MULTIWAVELENGTH BRILLOUIN/ERBIUM FIBER LASER ASSISTED BY BISMUTH-BASED

ERBIUM-DOPED FIBER AMPLIFIER

NURUL SHAHRIZAN SHAHABUDDIN

PHYSICS DEPARTMENT

FACULTY OF SCIENCE

UNIVERSITY OF MALAYA

KUALA LUMPUR

2009

MULTIWAVELENGTH BRILLOUIN/ERBIUM FIBER LASER ASSISTED BY BISMUTH-BASED

ERBIUM-DOPED FIBER AMPLIFIER

NURUL SHAHRIZAN SHAHABUDDIN

SUBMISSION OF DISSERTATION FOR THE FULFILMENT OF THE DEGREE OF MASTER

OF SCIENCE

PHYSICS DEPARTMENT

FACULTY OF SCIENCE

UNIVERSITY OF MALAYA

KUALA LUMPUR

2009

i

ABSTRACT

The thesis focuses on the exploitation of nonlinear effects in single-mode fiber

(SMF), mainly the stimulated Brillouin scattering (SBS) to generate multiwavelength

signal in a Bismuth-based Brillouin/Erbium fiber laser (BEFL). A 2.15 m Bismuth-

based Erbium-doped fiber (Bi-EDF) with Erbium ion concentration of 3250 wt. ppm. is

used as a linear gain medium in the BEFL . The characteristic of the Bismuth-based

Erbium-doped fiber amplifier (Bi-EDFA) is studied. The amplifier exhibits gain in 1565

to 1600 nm region when the Bi-EDF is pumped with two laser diodes of wavelength

1480 nm with 100 mW pump power each. Double-pass Bi-EDFA shows better gain

performance but higher noise figure than single-pass Bi-EDFA. Thus, single-pass Bi-

EDFA is preferred to be employed in the BEFL system.

In this thesis, the BEFL signal is generated in ring and linear cavities with

channel spacing of 0.09 nm. The injected Brillouin pump (BP) wavelength and power as

well as the power of 1480 nm pumps and effective cavity loss in the cavity exhibited

great effect on the number of wavelengths and output power of the generated

wavelength comb. The linear cavity BEFL exhibits a lower threshold power compared

to the ring configuration. Linear cavity shows improvement on the number of lines

generated. Three linear cavity designs are demonstrated. The third linear cavity BEFL

with lower cavity loss could generate up to 50 lines compared to 13 Stokes and anti-

Stokes lines generated by ring cavity BEFL. The stable output laser comb of 50 lines is

obtained at a BP wavelength of 1568.2 nm and BP power of 5 dBm and two 1480 nm

pumps power at 120 mW. The number of lines increases as the BP power increases

from 3 to 8 dBm. The Stokes lines are generated as long as the gain is sufficient to

support the cascading process.

ii

The multiwavelength BEFL signal has potential applications in wavelength

division multiplexing (WDM), optical fiber sensor system, optical component testing

and spectroscopy applications.

iii

ABSTRAK

Tesis ini memfokuskan kepada eksploitasi kesan taklinear di dalam

gentian mod tunggal (SMF), terutamanya kesan serakan Brillouin dirangsang (SBS)

untuk menjana isyarat pelbagai panjang gelombang di dalam laser gentian

Brillouin/Erbium (BEFL) berasaskan Bismuth. Gentian berasaskan Bismuth berdopan

Erbium (Bi-EDF) sepanjang 2.15 m yang mempunyai kepekatan ion Erbium sebanyak

3250 wt. ppm. digunakan sebagai bahantara amplifier linear di dalam BEFL. Ciri-ciri

amplifier gentian berasaskan Bismuth berdopan Erbium (Bi-EDFA) dikaji. Amplifier

ini mempamerkan gandaan di dalam kawasan 1565 hingga 1600 nm apabila Bi-EDF ini

dipam oleh dua diod laser, tiap-tiap satu berkuasa pam 100 mW dengan panjang

gelombang 1480 nm. Bi-EDFA laluan berganda dua menunjukkan prestasi gandaan

lebih baik tetapi angka hingar lebih tinggi berbanding dengan Bi-EDFA laluan tunggal.

Oleh itu, Bi-EDFA laluan tunggal lebih diutamakan untuk digunakan di dalam sistem

BEFL.

Di dalam thesis ini, isyarat BEFL dijana di dalam kaviti gelang dan linear

dengan ruang saluran 0.09 nm. Panjang gelombang dan kuasa pam Brillouin (BP) yang

disuntik serta kuasa pam 1480 nm dan kehilangan berkesan kaviti di dalam kaviti

mempamerkan kesan besar ke atas bilangan panjang gelombang dan kuasa keluaran

panjang gelombang sikat yang dijana. BEFL kaviti linear mempamerkan ambang kuasa

lebih rendah berbanding konfigurasi gelang. Kaviti linear mempamerkan peningkatan

ke atas bilangan garis yang dijana. Tiga kaviti linear telah ditunjukkan. BEFL kaviti

linear ketiga dengan kehilangan berkesan kaviti yang lebih rendah dapat menjana

sehingga 50 garis berbanding 13 garis Stokes dan anti-Stokes yang dijana oleh BEFL

kaviti gelang. Sikat keluaran laser 50 garis yang stabil didapati pada panjang gelombang

iv

BP 1568.2 nm dan kuasa BP 5 dBm dan kuasa dua pam 1480 nm pada 120 mW.

Bilangan garis meningkat apabila kuasa BP meningkat daripada 3 kepada 8 dBm. Garis

Stokes akan dijana selagi gandaan mencukupi untuk menyokong proses melata.

Isyarat pelbagai gelombang BEFL mempunyai potensi aplikasi di dalam sistem

pembahagian multipleks panjang gelombang, sistem pengesan gentian optik, ujian

komponen optik dan aplikasi spektroskopi.

v

ACKNOWLEDGEMENT

It is a pleasure to thank the many people who made this thesis possible. It is

difficult to overstate my gratitude and appreciation to my supervisors, Prof. Dr. Harith

Ahmad and Assoc. Prof. Dr. Sulaiman. With their enthusiasm, their inspiration, and

their great efforts to explain things clearly and simply, they helped to make the

researches fun for me. I really appreciate the opportunity given to me to do research in

Photonics Laboratory.

To all lab members, thank you for providing a stimulating and fun environment

in which to learn and grow. Special thanks my labmates Sharife Shahi and

Mohammadreza Rezazadeh who have taught me the fundamental elements and

mathematical aspects in Photonics Physics.

Big thanks to my entire extended family for providing a loving environment for

me. My parents, sister, and aunts for helping me get through the difficult times, and for

all the emotional support, comraderie, entertainment, and caring they provided.

I dedicate this work to Allah, the most Gracious and the most Merciful and

Prophet Muhammad, the Messenger of Allah (peace be upon him).

vi

CONTENTS

ABSTRACT

ABSTRAK

ACKNOWLEDGEMENT

LIST OF ABBREVIATION

i

iii

v

ix

CHAPTER 1 INTRODUCTION

1.1

1.2

1.3

1.4

1.5

Overview of The Development of Fiber Optic

Communication

Optical Fiber Amplifier

Multiwavelength Fiber Laser

Objective of the Study

Thesis Overview

1

5

9

10

10

CHAPTER 2 THEORETICAL BACKGROUND

2.1

2.2

2.3

2.4

2.5

Introduction

Doped Glass Structure and Properties

2.2.1 Bismuth-based Erbium-doped Fiber

Optical Amplifier

2.3.1 Erbium-doped Fiber Amplifier Operating

Principle

2.3.2 EDFA Characteristics

Non-linear Effect in Single-Mode Fiber

2.4.1 Principles of Stimulated Brillouin Scattering

Fiber Laser

19

21

24

27

29

32

38

44

49

vii

CHAPTER 3 BISMUTH-BASED ERBIUM-DOPED FIBER

AMPLIFIER

3.1

3.2

3.3

3.4

Introduction

Bismuth-based Erbium-doped Fiber

Characterization of the Single-pass and Double-pass EDFA

Summary

64

65

69

76

CHAPTER 4 BISMUTH-BASED ERBIUM-DOPED FIBER RING LASER

4.1

4.2

4.3

4.4

4.5

Introduction

SBS Observation in Single-mode Fiber

Single Frequency BEFL

Multiwavelength BEFL (MWBEFL)

Summary

80

80

83

87

93

CHAPTER 5: ENHANCED MULTIWAVELENGTH BISMUTH-BASED BRILLOUIN ERBIUM FIBER LASER

5.1

5.2

5.3

5.4

Introduction

Linear Cavity Bismuth-based Brillouin/Erbium Fiber Laser

5.2.1 Employing 25 km SMF as Nonlinear Gain

Medium

5.2.2 PMF as the Nonlinear Gain Medium in the

Linear Cavity BEFL

5.2.3 Comparison between Ring Cavity and Linear

Cavity BEFL

Enhanced Linear Cavity Bismuth-based

Brillouin/Erbium Fiber Laser

The Third Linear Cavity BEFL Design

97

97

102

103

104

106

113

viii

5.5 Summary 121

CHAPTER 6: CONCLUSION AND FUTURE WORKS

6.1

6.2

Conclusion

Future Works

124

128

LIST OF PUBLICATIONS 129

ix

LIST OF ABBREVIATIONS

Bi-EDFBi-EDFABEFLBPCDMCPMDWDMDFBEDFEDFAFRAFTTHFWMHSBBHUCMCVDNANFNGNOCDMAOSAPIQPCESBSSOASMFSNRSPM SRSTDMTLSWDM

Bismuth-based Erbium-doped FiberBismuth-based Erbium-doped Fiber AmplifierBrillouin/Erbium-doped Fiber LaserBrillouin PumpCode-division MultiplexingCross-Phase ModulationDense Wavelength Division MultiplexingDistributed Feed-Back (DFB)Erbium-doped FiberErbium-doped Fiber AmplifierFiber Raman AmplifierFiber-to-the-home Four-Wave MixingHigh Speed Broadband Homogenous UpconversionModified Chemical Vapor DepositionNumerical ApertureNoise FigureNext-Generation NetworkOptical Code Division Multiple AccessOptical Spectrum AnalyzerPair-induced QuenchingPower Conversion EfficiencyStimulated Brillouin ScatteringSemiconductor Optical AmplifierSingle-mode FiberSignal-to-noise-ratioSelf-Phase Modulation Stimulated Raman ScatteringTime-division multiplexingTunable Laser SourceWavelength-Division Multiplexing

1

CHAPTER 1

INTRODUCTION

1.1 Overview of The Development of Fiber Optic Communication

The change of telecommunication traffic in worldwide from analog signals to

digital signals has lowered the cost of communication services such as telephone call

and internet. This requires a better network service in which the conventional copper

wires are replaced by fiber optic cables. Fiber optic cable is often found in backbone

networks because of its wide bandwidth and cost-effective. Figure 1.1 shows the

evolution of optical communication system capacity within the period of 25 years [1].

As depicted in the figure, the commercial deployment of optical communication

systems followed the research and development phase closely. The progress has indeed

been rapidly increased in the bit rate by a factor of 100,000 over a period of less than

25 years.

The key in designing optical communication networks in order to exploit the

fiber's huge bandwidth is to introduce concurrency among multiple user transmissions

into the network architectures and protocols. In an optical communication network, this

concurrency may be provided according to either wavelength or frequency

[wavelength-division multiplexing (WDM)], time slots [time-division multiplexing

(TDM)], or wave shape [spread spectrum, code-division multiplexing (CDM)]. TDM is

limited by the speed of the time-multiplexing and demultiplexing components and

CDM suffers inter-channel interference as the number of channel increases. WDM is

the current favorite multiplexing technology for optical communication networks.

2

Figure 1.1 Increase in the capacity of optical communication systems realized after 1980. The commercial systems ( ) follow research demonstrations ( ) with a few-year lag. The change in the slope after 1992 is due to the advent of WDM technology[1].

Earlier network in 1980’s, the first generation optical technology started to

replace the copper technology in the backbone network. The system employed single

wavelength operation at wavelength of 0.8 m [2,3]. Then, second generation optical

networks operated at wavelength of 1.3m has increased the network capacity from 45

Mbps to 1.7 Gbps. This is followed by the third generation which started to operate at

1550 nm wavelength region. The fourth generation of lightwave systems makes use of

optical amplification for increasing the repeater spacing and of wavelength-division

multiplexing (WDM) for increasing the bit rate. As evident from different slopes in

Figure 1.1 before and after 1992, the advent of the WDM technique started a revolution

that resulted in doubling of the system capacity every 6 months. By 1996, the

transmission over 11,300 km at a bit rate of 5 Gbps had been demonstrated by using

actual submarine cables [4]. In 1998, utilising wavelength-division multiplexing

3

(WDM), Alcatel has conducted 6150 km transmission experiment with 32 channels,

each at 10 Gbps, over a single fiber, giving an overall capacity of 320 Gbps per fiber

[5]. This experiment is the first reported demonstration of 32 x 10 Gbps transmission

over transoceanic distances with such efficient use of the optical spectrum, using

narrow channel spacing, at 0.4 nm, and 10 Gbps bit rate. The experiment was based on

the WDM technique already applied in Alcatel's OALW40 (2.5 Gbps x 16) and

OALW160 (10 Gbps x 16) repeatered submarine systems families and currently being

implemented on several systems such as Southern Cross and Japan-US. Suyama et. al

from Fujitsu has demonstrated an ultra-large capacity (10.7 Gbps x 66) optical

submarine network system. STM-64 signal is converted to a 10.66 Gbps signal before

being multiplexed into WDM optical line signal [6].

