SU ZA IRI BIN DAUD
SUZAIRI BIN DAUD
A thesis submitted in fulfillment o f the requirements for the award o f the degree of
Master o f Science (Physics)
Faculty o f Science Universiti Teknologi Malaysia
A C K N O W LE D G E M E N T
Praised be to Allah s . w . t . Peace and Blessing be to Prophet Muhammad s.a.w...
In the preparation o f this thesis, I was in contact with numerous researchers and academicians. They have contributed towards my understanding and thoughts of the Physics o f 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. W ithout their continued support and interest, the completion o f thesis would definitely be impossible.
I wish to express my sincere thanks to the staff o f Photonics Research Laboratory, Fibre Optics Laboratory, Optoelectronics Laboratory, librians o f PSZ, and all staffs from Institute o f Advanced Photonics Science and Faculty o f 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.
A BSTRA CT
A BSTR A K
C H A P T E R T IT L E PA G E
D EC LA R A TIO N ii
D ED IC A TIO N iii
A C K N O W LE D G E M E N T iv
A B STR A C T v
A BSTRA K vi
TA BLE O F C O N TEN TS vii
L IST O F TABLES x
L IST O F F IG U R ES xi
L IST O F SYM BOLS xiii
L IST O F A PPEN D IC ES xvi
1 IN T R O D U C TIO N 1
1.1 Introduction 1
1.2 Background o f Study 2
1.3 Problem Statements 4
1.4 Objective 4
1.5 Scope o f Study 4
1.6 Significance o f Study 5
1.7 Organization o f Study 5
LIT E R A T U R E R E V IE W 2.1 Introduction
2.2 Historical Perspective of Fibre Bragg Grating
2.3 Fibre Bragg Grating 8
2.4 Fibre Bragg Grating as a Sensor 10
2.5 Overview o f Fibre Bragg Grating for Temperature Sensing 11 2.6 Advantages o f Fibre Bragg Grating Sensor 13 2.7 Sun Radiation and Spectral Wavelength 15 2.7.1 Composition o f Solar Radiation 16
3 T H E O R Y 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 o f Fibre Bragg Grating 22
3.4 Properties o f Fibre Bragg Grating 22
3.4.1 Bragg W avelength 23
3.4.2 Bragg Grating Bandwidth 24
3.4.3 Bragg Grating Reflectivity 26
3.5 Optical Response o f 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 o f 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 EX PE R IM E N T A L SET-U P 37
4.1 Introduction 37
4.2 M easurement o f Fibre Loss 37
4.3 Design o f Fibre Bragg Grating Temperature Sensing 39
4.3.1 Tunable Laser Source 40
4.3.3 Fibre Optic Coupler 42 4.4 M easurement o f Fibre Bragg Grating for Temperature
Sensing 43
4.4.1 M easurement o f Transmission Spectrum 43 4.4.2 M easurement o f Reflection Spectrum 44
5 R ESU L T AND D ISCU SSIO N 47
5.1 Introduction 47
5.2 Fibre Loss M easurement 47
5.2.1 Fibre Loss M easurement by Experiment 48 5.2.2 Fibre Loss M easurement by Calculation 48 5.3 FBG Reflection and Transmission Spectra at Room
Temperature 50
5.4 Result o f Fibre Bragg Grating for Temperature Sensing 52 5.4.1 Effect o f Different Placement Height on FBG
Sensitivity 56
5.4.2 Effect o f Focusing Element on FBG Sensitivity 58 5.4.2.1 Effect o f Convex Lens Focusing on FBG
Sensitivity 59
5.4.2.2 Effect o f Hand Lens Focusing on FBG
Sensitivity 60
5.4.2.3 Comparison o f Different Focusing Elements 62 5.4.3 Bandwidth o f Fibre Bragg Grating 65 5.4.6 Reflectivity o f Fibre Bragg Grating 66
5.5 Interpretation and Discussion 67
6 C O N C LU SIO N 70
6.1 Conclusion 70
6.2 Recommendation for Future W ork 71
R E FE R E N C E S Appendices A-I
L IS T O F TABLES
TA BLE NO. T IT L E PA G E
Table 4.1 Details o f focusing elements 39
Table 5.1 Power meter reading o f fibre loss 48
Table 5.2 Sensitivity o f the FBG at different height 58 Table 5.3 Effect o f focusing convex lens on the sensitivity o f FBG at
different placement height from the rooftop floor 60 Table 5.4 Effect o f focusing convex lens on the sensitivity o f FBG at
different positions from the rooftop floor 62
Table 5.5 Sensitivities o f FBG for different focusing elements based on
transmission spectrum 65
Table 5.6 Sensitivities o f FBG for different focusing elements based on
FIG U R E NO. T IT L E PA G E
Figure 2.1 Reflected and transmitted light in the FBG 9
Figure 2.2 The FBG system 10
Figure 3.1 Illustration o f contradirectional coupling 18
Figure 3.2 Schematic o f enlarge FBG 23
Figure 3.3 Calculation o f 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 o f 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 o f experimental set-up to measure transmission spectrum44 Figure 4.7 Schematic o f experimental set-up to measure reflection spectrum 45
Figure 4.8 Close-up o f 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 5.6 Shift o f Bragg wavelength in different temperature for transmission
spectrum 57
Figure 5.7 Shift o f Bragg wavelength in different temperature for reflection spectrum
Figure 5.8 Effect o f a focusing convex lens on the Bragg wavelength for
different temperature from the transmission spectrum 59 Figure 5.9 Effect o f focusing convex lens on the Bragg wavelength shift from
the reflection spectrum 59
Figure 5.10 Effect o f hand lens focusing on the Bragg wavelength shift at
different temperature obtained from the transmission spectrum 61 Figure 5.11 Effect o f hand lens focusing on the Bragg wavelength shift at
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
FW H M - 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 o f the forward-propagating wave k 2 - Modal wavevector o f 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
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 o f 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 o f length
dneff - Change o f refractive index
An - Dept of index modulation
A T - Temperature change
AXb - Bragg wavelength shift
°C - Degree Celsius
°F - Degree Farenheigh
L IS T O F A PPEN D IC ES
A PPEN D IX T IT L E PA G E
A Standard sheet o f MG9638A wavelength variable laser source 77 B Wavelength standard o f MS9710B Optical Spectrum Analyzer 78
C Details o f fibre optic coupler 79
IN T R O D U C TIO N
1.1 In tro d u ctio n
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).
The most important innovation o f 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 FB G ’s position from the rooftop floor using different focusing elements at periods of the day. The research will examine the effect o f 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 B ackground 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.
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 o f the recognized pioneering work about FBG and its applications were only published a decade later after it’s discovery by a group o f 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.
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 o f FBG? How does the focusing elements such as convex lens and hand lens affect the sensitivity o f 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 O bjective
The objective o f 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 o f focusing elements on the FBG sensor head based on the transmission and reflection spectrum.
1.5 Scope of Study
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 o f the FBG will be analyzed in terms o f 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 o f fields, such as medical, construction, manufacturing, industry, and many more.
1.7 O rganization of Study
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