ARRAYED WAVEGUIDE GRATING

YASSIR ABDELRAHIM MOHAMED ABDELAZIZ

A project report submitted in partial fulfillment of the

requirements for the award of the degree of

Master of Engineering (Electrical-Electronics & Telecommunications)

Faculty of Electrical Engineering

Universiti Teknologi Malaysia

MAY 2008

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TO

My beloved father and mother.

My brothers (Waleed, Hisham and Tariq)

My lovely sister (Hiba).

My Uncles and Aunts.

All those I love.

All my friends.

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ACKNOWLEDGEMENT

First of all, I thank ALLAH SWT for giving me the strength and power

to finish this thesis. Also I would like to acknowledge my real and deep

appreciation to Assoc. Prof. Dr. Abu Sahmah bin Mohd Supa’at for his

kindness constant endeavor, guidance, help and encouraging me to work hard

towards making a good thesis.

I am also grateful to Eng. Mohamed Khalfallah Hassan for his

important ideas, thoughts; he always has a heartfelt feelings and careful about

the thesis and considered it as his. Also I would like to thank Eng. Abdulaziz

for his advices and help during accomplishment of the project.

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ABSTRACT

If compared to other kinds of communication networks, it is found that

fiber optics networks are the most rewarding communication networks, since 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.

Arrayed waveguide grating is one of the component that serves as a multiplexer

and de-multiplexer, also if connected with other device can work as an add

drop multiplexer for dense wave length division multiplexing, This is due to the

fact that AWG-based devices have been proven to be capable of precisely de-

multiplexing a high number of channels, with relatively low loss. The purpose

of this project is to investigate the operation function of the arrayed waveguide

grating and understand the performance parameters of the device, and then we

investigate those performance parameters and try to find out how those

parameters can affect crosstalk of the device. We make our study about only

the crosstalk because crosstalk is the most important parameter that can affect

the overall performance of the device. We used the waveguide spacing as a key

that relates all other parameters to the crosstalk, since there is an inverse

relationship between the spacing and the crosstalk. The result from this project

is that there should be a tradeoff between the crosstalk and the waveguide

spacing, where small waveguide spacing is required for compact design, and at

the same time whenever the spacing is small crosstalk will be high.

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ABSTRAK

Jika dibandingkan dengan lain-lain kaedah rangkaian komunikasi, rangkaian

fibre (gentian) optik merupakan kaedah yang mempunyai potensi tinggi sejak

dilaksanakan penggunaannya lagi. Sejak gentian adalah mencukupi untuk membaikan

kemudahan kepada pengguna-pengguna telekomunikasi (permintaan penghantaran data-

data besar) di dunia. Gentian optik merupakan saluran penghantaran berasaskan cahaya

menggunakan kaca senipis helaian rambut. Penggunaan rangkaian optikal secara meluas

menyebabkan peningkatan terhadap komponen-komponen yang memandu gelombang

optikal untuk kebaikan bagi proses ‘multiplexing’ dan ‘routing’. Kekisi pemandu

gelombang teratur merupakan salah satu komponen yang digunakan sebagai

‘multiplexer’ dan ‘de-multiplexer’, disamping bertindak sebagai pembolehubah

‘multiplexer’ jika disambung dengan peralatan lain. Untuk mengukur ketebalan

gelombang mengikut fakta, peralatan berasaskan AWG telah terbukti berupaya

‘demultiplex’ bilangan yang besar pada saluran-saluran dengan tepat dan kurang

kehilangan data. Objektif projek ini adalah untuk menyiasat fungsi pengoperasian bagi

kekisi pengawal gelombang teratur dan memahami serta menyiasat keupayaan

parameter-parameter pada perkakasan bagi mendapatkan bukti bagaimana setiap

parameter boleh mengakibatkan ‘crosstalk’ pada setiap perkakasan. Pemilihan

‘crosstalk’ adalah disebabkan oleh kepentingan yang boleh mengakibatkan kesan

terhadap keupayaan keseluruhan perkakasan. Ruang-ruang pemandu gelombang

dijadikan kunci bagi setiap parameter sehingga terdapat hubungan songsang di antara

ruang-ruang dan ‘crosstalk’. Hasil daripada projek ini, terdapat hubungan timbal balik

diantara ‘crocctalk’ dan ruang-ruang pemandu gelombang. Ruangan pemandu

gelombang yang kecil diperlukan untuk rekabentuk yang padat dan pada masa yang

sama apabila ruangan kecil maka ‘crosstalk’ akan menjadi tinggi.

