DEVELOPMENT OF FIBER BRAGG GRATING BASED TEMPERATURE

SENSOR SYSTEM

LIAU QIAN YU

UNIVERSITI TEKNOLOGI MALAYSIA

“I hereby declare that I have read this thesis and in my opinion this thesis is

sufficient in term of scope and quality for the award of the degree of Bachelor of

Engineering (Electrical-Telecommunication).”

Signature : …………………………………….

Name of Supervisor : ……………………………………

Date : …………………………….............

DR. ASRUL IZAM BIN AZMI

DEVELOPMENT OF FIBER BRAGG GRATING BASED TEMPERATURE

SENSOR SYSTEM

LIAU QIAN YU

A thesis submitted in fulfilment of the requirements for the award of the degree of

Bachelor of Engineering (Electrical-Telecommunication)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

JUNE 2013

ii

I declare that this thesis entitled “Development of Fiber Bragg Grating Based

Temperature Sensor System” 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 : …………………………………….

Name of Student : ……………………………………

Date : …………………………….............

LIAU QIAN YU

iii

DEDICATION

To My beloved Mother

For her endless support

My beloved Father

For his worship and faith in me

My beloved Brother

For motivating me

My beloved Friends

This thesis is dedicated to them

iv

ACKNOWLEDGEMENT

I wish to express my deepest gratitude to my supervisor, Dr. Asrul Izam Bin

Azmi for the patience, supervision, and support I received from him during this

project. I would not have completed my project successfully without his guidance.

Also I would like to heartily thank PHD student, Siti Musliha and technician, Mr.

Nasir who offered helpful advice and support to me. Finally, my warmest

appreciation goes to my family, who gave me their support and encouragement

during this project.

v

ABSTRACT

The development of Fiber Bragg Grating (FBG) sensing technique has

improved significantly especially in the sensor head design and real-time data

acquisition technique. FBG is basically an optical filter that reflects or transmits light

at particular predetermined wavelengths. The FBG wavelength is sensitive to several

measurands including strain, temperature and pressure. This project presents the

development of a Labview program for real-time data acquisition of FBG

temperature sensor system. The Labview program is capable to record the FBG

wavelength shift from an Optical Spectrum Analyzer (OSA), subsequently calculate

the corresponding temperature change and record the important parameters of the

FBG. This project also presents the development of a simple and cost effective

packaging technique that further enhances the performances of the FBG sensor. The

packaged sensor overcomes the nonuniform heat distribution of a bare fiber that

causes eccentric response of FBG spectrum. Therefore, the packaged fiber could be

operated for a localized area with high temperature differential. The packaging also

compensates the unwanted strain effect from the surrounding which makes

temperature measurement become more accurate. The experimental works have been

successfully carried out to demonstrate the system operation and the packaging

functionalities.

vi

ABSTRAK

Perkembangan penderiaan teknik Fiber Bragg Grating (FBG) telah meningkat

dengan ketara terutamanya dalam reka bentuk kepala penderia dan teknik

pemerolehan data masa nyata. FBG asasnya merupakan fiber optik yang memantul

atau menghantar cahaya pada panjang gelombang tertentu yang telah ditetapkan.

Panjang gelombang FBG sensitif kepada daya, suhu dan tekanan. Projek ini

menggambarkan pembangunan satu pengaturcaraan LabVIEW untuk pemerolehan

data masa nyata sistem penderiaan suhu FBG. Pengaturcaraan LabVIEW ini mampu

merekodkan perubahan gelombang FBG dari Spectrum Analyser optik (OSA),

kemudiannya mengira perubahan suhu dan merekodkan parameter penting FBG.

Projek ini juga menggambarkan pembangunan satu teknik pembungkusan yang

mudah dan berkesan tetapi dengan kos yang rendah untuk meningkatkan prestasi

penderiaan suhu FBG ini. Penderiaan yang dibungkus mengatasi pengedaran haba

tak seragam daripada fiber kosong yang menyebabkan keanehan pada spectrum FBG.

Pembungkusan juga mengkompensasi kesan tekanan yang tidak diingini dari

sekeliling untuk meningkatkan ketepatan pengukuran suhu pada penderia FBG.

Kesimpulannya, kerja-kerja eksperimen telah berjaya dilaksanakan untuk

menunjukkan operasi sistem dan kefungsian pembungkusan.

vii

TABLE OF CONTENTS

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENTS iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES x

