aliya ashraf - eprints.utm.myeprints.utm.my/id/eprint/6093/1/aliyaashrafmfke2007.pdf · padat) ke...
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To my beloved family and friends.
.."Only those who dare to fail greatly, can ever achieve success greatly."
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ACKNOWLEDGEMENT
First and foremost, I thank God the Almighty for giving me strength to finish this
thesis. I wish to express my sincere appreciation to my supervisor, Dr. Abu Sahmah bin
Mohd Supa’at for encouragement, guidance, critics and friendship. His mentorship style,
guiding rather than leading, has allowed me to strengthen my intellectual wings and
tackle problems on my own. When I needed it,he was always ready with a helpful
suggestion as well as explanations. I am very thankful to my family members for all
their support and love even though they were not around. I am also very grateful for the
encouragement provided by my friends who have provided assistance and support at
various occasions.
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ABSTRACT
In terms of performance, communication by optical fiber is potentially the most rewarding of all communications. It has been suggested that if the full potential of fiber optic communication is realized, a single fiber would be sufficient to serve the needs of telecommunication users (heavy data traffic demands) throughout the world. Fiber optics is the channeled transmission of light through hair thin glass. The explosive growth of optical networks has brought forward an increased need for guided wave optical components for the sake of multiplexing and routing. Beam splitters are a basic element of many optical fiber communication systems often providing a Y-junction by which signals separate sources can be combined, or the received power divided between two channels. The purpose of this project is to investigate how an asymmetric Y-junction behaves as a power splitter using switching function called Thermo Optic Effect. The focus is made on the polymers used for making various layers of waveguide, geometry and design parameters of waveguide which make it better than other Y-splitters. Polymers are relatively cheap starting material and can be processed from solution, which offers additional potential for cost savings compared to other technologies and also have the advantage of having large thermo optic coefficient range. The polymers used are polyurethane (thermal coefficient: -3.3 x 10-4 K-1, thermal conductivity: -0.19 W m-1K-1) and PMMA (thermal coefficient: -1.2 x 10-4 K-1, thermal conductivity: -0.17 W m-1K-1). The 2D thermal analysis is made on buried type waveguide. The analysis is based on how heating of one of the arms change its refractive index leading to low crosstalk, insertion loss, low driving power, coupling length and optimum switching characteristics. All simulations are done using BPM (Beam Propagation Method) and FEMLAB (Finite Element Method) software. The results from this project is a structure of Y-junction Power Splitter which is compact in size, less power consumption, low crosstalk and insertion loss by varying various parameters like branching angle, spacing between the two arms of Y-junction, refractive index change of one of the arms by means of heating phenomenon and switching temperature.
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ABSTRAK
Dari segi prestasi, komunikasi melalui fiber optik berpotensi memberikan ganjaran yang paling memuaskan untuk semua komunikasi. Cadangan pernah diutarakan sekiranya potensi fiber optik direalisasikan sepenuhnya, satu fiber mungkin sudah cukup untuk menampung keperluan pengguna telekomunikasi (permintaan trafik terhadap data yang padat) ke seluruh dunia. Fiber optik adalah saluran yang menghantar cahaya melalui kaca yang nipis. Ledakan pembangunan rangkaian fiber telah membawa kepada peningkatan terhadap penggunaan komponen optikal gelombang terpandu untuk permultipleks dan laluan (routing). Pecahan cahaya adalah elemen asas dalam kebanyakan sistem komunikasi fiber optik yang menjadikan penghubung-Y boleh berfungsi menyatukan isyarat yang datang dari sumber yang berasingan atau berupaya membahagikan kuasa yang diterima antara dua saluran. Tujuan projek ini adalah untuk mengkaji bagaimana penghubung-Y bertindak sebagai pecahan kuasa dengan menggunakan fungsi pensuisan yang dikenali sebagai Kesan Kepanasan Optik. Penekanan diberikan kepada penggunaan polimer dalam penghasilan kepelbagaian lapisan gelombang pandu, serta geometri dan parameter rekabentuk gelombang pandu yang mana menjadikan ia lebih baik berbanding penghubung-Y yang lain. Polimer merupakan material pengasas yang murah, yang mana berpotensi dalam penjimatan kos berbanding teknologi yang lain, dan juga memberikan kelebihan dengan mempunyai had pekali kepanasan optic yang besar. Polimer yang digunakan adalah poliureten (pekali kepanasan: -3.3 x 10-4 K-1, kealiran kepanasan: -0.19 W m-1K-1) dan PMMA (pekali kepanasan: -1.2 x 10-4 K-1, kealiran kepanasan: -0.17 W m-1K-1). Analisis kepanasan 2D akan dilaksanakan ke atas gelombang pandu yang tersembunyi. Analisis ini dibuat berdasarkan kepada bagaimana pemanasan pada satu lengan mengubah indeks pembiasan yang kemudiannya membawa kepada perbicaraan bersilang yang rendah, kehilangan penyisipan yang rendah, kuasa terpandu yang rendah, panjang perangkai dan ciri-ciri pensuisan optimum. Semua simulasi dilakukan menggunanakan perisian BPM (kaedah penyebaran cahaya) and FEMLAB (Keadah Elemen Terhad ).
