UNIVERSITI PUTRA MALAYSIA

DESIGN AND DEVELOPMENT OF A MULTI-WAVELENGTH BRILLOUIN/ERBIUM LONG-BAND FABRY PEROT FIBER LASER

TARAK ALI HADDUD.

FK 2004 68

DESIGN AND DEVELOPMENT OF A MULTI-WAVELENGTH BRILLOUINI ERBIUM LONG-BAND FABRY PEROT FIBER LASER

TARAK ALI HADDUD

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

May 2004

In the name of God, Most Gracious, Most Merciful

Dedication to My wife, my daughters, my brothers and sisters

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

DESIGN AND DEVELOPMENT OF A MULTI-WAVELENGTH BRILLOUINI ERBIUM FABRY PEROT FIBER LASER

BY

TARAK ALI HADDUD

May 2004

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

Faculty: Engineering

Multi-wavelength fiber lasers operating in the long-wavelength band (L-band)

wavelength region are needed as laser source for applications such as wavelength-

division-multiplexed (WDM) communication systems, fiber sensors, spectroscopy and

optical fiber gyroscopes.

Multi-wavelength fiber lasers are emerging as attractive alternative technologies for

wavelength-selectable WDM. They are used as source for direct compatibility with the

fiber-optic transmission medium, excellent amplifying properties of rare-earth doped

fibers, Brillouin gain, rapidly continuing progress in novel fiber gain media, maturity and

robustness of the laser diode pumps used and availability of fiber-based components.

iii

The multi-wavelength laser applications of interest to this work have distinct performance

requirement. This requirement is the need for wide tunability (the ability to tune the

lasing emission through a wide range of wavelengths).

In this thesis, the design and development of a multi channel continuous wave Brillouid

Erbium L-band fiber laser (BEFL), with novel loop mirror configuration, has been

studied experimentally. The design parameters of multi-wavelength fiber laser (launched

pump powers, single mode optical fiber lengths and Brillouin wavelengths), the multi

wavelength Brillouiderbium linear fiber laser configuration with bi-directional pumping

is demonstrated. Throughout this work different lengths of single mode optical fiber with

various launched pump powers and Brillouin wavelengths have been examined to extract

the optimum output performance of a BEFL. The performance of the BEFL is presented

in terms of threshold pump power, output peak power, tuning range and the number of

Stokes. This development fiber loop configuration exhibits considerably high

performance. Twenty-four of Stokes with 10 nrn tuning range and efficient output powers

were obtained.

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

REKABENTUK DAN PEMBANGUNAN JARAK GELOMBANG-MULTI BRILLOUINIERBIUM FABRY PEROT FIBER LASER

Oleh

TARAK ALI HADDUD

Mei 2004

Pengerusi : Professor Madya Mohd Adzir Mahdi, Ph.D.

Fakulti : Kejuruteraan

Laser Jarak Gelombang -Berbilang Fiber yang beroperasi pada kawasan jarak gelombang

jalur jarak gelombang panjang (jalur-L) diperlukan sebagai sumber laser untuk aplikasi-

aplikasi seperti sistem komunikasi pemultipleks bahagi jarak gelombang(WDM), deria

fiber, spektroskopi dan fiber gyroscopes optikal.

Laser fiber jarak gelombang-multi sedang bangkit sebagai daya tarikan teknologi

alternatif untuk pilihan-jarak gelombang WDM. Ia menjadi sumber yang sesuai bagi

perantraan transmisi fiber optik, ciri penguat terbaik bagi fiber terdop jarang-bumi,

perolehan Brillouin, kesinambungan kemajuan yang pantas dalam media perolehan fiber

novel, pengunaan kematangan dan keteguhan pam laser diod and komponen berasaskan

fiber yang sedia ada.

Keperluan aplikasi laser jar& gelombang-berbilang dalam hasil kerja ini mempunyai

keperluan prestasi yang jelas. Keperluan ini adalah untuk talaan lebar (keupayaan

mentala pancaran laser jarak gelombang pada jarak yang lebar).

