
atoms to radiation, the energy in transition may dissipate in the form of heat or photons.
This condition elevates the temperature of the laser rod; the elevated temperature in the
rod tends to reduce the photon emission. So that, to prevent from overheating, the
lasing rod is cooled either by circulation of air or distilled water through the laser head
(Muncheryan, 1983).
Since Nd:YAG laser is the most powerful laser in this category, our study will
be directed to a system containing a Nd:YAG laser rod in the laser head. Thus, it is
important that during the planning stages of a laser system, careful measurement
includes water temperature, quality and flow rate must be made to provide a suitable
cooling system (Muncheryan, 1979).
1.2
Thermal Loading of lamp-pumped Nd:YAG Lasers
Consider a typical continuous wave (cw) Nd:YAG laser with an output power of
300 watts and input power of 12 kilowatts. Assuming a quantum efficiency of 50%
(low) this means that 600 watts are absorbed in the laser. Thus 11,400 watts are not
absorbed in the laser. The majority of this power is optical power from the lamps
outside the pump bands of the laser. This excess power is absorbed by the cavity and
by the lamps, thus dramatically increases the temperature of the laser. Roughly 10% to
20% of the electrical power will be dissipated as heat through the electrodes and 30% to
50% as heat through the envelope. In addition to causing mechanical overheating
problem (seals and so on), thermal gradient will cause thermal focusing in the laser rod
(Kuhn, 1998).
Typically the lamps, the cavity, and the Nd:YAG rod are cooled by water. The
usual pattern is to first take the incoming cold water and confine it to the region of the
laser rod with a flow tube. This will remove the heat deposited in the rod that is not
converted into laser light. Next, the water is allowed to flow through the major part of
the laser cavity to remove the heat deposited in the reflectors and in the cavity walls.
Finally, the water can be confined to the region around the lamps with a flow tube. This
removes the heat absorbed in the quartz envelope.
Many variations on this theme are possible depending on the total power
dissipation in the laser. For example, in extremely high average power lasers, a water
cooling loop is provided through the electrodes to avoid destroying them. In very low
power lasers, water cooling may only be provided over the lamps. In some extremely
low power lasers, it may even be possible to use air cooling (Kuhn, 1998).
1.3
Previous Research
Advanced Nd laser application which requires increasingly higher average
output power necessitate operating near the stress-fracture limit, i.e., a regime in which
output power is limited by the possibility of material fracture arising from thermally
induced stresses in the laser medium (Eggleston et al., 1984; Emmett et al., 1984).
Mangir and Rockwell (1986) have found large variations in the heat generation
accompanying flashlamp pumping of various types of Nd-doped phosphate glass and
Yittrium Aluminium Garnet (YAG). According to Chen et al., (1990) thermal effects in
flashlamp-pumped Nd:YAG lasers arise from the fact that nearly ten percent of the
flashlamp energy is converted to heat in the laser medium, while about three percent is
stored in the inversion as useful gain at the time of lasing. This heating is due to the
sizeable quantum defect between the pump spectrum and the lasing wavelength, and
quenching mechanisms.
A new mode of laser operation is proposed by Bowman (1999) which should
result in little or no heat generations within solid state laser materials. The technique
utilizes balanced spontaneous and stimulated emission within the laser medium. The
result would be a radiation balanced laser device in which no excess heat is generated
because of the average quantum defect of the radiation process is adjusted to zero. If
such a laser device can be realized, much higher average powers systems should be
possible without many of the thermal and beam quality issues that limit conventional
solid state laser.
From year 2000 onwards, the research on thermal heating in solid state laser was
more focused on diode pumped solid state laser which was found to have many
advantages over lamp-pumped solid state laser. The advantages include high system
efficiency and component lifetime and also reduction of thermal load of the solid state
laser material (Koechner, 1988). Usievich et.al, (2001) present a paper that discloses an
analytical method which delivers the exact temperature distribution in a
circularly cylindrical symmetrical, longitudinally, and transversely nonuniform heat
source distribution and circularly symmetrical cooling means. The analytical
expressions obtained for the temperature distribution open the way to a better
understanding of thermal phenomena and represent a fast tool for solid state laser
design and optimization.
1.4
Problem Statement
When laser rod was pumped by flashlamp, the temperatures of the rod will
increase and the rod will expand. Such expansion will result in the change of length of
the laser cavity and may cause overheating on laser equipment. To prevent the laser rod
from experiencing drastic changes, it needs to be controlled by developing a cooling
system and monitoring the laser chamber temperatures during the pumping process.
1.5
Research Objective
The main objectives of this research are listed as follows:
1.
To develop a cooling system.
2.
To develop a laser chamber.
3.
To measure the circulation of cooling system over the laser head.
4.
To analyze dissipation of heat at different points on laser chamber during
pumping process.
1.6
Research Scope
In this study, a water cooling system and laser chamber for high power Nd:YAG
solid state laser are developed. The input power of the flashlamp used for pumping is
1.6 kW. The measured parameters of water cooling system are including water
temperature, quality and flow rate. A laser chamber is set-up which comprised of a
laser rod, flashlamp, heat sink and stainless steel blocks. A laser house is built inclusive
of electric and water piping system. The water cooling system is installed in the laser
chamber and the circulation is tested. The laser rod is pumped with flashlamp and the
temperatures at different points which include the flashlamp, stainless steel block and
chamber heat sink of the laser chamber are measured within an hour.
1.7
Thesis Outline
This thesis consists of seven chapters. The first chapter reviews some of
previous research related to thermal heating in solid state laser. This chapter also
contains the objectives of the research under taken.
Chapter II covers the literature review related to the research work. This
includes the fundamental of solid state laser, thermal effect in solid state laser, the
fundamental of heat transfer and description of several types of cooling techniques used
in solid state lasers.
Chapter III describes the preparation of materials, development of water cooling
system and facilities involved in the research; and also describe the technique used to
measure temperature.
In chapter IV the measurement of various parameters of a water cooling system
is discussed. Two parameters are tested which include water temperature and water
quality in order to ensure the cooling system is appropriate for chilling of the laser rod.
The development of a laser chamber is explained in chapter V. This involves
the development of the component in the laser chamber, the design of the laser house
inclusive of an electric and water piping system and testing of the circulation of water
cooling system in the laser chamber.
Pumping of the laser rod is discussed in chapter VI. The temperature was
monitored at different part of the laser chamber. The amount of heat dissipation was
estimated based on the temperature information. The measurement was carried out with
and without chilling of the distilled water.
Finally, some conclusions of the project are drawn in chapter VII. These
include summary of the project and discussion of the problems encountered during the
work of the project; and finally last but not least, works to be carried out in the near
future are suggested.
CHAPTER 2
LITERATURE REVIEW
2.1
Introduction
In solid-state lasers only a small fraction of electrical input is converted to laser
radiation, the remainder of the input power is converted to heat. Solid state lasers
require cooling of the pump source, active medium and in case of flashlamp pumping,
cooling of the pump enclosure. In this chapter the fundamental of a solid state laser,
thermal effect in solid state lasers, fundamental of heat transfer and several types of
cooling techniques used in solid state lasers are treated.
2.2
Solid State Laser
A solid state laser is one in which the atoms that emit light are fixed within a
crystal or glassy material (Hecht, 1992). The first demonstration of laser action by
Maiman was achieved in 1960 using ruby (Cr3+Al2O3). In 1960, Johnson and Nassau
demonstrated the first solid state neodymium laser, in which the neodymium ion was a
dopant in calcium tungstate (CaWO4). Elias Snitzer demonstrated the first neodymiumglass laser at American Optical Centre in the same year. In 1963, the best choice of
neodymium host for most commercial applications- yttrium aluminium garnet (YAG),
was demonstrated as a laser material by Geutic, Marcos and Van Ultert (Koechner and
Bass, 2003).
Compared to other lasers, the solid state lasers have several advantages. The
solid state lasers can operate in continuous wave (cw), pulsed, Q-switched and modelocked modes to obtain high average power, high pulse repetition rate, high pulse
energy and high peak power. The average power of 4 kW has commercially been
achieved with modular construction YAG lasers. The peak power of 1013-1014 W has
also been obtained (Kuhn, 1998). More than 100 solid state materials can produce laser
beams. Most of these beams range in the visible and near infrared regions of the
electro-magnetic spectrum. The UV wavelength has also been achieved by harmonic
generators due to the advert of new non-linear materials and high beam quality obtained
from diode pumped lasers. Significant progress has been made in the development of
the tunable solid state lasers (Gan, 1995). Laser beams produced by some solid state
lasers can be developed with optical fibre, which makes lasers more flexible and
applicable in dangerous or difficult to access processing environments. Solid state
lasers are also more compact and have lower maintenance compared to high power CO2
lasers and excimer lasers (Sintec, 2000).
Solid state lasers are widely used for various applications, for examples in pure
science, medicine, diagnostics and entertainment. It is expected, based on the trend
observed in the past, that in general the use of various types of lasers will increase and
dominate many fields. In particular, it is expected that by improving the reliability of
the existing lasers or by introducing solutions needed, the market size of solid state
lasers may increase significantly (Kalisky, 1999).
2.3
Basic Construction of Solid State Lasers
A typical solid state laser usually consists of a laser rod (gain medium), a
pumping cavity, two mirrors and a power supply as shown in Figure 2.1. The gain
medium is placed in a reflective pumping cavity. Inside the cavity is an elliptical space
with the rod (gain medium) at one focus of the ellipse and a flashlamp at the other
focus. Light from an external source, pulsed flashlamp, a bright continuous arc lamp or
another laser enters the laser rod and excites the light emitting atoms. Ideally all the
light emitted by the lamp are coupled into the rod by the cavity. The optical resonator
consists of two mirrors mounted separately from the lasing medium. The cavity mirrors
form a resonant cavity around the inverted population in the laser rod, providing the
feedback needed to generate a laser beam that emerges through the output mirror
(Hecht, 1992). The cooling system is necessary since most of the light energy from the
lamp is lost as heat (LEOT, 2001).
Reflective cavity – focuses
pump light onto laser rod
Rear
mirror
Laser rod
Pump light
Lamp light source
Output
mirror
Laser
beam
Lamp
Power
Supply
Figure 2.1: A generic solid state laser [Hecht, 1992]
One of the most important elements in solid state lasers is a pumping cavity.
Besides providing good coupling between the pumping source and the absorbing
material, it is also responsible for the pump density distribution in the laser element
which influences the uniformity, divergence and optical distortion of the output beam
(Koechner, 1976). Figure 2.2 shows some of the typical pumping cavities.
(a)
(b)
(c)
(d)
Lamp
Rod
Figure 2.2: Typical Pumping Cavities ;(a) Single ellipse, (b) Double ellipse, (c)
Circular Cylinder, (d) Close-warp [Sintec, 2000]
Among the pumping cavities, the elliptical cavities have been most extensively
used in the development of solid state lasers (Sintec, 2000). In this configuration, a
linear lamp and a laser rod, possibly with different radii, are placed at the foci of an
elliptical cylinder, as shown in Fig.2.2(a). This geometry usually results in the most
efficient cavity. This cavity has further advantage that it minimizes the weight and size
of laser head. Since the energy delivered to a discharge lamp is limited, scheme to
focus the energy from many lamps onto a single crystal are attractive. Fig. 2.2(b) shows
two partial elliptical cylinders having one common axis at which crystal is placed. Fig
2.2(c) and 2.2(d) show example of closed-coupled non focusing pump cavities. The
lamp and rod are placed as close together as possible, and reflector closely surrounds
the lamp and rod. The reflector can be circular or oval in cross section. Fig 2.2(d) type
is often used in laboratory setups of low-repetition rate pulsed lasers. The efficiency of
the closely wrapped cavity is found to be about as good as when an elliptical cylinder is
used. The advantage of these pump cavity is fabrication simplicity (Sintec, 2000).