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TABLE OF CONTENTS
CHAPTER TITLE
TITLE PAGE
PAGE NO i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGEMENT
iv
ABSTRACT
v
ABSTRAK
vi
TABLE OF CONTENTS
vii
LIST OF FIGURES LIST OF TABLES
xi
xv
LIST OF SYMBOLS
xvi
1 INTRODUCTION
1
1.1 Objective of this Project
1
1.2 Scope of this Project
3
1.3 Problem Statement
4
1.4 Project Background
5
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1.5 Overview of this Project
8
2 OPTICAL WAVEGUIDES
9
2.1 Optical Fiber transmission
9
2.1.1 Ray Theory 2.1.2 Electromagnetic Mode theory 2.1.3 Maxwell Equations
10
14
14
2.2 Optical waveguide structure 2.3 Polymer Waveguides 2.3.1 Material Thermal Propagation
17
19
20
3 ASYMMETRIC Y-JUNCTION POWER SPLITTER
22
3.1 Integrated Optics
22
3.2 Literature Review
24
3.3 Operational Principle
26
3.3.1 Geometry of Y-junction
31
4 MODELING AND DESIGN OF WAVEGUIDE
33
4.1 Numerical methods
33
4.1.1 Finite Difference Method
34
4.1.1.1 Crank Nicolson Method
36
4.1.2 Finite Element Method 37
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4.2 Heat Transfer Equation
40
4.3 Simulation Guide
42
4.3.1 Setting up the model
42
4.3.2 Options and settings
44
4.3.3 Draw Mode
45
4.3.4 Boundary Mode
46
4.3.5 Sub Domain Mode
48
4.3.6 Mesh Mode
49
5 RESULTS AND DISCUSSION
51
5.1 Project Methodology
51
5.2 Propagation of light through device
54
5.2.1 Light through parallel arms
54
5.2.2 Light through the S-bends
56
5.3 Electric field generated
58
5.4 Thermal Distribution
62
5.5 Optimum Configuration of Y-junction
66
5.5.1 Mode Conversion factor and Refractive index
66
5.5.2 Crosstalk and Mode Conversion Factor
68
5.5.3 Attenuation in S-bend and Heating power
69
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5.5.4 Crosstalk and Heating
Power(Coupling Efficiency)
70
6 FUTURE WORK AND CONCLUSION
71
6.1 Thermal Analysis Model 6.2 Thermal Coupling Model
71
72
6.3 Future Work
73
6.3.1 Variable Optical Attenuator
73
REFERENCES
6.3.2 Double Digital Optical Switch (DDOS)
75
76
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LIST OF FIGURES
FIGURE NO. TITLE
PAGE NO.
2.1 Cross section of a planar waveguide
10
2.2 Zigzag ray picture for light waves in planar wave guiding structure showing radiation mode
11
2.3 Zigzag ray picture for light waves in a planar wave guiding structure showing guided modes
12
2.4 Vector triangle describing the relationship between β, k and ҝ
13
2.5a Typical dielectric waveguides: a)Strip Loaded waveguide structure. b) Ridge waveguide structure. c)Air-clad-rib waveguide structure d) Buried type waveguide structure .e) Embedded waveguide structure
