OPTICAL Y-JUNCTION POWER SPLITTER

ALIYA ASHRAF

UNIVERSITI TEKNOLOGI MALAYSIA

iii

To my beloved family and friends.

.."Only those who dare to fail greatly, can ever achieve success greatly."

iv

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.

v

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.

vi

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 ).

vii

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

viii

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

ix

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

x

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

xi

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

xii

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

xiii

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

xiv

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

xv

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

xvi

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

xvii

Θ

- incident angle

λ

- wavelength

ρ

- density capacity

ε

- relative capacity

γ

- coefficient of volume expansion

Ld

- Distance of symmetry in waveguide

1

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

2

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

3

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.

4

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

5

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.

6

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

7

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.

8

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.