CHARACTERIZATION OF FIBER OPTIC SENSOR FOR LIQUID

REFRACTIVE INDEX MONITORING

DIANAY SHAFINA BINTI SHAFEI

UNIVERSITI TEKNOLOGI MALAYSIA

CHARACTERIZATION OF FIBER OPTIC SENSOR FOR LIQUID REFRACTIVE

INDEX MONITORING

DIANAY SHAFINA BINTI SHAFEI

A thesis submitted in fulfillment of the

requirement for the award of degree of

Master of Science (Physics)

Faculty of Science

Universiti Teknologi Malaysia

MARCH 2015

iii

Special dedication for my beloved mother, father, brother, sister and husband. May

Allah shower his blessings to all of us.

Amin.

iv

ACKNOWLEDGEMENT

In the name of Allah, Most Gracious, Most Merciful. Praise be to Allah, the

Cherisher and Sustainer of the worlds. For His Mercy has given me the strength and time

to complete this project.

I would like to express my appreciation to my respected supervisors, Dr. Asiah

binti Yahaya and Dr. Raja Kamarulzaman bin Raja Ibrahim for their supervision,

guidance, enjoyable discussion and motivation throughout this study. Besides, I have

much pleasure to those who have assisted me in various ways in carrying out the

experimental works. There are my lab technicians, Mr Nasir bin Abu Bakar, Mr Asmawi

and also the researcher’s group in Advanced Photonics Science Institute (APSI).

My thanks are also due to Government of Malaysia through Research Student

Grant (RSG) vote Q.J130000.2626.07J69 for giving us financial support.

Last but not least, my appreciations go to my beloved family and my all best

friends for continuing support, patience throughout the present work and who have

favored me with correspondence, I have much pleasure in expressing my obligation. May

Allah bless those who have involved in this project.

v

ABSTRACT

This research was conducted to characterize the fiber optic sensor for liquid refractive

index monitoring. The sensor system consists of a light source, a 2x2 single mode fused

fiber coupler of different coupling ratios and a detector. Diode laser operating at

wavelength 1300 nm and 1550 nm was used as a light source. Fresnel’s Law of reflection

from the fiber-air and fiber-liquid interface was the fundamental of this work. The total

reflected light intensity from both coupler outputs was measured in terms of its power. A

simple test on the feasibility of the sensor for this type of measurement was conducted

and it was found that this sensor was sensitive enough and has a reasonable working range

to work with. The refractive index of a solution is a function of its concentration,

temperature and also the wavelength of the incident light. In this work the relation of

these parameters were investigated. The solution of NaCl was selected as a liquid sample.

Various concentrations of salt solution ranging from 0 % - 24 % were tested and were

heated from 30 ºC - 90 ºC to observe the change in refractive index. The results revealed

that the refractive index of liquid solutions varies linearly with its concentration and

temperature. Performance of the sensor system by using single mode fused fiber coupler

with different coupling ratio was studied. It was observed that the fiber coupler with 50:50

indicated the highest sensitivity to be used in this sensor system.

vi

ABSTRAK

Kajian ini telah dijalankan untuk mencirikan pengesan gentian optik untuk mengukur

indeks biasan cecair. Sistem pengesan terdiri daripada sumber cahaya, 2x2 gentian

pengganding mod tunggal dengan nisbah gandingan yang berbeza dan alat pengesan.

Diod laser dengan panjang gelombang 1300 nm dan 1550 nm telah digunakan sebagai

sumber cahaya. Hukum pantulan Fresnel di antara muka gentian-udara dan gentian-cecair

adalah asas kepada kajian ini. Jumlah keamatan cahaya terpantul dari kedua-dua output

pengganding telah diukur berpandukan kepada kuasanya. Suatu ujian yang mudah

mengenai kesesuaian pengesan untuk jenis pengukuran ini telah dijalankan dan didapati

bahawa pengesan ini sensitif dan mempunyai tahap kebolehan yang sesuai untuk

dijadikan pengesan. Indeks biasan cecair adalah fungsi kepada kepekatan, suhu dan juga

panjang gelombang sumber cahaya yang digunakan. Dalam kajian ini, hubungan diantara

parameter ini telah dikenalpasti. Cecair NaCl telah dipilih sebagai sampel cecair. Larutan

NaCl yang mempunyai pelbagai kepekatan dari 0 % - 24 % telah diuji dan larutan tersebut

telah dipanaskan dengan suhu dari 30 ºC - 90 ºC untuk melihat perubahan indeks biasan.