WDM technology provides enhancements such as increased network capacity

and greater network flexibility. The WDM technology enhances the networking

capabilities. WDM allows undersea networks to use the wavelength layer to add and

drop more traffic capacity at more landing points, while keeping the number of fiber

pairs in the system to a minimum. This feature is a result of adding wavelength-

selective filtering capabilities to undersea branching units in transoceanic systems. The

key challenge in the design of these systems is how to achieve a large number of

wavelengths over distances as large as 12,000 km. Special care must be taken in

choosing the dispersion map of each fiber path and in the spacing of the wavelengths

[7]. In the future, this may increase to an STM-64 (10 Gbps) per wavelength.

Fiber optic cable system implementation in Malaysia connects all states of

Malaysia via underwater route or terrestrial link. TIME Submarine-Based Network lies

on Malacca Straits and South China Sea provides links for peninsular towns and cities,

Land-Based Network along Federal Road connects East Coast of peninsular and along

North-South PLUS Expressway, Telekom Malaysia cable system connecting big cities

4

and goes underwater connecting Kuantan and Kota Kinabalu. For international

connection, SEA-ME-WE-3 network, which is one of an important global undersea

fiber optic infrastructure, connects Malaysia to the other parts of the world. The SEA-

ME-WE-3 Cable System uses WDM technology to do undersea routing of wavelengths

featuring 40 Gbps capacity (2.5 Gbps x 8 x 2 fiber pairs) [6]. The SEA-ME-WE-3

uses undersea wavelength add/drop multiplexing to realize a complex traffic

connectivity over two pairs of undersea fiber. This network has a capacity of up to eight

wavelengths on each fiber with each wavelength carrying an STM-16. This network

has a trunk and branch cable topology and uses undersea wavelength add/drop

multiplexing Branching Units. The add/drop capability allows efficient allocation of the

full capacity on each individual fiber pair to separate countries, either directly on the

network or indirectly connected via transit facilities. This configuration results in a very

high degree of traffic sovereignty and security.

In Malaysia, High Speed Broadband (HSBB) project to be carried out by

Telekom Malaysia is an indication of the rapid optical communication development in

Malaysia. HSBB services using three main technologies, i.e. fiber-to-the-home (FTTH),

Ethernet-to-the-home and Very High Speed Digital Subscriber Line 2. In addition, TM

is rolling out its Next-Generation Network (NGN) core backbone network based on an

all IP Platform as well as grow the nation's global capacities by building new

international gateways for enhanced connectivity and network efficiency.

5

1.2 Optical Fiber Amplifier

In a network system, as a signal travels in the optical fiber, the signal amplitude

degrades because the energy of the signal dissipates due to scattering and absorption in

the optical fiber. Previously, to overcome the loss limitation, optoelectronic repeaters

are employed, in which the optical signal is first converted into an electric current and

then regenerated using a transmitter. Such regenerating process become quite complex

and expensive for wavelength-division multiplexed (WDM) lightwave systems. The

complexity of regenerating process makes optical amplifiers more attractive to replace

the regenerators. Optical amplifiers can compensate for loss in transmission optical

fiber and in optical devices (such as optical add/drop multiplexers and optical switches)

that make up a network and enable greater network scale and transmission distances.

Optical amplifiers allow easy integration of the signal because they are bit rate

independent and also not affected by the type of modulation. The amplifier is able to

amplify multiplexed signals simultaneously, have high temperature stability and have

low insertion loss, thus, improves system flexibility and functionality. For long-haul

systems, the amplifiers which replace electronic regenerators are called in-line

amplifiers. Amplifier can also be placed at the transmitter module or at the receiver end

acting as a power booster and a pre-amplifier, respectively. A power amplifier can

increase the transmission distance by 100 km or more depending on the amplifier gain

and fiber losses. Transmission distance can also be increased by putting an amplifier

just before the receiver to boost the received power. Such amplifiers are called optical

preamplifiers and are commonly used to improve the receiver sensitivity.

Optical fiber amplifiers are essential for increasing the scale and performance of

the optical communication systems. They play a major role in determining the

operating wavelength region of Metro, Regional, and Ultra Long-Haul (ULH)

6

networks. Greater capacity per fiber could be achieved if optical fiber amplifiers

(EDFAs) are used in WDM system. The amplifier should have high gain, high power

conversion efficiency (PCE) and high saturated output power as well as low noise

figure. Optimization of optical amplifier deployment in a network requires a balance

between amplifier spacing and optical signal-to-noise-ratio (SNR) performance. The

optimization is needed sustain the communication signal from possible transmission

interruption arises due to dispersion and noise. Enhancement in amplifier gain and SNR

reduces the number of amplifiers needed in a network [8].

Types of Optical Amplifiers

There are three types of optical amplifiers; rare-earth doped fiber amplifier,

semiconductor amplifier and Raman amplifier. Rare-earth doped amplifier incorporates

rare-earth elements such as erbium, praseodymium, thulium and neodymium as dopants

in the glass matrix of an optical fiber. Semiconductor amplifier uses an active medium

made up of an alloy of semiconductor elements such as phosphorus, gallium, indium

and arsenic. A fiber-based Raman amplifier uses stimulated Raman scattering (SRS)

occurring in silica fibers when an intense pump beam propagates through it [9-12].

A rare-earth doped fiber amplifier type, Erbium-doped fiber amplifier (EDFA)

is commonly employed in the optical communication system. Amplification band

depends on the type of rare-earth ion doped [13]. The EDFA provides amplification in

S (1460-1529 nm), C (1530-1564 nm) and L (1565-1624 nm) bands region while

Thulium-doped fiber amplifier corresponds to S and U (1625-1675 nm) bands and

Praseodymium to O-band (1260-1359 nm).

Ultra large-capacity dense wavelength division multiplexing (DWDM) and

wideband coarse wavelength division multiplexing (CWDM) are expected to be

7

deployed in transmission systems in the near future to provide systems with

transmission rates of several terabits per second and inexpensive short/medium-

distance transmission systems, respectively. To support these systems, as well as access

networks and local area networks, broader bandwidth is needed to support more

number of channels. The amplification bandwidth of rare-earth-doped optical fiber

amplifiers must be expanded beyond the C band (1530–1565 nm). Thus, extending the

bandwidth to L and S bands is desired. The transmission bandwidth can be expanded

either by connecting optical fiber amplifiers having different amplification bandwidths

in parallel or expanding the amplification bandwidth itself by replacing the silica fiber

used in EDFAs with a new glass fiber such as tellurite [14-19] or bismuth fiber [20-21].

Bismuth-based EDFA (Bi-EDFA) which provides L-band amplification is investigated

in this thesis.

Amplification in L-band

L-band EDFA operates within the wavelength region approximately between

1560 to 1620 nm, which lies at the tail of the erbium gain window where inversion is

low. The emission and absorption coefficients in the L-band are also smaller than that

in C-band. These smaller coefficients along with the low average inversion cause the L-

band gain coefficient to be significantly smaller than the C-band. Thus, in order to

exploit this region to achieve high gain, the length of EDF is around 7 to 8 times longer

than that in the conventional amplifier [22]. Longer EDF leads to lower power

conversion efficiency caused by the higher fiber loss. Many research were carried out

to develop EDF with high Er3+ concentration and thus, exhibits low loss characteristic

[23-25]. Central Glass & Ceramic Research Institute, India [26] managed to fabricate

Silica EDF with Er3+ concentration of 900 ppm using a Modified Chemical Vapor

8

Deposition (MCVD) process in conjunction with a solution doping technique. The 15

m Silica EDF could provide gain from wavelength of 1565 until 1595 nm [26-29]

instead of the previous EDFA consisting of 50 m Silica EDF with concentration of 400

ppm to provide amplification in L-band region [30]. Bismuth-EDF (Bi-EDF) exhibits

wide emission bandwidth with strong emission probability in 1550 nm region as shown

in Figure 1.2 [31]. Bi-EDFA has been widely studied [32-38] and applied in fiber lasers

[39-40].

Figure 1.2 Bismuth oxide glass exhibits broader emission than silica glass [31].

The Bi-EDF can be used to generate multiwavelength laser in Brillouin/Erbium

fiber laser (BEFL). A brief introduction on multiwavelength fiber laser is given in the

next section.

9

1.3 Multiwavelength Fiber Laser

Multiwavelength fiber lasers have recently attracted much attention because

they have potential applications in wavelength-division-multiplexed (WDM) systems,

optical code division multiple access (OCDMA) systems, fiber sensor systems and

optical test equipment [41-45]. WDM corresponds to the scheme in which multiple

optical carriers at different wavelengths are modulated by using independent electrical

bit streams (which may themselves use TDM and FDM techniques in the electrical

domain) and are then transmitted over the same fiber. The optical signal at the receiver

is demultiplexed into separate channels by using an optical technique. WDM has the

potential for exploiting the large bandwidth offered by optical fibers. Considerable

attention was directed during the 1980s toward reducing the channel spacing, and

multichannel systems with a channel spacing of less than 0.1 nm had been

demonstrated by 1990 [46]. However in 1990s, WDM systems were developed most

aggressively [47]. Commercial WDM systems first appeared around 1995, and their

total capacity exceeded 1.6 Tbps by the year 2000.

Many techniques have been implemented to generate multiwavelength signal

such as using DFB laser array [48], spatial hole burning [49], independent gain media

[50], frequency shift [51] and phase modulator [52]. A technique known as spectral

slicing uses a broad emission spectrum of an SOA applicable to provide multiple WDM

channels has been reported Sagnac loop mirror is used as the wavelength selective

component [53]. Nonlinear effect in single mode fiber can be utilized towards

multiwavelength generation. Stimulated Brillouin scattering (SBS) effect in SMF

enables multiwavelength generation in BEFL [54]. SBS is a nonlinear effect that results

from the interaction between intense pump light and acoustic waves in a SMF, thus

giving rise to backward propagating frequency shifted light. The thermally excited

10

acoustic waves generate an index grating that co-propagates with the pump at the

acoustic velocity in the SMF. This moving grating reflects the pump light and causes

the backscattered light to experience a frequency downshift of 11 GHz giving rise to

the generation of stable and multiple Brillouin wavelengths with constant spacing and

narrow linewidth at room temperature [55]. BEFL combines the gain from Stimulated

Brillouin scattering (SBS) and EDF. BEFL with 11 GHz Stokes shift in ring cavity and

linear cavity BEFL have been demonstrated [56-58].

1.4 Objective of the Study

This research aims to provide solution to the current need for more channels in

the communication system. The research intends to generate multiwavelength signal in

L-band region using BEFL system. Generally, more output channels generated are

preferable as this could cater more channels. The optimum design of the BEFL

configuration to generate most number of channels is to be determined.

1.5 Thesis Overview

This thesis focuses on multiwavelength lines generation motivated by the

increasing needs for more channels in communication system. The key issue is to

generate the multiwavelength signal in L-band as C-band is exhausting as well as to

make a compact BEFL system. Bi-EDFA utilizes short length of fiber to enable the

realization of compact EDFA and compact BEFL.

The first chapter makes an introduction to the optical fiber communication

evolution where WDM system is used to provide high capacity communication system.

Chapter 2 provides the elementary concepts behind a BEFL. This chapter discusses the

11

theoretical background of amplification of light in EDFA and stimulated Brillouin

scattering in multiwavelength BEFL. Chapter 3 is devoted to introduce Bi-EDFA which

will be applied in the BEFL. The characterization of single-pass and double-pass Bi-

EDFA is described. Chapter 4 then focuses on the multiwavelength generation in

single-mode fiber (SMF) acting as a non-linear gain medium, which then was applied

in a ring cavity BEFL. Chapter 5 presents the enhanced BEFL utilizing linear cavity

designs to generate more lines. Chapter 6 concludes the findings of this research.

12

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13

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14

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19

CHAPTER 2

THEORETICAL BACKGROUND

2.1 Introduction

The rapid growth in the use of distributed information systems such as the

Internet stimulates the demand for high data transmission rates. Currently, the highest

data rates are achieved using silica glass fiber as a transmission medium. Glass fiber

contains a high refractive index core, surrounded by a lower refractive index cladding

layer. This structure functions as an optical waveguide: light is confined in the fiber

core by total internal reflection, allowing the transmission of optical signals over large

distances. Long distance data transfer also requires high transparency. In the

wavelength region of optimum transmission of glass fiber, occurring around 1550 nm,

modern fiber exhibits losses as low as 0.2 dB/km. Nevertheless, for transmission over

large distances the optical signal needs to be amplified at regular intervals in order to

maintain sufficient light intensity. This is done using Erbium-doped fiber amplifier

(EDFA) that operates at a wavelength region of 1550 nm. This wavelength region is

referred as a Conventional band (C-band) region.