The laser material is shaped into a cylindrical rod whose ends is round and
polished to be plane parallel. When the rod is placed between two mirrors facing each
other, and is strongly irradiated by an intense light source around it, a laser radiation is
emitted. The rod ends are usually anti-reflection coated for the Nd:YAG wavelength of
1064 nm. The rod ends are held in place and sealed by O-rings in the ends of the rod
holders to protect them from the pump light (Sintec, 2000).
2.4
The Nd:YAG Laser
The primary commercial example of a conventional solid state laser is Nd:YAG.
Nd:YAG lasing at 1.064 µm can be frequency doubled to 532 nm, tripled to 355 nm and
quadrupled to 266 nm. Continuous wave Nd:YAG lasers are available in power levels
up to several hundred watts and pulsed Nd:YAG lasers are available with pulse energies
up to a few several joules per pulse (Kuhn, 1998).
YAG has a combination of desirable properties as a host medium for Nd3+ ions:
it has relatively high thermal conductivity, which allows it to disperse the waste heat
from the optical pumping process; it has high mechanical strength, and can be grown as
crystals of large size with good quality. The Nd3+ ions substitute within the YAG lattice
in a single site so the emission and absorption lines are homogeneously broadened.
Typical Nd3+ doping densities range up to 1% (Davis, 1996).
2.4.1
Principle of Operation
Lasing is dependent on the rapid transitions from the lower lasing level to the
ground state by radiationless transition. When the rod temperature is low, these
transitions will occur at high rate. Hence, lasing efficiency depends mainly on cooling
efficiency. Higher output powers can be achieved by having lower operating
temperature. This explains why cooling systems are generally operated at temperature
just above the threshold of this effect (Sintec, 2000).
Figure 2.3: Energy level system of Nd:YAG laser [Wilson et.al, 1987]
Figure 2.3 shows the energy levels involved in the Nd:YAG laser action. The
number of 0, 1, 2 and 3 in Figure 2.3 represent the energy level E0, E1, E2, and E3 It is
essentially a four levels system, with the lasing transition taking place between the 4F3/2
(E2) and 4I11/2 states (E1). The terminal state E1 is sufficiently far above the ground state
to be practically empty at room temperature. The initial and final states are split into 2
and 6 crystal field level respectively, so that several lasing wavelengths are possible.
The most powerful of these occurs at 1.064 µm and this is usually the one used (Wilson
et al,1987).
If (E1 – E0) is large compared with the thermal energy, kT, at the temperature of
operation, then the populations of the levels E1, E2, and E3 are all effectively zero before
pumping commences. Thus a population inversion can readily be achieved between
levels E1 and E2, again the level E3 may be broad for effective pumping. Pumping
excites atoms from the ground state into level E3, whence they decay rapidly into the
metastable level E2, so that N2 increases rapidly to give inversion population inversion
between E2 and E1. If the lifetime of the transition from level E1 to level E0 is short,
then the population inversion can be maintained easily with modest pumping.
Nd:YAG laser pumping requirements are modest, and for pulsed operation it can
be met with a fairly simple flashlamp and reflecting cavity. To avoid overheating and
subsequent damage of the laser rod, cooling air may be blown through the cavity.
Continuous wave (cw) operation is also possible; the most popular pumping source
being the quartz-halogen lamp. The most effective pumping bands lie between 700 nm
and 900 nm in wavelength, which are reasonably close to the peak output of the lamps
which occur at about 1 µm. Even so only a few percent of the total radiation emitted is
usefully absorbed in the laser material. Consequently the overall power efficiency (it
also applies to pulsed lasers) is fairly low. CW outputs of up to several hundred watts
are possible, which implies that large amounts of waste heat will be produced,
necessitating water cooling of the laser rod (Wilson et al,1987).
2.5 Energy Transfer Processes in Solid State Lasers
Figure 2.4 shows the flow chart of simplified way the energy balance in a laser
system. The electrical input power supplied to the lamp is either dissipated as heat by
the lamp envelope and electrodes or emitted as radiation. A portion of the radiation will
be absorbed by the metal surfaces of the pump cavity. The radiation reflected from the
walls will be either absorbed by the lasing medium or will return to lamp. The light
which is absorbed by the lamp will add energy to the radiation process in the same way
as the electrical does, and the returned light will be radiated with the same efficiency as
the power supplied electrically. One consequence of the reabsorption is that a lamp,
when enclosed in the pumping cavity, is operated under a higher thermal loading
resulting in shorter lamp life than when operated in the open for the same electrical
input power. Since most laser cavities are liquid cooled, a distinction is made by the
radiation actually absorbed by the surrounding cooling liquids and flow tube. The
pump power absorbed by the laser rod causes stimulated emission and fluorescence at
the laser wavelength and other main emission bands. The remainder is dissipated as
heat by the laser material (Koechner, 1988).
External Power
Lamp input
Heat loss by lamp
Loss from source & transfer
Light
Light absorbed by pumping cavity
Light absorbed by coolant and flowtube
Light absorbed by lamp
Light absorbed by laser rod
Heat loss by rod
Energy loss by fluorescence
Stimulated emission
Laser beam
Optical losses
Figure 2.4: The flowchart of energy transfer in solid state lasers [Koechner, 1988]
2.6 Thermo-optics Effects
The optical pumping process in a solid state laser material is associated with the
generation of heat for a number of reasons:
(a) the energy difference of photons between the pump band and the upper laser
level is lost as heat to the host lattice; similarly, the energy difference between
the lower laser level and the ground state is thermalized. The difference
between the pump and laser photon energies, termed quantum defect heating, is
the major source of heating in solid-state lasers.
(b) In addition, nonradiative relaxation from the upper level to the ground state, due
to concentration quenching, and nonradiative relaxation from the pump band to
the ground state will generate heat in the active medium.
(c) In flashlamp-pumped systems, the broad spectral distribution of the pump
source causes a certain amount of background absorption by the laser host
material, particularly in the ultraviolet and infrared regions of the lamp
spectrum. Absorption of lamp radiation by impurity atoms and color centers can
further increase heating.
The temperature gradients set-up in the gain material as a result of heating can lead
to stress fracture, which represents the ultimate limit in average power obtainable from
a laser material. Below the stress fracture limit, thermal lensing and birefringence
adversely affect output beam quality. Also, due to thermal lensing, the operating point
of the resonator within the stability diagram becomes a function of input power.
Therefore the output beam quality and mode structure are power dependent because the
thermal lens can only be compensated for one input power level (Koechner et.al, 2003).
Efficient heat removal and the reduction of the thermal effects that are caused by
the temperature gradients across the active area of the laser medium usually dominate
design considerations for high average power systems.
2.7
Fundamental of Heat Transfer
Since pumping process involves heat transfer, it would be better to discuss the
fundamental of thermodynamics including the first and second law. The first law of
thermodynamics involves the conservation of energy. It states that energy can be
neither created nor destroyed; it can only change forms (Cengel et al., 1998).
Transferring heat energy is subject to the second law of thermodynamics. The second
law (for a closed system) states that for a spontaneous process there is a net increase in
entropy that is a measure of the disorder that exists in a system (Martin, 1999). The
first and second laws of thermodynamics govern the various modes of heat transfer:
conduction, convection and radiation.
2.8
Modes of Heat Transfer
Heat transfer is traditionally divided into three elementary forms: conduction,
convection and radiation. Each of this represents a way in which energy flows from a
hotter body to a cooler body. The rate at which heat flow maybe represented by the
.
symbol Q . In general, the amount of energy which flows as heat during the interval
between t1 and t2 is given by (Espinola, 1994)
t2 .
Q = ∫ Q dt
t1
(2.1)
In most cases, we will be studying constant heat flow, so
.
Q = Q(t 2 − t1 )
(2.2)
2.8.1 Conduction
In conduction, heat flows from a regions of higher temperature to a region of
lower temperature. This occurs within solid, liquid, or gaseous mediums or between
different mediums that make direct physical contact with each other (Kreith, 1973).
The transfer of the energy of motion between adjacent molecules conducts the heat. In a
gas, 'hotter' molecules, have greater energy and motions, and impart energy to adjacent
molecules at lower energy levels. This type of transfer occurs to some extent in all
solids, gases or liquids in which a temperature gradient exists. In conduction, energy
can also be transferred by "free" electrons, which is important in metallic solids
(Geankopolis, 1993). Examples of conduction are heat transfer through the surfaces of
cold plates or through the walls of refrigerator. In this particular project, stainless steel
blocks, would absorb excess heat during pumping process, by conduction.
2.8.2
Convection
In convection, the combined action of heat conduction, energy storage, and
mixing motion serve to transport energy. Convection is most important as the
mechanism of energy transfer between a solid surface and a liquid or a gas (Kreith,
1973). In forced-convection heat transfer, a pump, fan, or other mechanism forces a
fluid to flow past a solid surface. In natural or free convection, warmer or cooler fluid
next to the solid surface causes a circulation because of density differences resulting
from the temperature differences in the fluid (Geankopolis, 1993). An example of free
convection is the loss of heat into ambient air via the fins of a heat exchanger. If a fan is
used to circulate the air over the heat exchanger fins, this becomes an example of forced
convection. In the experiment carried out, circulation of water cooling system either in
external or internal is also in category of forced convection system.
2.8.3
Radiation
In radiation, heat flows from a higher temperature body to a lower temperature
body when the bodies are separated in space, even across a vacuum (Kreith, 1973). The
same laws that govern the transfer of light also govern the transfer of heat. Solids and
liquids tend to absorb the radiation being transferred through it, hence radiation is
important mainly in transfer through space or gases (Geankopolis, 1993). Examples of
radiation include the transfer of heat from the sun to the earth, and from a quartz
flashlamp to a cool object (laser rod) that requires warming.
2.9
Cooling Techniques in Solid State Lasers
Cooling can be defined as a process of removing heat from an enclosed space or
material and maintaining that space or material at a temperature lower than its
surrounding. As heat is removed, a space or material becomes colder. The more heat is
removed, the colder the object becomes (Air Conditioning and Refrigeration Inst,
1997).
Most laboratory and industrial lamp pumped which require high power lasers,
usually used liquid cooling to cool the laser head. Purely convective air cooling
suffices for very low power lamp lasers, which are used where portability is a
paramount concern. Some low power low-repetition rate lamp pumped lasers also use
forced air cooling (Hecht, 1992). Cooling techniques that usually applied to solid state
laser are liquid cooling, air or gas cooling and convective cooling (Koechner, 1976).
2.9.1 Liquid Cooling
The primary purpose of the liquid used in the cooling system is to remove the
heat generated in the laser rod, pump source and laser cavity. Sometimes the coolant
serves additional functions, such as index matching between laser rod and coolant,
thereby reducing internal reflections for depumping modes, or as a filter to remove
undesirable pump radiation. The coolant is forced under pressure to flow over the rod
and lamp surfaces. These elements are located either inside flow tube or in cooling
chambers machined out of the main body of a laser head. The temperature difference
between the part to be cooled and the liquid is a function of the velocity and the cooling
properties of the flowing fluid.
