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SUZAIRI BIN DAUD
UNIVERSITI TEKNOLOGI MALAYSIA
SUZAIRI BIN DAUD
A thesis submitted in fulfillment of the
requirements for the award of the degree of
Master of Science (Physics)
Faculty of Science
Universiti Teknologi Malaysia
DECEMBER 2010
To my beloved family...
ACKNOW LEDGEM ENT
Praised be to Allah s .w .t . Peace and Blessing be to Prophet Muhammad s.a.w...
In the preparation of this thesis, I was in contact with numerous researchers
and academicians. They have contributed towards my understanding and thoughts of
the Physics of sensors. In particular, I wish to express my sincere appreciation and
gratitude to my main supervisor, Prof. Dr. Jalil Ali, for his encouragement, guidance,
and friendship. I am also very thankful to my co-supervisor, Dr. Saktioto for his
guidance and motivations. Without their continued support and interest, the
completion of thesis would definitely be impossible.
I wish to express my sincere thanks to the staff of Photonics Research
Laboratory, Fibre Optics Laboratory, Optoelectronics Laboratory, librians of PSZ,
and all staffs from Institute of Advanced Photonics Science and Faculty of Science,
UTM, especially Mr. Ab. Rasid Isnin and Mr. Nasir Abu Bakar, for their help and
cooperation. My sincere appreciation also extends to all my friends who have
provided assistance and various occasions. Their views and suggestions have been
useful indeed.
Last but not least, I am very grateful to all my family members, especially my
sister, Mrs. Faridah Daud for her motivation, financial support, and patience. Also
thanks to the Ministry of Science, Technology and Innovation for supporting this
research via FRGS vote no. 78452.
ABSTRACT
A prototype outdoor fibre Bragg grating (FBG) temperature sensing system
based on a commercial FBG has been designed, constructed, developed, and its
performance evaluated. The commercial FBG sensor head has a center wavelength of
1553.865 nm, 0.24 nm bandwidth, >97 % reflectivity, and a length of (3.0+0.1) cm.
In order to reduce the optical losses of the FBG system, the shortest optical fibre path
used was (55.0+0.1) m. The temperature sensitivity of the system was evaluated for
different placement heights from the rooftop floor with and without the presence of
focusing elements. The TLS was used to provide the broadband light source via a
fibre optic cable of wavelength 1550 nm. The OSA was used to display the
transmission and reflection spectrum from which the Bragg wavelength, XB,
bandwidth, and power dip are obtained. The transmission spectrum was obtained
through direct connection to the FBG and the reflection spectrum using a (2x2) 3dB
coupler. The Bragg wavelength change with temperature changes was used to
determine the FBG system’s performance. Results obtained shows that AXB is
directly proportional to temperature changes for both transmission and reflection
spectrum with and without focusing elements at different placement heights. The
gradient of AXB versus temperature provides the sensitivity of the FBG system. The
average sensitivity of the FBG sensor system measured was 10.0 pm/°C without any
focusing element. It has an average of 12.5 pm/°C when the FBG sensor head was
focused with a convex lens and 13.9 pm/°C when focused with a hand lens. The FBG
sensor system sensitivity was increased when a hand lens was used. The sensitivity is
constant irrespective of the FBG sensor head position from the floor. Thus, a
prototype FBG sensing system has been developed for outdoor temperature
measurements.
ABSTRAK
Satu prototaip gentian parutan Bragg (FBG) bagi sistem penderia suhu di
kawasan terbuka menggunakan FBG komersil telah direkabentuk, dibangunkan,
dimajukan, dan prestasinya dinilai. Panjang deria FBG komersil tersebut mempunyai
panjang gelombang Bragg 1553.865 nm, jalur lebar 0.24 nm, pantulan >97%, dan
(3.0±0.1) sm panjang. Bagi mengurangkan jumlah kehilangan pada sistem ini,
gentian optik sepanjang (55.0±0.1) m telah digunakan. Kepekaan suhu sistem ini
dinilai pada ketinggian yang berbeza dari lantai bumbung dengan menggunakan
kanta penumpu dan tanpa meggunakan sebarang elemen penumpu. TLS
memancarkan sumber cahaya pada nilai tertentu menerusi kabel gentian optik
dengan panjang gelombang 1550 nm. OSA pula memaparkan spektrum pancaran dan
pantulan yang mana panjang gelombang Bragg, AXB, jalur lebar, dan kuasa junam
dapat diukur. Spektrum pemancar diperoleh melalui penyambungan terus dengan
FBG dan spektrum pantulan pula diperoleh dengan menggunakan gandingan (2x2)
3dB gentian optik. Perubahan panjang gelombang Bragg terhadap perubahan suhu
persekitaran digunakan untuk menilai kepekaannya. Keputusan yang diperoleh
menunjukkan perubahan panjang gelombang Bragg adalah berkadar langsung
dengan perubahan suhu bagi kedua-dua spektrum pancaran dan pantulan, dengan
atau tanpa elemen pemfokus, pada ketinggian berbeza. Kecerunan graf AXB lawan
suhu memberi nilai kepekaan sistem tersebut. Purata kepekaan sistem penderia FBG
tersebut adalah 10.0 pm/°C bagi sistem tanpa elemen pemfokus. Purata kepekaannya
bernilai 12.5 pm/°C apabila penderia tersebut ditumpukan dengan kanta cembung
dan 13.9 pm/°C bagi sistem yang ditumpukan dengan kanta tangan. Kepekaan sistem
penderia FBG tersebut meningkat apabila kanta tengan digunakan. Kepekaannya
adalah malar dengan tidak dipengaruhi oleh kedudukan FBG tersebut. Maka, satu
prototaip sistem penderia FBG bagi pengukuran suhu luar telah berjaya dibangunkan.
