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DESIGN OF OPTICAL SENSOR FOR WATER LEVEL DETECTION BASED ON FRESNEL REFLECTION

BY

SUNITY HALDER

A dissertation submitted in fulfilment of the requirement for the degree of Master of Science (Communication

Engineering)

Kulliyyah of Engineering

International Islamic University Malaysia

AUGUST 2018

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ABSTRACT

Optical fibers are used most often to transmit light between the two ends of the fiber and wide usage in fiber optic communication. The system permits signal transmission over longer distances with a higher bandwidth and data rates in comparison to the conventional electrical based communication system. In the process of measuring the liquid level, fiber optic sensor is suitable to sense, detect and monitor the measurement. Fresnel-reflection-based fiber optic sensor is capable to become a water level detector mechanism based on the difference of the fiber core refractive index to its end interface with air and water. In this research, Optisystem software is used to simulate the optical power range difference at the receiver side according to the design water level resolution and different branch of a river. At the fiber end sensing point of eight levels, the received power shows a linear power difference of - 22.48 dBm to -36.48 dBm between levels for both signal wavelengths 1310 nm and 1550 nm at a fixed fiber length L1. As the fiber length is varied with two different river branches respectively, a total length of 150 km is achieved for 0 dBm signal launch at a transmitter side. OSNR level from 45.52 dBm to 77.52 dBm for both wavelengths at the 150 km transmission length is acceptable for the receiver to determine the power level difference that indicates the water level consequently. The determination of the power level range and transmission length serves as an indicator for the developed water level sensor to be deployed in the diverse structure of drain and river water level monitoring. It is also served for a practical remote river level monitoring mechanism as part of the natural disaster early warning system of any residential or business area along the river that could save the life.

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ثحبلا ةصلاخ

لكشب مدختستو فايللأا فيرط ينب ءوضلا لقنل نايحلأا مظعم في ةيرصبلا فايللأا مادختسا متي عم لوطأ تافاسم برع تاراشلإا لاسرإب ماظنلا حمسي .ةيرصبلا فايللأا تلااصتا في عساو في كلذكو .يديلقتلا يكلسلالا لاصتلاا ماظنب ًةنراقم ىلعأ تانايب لدعمو ىلعأ يددرت قاطن بسانم ةيرصبلا فايللأا رعشتسم نوكي ،لئاسلا ىوتسم سايق ةيلمع ًا

وفشكو سايقلا سسحتل

.وتبقارمو ىلع مئاقلا ةيرصبلا فايللأا رعشتسلم نكيم

Fresnel نع فشكلل ةيلآ حبصي نأ

ءانب ءالما ىوتسم ًا

لإا رشؤم فلاتخا ىلع ءاولها عم ةيفرطلا وتهجاو ىدل فايللأل يساسلأا راسكن

ـلا جمانرب مادختسا متي ، ثحبلا اذى في .ءالماو Opti system

ةقاطلا قاطن قرف ةاكالمح

ًاقفو لِبقتسلما بناج في ةيئوضلا فلاتخاو هايلما ىوتسم عضو تابثل

ةطقن في .رهنلا عرف

راعشتسلاا ت ،تايوتسم ةينامثلل فايللأا ةيانه ةعقاولا

ظ ِه حواتري يطخ ةقاط قرف ةلبقتسلما ةردقلا ر

ينب 44.6:

dBm لىإ

58.6:

- dBm ةراشلإا تاجوم نم لكل تايوتسلما ينب 3531

و ترمونان 3771

فايللأا نم تباث لوط دنع ترمونان (

L1 ).

لأا لاوطا فلاتخا عم لإ فايل

يننث

لوط لياجمإ ينمأت َّتم ،لياوتلا ىلع ةفلتخلما رهنلا عورف نم 371

ك ولي م تر ةراشإ قلاطلإ 1

dBm ـلا ىوتسم .لسرلما بناج دنع

OSNR نم

dBm 45.52 لىإ

99.74 dBm

لاسرإ لوطب تاجولما لاوطأ نم لكل 371

ترموليك قرف ديدحتل لِبقتسملل ةبسنلاب ًلاوبقم برتعي

دقلا ىوتسم لوطو ةردقلا ىوتسم قاطن ديدتح نإ .كلذ ىلع ًاءانب ءالما ىوتسم لىإ يرشي يذلا ةر

