UNIVERSITI PUTRA MALAYSIA

DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM

MOHD SHAHNAN BIN ZAINAL ABIDIN

FK 2004 94

DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM

By

MOHD SHAHNAN BIN ZAINAL ABIDIN

Thesis Submitted to the School of Graduate Studies, Universiti Putra Malaysia, in Fulfilment of the Requirements for the Degree of Master of Science

December 2004

DEDICATION

To

My Mother; SitiKhadijah Bt. Kila

My Father;

Zainal Abidin Bin Yahaya

Brothers and Sister; Mohd Faizal, Mohd Haris, Mohd Rafie, Siti Nurathirah

Relatives, Friends, Teachers and Neighbours

Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of the requirement for the degree of Master of Science

DESIGN AND DEVELOPMENT OF REMOTELY PUMPED ERBIUM DOPED FIBER AMPLIFIER TRANSMISSION SYSTEM

By

MOHD SHAHNAN BIN ZAINAL ABIDIN

December 2004

Chairman: Associate Professor Mohd Adzir Mahdi, Ph.D. Faculty: Engineering

Erbium Doped Fiber Amplifier (EDFA) has been deployed extensively in optical

communication systems especially for a long haul transmission link. Basically,

EDFA requires a local pump laser for its optical amplification. This design will

encounter problem if the amplifier is located at the middle of transmission line where

power supply for the pump laser is unavailable. Therefore, remotely pumped

amplifier can overcome the problem by injecting the pump light from either side of

the transmission ends; transmitter or receiver.

This dissertation reveals a new technique of designing a repeaterless transmission

system using a remotely pumped EDFA. By varying the length of transmission fiber

before and after EDFA, its location can be optimized for a specific pump power. A

bit error rate is used as the main performance parameter and its threshold value is set

at better than 10-10. The optimized location of EDFA will lead to the maximum

transmission distance where it is found that the location of EDFA is closer to the

receiver side.

ii

In conclusion, the EDFA location on transmission line using remotely pumped

technique gives major impact to the system’s performance.

iii

Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai memenuhi keperluan untuk ijazah Master Sains

REKABENTUK DAN PEMBANGUNAN SISTEM TRANSMISI PENGUAT

GENTIAN TERDOP ERBIUM DIPAM SECARA JAUH

Oleh

MOHD SHAHNAN BIN ZAINAL ABIDIN

Disember 2004

Pengerusi: Profesor Madya Mohd Adzir Mahdi, Ph.D. Fakulti: Kejuruteraan Penguat Gentian Terdop Erbium (EDFA) telah digunakan secara meluas di dalam

sistem komunikasi optikal terutamanya untuk sistem talian transmisi panjang. Secara

dasarnya, EDFA memerlukan laser pam setempat untuk penguatan optiknya.

Rekabentuk ini akan menghadapi masalah jika penguat itu diletakkan di tengah talian

transmisi di mana bekalan kuasa untuk laser pam tidak dapat diperolehi. Oleh itu,

penguat dipam secara jauh mampu mengatasi masalah ini dengan menyuntik cahaya

pam daripada mana-mana belah hujung transmisi; penghantar atau penerima.

Tesis ini mendedahkan suatu teknik baru dalam merekabentuk sistem transmisi tanpa

ulangan menggunakan EDFA yang dipam secara jauh. Dengan mengubah panjang

gentian transmisi sebelum dan selepas EDFA, lokasinya boleh dioptimumkan bagi

kuasa pam yang spesifik. Kadar kesilapan bit diguna sebagai parameter prestasi dan

nilai ambang ditetapkan kepada lebih baik daripada 10-10. Lokasi EDFA yang

optimum akan membawa kepada jarak transmisi maksimum yang mana didapati

bahawa lokasi EDFA tersebut adalah lebih dekat kepada sebelah penerima.

iv

Kesimpulannya, lokasi EDFA pada talian transmisi menggunakan teknik pam secara

jauh memberi kesan yang besar kepada prestasi sistem.

v

ACKNOWLEDGEMENTS

Alhamdulillah, thanks to ALLAH the almighty. All blessing to Prophet Muhammad,

may Allah bless upon him.

My acknowledgement firstly goes to supervisor Associate Professor Dr. Mohd Adzir

Mahdi, for his continuous and valuable advices since the early stages of the research.

His professional thoughts, comments and opinions in have guided me on a right

track. I thank also to both members of the supervisory committee, Mr. Mohd Hanif

Yaacob and Mrs. Siti Barirah Ahmad Anas.