Keputusan menunjukkan indeks biasan cecair berubah secara linear dengan kepekatan

dan suhu cecair. Prestasi sistem pengesan dengan menggunakan gentian pengganding

mod tunggal yang mempunyai nisbah gandingan yang berbeza telah dibandingkan.

Keputusan menunjukkan gentian pengganding yang mempunyai nisbah gandingan 50:50

mencatat kepekaan yang paling tinggi untuk digunakan di dalam sistem pengesan ini.

vii

TABLE OF CONTENT

CHAPTER TITLE PAGE

DECLARATION ii

DEDICATION iii

ACKNOWLEDGEMENT iv

ABSTRACT v

ABSTRAK vi

TABLE OF CONTENTS vii

LIST OF TABLES xi

LIST OF FIGURES xii

LIST OF SYMBOLS xvi

LIST OF ABBREVIATIONS xviii

LIST OF APPENDICES

xix

1 INTRODUCTION 1

1.1 Introduction 1

1.2 Background of the Study 2

1.3 Statement of the Problem 3

1.4 Objective of the Study 4

1.5 Scope of the Study 4

1.6 Significant of the Study

5

viii

2 LITERATURE REVIEW 6

2.1 Introduction 6

2.2 Relationship of Refractive Index with Temperature,

Concentration and Wavelength 7

2.3 Basic of Optical Fiber 9

2.4 Fiber Optic Sensor Technology 10

2.5 Optical Modulation Mechanism 12

2.5.1 Intensity Modulated Fiber Optic Sensor 12

2.6 Fresnel’s Law of Reflection 13

2.7 Fiber Optic Coupler Technology 16

2.7.1 Coupling Ratio 18

2.7.2 Characteristic of Fiber Coupler 18

2.8 Light Source 20

2.9 Optical Detector 21

2.10 Works relating Fiber Optics and Refractive Index

Measurement

23

3 RESEARCH METHODOLOGY 28

3.1 Introduction 28

3.2 Fabrication of Fiber Optic Coupler 29

3.3 Characterization of Sensor System 30

3.3.1 Fiber Ends Preparation of the Single Mode

Fiber Coupler 30

3.3.2 Characteristics of the selected Fiber couplers 31

3.3.3 Liquid Samples Preparation 33

ix

3.3.4 Relation between Power of the Light Source

and Reflected Light Intensity

34

3.3.5 Determination the Depth of Immersion 35

3.3.6 Checking the Sensor Sensitivity 35

3.4 The Measurement System 36

3.4.1 Sensor Set up 36

3.4.1.1 Power meter as a detector 37

3.4.1.2 Photodiode as a detector 37

3.5 Method of Data Measurement 38

3.5.1 Variation of Refractive Index of Solution with

Wavelength of Incident Light 40

3.5.2 Variation of Refractive Index of Solution with

Concentration 40

3.5.3 Variation of Refractive Index of Solution with

Temperature of Solute 42

3.6 Calibration Curve 42

3.6.1 The Measurement of the Refractive Index

using Refractormeter 42

3.7 Measurement of Liquid Refractive Index using

OTDR 44

4

RESULT AND DISCUSSION 46

4.1 Introduction 46

4.2 Results of Sensor Characterization 47

4.2.1 Sensing Performance of Different Ratio of

Fiber Coupler 47

x

4.2.2 Depth of Immersion 48

4.2.3 Relation between Power of the Light Source

and Reflected Light Intensity

49

4.2.4 Sensitivity of the Sensor System

51

4.3 Measurement of Refractive Index with Reflected

Intensity 53

4.3.1 Concentration Relation 54

4.3.2 Temperature Relation 59

4.3.3 Wavelength Relation 62

4.4 Measurement of Liquid Refractive Index using

OTDR 65

4.5 Performance of Fiber-Couplers for Refractive Index

Measurement 68

4.6 Calibration Curve for the Sensor

72

5 CONCLUSION AND RECOMMENDATION 80

5.1 Conclusion 80

5.2 Recommendation

82

REFERENCES 83

Appendices A- F 87-95

xi

LIST OF TABLES

TABLE NO. TITLE

PAGE

3.1 Concentration of the liquid sample 34

3.2 Configuration between two probes 36

4.1 The outputs of the two ports using

powermeter as a detector 47

4.2 The outputs of two port using photodiode as

a detector 47

4.3 Reflected Intensity in different media 52

xii

LIST OF FIGURES

FIGURE NO. TITLE

PAGE

2.1 Graph of RI (solid lines) and group index (dotted lines)

of silica against wavelength 9

2.2 Fiber optic structures 10

2.3(a) Extrinsic optical fiber sensor 11

2.3(b) Intrinsic fiber optic sensor 11