LIST OF FIGURES xi

LIST OF ABBREVIATIONS xiii

LIST OF SYMBOLS xiv

LIST OF APPENDICES xv

1 INTRODUCTION 1

1.1 Motivation and Significance of Work 1

1.2 Problem Statements 1

1.3 Project Objectives 2

1.4 Project Scope 4

1.5 Work Breakdown 4

1.6 Work Flow 6

1.7 Gantt Chart 8

1.8 Thesis Organization 10

viii

2 LITERATURE REVIEW 11

2.1 Optical Fiber 12

2.2 Fiber Optic Sensor 13

2.2.1 Extrinsic and Intrinsic Fiber Optic Sensor 13

2.2.2 Intensity Modulated Sensor and Phase

Modulated Sensor

15

2.2.3 Advantages of FOS over Conventional

Electrical Sensor

15

2.3 Fiber Bragg Gratings 16

2.3.1 Properties of FBG 16

2.3.2 Types of FBG 19

2.3.3 Applications of FBG Sensors 21

2.4 Packaged Technique of FBG Temperature Sensor 22

3 METHODOLOGY 24

3.1 Experimental Setup of FBG Temperature Sensor 25

3.1.1 Process of Splicing Fiber 27

3.1.2 National Instruments GPIB-USB Cable 29

3.2 Labview Program 30

3.3 Fabrication of FBG Sensor Head Packaging 32

3.4 Tests of the FBG Temperature Sensor 34

4 RESULT AND DISCUSSION 36

4.1 Uniform Heat Test 37

4.2 Nonuniform Heat Test 40

5 CONCLUSION AND RECOMMENDATION 44

5.1 Conclusion 44

5.2 Recommendation 45

ix

REFERENCES 46

Appendices A 48

x

LIST OF TABLES

TABLE NO. TITLE PAGE

3.1 Specification of the Equipments and Component of the

Experimental Setup

26

3.2 Description of Saved Text Document 31

3.3 Specification of the Packaging Materials 33

xi

LIST OF FIGURES

FIGURES NO. TITLE PAGE

1.1 Work breakdown 5

1.2 Work Flow 7

1.3 Gantt chart for first semester 8

1.4 Gantt chart for second semester 9

2.1 Schematic of optical fiber 12

2.2 Fiber optic sensor system 13

2.3 Extrinsic FOS 14

2.4 Intrinsic FOS 14

2.5 Schematic diagram of structure and spectral response of

FBG

17

2.6 Common Bragg grating 19

2.7 Blazed Bragg Grating 20

2.8 (a) Chirped Bragg grating with aperiodic grading planes

spacing

20

2.8 (b) Chirped Bragg grating with a cascade of several

gratings with increasing period

20

3.1 Experimental setup 25

3.2 Process of splicing fiber 27

3.3 Fiber stripper 27

3.4 Alcohol and wipes 28

3.5 Fiber cleaver 28

3.6 Fiber splicer 29

3.7 National Instruments GPIB-USB cable 29

xii

3.8 Graphical user interface of the Labview data

acquisition program

30

3.9 Example of the saved data files of the Labview

program

31

3.10 Schematic of the sensor head packaging 32

3.11 The packaged FBG sensor head 34

3.12 Flow of the experiments 35

4.1 Temperature characteristics curve of bare and packaged

sensor

38

4.2 (a) Temperature measurement of bare FBG sensor 39

4.2 (b) Temperature measurement of packaged FBG sensor 40

4.3 (a) FBG transmission spectra at different stages of

experiment of bare FBG sensor in nonuniform heat test

41

4.3 (b) FBG transmission spectra at different stages of

experiment of packaged FBG sensor in nonuniform

heat test

42

4.4 Temperature measurement of packaged FBG in

nonuniform heat test

43

xiii

LIST OF ABBREVIATIONS

FBG - Fiber Bragg Grating

FOS - Fiber Optic Sensor

EMI - Electro-Magnetic Interference

FOG - Fiber-optic gyroscope

WDM - Wavelength Division Multiplex

TDM - Time Division Multiplex

UV - Ultraviolet

SMF - Single Mode Fiber

OSA - Optical Spectrum Analyzer

GPIB - General Purpose Interface Bus

USB - Universal Serial Bus

I/O - Input and Output

SLD - Superluminescent Diode

DSP - Digital Signal Processing

xiv

LIST OF SYMBOLS

n - Refractive index

Λ - Grating period

λB - Bragg wavelength

λ Wavelength

neff - Effective refractive index

𝛥𝜆𝐵 - Shift of Bragg wavelength

𝑝𝑒 - Effective strain-optic constant

p11 and p12 - Strain-optic tensor

ν - Poisson’s ratio

𝛥𝑇 - Change of temperature

αΛ - Thermal expansion coefficient

αn - Thermo-optic coefficient

θ - Degree of angle

α - Thermal expansion coefficient

ξ - Thermo-optic coefficient

αsub - Thermal expansion coefficient of packaging material

peff - Effective photo-elastic coefficient

°C - Degree Celcius

xv

LIST OF APPENDICES

APPENDIX TITLE PAGE

A Block diagram of the Labview program 48

CHAPTER 1

INTRODUCTION

1.1 Motivation and Significance of Work

In the past several decades, the development of fiber optic sensor (FOS) has

expanded and improved significantly. Peoples are showing great interest on FOS

because the attractive advantages of FOS such as low loss, low cost, low

maintenance, small size, lightweight, high bandwidth, EMI immunity, safety, etc.

Therefore, there are a number of intensive researches on developing the technique of

FOS such as Fiber Bragg grating (FBG), Fiber-optic gyroscopes (FOG), fiber-optic

current sensors, etc. Among these types of techniques, FBG is the most widely used

technique.

FBG has highly accepted and practiced in various of sensing and monitoring

fields such as industrial sensing, bio-medical device, mechanical and civil

engineering, aerospace, oil & gas, etc. Compared with other techniques, FBG shows

a number of distinguishing advantages: Wavelength Encoded; Self-referencing;

Linear output; small and lightweight; WDM & TDM Multiplexing; Mass producible;

durable; Single-& Multi-Point Sensing. Due to these superior advantages, FBG

shows enormous potential of temperature, strain, pressure and radiation effect of

sensing in smart structures and polymeric materials. FBG-based sensors have

2

become one of the most popular in the market now because of their relatively low

cost, flexibility of design to Single or Multi-Point sensing arrays and easy to

multiplex.

FBG sensors are high temperature tolerance therefore, it also being used in

remote sensing (oil wells, power cables, pipelines, space stations, etc.) which have to

stand the high temperature. Due to this FBG temperature sensors are widely used in

oil & gas industry. Since oil & gas industry is one of the important industry in our

country it is important or necessary to develop and improve the performance of FBG

temperature sensing system. Before begin the research regarding improve the

performance and develop new approaches of FBG temperature sensing system

research on the basis FBG temperature sensing system is required.