A special gratitude to all my friends in Photonic and Fiber Optics System

Laboratory, Engineering Faculty, University Putra Malaysia. Your constant support

gives additional strength to complete the thesis. Not to forget as well staffs of

Kulliyyah Engineering, International Islamic University Malaysia.

Millions of appreciations are bound for Wan Farha Bt. Wan Abdul Fatah for the

inspiration from the beginning of the research. Finally to my parent, Siti Khadijah Bt.

Kila and Zainal Abidin Bin Yahaya, for the understanding until the end of my study.

vi

I certify that an Examination Committee met on 30th December 2004 to conduct the final examination of Mohd Shahnan bin Zainal Abidin on his Master of Science thesis entitled “Design and Development of Remotely Pumped Erbium Doped Fiber Amplifier Transmission System” in accordance with Universiti Pertanian Malaysia (Higher Degree) Act 1980 and Universiti Pertanian Malaysia (Higher Degree) Regulations 1981. The Committee recommends that the candidate be awarded the relevant degree. Members of the Examination Committee are as follows: Khairi Yusof, Ph.D. Lecturer Faculty of Engineering Universiti Putra Malaysia (Chairman) Mohamad Khazani Abdullah, Ph.D. Associate Professor Faculty of Engineering Universiti Putra Malaysia (Member) Syed Javaid Iqbal, Ph.D. Lecturer Faculty of Engineering Universiti Putra Malaysia (Member) Harith Ahmad, Ph.D. Professor Faculty of Science Universiti Malaya (Independent Examiner) __________________________________ GULAM RUSUL RAHMAT ALI, Ph.D. Professor/Deputy Dean School of Graduate Studies Universiti Putra Malaysia Date:

vii

This thesis submitted to the Senate of Universiti Putra Malaysia and has been accepted as fulfilment of the requirement for the degree of Master of Science. The members of Supervisory Committee are as follows: MOHD ADZIR MAHDI, Ph.D. Associates Professor Faculty of Engineering Universiti Putra Malaysia (Chairman) MOHD HANIF BIN YAACOB Lecturer Faculty of Engineering Universiti Putra Malaysia (Member) SITI BARIRAH BT AHMAD ANAS Lecturer Faculty of Engineering Universiti Putra Malaysia (Member)

_______________________ AINI IDERIS, PhD Professor/Dean School of Graduate Studies Universiti Putra Malaysia Date:

viii

DECLARATION

I hereby declare that the thesis is based on my original work except for the quotation and citations which have been duly acknowledged. I also declare that it is not been previously or concurrently submitted for any other degree at UPM or other institutions.

_____________________________________ MOHD SHAHNAN BIN ZAINAL ABIDIN Date:

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TABLE OF CONTENTS Page ABSTRACT ii ABSTRAK iv ACKNOWLEDGEMENTS vi APPROVAL vii DECLARATION ix LIST OF TABLES xii LIST OF FIGURES xiii LIST OF ABBREVIATIONS xvi CHAPTERS 1 INTRODUCTION 1 1.1 Background 1 1.2 Optical Communication System 2 1.3 Optical Amplifiers 3 1.3.1 Earth Rare Doped Fiber Amplifier 5 1.3.2 Optical Amplifier Classification 5 1.4 Problem Statement and Critical Review 8 1.5 Research Objectives 9 1.6 Scope of the Work 10 1.7 Organization of the Thesis 11 2 THEORY 12 2.1 Introduction 12 2.2 Optical Amplification Fundamental 12 2.3 Energy level of Erbium Ions 16 2.4 Three Level Laser System 18 2.4.1 Three Level Atomic Rate Equations System 18 2.4.2 Signal with Amplified Spontaneous Emission Power Rate

Equations 22

2.4.3 Pump Power Rate Equations 26 2.5 Reduction to Two Level System from Three Level System 28 2.6 Repeaterless Transmission System 30 2.6.1 System Development 30 2.6.2 Applications 31 2.6.3 System Technologies 32 2.6.4 System Configuration 38 2.7 Summary 40 3 RESEARCH METHODOLOGY 41 3.1 Introduction 41 3.2 Experimental Setup 41 3.3 Components in Remotely Pumped EDFA 43 3.3.1 Isolator 44 3.3.2 Wavelength Selective Coupler 44 3.3.3 1480 nm Pump Laser 45