18
3.1 All of the light couples with the wider channel and very less light couples with narrower asymmetric channel.
28
3.2 An electrode used on channel 1 to change its refractive index leading to thermo coupling.
29
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3.3 The vertical cross section through a DOS
31
4.1 Finite Difference mesh for modeling of a rib Waveguide
34
4.2 Locating nodes (a) on centre of a mesh cell, or (b) on mesh points
35
4.4 Modeling of a buried waveguide using a Finite Element mesh
37
4.5 The buried type waveguide is divided into sub domain region, which are triangles
38
4.6 Model Navigator showing five tabbed pages
42
4.7 Model Navigator showing application modes
43
4.8 Add/Edit Constant dialog box
44
4.9 Axes/Grid settings
45
4.10 Draw Mode for buried type waveguide structure
46
4.11 Boundary settings
47
4.12 Insulated boundaries and uninsulated boundary (red)
47
4.13 Sub domain settings
48
4.14 Sub domain settings showing the selected domain (heater in red)
49
4.15 Mesh mode settings
50
5.1 Flowchart defining the procedural work done in this project
52
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5.2 The top view of the buried type Y-junction power splitter (using BPM)
54
5.3 Simulation for Y-junction
55
5.4a) The top view of the S-bend of buried type Y- junction power splitter (using BPM)
56
5.4b) Simulation for parallel arms.
57
5.5a) The transverse normalized mode field distribution of mode0 at Ld =2.5mm,Ld is length of the device .
59
5.5b) Mode1 at Ld=4.5mm,X=10µm
60
5.5c) The fundamental exclusively excited in the non heated arm.
61
5.6 2D thermal distribution with core dimension 2.5µm× 5µm and H= 2.5µm, at heater temperature 300K using FEMLAB 2.0
62
5.7a) Temperature profile plot at z=4mm with fixed heater distance, H=2.5m.
63
5.7b) Graph plotted shows waveguide temperature versus boundaries of the core (S-bend at z=4mm) with fixed heater distance, H=2.5m.
64
5.8a) Temperature profile plot at the gap between two parallel arms of the waveguide) with fixed heater distance, H=2.5m.
65
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5.8b) Graph plotted shows waveguide temperature versus
boundaries of the core (gap between two parallel arms of the waveguide) with fixed heater distance , H=2.5m.
65
5.9a) Mode conversion factor And Refractive index Change
67
5.9b) Plot showing relationship between Crosstalk and Mode conversion factor
68
5.9c) Plot showing relation between Attenuation in S- bend And Heating Power
69
5.9d) Plot showing the relationship between Crosstalk and Heating Power(Coupling Efficiency)
70
6.1 Top view of DOS with S-bend VOA
74
6.2 Evolution from DOS to DDOS
75
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LIST OF TABLES
TABLE NO.
TITLE PAGE NO.
1.1 The typical properties of waveguide material used in integrated optics is mentioned in the tabulated form.
7
3.1 The coupling ratio related to insertion loss
28
5.1 Opto-thermal parameters for utilized materials (Data Sheet, Technology Evaluation Report for Optical Interconnections and Enabling Technologies)
53
5.2 Numerical values for the design parameters of designed DOS.
53
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LIST OF SYMBOLS
A
- contact area
C
- heat capacity
d
- waveguide arm spacing
K
- thermal coupling estimation
k
- thermal conductivity
H
- distance from heater to waveguide core
h
- thickness of core arm
L
- Length of device
η
- refractive index
Q
- rate of heat flow
t
- thickness of the substrate
W
- Heater width
w
- Core arm width
β
- propagation constant
µ
- relative permittivity
∆
- temperature ratio difference
Φ
- change in polarizabilty with temperature
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Θ
- incident angle
λ
- wavelength
ρ
- density capacity
ε
- relative capacity
γ
- coefficient of volume expansion
Ld
- Distance of symmetry in waveguide
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CHAPTER 1
INTRODUCTION
1.1 Objective Of This Project
Today, optical network is developing rapidly as growing capacity demand in
telecommunication system is increasing. Optical transmission systems are typically
running at data rates of 2.5Gbits/s or 10Gbits/s per channel. There are many advantages
for designing switching elements using optical components. These advantages include
decreased switching time (less than 1/10 of a pico second (1012)), less cross talk and
interference, increased reliability, increased fault tolerance, enhanced transmission
capacity, economical broadband transport network construction, enhanced cross-connect
node throughput, and flexible service provisioning.
There exists a demand of combining information from separate channels,
transmission of the combined signals over a single optical fiber link and separation of
the individual channels at the receiver prior to routing to their individual destinations.
Hence, the application of integrated optics in this area is to provide optical methods for
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multiplexing, modulation and routing. More recently, interest has grown in these devices
to divide or combine optical signals for application within optical fiber information
system including data buses, local area networks, computer network links and
telecommunication access networks. These functions may be performed with a
combination of optical beam splitters, switches, modulators, filters, sources and
detectors.