This research seeks to develop a real-time data acquisition of FBG

temperature sensor system and presents a simple and economical sensor head

packaging technique to improve the sensing performance of the temperature sensor.

A comprehensive literature review that provides the issues and knowledge to develop

the FBG temperature sensor system will be covered in this research.

1.2 Problem Statements

First and foremost, a real-time acquisition programming is needed to

interrogate optical Fiber Bragg Grating (FBG) temperature sensor. This program

must able to calculate the temperature change from the shift of Bragg grating

wavelength and record the shape of reflected/transmission spectrum. It is important

in determining the accuracy of the sensor.

In the reality, the bare FBG sensor cannot directly be applied in practical

environment because of the following reasons:

Optical fiber is made of glass, therefore it is extremely fragile.

3

Experience the cross-effect from other measurands such as strain and

pressure.

Low sensitivity and accuracy.

Less linearity and repeatability on the response characteristics of sensor.

For the case of temperature sensor, a nonuniform heat distribution on a bare

FBG sensor that causes eccentric behaviour of FBG spectrum also will drive the

system to make an error in determine the temperature change. To develop a

practicable FBG sensor, these issues must be overcome. Therefore, a packaging

technique for the FBG sensor head using suitable material is required. The packaging

technique also will compensate the cross-effects of the strain from surrounding.

Moreover, the sensitivity of the FBG sensor will be increased due to the higher

thermal expansion of the packaging material.

1.3 Project Objectives

The objectives of this project are to:

i. Develop a Labview programming for real-time data acquisition of

FBG temperature sensor system.

ii. Develop a simple and cost effective packaging technique that further

enhances the performances of the FBG sensor.

iii. Experiment the packaged sensor to verify the functionalities of the

developed packaging technique.

4

1.4 Project Scope

The scope of this project will include three aspects, which are software,

hardware, and experiment. In the software aspect, a Labview based real-time data

acquisition programming is to be developed. For the hardware aspect, a packaging

for FBG temperature sensor head has to be fabricated. Lastly, the bare and the

packaged FBG temperature sensor have to undergo two test, which are the uniform

test and nonuniform test.

1.5 Work Breakdown

To achieve all of the objectives in this project three categories of tasks have

to be done as shown in the Figure 1.1. The three categories are literature review,

design, and implementation. In the earlier stage of this project, I have to do the

literature review on the topics related to my project. The sources of my literature

review included books, articles, scientific journals and thesises. From the literature

review I able to gain sufficient ideas, methods, algorithms and theories for proceed to

my project designs.

Firstly, I have to design a Fiber Bragg Grating (FBG) based temperature

sensor system, the implemented system is used to test the performance of the FBG

temperature sensor. A labview based real-time acquisition coding is developed and

interfaced with the FBG temperature sensor system to calculate the corresponding

temperature change and record the important parameter of the transmitted spectrum.

Those algorithms related to the FBG temperature sensor are applied to the Labview

coding for the calculation of the temperature change relative to the wavelength shift.

Finally, the proposed FBG temperature sensor head design is fabricated to enhance

the performance of the temperature sensor. In the process of designing the sensor

5

head packaging the tradeoffs on the budget and the performance of the chosen

materials is the significant issue.

Figure 1.1: Work breakdown

Main Process

Literature Review

Fiber Optic Sensor

Fiber Bragg Grating (FBG)

FBG Temperature Sensor

FBG Sensor Head Packaging Technique

Labview Software

Design

FBG Temperature Sensor System

Software Algorithm

Architecture of Sensor Head Packaging

Implementation

Labview Coding

System Setup

Fabrication of Sensor Head Packaging

6

1.6 Work Flow

The workflow of this project was simplified and shown in the Figure 1.2. The

procedures shown in the figure are continuing. This project is completed within two

semesters that is approximately one year.

The foremost for the workflow of this project is the literature review and

theoretical study related to this project. Next, I proceed to development of Labview

programming and also FBG temperature sensor system. The programming and the

system are tested and optimized to improve the performance of the FBG sensor

system. The characteristics response and transmitted spectrum of bare FBG sensor

are recorded before the fabrication of the sensor head packaging on the bare FBG

sensor head. Similarly, I tested the packaged FBG sensor and optimized its

performance again.

After the FBG temperature sensor system is completed and the performance

of the FBG sensor is satisfied, I presented the outcome of this project and also

demonstrated it on the Telecommunication Exhibition 2013 (TED 2013).

7

Figure 1.2: Work Flow

Literature Review and Theoretical Study

Development of Labview Programming

Development of FBG Temperature System

Testing and Optimization

Fabrication of Sensor Head Packaging

Testing and Optimization

Presentation and Demonstration

Report Writing

8

1.7 Gantt Chart

These following gantt charts shown the timetable for this project in the two

semester’s time.

Weeks

Activities

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Final Year

Project

briefing

Brief idea of

project

Literature

review and

theoretical

study

Project

proposal and

planning

Revise of

project

proposal

Labview

programming

design

Hardware

design

Presentation

Report of

proposed

project

Figure 1.3: Gantt chart for first semester

9

Weeks

Activities

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Development

of Labview

programming

Development

of the system

setup

Fabrication

of the

hardware

Testing and

optimization

Experiment

Collecting

and

analysing

data

Presentation

Report of the

project

Figure 1.4: Gantt chart for second semester

10

1.8 Thesis Organization

There are five chapters in this thesis. Chapter 1 is the introduction of the

project and work planning for the project. Subsequently, Chapter 2 represents the

general background of optical fiber systems which included the overviews of the

fiber optic sensor, basic principles of Fiber Bragg Grating, and the packaged

technique of FBG sensor. In Chapter 3, the approach of the project is explained

clearly. Following chapter presents the result and discussion of this project. Finally,

Chapter 5 will conclude this project and proposed some recommendation for future

research.