Dalam tesis ini, rekabentuk dan pembangunan kesinambungan gelombang Brillouin

Saluran berbilangfiaser fiber Erbium jalur-L (BEFL), beserta konfigurasi cermin gelung

novel, telah dikaji secara eksperimen. Rekabentuk parameter bagi laser fiber jarak

gelombang berbilang(kuasa pam yang dilancarkan, mod berasingan jarak optikal fiber

dan jarak gelombang Brillouin), konfigurasi jarak gelomabang Brillouid laser fiber

erbium lurus bersama pengepaman dua-arah telah ditunjukkan. Melalui hasil kerja ini

optikal fiber mode berasingan yang mempunyai jarak yang berbeza berserta

keperlbagaian kuasa pam yang dilancarkan dan jarak gelombang Brillouin telah diperiksa

untuk mendapatkan output prestasi yang optimum bagi BEFL. Prestasi BEFL

dipersembahkan dalam bentuk ambang kuasa pam, kuasa puncak output, jarak talaan dan

bilangan Stokes. Konfigurasi pembangunan gelung fiber menghasilkan prestasi yang

agak tinggi. Dua puluh empat Stokes yang mempunyai jarak talaan lOnm dan kuasa

output yang efektif diperolehi

ACKNOWLEGEMENTS

I wish to forward my gratitude and sincere thanks to my supervisor, Assoc. Prof. Dr.

Mohd. Adzir Mahdi for his guidance and suggestions throughout the study. And for the

preparation and completion of this thesis. I also, wish to extend special thanks to my co-

supervisor, Assoc. Prof. Dr. Mohd. Khazani Abdullah for giving me the chance to do my

research in photonics laboratory

My thanks are extended to all staffs of photonics laboratory who have contributed to the

full completion and success of this study.

Last but not least, I would like to thank my wife and all my family who gave me

encouragements and support in this endeavor.

vii

I certify that an Examination Committee met on 24' May 2004 to conduct the final examination of Tarak Ali Haddud on his Master of Science thesis entitled "Design and Development of a Multi-Wavelength Brillouin/Erbium Long-Band Fabry Perot Fiber Laser" in accordance with Universiti Putra 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:

Syed Abdul Rahman Mohamed, MS. Lecturer Faculty of Engineering Universiti Putra Malaysia (Chairman)

Borhanuddin Mohd. Ali, Ph.D. Professor Faculty of Engineering Universiti Putra Malaysia (Member)

Adznan Jantan, PLD. Associate Professor Faculty of Engineering Universiti Putra Malaysia (Member)

Kaharuddin Dimyati, Ph.D, Associate Professor Faculty of Engineering Universiti Malaya (Independent Examiner)

P r o f e s s o r / ~ e ~ ~ t ~ be& School of Graduate Studies Universiti Putra Malaysia

Date : 2 6 AUG 2004

viii

This thesis submitted to the Senate of Universiti Putra Malaysia and has been accepted as fblfilment on the requirement for the degree of Master of Science.The members of the supervisor committee are as follows:

Mohd. Adzir Mahdi, Ph.D. Associate Professor Faculty of Engineering Universiti Putra Malaysia (Chairman)

Mohd. Khazani Abdullah, Ph.D. Associate Professor Faculty of Engineering Universiti Putra Malaysia (Member)

AINI IDERIS, Ph.D. ProfessodDean School of Graduate Studies Universiti Putra Malaysia

Date: 10 SEP 2004

DECLARATION

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

TARAK ALI HADDUD

TABLE OF CONTENTS

Page

DEDICATION. ABSTRACT. ABSTRAK ACKNOWLEDGMENTS APPROVAL DECLARATION. LIST OF FIGURES LIST OF ABBREVIATIONS

1 INTRODUCTION

1.1 Multi Wavelength Fiber Lasers Background 1.2 Hybrid Brillouin/Erbium Fiber Lasers 1.3 Uses and Merits 1.4 Problem Statement and Motivation 1.5 Research Objectives 1.6 Organization of The Thesis