فرصلا هايم ىوتسم ةبقارلم ةعونتلما ةينبلا في ءالما ىوتسلم رعشتسم ريوطتل رشؤمك لمعي لاسرلإا يم امك .رانهلأاو ومادختسا نك

ك رهنلا ىوتسم نع ةديعب ةبقارم ةيلآ نوكتل

ًا ءزج راذنلإا ماظن نم

رهنلا لوط ىلع ةيراتج وأ ةينكس ةقطنم يأ في ةيعيبطلا ثراوكلل ركبلما تيلاو

ةايلحا ذقنت نأ نكيم

.

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APPROVAL PAGE

I certify that I have supervised and read this study and that in my opinion; it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Master of Science in (Communication Engineering).

………

Mohd Shahnan Zainal Abidin Supervisor

I certify that I have read this study and that in my opinion it conforms to acceptable standards of scholarly presentation and is fully adequate, in scope and quality, as a dissertation for the degree of Master of Science in (Communication Engineering).

………

Norazlina Binti Saidin Internal Examiner

………

Suriza Ahmad Zabidi Internal Examiner

This dissertation was submitted to the Department of Electrical and Computer Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science in (Communication Engineering).

………

Mohamed Hadi Habaebi

Head, Department of Electrical and Computer Engineering

This dissertation was submitted to the Kulliyyah of Engineering and is accepted as a fulfilment of the requirement for the degree of Master of Science in (Communication Engineering).

………

Erry Yulian Triblas Adesta Dean, Kulliyyah of Engineering

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DECLARATION

I hereby declare that this dissertation is the result of my own investigations, except where otherwise stated. I also declare that it has not been previously or concurrently submitted for any other degrees at IIUM or other institutions.

Sunity Halder

Signature ... Date ...

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INTERNATIONAL ISLAMIC UNIVERSITY MALAYSIA

DECLARATION OF COPYRIGHT AND AFFIRMATION OF FAIR USE OF UNPUBLISHED RESEARCH

DESIGN OF OPTICAL SENSOR FOR WATER LEVEL DETECTION BASED ON FRESNEL REFLECTION

I declare that the copyright holders of this dissertation are jointly owned by the student and IIUM.

Copyright © 2018 by Sunity Halder and International Islamic University Malaysia. All rights reserved.

No part of this unpublished research may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder except as provided below

1. Any material contained in or derived from this unpublished research may only be used by others in their writing with due acknowledgement.

2. IIUM or its library will have the right to make and transmit copies (print or electronic) for institutional and academic purposes.

3. The IIUM library will have the right to make, store in a retrieved system and supply copies of this unpublished research if requested by other universities and research libraries.

By signing this form, I acknowledged that I have read and understand the IIUM Intellectual Property Right and Commercialization policy.

Affirmed by Sunity Halder

……..……….. ………..

Signature Date

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This thesis is dedicated to my family

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ACKNOWLEDGEMENTS

In the name of Allah, Most Gracious, Most Compassionate. All Praise be to Allah (S.W.T.), the Almighty for bestowing His Bounty and Mercy, Solawat and Salam to our beloved prophet (P.B.U.H).

First and foremost, all my du’a and gratitude make to Allah the Almighty for granting me the nikmah and abilities to pursue my post graduate studies. With His blessings, I can withstand all the obstacles and hardships throughout the journey in completing this research. With his favors, I can carry out and complete this thesis.

I would like to express my sincere gratitude and respect to my supervisor Dr Mohd Shahanan Zainal Abidin for his constructive suggestions, guidelines and continuous supervision. Also, I like to give thank and respect to my co-supervisor Dr Belal Ahmed Hamida for his many invaluable suggestions, discussion and support.

I would like to pay my thanks to the Department of Electrical and Computer Engineering and Research Management Center, International Islamic University for various laboratory supports, financial supports and cooperation. special thanks to all my friends who have helped me through various suggestions and inspirations. Finally, I wish to express my love and gratitude to my beloved family members, my parents and husband especially for their understanding, continuous encourage and support me, and love throughout the study periods. Besides, I would like to acknowledge people in Post Graduate Lab who have helped and encourage me a lot during the period of my studies. Their encouragement and support have helped me a lot in increasing my self- esteem and all sort of challenge I encountered seemed bearable.