CHAPTER TITLE PAGE
DECLARATION ii
DEDICATION iii
ACKNOW LEDGEM ENT iv
ABSTRACT v
ABSTRAK vi
TABLE OF CONTENTS vii
LIST OF TABLES x
LIST OF FIGURES xi
LIST OF SYMBOLS xiii
LIST OF APPENDICES xvi
1 INTRODUCTION 1
1.1 Introduction 1
1.2 Background of Study 2
1.3 Problem Statements 4
1.4 Objective 4
1.5 Scope of Study 4
1.6 Significance of Study 5
1.7 Organization of Study 5
LITERATURE REVIEW
2.1 Introduction
2.2 Historical Perspective of Fibre Bragg Grating
7
7
2.3 Fibre Bragg Grating 8
2.4 Fibre Bragg Grating as a Sensor 10
2.5 Overview of Fibre Bragg Grating for Temperature Sensing 11
2.6 Advantages of Fibre Bragg Grating Sensor 13
2.7 Sun Radiation and Spectral Wavelength 15
2.7.1 Composition of Solar Radiation 16
3 THEORY 17
3.1 Introduction 17
3.2 Coupled-Mode Theory 17
3.2.1 Coupled-Mode Equations 19
3.2.2 Coupling Coefficient 20
3.3 Principle of Fibre Bragg Grating 22
3.4 Properties of Fibre Bragg Grating 22
3.4.1 Bragg Wavelength 23
3.4.2 Bragg Grating Bandwidth 24
3.4.3 Bragg Grating Reflectivity 26
3.5 Optical Response of Fibre Bragg Grating 28
3.5.1 Optical Response to Wavelength 29
3.5.2 Optical Response to Strain 29
3.5.3 Optical Response to Temperature 31
3.6 Characteristics of Fibre Bragg Grating 32
3.6.1 Bragg Condition 33
3.6.2 Transmission and Reflection in Fibre Bragg Grating 34
3.6.3 Effective Refractive Index 35
3.6.4 Grating Period 36
4 EXPERIM ENTAL SET-UP 37
4.1 Introduction 37
4.2 Measurement of Fibre Loss 37
4.3 Design of Fibre Bragg Grating Temperature Sensing 39
4.3.1 Tunable Laser Source 40
4.3.2 Optical Spectrum Analyzer 41
4.3.3 Fibre Optic Coupler 42
4.4 Measurement of Fibre Bragg Grating for Temperature
Sensing 43
4.4.1 Measurement of Transmission Spectrum 43
4.4.2 Measurement of Reflection Spectrum 44
5 RESULT AND DISCUSSION 47
5.1 Introduction 47
5.2 Fibre Loss Measurement 47
5.2.1 Fibre Loss Measurement by Experiment 48
5.2.2 Fibre Loss Measurement by Calculation 48
5.3 FBG Reflection and Transmission Spectra at Room
Temperature 50
5.4 Result of Fibre Bragg Grating for Temperature Sensing 52
5.4.1 Effect of Different Placement Height on FBG
Sensitivity 56
5.4.2 Effect of Focusing Element on FBG Sensitivity 58
5.4.2.1 Effect of Convex Lens Focusing on FBG
Sensitivity 59
5.4.2.2 Effect of Hand Lens Focusing on FBG
Sensitivity 60
5.4.2.3 Comparison of Different Focusing Elements 62
5.4.3 Bandwidth of Fibre Bragg Grating 65
5.4.6 Reflectivity of Fibre Bragg Grating 66
5.5 Interpretation and Discussion 67
6 CONCLUSION 70
6.1 Conclusion 70
6.2 Recommendation for Future Work 71
REFERENCES
Appendices A-I
72
77-91
LIST OF TABLES
TABLE NO. TITLE PAGE
Table 4.1 Details of focusing elements 39
Table 5.1 Power meter reading of fibre loss 48
Table 5.2 Sensitivity of the FBG at different height 58
Table 5.3 Effect of focusing convex lens on the sensitivity of FBG at
different placement height from the rooftop floor 60
Table 5.4 Effect of focusing convex lens on the sensitivity of FBG at
different positions from the rooftop floor 62
Table 5.5 Sensitivities of FBG for different focusing elements based on
transmission spectrum 65
Table 5.6 Sensitivities of FBG for different focusing elements based on
reflection spectrum 65
FIGURE NO. TITLE PAGE
Figure 2.1 Reflected and transmitted light in the FBG 9
Figure 2.2 The FBG system 10
Figure 3.1 Illustration of contradirectional coupling 18
Figure 3.2 Schematic of enlarge FBG 23
Figure 3.3 Calculation of FWHM using reflection spectrum 24
Figure 3.4 The FBG system 32
Figure 3.5 Transmission and reflection in FBG 35
Figure 4.1 Experimental set-up for fibre loss measurement 38
Figure 4.2 Schematic of the basic experimental set-up 39
Figure 4.3 Tunable Laser Source model MG9638A 40
Figure 4.4 Optical Spectrum Analyzer model MS9710B 41
Figure 4.5 3-dB fibre optic coupler 42
Figure 4.6 Schematic of experimental set-up to measure transmission spectrum44