2 LITERATURE REVIEW AND BACKGROUND

2.1 Introduction. 2.2 Background of Erbium-Doped Fiber Lasers 2.3 Background of Brillouin Fiber Lasers.

2.3.1 Scattering of Light 2.3.2 Rayleigh Effect 2.3.3 Brillouin Effect 2.3.4 Spontaneous Brillouin Scattering 2.3.5 Simulated Brillouin Scattering

2.4 Previous Work. 2.4.1 General Multi-wavelength Laser Sources 2.4.2 Multi Wavelength Brillouinlerbium Fiber Laser Sources

2.5 Type of CW Multi-wavelengths Fiber Laser Resonators 2.6 Summery

3 METHODOLOGY

3.1 Introduction 3.2 Stimulated BriIlouin Scattering

3.2.1 Effect of SMF length on Reflected Power (Stokes)

ii iii v

vii viii

X ... X l l l

xvi

3.2.2 Effect of SMF Length on Throughput Power 3.3 Design of L-band Multi-Wavelength Brillouinlerbium Linear Fiber

Laser with Bi-directional Pumping Scheme 3.3.1 Brillouin/Erbium Fiber Laser Parameters and Related

Components 22 3.3.2 Experimental Setup and Schematic Diagram of Cavity Losses

Measurement 30 3.4 Summery. 33

RESULTS AND DISCUSSION

4.1 Introduction 4.2 Stimulated Brillouin Scattering

4.2.1 Effect SMF Length on Reflected Power 4.2.2 Effect SMF Length on Throughput Power

4.3 L-Band Multi-Wavelength Brillouin/Erbium Linear Fiber Laser With Bi-directional Pumping Scheme 4.3.1 Optimum SMF Length of the BEFL System 4.3.2 Tuning Range and Flatness of BEFL System 4.3.3 Multiple Stokes of BEFL System 4.3.4 Output Power of BEFL System 4.3.5 Stokes Signal Threshold 4.3.6 Cavity Losses

5 CONCLUSION AND FUTURE WORK

5.1 Conclusion. 5.2 Future work

REFERENCES BIODATA OF THE AUTHOR

xii

LIST OF FIGURES

Figure Page

Light Scattering Without Change in Frequency (Rayleigh Effect). 8

Different Wave Movement in the SMF. 9

Impact of Electric Field on Material Properties.

An Acoustic Wave Phenomenon in Electrostrictive Material.

Loop Process of Stimulated Brillouin Scattering (SBS).

Fiber Laser Configurations, (a) Linear Cavity (F-P) and (b) Ring Cavity 18

Experimented Setup of Stimulated Brillouin Scattering (SBS) in Single Mode Fiber 21

Schematic of Power-Based Coupler

Three Port-Optical Fiber Circulator

Schematic of h- Based Coupler.

Absorption Spectrum of an Erbium Doped Silica Fiber [Thompson,200 11

Experimental Setup of Multi Wavelengths Brillouin/Erbium Linear Fiber Laser with Bi-Directional Pump Power in L-Band. 3 1

Schematic Diagram for Measurement of Cavity Losses.

Stoke Output vs Brillouin Pump Power at Different SMF Lengths

3 D Picture Shows Stokes Output Vs Brillouin Pump Power At 25 km SMF Length as Measured On OSA.

Throughput vs Brillouin Pump Power at Different SMF Lengths.