At low velocities, the flow is laminar and most of the temperature drop is due to
pure conduction across a stationary boundary layer at the liquid interface. For higher
velocities, the flow becomes turbulent, leading to a more efficient heat transfer process
with a subsequent lower temperature drop. Turbulent flow requires a greater pressure
differential for the same volume flow, but the necessary differential usually is still small
compared to the total pressure difference associated with the complete cooling system
(Koechner, 1976).
Water is preferably used as a coolant for solid state lasers. From purely heat
transfer considerations, water is by far the best fluid. As compared to the other coolants
it has the highest specific heat and thermal conductivity and the lowest viscosity. Water
has the additional advantage over all other coolants that it is chemically stable under
intensive ultraviolet radiation (Teppo, 1975).
With the exception of lasers cooled directly by tap water, a closed-loop cooling
system is employed which consists in its most basic form of at least a liquid pump, a
heat exchanger and a reservoir. Commercially available coolers contain, in addition to
these components, a particle filter, a demineralizer, gauges and sensors for monitoring
flow temperature and pressure. If common tap water is used, periodic cleaning is
necessary to remove deposition of organic and mineral deposits. In closed-loop
systems, if demineralization and filtering are employed, the need to clean surfaces
exposed to the cooling fluid is essentially eliminated (Koechner, 1976).
The heat exchanger removes heat from the closed-loop system by thermal
coupling to an outside heat sink. This can be established in several ways:
(1) Water cooler with a liquid to air heat exchanger
(2) Water cooler with a liquid to liquid heat exchanger
(3) Water cooler with a refrigeration unit
2.9.1.1 Water Cooler with a Liquid-to-Air Heat Exchanger
Liquid-to-air heat exchangers transfer heat from hydraulic fluid to ambient air.
Working much like an automobile radiator, they allow air to be passed over finned
tubes containing the hot liquid. The finned tubes can be made of aluminum, copper,
steel, or stainless steel, and are brazed or roller expanded to the header tank. Air is
moved through the core by forced or induced-draft fans. Air-cooled exchangers are
most commonly used where water is costly or unavailable in sufficient quantities to
dissipate the required heat, or where a portable heat exchanger is required. In some
instances, they have been used to help supply plant heating requirements during winter
months.
Typically, liquid-to-air exchangers are larger, heavier, and noisier than liquid-toliquid units. In return, they operate without necessity for water and they are portable.
They require ambient air temperature at least 10 to 15 °C below the required oil output
temperature for efficient operation. The only requirement for long life is that the fins
must be protected from clogging and dirty environments; a single mesh (window
screen) overlay avoids fin clogging and provides for easy cleaning (Kren et.al, 2000).
2.9.1.2 Water Cooler with a Liquid-to-Liquid Heat Exchanger
In a liquid-to-liquid heat exchanger the heat generated within the closed loop is
exhausted to external water. Figure 2.5 shows the plumbing diagram of a typical cooler
with a liquid-to-liquid heat exchanger. The water flows from the reservoir to a
centrifugal pump, through a heat exchanger into the laser head, and back again in the
reservoir. This sequence of components minimizes the static pressure in the laser head.
The temperature of the closed-loop water is regulated by a control valve in the external
supply line. The valve probe is located in the reservoir. As the temperature at the
reservoir increases, the valve is opened, thereby allowing more external cooling water
flow through the heat exchanger. The system contains an in-line honeycomb filter to
remove particulate matter and a bypass demineralizer which will maintain low electrical
conductivity in the water and minimize corrosion. The return line is monitored by a
low-flow interlock and an over-temperature switch. If flow falls below a preset value or
the discharge temperature exceeds a certain limit, the interlock turns off the power
supply. The system also contains gauges to display temperature in the return line and
the discharge pressure (Koechner, 1976).
Figure 2.5: Schematic of a water cooler containing a water to water heat exchanger
[Koechner, 1976]
2.9.1.3 Water Cooler with a Refrigeration Unit
In both types of cooler that mention earlier, the temperature of the closed loop
can be regulated only over a relatively narrow temperature range. Furthermore, the
temperature of the cooling loop is always above ambient air temperature or the
temperature of the external cooling water. Figure 2.6 shows a diagram of a cooler
which maintains the cooling water at a precise, reproducible temperature which is
independent of the ambient air or water temperature. This cooler contains a
thermostatically controlled refrigeration stage between the heat exchanger and a water
cooled condenser by means of a compressor. Any changes in the heat dissipation of the
load, or temperature variations in the external lines, are compensated by a hot gas
bypass valves which regulates the amount of refrigeration. Smaller units contain air
cooled condensers, thus eliminating the need for an external water source (Koechner,
1976).
Figure 2.6: Schematic of a water cooler employing a refrigeration unit
[Koechner, 1976]
2.9.2
Air or Gas Cooling
In a low average power lasers, especially portable systems, forced air is
sometimes used to cool the laser rod and flashlamp. Air flow is generated by
employing miniature axial or centrifugal blowers or fans which have been designed for
air cooling of electronic equipment. The air flow required for cooling the laser head is
calculated from the dissipated heat and the maximum temperature difference along the
air stream. Figure 2.7 shows an example of an air-cooled Nd:YAG laser. A vane axial
fan located upstream generates an air flow which passes through the pump cavity and
over the rod and flashlamp. Besides convection cooling, the laser rod is also cooled by
conduction into a copper heat sink which it is mounted (Rundle, 1975).
Figure 2.7: Laser head of a small air-cooled Nd:YAG laser [Korad KYM, 1975]
A cooling system which has been employed very successfully in small military
laser systems is based on the use of compressed dry nitrogen as the coolant medium to
transfer the heat generated in the laser cavity to the ambient air. The compressed
nitrogen is circulated through the laser pumping cavity by means of an axial flow
blower. Nitrogen exhausted from the cavity is then ducted through fins of a heat
exchanger, where it gives up the energy picked up in the laser cavity. The cooled
nitrogen is then ducted back through the cavity again to complete the nitrogen cooling
loop. A fan provides the required air flow through the heat exchanger (Teppo,1975).
2.9.3
Conductive Cooling
In a variety of commercial and military systems the laser rod is mounted directly
to a heat sink, as shown schematically in Figure 2.8. Good conduction cooling of the
laser element requires intimate thermal conduct between the laser rod and the heat sink.
The laser rod can be mechanically clamped, soldered or bonded to the heat sink. If the
laser rod is mechanically clamped to a heat sink, a temperature gradient across the rodclamp interface will develop (Koechner, 1976).
Pump reflector
Flashlamp
Ruby, radius r
θ
Gallium surface
Pedestal
Figure 2.8: Typical geometry of a conductively cooled laser rod [Koechner, 1976]
In a space-borne Nd:YAG laser system the rod was soldered to a mounting
structure of pure niobium, having a thermal coefficient of expansion which closely
matches that of Nd:YAG. The mounting surface for the rod is a groove to provide
intimate contact with the rod over 900 of its periphery. The rod was soldered to the heat
sink with indium solder after the contact surfaces were gold-plated. The heat was
removed from the rod mounting structure by a heat pipe. A fin-type radiator at the
condensor end of the heat pipe radiated the heat into space (Foster and Kirk, 1971).
In another technique, a small Nd:YAG is bonded to a copper heat sink by means
of a silver-filled epoxy adhesive. In this case, ultraviolet-free flashlamps must be
employed to avoid decomposition of the epoxy. The temperature distribution in a
conductively cooled rod has been calculated with the assumption that is uniformly
produced by absorption of radiation and no losses occur except by conduction through
the contact surface of the heat sink (Kaplan, 1964).
2.10
Summary
In this research, the technique used to cool the laser rod and flashlamp in laser
chamber was liquid cooling. This is due to the high heat dissipation (the power input
level of flashlamp used was 1.6 KW), which would be radiated into the laboratory or
operating environment of the laser. Water is an effective and reliable means of cooling
high power laser systems. The development of the water cooling system in this
research will be describes more in the next chapter.
CHAPTER 3
DEVELOPMENT OF A WATER COOLING SYSTEM
3.1
Introduction
In this chapter, the development of water cooling system and laser chamber is
described. In general, a water cooling system is used to circulate cooled distilled water
through a flashlamp and laser rod contained in the laser chamber. Temperature of laser
chamber then is monitored during laser pumping process.
3.2
Water Cooling System
This section describes the development of cooling system in the project. The
type of coolants chosen in the study is the liquid cooling. Liquid cooling offers several
advantages as a removal mechanism. First, in applications where power densities
exceed the limits of air-cooling, liquid cooling is the only practical heat removal
mechanism. Second, liquid cooling offers a high-performance cooling solution and
results in a compact design. Further, liquid cooling also offers better control over
changes in the heat load and higher reliability (Kelkar, 2002). It also provides critical
temperature monitoring of chamber components and is cleaner than air cooling
(American Laser Technique, 2001). In the development, the water cooling system was
divided into two parts that are external and internal cooler.
The type of cooling cycle process involves is a closed loop system. This system
used similar concept as applied in an ordinary refrigerator. The system provides a
constant supply of chilled and pressurized water. The schematic diagram of whole
cooling system including external system and internal system is shown in Figure 3.1.
Laser
chamber
Internal
Cooler
External
Cooler
Figure 3.1: The schematic diagram of whole cooling system
3.2.1
External Cooler
An external cooler is used to cool the distilled water in the internal cooler. It
uses tap water which circulates through a pair of cooling coils mounted inside the tank.
This particular cooler is located outside the lab to reduce vibration created by the
compressor and other related components and also for easy maintenance.
The process involved is a refrigeration cycle and it works continuously. This
type of process can maintain the cooling water at a precise, reproducible temperature,
which is independent of the ambient air, or water temperature (Koechner, 1976).
Refrigerant moves from the compressor to the condenser, then through a metering
device consisting of oil receiver tank, filter drier and expansion valve, eventually to an
evaporator (cooling coil), and the cycle then repeats. The circulation cycle in external
cooler is shown in Figure 3.2.
Compressor
Condenser
Metering Device
(oil receiver tank, filter drier
and expansion valve)
Cooling coil
(Evaporator)
Figure 3.2: The circulation in external cooler
The refrigeration cycle process used to cool a tap water in external cooler begins
with the compressor. The compressor compresses the refrigerant and receives low
pressure gas from the evaporator and converts it to high pressure gas. As the gas is
compressed, the temperature rises. The hot refrigerant gas then flows to the condenser.
The condenser is a heat exchanger that uses a colder fluid, an ambient air, to cool the
refrigerant. As refrigerant flows through this heat exchanger, it condenses to a hot
liquid. Liquid refrigerant exits the condenser and flows to the system's expansion valve.