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TABLE OF CONTENTS

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 xv

1 INTRODUCTION 1

1.1 Objective if the project

1.2 Scope of Project

1.3 Statement of Problem

1.4 Project Background

1.4.1 Forms of Photonic Network

1.5 Overview of the thesis

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2 OPTICAL COMMUNICATION AND Waveguides

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2.1 Fiber Optic Communication 10

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2.1.1 Applications

2.1.2 History

2.2.2 Technology

2.2 Fiber Optic System

2.2.1 Transmitters

2.2.2 Fiber

2.2.3 Amplifiers

2.2.4 Receivers

2.3 Wavelength Division Multiplexing and Dense

Wavelength Division Multiplexing

2.4 Bandwidth Distance Product

2.5 Limitations

2.5.1 Dispersion

2.5.2 Attenuation

2.5.3 Transmission Window

2.5.4 Regeneration

2.6 Last Mile

2.6.1 Comparison with Electrical Transmission

2.7 Optical Waveguide

2.7.1 Optical Waveguide Structure

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ARRAYED WAVEGUIDE GRATING

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3.1 Arrayed Waveguide Grating

3.2 AWG Tolerance

3.3 Current Design Process of an AWG (Basic De-

multiplexer)

3.3.1 Introduction

3.3.2 Receiver Waveguide Spacing

3.3.3 Free Propagation Region(FPR) Length

3.3.4 Arrayed Waveguide Length Increment

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3.3.5 Arrayed Waveguide Aperture Width

3.3.6 Free Spectral Range(FSR)

3.4 Issues Affecting the Performance of the AWG

3.4.1 Crosstalk

3.4.2 Insertion Loss

3.4.3 Polarization Dependent

3.4.4 Pass band Shape

3.4.5 Pass band Position

3.5 Summary

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ARRAYED WAVEGUIDE GRATING DESIGN

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4.1 Basic design considerations and design parameters

4.1.1 Wavelength dispersion angle and distance

4.1.2 Free spectral range

4.1.3 The channel spacing and focal length

4.1.4 The maximum number of the wavelength

channels

4.2 Performance parameters

4.3 Design and simulation

4.3.1 General Design Procedure

4.3.1.1 Channel spacing and the number of

ports

4.3.1.2 Free spectral range and the

diffraction order

4.3.1.3 Length difference

4.3.1.4 Pitches and shift positions

4.3.1.5 Focusing length

4.3.1.6 Number of the array waveguides

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5 RESULTS, ANALYSIS AND DISCUSSION 56

5.1 Project Methodology

5.2 AWG design Steps

5.3 Design Specifications

5.4 Results

5.4.1 Relation between the waveguide spacing and

Crosstalk

5.4.2 The relation between number of channels and

diffraction order relation

5.4.3 The relation between Waveguide spacing ∆x

and the diffraction order

5.4.4 Relation between focal length of the free

propagation region and waveguide spacing

relation

5.4.5 Relation between loss non uniformity and the

frequency shift corresponding the outermost

output waveguide

5.4.5 The relation between the Gaussian parameter

σg and the loss non uniformity Lu

5.5 Conclusion

5.6 Future Work

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REFERENCE 71

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LIST OF TABLES

TABLE TITLE PAGE

1.1 Comparisons of WDM de‐multiplexing technologies     2 

2.1 Transmission window brands 24

5.1 Design parameters 61

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LIST OF FIGURES

FIGURE NO. TITLE PAGE 1.1 Add drop Multiplexer 6 1.2 An Optical Cross Connect (OXC) employing a space

division switch for each wavelength.

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1.3 Passive optical Network 7 1.4 A Long Haul Network, utilizing Add-Drop Multiplexers

(ADMs) and Optical Cross Connects (OXCs)

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2.1 Typical dielectric waveguide 32 3.1 AWG general structure 34 3.2 The Input / Output Free Propagation Region 39 3.3 Passband shape 44 5.1 Flowchart of the methodology of the project 57 5.2 General flow of the design 59 5.3 Waveguide spacing and crosstalk relation

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5.4 Number of wavelength channel and diffraction order

relationship

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5.5 Waveguide spacing and diffraction order relationship

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5.6 Focal length and waveguide spacing relationships.