Optical fiber amplifiers are important system components in the construction of

optical-based dense wavelength division multiplexing (DWDM) networks. They can

compensate loss in transmission optical fiber and in optical devices (such as optical

add/drop multiplexers and optical switches) that make up a network and enable greater

network scale and transmission distances. The amplification bandwidth or operating

wavelength region of the optical fiber amplifiers determines the operating wavelength

region of the network. Thus, to create large-capacity photonic networks, we must

expand the bandwidth of optical fiber amplifiers. Efforts to increase network capacity

20

are being combined with efforts to expand the optical fiber amplification bandwidth to

the C-band (1530–1565 nm), L-band (1565– 1625 nm), S-band (1460–1530 nm) and O-

band (1260-1360 nm) [1-7]. Multiple hosts material such as tellurite-based fiber [8],

antimony silicate fiber [9], phosphorous co-doped silica based fiber [10], lanthanum co-

doped bismuth-based erbium-doped fiber [11] have been investigated to improve the

amplification characteristics of the EDFA. Bismuth-based EDFA (Bi-EDFA) is a

promising candidate for broadband signal amplification around 1550 nm [12-15].

The information is transmitted as a series of light pulses that travel through the

fiber at approximately 200,000 km/s. Due to the high frequency of light, the amount of

data that can be transmitted is tremendous and the capacity keeps increasing.

Wavelength Division Multiplexing (WDM) technology enables the transmission

capacity to be increased by dedicating optical signals to different wavelengths in an

optical fiber. The SEA-ME-WE-3 Cable System which connects Malaysia

internationally uses WDM technology to do undersea routing of wavelengths featuring

40 Gbps capacity (2.5 Gbps x 8 x 2 fiber pairs) [16]. Many efforts have been carried

out to generate multiwavelength signal such as using Distributed Feed-Back (DFB)

laser array [17], spatial hole burning [18], independent gain media [19], frequency shift

[20], phase modulator [21] and Sagnac loop mirror [22]. Multiwavelength generation

employing third-order optical nonlinearity in fiber lasers has received great interest [23-

29].

21

2.2 Doped Glass Structure and Properties

Glass is suitable for the laser host because of its optical quality, transparency,

low birefringence, high optical damage threshold, thermal shock resistance, weak

refractive index nonlinearity, high energy storage and power extraction capacities,

varieties of possible composition, size and shape scalability and low cost of raw

materials. Generally glass is an inorganic product of fusion cooled to a rigid condition

without crystallizing. The structural organization of glass is well defined at the scale of

few atoms, but completely random, asymmetric and aperiodic at a larger scale. The

atomic arrangement of silica glass is similar to the crystalline form of silica, but leads

to the lack of long-range periodicity or symmetry. The most common is the silica

tetrahedron (SiO4)2-. Other usual glass formers are SiO2, GeO2, B2O3, and P2O5. The

glass lattice is built from basic structural units made of network former atoms. Network

formers are all capable of forming a three-dimensional network with oxygen, thus

providing very strong covalent bonds that give glasses their characteristic properties. In

silica glass, the tetrahedron units are tightly connected to form a three dimensional,

disorder lattice.

Other compounds such as alkaline or alkaline earths can be added to the glass as

network modifiers. These modifiers can cause former bridging ions to become

nonbridging to break the lattice and results in a looser network structure. The addition

allows glass to be processed at workable temperatures. Oxides of alkali metals and

alkali-earth metals such as LIO, Na2O, K2O, Rb2O, Cs2O, CaO, SrO and BaO are

typically used as network modifiers. Network modifier such as Na+ and Al3+ are used to

facilitate the incorporation of trivalent rare earth ions in glass, as their size near 1

angstrom is substantially greater than that of the basic network formers. Figure 2.1

shows the structure of typical alkali silicate glass.

22

Figure 2.1 Structure of alkali silicate glass.

Other element such as germanium, aluminum, phosphorus, or boron may

substitute a number of silica sites in fused silica. If silica is replaced by germanium,

tetrahedral structure is retained and its properties are similar to fused silica glass. The

presence of germanium increases the refractive index over that of the cladding glass

and enables a wide range of guiding structures to be made. In fiber for

telecommunications, aluminum has been used as an alternative to germanium for

increasing the refractive index of the core. Rare earth ions more specifically bivalent or

trivalent lanthanides, have been used as activators mostly in laser crystals. From the

electronic configuration of trivalent ions, one or two electrons are removed from the 4f

and 6f shells respectively, a consequence of the energetic sequence in which electron

fill up the subshells. N-1 inner electrons at 4f have been shielded by outermost shells

5s, 5p cause 4f → 4f laser transitions of rare earth solid state laser materials to exhibit

23

relatively sharp lines. Another consequence of this shielding effect is that the

spectroscopic characteristics of the 4f → 4f transitions are weakly sensitive to the type

of host. In amplifier device applications, the weak perturbations induced by the host

actually represent important effects.

The most common glass used for erbium-doping is silicon dioxide (SiO2) or

silica. Silica glass has the lowest crystallization rate compared to all other phases of

glass. The single bridging oxygen is replaced by two non-bridging oxygen ions bonded

to alkali ions. A high concentration of non-bridging oxygen groups allows the

incorporation of a small concentration of rare earth ions which is the basic process of

doping fibers. Figure 2.2 shows the illustration of silica glass, with the non-bridging

ions which is connected to alkali ions R+. In pure silica glass, where no network

modifiers exist, a very rigid structure is formed due to the inexistence of non-bridging

Si-O- groups. The inclusion of rare earth ions is made difficult by the tendency of these

ions to cluster together, effectively reducing radiative transitions, which fundamentally

limit their concentration [30]. However, the clustering problem can be compensated by

modifying the host glass with compositions such as P2O5 [31] and Al2O3 [32] or by

using a different glass host such as Bismuth trioxide (B2O3) [33] and tellurite glass

[34].

Figure 2.2 Illustration of pure silica glass.

O

O

O

O

O

O

O

O

OSi Si

R+

R+

R+

R+

24

2.2.1 Bismuth-based Erbium-doped Fiber

Erbium-doped fiber (EDF) is the key element in the EDFA. The erbium ions

added in silica core contribute to the optical signal amplification through stimulated

emission. Short, highly doped EDF enables the realization of compact EDFA. With

shorter EDF, an erbium-doped fiber laser will have lower cavity loss and lower

dispersion which introduced by the EDF. Bismuth-based glass has several advantages

over conventional silica-based glass as host material for Erbium-doped glass. Its ability

to disperse Erbium ions has allowed Erbium ion doping of more than 1,000 ppm

without significant concentration quenching effect. Whereas in silica-based glass, the

concentration quenching of Er3+ ions begin at less than 1000 wt ppm, leading to longer

effective Si-EDF length [35].

The higher refractive index and with large Judd-Ofelt intensity parameter, Ω6,

of Bismuth-based glass result larger emission cross section and broader emission

bandwidth than tellurite glass [36-37]. However, due to high refractive index, the 4I13/2

level lifetime is relatively shorter in Bismuth-based glass. Erbium ion concentration

affects the EDF performance. High concentration of erbium ions in Si-EDF, may result

in pair-induced quenching (PIQ) effects [30], therefore reduces the pump power

conversion efficiency (PCE) and increases the noise figure (NF) for an EDFA. This is

because when the distance between the Er3+ ions decreases (due to high concentration),

any two adjacent ions excited to the 4I13/2 lasing state of erbium will interact in a

process known as cooperative upconversion, whereby energy is transmitted from one

ion (which makes a transition to the 4I15/2 ground state) to the other ion (which is

excited to the 4I9/2 state). The ion excited to the 4I9/2 state decays through a process of

nonradiative transition (or multiphonon decay) to the 4I13/2 state and does not contribute

to signal amplification.

25

The host glass of the Bismuth-based EDF (Bi-EDF) is Bismuth trioxide [38].

Bismuth Oxide takes the fundamental crystalline structure of Bi2O3 which the basic

structure of Bismuth Oxide is shown in Figure 2.3. The physical form of Bismuth

Oxide is a yellowish powder and has a melting temperature of 817˚C, lower than silica

glass. Thus, Bismuth oxide glass fiber can be fusion spliced using standard fiber fusion

splicer [13,39].

Figure 2.3 Basic structure of Bismuth Oxide [40].

Figure 2.4 shows the illustration of distribution of erbium ions in bismuth-based

glass. Bi-EDF co-doped with Lanthanum (La) can have much higher concentration of

erbium ions than Si-EDF but with negligible ion quenching effect. This is because the

La ions extend the distance between Er ions and reduce the concentration quenching

significantly [41]. Bi-EDF increases the limit of the erbium doping concentration that is

imposed by concentration quenching in the EDF. Bi-EDF co-doped with Lanthanum

(La) decreases the concentration quenching of the erbium ions in the fiber. The Bi-

EDFA is expected to have a broad and flat 4I13/2-4I15/2 emission at wavelength region

around 1560-1610 nm.

Bi

Bi

Bi

Bi

26

Figure 2.4 Illustration on the distribution of erbium ions in bismuth-based glass

[40].

Figure 2.5 shows the emission spectrum of the Bismuth oxide glass, which exhibits

broader emission compared to silica-EDF [39]. The wider amplification band is due to the

smaller vibration energy of the bismuth glass lattice, which resulted in a larger emission

spectrum and lower excited state absorption in the extended L-band region [42, 43].

Figure 2.5 Emission spectrum of Bismuth oxide glass and silica glass [39].

27

The short length Lanthanum co-doped Bi-EDF can provide wideband

amplification. This makes it a suitable candidate for compact EDFA and EDFA based

devices such as fiber laser. In this thesis, the Bi-EDF is applied in fiber laser.

2.3 Optical Amplifier

The general mechanism of amplification in optical amplifiers is stimulated

emission, the same mechanism is also used by the lasers. When a material is exposed to

light, the atoms absorb the photons and end up in excited states. This process is known

as absorption. These excited atoms eventually return to their normal “ground” state and

the light energy is released. This phenomenon is called light emission. The light

emission can be spontaneous with no phase relationship among the emitted photons.

This phenomenon is called spontaneous emission. Stimulated emission, however, is

initiated by an existing photon. As a result of this phenomenon, the emitted photon

matches the incident photon in frequency as well as phase and constructive interference

takes place which lead to an amplification of the incident lightwave signal. Common

types of optical amplifiers are Semiconductor Laser Amplifier (SLA), fiber Raman

Amplifier (FRA), fiber Brillouin Amplifier (FBA) and EDFA.

SLA is basically a semiconductor laser without a feedback. For stimulated

emission to occur, population inversion condition is necessary. That is, number of

atoms in the excited state must be higher than the number of atoms in the ground state.

In the SLA, population inversion condition is achieved by external current injection

[44]. Although several potential applications of SLAs have been demonstrated, they

need to overcome several drawbacks before their use becomes practical. A few among

them are polarization sensitivity, inter-channel cross-talk, and a large coupling loss.

28

SLAs can have chip gain as high as 30 - 35 dB but the usable gain is reduced to 8 -10

dB because of the large coupling loss occurring at the input and output ends. FRA uses

stimulated Raman scattering (SRS) occurring in silica fibers when an intense pump

beam propagates through it [45]. In SRS, the incident pump photon loses its energy to

create another photon at lower frequency (higher wavelength); the remaining energy is

absorbed by the medium in the form of molecular vibrations. FRAs are pumped

optically unlike SLAs which are pumped electrically. Moreover, the population

inversion condition is not required in this case. The broad bandwidth of FRA is

extremely useful for amplifying several channels simultaneously. However, the

applications of FRAs are limited by amplifier noise associated with spontaneous

Raman scattering which occurs over a wide frequency range (> 5 THz). FBAs function

essentially on the same operating principle as FRAs except that the optical gain is

provided by stimulated Brillouin scattering (SBS) [46]. These differences exist due to a

relatively small value of the ratio of the acoustic velocity in silica and the velocity of

light. These amplifiers are less suitable as power amplifiers, preamplifiers, or in-line

amplifiers because of their narrow spectrum.

Doped fiber amplifiers are fabricated by doping the fiber by rare earth ions.

Different rare earth ions, such as erbium, holmium, neodymium, samarium, thulium,

and ytterbium, can be doped to achieve amplification at different wavelength regions

ranging from visible to infrared region. EDFA is the most commonly used all-optical

amplifier because of its excellent amplification properties near 1550 nm, the

wavelength region in which the fiber loss is minimum [47]. High gain and low noise

performance of EDFAs make them integral components in most of the important

applications in optical fiber communication systems.