D Picture State the Throughput Power vs Bp at 25 km SMF Length as Measured on OSA. 38

Number of Channels vs SMF Lengths at Different Brillouin Wavelength

xiii

4.6 Tuning Range of BEFL Outputs at 10.8 km SMF Length, 180 mW Pump Power and 2.86 mW Brillouin Pump as Measured by Using OSA. 4 1

4.7 Tuning Range of BEFL Outputs at 4 km SMF Length, 180 mW Pump Power and 2.86 mW Brillouin Pump as Measured by Using OSA 42

4.8 Total Stokes Output vs Brillouin Wavelength at 180 mW Pump Power, 10.8 km SMF Length and Different Brillouin Pump Power

4.9 Number of Stokes vs Brillouin Wavelength at Different Brillouin Pump Power. 45

4.10 Optical Spectrum for 24 Stokes as Measured by OSA (with 0.Olnm Resolution). 46

4.11 Number of Stokes vs 980 nm Pump Power at Different Brillouin Pump Power.

4.12 Number of Stokes vs Brillouin Pump Power at Different 980 nm Pump Power. 48

4.13 The Presence of the Self-lasing Cavity Modes Together with the Brillouin Stokes at Srnall-Injected Brillouin Pump Power. 50

4.14 Number of Stokes vs Brillouin Pump Power at Different Brillouin Wavelengths

4.15 Optical Spectrum for Stokes as Measured by OSA at Bp, (a) 0.58 mW, (b) 1.47 mW 53

4.16 Total Stokes Output Power vs SMF Lengths at Different Brillouin Wavelength 55

4.17 Stokes Total Output Power vs 980 nrn Pump Power at Different Brillouin Pump Power

4.18 Stokes Output Power vs Brillouin Pump Power at Different 980 nm Pump Power 58

4.19 Total Stokes Power Against Brillouin Pumps Power at Different Brillouin Pump Wavelength and 980 nm Pump Power was Set at 180 mw. 59

xiv

4.20 Output Spectrum of the First Brillouin Stokes for the BP Wavelength at 1598 nm and the BP Power was Fixed to 2.86 mW; the 980 nm Pump Power was (a) 60 mW and (b) 80 mW 6 1

4.21 980 nm Pump Power Threshold vs Brillouin Pump Power.

4.22 Setup Losses at 10.8 krn SMF Length as Measured by OSA.

LIST OF ABBREVIATIONS

BP

DFB

hB

CW

EDF

EDFL

ESA

FBG

FP

FWM

LED

OSA

SBS

SMF

TLS

WDM

PP

Brillouin Pump

Distributed Feed-Back

Brillouin Wavelength

Continuous Wave

Erbium Doped Fiber

Erbium Doped Fiber Laser

Excited State Absorption

Fiber Bragg Grating

Fabry-Perot

Four- Wave Multiplexed

Laser Emitting Diode

Optical Spectrum Analyzer

Stimulated Brillouin Scattering

Single Mode Fiber

Tunable Laser Source

Wavelength Division Multiplexing

980 nm Pump Power

xvi

INTRODUCTION

1.1 Multi Wavelength Fiber Lasers Background

The light is widely used to transmit information over large distance because of its

security, reliability and huge bandwidth. One of the most important devices in optical

communication systems is laser source. It is used to launch light into fibers. There are

many types of laser that can be used for optical communication systems. In general, fiber

lasers have many advantages over other types of lasers. When compared to many solid-

state and gas laser systems; fiber lasers have high output power, simpler, more compact

and can be pumped with laser diodes. Moreover, when compared to laser diodes, fiber

lasers are spectrally cleaner and can be modulated with less chirp and signal distortion.

Also, the intense pumping availability in rare-earth doped, single mode fibers lead to

lower threshold three-level laser operation, and improve efficiency, just as tighter

confinement improves laser diode performance [Li, 20003.

In addition, fiber-to-fiber compatibility is a distinct advantage in optical communication

systems [Talaverano, 20011. However, the multiple wavelength laser sources with narrow

wavelength channel spacing are of high potential for use in dense wavelength division

multiplexing (DWDM) transmission systems. As optical transmission system capacity of

optical communication system approaching a few Terabit per second through DWDM

technique, multi wavelength source technology becomes more important [Krauss,

20021. Much effort has been focused to achieve simultaneous multi wavelength

operation. The use of erbium-doped fiber laser (EDFL) has been demonstrated to achieve

multiple wavelength outputs [Yamashita, 19961. The selection of their operation

wavelengths has been achieved by using different methods; see more explanation in

Chapter 2.