x

3.3.4 Erbium Doped Fiber 46 3.3.5 Variable Optical Attenuator 47 3.3.6 Test Equipment and Measurement Devices 48 3.3.7 Optical Spectrum Analyzer 49 3.3.8 Tunable Laser Source 49 3.4 Characterization of Erbium Doped Fiber Amplifier 50 3.5 Repeaterless Transmission System Using Remotely Pumped EDFA 52 3.5.1 Fiber Background Loss 52 3.5.2 Bit Error Rate Test 54 3.5.3 Maximum Transmission Distance 55 3.5.4 EDFA Location 56 3.5.5 Pump Power Requirement 56 3.6 Parameter Under Study 57 3.6.1 Design Parameter 57 3.6.2 Performance Parameter 59 3.7 Summary 60 4 RESULTS AND DISCUSSION 61 4.1 Introduction 61 4.2 Erbium Doped Fiber Amplifier Characterization 61 4.2.1 Pump Power Effect on Gain and Noise Figure 61 4.2.2 Pump Power Effect on Output Power 64 4.2.3 Operating Range of Pump Power 65 4.2.4 Input Power Effect on Gain and Noise Figure 67 4.2.5 Input Power Effect on Output Power and OSNR 70 4.2.6 Wavelength Effect on System Gain and Noise Figure 71 4.3 EDFA Transmission Performance 73 4.3.1 EDFA Position on Transmission Line 74 4.3.2 Optimum Parameter for Specific Transmission Total

Distance 82

4.3.3 Effect of System Upgrade to OSNR Performance 89 4.3.4 Gain Improvement over Different Wavelength 91 4.4 Summary 92 5 CONCLUSION AND FUTURE WORKS 94 5.1 Conclusion 94 5.2 Future Work 95 REFERENCES 97 BIODATA OF THE AUTHOR 103 PUBLICATIONS 104

xi

LIST OF TABLES

Table Page

3.1 Specifications of erbium doped fiber used for the experiments 47

3.2 Required test time for BER measurement at different transmission speed

55

4.1 Fiber length before and after EDFA associated with maximum

total transmission distance and percentage of amplifier location from transmitter side

77

4.2 Performance parameter summary of remotely pumped EDFA

transmission system 82

xii

LIST OF FIGURES

Figure Page

1.1 Optical fiber amplifier classification; (a) Post amplifier (b) Inline amplifier (c) Preamplifier

7

1.2 Scope of research work 10

2.1 Energy level diagram 13

2.2 Atom with respective energy level (a) with external energy

and (b) releases photon 15

2.3 Energy levels of erbium ions with possible pump bands. 16

2.4 Energy level system corresponding to three-level system. 19

2.5 Cylindrical fiber cross section 22

2.6 Energy level of two-level system 29

2.7 Signal evolution of distributed amplifier for different pumping

schemes 33

2.8 Signal power evolution experienced by remotely pumped

EDFA systems 35

2.9 Remotely pumped EDFA incorporating circulators to achieve

double pass amplification 36

2.10 Additional pumping line utilized providing more pump power

to remotely pumped EDFA 36

2.11 Deployment of filter within pumping line 37

2.12 Various configuration of repeaterless transmission systems 38

2.13 Signal power evolution over transmission distance for

different repeaterless systems 39

3.1 Experimental setup of EDFA 42

3.2 Setup of EDFA experiment for repeaterless transmission

system using remotely pumped EDFA 43

3.3 Basic operation of isolator 44 3.4 Inputs and output associates with wavelength selective

coupler 45

xiii

3.5 Structure of pump laser module 46

3.6 Experimental setup associating transmitter, receiver and BERT

48

3.7 Output power response for a current applied to 1480 nm pump

laser 50

3.8 Screenshot from OSA for measurement technique for gain,

ASE level, noise figure and output power. 51

3.9 Fiber loss associates with respective wavelength of signal 54

4.1 Gain and noise figure characteristics using single-pass EDFA

at different signal powers 62

4.2 Output power against pump power for various input power 65

4.3 Gain Conversion Efficiency over consumed pump power for a

range of input signal power from - 40 to - 4 dBm 66

4.4(a) Gain and noise figure against various pump power for -40

dBm to 0 dBm input power 68

4.4(b) ASE level over signal wavelengths at input power of -30 dBm

at 40 mW pump power 69

4.5(a) Output power and OSNR against input power for 30, 40 and

50 mW pump power 70

4.5(b) ASE level over signal wavelengths at input power of 0 dBm at

40 mW pump power 71

4.6 Gain and noise figure variation over wavelength. Input power

is set to -20 dBm for 30, 40 and 50 mW 73

4.7 EDFA position configuration on remotely-pumped

transmission experiment 74

4.8 Maximum transmission length at BER better than 10-10 with

respect of EDFA location on transmission line for pump powers of 30, 40 and 50 mW. Launch power from transmitter side is set to be 0 dBm