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TABLE OF CONTENTS

Abstract ... ii

Abstract In Arabic ... iii

Approval Page ... iv

Declaration ... v

Copyright Page ... vi

Dedication ... vii

Acknowledgements ... viii

List Of Tables ... xi

List Of Figures ... xii

List Of Abbreviations ... xiv

CHAPTER ONE: INTRODUCTION ... 1

1.1 Overview... 1

1.2 Problem Statement ... 3

1.3 Objective ... 4

1.4 Methodology ... 5

1.4.1 Literature Research ... 5

1.4.2 Design and Simulation Using Optisystem ... 5

1.4.3 Performance Analysis of the Research ... 5

1.5 Scope... 6

1.6 Organization Of Dissertation ... 6

CHAPTER TWO: LITERATURE REVIEW ... 8

2.1 Introduction... 8

2.2 Fiber Optics And Transmission ... 8

2.3 Fresnel Based Reflection ... 11

2.4 Existing Optical Fiber Sensor ... 14

2.4.1 Fiber-Optic based Temperature Sensor ... 14

2.4.2 Fiber-Optic based Liquid-Level Sensor ... 15

2.4.3 Time Domain Reflectometer Sensor (TDR) ... 17

2.4.4 Optical Fiber Refractive Index Sensor (RI) ... 18

2.4.5 Fiber-Optic Based Water Level Sensor... 19

2.5 Optical Sensor In Different Application ... 22

2.5.1 Environment ... 22

2.5.2 Biological and Chemical ... 23

CHAPTER THREE: RESEARCH METHODOLOGY ... 25

3.1 Overview... 25

3.2 Optisystem Simulation Software ... 25

3.3 Main Components Used ... 27

3.3.1 Optical Transmitter ... 27

3.3.2 Optical Laser ... 28

3.3.3 Splitter and Combiner ... 28

3.3.4 Wavelength Division Multiplexing and De-Multiplexing ... 29

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3.3.5 Circulator... 30

3.3.6 Reflector-Bidirectional ... 30

3.3.7 Optical Power Meter ... 31

3.3.8 Optical Signal to Noise Ratio (OSNR) ... 31

3.3.9 Optical Spectrum Analyzer (OSA) ... 32

3.4 Optical Software Simulation Tools ... 32

3.5 Design Of The Project ... 33

3.6 Summary ... 39

CHAPTER FOUR: RESULT AND ANALYSIS ... 40

4.1 Overview... 40

4.2 Power ... 40

4.3 Optical Fiber Length ... 42

4.4 Comparison With A Different Wavelength ... 43

4.5 Comparison Of Osnr Performance ... 44

4.6 Comparison Using Different Power Splitter/Combiner ... 46

4.7 Summary ... 47

CHAPTER FIVE: CONCLUSION AND FUTURE WORK ... 49

5.1 Conclusion ... 49

5.2 Future Work ... 50

REFERENCES ... 51

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LIST OF TABLES

Table 4.1 Power Received at Sensing Point 47

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LIST OF FIGURES

Figure 1.1 Flood Management Chart of Scope 6

Figure 2.1 Physical Appearance of Fiber Optic 9

Figure 2.2 Total Internal Reflection occurs 10

Figure 2.3 Micro-Bending Losses and Macro-Bending Loss 11

Figure 2.4 Variable used in Fresnel Reflection 12

Figure 2.5 Reflection and refraction of the light 13

Figure 2.6 Diagram of a FBG-based liquid-level sensor system (a) and (b)

structure of the cantilever rod and (b) experimental setup. 16 Figure 2.7 The configuration of the TDR-based system (C. B., 2008) 17 Figure 2.8 Experimental setup of the multipoint measurement RI sensor 19 Figure 2.9 Experimental setup employed for the simultaneous

measurement of temperature and water level using the fiber-

optic water level sensor 20

Figure 2.10 The sensor head and Schematic diagram of the testing

equipment 21

Figure 2.11 Diagram of discrete Liquid Propellant Level Sensor System Simple system construction indicates feasibility of any desired

resolution for fluid levels 21

Figure 2.12 Diagram of discrete Liquid Propellant Level Sensor System Simple system construction indicates feasibility of any desired