Figure 4.7 Schematic of experimental set-up to measure reflection spectrum 45
Figure 4.8 Close-up of the experimental set-up 46
Figure 4.9 Experimental set-up for FBG temperature sensing 46
Figure 5.1 Reflection spectrum provided by the manufacturer, QPS
Photonics Inc, Canada 50
Figure 5.2 Reflection spectrum at 23 °C 51
Figure 5.3 Transmission spectrum at 23 °C 51
Figure 5.4 Examples of transmission and reflection spectrum 54
Figure 5.5 Examples of calculation of Bragg wavelength shift 55
Figure 5.6 Shift of Bragg wavelength in different temperature for transmission
spectrum 57
Figure 5.7 Shift of Bragg wavelength in different temperature for reflection
spectrum
Figure 5.8 Effect of a focusing convex lens on the Bragg wavelength for
different temperature from the transmission spectrum 59
Figure 5.9 Effect of focusing convex lens on the Bragg wavelength shift from
the reflection spectrum 59
Figure 5.10 Effect of hand lens focusing on the Bragg wavelength shift at
different temperature obtained from the transmission spectrum 61
Figure 5.11 Effect of hand lens focusing on the Bragg wavelength shift at
different temperature obtained from the reflection spectrum 61
Figure 5.12 Comparison of Bragg wavelength shift at 2.0 cm height 62
Figure 5.13 Comparison of Bragg wavelength shift at 30.0 cm height 63
Figure 5.14 Mean sensitivity of FBG for unfocused system 64
Figure 5.15 Mean sensitivity of FBG that focused with convex lens 64
Figure 5.16 Mean sensitivity of FBG that focused with hand lens 64
Figure 5.17 Bandwidth versus temperature for different FBG placement height 66
Figure 5.18 Reflectivity of FBG at different temperature 67
a - Core radius
b - Wavelength position at 0 °C
CMT - Coupled-mode theory
d - Power dip
dB - Decibel
DNA - Acid deoksibonukleid
E - Electric field
EMI - Electromagnetic interference
FBG - Fibre Bragg grating
FBGs - Fibre Bragg gratings
FOS - Fibre optic sensor
FWHM - Full width at half maximum
H - Magnetic field
HiBi - High birefringent
K - Grating momentum vector
k - Propagating constant vector
k - Propagating constant
k ! - Modal wavevector of the forward-propagating wave
k 2 - Modal wavevector of the backward-propagating wave
L - Grating length
MHz - Mega Hertz
Mp - Fraction of fibre mode power
N - Number of grating plane
NA - Numerical aperture
nm - nanometer
ncl - Cladding refractive index
nco - Core average index
neff - Effective refractive index
no - Average refractive index
ni - Refractive index of fibre core
n2 - Refractive index of fibre cladding
OSA - Optical Spectrum Analyzer
pm/°C - pikometer per degree Celsius
R - Reflectivity
R(L,X) - Reflectivity in the function of length and wavelength
s - Fringe visibility of the index change
T - Temperature
TLS - Tunable Laser Source
UV - Ultraviolet
UVA - Ultraviolet A
UVB - Ultraviolet B
UVC - Ultraviolet C
z - Distance along the fibre longitudinal axis
e - Permittivity
U - Permeability
ez - Strain
a - Thermo-expansion coefficient
£ - Thermo-optic coefficient
Q - Coupling coefficient
A - Grating period
Ag - Grating spacing
Apm - Phase mask period
X - Wavelength
XB - Bragg wavelength
Xin - Incident light
XB,0 - Nominal Bragg wavelength
Xo - Initial wavelength
SL - Change of length
dneff - Change of refractive index
Ak - Detuning wavevector
An - Dept of index modulation
A T - Temperature change
AXb - Bragg wavelength shift
°C - Degree Celsius
°F - Degree Farenheigh
% - Percentage
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Standard sheet of MG9638A wavelength variable laser source 77
B Wavelength standard of MS9710B Optical Spectrum Analyzer 78
C Details of fibre optic coupler 79
D Data of transmission spectrum without focusing element 80
E Data of reflection spectrum without focusing element 82
F Data of transmission spectrum with convex lens focusing element 84
G Data of reflection spectrum with convex lens focusing element 86