CHAPTER 2

LITERATURE REVIEW

In the early 1970’s researchers studying multi-access single-fiber

communications architectures were first realize that light guided by an optical fiber

could be modulated by direct physical perturbation and that this could be utilized in a

variety of sensing applications [1]. After this discovery, various ideas and techniques

regarding the fiber optic sensor have been proposed by researchers. The inherent

advantages of fiber optic sensors, which include lightweight, small size, electrically

passive, low power ,nonconductive, resistant to electromagnetic interference, high

sensitivity, high bandwidth, and their environmental ruggedness, were successfully

make it accepted and commercial by the market [2].

12

2.1 Optical Fiber

Basically optical fiber is a transparent and flexible fiber made of silica or

plastic. Geometrically, it is thinner than a human hair. It functions as a waveguide of

light, to transmit light between the two ends of fiber with a very little light loss at

certain wavelengths. The schematic of optical fiber in Figure 2.1, which shows a

transparent core with a refractive index n1, is surrounded by a transparent cladding

of slightly lower refractive index n2 [3]. The interference between the core and

cladding is acts as a perfect mirror. The light is transmitted within the core by total

internal reflection. The outer buffer coating provides extra protection to the fiber

from the external conditions and physical damage.

Figure 2.1: Schematic of optical fiber

13

2.2 Fiber Optic Sensor

Figure 2.2 is illustrated the basic of fiber optic sensor system. The light from

the light source is transmitted to a modulated region by optical fiber. In the

modulated region, the light is modulated by physical, chemical, or biomedical

phenomena, and the light is transmitted back to a detector and therein the light is

demodulated [4].

Figure 2.2: Fiber optic sensor system

2.2.1 Extrinsic and Intrinsic Fiber Optic Sensor

Fiber optic sensor (FOS) basically grouped into two classes which is extrinsic

and intrinsic. The extrinsic FOS is basically optical sensor where the light leaves the

fiber, passes through some external transduction element, and is then recoupled back

into fiber. The intrinsic FOS is the optical sensor where the modulation of the optical

14

signal occurs while the light is guided within the fiber. In the simpler word, the

external transduction element in the extrinsic FOS is responsive to the measurand,

whereas the fiber of intrinsic FOS itself is sensitized to the measurand field. The

benefit of extrinsic FOS is their ability to reach places which is unable to access,

such as the temperature inside aircraft jet engines or electrical transformers. The

major advantage of intrinsic FOS is they can be used in the applications that have to

be prohibitive by electrical signal.

Figure 2.3: Extrinsic FOS

Figure 2.4: Intrinsic FOS

15

2.2.2 Intensity Modulated Sensor and Phase Modulated Sensor

According to principle of operation the extrinsic and intrinsic FOS can be

further divided in two categories, there are intensity modulated and the phase

modulated sensors. Intensity modulated sensors works by letting a physical

disturbance cause a change in the received light through an optical fiber. Phase

modulated sensors or interferometric work by comparing the phase of light in the

sensing fiber with a reference fiber in an interferometer. Phase modulated sensors are

much more expensive than intensity modulated sensors because they required

advanced electronics to process the information, but it is much more accurate than

the intensity modulated sensors. However, most of the applications is not required

extreme sensitivity, therefore intensity modulated sensors are more common in the

market by a more reasonable and competitive price.

2.2.3 Advantages of FOS over Conventional Electrical Sensor

Electrical sensors such as foil strain gages, thermocouples, and vibrating

wires that have been used in sensing and monitoring fields for decades have shown

their ineffective and limitation over FOS.

The nonconductive and noncorrosive natures of the fiber make the FOS more

suitable for outdoor than conventional electrical sensors. FOS also can safely

perform sensor measurements near high voltages, high sources of electromagnetic

interference, and in explosive environments because they are nonconductive,

electrically passive, immune to EMI and lightweight. Others characteristics of FOS

make it more effective compared to conventional electrical sensors are long-term

deployments, distributed systems and able to measure over long distance (up to 10km)

without any loss in accuracy.

16

2.3 Fiber Bragg Gratings

Fiber Optic Sensors (FOS) have been increased acceptance and widespread

use. The FBG based sensors are the most widely known and popular among the

multitude of sensor types. Their intrinsic capability to measure a multitude of

parameters such as strain, temperature, pressure, chemical, and biological agents-and

many others-coupled with their flexibility of design to be used as single point or

multi-point sensing arrays and their relative low cost, make of FBGs ideal devices to

be adopted for a multimode of different sensing applications and implemented in

different fields and industries [5].

2.3.1 Properties of FBG

FBG is composed of periodic changes of the refractive index that is formed

from exposure to an intense UV interference pattern in the core of an optical fiber [6].

Specific predetermined narrowband wavelength called the Bragg wavelength is

reflected by the grating structure as shown in the Figure 2.5.

17

Figure 2.5: Schematic diagram of structure and spectral response of FBG

The sensing principle used in the FBG based sensor system is to monitor the

shift in the Bragg wavelength with the changes in the measurand. The Bragg

wavelength, or resonance condition of a grating, is given by the expression [7]

λB = 2 neff Λ (2.1)

where neff is the effective refractive index of the core and Λ is the grating period of

the grating structure.

A unique property of FBG is that the back-reflected Bragg grating

wavelength, 𝜆𝛣 , will shifted when the surrounding temperature, strain or other

environment factor is changing. This happened because when the environment factor

is changing the effective refractive index, n and grating period, Λ also changing.