29

2.3.1 Erbium-doped Fiber Amplifier Operating Principle

EDF is the gain medium for EDFA, which is made by doping erbium ions into

the core of alumino-germano-silicate glass fibers. The cladding is made from the

phosphate-silicate glass. Erbium is a rare earth element belonging to the group of the

Lanthanides. When embedded in a solid, erbium generally assumes the trivalent Er3+

state, which has an electronic configuration [Xe]-4f11. The Er3+ ion has an incompletely

filled 4f-shell, allowing for different electronic configurations with different energies

due to spin-spin and spin-orbit interactions. Radiative transitions between most of these

energy levels are parity forbidden for free Er3+ ions. When Er is incorporated in a solid

however, the surrounding material perturbs the 4f wave functions. This has two

important consequences. Firstly, the host material can introduce odd-parity character in

the Er 4f wave functions, making radiative transitions weakly allowed. Secondly, the

host material causes Stark-splitting of the different energy levels, which results in a

broadening of the optical transitions. Figure 2.6 shows a schematic level diagram of the

Stark-split Er3+ energy levels, labelled using Russell-Saunders notation. Since radiative

transitions in Er3+ allowed are weak, the cross sections for optical excitation and

stimulated emission are quite small, typically on the order of 10-21 cm2, and the

radiative lifetimes of the excited states are long, up to several milliseconds.

Electrons in the 4I11/2 state will de-excite to 4I13/2 state through a non-radiative,

phonon relaxation process can be used to obtain stimulated emission from metastable

excited state, 4I13/2 to ground state, 4I15/2. From the metastable 4I13/2 state, electrons will

decay to the ground state by emitting a photon with frequency corresponds to the

energy difference between 4I13/2 and 4I15/2 state, obeying equation:

hvE (2.1)

30

where E is the energy between the two states, h is the Planck’s constant, and v is the

frequency of the photon emitted.

From the equation of frequency,

c

v (2.2)

where c is the speed of light in vacuum and λ is the wavelength, the photons emitted

have a wavelength characteristic around the 1550 nm region. Due to various line

broadening effects [48], namely Stark’s splitting of the 4I13/2 and 4I15/2 energy level, the

1550 nm emission band of Er3+ ion ranges from 1525 nm to 1565 nm.

The lifetime of 4I13/2 level is influenced by the phonon energy of the glass host

[49]. The lower the phonon energy of the glass host, the more phonons is needed to

bridge the energy gap between the 4I13/2 level and the lower 4I15/2 level and

consequently the lower the probability of nonradiative transition rate between the two

levels. This translates to longer radiative lifetime and quantum efficiency.

Erbium can be pumped directly into the first excited manifold using a 1480 nm

diode laser, or via one of the higher lying absorption lines, for example using a 980 nm

diode laser. The absorption of pump photons excites ion to higher energy states. The

excited ions dissipate this acquired energy through radiative emission of a photon or by

converting the energy into lattice vibrations or phonons. The tendency to radiate a

photon when jumping to lower energy levels increases with the energy gap. Therefore,

the transition between (4I13/2) and (4I15/2) is predominantly radiative resulting in the

emission of optical output in the 1550 nm region. Thus, a 1550 nm signal traveling

through the EDF will then induce stimulated emission from the first excited state to the

ground state, resulting in signal amplification.

31

Figure 2.6 Schematic representation of the Er3+ intra 4f energy levels. Figure (a)

shows the 1550 nm transition, the upward arrows indicate excitation using 1480 nm

pump light and 980 nm pump light respectively. Figure (b) show the process of co-

operative upconversion, where interaction between two excited Er3+ ions leads to the

population of higher lying energy levels. Figures (c) and (d) show the process of

excited state absorption of a 1480 nm or a 980 nm pump photon respectively [48].

Upconversion process via interparticle interactions are the main cause of erbium

doped fiber gain degradation. Figure 2.6 (b) shows the energy level diagram of erbium

ions with upconversion process. In this process one initially excited 4I13/2 erbium ion

(donor) donates its energy to a neighbour excited erbium ion (acceptor), producing one

upconverted ion and one ground-state ion (4I15/2). The upconverted ion then

nonradiatively relaxed rapidly to the initial state 4I13/2. There are two different kinds of

upconversion processes. The first one is the homogenous upconversion (HUC) in which

the ions are uniformly distributed and the energy transfers from one ion to its neighbour

with a characteristic time of a few milliseconds. The second one is the homogenous

upconversion or pair-induced quenching (PIQ) in which the ions are not uniformly

32

distributed and the energy transfer happens rapidly between two adjacent excited paired

ions with a characteristic time of a few microseconds [30,50,51]. Therefore, the PIQ is

the dominant upconversion process in high-concentration EDFs.

2.3.2 EDFA Characteristics

Signal Gain

One of the most important parameter in optical amplifier is to achieve high gain.

However, gain is actually limited by several physical effects such as the limit due to the

energy conversion principle and the finite number or erbium ions existing in the

medium. In practice, EDFA gain properties are also limited by commonly called

second-order physical effects including pump excited-state absorption (ESA), self-

saturation by amplified spontaneous emission (ASE), concentration quenching, and

inhomogeneous broadening. Therefore, the optical gain, G is defined as

in

ASEout

P

PPG

(2.3)

where Pin and Pout are the amplifier input and output signal powers respectively and

PASE is the ASE power. An understanding of the net amplifier gain can be derived from

an analysis of the gain from an individual ‘slice’ along the fiber. An ASE-free two-

level approximation is assumed [52]. An optical amplifier is actually concatenation of

many amplifier segment incremental of length, Δz. The gain is composed of the

contribution of all gain elements, g(z) along the amplifier fiber:

zLzgzzgzzg

z

neeeG

)()()(

0

...lim21

(2.4)

33

where g(z), the gain element, is given as

)()()( 12 zNzNzg aes (2.5)

In an actual amplifier, the absorption of pump photons is limited by the finite number

of erbium ions existing in the medium that is in the ground state population N1. The

gain is also dependent on the metastable level population density, N2, the stimulated

emission σe and absorption cross-section, σa, and the confinement overlap integrals

factor, Гs. Both emission and absorption represent the strength of the transition or the

ability to produce gain or absorption respectively. From equations 2.3 and 2.4, the

signal gain corresponding to the three-level laser medium of length L is given by:

LNNG aes ])[][(exp 12 (2.6)

For the maximum signal gain which occurs during complete inversion, N1 ≈ 0:

)exp( 2max LNG es (2.7)

Figure 2.7 shows the spectral information required for determination of EDFA gain.

The gain can also be determined by the difference between output power and input

power.

34

Figure 2.7 Spectral information required for the determination of gain [40].

Amplified Spontaneous Emission

An EDFA would amplify the input signal by its gain and produce no additional

output. However, EDFA also produces amplified spontaneous emission produced by

the signal source. As the ions have a finite excited state lifetime (τ = 10 rms), some of

the ions spontaneously return to ground state. These photons have no coherence

characteristics with respect to the incoming signal light, as opposed to a photon

generated by stimulated emission. This spontaneously emitted photon can be amplified

as it travels down the fiber and stimulated the emission of more photons from excited

ions, photons that belong to the same mode of the electromagnetic field as the original

spontaneous photon.

This process occurs at any frequency within the fluorescence spectrum of the

amplifier transitions. This reduces the gain from the amplifier. It takes away photons

that would otherwise participate in stimulated emission with the signal photons. This

Input

Output

Power

(dBm)

Input power

Output power

Gain (dB) = Output power (dBm)-Input power (dBm)

Wavelength (nm)

35

background noise is usually referred to as amplified spontaneous emission, or ASE.

The ASE limits the total amount of gain available from the amplifier. The generated

ASE power propagates in both directions along the fiber, co-propagating and counter-

propagating with the pump power. In the unsaturated region (small signal gain regime),

the output ASE in a given bandwidth Δ of an amplifier with gain G can be expressed

as:

vGhvnGhvnP seqsspASE )1( (2.8)

where spn and

eqn are the spontaneous emission factor and the equivalent input noise,

respectively, corresponding to forward and backward propagation directions.

The output spectrum contains spontaneous emission from both the source and

the EDFA under test, so the EDFA ASE cannot be determined directly from the output

spectrum measurement. The calculation of EDFA noise figure requires that the portion

of the output ASE level that is generated by the EDFA is known. This is calculated as

the difference between the output spontaneous emission power and the equivalent

source spontaneous emission power at the amplifier output.

Noise Figure

The optical noise figure represents a measure of the signal-to-noise ratio (SNR)

degradation experienced by the signal after passing through the amplifier. The original

formula of noise figure proposed by Friis [53] and standardized by IEEE [54] defines it

as:

out

in

SNR

SNRNF (2.9)

36

where the SNRin and SNRout are SNR at input and output of an amplifier respectively.

The SNRs are referred to the output of an ideal photo detector, which is capable of

converting each photon of incident light into electrical current, which means a 100%

quantum efficiency. When SNRin > SNRout, the amplifier optical noise figure is always

greater than unity due to the property that optical amplifier cannot improve the signal

SNR [48].

A commonly used definition of noise figure is the quantum-beat-noise-limited

noise figure, which excludes the output SNR due to spontaneous-spontaneous beat

noise, ASE shot noise. That is [48]

GvhvG

PNF

s

ase 1

(2.10)

where Pase ASE, h is Planck’s constant, v is signal frequency, ∆vs is optical frequency

band of photo detector system and G is signal gain. The first term in the right hand side

of the above equation is the signal-spontaneous beat noise and the second term is the

signal shot noise. From the equation, the degrading of noise figure is mainly caused by

the Pase. The Pase is summed over all the spatial modes that the fiber supports in an

optical bandwidth, B0 and have two propagating modes of polarization with the Pase and

given by [53]:

0]1)([2 BzGhvnP spase (2.11)

where nsp is spontaneous emission factor and defined as

12

2

NN

Nn

ae

esp

(2.12)

where σe is stimulated emission cross section, σa is absorption cross section, N1 and N2

are ground state population density and metastable state population density

37

respectively, nsp measures the quality of inversion of the EDFA and become unity

(minimum) when complete inversion (N1~0) occurs. The noise figure then can express

in relationship with the spontaneous emission factor is given by [44]

)(

1]1)([2)(

zGG

zGnzNF sp

(2.13)

In the high gain limit, G >> 1, the signal noise can be ignores. Therefore the noise

figure reduces to

spnNF 2 (2.14)

The noise figure of the high gain amplifier is always greater than 2 dB, or 3 dB.

The 3 dB quantum limit noise figure only can achieve with a high gain fully inverted

amplifier. Figure 2.8 shows the spectral information required for determination of

EDFA noise. From the figure, the noise figure is the difference between ideal output

and actual output.

Figure 2.8 Spectral information required for the determination of noise figure.

Input

Actual Output

Power

Wavelength

Input signal x Gain

Ideal Output

Gain

Output Signal to Noise Ratio

EDFAnoise

Source Spontaneous Emission (Nin)

Nin x Gain + EDFAnoise

Nin x Gain

38

2.4 Non-linear Effect in Single-Mode Fiber

The propagation of light, like other electromagnetic wave is related to electricity

and magnetism governed by Maxwell’s equations [55]:

0

E

t

B

E -x (2.15)

0 B

εμx 00 JEB

t

where E, B, and J are electric field, magnetic field and total current density,

respectively, while 0 is 0 are vacuum permeability and vacuum permittivity,

respectively. The propagation of light waves is described by the wave equation, derived

from the Maxwell equations with the absence of charges and currents,

0tc

12

2

20

2

EE (2.16)

where c0 is the velocity of light in vacuum.

The presence of matter alters the popagation. A polarization field P appears, describing

the reaction of the material to the wave.

2

2

02

2

22

tμ

tc

1

PEE (2.17)

where c is the velocity of light in medium.

The induced polarization contains linear-optical effects (the absorption

coefficient and refractive index) and also nonlinear-optical effects. At low intensity (or

low field strength), the induced polarization is proportional to the electric field.

39

EP (1)χε0 (2.18)

The linear susceptibility, χ (1), describes the linear-optical effects.

However as the intensity of the applied field increases the dipole response

becomes non-linear. The optical non-linear effects occur in the situation where the

optical intensity within a dielectric becomes sufficiently high such that the motion of

bound electrons becomes anharmonic in response to the applied electromagnetic field.

This causes changes to the fundamental optical properties of the dielectric, and hence

alters the way in which light propagates through the material. The way in which the

electromagnetic field propagates through a dielectric is governed by the polarization

field P, which can be expressed as

P=εo[ (1).E + (2):EE + (3):EEE+...] (2.19)

where εo is the vacuum permittivity and (1) (j+1,2,3,…) is the jth order susceptibility.

The linear susceptibility of (1) is the dominant contribution to P, while the 2nd order

susceptibility (2) is responsible for certain nonlinear effects such as second harmonic

and sum-frequency generation. However (2) is diminishes in silica glasses because

SiO2 is a symmetrical molecule. The final nonlinear order in the equation, (3), is

responsible for third harmonic generation, four-wave mixing and nonlinear refractions.