The expansion valve is used to create a pressure drop. The temperature and
boiling point of liquids decrease as the pressure decreases. Some refrigerant liquid
vaporizes and the temperature of the liquid-gas mixture drops. The cool refrigerant then
flows to the evaporator. The refrigerant enters the evaporator as a low temperature gasliquid mixture. By designing, the temperature of the heat source is always higher than
the refrigerant's boiling point. In the evaporator, the refrigerant vaporizes as it absorbs
heat from the heat source. The refrigerant's temperature remains constant as it
vaporizes. The refrigerant then exits the evaporator as a gas, enters the compressor and
the cycle starts again. The thermostat responds to the temperature of water. From this
process, a tap water in external cooler is cooled continuously. A cooled tap water then
was flow to internal cooler through a water pipe that connected between two systems.
This cooled tap water is circulated through cooling coil that mounted in internal cooler
reservoir to cool distilled water inside.
The schematic diagram of whole external cooler is shown in Figure 3.3.
Photograph of the components used to develop the external cooler is shown in Figure
3.4. The whole external cooler system is shown in Figure 3.5.
Oil receiver
tank
Condenser
Expansion
valve
Filter drier
Water tank
Compressor
Cooling coil
To Internal
coolant
controller
box
Thermostat
Refrigerant lines
Water lines
Figure 3.3: The schematic diagram of external cooler
Oil Receiver Tank
Condenser
(Fan Coil
Unit)
Controller Box
Compressor (R-22)
Filter drier
Expansion Valve
Figure 3.4: A photograph of compressor based refrigeration system components
Water tank
nlet
utlet
Outle
Compressor based refrigeration
Figure 3.5: A photograph of whole system of external cooler
3.2.2
Internal Cooler
This internal cooler is used to circulate the distilled water into the laser chamber
equipment. It is located inside the lab and at a lower position from the laser chamber
equipment. The stainless steel reservoir is used to keep the distilled water that is
pumped to laser chamber. It dimension is 42 x 6 x 8 cm. The cooling coil which is
located inside the reservoir is used as a medium to flow a cooled tap water from an
external cooler to cool the distilled water in the reservoir.
The internal cooler uses distilled water to cool both the flashlamp and the laser
rod in the chamber. Tap water meets the needs of most liquid-cooling applications.
However, distilled water has chemical and electrical properties that make it the optimal
choice for cooling when the liquid circuit contains micro-channels or when sensitive
electronics are involved. As the name implies, distilled water has an extremely low
concentration of ions, which imparts important performance attributes. Firstly it
eliminates mineral deposits that block the coolant flow. This will degrade cooling
efficiency and system operating performance. Secondly it eliminates the risk of
electrical arcing due to static charge build up from the circulating coolant. The arc can
damage sensitive control electronics in the equipment being cooled. The lack of ions in
distilled water eliminates both of these problems (LYTRON Application Note, 2002).
The temperature of distilled water in reservoir was monitored by a pocket digital
thermometer, which was put inside it. The type of thermometer used was pocket digital
thermometer 310 from TPI Inc. It can measure temperature either in Fahrenheit and
Celsius. The range of temperature measurement is from –50 0C up to 150 0C. This is
appropriate since the distilled water required to be cooled is in the range of 15-25 0C
only.
The coolant is pumped by a pump from the reservoir up to the laser head via
plastic tubes. The length of the plastic tube is almost 3 m. An internal cooler normally
is placed a distance away from the laser head, and must be in the lower position to
prevent overflow when the laser head is not in operation. The coolant was circulated
through laser rod and flashlamp located in laser chamber, then return to the reservoir.
The schematic diagram of the whole internal cooler system is shown in Figure
3.6.
Laser head
Temperature
sensor
Cooling
coil
To
external
coolant
Reservoir
Water
pump
Water line
Figure 3.6: The schematic diagram of internal cooler
3.3
Laser Chamber
All solid state laser systems require a pump chamber. The laser pump chamber
is the heart of the laser system. It is where electrical energy is converted to coherent
light at the laser rod's specified wavelength. The laser chamber is designed to be
assembled with water cooling compartment.
Neodymium-doped, yttrium aluminium garnet (Nd:YAG) laser rod is used in an
optical configuration to produce high power, low divergence beam. The optical head
contains a 75 mm x 4 mm diameter oscillator rod with a single flashlamp enclosed
within a heat sink reflector. This features a glazed ceramic reflector (chamber heat
sink), to provide diffuse coupling of the pulsed pump light- and is flooded with
circulating distilled water to cool the flashlamp and the laser rod. The detail of the
development of laser chamber will be described in chapter five.
3.4
Lab Recorder (LR)
In this project, a Lab recorder (LR) was used to monitor the temperature of the
laser head during pumping process. The LR was interfaced to a Personal Computer
(PC). The Lab Recorder model LR4200E from YOKOGAWA was used in this
experiment. Any DC voltage, thermocouple or RTD input could be selected for each
channel. RS-232C interfaces bi-directional communication was used in which it
allowed data output and panel setting through computer. Communication input was
analog-recorded, enabling raw measured data and communication input data to be
recorded simultaneously.
LR PC software was used to read measurement data directly from the Lab
Recorder. The software sets and controls the LR recorder. It periodically acquires data
measured by an LR recorder to a personal computer, and saves it to a hard disk. It is
also capable of displaying waveforms and digital values in real time while data logging
is taking place (LR PC Software Manual,1997).
To calibrate the Lab Recorder, the thermometer and the twisted end of
thermocouple (type T) when connected to Lab Recorder were immersed in the water as
shown in Figure 3.7. The water was heated with Gallenkemp Regulator Hotplate. The
data were taken from the thermometer with 5 0C increment starting from 0 0C to 95 0C.
At the same time temperature reading from the thermocouple was taken. The result of
the calibration is shown in Appendix A.
Thermometer
Thermocouple
‘T’
Retort
stand
Personal Computer (PC)
Beaker
LR
Water
Hot plate
Figure 3.7: The schematic diagram for Lab Recorder calibration
3.5
Monitoring Laser Chamber
The temperature measurement during the pumping process was measured using
thermocouple type ‘T’ which was interfaced to a Personal Computer (PC) via a Lab
Recorder (LR). The measurement was taken three times a day. The duration of the
experiment was about one hour. An average was calculated by dividing the total by
three. Two conditions were considered. First condition, the temperature was taken
during pumping process with water coolant circulated. Second condition, is without
circulating water coolant. The data were measured at three different parts of pumping
chamber, which include a stainless steel block, a chamber heat sink and a flashlamp.
During pumping process, laser chamber is the part of laser which received the
most excess heat. Flashlamp is a source for optical pumping. The white light radiated
from the flashlamp is used to excite a laser rod. However not all the energies are used.
In fact only some small portion responsible to absorb and excited population on the
laser rod. The rest of the heat, dissipated to various compartments in the laser chamber.
This includes especially the laser rod and flashlamp holder, which made from metal
(which easy to conduct heat), stainless steel. Beside the holder, the laser rod and the
flashlamp are enclosed in a heat sink, which was made from ceramic (insulator).
However, this heat sink also absorbs excess heat. Thus these two components, that are
laser holder and the heat sink, are responsible for absorbing most of the excess heat.
Consequently these two parts are monitored.
The quantities of heat received by the laser chamber were determined by
measuring the temperature at those particular points. The total heat given by the
flashlamp was monitored by measuring the temperature at the cathode and anode.
A thermocouple type T was employed as a temperature sensor. The sensor was
interfaced to a personal computer via a Lab Recorder (LR). The thermocouple was
connected to the specific point like, stainless steel block, heat sink and electrode, due to
the reason mention previously. The pumping process was running for one hour. It start
from room temperature up to the constant reading obtained from temperature
measurements.
The pumping process was operated at two conditions. The first condition
involves with the laser chamber which was chilled. In the second condition, the water
is still running but has not been cooled. The temperature was taken three times at each
different location on the laser chamber as mentioned before, and the average was
calculated.
The schematic diagram of the experimental set-up for temperature measurement
is shown in Figure 3.8.
Personal Computer (PC)
Thermocouple type ‘T’
Lab
Recorder
(LR)
Pumping
Chamber
Power
Supply
Cooling System
Figure 3.8: Experimental set-up for temperature measurement of pumping system
CHAPTER 4
CHARACTERIZATION OF THE WATER COOLING SYSTEM
4.1
Introduction
Conditions of cooling water can have significant effect on the operating
performance of equipment. In lasers, water conditions can affect beam pointing, power
stability, warm-up characteristics and plasma tube lifetime (Rabiah, 1998). Careful
selection, design and monitoring of multiple cooling water systems will minimize any
long-term problems that might other wise occur. It is unwise to assume that available
water is adequate for equipment cooling needs without first understanding the critical
variable involved. These variables include water temperature and water quality. In this
chapter, the developed cooling system was characterized to ensure it can provide a good
system to cool a Nd:YAG laser chamber.
4.2
Water Temperature
Water temperature stability is important since temperature fluctuations may
cause thermal stresses on the equipment which could cause shortage of life, poor beam
pointing on lasers, poor power stability and equipment damage (Rabiah, 1998). In this
study, temperatures of the water cooling system in the external and internal systems
were measured. Initially the temperature measurement was taken at the time the system
was switched on up to the point where the minimum of cooling temperature was
achieved. The temperature of internal coolant was taken simultaneously with the data
of temperature from external coolant.
4.2.1
Temperature of External Coolant
The temperature of external coolant was tested by using thermometer based on
mercury. The temperature was measured from the time the system was on, to a point
when the minimum of cooling temperature that is 18.0 ± 0.5 0C, was achieved. The
reading was taken for three sequential days. The obtained data are listed in Table 4.1.
These data are used to plot graph of temperature against time such as shown in Figure
4.1.
Table 4.1: Temperature variation of external coolant with time
Temperature ( θ ± 0.5oC )
Time (min)
(± 0.01s)
Day 1
Day 2
Day 3
Average
15.0
25.0
24.0
26.0
25.0
30.0
24.0
24.0
24.0
24.0
45.0
26.0
23.0
23.0
24.0
60.0
24.0
23.0
25.0
23.0
75.0
23.0
22.0
24.0
23.0
90.0
22.0
21.0
23.0
22.0
105.0
22.0
21.0
23.0
22.0
120.0
21.0
21.0
22.0
21.3
135.0
21.0
20.0
21.0
21.0
150.0
21.0
20.0
21.0
20.6
165.0
21.0
20.0
20.0
20.3
180.0
20.0
19.0
20.0
19.7
195.0
20.0
18.0
19.0
19.0
210.0
20.0
18.0
19.0
19.0
225.0
19.0
19.0
19.0
18.0
240.0
19.0
17.0
18.0
18.0
255.0
19.0
17.0
18.0
18.0
270.0
18.0
18.0
18.0
18.0
285.0
18.0
18.0
18.0
18.0
300.0
18.0
18.0
18.0
18.0
27
Temperature(0C)
25
23
21
19
17
15
0
50
100
150
200
250
300
350
Time(min)
Figure 4.1: Variation in the temperature of external coolant with respect to time
The graph of Figure 4.1 shows that the temperature of the external coolant
exponentially decreasing upon time. The minimum temperature was achieved after four
hours operating. This means that, the cooling system must be operated four hour earlier
before operating a laser system. This duration is considered to be quite a long period of
waiting. This is possibly due to the utilization of a bigger size of the water tank or
reservoir. The problem may be overcome by using a smaller tank.