The shift in Bragg wavelength with strain and temperature changes is

expressed as

𝛥𝜆𝐵 = 2(𝛬𝜕𝑛𝑒𝑓𝑓

𝜕𝑙+ 𝑛𝑒𝑓𝑓

𝜕𝛬

𝜕𝑙)𝛥𝑙 + 2(𝛬

𝜕𝑛𝑒𝑓𝑓

𝜕𝑇+ 𝑛𝑒𝑓𝑓

𝜕𝛬

𝜕𝑇)𝛥𝑇 (2.2)

18

The first term in (2.2) represents the strain effect on an optical fiber. This

corresponds to a change in the grating spacing and the strain-optic induced change in

the refractive index. The strain effect term may be expressed as

𝛥𝜆𝛣 = 𝜆𝛣(1 − 𝑝𝑒)𝜀𝑧 (2.3)

Where 𝑝𝑒 is an effective strain-optic constant defined as

𝑝𝑒 =𝑛𝑒𝑓𝑓

2

2[𝑝12 − 𝜈 𝑝11 + 𝑝12 ] (2.4)

p11 and p12 are components of the strain-optic tensor and ν is the poisson’s ratio. For

a typical germanosilicate optical fiber p11 = 0.113, p12 = 0.252 , ν = 0.16 , and

neff = 1.482.

The second term in (2.2) represents the effect of temperature on an optical

fiber. A shift in the Bragg wavelength due to thermal expansion changes the grating

spacing and the index of refraction. This fractional wavelength shift change 𝛥𝑇 may

be written as

𝛥𝜆𝛣 = 𝜆𝛣(𝛼𝛬 + 𝛼𝑛)𝛥𝑇 (2.5)

Where αΛ = 1

Λ (

∂Λ

∂𝑇) is the thermal expansion coefficient for the fiber

(approximately 0.55x 10-6

for silica). The αn = 1

neff (

∂neff

∂𝑇) represents the thermo-

optic coefficient, which is approximately equal to 8.6 x 10-6

for the Germania-doped,

silica-core fiber.

Based on the equations, we can notice that the Bragg wavelength has linear

relationship with the environment factors. Therefore, FBG has been regarded as an

excellent element to be used as sensors to measure strain, temperature, and other

physical parameters.

19

2.3.2 Types of FBG

There are three types of fiber Bragg grating structures: the common Bragg

grating, blazed Bragg grating, and chirped Bragg grating. The differences between

these types of FBG are their spacing between grating plane and the angle between

the grating planes with the fiber axis.

The simplest and most used FBG is the common Bragg grating. The common

Bragg grating has a constant spacing between the grating planes. For the common

Bragg grating the parameters it depending is grating length and magnitude of

induced index change. Since the wavelength-encoded characteristics of common

Bragg grating, it considered as excellent strain and temperature sensing devices.

Figure 2.6: Common Bragg grating

The grating planes of blazed Bragg grating have an angle less than 90 degrees

respect to the fiber axis. The angle in between the grating planes and the fiber axis

and the strength of the index modulation determined the coupling efficiency and the

bandwidth of the trapped out light. The main applications of blazed Bragg grating are

mode conversion and acting as gain flatteners in amplifiers.

20

Figure 2.7: Blazed Bragg Grating

The chirped Bragg grating has an aperiodic spacing in between the grating

planes, displaying a monotonic increase in the spacing between the grating planes.

There are certain characteristic properties offered by chirped Bragg grating that are

considered advantages for specific applications in telecommunications and sensor

technology, such as dispersion compensation and the stable synthesis of multiple

wavelength sources.

Figure 2.8: (a) Chirped Bragg grating with aperiodic grading planes spacing (b)

Chirped Bragg grating with a cascade of several gratings with increasing period

21

2.3.3 Applications of FBG Sensors

The features and benefits of FBG sensors such as nonconductive, electrically

passive, immune to electromagnetic interference (EMI), and daisy chain multiple

sensors on a single fiber have proven that FBG sensors can work much effective

under challenging environment or long distances compare to the conventional

electrical sensors.

Nowadays, FBG optical sensing has been benefited in many areas such as

energy, petroleum, civil infrastructure, and transportation monitoring. This trend is

increasing significantly because the FBG optical sensing is lightweight, simple to

install, suitable for outdoor applications because the nonconductive and noncorrosive

nature, ability to work under hazardous gases and voltages, and immunity to EMI

removes the need for expensive and often difficult signal conditioning required for

measurements near noisy sources such as power transformers.

In the energy area, FBG optical sensing used for monitoring the structural

integrity of a wind turbine blade with electrical sensors would often result in noisy

measurements because of long copper lead wires. With optical sensing, accurate and

noise-free strain measurements on wind turbine blades are possible. Furthermore, the

nonconductive and distributed nature of optical fibers lends too many uses in oil and

gas applications, including pipeline monitoring and downhole monitoring.

FBG optical sensing is drawing attention from civil infrastructure too. The

monitoring of infrastructure such as bridge and building based on the electrical

sensors often faces significant environmental challenges. An electrical monitoring

system would require the installation of countless wires, a lightning grounding

system, and periodic calibration and maintenance services. With an optical sensing

solution, these downfalls are all eliminated. Therefore, optical fiber sensing system is

widely used in large building monitoring, bridge and road monitoring, airport landing

strip load monitoring and dam monitoring.

22

To ensure the operation of transportation such as airplanes, railways, and

ships, are in the proper and safe condition the monitoring system in the transportation

area is very important. However, weight, size, and harsh environmental requirements

can pose significant challenges to implementing an electrical monitoring system.

FBG optical sensors alleviate these challenges because of the longevity and ease of

installation of FBG optical sensors and lack of need for external calibration, these

sensing systems can be deployed reliably for decades without needing any

maintenance - this is especially beneficial for long-term railway and ship hull

monitoring. The ability to have multiple sensors on a single, very thin fiber

dramatically reduces the weight of the monitoring system, which is especially

important in aerospace applications.