The nonlinear effects in optical fiber occur either due to intensity dependence of

refractive index of the medium or due to inelastic-scattering phenomenon are known as

non-linear refraction [56] and Stimulated Scattering [57], respectively. Elastic here

signifies that no energy is exchanged between the electromagnetic field and the

dielectric medium, which means the photons maintain their energy and they have the

same frequency as the incident light. The non-linear refraction in which the intensity is

dependent to the refractive index is responsible for the Kerr-effect. The Kerr-

nonlinearity manifests itself in three non-linear effects; four-wave mixing, self-phase

40

modulation (SPM) and cross-phase modulation (XPM). SPM causes a change in

refractive index due to high intensity by an optical pulse. The most prominent effect of

SPM on a single pulse is the phenomena of spectral broadening which refers to the way

in which different intensities of a monochromatic pulse travel at different speeds due to

the intensity dependence of the refractive index. This causes the leading edge of the

pulse to have a longer wavelength, whilst the trailing edge bunches up to have a shorter

wavelength. The net effect is a pulse made up of a range of frequencies beating

together. Such broadening distorts the optical pulse, and is referred to as SPM induced

chirp. The non-linear phase shift encountered can also lead to wave mixing through the

satisfaction of what is known as the phase matching condition. SPM can also occur

when two co-polarized spectrally overlapping beams beat together along an optical

fiber. The envelope function looks like a train of pulses that can experience spectral

broadening. Cross-phase modulation (XPM) is the result of the same physical

principles as SPM. It is the effect of one pulse causing phase modulation in other

overlapping pulses propagating simultaneously within the fiber. Each pulse is modified

according to the aggregate intensity. Spectral broadening induced by XPM can be

important for spectrally close signals where it can lead to spectral overlap between the

channels. This effect could be severe in multi-channel systems. Four-wave mixing

(FWM) occurs when two or more frequencies (or, equivalently, wavelengths) of light

propagate through an optical fiber together and generates light at a new frequency. The

generated light utilizes optical power from the original frequencies provided that phase

matching is satisfied.

FWM is the process whereby signal waves interact to produce new waves at

shifted frequencies. Quantum mechanically, it can be described as the effect where two

photons annihilating one another creating two new photons at shifted frequencies that

satisfy the conservation of net energy and momentum. For the effect to occur efficiently

41

phase matching must be achieved. This condition refers to the requirement that the sum

of the initial frequencies must be equal to the sum of the mixing products, and that the

overall momentum must be conserved. If three optical fields with carrier frequencies

ω1, ω2 and ω3, copropagate inside the fiber simultaneously, (χ(3)) generates a fourth

field with frequency ω4, which is related to other frequencies by a relation,

ω4 = ω1 ω2 ω3 (2.20)

Figure 2.9 illustrates the FWM process which generates of a number of extra

frequencies from the interaction between light at two or three incident frequencies. The

relation that gives the frequency of the generated wave, ijk = ωi + ωj – ωk [58].

(a) (b) Figure 2.9 Additional frequencies generated through FWM in the partially degenerate (a) and non-degenerate case (b) [58].

Figure 2.9(a) shows a simple example of mixing of two waves at frequency ω1 and ω2.

When these waves mixed up, they generate sidebands at 112 or (2ω1−ω2) and 221 or

(2ω2−ω1). Similarly, three co-propagating waves will create nine new optical sideband

waves at frequencies given by ijk = ωi + ωj – ωk. These sidebands travel along with

original waves and will grow at the expense of signal-strength depletion. In general, for

N-wavelengths launched into fiber, the number of generated mixed products M is [59],

M = N2/2 • (N − 1) (2.21)

42

The efficiency of the FWM depends on fiber dispersion and the channel spacing. Since

the dispersion varies with wavelength, the signal waves and the generated waves have

different group velocities. This destroys the phase matching of interacting waves and

lowers the efficiency of power transfer to newly generated frequencies. The higher the

group velocity mismatch and wider the channel spacing, the lower the four wave

mixing effect [60]. FWM responsibles for the anti-Stokes generation in

multiwavelength fiber lasers [61].

The inelastic nonlinear effect comprises of two inelastic scattering effects

namely Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS).

Raman and Brillouin scattering are inelastic processes in which part of the power is lost

from an optical wave and absorbed by the transmission medium while the remaining

energy is re-emitted as a wave of lower frequency. Raman scattering arises from the

interaction of light with the vibrational excitation modes of silica molecules in the

scattering medium; equivalently this can be considered as the scattering of light from

optical phonons which then generate high energy optical phonons in SRS. On the other

hand, Brillouin scattering arises from the interaction of light with propagating density

waves or acoustic phonons [62, 63] and generate lower energy acoustical phonons. The

conversion of an incident photon into a lower energy scattered photon plus a phonon of

vibrational energy obeys the conservation of energy and momentum. The energy and

momentum before and after scattering must be equal, in which the incident photon

energy is shared between the phonon and the scattered photon. Since the frequency of

an optical wave is proportional to its energy, the photon produced by the scattering

event has a lower frequency than the incident photon. This frequency downshifted

wave is called as the Stokes wave. Typical values of the pump-Stokes frequency

difference are 10 GHz (~0.1-nm at 1550-nm) for SBS and 13 THz (~110-nm at 1550-

nm) for SRS [62]. Another key distinction between the two effects is that the scattered

43

wave due to SBS travels predominantly backwards. The SBS Stokes wave emerges

from the input end of the fiber whereas the Stokes wave due to SRS travels forwards

with the pump wave.

Spontaneous Raman and Brillouin scattering have been observed in bulk

material such as quartz and silica [64, 65]. The intensity of the scattered wave is

dependent on the angle of scattering and the optical power density in the material. The

growth of the Stokes wave is proportional to the product of the scattering gain

coefficient, the intensity of the pump wave and the intensity of any Stokes wave

present. In bulk media the Stokes wave quickly disperses as it propagates away from

the point of generation. However, single mode optical fiber will support low-loss

propagation for waves travelling almost parallel to the fiber axis. Consequently,

scattered radiation in either the forward or backward directions relative to the incident

wave will be guided within the fiber and will co-propagate with the pump wave over

long distances. Under these circumstances, it is possible for the Stokes wave to

continue to interact efficiently with the pump wave and exponential growth in the

downshifted optical power occurs. By gradually increasing the pump power launched

into one end of the fiber, there will be a gradual increase in Stokes power through

spontaneous scattering. Exponential growth in the Stoke power may occur if the pump

power is then increased further. The input pump power at which the Stokes wave

increases rapidly as a function of pump power is termed the stimulated scattering

threshold. Both SBS and SRS have so-called threshold pump powers above which

power transfer to the Stokes wave increases rapidly. In SBS this means that the amount

of optical power leaving the far end of the fiber no longer increases linearly with the

input power. The maximum launch power is clamped and excess power is simply

reflected back out of the fiber.

44

In this thesis, the SBS becomes a focus of interest as it has been found to be a

valuable tool in applications such as Brillouin laser, amplifier and Brillouin/Erbium

fiber laser (BEFL). The principle of SBS is discussed in the following section.

2.4.1 Principles of Stimulated Brillouin Scattering

With a sufficient input pump power, SBS converts the pumped light in the fiber

to a scattered, Stokes-shifted (downshifted) reflection. When the narrow linewidth and

high powered signal propagates through the optical fiber, it will begin to generate

acoustic waves that travel in the same direction as the pump wave as shown in Figure

2.10. The acoustic wave has a wavelength approximately half of the optical wavelength

and travels at the speed of sound in the fiber. This phenomenon arises from the

interaction between the optical field and acoustic phonons in the fiber, driven through

an electrostrictive process where the medium becomes denser in regions of high optical

density. As shown in Figure 2.11 an incident optical field of sufficient intensity

interferes with ubiquitously scattered optical fields, which give rise to density and

pressure variations (electrostriction) [63]. The incident optical field then scatters off the

refractive index perturbations as a result of the aforementioned density variations. The

scattered light is Stokes shifted and will add constructively with the Stokes radiation

which produced the acoustic disturbance. The incident light can add energy to the

acoustic waves as it interferes with the scattered Stokes light and thus significantly

increase the probability of scattering more of the incident light through Bragg

diffraction. Since energy and momentum are conserved during these scattering events,

the frequency, ΩB and wave vector of the pump (incident), scattered and acoustic fields,

q are given by

45

ΩB = ωp – ωS (2.22)

q = kp - kS (2.23)

where ωp,S and kp,S are the optical frequencies and wave vectors of the pump and

Stokes shifted fields, respectively. The Brillouin frequency, ΩB, and the wave vector of

the acoustic field, q, are related by the phonon dispersion relation

ΩB = | q | υA ≈ 2 υA | kp| sin (θ/2) (2.24)

where υA is the speed of sound in the medium and θ is the angle between the pump and

Stokes fields. However, in a SMF the relevant θ values are 0 and π, hence the Brillouin

frequency shift is given by

νB = 2 n υA / λp (2.25)

where n is the refractive index of the medium, υA is the acoustic wave velocity and λp is

the pump wavelength. Using c = vλ, the Brillouin frequency shift can be given as a

Brillouin wavelength shift by

c

2p

B

)( νB (2.26)

where c is the speed of light in a vacuum. In a silica-based fiber, the main component is

SiO2, and therefore υA = 5.96 km/s, n=1.45, and λp=1550 nm. From equation 2.25, the

Brillouin frequency shift, νB = 11 GHz. Substituting νB into equation 2.26, gives out

Brillouin wavelength shift λB = 0.09 nm. Typical values of the Brillouin frequency shift

in SMF at 1550 nm wavelength region is approximately between 9 to 12 GHz,

depending on the fiber materials and structure [66].

46

Figure 2.10 Generation of SBS due to the interaction of the injected light with the traveling acoustic wave that acts as a traveling Bragg grating.

Figure 2.11 Schematic diagram of the SBS process in an optical fiber [63].

The Brillouin shift is determined by the velocity of the acoustic grating along

the fiber and is therefore dependent on the mechanical properties of the fiber such as

the elasto-optic coefficient, applied strain and ambient temperature [67, 68]. The

frequency shift has also been demonstrated to be dependent on the dopant

concentrations in the core and cladding of the fiber [69, 70]. SBS has certain interesting

traits, such as low threshold and large Brillouin gain. The threshold power for SBS to

occur can be as low as 1 mW (depending on the fiber length, operating wavelength and

Velocity = 5900ms-1

Brillouin Pump, V0

Cladding

Cladding

Backscattered

light, ν0- νb Core

Acoustic wave, νB = 11 GHz

47

linewidth of the pump source) [71]. Thus the low Brillouin threshold makes SBS a

dominant nonlinear process in the optical fiber.

Brillouin Threshold

The nonlinear effects depend on transmission length. The longer the fiber link

length, the more the light interaction and greater the nonlinear effect. As the optical

beam propagates along the link length, its power decreases because of fiber attenuation.

The effective length (Leff ) is the length, up to which power is assumed to be constant is

defined as

))αLexp(1(

effL

(2.27)

where L is the fiber length and α is the attenuation of the fiber in neper/km. The

attenuation coefficient is assumed to be identical for the pump and Stokes waves since

they are so closely spaced in frequency.

The peak value of the Brillouin gain coefficient, gSBS , is dependent on the

material properties of fiber the spectral width of the pump and any modulation scheme

applied. For a pump of spectral width p (FWHM), the peak Brillouin gain coefficient

is given by [72]

PB

B

BB

0

3p

c

212

p8n4π

SBSg (2.28)

where n is the refractive index of the medium, p12 is the dimensionless longitudinal

elasto-optic coefficient, c is the speed of light in a vacuum (m/s), 0 is the material

density (kg/m3) and p is the peak frequency of the pump wave (Hz). The

symbol represents the convolution of the pump and Brillouin linewidths. For

Gaussian profiles, the convolution equates to

48

1/2)2PΔν2

BΔν(P

ΔνB

ν (2.29)

whereas for the more common assumption of Lorentzian profiles,

)B

ΔνP

Δν(P

ΔνB

ν (2.30)

The gain coefficient of the backscattered wave, gB(), is commonly approximated by a

Lorentzian function of the pump-Stokes frequency separation centred on B and given

by:

/2

BΔν /

Bνν1

21

Bg

SBSg (2.31)

For the purpose of estimating the Brillouin threshold, pump depletion (due to SBS) is

neglected. The intensity of the Stokes wave backscattered by a fiber of length L can be

written as [72]

αL)effAeffL0P

B(L)exp(gsI(0)sI (2.32)

where P0 = Ip(0)Aeff is the input pump power and Aeff is the effective core area.