4.2.2
Temperature of Internal Coolant
The temperature of internal coolant was taken at the same time with the
temperature from external coolant. The data were also taken for three days and the
average value was calculated. The obtainable data from this experiment are listed in
Table 4.2. The data are used to plot graph of temperature versus time such as shown in
Figure 4.2:-
Table 4.2: Temperature variation of internal coolant with time
Temperature (θ ± 0.05oC)
Time (min)
(± 0.01s)
Day 1
Day 2
Day 3
Average
15.0
25.30
25.20
25.10
25.20
30.0
25.00
24.50
24.90
24.80
45.0
24.50
22.20
23.50
23.40
60.0
24.00
22.20
23.40
23.20
75.0
23.20
22.00
23.20
22.80
90.0
23.00
21.80
22.40
22.40
105.0
22.80
21.40
21.80
22.00
120.0
22.40
21.20
21.80
21.80
135.0
22.20
21.00
21.00
21.40
150.0
22.00
20.80
20.20
21.00
165.0
21.80
20.40
20.20
20.80
180.0
21.40
20.20
19.60
20.40
195.0
20.40
20.00
20.20
20.00
210.0
20.00
19.80
19.60
19.80
225.0
19.80
19.40
19.00
19.40
240.0
19.40
19.20
19.00
19.20
255.0
19.00
19.00
19.00
19.00
270.0
18.80
18.60
18.70
18.70
285.0
18.60
18.40
18.50
18.50
300.0
18.20
18.00
17.80
18.00
27
Temperature(0C)
25
23
21
19
17
15
0
50
100
150
200
250
300
350
Time(min)
Figure 4.2: Variation in the temperature of internal coolant with respect to time
The curve obtained from the graph in Figure 4.2, shows that the temperature
inversely proportional with respect to the time.
The minimum temperature was
achieved at the same value with external coolant that is 18.00 ± 0.05 oC. It shows that
the configuration of the graph is almost similar between the two systems. Both water
cooling systems are found to have an exponentially decrease cooling temperature
profile.
However the curvature of the graph is different between the two systems. The
temperature for internal coolant is found to have a more drastic and smoother profile.
The difference arises possibly due to the different type of water and size of tank used in
both systems. The internal system used distilled water, which is much more clean
compared to tap water used in external system which is known to contain more
impurities and other chemical agents like chlorine. In addition the temperature reading
of internal cooling system is taken using a digital thermometer. So that, the
measurement is considered more accurate. In term of tank sizes, the internal one has a
smaller size and made from metal whereas the external tank has bigger size and made
from fibre glass. Furthermore, the external coolant is exposed with different kind of
atmosphere environment. The daily temperature outside the lab is not consistent.
Weather changes, for example rainy or hot summer days, can contribute to temperature
changes for external system. These conditions are in marked contrast to the internal
coolant system which has more stable temperatures.
4.3
Water Quality
Poor water quality can cause a wide variety of equipment failures, mineral
build-up or corrosion of the inner tubes. The constituents of the processed cooling
water that are of most concern are pH levels and resistivity. The pH of distilled water
for the water cooling system was measured using pH meter CyberScan pH500 from
EUTOCH Instruments made in Singapore, and water conductivity was measured using
a conductivity meter YSI 30 from YSI Incorporated USA. These two variables were
used to determine the water quality. Both parameters were taken with respect to
temperatures in the range of 10-60 0C which are considered as an appropriate
temperature range during laser pumping process. The measurement for water quality is
only considered for the distilled water used in internal coolant because this is the water
that will circulate through the laser chamber which contains a laser rod and a flashlamp.
4.3.1
The pH Level
In this experiment, the pH of the distilled water used in internal coolant was
measured as the water temperature changes. The data collected are listed in Table 4.3.
The data are used to plot a graph pH level versus water temperature. The graph is
shown in Figure 4.3.
Table 4.3: pH of distilled water at various temperatures
Water
pH Level (± 0.005)
Temperature
(θ ± 0.5 oC)
Experiment 1 Experiment 2 Experiment 3
Average
10.0
6.540
6.560
6.5500
6.550
15.0
6.650
6.680
6.690
6.673
20.0
6.780
6.800
6.740
6.773
25.0
6.780
6.800
6.760
6.780
30.0
6.840
6.860
6.780
6.827
35.0
6.880
6.880
6.840
6.867
40.0
6.880
6.890
6.860
6.877
45.0
6.890
6.890
6.880
6.887
50.0
6.880
7.000
6.890
6.923
55.0
6.890
7.000
6.940
6.943
60.0
6.880
6.870
6.890
6.880
8
7.5
pH
7
6.5
6
5.5
5
0
10
20
30
40
50
60
0
Water Temperature ( C)
Figure 4.3: Graph pH versus temperature for distilled water
Figure 4.3 shows that the pH level for the distilled water stays almost constant
with change in temperature. It is worth noting that, a pH level between 6.0 to 8.0 is
considered as a stable pH value for cooling process (Coherent, 2003). Water systems
that are either acidic (pH values of less than 7.0), or basic (pH values of greater than
7.0), can lead to electrochemical corrosion depending upon the metals that are present
in the cooling system (Rabiah, 1998). In this case, the pH level for distilled water was
found within 6.550 ± 0.005 to 6.943 ± 0.005 corresponding to the temperatures in the
range of 10 oC to 60 oC. The pH level obtained from the internal system is almost near
7, means that, the water is quite clean and safe to circulate through laser rod and
flashlamp in laser chamber. Meanwhile to avoid the failures of component inside laser
chamber, frequent checks of the water coolant are necessary to ensure the pH level of
the distilled water used.
4.3.2
Conductivity and Resistivity of Water
Conductivity and resistivity are parameters to measure the ability of a fluid to
conduct electrical current. Conductivity is simply the reciprocal of resistivity:
conductivity microsiemen (µS) = 1/ resistivity, megaohm meter (MΩm). In practice,
conductivity units are typically used when referring to water ranging from drinking
water to sea water, while resistivity units are reserved for ultra pure water such as
deionized or reverse-osmosis water (Paparone, 2004). Resistivity is a measure of the
concentration of dissolved minerals and metals in solution. Using the equation
conductivity (µS) = 1/ resistivity (MΩm), the resistivity of distilled water was
determined.
In this particular experiment, the conductivity of distilled water was measured at
different water temperature. The collected data of the measurement are listed in Table
4.4. The data were used to calculate a resistivity of data for distilled water as listed in
Table 4.5. This calculated resistivity is used to plot a graph as shown in Figure 4.4.
Table 4.4: Conductivity of distilled water at various temperatures
Conductivity of distilled water ( ± 0.05 µS)
Temperature
(θ ± 0.5oC)
Experiment 1 Experiment 2 Experiment 3
Average
10.0
8.30
9.00
8.80
8.70
15.0
8.10
8.80
8.60
8.50
20.0
7.80
8.60
8.40
8.27
25.0
7.80
8.20
8.00
8.00
30.0
7.60
8.50
7.60
7.90
35.0
7.40
8.40
7.40
7.73
40.0
7.40
8.20
7.40
7.73
45.0
7.50
8.20
7.40
7.70
50.0
7.40
8.20
7.40
7.67
55.0
7.40
8.00
7.40
7.60
60.0
7.40
8.00
7.40
7.60
Table 4.5: Resistivity of distilled water as the reciprocal of conductivity at various
temperatures
Resistivity
Temperature
Conductivity
( 1/ Conductivity)
(θ ± 0.5oC)
( ± 0.05 µS)
(± 0.01 MΩm)
10.0
8.70
0.11
15.0
8.50
0.12
20.0
8.27
0.12
25.0
8.00
0.13
30.0
7.90
0.13
35.0
7.73
0.13
40.0
7.73
0.13
45.0
7.70
0.13
50.0
7.67
0.13
55.0
7.60
0.13
60.0
7.60
0.13
0.15
0.145
Resistivity(M Ωm)
0.14
0.135
0.13
0.125
0.12
0.115
0.11
0.105
0.1
0
10
20
30
40
50
60
0
Temperature( C)
Figure 4.4: Graph resistivity versus temperature of distilled water
It is found from Figure 4.4 that, the resistivity of the distilled water increases
drastically in the range of 10 to 20 0C. Thereafter, it remains constant even when
the temperature is continuously increased to 60 0C. The resistivity for distilled
water obtained is in the range of 0.11 ± 0.01 to 0.13 ± 0.01 MΩm. The suitable
range of resistivity level for a process of cooling is in between 50 kΩm to 2 MΩm
(Coherent, 2003). Water with very low resistivity corresponds to high concentration
of dissolved salts. This condition can result in the formation of deposits on the
envelope of the plasma tube, which gradually will result in the formation of an
insulating layer of scale between the tube and cooling water. This can lead to poor
cooling and possible tube failure due to cracking of the flow tube (Coherent Laser,
2003). However, the resistivity of the distilled water used in this cooling system is
considered quite high
( 0.11 – 0.13 ) ± 0.01 MΩm. Hence the conditions of poor cooling system and the
failure of flow tube are potentially low.
4.5
Summary
A water cooling system for Nd:YAG was successfully developed. The
characteristics of water cooling system are: first it has a minimum temperature of 18.00
± 0.05 oC, second the pH level of distilled water is in the range of (6.550 – 6.943) ±
0.005 which is in the stable range and third it possesses distilled water with quite low
resistivity in the range of (0.11-0.13 ) ± 0.01 MΩm.
CHAPTER 5
DEVELOPMENT OF A LASER CHAMBER
5.1
Introduction
All solid state laser systems require a pump chamber. The pump chamber is the
heart of the laser system. It is where electrical energy is converted to coherent light at
the specified wavelength of laser rod. In this research, the laser chamber is designed
and installed with water cooling system.
Basically a laser pump chamber is made up of:
•
An alumina diffuse reflector
•
A parallel cooling path
•
High UV absorbing flow tubes
•
O-rings
•
A Lamp
•
A Laser rod
In a typical pulsed solid-state laser, pump energy is delivered to the solid state
medium via a flashlamp, which is essentially a very bright strobe light. Usually, the
flashlamp and the solid state medium are rod-shaped and of equal diameters, and are
positioned at the two focus of a reflective elliptical cavity (called a resonator or pump
chamber), thus ensuring that all photons emitted from the flashlamp will find their way
to the rod for maximum efficiency. Photons created by stimulated emission "resonate"
between the highly reflective (HR) mirror and the output coupler (OC), or partially
reflective mirror, and on to the delivery device (Shore, 2000).
5.2
Laser Chamber
Solid state laser is controlled by reflectivity of laser pump cavity. As far as laser
is concerned, important role of cavity (usually called light-focusing cavity) must reflect
irradiation of pump light source back towards laser-producing material source. In this
research, it features a glazed ceramic reflector (chamber heat sink), to provide diffuse
coupling of the pulsed pump light- and is flooded with circulating distilled water to cool
the flashlamp and the laser rod.
The design of pumping chamber consists of a Nd:YAG laser rod, flashlamp,
flow tube, two stainless steel blocks, chamber heat sink, base plate and all stainless steel
screws and clamps. All components in the laser chamber were designed according to
the size of laser rod and flashlamp that had been manufactured by KENTEK Laser
Corp.
5.2.1
Nd:YAG laser rod
Neodymium-doped, yttrium aluminium garnet (Nd:YAG) laser rods are used in an
optical configuration to produce high power, low divergence beam. Neodymium-doped
yttrium aluminum garnet (Nd:YAG) possesses a combination of properties uniquely
favorable for laser operation. The YAG host is hard, of good optical quality and has a
high thermal conductivity. The dimension of laser rod used was 75 mm x 4 mm with
1% of impurities of Nd and 1064 nm anti-reflected (AR) coating on both ends. It is
manufactured by KENTEK Laser Corp. The photograph of the Nd:YAG laser rod
utilized in this particular equipment is shown in Figure 5.1.