2.4 Packaged Technique of FBG Temperature Sensor

The bare FBG sensor is not practicable in the reality applications, because the

optical fiber is slim, fragile and brittle. Moreover, the sensitivity of bare FBG sensor

head is lower than packaged FBG sensor. The sensitivity of FBG temperature sensor

should be enhancing by appropriate packaging technique and to facilitate detection of

sensor. The packaging technique can also weaken the impact of strain or pressure to

the Bragg wavelength, and reduce the sensitivity of strain and pressure [8].

The relative shift of Bragg grating wavelength due to the temperature

perturbation can be expressed as [9]

ΔλB = (α +ξ) ·λB·ΔT (2.6)

where the α is thermal expansion coefficient of optical fiber, the ξ is thermo-optic

coefficient of optical fiber, and ΔT is the change in external temperature at the FBG

zone. For a general germanosilicate fiber, the value of α is 0.55x 10-6

/ °C，and the

value of ξ is 6.67x 10-6

/ °C [9].

23

However, the thermal expansion coefficient will alter when the bare FBG are

packaged by some material. Therefore, the relative shift of packaged Bragg grating

wavelength due to the with the variation of temperature can be expressed as [9]

ΔλB = [(α +ξ) + (1-peff) (αsub -α)] ·λB·ΔT (2.7)

where the αsub is thermal expansion coefficient of packaging material and the peff is

effective photo-elastic coefficient and assume to be 0.22.

The sensitivity coefficient of FBG packaged is higher if the larger expansion

coefficient of packaging material is used [10]. Therefore, to attain higher temperature

sensitivity coefficient of FBG temperature sensor, we can select the packaging

material of larger thermal expansion coefficient.

CHAPTER 3

METHODOLOGY

This chapter describes the approach of this project in term of the setup of the

FBG temperature sensor system, the capability of the developed Labview based real-

time data acquisition coding, the details of the materials and the procedures for

fabricate the proposed sensor head packaging, and also the procedures of testing the

FBG temperature sensor.

25

3.1 Experimental Setup of FBG Temperature Sensor

The FBG temperature system consists of a tunable laser source which supply

optical source to the sensing FBG. The sensing FBG was fabricated on SMF-28

hydrogen loaded fiber by UV scanning through a phase mask with no apodization

applied. The grating was written by a 244 nm frequency doubled Argon ion laser at

output power of 150 mW and scanning speed of 3 mm/min [11]. The processes of

splicing the FBG into to the setup are explained in following section. The transmitted

optical spectrums from the sensing FBG are transmitting to the Optical Spectrum

Analyzer (OSA) MS9710B. The OSA passes the FBG transmission spectrum to the

Labview program in the desktop computer through the National Instrument GPIB-

USB cable. The Labview program is capable to calculate the corresponding

temperature change from the wavelength shift of the transmitted FBG spectrum from

the OSA and in the meantime record those important spectrum parameters for further

analysis purpose. The Table 3.1 lists out the specification of the equipments and

components of this system.

Figure 3.1: Experimental setup

26

Table 3.1: Specification of the Equipments and Component of the Experimental

Setup

Equipment / Component Specification

Fiber Bragg Grating (FBG) Single Mode Fiber (SMF-28)

Central Wavelength : 1528nm

Length: 20mm

Peak reflectivity: ~ 99%

Tunable laser source 1550nm SLD Light Source

Operating Wavelength : 1400-1600nm

MS9710B Optical spectrum analyzer

(OSA)

Measurement range : 600 – 1750nm

Wavelength accuracy : 50pm(1530-

1570nm)

NI GPIB-USB Cable Full speed12 Mb/s USB Signaling

Transfer Rates Greater than 880 kbytes/s

27

3.1.1 Process of Splicing Fiber

Figure 3.2: Process of splicing fiber

The first step for slicing fiber is removing the protective polymer coating

around optical fiber and we called this act as stripping. We used fiber stripper to

stripping fiber coating.

Figure 3.3: Fiber stripper

After the fiber being stripped, the bare fiber is cleaned using alcohol and

wipes.

Stripping the Fiber

Cleaning the Fiber

Cleaving the Fiber

Splicing the fiber

28

Figure 3.4: Alcohol and wipes

The bare fiber is then cleaved using fiber cleaver, so that the endface is

perfectly flat and perpendicular to the axis of the fiber. This step is very important,

because the splice loss is a direct function of the angles and quality of the two fiber-

end faces and as the degrees of the cleave angle is closer to 90° the lower is the splice

loss.

Figure 3.5: Fiber cleaver

Finally, we used the fiber splicer to make either core or cladding of the two

cleaved bare fiber are aligned to each other. The fiber splicer aligns the cleaved

automatically.

29

Figure 3.6: Fiber splicer

3.1.2 National Instruments GPIB-USB Cable

The National Instruments compact GPIB-USB transforms any computer with

USB port into a full-function, plug-and-play IEEE-488.3 Controller. It is ideal for

computer or other applications that has no available internal I/O slots.

Figure 3.7: National Instruments GPIB-USB cable

30

3.2 Labview Program

As mentioned the Labview program is capable to record the FBG wavelength

shift from an OSA and calculate the corresponding temperature change and record

the important parameters of the FBG spectrum. The real time data is continuous

acquired and saved in the format of Text Documents as shown in the Figure 3.3.The

description for each of saved file is presented in Table 3.2. The transmitted FBG

spectrum is displayed on the screen and the scanning ends only when the program is

terminated by user. This Labview program performed the data acquisition process in

the Digital Signal Processing (DSP). The scanning range is set by the user, in this

project I set the scanning range as 2nm. Within the scanning range 501 sampling

point are taken, therefore the resolution of this program is approximate to 4.0pm.