A number of definitions of the threshold pump power for SBS in optical fibers

have been defined [62, 73]. The SBS threshold has been variously defined based on the

change in slope of the output power due to pump depletion, or the maximum change in

slope (second derivative) of the reflected power curve (in linear dimensions). However,

the SBS power threshold can also be defined to be the input power where the reflected

power is equal to some fraction, η, of the pump [63],

Preflect = η x Pin ( 2.33)

Here, the Brillouin threshold is considered as the critical pump power for which

the Stokes backscattered power becomes equal to the pump power, which can be

expressed using the effective length, Leff. The effect of nonlinearity grows with

intensity in fiber and the intensity is inversely proportional to area of the core. Since the

49

power is not uniformly distributed within the cross-section of the fiber, it is reasonable

to use effective cross-sectional area (Aeff). The Aeff is related to the actual area, A and

the cross-sectional distribution of intensity. In conventional step-index fibers, the mode

field is well approximated by a Gaussian function of radius w at the 1 /e amplitude

points. In this case, the effective area can be shown simply to be

)(πwA 2eff (2.34)

where 2w(l) is the mode field diameter (MFD) of the fiber at wavelength . Mode field

diameter is a well-established parameter with recognised measurement procedures.

However, for fibers that do not have simple step-index geometry such as dispersion-

shifted and dispersion-flattened fibers, the mode field cannot be approximated by a

Gaussian function.

eff0

eff

eff

thth Lg

19A

A

IP (2.35)

where the g0 is the Brillouin gain coefficient. Typically, for gSBS 5x10-11 m/W, Leff

25 km and Aeff 50m2, Pth 1mW.

2.5 Fiber Laser

Generally, fiber laser consists of a doped fiber as gain medium within the fiber

resonator. Fiber laser can be realized in two configurations; ring cavity and linear

cavity. The standing wave linear cavity or Fabry-Perot cavity is realized by placing the

gain medium between two high reflecting mirrors. Various types of mirrors are used at

two ends of the linear fiber laser resonators; dielectric mirror, loop mirror, fiber Bragg

grating, dielectric coating or WDM coupler. Linear cavity design has lower pump

threshold due to low cavity losses. For a standard two mirror standing wave laser

50

resonator, the round trip gain G = exp(g) at the signal wavelength s can be written as

[74, 75]

g = 2SL [x0e(s) - (1-x0)a (s)] + ln(R1) + ln(R2) + ln(1-) (2.36)

where S denotes the signal modal confinement factor, L is the fiber length, is the

erbium ion concentration, a and e are the absorption and emission cross section

respectively, is the round trip fractional lumped component loss, and x0 is the mean

fraction of excited erbium ions. R1 and R2 are the effective mirror fractional power

reflectivities. The power of laser radiation Ps, can be determined with obeying the

conservation of the photon number [74].

Ps = [Pp(1-A) - Pth ](hs/hp) (2.37)

where Pp is the pump power, Pth is pump power loss by spontaneous emission, A is the

single pass-loss, hp is the pump-photon energy and hs is the energy of a photon at the

laser frequency.

In contrast to a standing-wave laser resonator, the travelling ring resonator in a

form of a ring allows for two different propagation directions of the intracavity light.

Usually, unidirectional operation (where light propagates only in one of the two

possible directions) is enforced by introducing an element into the resonator which

leads to different losses for the propagation directions. Ring configuration allows

mirror free operation and total integration of component. In ring configuration, the laser

propagating obeys equation (2.36). Above threshold, the lasing condition where g=0,

will be satisfied over a range of wavelengths. However, the oscillating wavelength of

the laser depends on x0 because of its wavelength dependent absorption and emission

cross-section. Thus the lasing wavelength is determined by the choice of x0, the mean

fraction of excited erbium ions, which results in a maximum gain cross-section, g and

it is given by

g =[x0e(s) - (1-x0)a (s)] (2.38)

51

2.5.1 Brillouin/Erbium Fiber Laser

Since Cowle and Stepanov firstly reported the BEFL [76], different kinds of

BEFLs have been developed including ring, linear and figure-of-eight cavity

configurations [22-25, 77]. Instead of using only single gain medium such as the rare-

earth doped fiber, a BEFL employs two gain media, namely from the non-linear gain in

SMF and the linear gain from the EDF. A ring cavity of BEFL is shown in Figure 2.12.

When the SMF is pumped with a narrow-linewidth laser source which is also known as

a Brillouin pump (BP), a Stokes-shifted wave is generated in the reverse direction

which is then amplified by the EDF. The potential for this type of laser is that the

wavelength of the resulting laser can be determined very accurately due to the known

frequency shift from the pump signal. By pumping the EDF with a 1480 or 980 nm

laser diode (in most cases) gain can be produced to overcome the resonator loss. When

the BP wavelength is set close to the maximum gain produced by the EDF, lasing will

occur at the Stokes-shifted wavelength.

52

Figure 2.12 Schematics of the BEFL configuration.

Figure 2.13 depicts schematically the BEFL operation. The broad-band gain

with the maximum peak wavelength of x is generated by the EDF, while narrow-band

gain is generated from the SBS process in the SMS at a wavelength of y as shown in

Figure 2.13(a). If the total gain of wavelength y is greater than that of wavelength x and

is equal to the threshold gain of gth, then lasing actions will commence due to the

combination of the two gain media. However, if the peak not near the wavelength x, but

instead at a different wavelength z as shown in Figure 2.13(b), then the lasing will

occur at wavelength x nonetheless, but will only be generated by the gain of the EDF.

53

(a)

(b)

Figure 2.14 Schematics of two BEFL operations; (a) the Brillouin gain reaches the threshold gth and (b) the Brillouin gain below the threshold.

Since Brillouin gain is relatively small as compared to the EDF gain, the BEFL

must operate at a wavelength at which EDFL would operate without Brillouin gain.

BEFL operates in a manner different to that of EDFL and BFL in which it combines

both the two characteristics. The wavelength operation is determined from the Stokes

shift frequency, which is to BFL, and above threshold, the characteristics are similar to

54

those of EDFL in which most of the BEFL power in the output is extracted from LD

pump.

The conversion efficiency of the laser can be determined by the amount of

pump power that is converted to output. As the BEFL comprises of two gain media,

thus each gain media will contribute to the pump power and the overall conversion

efficiency can be written as [57]

BE

outBE II

I)I,η(I

(2.39)

where Iout, IE and IB are the laser output, EDF pump and BP intensities respectively.

The output intensity depends on the pumping level of the EDF and SMF and can be

written as follows:

)R1ln(IIIηIηIηI satBEEBBBEEout (2.40)

where Isat is the saturation intensity, R is the coupler ratio, ηE, ηB, ηEB are erbium,

Brillouin efficiency and cross efficiency of erbium/Brillouin respectively.

Various BEFL schemes will be proposed in the next chapters and will be

thoroughly studied.

55

References

[1] N. Sugimoto, "Novel short-length EDF for C+L band amplification," OAA

2000, Quebec, Canada, 9-1 2 July, 2000, paper PD-3.

[2] M. Yamada and M. Shimizu, “Ultra-wideband Amplification Technologies for

Optical Fiber Amplifiers”, NTT Technical Review, vol. 1, no. 3, pp. 80-84,

2003.

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64

CHAPTER 3

BISMUTH-BASED ERBIUM-DOPED FIBER AMPLIFIER

3.1 Introduction

Long-wavelength band (L-band) region is receiving big interest to meet the

demand for the transmission system due to the exhausting limited bandwidth in

conventional-band (C-band). Many studies are done on Erbium-doped fiber amplifier

(EDFA) to achieve broader operation bandwidth. EDFA such as Tellurite-based

EDFAs were reported for broadband amplifications [1]. It reported flat and broadband

gain of 75 nm from 1535 to 1610 nm with average output power of 18.5 dBm for input

power of 0 dBm. However, the Tellurite-based EDF needs more complicated splicing

technique to splice with standard communication fiber (SiO2 fiber) such as asymmetric

splicing technique whereas Bismuth-based EDFA (Bi-EDFA) can be spliced using

standard fusion splicing technique to SiO2

fiber. Bi-EDFA is a promising candidate for

broadband signal amplification around 1550 nm [2-5]. CW signal amplification over

the range of 1520-1620 nm has been demonstrated in a short length of Bismuth-based

EDF (Bi-EDF) [2]. Recently, increased interest has been shown in the development of

compact fiber amplifiers with a short gain medium length. In compensation for the

shorter gain medium length, such amplifiers necessitate a higher erbium ion

concentration in the gain medium. However, in case of silica-based erbium-doped fiber

(Si-EDF), a high concentration of erbium ions may result in pair-induced quenching

(PIQ) effects [6], which potentially reduces the pump power conversion efficiency

(PCE) and increases the noise figure (NF) for an EDFA. This is because when the

distance between the Er3+ ions decreases (due to high concentration), any two adjacent

65

ions excited to the 4I13/2 lasing state of erbium will interact in a process known as

cooperative upconversion, whereby energy is transmitted from one ion (which makes a

transition to the 4I15/2 ground state) to the other ion (which is excited to the 4I9/2 state).

The ion excited to the 4I9/2 state decays through a process of nonradiative transition (or

multiphonon decay) to the 4I13/2 state and does not contribute to signal amplification. To

increase the limit of the erbium doping concentration that is imposed by concentration

quenching in the EDF, several techniques such as co-doping the EDF with Ytterbium

[7] and using a Bi-EDF [8] can be used. In the first technique, the energy transfer

between the excited states of ytterbium (Yb3+) and erbium (Er3+) is utilized to form a

population inversion between the lasing levels of Er3+ and subsequently, signal

amplification via stimulated emission. The second technique utilizes a Bi-EDF co-

doped with Lanthanum (La) to decrease the concentration quenching of the erbium ions

in the fiber. The Bi-EDFA has a broad and flat 4I13/2-4I15/2 emission of Er3+ ions around

1560-1610 nm. The wider amplification band compare to the Si-EDF is due to the

smaller vibration energy of the bismuth glass lattice, in turn, larger emission and lower

excited state absorption in the extended L-band region [9-11].

In this chapter, two designs of Bi-EDFA are demonstrated, namely the single-

pass EDFA and double-pass EDFA. The double-pass amplifier’s performance is

evaluated against the performance of the single pass amplifier for comparison purpose.

3.2 Bismuth-based Erbium-doped Fiber

The host glass of the Bi-EDF is Bismuth trioxide [12]. The high concentration

erbium ion can be doped in this glass without a significant concentration quenching

effect. The high refractive index of Bi2O3 broadens the emission spectrum of erbium

66

ions [13]. Therefore this fiber allows a great broadband transmission capability and

ideal for compact amplifier applications. The Bi-EDF used in this dissertation work is

manufactured by Asahi Glass Company (AGC), Japan by product code of T1L. The

specification of the Bi-EDF is shown in Table 3.1 [13]. The 2.15 m Bi-EDF has an

Erbium ion concentration of 3,250 wt-ppm, which is obtained from the weight ratio of

erbium oxide (Er2O3) to other starting materials (Bismuth oxide) during the fabrication

process, hence the concentration is mentioned as “weight parts per million” (wt. ppm).

The Bi-EDF has a cladding diameter of 125 µm and La ion concentration of 4.4 wt%.

The mode field diameter (MFD) of the fiber is measured to be 6.2 µm at 1550 nm. The

refractive index of the core and the numerical aperture (NA) of the Bi-EDF at 1550 nm

are 2.03 and 0.20, respectively.

The MFD of the spliced fiber must be matched to reduce the splicing loss [12].

Thus, the Bi-EDF is fusion-spliced to high NA fibers (Corning HI980). Angled-

cleaving is applied during splicing to suppress the reflection effect due to the large

refractive index difference between the Bi-EDF and the SiO2 based fiber [14].

67

Table 3.1 Specification of Bismuth-based EDF [13].

Parameters Bi-EDF

Erbium ions concentration [wt. ppm] 3250

Co-dopants La

Peak absorption around 980 nm [dB/m] 73

Peak absorption around 1480 nm [dB/m] 83

Peak absorption around 1530 nm [dB/m] 133

Maximum background loss at 1300 nm [dB/m] < 1

Return loss at splice point [dB] > 55

Maximum splicing loss per splice point [dB] < 1

Numerical aperture (NA) 0.20

Mode-field diameter at 1550 nm [μm] 6.2

Cutoff wavelength [nm] <1450

Cladding diameter [μm] 125

Coating diameter [μm] 250

Core/cladding concentricity [μm] < 1

Core/cladding refractive index at 1550 nm 2.03/2.02

68

The Bi-EDF is co-doped with lanthanum to suppress concentration quenching

of erbium ions by increasing the distance of Erbium ions [15]. The slightly large MFD

of 6.2 (0.2 µm larger than typical Si-EDF) and lower NA (0.02 lower than the Si-EDF)

leads to higher single-mode cut-off wavelength. Advantage of the larger MFD is its

corresponding larger mode area which increases energy storage per unit length of the

Bi-EDF as well as increasing nonlinear effect threshold. Bi-EDFA can be highly doped

with Erbium without suffering from the ion-quenching and clustering effects commonly

limiting the conventional Si-EDFA. Therefore, only a short length of Bi-EDF is

required to provide gain in extended L-band compared to that of the much longer

conventional Si-EDFA. The short length of fiber implies a short interaction length and

hence much lower accumulated dispersion and nonlinearity.