Beam splitters are a basic element of many optical fiber communication systems
often providing a Y-junction by which signals from separate sources can be combined,
or the received power divided between two or more channels. The objectives of this
project are, therefore:
Model formulation of Y-junction which involves study of Y-junction geometry,
which makes it better than other beam splitters. To study how the length of
switch effects the operation and behavioral characteristics of switch.
Generation of Electric field modes within the waveguide.
Simulation of propagation of light through Y-junction.
Other optimum configurations like:
• Branching /Opening angle formed exactly at the junction of Y-channel.
• Switching characteristics like switching temperature, switching power,
switching functions(Thermo optic effect, Acoustic optic effect, magneto
optic effect, electro optic effect etc)
• Power consumption meaning how much power is required for thermal
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coupling to occur.
• Low crosstalk in Y-junction for lossless transmission through it.
1.2 Scope Of This Project
The aim of this project is twofold. Firstly, to study the effect of temperature on
the refractive index of the thermo-optic waveguide. The waveguide is heated by means
of a thin electrode which is placed on one of the channels of Y-junction. The electrode
leads to heating of one of the channels of Y- junction, in turn changing the density of
polymer, increasing the polarizability and variation in optical path from one channel to
other of Y-junction. In this project, the buried waveguide is considered and the
phenomenon of thermo coupling will be studied seriously.
The second investigation is to gain insight of the various design parameters of
the Y-channel power splitter. Here, we would determine how by varying he
opening/branching angle between to arms of Y-channel
• Effect the total length of the power splitter.
• Effect the power transmission in two channels of Y-junction.
The efforts will be made to keep the size of Y-junction power splitter small.
However, very short switch can lead to worst cases of crosstalk of about 2.25dB (which
in turn indicates the influence of interferometer effects on the device).All simulations are
done by means of software named MATLAB and BPM.
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1.3 Problem Statement
The use of visible optical carrier waves or light for communication has been
common for many years. Simple systems like signal fibers, reflecting mirrors and
signaling lamps have provided successful information transfer. The communication
using optical carrier wave guided along a glass fiber has a number of extremely
attractive features, several of which were apparent when the technique was originally
conceived. These features include enormous potential bandwidth, small size, and weight,
electrical isolation, low transmission loss, signal security, reliability, flexibility, potential
low cost etc.
Due to these enormous features of Optical fiber communication in comparison to
conventional electrical communication, there is seen an increase in the complexity of
Optical modules. So, the demand of time is to provide structure that have below
mentioned features.
Firstly, Optical device with simple compact structure meaning small in size and
less in weight. The industry will need to more to new technologies, such as flip chip
instead of wire bond and multi chip modules to reduce package size. Secondly, there
exists a need to keep the excess loss associated to Optical module very low so that the
obtained output power is approximately equal to the input power. Next is the demand to
manufacture Optical structures with less polarization dependence. However, this can be
achieved by making using of an appropriate switching function like thermo-optic effect
which is polarization insensitive and can be easily implemented by usage of electrically
driven by micro heaters. Finally, the Optical module should be such which can provide
NxN structure for power splitting and combining if required. Sometimes a switching unit
does not provide the desired value of some parameter; the Optical module must be such
that by cascading it into various stages, the problem is overcome. For instance, if the
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crosstalk of a single 1x2 switching unit(for example, a DOS) is not sufficient, it can be
improved by cascading several of these devices and thereby multiplying the crosstalk(in
dB) by the number of cascaded stages.
This project use polymer material to build Optical waveguide. Since polymer
has large range of thermo- optic coefficient, we will apply the thermo optic effect to
overcome the above stated problem statement.
1.4 Project Background
The term Integrated Optics came into being since 1969 and was first discovered
by Miller. The concept of Integrated Optics involves the realization of optical and
electro-optical elements which may be integrated in large numbers on to a single
substrate. The need of Integrated Optics is there because most of the equipments today
are still based on electronic signals meaning that optical signal has to be converted first
into electrical signal, amplified, regenerated or switched and then reconverted to optical
signals. It is called Optical-to-Electronics-to-Optical (OEO) conversion. This is well
explained in Chapter 3.
There are two major kinds of Integrated Optical Switch namely, Interferometer
switch like Directional Coupler, Mach Zehnder etc and Digital Optical switch. For the
concern of this project we consider DOS because of it’s low polarization and wavelength
insensitivity. Also unlike Interferometer switches, DOS doesn’t require tight control of
biasing condition and less sensitive to heat.