2.4(a) Reflection type 13

2.4(b) Transmission type 13

2.5 Fresnel’s Law at medium interface 14

2.6 Graph of reflectivity relation to the angle of incidence

between two medium interfaces 16

2.7 Directional coupler consisting of two fibers whose

core is brought close to each other 17

2.8 Characteristics of coupler 19

2.9 Graph of optical output power versus input drive

current

for laser diode 21

2.10 Photodiode model 22

2.11 Graph of RI of sucrose versus a variation of

concentration 24

2.12 Schematic demonstration of the light paths at the end of

a fiber 25

xiii

2.13 Temporal response showing the sensors performance as

a function of refractive index 26

2.14 Graph of the comparison of theoretical and measured

reflectance for various refractive index of media 27

3.1 Flow chart of the fabrication of fiber-based coupler 29

3.2 Twisting and pulling the fiber 29

3.3 Photograph of cleaved glass fiber(x50) 31

3.4 Fiber based coupler 31

3.5 The experiment set up for measuring output power P1 32

3.6 The experiment set up for measuring output power P2 33

3.7 Schematic experimental set up for reflected light

intensity measurement using fiber coupler 37

3.8 Schematic experimental set up for reflected light

intensity measurement using fiber coupler 37

3.9 The experimental set up of the sensor system 38

3.10 Reflected light intensity from powermeter under 5 time

measurements or distilled water at room temperature 39

3.11 Reflected light pulse display on oscilloscope 41

3.12 Graph of refractive index versus concentration for salt

solution 43

3.13 Schematic experimental set up for reflectance

measurement using OTDR 44

3.14 Reflectance measurements from OTDR trace 45

4.1 Graph of output power of reflected light intensity

versus depth of fiber end immersed in liquid

49

4.2 Graph of reflected output power of light intensity

versus input power for 50:50,70:30 and 90:10 coupling

ratio

50

4.3 Graph of output voltage of reflected light intensity

versus injection current for 50:50, 70:30 and 90:10

coupling ratio 50

xiv

4.4 Sensor responses for 50:50 coupler at different

temperature of salt solution 55

4.5 Sensor responses for 70:30 coupler at different

temperature of salt solution 55

4.6 Sensor responses for 90:10 coupler at different

temperature of salt solution 56

4.7 Graph of normalized intensity versus concentration of

sucrose 58

4.8 Graph of normalized intensity versus concentration of

ethanol 58

4.9 Sensor responses for 50:50 coupler at different

concentration of salt solution 59

4.10 Sensor respond for 70:30 coupler at different

concentration of salt solution 60

4.11 Sensor respond for 90:10 coupler at different

concentration of salt solution 60

4.12 Graph of output power of reflected light intensity

versus concentration for different operating wavelength

at room temperature (22.9 ± 0.1°C) for salt solution for

50:50 coupling ratio

62

4.13 Graph of output power versus temperature at different

operating wavelength for distilled water (0% mass of

solvent) for 50:50 coupling ratio 63

4.14 Represent the measured refractive index versus

temperature of distilled water at wavelength 1310 nm

and 1551 nm 64

4.15 OTDR trace for salt solution in range 3%-15%

(zoom=14x) 65

4.16 Graph of reflectance versus concentration of salt

solution for temperature in range 30ºC-90ºC 66

4.17 Graph of reflectance versus temperature of salt solution

for concentration in range 3%-24% 67

4.18(a) Graph of output power of reflected light intensity

versus temperature for different coupling ratio of fiber

coupler for distilled water using power meter 68

xv

4.18(b) Graph of output voltage of reflected light intensity

versus temperature for different coupling ratio of fiber