Figure 3.1 exhibits the comparison of fluorescence or amplified spontaneous

emission (ASE) spectra between the Bi-EDFA and Si-EDFA. The 50 m Si-EDF used in

the experiment has an Erbium ion concentration of 400 ppm, cut-off wavelength of 962

nm and NA of 0.24. The Bi-EDF length is fixed at 2.15 m, which is more than 20 times

shorter than that of the Si-EDF. The 1480 nm pump power is optimized so that both

EDFAs exhibit almost the same level of ASE spectrum as shown in the figure.

The ASE spectrum gives a good indication of EDFA’s gain profile. The erbium

ions are excited to higher energy level when they are pumped by 1480 nm laser diode

to create a population inversion. Higher ASE level indicates higher population

inversion and consequently, higher gain. Thus, Bi-EDFA is expected to give higher

gain in the extended L-band region compared to the Si-EDFA. Figure 3.1 shows that

Bi-EDFA has higher gain at longer wavelength and lower gain at shorter wavelength.

This is due to the energy transfer from shorter wavelength to a longer wavelength,

which occurs with a higher number of erbium ions.

69

As shown in Figure 3.1, the ASE level is so much higher in Bi-EDFA compared

to the Si-EDFA at wavelength region above 1600 nm. This is due to the excited state

absorption (ESA) effect which is lower in the Bi-EDFA.

-85

-70

-55

-40

-25

-10

1520 1540 1560 1580 1600 1620Wavelength (nm)

Po

wer

(dB

m)

Bi-EDF (2.15 m)

Si-EDF (50 m)

Figure 3.1 ASE spectrum comparison between Bi-EDFA and Si-EDFA.

3.3 Characterization of the Single-pass and Double-pass Bismuth-based EDFA

Double-pass technique is a method to enhance the amplifier gain. In double-

pass EDFA, the forward ASE and signal are retro-passed back into the EDF by an

optical circulator. The gain enhancement is attributed to the double-propagation of the

signal in the EDF that increases the effective EDF length. The gain of the single-pass

EDFA as function of signal wavelength, λ is governed approximately by [17]

G(λ) = exp {([α(λ) + g(λ)]ñ - [ α(λ)+ λ(λ)]L} (3.1)

70

where G(λ) is the single-pass gain, L is the EDF length, α, g, λ, are EDF absorption,

emission and intrinsic loss, respectively. Average population inversion, ñ over the

amplifier optical length, L is defined as

dzn(z)L

1n~

L

0 (3.2)

where n(z) is the fractional metastable population inversion.

In double-pass, the signal is amplified in the EDF section twice in different

directions and the gains in both directions are assumed to be the same due to identical ñ

and L [18], the total gain is

Gt (λ) = L’ (λ) G2 (λ) (3.3)

where L’(λ) is the total loss of the feedback loop. From Equation (3.1) and (3.3), the

effective EDF length of the double-pass is twice the physical length. If L’(λ) is small,

the gain enhancement must be obtained for the double-pass system, caused by the

increase of the effective EDF length. This is the result of the reuse of the backward

ASE facilitated by the feedback loop.

On the other hand, the double-pass system has a higher noise figure compared

to that of the single-pass system. The reason for this noise figure penalty can be

explained below. The double-pass amplifier can be viewed as a bi-directional amplifier,

formed by two related cascaded unidirectional amplifiers, with single direction

pumping. The total noise figure of the double-pass EDFA is given by [19]

)(L')G(

1-)(NF

)(

1-))(L'(1/ )(NFNF 2

1t

G (3.4)

where NF1 (λ) and NF2 (λ) are the NF of the first and the second stage of the double-

pass EDFA. They are determined by the pump conditions, the total input signal powers

and the relative power between forward and backward signals [17]. Equation (3.4)

71

shows that the total noise figure is made up of the noise at the first stage EDFA, the

insertion loss of the feedback loop (C2) and the accumulation of the cascaded

amplifiers.

In this section, the performance of the single-pass and double-pass of the Bi-

EDFA will be investigated. Figure 3.2 shows the single-pass Bi-EDFA setup. The Bi-

EDF is bi-directionally pumped by 1480 nm laser diode. Two wavelength division

multiplexers (WDM) are employed to combine the pump and the signal. The Bi-EDF

has a length of 215 cm with an erbium concentration of 3,250 ppm and a cut-off

wavelength of 1440 nm as well as pump absorption of 83 dB/m at 1480 nm. The Bi-

EDF is spliced to the single-mode fiber (SMF) utilizing angled cleaving to suppress the

reflection from the index difference between the Bi-EDF and silica fiber. In the

experiment, the powers of P1 and P2 are fixed at 120 and 80 mW respectively. Tunable

laser source (TLS) is used in conjunction with optical spectrum analyzer (OSA) to

characterize gain and noise figure.

Figure 3.3 shows the double-pass Bi-EDFA configuration. The double-pass Bi-

EDFA utilizes an optical circulator as a reflector and the other components are similar

with the single-pass one. The optical circulator OC2 is located at the output end of the

EDF whereby ports 3 and 1 are connected together. The powers of P1 and P2 are also

fixed at 120 and 80 mW respectively. The amplified signal is routed into an OSA via

Port 3 of optical circulator OC1. The double-pass amplifier configuration performance

is compared with the single-pass amplifier configuration.

72

Figure 3.2 Single-pass Bismuth-based EDFA.

Figure 3.3 Double-pass Bismuth-based EDFA.

Result and discussion

Figure 3.4 compares the ASE spectra for both the single-pass and double-pass

amplifiers. As illustrated in Figure 3.4, the double-pass amplifier exhibits a higher ASE

power as compared to the ASE power of the single-pass amplifier. The ASE power

improves by more than 8 dB at the wavelength region of 1530 to 1600 nm. The

maximum ASE power improvement of 13.6 dB is obtained at a wavelength of

approximately 1570 nm as shown in Figure 3.4. This improvement is attributed to the

double propagation of the spontaneously emitted light in the Bi-EDF, which increases

the population inversion and in turn increases the ASE power.

73

-55

-45

-35

-25

-15

1520 1540 1560 1580 1600 1620

Wavelength (nm)

Po

we

r (d

Bm

)

Double pass

Single pass

Figure 3.4 ASE spectrum for single-pass and double-pass Bismuth-based EDFA.

Figure 3.5 shows the gain spectra for the single-pass and double-pass

configurations at two different input signal powers, -30 and 0 dBm. As shown in Figure

3.5, the double-pass amplifier shows a higher small signal (-30 dBm) gain as compared

to the single-pass amplifier. The small signal gain increases by more than 9 dB at the

wavelength region from 1565 to 1600 nm due to the double propagation of the signal in

the EDF, which increases the population inversion. The maximum gain enhancement of

11 dB is obtained at a wavelength of 1570 nm, which also shows the highest ASE

enhancement as shown in Figure 3.5. However, at a high input signal (0 dBm), the gain

from both configurations remains unchanged except at wavelengths longer than 1605

nm, in which the single-pass configuration gain is observed to be higher than the gain

of the double-pass configuration. This is attributed to the excited state absorption

phenomenon, which is more pronounced in the double-pass configuration. In addition,

Figure 3.5 also shows an improvement in the gain flatness as the input signal power

74

increases. This is attributed to the Bi-EDF, which increasingly suppresses the gain at

the 1570 nm wavelength region at high input powers.

Figure 3.5 Comparison of gain between double-pass (DP) and single-pass (SP) amplifiers.

Figure 3.6 shows the noise figure (NF) spectra for both the single-pass and

double-pass amplifier configurations. As shown in Figure 3.6, the double-pass

amplifier exhibits lower small signal (-30 dBm) noise figure compared to the small

signal noise figure of the single-pass amplifier at wavelengths longer than 1575 nm.

However, a NF penalty is observed at a high input signal power (0 dBm). The NF

penalty varies from 3.4 to 5.7 dB within the wavelength region of 1565 to 1600 nm.

This is attributed to the reduction of the high input power gain which in turn affects the

NF as described in the standard NF equation as shown in Equation (3.5).

GhυB

P

G

1NF

w

ase (3.5)

75

The NF penalty is also attributed to the higher counter-propagating ASE at the

input part of the amplifier. This reduces the population inversion at the input part of the

amplifier and therefore increases the noise figure particularly at the high input signal

power. Therefore, the double-pass Bi-EDFA is not suitable for power amplifier

applications. These results show that the employment of the Bi-EDF in conjunction

with the double-pass configuration system will play an important role in the

development of a compact L-band EDFA for applications as an inline amplifier.

However, the single-pass configuration is preferred for fiber laser application, which

requires lower noise figure for better performance.

Figure 3.6 Comparison of noise figure between double-pass and single-pass amplifiers.

76

3.4 Summary

Bi-EDFA exhibits better performance for amplification in extended L-band

compared to Si-EDFA. An efficient L-band EDFA with high gain characteristics using

a Bi-EDF in the double-pass configuration has been demonstrated. The amplifier

utilizes the double-propagation of the signal provided using an optical circulator at the

output end of the Bi-EDF and has obtained improved gain characteristics as compared

to an amplifier of single-pass configuration. This amplifier provides a gain as high as

30 dB using a 215 cm Bi-EDF pumped by two 1480 nm pump signals totaling 200 mW

in power. In comparison to the single-pass configuration, this amplifier has a gain

enhancement of more than 9 dB from 1565 to 1600 nm wavelength for small signal

gain. This proposed amplifier will play an important role in development of a compact

EDFA that operates in the L-band region. However, the double pass amplifier suffers a

high noise figure penalty at high input signal powers. Thus, the single-pass amplifier is

preferred to be employed in multiwavelength BEFL system in the next chapter because

high NF degradation in double-pass amplifier leads to low SNR. Multiwavelength

signals will be buried in the noise or easily experience intersysmbol interference or

crosstalk.

77

References

[1] Yasutake Ohishi, Atsushi Mori, Makoto Yamada, Hirotaka Ono, Yoshiki

Nishida, and Kiyoshi Oikawa, "Gain characteristics of tellurite based erbium-

doped fiber amplifiers for 1.5 µm broadband amplification," Optics Letters, vol.

23, no. 4, pp. 274, 1998.

[2] N. Sugimoto, "Ultrafast Optical Switches and Wavelength Division

Multiplexing (WDM) Amplifiers Based on Bismuth Oxide Glasses," J. Am.

Ceram. Soc. vol. 85, pp. 1083-1088, 2002.

[3] Y. Kuroiwa, N. Sugimoto, K. Ochiai, S. Ohara, Y. Fukasawa, S. Ito, S. Tanabe,

and T. Hanada, "Fusion Spliceable and High Efficient Bi2O3-based EDF for

short length and broadband Amplification pumped at 1480 nm," in Proc. 26th

Optical Fiber Communication Conference (OFC 2001), TuI5, Anaheim, March

2001.

[4] K. Taira, K. Kikuchi, and N. Sugimoto, "Dispersion and pulse amplification

characteristics of Bismuth Oxidebased Erbium doped fiber amplifiers," in Proc.

Optical Amplifies and Applications Conference (OAA 2002), paper OTuC2,

Vancouver, July 2002.

[5] H. Sotobayashi, J.T. Gopinath, and E. P. Ippen, "23 cm long Bi2O3-based

EDFA for picosecond pulse amplification with 80 nm gain bandwidth," IEE

Electron. Lett., vol. 39, no. 19, pp. 1374-1375, 2003.

[6] E. Delevaque, T. Georges, M. Monerie, P. Lamouler and J. F. Bayon,

“Modeling of pair-induced quenching in erbium-doped silicate fibers”, IEEE

Photon. Technol. Lett., vol. 5, pp. 73-75, 1993.

[7] S. W. Harun, H. A. Abdul-Rashid, S. Z. Muhd-Yassin, M. K. Abd-Rahman, M.

R. Tamjis and H. Ahmad, “Dual-stage Er/Yb doped fiber amplifier for gain and

78

noise figure enhancements”, IEICE Electron. Express, vol. 3, no. 23, pp. 517-

521, 2006.

[8] B. O. Guan, H. Y. Tam, S. Y. Liu, P. K. A. Wai and N. Sugimoto, “Ultra-

wideband bismuth-based EDFA for DWDM systems”, Optoelectronics,

Proceedings of the Sixth Chinese Symposium, pp. 147 – 149, 2003.

[9] B. O. Guan, H. Y. Tam, S. Y. Liu, P. K. A. Wai and N. Sugimoto, “Ultrawide-

Band La-codoped Bi2O3-Based EDFA for L-Band DWDM Systems”, IEEE

Photonics Technology Letters, vol. 15, no. 11, pp. 1525-1527, 2003.

[10] B. Peng, X. M. Qiu, L. Jiang, Z. C. Fan and W. Huang, "High-quantum-

efficiency erbium-doped optical fiber and the effective deactivator”, Applied

Physics Letters, vol. 85, no. 11, pp. 1910-1912, 2004.

[11] S. Tanabe, N. Sugimoto, S. Ito and T. Hanada, “Broad-band 1.5 m emission of

Er3+ ions in bismuth-based oxide glasses for potential WDM amplifier”, Journal

of Luminescene, vol 87-89, pp. 670-672, 2000.