H Data of transmission spectrum with hand lens focusing element 88
I Data of reflection spectrum with hand lens focusing element 90
INTRODUCTION
1.1 Introduction
Over the last five decades, fibre optic sensors (FOS) have emerged as modern
device in sensing and telecommunication technologies. FOS can be classified as
fluorescent/spectrally-based, intensity-based, or interferometric. Fluorescent-decay
temperature sensor, blackbody radiation temperature sensor, Fabry-Perot temperature
sensor, interferometric temperature sensor, polarimetric temperature sensor, and dual
mode temperature sensor are examples of sensors used in fibre optic temperature
sensing. Fibre Bragg grating (FBG) has the unique advantage as an fibre optic
temperature sensor as compared with all other temperature sensors (Neil, 1990).
FBG is used in measuring the temperature accurately in a variety of
environment (Yonghang, 2004) which includes in harsh environment, underground,
and also disaster places. Fibre Bragg grating is a type of distributed Bragg reflector
constructed in a short segment of optical fibre that reflects a particular wavelength of
light and transmits all the other wavelengths. This is achieved by appending a
periodic variation to the refractive index of the fibre core, which generates a
wavelength of specific dielectric mirror. Thus, an FBG can be use as an inline optical
filter to block certain wavelength or as a wavelength-specific reflector.
The most important innovation of the 1990’s in FOS is the development of
FBG sensors. Fibre Bragg grating can used as a sensor because of its excellent
unique properties. FBG emerges as an important technology in fibre optic sensing
due to its smart structure, excellent linear characteristics, immune to electromagnetic
interference (EMI), low fibre loss, and other outstanding advantages (Lai et al.,
2002). Nowadays, decent and sophisticated techniques in temperature measurement
become very important and necessary for safety precautions in industrial
requirements (Bowei, 2004).
The FBG sensitivity measurements will be assessed for different FBG’s
position from the rooftop floor using different focusing elements at periods of the
day. The research will examine the effect of different focusing elements, FBG
positions, and temperature variations on the sensor head of the FBG. Measurements
made will be based on the transmission and reflection spectrum of the FBG.
1.2 Background of Study
A sensor is a device that measures a physical quantity and converts it into a
signal which can be read by an instrument. For example, mercury in glass
thermometer converts the measured temperature into expansion and contraction of
the liquid which can be read on a calibrated glass tube. A thermocouple converts the
temperature measurement to an output voltage which can be read by a voltmeter. For
accuracy purposes, most sensors are calibrated against known standards.
Temperature is an important and most commonly measured parameter in
everyday applications. Traditionally, the semiconductor sensor, platinum resistance
sensor, thermistor, and thermocouple are most commonly utilized for temperature
measurements. For most applications, these conventional temperature sensors can be
adequate for its purpose. However, these temperature sensors are not reliable due to
the lack of intrinsic safety, their characteristics of electrically active, and poor
lifetime at excessive temperatures (Yonghang, 2004). Furthermore, these
conventional temperature sensors are all point sensors. In other words, they are
localized sensors, which can only provide temperature reading over a small area
rather than providing an overall temperature profile (Udd, 1995). These sensors are
also suitable for the passive multiplexing, but difficult in practice, due to size
limitations (Xiaopei et al., 2004).