Figure 5.1: The photograph of Nd:YAG laser rod
5.2.2
Flashlamp
Source of powerful light is in the form of a linear lamp and used to excite photon
emission in a solid-state laser. It dimension was 66 mm arc, with 137.2 mm glass and
4 mm x 6 mm envelope. It has 4.7 mm x 9.9 mm connectors Xenon and manufactured
by KENTEK Laser Corp. The photograph of flashlamp used to pump the laser rod is
shown in Figure 5.2.
Figure 5.2: The photograph of flashlamp
5.2.3
Flow tube
Material used was samarium quartz and with process flame polished ends. The
material was chosen because of its robustness with the temperature changes due to
heating from flashlamp and cooling from chiller inside the pumping chamber during
pumping process. The dimension of the flow tube is 70 mm x 10 mm. It functions is
used to enclose a laser rod, enriched reflection, confine and smoothing the flow of
coolant water, and the most important is to absorb ultraviolet (UV) light produced from
the xenon flash lamp. The photograph of the flow tube utilized in this experiment is
given in Figure 5.3.
Figure 5.3: The photograph of flow tube
5.2.4
Stainless steel block
Two blocks made from stainless steel were designed. The stainless steel was chosen
as a material because it resistance to corrosion and staining, low maintenance, relative
inexpense, and familiar luster make it an ideal material. They are used for holding
chamber heat sink, laser rod and flash lamp. Block 1 was designed with two holes to
hold laser rod and flash lamp. While block 2 was made with two holes at the side and
another two holes at the bottom. Two holes at the bottom were specifically made for
cooling purposes. The chilled water supplied from internal cooling system via L pipe
was attached with the block. This is to ensure that the chilled water can circulate
through laser rod and flashlamp. The dimension and drawing of these two blocks are
shown in Figure 5.4 and Figure 5.5 respectively.
Figure 5.4: Engineering drawing of block 1. (Dimensions are in millimeters (mm))
Figure 5.5: Engineering drawing of block 2. (Dimensions are in millimeters (mm))
The sizes of both blocks were quite large. The main objective was to ensure a
laser rod and flash lamp remained in stable position. It is also to prevent from
expansion of laser rod due to overheating during the pumping process. Both blocks 1
and 2 are shown in Figure 5.6.
Figure 5.6: The photograph of stainless steel blocks 1 and 2
5.2.5
Chamber heat sink
Chamber heat sink was needed to provide diffuse coupling of pulsed pump light. It
was made from ceramic (alumina). The configuration of 45oside view of laser chamber
was made for easy to clamp the chamber heat sink with the holder. The dimension and
schematic drawing of this chamber are shown in Figure 5.7, 5.9 and 5.10 while the
photograph is shown in Figure 5.8.
Figure 5.7: Dimension of top view of the chamber heat sink. (Dimensions are in
millimeters (mm))
Figure 5.8: The photograph of chamber heat sink
Figure 5.9: Dimension of side view of the chamber heat sink. (Dimensions are in
millimeters (mm))
Figure 5.10: Dimension of front view of the chamber heat sink. (Dimensions are in
millimeters (mm))
5.2.6
Base plate
It was used to hold the clamped pumping chamber in steady place. It is consist
of two holes which are needed to put ‘L’ shape pipe for a coolant to flow through the
flashlamp and laser rod. The technical drawing of these base plates is shown in Figure
5.11 and 5.12 respectively and the photograph is shown in Figure 5.13.
Figure 5.11: Dimension of top view of base plate. (Dimensions are in millimeters
(mm))
Figure 5.12: Dimension of side view of base plate. (Dimensions are in millimeters
(mm))
Figure 5.13: The photograph of base plate
5.2.7
Technical Accessories
Other related components such as O ring, block clamp and related screws are
used to assemble all components in the laser chamber. An O ring was utilized to seal
laser rod, flowtube and flashlamp in order to prevent water leaking during circulation
process of chilled water in the laser chamber. The photograph of assembling
components is depicted in Figure 5.14.
Figure 5.14: The photograph of O ring, block clamps and other screw used to assemble
laser chamber
5.3 Assembly of the Laser Chamber
The assembling of the pumping chamber began by attaching heat sink chamber into
two stainless steel blocks. The blocks were then screwed on the base plate. The end of
a flow tube inside the chamber heat sink was ensured tightened in between the two
blocks. A flashlamp was then slid into the pumping chamber. The polarity (positive
and negative side) of the lamp was identified by ensuring that it was placed correctly.
A great care must be taken not to touch the quartz body of the lamp. It is easily
contaminated and can cause early failure. Two screws and the flashlamp clamp plate
were seal with the ‘O’ ring at each end of the lamp to prevent leakage of water coolant
during circulation.
The laser rod is fixed to the rod tube with a watertight silicone rubber seal. The rod
was placed in the pumping chamber, by pushing an ‘O’ ring of the correct size over the
plain cylindrical end tubes. The rod must be carefully inserted into the pumping
chamber until the ‘O’ ring rests against the end block. The other ‘O’ ring, clamp plate
and screws were fitted at the other end and alternately screwed with tighten a little by a
little at a time. It was to avoid placing any bending forces on the rod, until all screws are
completely tight. The chamber clamps were screwed between two blocks for
completely attaching chamber heat sink with each block.
The schematic diagrams of the whole pumping chamber are shown in Figure 5.15
and Figure 5.16. The photograph of the laser chamber is shown in Figure 5.17 and
Figure 5.18.
Flashlamp clamp
Chamber Heat Sink
Stainless steel
block 2
Stainless steel
block 1
Flash lamp
Chamber
clamp
Base plate
Figure 5.15: The schematic diagram of complete pumping chamber from side view
Nd:YAG Laser rod
Stainless steel
block 2
Rod tube
Flashlamp clamp
Flow Tube
Stainless steel
block 1
Flash lamp
Laser rod
clamp
Base plate
Figure 5.16: The schematic diagram of pumping chamber without chamber heat sink
from side view
Flashlamp clamp
Chamber Heat Sink
Stainless steel block
Base plate
Figure 5.17: The photograph of pumping chamber (Side view)
Stainless steel block
Flashlamp clamp
Flash lamp
Nd:YAG rod
Rod clamp
Flow tube
Base plate
Figure 5.18: The photograph of pumping chamber without chamber heat sink (Top
view)
5.4
The Laser House
A house was designed in order to place a laser chamber in a stable condition and
to provide other facilities such as pumping and electrical facilities. The material used
for the house was stainless steel. The technical drawing of the house with its specific
dimensions is shown in Figure 5.19 – 5.20. The photograph of the whole house
including the sliding mounting and the laser chamber is depicted in Figure 5.21.
10
40
30
Ø 10
70
70
115
Ø
130
60
30
60
240
(a)
(b)
Figure 5.19: Technical drawing of front cover of laser house from (a) front view, (b)
side view. (Dimensions are in millimeters (mm))
50
110
20
20
10
10
25
35
10
70
10
240
(a)
600
110
160
130
700
(b)
45
35
70
10
700
(c)
Figure 5.20: Technical drawing of housing from (a) front view (b) top view (c) side
view. (Dimensions are in millimeters (mm))
(a)
(b)
(c)
Figure 5.21: The photograph of laser house (a) from top view without laser trail, (b)
from top view with laser trail and (c) from side view with the laser chamber
5.5
Piping
The L pipe is used as a connector to provide cooled distilled water from internal
cooling system circulated into the laser chamber. It was attached to base plate to one of
stainless steel block. It is made from stainless steel. The technical drawing and
photograph of this L pipe is shown in Figure 5.22. The photograph of the L shaped pipe
is manifested in Figure 5.23.
200
55
19
75
19
16
Figure 5.22: Dimension of side view of L pipe. (Dimensions are in millimeters (mm))
Figure 5.23: The photograph of L pipe
5.6
Testing the Circulation System
The cooling system was tested to ensure that the circulation of the distilled water
through the flashlamp and the laser rod in the laser chamber was smooth and contained
no leakage. A flow rate was measured in order to ensure a smooth circulation. Figure
5.24 shows a schematic diagram of all components involved in this circulation system.
It begins with an external cooling system which supplies a chilled water. The coolant
flows through a pair of cooling coil in an internal cooling system to cool the distilled
water. The distilled water then flows through a rubber hose that connects the internal
cooling system to the L pipe, which then goes into the laser house. The L pipe was
attached under the base plate to enter into the stainless steel block 2. The cooled
distilled water then circulates through the laser chamber, during pumping process.
Laser chamber
Laser house
Internal cooling
system
External cooling
system
Water line
Water flow in L pipe
inside laser house
Figure 5.24: Schematic diagram showing circulation of cooling system from coolant to
the laser head
A schematic diagram of the circulation of the distilled water in the laser
chamber is shown in Figure 5.24 (top view) and Figure 5.25 (side view). A cooled
distilled water flows through the laser rod and then through the flashlamp. This will
remove the heat deposited in the rod that is not converted into light, so that it can
prevent overheating in the laser chamber.
Inlet
hole
Block 1
Block 2
Flow tube
Outlet
hole
Laser rod
Flashlamp
Chamber heat sink
Water flow
Figure 5.25.: The flow path of distilled water in laser chamber during circulation (top
view)
Laser rod
Chamber heat
sink
Flow tube
Flashlamp
Ellipsoid space
in the heat sink.
chamber
Water flow
Inlet
Outlet
L pipe
Figure 5.26: The detail of the circulation system in laser chamber (concentrated in
ellipsoid space)
5.7
The Water Flow
The flow rate of the coolant to the laser head must be kept possibly constant.
The rate at which heat is conducted from the laser depends on the flow rate. Changes in
the flow rate cause temperature changes in the laser head, which may result in problems
such as drift in the laser output power and output beam direction (beam pointing). Long
term flow rate stability can be measured using a flow rate transducer or by manual
recording of data over time the laser is used. This should be done in a critical situation
when there is a suspected problem with the flow rate stability (Coherent Laser, 2003).
The water flow rate was determined by measuring the volume of water collected
in an interval of time, using beaker and stop watch. The water flow rate R, is
determined by dividing volume by time taken (Lytron, 2002)
R=
v
t
(5.1)
where v is volume (liter/l) and t is time (min).
If a flow meter is not available, a graduated container and a timer can be used to
determine the fluid flow rate by measuring the amount of fluid that has passed through
the system and dividing by the amount of time that has elapsed (Lytron, 2002). A
constant flow rate is essential when measuring the flow in this manner. The density of
the fluid should be used to convert the volumetric flow rate to mass flow rate.
In this experiment the volume of water taken is fixed to 2260.0 ± 0.5 ml, the
data is taken for five times with an average time taken of 13.80 ± 0.01 s. Using
equation (5.1), the flow rate of water entering the laser chamber is 9.83 ± 0.01 l/min.
The distilled water was circulated by using MARCH pump. The detail specification of
the pumped is listed in Table 5.1. The water coolant was circulated through the laser
rod, flashlamp and return back to the reservoir.