resolution for fluid levels 22

Figure 3.1 Optisystem Graphical User Interface (GUI) 26

Figure 3.2 Multiplexing System 29

Figure 3.3 Input and output reading of circulator 30

Figure 3.4 Output of the optical power meter 31

Figure 3.5 Diagram of Optical spectrum analyzer (OSA) 32

Figure 3.6 Model of water level detection system based on Fresnel

reflection 33

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Figure 3.7 Model of Water Level Sensor measurement system in river 34 Figure 3.8 Water Level Sensor measurement by using Optisystem 36 Figure 3.9 Reflectors used in the subsystem for branch 1 with 1x4 power

coupler 37

Figure 3.10 Reflectors used in the subsystem for branch 2 with 1x8 power

coupler 38

Figure 4.1 Power received Prx vs number of Sensing point (sp1 to sp8) 41 Figure 4.2 Diagram of 1550 nm received power, Prx over sensing points

(SP) for 8-reflectors for different fiber lengths 42 Figure 4.3 Comparison of received power Prx with fiber 1310 nn and 1550

nm length of wavelength 43

Figure 4.4 Diagram of OSNR architecture 44

Figure 4.5 Diagram of OSNR 45

Figure 4.6 Comparison of OSNR and with total wavelength of 1550 nm 46

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LIST OF ABBREVIATIONS

TIR Total Internal Reflection LED Light Emitting Diode TDR Time Domain Reflector

WDM Wavelength Division Multiplexing CW Laser Continuous Wave Laser

SMF Single Mode Fiber MMF Multi-Mode Fiber

RI Refractive Index

FBG Fiber Bragg Grating CW Laser Continuous Wave Laser OPM Optical Power Meter OSA Optical Spectrum Analyzer OSNR Optical Signal Noise to Ratio n1 Refractive index (high index) n2 Refractive index (low index)

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CHAPTER ONE INTRODUCTION

1.1 OVERVIEW

A flood is a natural occurrence where a piece of land or area typically a dry land, suddenly gets submerged under a water. Water flood can rapidly occur and recede quickly. It is caused by many ways such as heavy rain, river overflows, lakes and coastal flooding (large or massive strong winds towards the ocean carry rains in them).

It is important to note that water naturally flows from high areas to low lying areas.

This means low-lying areas may flood quickly before it begins to get to higher ground.

The change of optical fiber sensor has incredible progress throughout the last few centuries. Different types of fiber sensors have demonstrated and implanted in research which can be found in this literature. The optical fiber medium for laser signal transmission towards a sensing point is desirable due to some attractive structures that make optical fiber sensors (OFS) greater to electrical structures.

Approximately of the key benefits are low power consumption, capability to survive with large temperature, real-time monitoring, protection from electromagnetic interference and large distance between the transmitter and receiver.

The Fresnel reflection signal transmitted between the two-different medium with two different refractive indexes; fiber-water and fiber-air interface. Fresnel reflection happens in the coupling region between more than one medium those which has a various refractive index, and the light is reflected the original medium. The portion of optical power (R) whereas is reflected at the interface is given which n1 is the refractive index and the air is core of optical fiber and water material of n2

refractive index are 1.464 and 1.33 that contact the fiber core n2 is the refractive index

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of the contact material water is 1.33 that contacts the fiber core. The decreasing difference between air and water materials n1 and n2, which is included the Fresnel reflection also decreases. Therefore, decreases of reflected light whereas the optical power of broadcast rises. The optical properties light emitted from a light source if n2 can be varied by moving sure physical constraints. The physical contact material parameters measure by using sensing element.

A less quantity of fiber-optic sensors and water level indicators have been considered of using several sensing optical techniques for example light intensity modulation (Sohn, K., & Shim, J.,2009), wavelength shift (Wang, T., 2015) and optical power changes [Boon, J. D., & Brubaker, J. M. (2008). Fiber optic has caught the eye of the researcher and gets a great attention due to its outstanding characteristics (Nogueira, R., 2012). The fiber optic detecting applications are identified with the regulation power of propagating light. Fiber optic also has the characteristics that modulate the reflective intensity and able to measure the change of surrounding (ChSarma, K., 2008). This characteristic makes the fiber optic widely used in technology to detect and monitoring the liquid level. As the operation is based on an optical laser with a different wavelength, a single mode fiber, and Fresnel reflection, it would be reliable as an inexpensive water level that offer all the natural advantages of optical fiber sensors despite their simple and low-cost design. Fresnel’s equation clarifies transmission and reflection possessions of light which are emitting at the interface of two dielectric media.