76

4.9 Eye diagram of the transmission for pump powers of (a) 30

mW, (b) 40 mW and (c) 50 mW 76

4.10 Gain and noise figure performance over EDFA location 78

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4.11 OSNR and received power performance over EDFA location on transmission line

79

4.12 Consumed pump power associated with EDFA location, the

local pump power of 40 mW is used 81

4.13 Bit Error Rate for total transmission distance of 180 km. 83

4.14 BER for 196 km of total transmission distance for 40 mW

pump power injected at input EDFA 84

4.15 BER for 208 km transmission distance for 40 mW pump

power at EDFA input 86

4.16 BER for 215.18 km transmission distance for 40 mW pump

power at EDFA input 86

4.17 OSNR and received power over EDFA location at 208 km

total transmission system for 40 mW pump power at EDFA input

89

4.18 OSNR requirement for system upgrade from 2.5 to 10 Gbps 91

4.19 Gain and noise figure difference in respect to reference point

at 1550.3 nm 92

xv

LIST OF ABBREVIATIONS

ASE - Amplified Spontaneous Emission

BER - Bit Error Rate

BERT - Bit Error Rate Tester

DCF - Dispersion Compensating Fiber

DUT - Device Under Test

EDF - Erbium Doped Fiber

EDFA - Erbium Doped Fiber Amplifier

FEC - Forward Error Correction

G - Gain

GCE - Gain Conversion Efficiency

LD - Laser Diode

NA - Numerical Aperture

OSA - Optical Spectrum Analyzer

OSNR - Optical Signal to Noise Ratio

PRBS - Pseudo-Random Bit Sequence

PCE - Power Conversion Efficiency

PPM - Part Per Million

SOA - Semiconductor Optical Amplifier

TLS - Tunable Laser Source

VOA - Variable Optical Attenuator

WDM - Wavelength Division Multiplexing

WSC - Wavelength Selective Coupler

xvi

CHAPTER 1

INTRODUCTION

Telecommunication industries evolve rapidly since past years influenced by hungry

demanding competition among manufacturers. Various telecommunication

technologies have been created and innovated in order to achieve manufactures need

for a reliable telecommunication quality more than just voice services. Higher

bandwidths are required to satisfy a real time video on demand, live telecast and

interactive applications. An optical communication is seen as one of the enabling

technologies and being one of the key factors for the present and future applications.

This chapter will explain briefly about optical communication systems and its

development up to date. Later, optical amplifiers will be highlighted by focusing to

rare-earth doped fiber amplifier. Statement of related problem and scope of the

research to be conducted are discussed in details as well as its objectives to achieve.

1.1 Background

Optical fiber communication systems can fulfill the bandwidth need for a practical

long haul transmission distance. It is reliable in handling and transmitting data over

hundreds of kilometers with an acceptable bit error rate. Worldwide researchers

continuously update the technology to improve the system’s performance.

Typically, a link of optical fiber communication can be hundreds of kilometers and

some could be extended up to several thousands of kilometers with an additional

1

amplifiers and repeaters [1]. Data speed of each optical fiber transmission channel

could reach up to 40 Gb/s [2] and it is limited by polarization-mode and chromatic

dispersions, attenuation, and nonlinearity of the fiber [3]. Some schemes included

into the system to improve the transmission distance and bit rate further such as

Forward Error Correction (FEC), new remote pumping schemes and utilization of

dispersion compensating fibers [4]. Optical fiber communication growths

impressively with over 600 millions kilometers of fiber optics have been installed

worldwide [5] with USD 1.2 billion amounted only for equipment of optical

transport for the year of 2000 only [6].

1.2 Optical Communication System

Basically an optical communication system consists of a link of fiber optic as a

transmission medium between a transmitter and a receiver. Information is converted

from electrical to optical domain, modulated and multiplexed before injecting into

the fiber optic. Optical fiber carries the information by guiding the laser beam in its

core utilizing a total internal reflection requirement. At the receiver end, the signal is

converted back into an electrical domain by a photodetector, amplified and

demodulated to produce the original signal.

Between two transmission ends, an amplifier could be added to extend the link.

Previously, electrical amplifiers were utilized by converting optical signal into

electrical domain before it was amplified. Electrical amplifiers require external

power supply to operate and this would become a big problem if a submarine

transmission link is going to be deployed. The optical amplifier overcomes this

2

barrier since the system uses light to amplify the information signals all optically and

light can be send from a distance.

1.3 Optical Amplifiers

Traditionally, an electrical amplifier converts optical signal into electrical domain

first before converts back again into optical signal after amplifying it. The state of

signal changes several times as there are several stages of electrical amplifiers. This

would add more delay in transmission and would affect the overall system bit error

rates.