1.2 Hybrid Brillouin/Erbium Fiber Lasers

A general technique for producing a Brillouin fiber laser is to construct a critically

coupled fiber resonator, which is necessary because of the small magnitude of the

Brillouin gain [Agrawal, 19951. The disadvantage of the critically coupled Brillouin fiber

lasers presented in the small output power that can be achieved [Hill, 761, the requirement

of cavity matching to the pump signal, and the difficulty in incorporating intra-cavity

elements because of their associated loss. This type of laser can be compared with

erbium-doped fiber lasers, which operate with a high-gain medium that allows for

efficient operation of complex resonator structures. These EDFL's are suited for

operation with large output coupling, and can have an operating quantum efficiency of

>95% [Cowle, 19911.

The problem for a critically coupled resonator in a laser based on Brillouin gain can be

overcome by using an erbium-doped fiber amplifier to compensate for the resonator

losses while still originating lasing action from the Brillouin gain. The wavelength of the

resulting laser is accurately determined by both; the BEFL cavity resonances and the

wavelength of the Brillouin pump. Therefore, the exact frequency of the BEFL lasing is

not completely controlled by the injected Brillouin pump, but can be derived from the

position of the BEFL cavity resonance [Stepanov, 19971, and a large number of channels

can be achieved.

1.3 Uses and Merits

The multi-wavelength fiber lasers technology are well suited to industrial applications

because they are compact and reliable. They have many applications such as sources for

DWDM systems, sensor; laser gyroscopes [Zarinetchi, 19911 and current monitors

[Kung, 19961.

Other possible applications for the BEFL are optical-fiber characterization [Ohashi,

19921, narrow-band width amplification [Ferreira, 19941, distributed strain and

temperature measurement [Tateda, 1990 & Kurashima, 19901, fiber frequency shifter

[Kalli, 19911 pulse generation [Guy, 19951 and rnicrowave-frequency generation

[Culverhouse, 199 11. The multi-wavelength fiber laser systems investigated can be used

as an optical source to upgrade information carrying capacity of the communication

systems, to meeting the fast development of WDM technology.

1.4 Problem Statement and Motivation

The researchers continue their efforts toward the development multi-wavelength laser

sources. Moreover, producing more number of channels in a laser system is more

economical. The multi-wavelength lasers used today still suffering from the high cost and

complex structure. Then, the motivation of this thesis is to search for economic source to

. produce multi-channels. The BrillouinErbium fiber laser (BEFL) using Fabry-Perot

cavity and operates in the L-band region is used for this thesis. The principles of BEFL

system are based on rare earth active material (erbium doped-fiber), primary laser source

(laser diode), non linear Brillouin scattering (SMF), and feedback mechanism (loop fiber

mirror).

1.5 Research Objectives

The research work concentrates on the design, development and testing of

BrillouinErbium fiber laser system. Thus, the key objectives of the research work are as

the following:

1) To investigate the behaviors of stimulated Brillouin scattering in single mode

fibers.

2) To develop a multi-wavelength BrillouinErbium fiber laser using Fabry-Perot

cavity, in L-band region.

3) To the performance of the developed fiber laser based on the number of Stokes.