The water level monitoring sensor is a very important technology to monitor natural disasters, for example, storm and flooding damages, typhoons, and this system provide the alarm for an upcoming disaster. Recently, optical fiber technologies are also used to measure the water level or the liquid level. This optical water level

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detection sensor is very effective to save the human life and social society. The system failure rate is low because of its simple structure. It is easy to install and measure the water level detection of rivers, the pool’s and some other drainage system. Moreover, for the developing countries those are suffering from the environmental disaster and frequent flood every year, this system is necessary and crucial.

1.2 PROBLEM STATEMENT

From the economic point of view in Malaysia, the flood is the most destructive category of natural hazardous. The economic loss includes the personal property, business facilities, stocks, utilities, and carriages. These happen because of the level of water increase in a very short time without any warning. According to the loss of life in Malaysia due to a natural disaster, the flood is the first position tracked by an earthquake. In the early century, a lot of methods have been used to control the flood such as building a water dam, the tank, and the lake. Technology growth, various equipment has been invented to sense the flood level like flood monitoring sensing, water sensor, and microcontroller.

Some of the citizen in the rural area like in Sabah and Sarawak are living in the high-risk zone where there is no real-time warning mechanism. The percentage of the availability of technologies and network coverage to reach the rural area is very small and sometimes there is no access to electricity. Therefore, most of them become the victims of flood due to the lack of real-time early warning system and continuous rainfall.

A lot of researches have been done to how to overcome the water level detection problem. A microcontroller to detect the water level and the transmitter identifies the analogue signal of water and converts it into the digital signal. But,

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detection length from the monitoring centre the method needs electrical energy the distance between the terminals limit. Water level detection method varies such as Brillouin, Raman and Rayleigh scattering, but these techniques require much complex optical setup and arrangement.

In this research, Fresnel reflection-based water level sensor is developed to sense presence of the water. By dividing launch optical power by means of optical coupler to its output in stack levels, consequences of water level rising can be quantified. The level of water detection setup simulation which is carried out by Optisystem software. The setup performance is evaluated based on its transmission length, power received and sensing point levels. The higher the number of sensing point, the higher the water level resolution. The received power then will affect the length of the fiber that will be used in the system. The system can detect the increment of water at the upper stream by using Fresnel based reflection concept, and we can give the early warning system to the locality who living nearby the downstream.

1.3 OBJECTIVE

The goal of this research is to design an optical water level sensor of a flood warning system.

i. To investigate and design optical fiber water detection based on Fresnel reflection.

ii. To determine the range of a received power at receiver side due to variation of designed water level.

iii. To analyze power level and fiber length based on receiver power and OSNR of the Fresnel based optical water level sensor.

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5 1.4 METHODOLOGY

To attain the objectives stated above, the subsequent approach must be taken

1.4.1 Literature Research

The study of the research papers, journals, articles and the book chapter on flood detection sensor has been made. The other researchers introduced the flood detection sensor and flood monitoring system. In this research, the study of fiber sensor has been done as a new alternative, water level, detecting sensor the pre-flood warning system in terms of wavelength shifting and optical power detecting. All relevant information associated with optical fiber, an optical sensor, power, wavelength, and the length of the fiber discussed in the literature.

1.4.2 Design and Simulation Using Optisystem

After collecting all the related information, the next step is to set up the simulation model by Optisystem Software based on previous research. Based on few of parameters the flood detector system has been designed. The parameters such as, the power of CW Laser, the optimum length of optical fiber, the optimum number of reflectors. The water level detection design based on Fresnel reflection which relates to the received power signal, and wavelength

1.4.3 Performance Analysis of the Research

We run the performance and modelling of water level detection for an optical sensor based on Fresnel reflection and the simulation data was collected and analysed.

Moreover, the discussion and conclusion have been done based on the collected result.

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6 1.5 SCOPE

Fiber optic sensor will be used as the main component to detect the presence of water.