Optical fibre offers a number of distinguishing and excellent advantages over
conventional sensors. Fibre Bragg grating (FBG) sensor is one of the most important
and useful optical fibre sensors. The concept of FBG was discovered three decades
ago by Hill and his co-workers (Hill, 1978). However, most of the recognized
pioneering work about FBG and its applications were only published a decade later
after it’s discovery by a group of researchers at United Technology Research Centre
(Meltz et al., 1989).
FBG sensors are dielectric and virtually immune to electromagnetic
interference (EMI). It can withstand against hostile environment, including in high
and excessive temperature. It can be used to measure high temperature, until 1000 °C
(Hirayama et al., 2000). It is also utilized as a distributed sensor. In principle,
distributed sensors are different from point sensors. Here the parameter of interest is
measured with certain spatial resolution at any points along a single optical fibre.
They would permit the use of power sensors and represents more effective use of
optical fibre that are used for both measurements and data transmission, which
allows them to become more attractive.
FBG sensors can be incorporated into optical fibre cables. These sensors can
be embedded into new structure or surface bonded onto an existing structure. This
allows real time monitoring of structures (Tahir et al., 2005), ultimately leading to
truly ‘smart’, and provide fatigue data for subsequent analysis (Everal, 2000). FBGs
are able to play a crucial role in sensing technology, due to their unique smart
structure (Lai, 2002).
1.3 Problem Statem ent
The main motivation of this research is to design, construct, and develop
temperature sensor using a commercial fibre Bragg grating (FBG) sensor under
outdoor condition. To this end, a commercial germanium-doped silica fibre Bragg
grating is used. This research will examine the effect of temperature variations on the
characteristics of an outdoor FBG temperature sensor. How does the characteristics
of FBG sensor respond to changes under different environment conditions and
placement heights of FBG? How does the focusing elements such as convex lens and
hand lens affect the sensitivity of FBG? These investigations can be used to
determine the performance of FBG sensor for outdoor temperature measurement. Its
performance will be evaluated based on the transmission and reflection spectrum.
1.4 Objective
The objective of this research is to design, construct, and develop an outdoor
FBG temperature sensor system for different FBG placement heights from the
rooftop floor. The effect of FBG at different heights from the rooftop floor will be
evaluated. The research will also analyze the effect of focusing elements on the FBG
sensor head based on the transmission and reflection spectrum.
1.5 Scope of Study
This research starts with a literature review on FBG. The literature review
will focused on temperature sensing of FBG and its sensitivity. The theory involved
in temperature sensing will be discussed. The principle of FBG is based on coupled
mode theory (CMT). The FBG spectrum at room temperature were taken for both
transmission and reflection spectrum as the reference for Bragg wavelength shift.
The measurement of FBG temperature sensing was made at the different placement
heights of FBG with different focusing elements used at the sensor head. The convex
lens and hand lens were used as the focusing elements in this research. The
performance or sensitivity of the FBG will be analyzed in terms of the Bragg
wavelength shift.
1.6 Significance of Study
This research enables us to understand FBG temperature sensing performance
by examining the characteristics and properties of the FBG. Initially studies shows
that there are rapid fluctuations in outdoor temperature. The sudden change in
outdoor conditions such as temperature variation, rain, and effect of wind blowing
may affect the stability of the reading.
With the increasing needs for health monitoring in structures such as bridges,
tunnels, highways, dams, aircraft wings, and spacecraft fuel tanks, it is imperative to
design and develop an effective sensor system which can detect any sudden changes
in strain, pressure, and temperature. The practical challenges here may include
decoupling at different sensing parameters for long term thermal stability of the FBG
under outdoor environment. This enables us to apply the FBG temperature sensor in
a variety of fields, such as medical, construction, manufacturing, industry, and many
more.
1.7 Organization of Study
This thesis consists of six chapters and begins with a brief introduction of the
research background and work undertaken as described in Chapter 1. It includes the
background study, problem statement, objective, scope, and significance of the study.
The literature review on FBG fundamental, an overview of FBG for temperature
sensing, the properties of sun radiation and spectral wavelength are described in
Chapter 2. In Chapter 3, the theory of FBG will be discussed in detail. This includes
the coupled-mode theory, the properties, optical response, and characteristics of
FBG. Chapter 4 describes the FBG experimental set-up and the research
methodology used in this research. Chapter 5 gives the results, analysis, and
discusses the performance testing evaluation of FBG for outdoor temperature
sensing. The Bragg wavelength response with respect to the temperature variation is
analyzed. Parameters such as sensitivity, bandwidth, and reflectivity are analyzed for
different FBG’s height placement from the rooftop floor and for different focusing
elements placed on the FBG sensor head. Finally, Chapter 6 provides the conclusion
of this study and suggestion for further work.
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