coupler for distilled water using photodiode

69

4.19(a) Graph output power of reflected light intensity versus

concentration for different coupling ratio at room

temperature using power meter 71

4.19(b) Graph of output voltage of reflected light intensity

versus concentration for different coupling ratio at

room temperature using photodiode 71

4.20 Graph of output power of reflected light intensity

versus refractive index for 50:50 coupling ratio at room

temperature using power meter as a detector 73

4.21 Graph of output voltage of intensity versus refractive

index for 50:50 coupling ratio at room temperature

using photodiode as a detector 73

4.22 Graph of refractive index versus temperature of

solution for distilled water 74

4.23 Graph of refractive index versus the temperature for

NaCl–H2O solutions with different concentrations,

which ranged from 20 to 70 °C 75

4.24 Graph refractive index of tap water versus the

temperature at the wavelength of 1550nm 76

4.25 Graph of refractive index versus temperature of water 77

4.26 Graph of refractive index versus concentration for

sugar solution at room temperature using 50:50

coupling ratio of fiber coupler 78

4.27 Graph for the variation of refractive index with

concentration of sugar solution 79

xvi

LIST OF SYMBOLS

n - Refractive index

c - Speed of light in vacuum

v - Speed of light in medium

I - Incident ray

θi - Angle of incident

θr - Angle of reflection

θt - Angle of refraction

n1 - Refractive index of medium 1

n2 - Refractive index of medium 2

R - Reflectance

T - Transmission

RS - s-polarized light

RP - p-polarized light

μ - Permeability

μo - Vacuum permeability

Pout - Output power

Pin - Power input

L - Length oh the interaction region

- Coupling coefficient

xvii

dB - Decibel

Pexcess - Excess loss

Si - Silicon

Ge - Germanium

InGaAs - Indium Gallium Arsenide

C - Concentration

T - Temperature

- Wavelength

Vg - Group velocity

- Propagation constant

- Radian optical frequency

nf - Effective index

ng - Group index

- Optical dispersion

mV - Milivolt

xviii

LIST OF ABBREVIATIONS

EMI - Electromagnetic interference

CR - Coupling ratio

IL - Insertion loss

D - Directivity

RL - Return loss

FBT - Fused biconical taper

LED - Light emitting diode

SLD - Superluminescent diode

OPM - Optical power meter

NIST - National Institute of Standards and Technology

FBG - Fiber brag grating

RI - Refractive index

IMG - Index matching gel

xix

LIST OF APPENDICES

APPENDIX NO: TITLE PAGE

A Data sheet for each fiber coupler used 87

B The specification of InGaAs Photodiode used

followed by the basic of photodiode circuit,

amplified detector and spectral response by

DET01CFC

90

C The value of output power value for Port 1 and

Port 2 for 50:50, 70:30 and 90:10 coupling ratio

at 1300nm

92

D Performance characterizations and related

formula for CORNING SM28e fiber used 93

E The specification of Digital Handheld

Refractometer from Reichert Technologies

used 94

F The value of refractive index measured by

refractometer and fiber coupler system. 95

1

CHAPTER 1

INTRODUCTION

1.1 Introduction

Optical sensing can be described as a field of optoelectronics that has used optics

and electronics to sense variations in physical quantities through their interaction with

optical signals (A. H.Morshed, 1999). Generally, this field was focused on military and

aerospace usages during the late 1970s. Nowadays, improvement of optical sensors and

fiber optic sensors are been revolutionized with the development of optical device

invention and the proliferation of sensing applications. The rapid development in the field

of fiber optic sensors is mainly due to the flexibility in the application of optical fiber in

a various industries.