Every transmission medium introduces some signal losses of its signal power. The

signal is attenuated along the medium as it travels from transmitter to receiver end. In

an optical fiber communication, the attenuation causes the launched signal power

level to decrease mainly due to signal absorption by the fiber itself.

Optical amplifiers are designed in such a way that the weakened signal is boosted to

a specific power level for the next transmission sequent. As its name implies, optical

amplifiers operate in optical domain and maintain signal’s state along fiber within

the transmission distance. They eliminate a need for signal interconversion of

photons to electrons. Moreover, they offer a simple setup of a single in line

components arrangement which is practical for any kind of modulations and

transparent to any transmission speeds [7]. Multiple optical wavelengths can be

3

easily amplified over a certain bandwidth that is limited by the operating range of

optical amplifiers.

There are two main techniques to achieve optical amplification. Semiconductor

Optical Amplifier (SOA) uses a fundamental of stimulated emission to amplify an

optical information signal. The information signal is amplified in a semiconductor’s

active region where the injection current is applied to deliver the external energy to

pump electron at the conduction band. The signal stimulates the electron transition

and emits photons with the same energy and wavelength as the input’s [8]. SOA can

be used in both nonlinear and linear modes of operation [9, 10]. But on the other

hand, the SOA is unsuitable to be utilized in a repeaterless transmission system due

to its operation requirement for an inline electrical source.

The second type of optical amplification is the use of rare-earth doping material

inside the fiber. Essentially it is a spliced active fiber connected to a pump laser

within a transmission line [11]. It works on the principle of stimulated emission; the

pump laser is used to provide energy and to excite ions in a special doped fiber to an

upper energy level. Then, the ions are stimulated by photons of the information

signal and fall down to lower level of energy; subsequently, emits photon energy

exactly at the same wavelength of the signal.

4

1.3.1 Rare-Earth Doped Fiber Amplifier

Currently research works are concentrating more on the erbium dopant, particularly

in silica based fibers. This is due to the emission of Er3+ ion lying within a set of

wavelength around 1550 nm where the silica fiber exhibits the minimum attenuation

of the information signal. Erbium doped fiber amplifiers (EDFA) could provide gains

as high as 40 dB associated with low noise, as successfully demonstrated within a

pump power range of 50 to 100 mW [12-14].

The first rare earth doped material of Neodymium (Nd ) 3+ used into a single mode

fiber was demonstrated in 1983 by Bell Telephone Laboratories [15, 16]. Since then,

rare-earth doped fibers were fabricated in a variety of methods to suit the different

designs of amplifiers. The amount of dopant inside the fiber core ranging from 100 to

2000 parts per million (ppm) up to as high as 5000 ppm [17]. Various methods used

to provide a low loss optical fiber communication with an introduction of new

composition to improve the performance of amplifier, as well as uniformity of

doping along the longitudinal and transverse fiber axes [18-22].

1.3.2 Optical Amplifier Classification

Optical fiber amplifiers generally can be classified and categorized based on its

position in transmission lines which in turn shows their respective applications.

There are three basic types of amplifier; post, inline and preamplifiers.

5

1.3.2.1 Post-amplifier

Known as a booster and power amplifier, a post amplifier is located just after a

transmitter (Tx) before the launched signal goes down to fiber as depicted by Figure

1.1(a). As its location is near to the transmitter, the post amplifier handles relatively

high power signals compared to other types of optical amplifiers. Mainly the post

amplifier’s function is to boost the signal power to the highest level hence maximizes

the transmission distance. It differs than a normal amplifier as it relieves a need for a

transmitter to produce a maximum optical power.

1.3.2.2 Inline Amplifier

An inline amplifier as shown by Figure 1.1(b) is usually located in the middle of a

transmission span. It functions to compensate the power losses along the fiber link

and could be cascaded to extend the transmission length. As it amplifies the signal

several times in the case of cascaded system, the noise level is to be kept at a

minimal level. For a WDM system, its stability to produce a uniform gain among

wavelengths becomes its main goal.

6

Tx

RxTx

Tx Rx

Rx

(a)

(b)

(c)

Figure 1.1: Optical fiber amplifier classification; (a) Post amplifier (b) Inline amplifier (c) Preamplifier

1.3.2.3 Preamplifier

Preamplifier amplifies the incoming signal before entering a receiver (Rx) at the end

of the transmission as illustrated by Figure 1.1(c). Located just before the receiver, a

preamplifier increases a weak signal to an acceptable level for detection. Hence the

preamplifier is designed to have a good sensitivity, high gain and low noise level.

Receiver’s performance is not only limited by its own noise but also contributed by

the noise from preamplifier [23].

7