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5.7 Loss non uniformity Lu and the frequency shift

corresponding the outermost output waveguide ∆Vlu

relationship

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5.8 Relation between σg and Lu

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LIST OF SYMBOLS

λc - Central wavelength

m - Diffraction order

neff - effective refractive index

β - Propagation constant

RFPR - Radius of the free propagation region

∆ƒFSR - Free spectral range

ƒc - Central frequency

θmax - Maximum dispersion angle

Smax - distance to the outermost output waveguide

D - Required dispersion

ds - Displacement of the focal spot on the image plane

df - The change in frequency to cause this displacement

∆ℓ - Length increment

Ňg - Group refractive index

d - Pitch length

nc - Core refractive index

∆x - Waveguide spacing

∆λ - Channel Spacing

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Lf - Focal length of the FPR

N - Number of Channels

P - Number of Arrayed Waveguides

ωg - the mode field radius of the central guide

Lu - Loss non uniformity

∆Vlu - frequency shift corresponding the outermost output waveguide

σg - Gaussian Parameter

CHAPTER 1

INTRODUCTION

1.1 Objectives of this Project:

In recent years, arrayed waveguide gratings (AWG, also known as the

optical phased-array—Phasor, phased-array waveguide grating—PAWG, and

waveguide grating router—WGR) have become increasingly popular as

wavelength multiplexers/de-multiplexers (MUX/De-MUX) for dense

wavelength division multiplexing (DWDM). This is due to the fact that AWG-

based devices have been proven to be capable of precisely de-multiplexing a

high number of channels, with relatively low loss. Main features of the M (input) x N (output) AWG MUX/De-MUXes are low

fiber-to-fiber loss, narrow and accurate channel spacing, large channel number,

polarization insensitivity, high stability and reliability, and being suitable for

the mass production. Table 1 provides a summary and comparison of most

characteristics of WDM MUX/De-MUX technologies used in the current

WDM optical communications.

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Because the fabrication of the AWG is based on standardized

photolithographic techniques, the integration of the AWG offers many

advantages such as compactness, reliability, large fabrication tolerances and

significantly reduced fabrication and packaging costs. The inherent advantages

of the AWG also include precisely controlled channel spacing (easily matched

to the ITU grid), simple and accurate wavelength stabilization, and uniform

insertion loss.

Table.1.1 Comparisons of WDM de-multiplexing technologies

The devices are based on a fundamental principle of optics that light

waves of different wavelengths do not interfere with each other. If each channel

in an optical communication network makes use of light of a slightly different

wavelength, then the light from a large number of these channels can be carried

by a single optical fiber with negligible crosstalk between the channels. The

AWGs are used to multiplex channels of several wavelengths onto a single

optical fiber at the transmission end and are also used as de-multiplexers to

retrieve individual channels of different wavelengths at the receiving end of an

optical communication networks.

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The objectives of the project are therefore:

1. Design an arrayed waveguide grating device and its functions.

2. Studying the AWG performance parameters and their effects on

crosstalk.

3. Design an arrayed waveguide grating device AWG.

1.2 Scope of the Project:

The project has three aims which are:

1. To study and understand the function, concepts and operation of optical

devices used as multiplexing and de-multiplexing techniques for the optical

networks.

2. Studying some of the AWG performance parameters such as cross-talk,

insertion loss, free spectral range (FSR), and of course the polarization

dependence dispersion of the device.

3. Concerning the crosstalk as an important performance parameter and

finding out the parameters that can affect the crosstalk.

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1.3 Statement of the Problem:

Optical fiber is a popular carrier of long distance communications due

to its potential speed, flexibility and reliability. Attenuation and dispersion

problems in fiber, which limit the practical speed and distance of

communication, were partially resolved with the advent of the Erbium Doped

Fiber Amplifier (EDFA)[1],eliminating problems caused by attenuation.