Table 5.1: Physical properties of pump used by internal cooling system
Physical Properties
Value
Angular speed
3000 rpm
Power
49.7 W
Radius of the motor
42.6 mm
Manufactured
MARCH, MFG, Inc.
5.8
Summary
A laser pumping chamber for Nd:YAG was successfully developed. The system
includes a Nd:YAG laser rod, a linear flashlamp, a flow tube, a chamber heat sink, two
stainless steel blocks and a base plate. The pumping chamber was provided with the
water cooling system developed earlier. A circulation of distilled water through the
laser chamber was tested to ensure no leakage occurred during circulation. A flow rate
was calculated to be as 9.83 ± 0.01 liter / min during the pumping process.
CHAPTER 6
TEMPERATURE MONITORING DURING PUMPING PROCESS
6.0
Introduction
An appreciable amount of heat normally dissipates during a pumping process. This
may cause degradation of the laser performance and could damage other components.
Henceforth, it is desirable to cool the chamber during a pumping process. The aim is to
estimate the percentage of heat dissipated in different part of the laser chamber during a
pumping process, with and without cooling the distilled water.
Since the heating of the laser media severely limits its performance, this
measurement may have significant practical design consequences (Mangir and
Rockwell, 1986). Therefore, it is essential to quantify the heat generated accompanying
flashlamp pumping of Nd:YAG. In this chapter, the calculation of the heat loss during
pumping process is presented.
6.1
Heat Loss
The heats Q gained (or lost) at three different parts in laser chamber were
obtained by using the relation (Nelkon and Parker, 1980):
Q = mcp∆θ
(6.1)
where Q is the heat gained (J), m is the mass of object (g), cp is the specific heat (J/goC)
and ∆θ is the temperature change (oC). The power absorbed was calculated by dividing
heat with the exposure duration time, t
Pabsorbed =
Q
t
(6.2)
While the percentage of the power absorbed in different part of the laser chamber was
calculated using equation:
% Pabsorbed =
Pabsorbed
× 100%
Pi
(6.3)
where Pabsorbed is the power absorbed (W) and Pi is the power input (W).
During pumping process the heat is produced by a flashlamp. The ends of
flashlamp were connected to two electrodes made from cooper. The whole flashlamp
tube was held by stainless steel block and enclosed by heat sink. The heat sink is made
from ceramic and polished to shine to stand as a reflector. The physical properties of all
components in the laser chamber are listed in Table 6.1. The flashlamp used in this
experiment was supplied with 16kW input power (Lumonics, 1991).
Table 6.1: Physical Properties of the Laser Chamber elements [Cengel and Boles,
(1998), Applied Ceramics Inc, (2003)] and their respective temperature change with and
without chilled water cooling.
Physical properties
Items
Temperature
Mass
(± 0.005 g)
Specific heat, cp
o
(± 0.001 J/g C)
Temperature
change,
change,
o
θmax- θmin (± 0.05 C)
θmax- θmin (± 0.05 oC)
with chilled
without chilled
water
water
Flashlamp
6.440
0.386
59.80
66.10
Heat sink
683.380
0.879
2.40
24.00
Stainless
852.010
0.460
2.50
17.70
steel block
6.2
The Temperature Distribution of the Laser Chamber
In this project, the temperature distribution during pumping process was carried
out. Therefore the energy absorbed in the laser chamber could be estimated. The
temperature data was measured using the thermocouple type ‘T’ which was interfaced
to Personal Computer (PC) via a Lab Recorder (LR). The thermocouple was attached
to three parts of the laser chamber that were stainless steel block, ceramic chamber heat
sink and flashlamp. Two conditions were considered during the measurement, which
were performed with and without a water cooling system. In the first case, the study
was carried out by chilled distilled water which was circulated through the flashlamp
and flow tube. In the second condition, the water is still circulated in the laser chamber,
but not being chilled. The system was operated for an hour.
6.2.1
The Temperature Distribution of the Laser Chamber With Chilled Water
Temperature at different parts of the laser chamber while being cooled by
chilled distilled water is listed in Table 6.3.
Table 6.3: Temperature at different parts of laser chamber with chilled water operation
Time
(± 0.01 s)
0.0
60.0
120.0
180.0
240.0
300.0
360.0
420.0
480.0
540.0
600.0
660.0
720.0
780.0
840.0
900.0
960.0
1020.0
1080.0
1140.0
1200.0
1260.0
1320.0
1380.0
1440.0
1500.0
1560.0
1620.0
1680.0
1740.0
1800.0
1860.0
1920.0
1980.0
2040.0
2100.0
2160.0
Temperature, θ (± 0.05 oC)
Stainless steel
Flashlamp
Chamber heat
block
sink
27.10
27.90
28.80
29.30
29.70
30.10
30.20
30.40
30.40
30.40
30.40
30.40
30.40
30.30
30.30
30.20
30.20
30.20
30.10
30.10
30.10
30.10
30.00
30.00
30.00
30.00
29.90
29.90
29.90
29.90
29.90
29.90
29.90
29.80
29.90
29.80
29.80
33.80
56.50
68.40
74.50
79.40
82.90
84.90
87.60
88.90
89.80
90.50
91.10
91.50
92.00
92.30
92.50
92.70
92.80
93.00
93.00
93.20
93.20
93.20
93.20
93.20
93.10
93.30
93.30
93.30
93.30
93.40
93.00
93.50
93.40
93.50
93.50
93.50
25.70
26.10
26.60
27.00
27.50
27.90
28.20
28.40
28.50
28.60
28.70
28.70
28.70
28.80
28.90
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.80
28.70
28.70
28.70
28.70
28.60
28.60
28.60
28.60
Time
(± 0.01 s)
2220.0
2280.0
2340.0
2400.0
2460.0
2520.0
2580.0
2640.0
2700.0
2760.0
2820.0
2880.0
2940.0
3000.0
3060.0
3120.0
3180.0
3240.0
3300.0
3360.0
34200.
3480.0
3540.0
3600.0
Temperature, θ 0(± 0.05 oC)0
Stainless steel
Flashlamp
Chamber heat
block
sink
29.80
29.80
29.80
29.80
29.70
29.70
29.70
29.70
29.70
29.70
29.70
29.60
29.70
29.70
29.60
29.60
29.60
29.60
29.50
29.50
29.50
29.50
29.50
29.50
93.50
93.50
93.50
93.50
93.30
94.50
93.50
93.80
93.90
93.50
93.50
93.70
93.80
93.50
93.90
93.80
93.80
93.50
93.90
93.50
93.40
93.60
93.70
93.60
28.60
28.60
28.60
28.60
28.50
28.50
28.50
28.50
28.50
28.40
28.40
28.40
28.40
28.40
28.30
28.30
28.30
28.30
28.30
28.30
28.20
28.20
28.20
28.20
The result of temperature monitoring of the laser chamber by chilling the water
coolant is shown in Figure 6.1. The graph illustrates the temperature profile of the flash
lamp, chamber heat sink and stainless steel block. It is obvious that, the temperature of
the flashlamp increases drastically within the first 14 minutes; and remains constant as
the time proceeds. The maximum temperature obtained from the flashlamp is 93.6 ±
0.05 oC. The temperature profile for the flashlamp is much higher compared to the
other two tested components. This is due to the measurement of temperature was taken
directly onto the electrode, outside the laser chamber. Hence the heat does not
dissipated in the chilled water. However, the other two tested parts received the heat
transfer indirectly where some of the heat was dissipated through the circulated chilled
water.
100
90
O
Temperature( C)
80
70
60
50
40
30
20
10
0
0
1000
2000
3000
4000
Time(s)
Chamber heat sink
flashlamp
stainless steel block
Figure 6.1: Temperature profile at laser chamber when operated with chilled water.
The stainless steel block for example is uninsulated. The block is a metal; it can
easily conduct the heat away. On the other hand chamber heat sink is made from
ceramic; and thus an insulator. Less heat transfers during the circulation of chilled
water because most of the heat has been carried away by the chilled and running water.
Thus, only the left over heat was transferred to the heat sink and the stainless steel
block. As a result, the temperature profiles for both components almost similar and
relatively much lower as compared to the flashlamp temperature. The temperature
profiles for both cases slightly increase in the initial stage, but subsequently remain
constant throughout the one hour operation.
6.2.2
The Temperature Distribution of the Laser Chamber Without Water
Cooling
The result obtained from the second experiment without chilled the water is
shown in Figure 6.4. Such studied was carried in an attempt to predict what would
happen to the laser chamber, in case the cooling system is not operational. It is to
ensure whether it is safe to run the laser system, if the coolant is not functional. In this
investigation, the water was let to run, but the chilled water was switched off. The
temperatures are still detected at the same points on laser chamber which include the
flashlamp, stainless steel block and chamber heat sink.
Table 6.4: Temperature of laser chamber at different part without chilled water
Time
(± 0.01 s)
0.0
60.
120.0
180.0
240.0
300.0
360.0
420.0
480.
540.0
600.0
660.0
720.0
780.0
840.0
900.0
960.0
1020.0
1080.0
1140.0
1200.0
1260.0
1320.0
1380.0
1440.0
1500.0
1560.0
1620.0
1680.0
1740.0
1800.0
1860.0
1920.0
1980.0
2040.0
2100.0
2160.0
2280.0
Temperature, θ (± 0.05 oC)
Stainless steel
Flashlamp
Chamber heat
block
sink
26.90
28.20
29.20
29.90
30.40
31.00
31.70
32.30
32.70
33.40
33.90
34.30
34.70
35.40
35.70
36.30
36.80
37.20
37.80
38.10
38.60
39.10
39.40
39.90
40.20
40.60
41.20
41.50
41.90
42.20
42.60
43.00
43.40
43.60
44.00
44.50
44.70
44.90
36.10
58.50
68.50
77.00
78.70
79.60
82.30
83.90
85.80
87.10
87.80
89.20
89.70
90.00
90.30
90.40
90.30
90.70
90.90
91.30
91.90
92.30
92.60
92.90
93.50
93.90
94.20
93.50
94.90
95.20
95.80
95.90
96.00
96.20
96.50
96.80
96.90
97.50
24.50
24.90
25.80
26.40
27.20
27.60
28.30
28.70
29.30
29.70
30.10
30.80
31.10
31.60
32.10
32.50
32.90
33.40
33.80
34.10
34.60
34.90
35.40
35.80
36.20
36.50
36.90
37.40
37.70
38.10
38.40
38.70
39.10
39.50
39.80
40.30
40.60
40.80
Time
(± 0.01 s)
2340.0
2400.0
2460.0
2520.0
2580.0
2640.0
2700.0
2760.0
2820.0
2880.0
2940.0
3000.0
3060.0
3120.0
3180.0
3240.0
3300.0
3360.0
3420.0
3480.0
35400.
3600.0
Temperature, θ (± 0.05 oC)
Chamber heat
Flashlamp
Stainless steel
sink
block
45.20
45.50
45.90
46.30
46.60
46.90
47.30
47.60
47.90
48.20
48.50
48.80
49.40
49.40
49.60
50.10
50.40
50.70
50.90
50.90
50.90
50.90
97.70
97.90
97.90
98.10
98.40
98.60
99.00
99.60
99.60
99.70
100.10
100.20
100.90
100.70
101.20
101.20
101.40
101.40
101.60
101.70
102.10
102.20
41.20
41.60
41.90
42.10
42.30
42.60
43.10
43.30
43.70
43.90
44.30
44.40
44.70
44.20
43.70
43.40
43.20
42.90
42.70
42.50
42.50
42.20
120
O
Temperature( C)
100
80
60
40
20
0
0
1000
2000
3000
4000
Time(s)
Chamber heat sink
flashlamp
stainless steel block
Figure 6.2: Temperature Profile Without Chilled Water.