1.6 Organization of the Thesis

This thesis presents the design and development of multi wavelength Brillouin/Erbium

fiber laser (BEFL). The fiber Fabry Perot resonator is used to construct BEFL. The work

is divided into several chapters. A short introduction is given in Chapter 1. The

background of Erbium fiber laser, Brillouin fiber laser and multi wavelength fiber laser

with emphasis on multi-wavelength Brillouin/Erbium fiber lasers (BEFL) is presented in

Chapter 2. Multi wavelength Brillouin/erbium fiber lasers can be designed with a variety

of cavity configurations; the two most common are explained in this chapter. Chapter 3

will first come out with simple sketch to explain the effect of SMF length on output and

threshold power. Then, it goes on to describe the related fiber laser components, in order

to have a better understanding of the construction of the BEFL. A system configuration,

which is Brillouin/Erbium fiber loop mirror with two circulators on both of its ends as

loop back reflectors will be presented. This will allow bi-directional operation in the

cavity to be obtained without the use of optical isolators. Chapter 4 presents the

implementation of the Brillouin/Erbium fiber lasers, based on the resonator proposed in

Chapter 3. The experimental results of the BEFL including the output power, threshold,

and the number of Stokes will be presented in this Chapter. The thesis concludes with

Chapter 5, which summarizes the work done on Brillouin/Erbium fiber laser, loop mirror

configuration and the optimization of the performance parameters for this configuration.

Recommendations and suggestions for future works will be treated here.

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction

The concept of fiber laser is not new, but it is a surprising idea. The theoretical concept

behind fiber-laser operation is quite elegant. An optical fiber doped with a lasing material

is optically pumped by laser pump source (flash lamp or laser diode), and a laser output is

obtained from the fiber. This output laser can be varied according to the type of pump

source, the length of the active materials and some other parameters.

This chapter gives background information on Erbium-doped fiber and Brillouin effect in

SMF. Then, it reviews the general multi wavelength fiber lasers, with some emphasis on

the Brillouin/Erbium fiber lasers.

2.2 Background of Erbium-Doped Fiber Lasers.

Among rare-earth doped fiber lasers, erbium-doped fiber lasers at 1.5 pm region attracts

the most attention because it coincides with the least-loss (as low as 0.2 dB/Km [Kapron,

19701) region of silica fibers used for light wave communications. Erbium-doped fiber

lasers can be operated either in continuous wave or in pulse mode. Continuous wave

EDFLs are preferred for applications in multi-channel system, while pulsed mode EDFLs

performs better in Soliton communication systems. The techniques of Q-switching and

mode locking [Huang, 20001 can be used to produce high power short duration pulses,

which is beyond the scope of this thesis.

2.3 Brillouin Fiber-Background

While studying the types of light scattering, emphasis will be on Brillouin scattering that

has relation with the objective of this work.

2.3.1 Scattering of Light

When light travels through a transparent media, most part of it travels in a straight line in

direction of propgation, other than a small fraction of it that will be scattered. The

inhomogeneities (heterogeneity) of refractive index of the media are responsible for this

phenomenon. In addition, there are two more important reasons for this phenomenon,

Rayleigh and Brillouin effects.

2.3.2 Rayleigh Effect

It is related to the inhomogeneities due to the material structure. Air and glass are well

known examples where the small refractive index fluctuations induced by their

amorphous nature can scatter light in all directions [Palais, 19981. This is done without

changing the frequency of the scattered light; because the inhomogeneities are frozen in

the material structure, see Figure 2.1

Figure 2.1: Light Scattering Without Change in Frequency (Rayleigh Effect).

2.3.3 Brillouin Effect

French physicist LRon Brillouin was the first to study the diffusion of light by acoustic

waves in 1920. One of the distinctive features he observed was a change of frequency in

the scattered light. This effect, named after its discoverer, has remained for a long time

within the frame of purely academic research. But it went out of the lab shortly after the

advent of lasers and optical fibers. Its frrst "contribution" was indeed a negative one: it

was demonstrated that the Brillouin effect is the most drastic limitation encountered when

the light power within the fiber is increased. In other words, there is a power threshold

above which any additional light is back scattered due to its interaction with acoustic

waves. Since a few years, a handful of research groups have been trying to convert this

flaw - the relatively high efficiency of the Brillouin scattering in fibers - into an

advantage in developing multi wavelength fiber lasers.