From this, we can detect the level of the water in the river. The main parameters observed in this research are the received power and the different wavelength applied to detect the percentage of light reflection. Overall, this dissertation can be the alternative way to detect the flood and send an early warning. The main scope of the system shows Figure 1.1 a different way, which can be defined the flood management system.

Figure 1.1 Flood Management Chart of Scope

1.6 ORGANIZATION OF DISSERTATION

The dissertation report is organized in five chapters where each chapter will cover different scope.

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Chapter 1: This Chapter delivers of research work which has included the

objective, research problems, scope, and methodology.

Chapter 2: This chapter provides in-detail literature study and provides the all

related information, topics, and terms that will be used in the project. Besides that, advantages and disadvantages of this system and how to make it easy to implement for flood detection are discussed.

Chapter 3: In this chapter, it tells us about the method used in the process to

achieve the objectives of research. Optisystem software has been used to design the project.

Chapter 4: This chapter contains the simulation results and analysis in each

part of the model of an optical water level sensor of a flood warning system based on optical fiber length, wavelength shift, and optical power changes.

Chapter 5: After achieving the objectives, a summary of the dissertation and some suggestions are highlighted. Moreover, possible future work is also being outlined for future work planning.

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CHAPTER TWO LITERATURE REVIEW

2.1 INTRODUCTION

This chapter brings a research background of this project. It also covered the previous related research and encouraged to apply the Fresnel reflection based optical sensor to measure the water level detection. At first, the science behind the optical fiber and its transmission technology will be discussed. Then the existing Fresnel reflection sensor and other types of the optic sensor will be characterized. Finally, different types of an optical sensor for the water level detection system.

2.2 FIBER OPTICS AND TRANSMISSION

An optical fiber is a flexible, convenient data transmission medium. It carries a huge amount of data information like light signal, audio, video text, where a signal transmitted through a plastic or glass fiber. Fiber optic biosensor has started to implement (Leung, A., Shankar, P. M., & Mutharasan, R.,2008) in a different application in daily life and in research from the last two decades which already achieved a great advancement, which is immobialized tapered in fiber optic at 1310 nm and 1550 nm of stagnanr and condtion . In the early state, fiber optic has been used for the medical purpose as it transfers light and images for some medical applications and some encrypted data transmission in the military of the USA. After a few years, researchers realize that the benefits of using the optical fiber and a lot of research group started to implement an optical fiber in real life applications (Rosolem, J. B., Dini, Durelli, A. S., 2013). The optical fiber has three important parts which are the core, cladding, and coating. The detailed physical configuration of the fiber optic

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is shown in Figure 2.1. The core is the minimum part of the fiber optic which is in the middle of it and fabricated from glass or plastic silica. The next layer is cladding which covers the core and finally, a plastic or other outside layer cover the whole fiber for protection. Within the core, a huge amount of data in a form of light signals propagates with the speed of light (Ammar, M., Ahmad, 2015). Moreover, the cladding is the outer part of the core which its refractive is smaller than the core to allow the light travels to the core without any distraction or loss.

Figure 2.1 Physical Appearance of Fiber Optic

When, the light is reflected in the core and light travels from high index medium (n1) to the lower index medium (n2), it’s called reflection. The light is reflected that angle is called the critical angle (Ɵc). Once the light hit the cladding, by obeying the law of reflection, the core light will be reflected if the critical angle is greater than the normal angle

c)n1 sin = n2sin (2.1)

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The sine of an angle cannot exceed the directions which already shows the Snell’s Law equation above (2.1), so it must have n1>n2. From the equation (2.1) above, total internal reflection cannot happen when n1, than n2. The material absorption losses can be divided into two categories, which are intrinsic absorption and extrinsic absorption. Intrinsic absorption is an interaction of one or more component of glass, whereas the extrinsic absorption is caused by different impurities within the glass like transition metal or water. A scattering of light pulses when travels into the optical fiber and it affects to the bandwidth of the fiber dispersion which is defined in Figure 2.2,

Figure 2.2 Total Internal Reflection occurs

Moreover, the loss also occurs when the fiber is bending. Fiber bending is the phenomenon where the light energy is radiated at the bending point and some light passes through the optical fiber. There are two types of bending which are macro- bending and micro-bending. Macro-bending happened when the fiber is sharply bent, and some light passes through from it. From the information in Figure 2.3, those are

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