The features of the results are still similar with the first experiment. The heat
from the flashlamp of course becomes much higher. In fact the highest temperature
obtained was 102.2 ± 0.05 oC. Different results were obtained for both the block and
the heat sink. The temperature profiles are found to have almost linear relationship.
This is reasonable because more heat is transferred to both components, since the water
is not chilled.
6.3
Heat at different Part of the Laser Chamber
The temperature data obtained can be used to calculate the heat loss from the
flashlamp, and the heat absorbed by the stainless steel block and the heat sink. Using
Equation (6.2), the power loss by the flashlamp was calculated to be
0.04 ± 1.75 x 10-4 W in the case where the laser chamber was circulated by chilled
water. While 0.02 % power have been absorbed by the stainless steel block which is
used to stabilize the laser head. 0.03 % power has been absorbed by the heat sink.
The power loss by the flashlamp from the second experiment without the chilled
water was 0.05 ± 0.19 x 10-3 W . 0.25 % energy has been absorbed by heat sink and
0.12 % by stainless steel block. The detailed of calculated results of heat and power
absorbed by the flashlamp, heat sink and stainless steel block are shown in Table 6.5.
Calculation of percentage improvement between two experiments was made in
order to shown the efficiency of water cooling system applied. The calculation was
made using equation;
% Im provement =
Pabsorbed ( withoutchilledwater ) − Pabsorbed ( withchilledwater )
× 100
Pabsorbed ( withoutchilledwater )
The result shows that the flashlamp had 20% improvement during pumping
process with circulation of chilled water rather than without using chilled water. While
the stainless steel and chamber heat sink had showed higher percentage of improvement
which was 90% and 86% respectively. This indicates that the water cooling system that
provided in the system was effectively dissipating all the excess heat from the pumping
process.
However, when the results obtained with and without the chilled water were
compared, the excess heat was found to be ten times higher in the heat sink and almost
seven times in the stainless steel block, but relatively very small change occurred in the
flashlamp. Nevertheless, it was advisable to cool the water coolant for the sake of
safety and long life expectancy of the laser crystal and the flashlamp tube.
Table 6.5: The Result of Energy Absorbed by Different Part in the Laser Chamber during the Pumping Process.
Component
Flashlamp
Heat sink
With chilled water
Heat ,
Q (J)
Power absorbed,
Pabsorbed(W)
Percentage
Power
Absorbed (%)
Without chilled water
Heat ,
Q (J)
148.65 ± 0.63
0.04 ± 1.75 x 10-4
2.50 x 10-3
164.31 ± 0.68
1441.66 ± 31.65
0.40 ± 0.88 x 10-3
0.03
14416.58 ± 46.13
Power absorbed,
Pabsorbed(W)
Percentage
Power
Absorbed
(%)
Percentage of
improvement
(%)
0.05
± 0.19 x 10-3
4.00
± 1.28 x 10-2
3.13 x 10-3
20
0.25
90
Stainless
steel
block
979.81± 21.73
0.27 ± 6.05 x 103
0.02
6937.07± 34.68
1.93
± 9.65 x 10-3
0.12
86
6.4
Summary
The high percentage of improvement liberated from flashlamp, chamber heat
sink and stainless steel blocks indicated that the cooling system provided in the laser
chamber was very effective in carrying out the excess heat from pumping process.
Without chilled water, the absorption in the heat sink was found to be ten times
higher and seven times higher in the stainless steel block, in comparison to the
situation with chilled water coolant.
CHAPTER 7
CONCLUSION AND SUGGESTIONS
7.1
Conclusion
The objectives of this research have been successfully achieved. The
temperature distribution of the laser chamber during the pumping process has been
studied. The water cooling system and laser chamber for a solid state laser have
been successfully developed.
A water cooling system is used to circulate cooled distilled water through a
flashlamp and laser rod contained in the laser chamber. This water cooling system
was divided into two parts; external and internal cooler. An external cooler is used
to cool the distilled water in the internal cooler. Tap water was used in the external
cooler to circulate through a pair of cooling coils mounted inside the tank. This
external cooler was put outside the lab to reduce vibration created by the compressor
and other related components and also enhance maintenance purposes. While an
internal cooler was used to circulate the distilled water into the laser chamber
equipment. It is located inside the lab.
Several critical parameters which include water temperature, water quality
and resistivity were measured. These measurements were made to ensure the water
cooling system was appropriate for cooling a Nd:YAG laser system. The minimum
temperature that could be achieved for this water cooling system is 18.00 ± 0.05 0C.
The pH level of distilled water used was measured in the range of (6.550-6.943) ±
0.005 which was considered as the stable level. The distilled water also posses quite
high resistivity in the range of (0.11-0.13) ± 0.01 MΩm.
A laser pumping chamber for Nd:YAG was successfully developed. It is
here where electrical energy is converted to coherent light at the laser rod's specified
wavelength. The system include Nd:YAG laser rod, a linear flashlamp, a flow tube,
a chamber heat sink, two stainless steel blocks and a base plate. All components are
properly assembled to form a laser chamber. Piping and electrical power lines are
also included in this design. Piping provides path for the distilled water to flow.
Circulation of distilled water throughout the laser chamber was tested to ensure no
leaking occurred during the pumping process. The flow rate of the distilled water in
this circulation was found to be 9.83 ± 0.01 liter/ min.
The laser rod in the laser chamber was pumped using a flashlamp. The
measurement of temperature distribution during a pumping process was carried out.
The thermocouple was attached to three parts of the laser chamber consisting of
stainless steel block, ceramic chamber heat sink and flashlamp. The measurements
were done under two conditions. In the first condition, the distilled water was
cooled by the external cooler while in the second condition, the distilled water was
not cooled.
The heat liberated in the laser chamber was calculated. The result obtained
shows that the flashlamp had 20% improvement during pumping process with
circulation of chilled water rather than without using chilled water. On the other
hand the stainless steel and chamber heat sink had shown higher percentage of
improvement which was 90% and 86%, respectively. Without chilling distilled
water, the absorption in the heat sink was found to be ten times higher and seven
times higher in stainless steel block, in comparison the situation with chilled water.
This indicates that the cooling process with chilled water successfully prevents the
excess heat generated during the pumping process from heating the rest of the
components. This also means that the cooling system is very efficient and effective
in transferring the excess heat away from the laser chamber during the pumping
process.
7.2
Problem and Suggestions
In this study, the measurement of critical variables for water cooling system
only includes water temperature, water flow and water quality. For future works it
would be advantageous that the laser system is incorporated with an interlocking
system that would shut the laser off when the water temperature gets too high thus
preventing the laser from operating when the water temperature is too high. The
rate of water coolant that flows from the cooling system to the laser head should be
kept as constant as possible. Therefore, a digital flow rate transducer is required to
monitor the flow rate continuously.
Typically, a temperature control system is necessary in solid state laser to
enhance laser stability. As mentioned earlier, this research only covers the initial
stage of preparing the water cooling system and development of a laser chamber. In
additional, we have also developed a preliminary works on temperature control
system such as shown in Figure 7.1. However because of the time constraint and
involvement of extensive electronic circuits design, the work could not be carried
out. This could be performed in the future.
Basically the idea of our recent works is to ensure that the temperature
remain in the specific window, by controlling the speed of the water pump which
used to circulate distilled water through the laser head. A microcontroller based
control circuit using PIC16F870 is desired to be developed in order to control the
laser chamber and water temperature, water flow rate and speed of water pump. An
assembly language program needs to be written and programmed into the
microcontroller. This system is constructed by interfacing with Personal Computer
running Linux via RS-232 serial communication.
The digital flow transducer will be used to detect the water flow rate
continuously and display data measurement in Personal Computer (PC). The
temperature is detected by the thermocouple and displayed on the PC. By using this
system, a totally smart laser can be designed.
Flow Transducer
Solid state Laser Head
Water
cooling
system
Switch Mode
Power Supply
(SMPS)
PIC Microcontroller
Thermocouple
Water
pump
Switch
Water lines
Circuit lines
PC
Figure 7.1: The schematic diagram of the suggested design programmable
temperature controller in solid state laser head using microcontroller PIC16F870
The development of laser chamber and the measurement of variables for the
water cooling system had been done in this study which will be beneficial for future
researchers. The suggested idea and the information provided will lead to better
understanding. Last but not least, it is hopeful that this thesis would become a good
source of reference by future workers in this area.
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Lab Recorder Personal Computer
APPENDIX A
Data observed from calibration of Lab Recorder is listed in Table 1. The
graph is shown in Figure 1.
Table 1: Calibration data for Lab Recorder
Thermometer
measurement
± 0.5 (oC )
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
45.0
50.0
55.0
60.0
65.0
70.0
75.0
80.0
85.0
90.0
95.0
Lab Recorder measurement, θ (0C) ± 0.05
Average
θ1
θ2
θ3
0.20
5.20
10.50
16.00
20.80
25.80
31.00
35.90
41.00
45.60
50.90
56.10
60.80
65.80
70.20
75.50
81.00
85.80
90.80
95.80
0.30
5.40
10.50
15.60
20.60
25.40
30.80
35.80
40.80
45.70
50.90
55.90
60.40
65.40
70.20
75.90
81.00
85.90
90.90
95.60
0.10
5.20
10.80
15.30
20.80
25.40
30.40
36.40
40.80
45.30
50.40
55.60
61.20
65.20
70.10
75.80
80.90
86.00
90.60
95.60
0.20
5.30
10.60
15.60
20.70
25.50
30.70
36.00
40.90
45.50
50.70
55.90
60.80
65.50
70.20
75.70
81.00
85.90
90.80
95.70
Lab Recorder measurement vs thermometer measurement
Lab Recorder measurement (0C)
120
100
y = 1.0034x + 0.4971
80
60
40
20
0
0
20
40
60
80
0
Thermometer measurement ( C)
Figure 1: Calibration curve for Lab Recorder
100
PUBLICATIONS
1. Nor Aziawati Azahari, Noriah Bidin, Calibration of A Water Cooling System
for Nd:YAG Laser, Proceeding of Annual Fundamental Science Seminar
(AFSS 2004), 14-15 June 2004, Skudai, Johor.
2. Nor Aziawati Azahari, Noriah Bidin, Estimation of A Wasted Heat During
Nd:YAG Laser Pumping Process, Proceeding of Persidangan Fizik
Kebangsaan (PERFIK 2004), 5-7 October 2004, Seri Kembangan, Selangor.
3. Nor Aziawati Azahari, Johari Adnan, Ahmad Hadi Ali, Mohd Fairuz Jani,
Noriah Bidin, Development of A Programmable Switch Mode Power Supply
(SMPS) for Controlling Water Pump in Solid State Laser Pumping System
Using PIC16f870 Microcontroller, Proceeding of The XXI Regional
Conference and Workshop on Solid State Science and Technology
(RCWSST 2004), 10-13 October 2004, Kota Kinabalu, Sabah.