However, the dispersion qualities of an optical fiber still force a compromise

between transmission distance and bandwidth, making it necessary to refresh

high-speed signals at intervals using opto-electronic repeaters. Solving the

dispersion problem in this manner is expensive, due to the additional cost of

high-speed electronics, and maintaining and upgrading the link is made more

difficult and costly (especially with a buried or under-water link). A more

elegant solution is found using Dense Wavelength Division Multiplexing

(DWDM), which effectively increases the useable bandwidth in a system

without electronic repeaters, and allows realization of a true photonic network.

The recent surge in demand for photonic components that meet

economic criteria as well as technical requirements in the telecom and datacom

industries has opened the door for novel technologies that enable unique

functions and/or unconventional high-yield manufacturing without sacrificing

high performance.

The project concerns with following problems:

1. Design an AWG device that is able to multiplex large number of

wavelength into single optical fiber so as to increase the robustness and

capacity of transmission of optical networks.

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2. The device should be able to retrieve individual channels of different

wavelengths at the receiving end.

3. The AWG device is sensitive to the performance parameters such as the

crosstalk, diffraction order, loss non uniformity, polarization dependence loss,

insertion loss.

1.4 Project Background:

Dense Wavelength Division Multiplexing (DWDM) is an efficient

method where several channels, each carried by a different wavelength, are

transmitted through a single optical fiber, utilizing more of the available

bandwidth without increasing the effects of dispersion [2]. Each channel, since

it is effectively separated from the others, can be independent in protocol,

speed, and direction of communication. DWDM also helps realize an all-optical

network architecture where signals are routed according to wavelength without

the need for electro-optical conversion. As a result, this type of network is

potentially faster and more flexible, and can be less costly to maintain when

compared to other methods. Arrayed Waveguide Gratings (AWGs) are optical

wavelength (de)multiplexers used in DWDM. As well as performing basic

(de)multiplexing functions, they can be combined with other components to

create add/drop multiplexers, Figure 1.1, used to pipe single wavelengths on

and off the network, and Cross Connects, Figure 1.2, used for routing. These

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devices can be passive, where the signal routing is fixed according to

wavelength, or active, as in Figures 1.1 and 1.2, where optical switches are

utilized to dynamically route the signals. Both circuits shown are transparent to

the data format, can allow bi-directional transfer of information, and function

entirely in the optical domain. These functions allow the construction of

different transparent optical network topologies, examples of the three major

types of these are described in the following subsection.

Fig 1.1 An add drop multiplexer (ADM)

Fig 1.2: An Optical Cross Connect (OXC) employing a space division switch for

each wavelength.

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1.4.1 Photonic Network:

The simplest form of optical network is the point-to-point network.

Optical multiplexers and de-multiplexers are required at each end of the link. In

this Configuration, DWDM simply increases the number of channels available

through one fiber. Passive Optical Networks (PONs) (Figure 1.3) use a

wavelength (de)multiplexer as a passive optical router, each wavelength

servicing an Optical Network Unit (ONU). This allows the ONUs to share a

single long optical fiber link back to the central office (CO) [3].

Fig 1.3: A Passive Optical Network

Long haul networks tend to have more than one point where channels

are added to and removed from the system, for example to provide a bi-

directional channel to an optical network unit. To add and remove channels

optical add drop multiplexers (OADMs) and Optical Cross Connects (OXCs)

are utilized (Figures 1.1 and 1.2), to allow single channels to be individually

piped off the network, and to route channels between sections of the network

respectively. Figure 1.4 shows an example configuration of a Long Haul

network.

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Fig 1.4: A Long Haul Network, utilizing Add-Drop Multiplexers (ADMs) and

Optical Cross Connects (OXCs)

The Arrayed Waveguide Grating (AWG) plays a crucial role in the

realization of modern optical networks [4]. The next section introduces the

principles of operation of the AWG, and then examines the design process.

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1.5 Overview of this thesis:

In chapter 1 a quick introduction of the AWG and some background of

WDM and DWDM are discussed. The objective, scope of the project and the

problem statement are described.

Chapter 2 introduces the optical communications and optical

waveguides. It covers the applications, technology, transmitter, amplifiers,

receivers, WDM, dispersion and attenuations.

Chapter 3 introduces the AWG device, its operation, functions, and then

over viewing the performance parameters that affect the performance of the

device.

Chapter 4 taking these performance parameters deeply and finding out

their importance.

Chapter 5 is for the results discussion, analysis and results of the

project.