Figure 3.8: Graphical user interface of the Labview data acquisition program

31

Figure 3.9: Example of the saved data files of the Labview program

Table 3.2: Description of Saved Text Document

Name of the Text

Document

Description

delta_T.txt Temperature changes correspond to the shift of dip

wavelength.

delta_WL.txt Changes of the dip wavelength to the initial (first

detected) dip wavelength.

dip_wavelength.txt Dip wavelength of all the spectrum thought out the

sensing period.

level.txt Power level of the spectrum.

relative_time.txt The relative time of each spectrum being detected.

spectra.txt The entire spectrum pattern detected by the program.

time.txt The absolute time (according to the time of desktop) of

each spectrum being detected.

wavelength.txt The wavelength of each sampling points.

32

3.3 Fabrication of FBG Sensor Head Packaging

The copper tube used for packaging has the length of 4 cm, which create

additional 1 cm of length at both ends for 2 cm FBG. In the packaging procedure, the

bare FBG was loosely place at the center of a copper tube and then the position is

fixed by small amount of glue applied to the fiber ends. This step is to ensure the

grating section has a direct contact to the inner wall of the copper tube whilst

maintaining the flexibility of grating section. Then heat sink compound was slowly

injected into the slot of the copper tube. At the 1 cm buffer zone of both ends, the

rubber sleeves were used to give extra protection to the ingress/egress points which

are the most vulnerable parts. Finally, strong glue was applied to fix the position of

the rubber sleeves as well as contain the heat sink compound in the copper tube. The

glue was left to dry before the sensor can be tested. The copper tube is then cleaned

from the remaining of the heat sink compound. The FBG sensor and heat sink

compound consolidated very well with the copper tube and the surface cosmetic

looked very good after treatment.

Figure 3.10: Schematic of the sensor head packaging

The main aspect of packaging technique is the type of the coating material

with high thermal conductivity so that the heat can be conducted across the grating

structure fast and efficiently. Copper possesses many desirable properties for

packaging purpose compared with other conventional materials. A part from its high

thermal conductivity, copper has the advantage of corrosion resistant, good thermal

expansion, high allowable stress resistant and internal pressure resistant. With high

bulk modulus of 140 GPa, the packaged sensor will be insensitive the unwanted

strain originated from the surrounding.

33

The filler material for the copper tube that is used to enhance the contact

between the sensor and coating wall should be ready with suitable viscosity, so that

the process of injecting the material can be done with ease. More importantly, the

material should also possess relatively high thermal conductivity. As for this work, a

heat sink compound RS503-357with thermal conductivity of 0.65W/m°C is selected.

The cost for heat sink compound is increased for higher thermal conductivity. All

materials used in the experiments are selected so that it could sustain the minimum

and maximum expected temperatures to be measured. For this work, the sensor is

tested in boiling water, hence, the range of temperature is between the room

temperature and the boiling point of water i.e. 100 °C. The packaging of sensor to

sustain up to 180 °C, limited by the rubber sleeve operating temperature range. The

specifications of the materials being used in the proposed sensor head packaging is

listed in Table 3.3.

Table 3.3: Specification of the Packaging Material

Material Specification

Heat sink compound

RS503-357

Material : Zinc Oxide

Thermal conductivity : 0.65W/m°C

Boiling point : > 275°C

Melting point : 1970°C

Copper tube Thermal conductivity: 401W/m°C

Operating temperature range: -50 - 200°C

Tube inside diameter: 2.8mm

Tube outside diameter: 4mm

Bulk modulus: 140 GPa

Melting point: 1084 °C

Rubber sleeve Material : Silicone Rubber

Operating temperature range : -65 – 180°C

Sleeve diameter : 1mm

Wall thickness : 0.5mm

Glue Faster Super Glue, SG-F-3ML

34

Figure 3.11: The packaged FBG sensor head

3.4 Tests of the FBG Temperature Sensor

The experiments were carried out in two phases; the bare fiber sensor head

and packaged fiber sensor head. The same FBG with Bragg wavelength is 1528nm

was used throughout the experiment for fair comparison. The functionality of the

FBG temperature sensor was tested by continuous heating of the sensor underwater

from room temperature up to the boiling point. A thermometer was used as a

reference reading. Reading from FBG sensor and thermometer was compare based

on the relative time of the process. For both bare and packaged sensors, two cases

were examined; uniform heat test and nonuniform heat test.

In the uniform heat test, the whole sensor was submerged to the normal

operating condition where the heat can be evenly distributed in the whole grating

structure. On the other hand, for the nonuniform heat test, only half length of the

grating is submerged underwater. The partial heating on the grating will test the

35

performance and response of FBG sensor when there are anomalies of heat

distribution across the grating section.

Figure 3.12: Flow of the experiments

The bare FBG sensor is used in the uniform heat test and nonuniform heat test.

The bare FBG sensor is packaged by the proposed pakaging technique.

The packaged FBG sensor is used in the uniform heat test and nonuniform heat test.

The recorded spectrum data of the bare and packaged FBG sensor during the uniform and nonuniform heat test are analyzed and

compared.

CHAPTER 4

RESULT AND DISCUSSION

In this project, we are intent to enhance the performance of FBG temperature

sensor by applying the packaging technique. Therefore in this chapter we will

compare the performance of bare and packaged FBG temperature sensor. There are

two tests called uniform heat and nonuniform heat test to examine the performance

of FBG temperature sensor. In these two tests, the same FBG with Bragg wavelength

1528nm was used for fair comparison.