The rapid development of optical fiber-based sensing is due to it has several

advantages like small size, light weight, immunity to electromagnetic interference (EMI),

high temperature performance capability, environmental ruggedness, large bandwidth,

and the capability of distributed sensing (David R.G,2004). Optical fiber for sensing

applications are used to communicate with a sensor device or use a fiber as the sensor

2

itself to conduct monitoring of physical, biological and chemical properties, changes in

the subject or object of study. In optical fiber sensors, information is primarily conveyed

in all optical sensors by a change in phase, frequency(wavelength), polarization or

intensity depend on it modulation mechanism.

1.2 Background of the Study

Today, the revolution in optoelectronic industries has enabled the development of

fiber optic sensor that offers a series of advantages over conventional electronic sensors

(E.Udd, 2006). This development, in combination with advances in the industrial control

process technology, has opened the new field of fiber optic sensor in many field of

application to the world. By dealing with a simple monitoring system that only use optics

and electronics device, it can function as a good sensor to detect variation in physical

quantities of interest such as refractive index, pressure, density, temperature and many

others through their interaction with optical signal. However, refractive index

measurement using optical fiber sensors have attracted attention to be explored and

applied to various applications.

Refractive index (RI) is a basic optical property of material that describes an

important part of its interaction with electromagnetic radiation. It is an inherent

characteristic and very useful to find the physical parameter such as concentration,

pressure, density and so on.

3

Recently, the measurement of liquid refractive index has attracted much attention

due to its wide applications in basic research, biochemical analysis, environment and

contamination assessments and chemical industry. Due to the correlation between the

wavelength, concentration and temperature and the refractive index of a liquid, the optical

methods can be apply to measure the changes in the liquid refractive index. The fiber

optic technique based on Fresnel reflection is a very appropriate method since it’s very

close to refractive index of medium interface.

1.3 Problem Statement

At present, several methods with various principle of monitoring refractive index

of liquid are available including critical angle principle, surface plasma resonance (SPR)

and grating-based refractive index sensor. Most of them proved to be sensitive for certain

condition involved but in terms of the sensor preparation or fiber modification, they could

get rather tedious and complicated. Furthermore, each measurement has its own limitation

such as cannot be used at hard to reach area and only appropriate for certain liquid. In this

work, another approach is being looked into to see the viability of producing a simple and

direct measurement fiber sensor for monitoring the small changes of refractive index of

liquid. The sensor must be sensitive, flexible, small and handy which can be used in

remote and hard to reach and hazardous areas especially in industrial applications. The

realization of how important accurate refractive index of liquid contributes advantages in

our industries especially in monitoring or analyzing process control and the rapid growth

of fiber optic sensor technology is the reason why the this work is carried out.

4

1.4 Objectives of the Study

The objectives of this work are:

To develop a simple fiber coupler measurement system for liquid

refractive index monitoring

To optimize the sensor parameter such as coupling ratio, probe-sample

position, detector and light source

To investigate the total reflected light intensity to the change of liquid

refractive index and liquid level.

1.5 Scope of the Study

This work is based on Fresnel reflection at flat interface between two different

media which is air and liquid. The sensor system is based on the use of 2x2 single mode

fused fiber coupler with different coupling ratio. Two light sources of different

wavelength and two optical detectors namely the power meter and the photodiode are

used. Reflected light produced from Fresnel reflection at the interface was measured in

term of power due to change of liquid refractive index.

Parameters such as concentration, temperature and wavelength of incident light

that are related to the change of liquid refractive index were studied and their relations

were investigated. A light source with operating wavelength 1300 nm and 1550 nm was

used while a salt solution with concentration by mass in range 0%- 24% was tested as a

liquid sample. In order to study the sensor’s respond to temperature, the liquid sample

was heating in the range of 30ºC – 90ºC. The variation of reflected light intensity detected

by optical detectors then relate to the change in the refractive index due to the variation

of tested parameters.

5

1.6 Significant of the Study

The precise measurement of liquid refractive index is important to many fields

such as fundamental research, bio-chemical analysis, environments and contamination

assessment, diagnostics, foods, and chemical industry. The study of fiber optic sensor for

liquid refractive index monitoring is very important to produce a simple and high

sensitivity for many uses and applications such as monitoring the quality of frying oil,

investigate purity of honey and so on. It also contribute more efficient measurement

techniques for the improvement of the process control and to preserve the quality of final

products by offering remote sensing, with low disturbance and no explosion risks.

83

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