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With the beginning of Integrated Optics many waveguide fabrication techniques
have been proposed and used to form various optical waveguides on a variety of
substrates. The great variety of potential applications for integrated optical devices have
spurred intensive investigation of a large number of waveguide as well as substrate
materials over the past two decades. Initially, semi conductive substrates were studied
in the hope of facilitating easy interface with microelectronic or optoelectronic
components. Glass substrates were investigated with intended applications as advance
passive fiber-optical components. Electro-optical crystal substrates were explored for the
development of optical switches, modulators, and optical signal –processing devices.
Non linear optical materials were studied in the attempts to make high efficiency optical
harmonic generators. But none of them fulfill all desirable criteria at the same time. An
interesting alternative is polymers which offer some unique advantages.
Polymers are a relatively cheap starting material and can be processed from
solution, which offers additional potential for cost savings compared to other
technologies. Besides this polymers have wide range of refractive index from (-1 x 10-4
K-1 to -4x 10-4 K-1) which leads to power efficient dynamic components. Waveguide can
be designed with very large or very small index contrast between core and cladding
(0%-35%). Polymer can also have very low optical loss<0.1dB/cm at the
telecommunication wavelengths 1310nm and 1550 nm(John M. Senior, 1992).At present
polymers find their application widely in optical communication devices like switches,
couplers, filters, attenuators, polarization, controllers, dispersion compensators,
modulators, laser and amplifiers. Polymer materials have proved to have satisfactory
light-guiding characteristics. In principle, it is possible to achieve low optical
loss(infrared), high thermal and environmental stability, high thermo optic effects, low
thermal conductivity, good adhesion to metals and silica, and refractive index tailoring.
Furthermore modifying the chemical structure or doping the polymers with guest
molecules can tune the physical properties far simpler than in the case of semiconductors
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or dielectrics. Polymers with different functionalities can be integrated on the same chip
and offer a versatile platform. Additionally, the processing of polymers is usually
compatible with semiconductors or dielectrics, which allow hybrid integration.
Table 1.Some of the typical properties of waveguide material used in integrated optics is
mentioned in the tabulated form.
Propagation
Loss(dB/cm)
Refractive
Index
Index
Contrast(ηcor
ηclad)/(ηcor)
Birefringence T/O
Coef.
dn/dt
[K-1 ]
Max.
Modulation
Freq.
Silica 0.1 1.5 0-1.5% 10-4-10-2 10-5 1kHz(TO)
Silicon 0.1 3.5 70% 10-4-10-2 1.8x10-4 1kHz(TO)
Polymers 0.1 1.3-1.7 0-35% 10-6-10-2 -1x10-4
- 4x 10-4
1kHz(TO)
Lithium
Niobate
0.5 2.2 0-0.5% 10-2-10-1 10-5 40GHz(EO)
Indium
Phosphide
3 3.1 0-3% 10-3 0.8x10-4 40GHz(EO)
Furthermore modifying the chemical structure or doping the polymers with guest
molecules can tune the physical properties far simpler than in the case of semiconductors
or dielectrics. Polymers with different functionalities can be integrated on the same chip
and offer a versatile platform. Additionally, the processing of polymers is usually
compatible with semiconductors or dielectrics, which allow hybrid integration. Polymer
materials have proved to satisfy specific applications and are thus of wide interest
nowadays.
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In this project we consider Polyurethane as core material (η1= 1.573) and
Polymethylmethacrylate as cladding (η3 = 1.49 to 1.56). The Thermo optic effect will be
studied over the refractive index of core in this project and also Thermo coupling of
power in the two channels of Y-channel.
1.5 Overview Of This Project
This part of Introduction provides the work frame of Project 1. This report
consists of four Chapters including the Introduction.
Chapter2 holds the discussion about the phenomenon of light traveling through a
waveguide. Here, the propagation of light wave is expressed in terms of Maxwell
Equations. The various types of Optical waveguides are discussed. However, for this
project we consider buried type waveguide.
Chapter3 involves the methodology of the project. Here, the Operational
principle of thermo-optic waveguide is discussed. There are certain numerical methods
(formulas) which describe the relationship between many light propagation parameters
and further used for simulation purpose.
Chapter 4 is about the design and geometry of Optical Y-junction power splitter.
Chapter 5 holds the simulations for this project. All the results are produced using
FEMLAB and BPM software. Future work for this project is discussed in Chapter 6.