37

4.1 Uniform Heat Test

The intention of uniform heat test is to observe the performance and response

of bare and packaged FBG temperature sensor in the normal operating condition

where the heat can distribute evenly toward the whole grating structure. In this test,

the whole sensor was submerged underwater.

Figure 4.1 show the temperature sensitivity coefficient of the bare and

packaged FBG temperature sensor. The central Bragg wavelength of the bare FBG is

1528nm but it changed to 1528.46nm for the packaged FBG temperature sensor. This

is due to the stress residue created by the external packaging material. From the

Figure 4.1 the temperature sensitivity coefficient for the bare and packaged FBG

sensor are 10.05pm/°C and 10.09pm/°C, there is slightly increased but it is much

lower than the theoretical value. According to the equations 2.1 and 2.2, we assumed

the fiber is germonosilicate fiber, the thermal expansion coefficient, α of value is

0.55x 10-6

/°C，and the thermo-optic coefficient, ξ of value is 6.67x 10-6

/°C and

expansion coefficient of the copper is 17x10-6

/°C the theoretical value of temperature

sensitivity coefficient for bare and packaged FBG sensor are 11.03pm/°C and

30.64pm/°C. The big different between the theoretical and experiment result is

caused by the assumption of the value of α and ξ. The index change (α +ξ) is the

dominant effect in temperature sensitivity [12], but the type of fiber we used is not

germanosilicate fiber. The improvement of sensitivity for the packaged FBG sensor

is not significant, this is because the grating section is loosely placed inside the tube,

and effect of higher thermal expansion of the copper tube is not prominent. To

significantly enhance the sensitivity, strong contact between the FBG sensor and the

copper is required so that the effect of higher thermal expansion can be transfer to the

FBG.

38

Figure 4.1: Temperature characteristics curve of bare and packaged sensor

Series of measurement have been carried out to verify the temperature response

of bare and packaged sensor correspond to the actual temperature. The measurement

is then compared to the measurement obtained from thermometer. Figures 4.2(a) and

4.2(b) show the temperature measurement of bare fiber and packaged fiber,

respectively. The result clearly indicates the temperature measurements for bare and

packaged fiber are quite similar to the reading of thermometer. There are minor peak

fluctuations of the FBG spectrum that cause temperature fluctuation. One of the

reasons is that the low wavelength resolution of the OSA. Furthermore, the low

scanning speed of the OSA could also cause small irregularity to the measured

spectrum as the temperature could change during between each scanning step. By

using high speed tunable optical filter with high resolution, the aforementioned

problem can be minimized. Adjustment on the coefficient for the packaged fiber is

made to compensate the higher thermal expansion of the copper tube and the heat

sink compound. The higher thermal expansion of the copper tube also enhances of

39

the sensitivity of the packaged sensor. The increase of temperature is almost linear

with the relative time taken.

(a)

40

(b)

Figure 4.2: Temperature measurement of (a) bare FBG sensor, and (b)packaged

sensor

4.2 Nonuniform Heat Test

The nonuniform heat test is used to observe the performance and response of

the bare and packaged FBG temperature sensor in the anomalies heat distribution

condition. In the nonuniform heat test , only half of the grating is submerged

underwater to create the partial heating condition toward the FBG temperature sensor.

Figure 4.3(a) and 4.3(b) show the transmission spectra of bare and packaged

sensors for the nonuniform heat test. There are multiple peaks and spectrum

broadening in the bare fiber due to the nonlinear chirping of grating. Apparently, this

41

multiple peaks lead to error in temperature measurement. Meanwhile, for the

packaged sensor, the shape of transmission spectrum is undistorted by the

nonuniform heat distribution test. The result shown in Figure 4.4 verifies that the

measurement of packaged FBG sensor under nonuniform test is comparable to the

reading from thermometer. The functionality of the copper tube packaging for heat

conductor is evident as the sensors pick up the highest temperature from the

nonuniform heat test.

(a)

42

(b)

Figure 4.3: FBG transmission spectra at different stages of experiment of (a) bare

FBG sensor, and (b) packaged FBG sensor, in nonuniform heat test

43

Figure 4.4: Temperature measurement of packaged FBG in nonuniform heat test.

CHAPTER 5

CONCLUSION AND RECOMMENDATION

5.1 Conclusion

In this project, a Labview based data acquisition system for temperature

measurement of FBG sensor has been successfully developed. The temperature

measurement of the FBG sensor system is corresponding to the thermometer

measurement. However we find out that the spectrum of bare FBG sensor is distorted

in nonuniform heat test and could not give correct temperature reading. This is due to

the anomalies of grating period and refractive index distribution. This problem

managed to solve by apply the proposed packaging technique. Result shows that the

simple and economical packaging for FBG sensor head gives the correct temperature

reading while the bare FBG sensor gives error reading in the nonuniform heat test.

Furthermore, the sensitivity of the FBG sensor could further enhanced due to the

higher thermal expansion of the packaged material. This sensing performance in

conjunction with the existing advantages of fiber optic sensor is highly desirable by

many practical applications.

45

5.2 Recommendation

The sensing performance of FBG temperature can be further enhancing by

using packaged material that have higher thermal expansion, such as aluminium to

replace the copper tube. In order to improve the performance of FBG temperature

sensor, we can change to germonosilicate fiber or other fiber that will give higher

index change (α +ξ). This can increase the sensitivity of the FBG temperature sensor

significantly. Furthermore, we have to make sure the strong contact between the

FBG sensor and the copper so that the effect of higher thermal expansion can be

transfer to the FBG. This can be achieved by adhere the edge of the FBG sensor

when it is placed in the copper tube, to make sure it will contact to the copper tube

although the heat sink compound is injected into the slot of the copper tube.

46

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48

Appendices A