[12] S. Ohara, N. Sugimoto, K. Ochiai, H. Hayashi, Y, Fdasawa, T. Hirose and M.

Reyes, “Extra-Broadband and Highly Eficient Short length Bi2O3-based EDF”,

OFC 2003, paper FB8, vol. 2, pp. 635-637

[13] “Introduction of high performance EDF based on bismuth oxide host glass”,

Technical Bulletin Bi-EDF, Asahi Glass co. Ltd., Nov 2002.

[14] N. Sugitomo, “Recent Progress in Bi-EDF Technology”, Asahi Glass Co. Ltd.,

2005.

[15] A. Keiichi, T. Yoshio, S. Tsuneo and Y. Takeshi, “Erbium Lanthanum co-doped

fibers for L-band amplifier with high efficiency, low nonlinearity and low NF”,

Optical Fiber Communication Conference and Exhibition, vol. 2, pp. TuA6-1 –

TuA6-3, 2001.

79

[16] N. Sugimoto, "Ultrafast Optical Switches and Wavelength Division

Multiplexing (WDM) Amplifiers Based on Bismuth Oxide Glasses," J. Am.

Ceram. Soc. vol. 85, pp. 1083-1088, 2002.

[17] C. R. Giles and E. Desurvire, “Modelling erbium-doped fiber amplifier,” J.

Lightwave Technology, vol. 9, pp. 271-283, 1991.

[18] Q. Mao, J. Wang, X. Sun and M. Zhang, “A theoretical analysis of

amplification characteristics of bi-directional erbium-doped fiber amplifier,”

Opt. Commun., 159, pp. 149-157, 1999.

[19] E. Desurvire, “Erbium-doped fiber amplifier: Principles and Applications,”

John Wiley and Sons Inc., New York, pp. 626, 1994.

80

CHAPTER 4

BISMUTH-BASED BRILLOUIN ERBIUM FIBER RING LASER

4.1 Introduction

A multiwavelength generation was demonstrated using a Brillouin/Erbium fiber

laser (BEFL) with stimulated Brillouin scattering (SBS) [1-4]. In these works, an

optical fiber laser is used as a gain medium to generate a stable 11 GHz equally spaced

multiwavelength signal. Brillouin multiwavelength generation has two distinct

advantages over other multiwavelength method; a constant spacing and narrow

linewidth [4-6]. In this chapter, a ring cavity BEFL is presented. The BEFL utilizes

single-mode fiber (SMF) and Bismuth-based EDF (Bi-EDF) as nonlinear and linear

gain medium. The Bi-EDF provides a gain to amplifiy the Stokes generated by the

SBS. On the other hand, the nonlinear Brillouin gain determines the operating

wavelength of the laser. The SBS effect in SMF will be demonstrated in the next

section.

4.2 SBS Observation in Single-Mode Fiber

The experimental setup to demonstrate stimulated Brillouin scattering in a 25

km long SMF is shown in Figure 4.1. The SMF has loss characteristics of 0.196 dB/km

at 1550 nm. A tunable laser source (TLS) is used to act as Brillouin pump (BP). The

narrow linewidth BP enables a narrow linewidth gain to be generated at a frequency

shifted from BP frequency by the Stokes shift in the SMF. The spectrum of the

81

backreflected signal and the residual pump signal are measured at Out 2 and Out 1,

respectively. An optical circulator, in which port 3 connected to an optical spectrum

analyzer (OSA) is used to allow the backreflected signal from SMF to reach Out 2.

Figure 4.1 Experimental setup to observe SBS effect in SMF.

The BP signal at wavelength of 1570 nm is injected into the SMF via the optical

circulator. Figure 4.2 shows the residual BP spectrum measured at Out 1 at various BP

power. The power of the residual BP is obtained at about 0 dBm with the input BP

signal power of 6 dBm. The 6 dB loss is due to the optical circulator insertion, splices

and common losses.

Brillouin pump

Out 2

13

2Out 1

Circulator SMF 25km

82

Figure 4.2 The spectrum at Out 1 for multiple input power.

Backreflected Stokes is generated through the electrostriction process. The

reflected part of the BP signal and the generated Brillouin Stokes travel in opposite

direction of BP as shown in Figure 4.3. As shown in the figure, the spacing between the

BP and the Stokes is about 0.09 nm. The BP power is set at 4 dBm. The spacing is the

Brillouin shift which corresponds to the equation

|B| = (2ηVA/p) sin (θ/2) (4.1)

where B is the Brillouin shift, η is the refractive index of the core, VA is the acoustic

velocity in the fiber and p is the pump wavelength. Since scattering in backward

direction, the angle θ is π. Using VA = 5.96 km/s and η 1.45 as typical values for silica

fibers, the shift is found to be 11 GHz or 0.09 nm at 1570 nm region. An anti-Stokes is

also observed as shown in Figure 4.3, which is due to the four wave mixing effect

83

between the BP and the Stokes signals [6,7]. The SBS effect will be used in the

following section to generate BEFL.

Figure 4.3 Anti-Stokes, Brillouin pump and Stokes at Out 2 with BP wavelength of 1570 nm.

4.3 Single Frequency BEFL

The performance of a single frequency BEFL with ring cavity has been

investigated. In this section, a Bi-EDFA is used to amplify the Brillouin gain to

overcome the high loss in the cavity so that the generated Brillouin gain can initiate

laser action. The Bi-EDF provides gain at 1565-1600 nm wavelength region.

The BEFL system setup is illustrated in Figure 4.4. The system comprises of a

Bi-EDF approximately 215 cm long, a SMF approximately 25 km, two 1480 nm laser

diodes, two wavelength division multiplexers (WDM) and a 90/10 coupler. The Bi-

84

EDF has an erbium concentration of 3,250 ppm and a cut-off wavelength at 1440 nm,

with a pump absorption of 83 dB/m at 1480 nm. The Bi-EDF was spliced to the SMF

using angled cleaving to suppress the reflection from the index difference between the

bismuth fiber and silica fiber. The Bi-EDF was pumped bidirectionally using two 1480

nm lasers to serve as the linear gain medium. A narrow linewidth signal from a tunable

laser source serves as the BP is injected into the SMF through port 1 to port 2 of the

optical circulator in a clockwise direction. The BP can generate additional gain in the

SMF as long as the BP has a linewidth smaller than that of the Brillouin gain bandwidth

of 10 MHz typically [8,9]. The optical circulator is also used to force unidirectional

operation of the laser in the cavity. The laser light oscillates only in the counter-

clockwise direction. Oscillation in the clockwise direction is prohibited. The output

from 10 dB coupler is measured and characterized by an OSA with a resolution of

0.015 nm.

85

Figure 4.4 The experimental setup of single cavity BEFL producing single frequency output.

Figure 4.5 shows the spectrum of BEFL with different 1480 nm pump power

ranging from 40 mW to 140 mW each. The BP signal is injected at wavelength of 1573

nm which is coincided with the operating wavelength of the erbium-doped fiber laser

(without BP). The BEFL signal at a Stokes shifted wavelength has higher magnitude

than the magnitude of the BP signal. The shift is about 0.09 nm which corresponds to

11 GHz Stokes shift frequency. The presence of BP and anti-stokes signals are

indistinguished for 1480 nm pump power exceeding 60 mW each. The injected 1480

nm pump powers are converted to the BEFL signal. The bidirectional pumped Bi-EDF

generates a gain at L-band region ranging from 1560 nm to 1610 nm. The gain is used

to amplify the generated Stokes by SBS to overcome the cavity loss. The BEFL peak

increases as the 1480 nm pump power increases. At high pump power, a leakage of BP

is suppressed by the Stokes, thus only one line is observed as shown in Figure 4.5.

86

Figure 4.5 The output of single frequency BEFL with varied 1480 nm pump powers.

Figure 4.6 shows the output power level against the 1480 nm pump power. The

output power increased when the pump power increased until it reached pump

threshold of 60 mW. At pump power below threshold, the gain provided by Bi-EDF

amplifier is small and is insufficient to compensate for the loss in the ring cavity.

Therefore, no lasing is observed below the threshold. The peak power remains constant

although the pump power keeps increasing after the threshold. The maximum output

power is obtained at 4 dBm.

87

Figure 4.6 The output power with varied pump power.

4.4 Multiwavelength BEFL (MWBEFL)

Multiple wavelength generation using ring cavity configuration is demonstrated.

The ring cavity BEFL also uses the combination of linear gain from EDF and nonlinear

gain from SMF to generate Stokes. The MWBEFL employs the cascading technique to

generate multiple Stokes signal. This is done using two 3-dB couplers as the looping

arm to loop in the Stokes signal that was previously generated in the cavity to re-inject

the signal into the SMF.

The multiwavelength BEFL system setup is illustrated in Figure 4.7 which

comprises of similar component as the single wavelength configuration of Figure 4.4. A

looping arm is added in this set-up to enable a cascading process for multiwavelength

generation. In the experiment, the SMF length is varied from 25 to 75 km.

88

Figure 4.7 All fibered BEFL Experimental setup

The BP signal will generate additional gain in the counter-clockwise direction at

a frequency downshifted from the BP by the Stokes shift in the SMF. This Brillouin

gain is then routed into the bi-directionally pumped Bi-EDF through port 2 to port 3 of

the optical circulator to generate the first Stokes at a frequency of approximately 11

GHz downshifted from the BP. The signal is amplified as it propagates through the Bi-

EDF. Two 3-dB couplers are used to form a loop with the SMF for the successive

Stokes wave generation through SBS. The 3-dB portion of the BEFL signal traveling in

a counter-clockwise direction is reinjected at the other end of the SMF in a clockwise

direction to act as a BP and generate subsequent Stokes signal, each one being 11 GHz

downshifted from the other. The output power of the BEFL is tapped out from the 10%

output coupler. The output is measured and characterized by an OSA with a resolution

of 0.015 nm.

89

The operating wavelength region of the BEFL is determined by the EDF gain

which is used to amplify the Brillouin gain and compensate for the cavity loss. Figure

4.8 shows the EDF laser spectrum or a free-running (without BP) spectrum of the

BEFL at various SMF lengths. The SMF has loss characteristic of 0.196 dB/km at 1550

nm. Thus, the 75 km SMF exhibits the highest loss of 14.7 dB while the 25 km exhibits

loss of only 4.9 dB. Thus, the cavity with 75 km SMF has highest loss. The free-

running lasers are obtained within the wavelength region between 1566 and 1570 nm

depending on the SMF length. The laser is generated at the shortest wavelength of 1566

nm with the SMF length of 75 km while the laser for 50 km SMF is located between

those of 75 km and 25 km SMFs. This is because cavity losses contribute to the

deviation of the lasing region. The lasing region is determined by the net gain in the

system, which is equivalent to the difference between the EDF gain and the cavity loss.

The lasing gain region of the free-running spectrum of the BEFL appears to be at

longer wavelength when the cavity loss is smaller [6]. In this case, the shortest SMF

length of 25 km exhibits the smallest cavity loss that in turn contributes to the longest

lasing wavelength of approximately 1570 nm. In addition, the 25 km SMF also exhibits

the highest lasing gain peak power of approximately -5.362 dBm.

90

Figure 4.8 Free running spectrum of the BEFL at various SMF lengths.

The BEFL operating wavelength must be close to the wavelength of the same

resonator operating as a free running BEFL without the BP. This region has the highest

net gain. In the experiment, the BP signal is launched into the SMF at a wavelength of

1573 nm, which is close to the free-running BEFL wavelength region. The Bi-EDFA

has a broad and flat 4I13/2-4I15/2 emission of Er3+ ions around 1560-1610 nm. Therefore

the BEFL operates at around 1573 nm, instead of around 1600 nm in the case of using a

Si-EDF [10].

Figure 4.9 shows the effect of the SMF length towards the generated Stokes

line. The BP is set at 1573 nm, which is optimized for the highest number of lines and

power, while the powers of both pumps are set at maximum of 140 mW. At the shortest

SMF length of 25 km, more than 16 lines of the BEFL are produced including the anti-

Stokes lines. However, the number of lines decreases as the SMF length increases. The

line spacing is approximately 0.09 nm corresponding to 11 GHz. The 3-dB bandwidth

91

of each line is about 0.02 nm, limited by the OSA resolution of 0.015 nm. The first line

(with the highest power) in the spectrum is the wavelength of BP, while subsequent

lines are the generated Brillouin Stokes wavelengths. Thus, the generation of a

multiwavelength fiber laser seems to be possible through the combination of traveling-

waves generated by Brillouin gain and Bi-EDF gain.

Figure 4.9 BEFL output spectra for different SMF lengths.

The length of the SMF contributes to the cavity loss whereby a 75 km SMF

exhibits a higher cavity loss compared to the 50 km and 25 km SMF lengths. The gain

of the laser system decreases as the cavity loss increases, in turn reducing the output

power and the number of lines generated. As such, the output power of the BEFL with

a 75 km long SMF declines abruptly after the 5th Stokes. In addition, the power of

subsequent Stokes lines is always lower than previous Stokes because each of the

subsequent Stokes is generated with energy from the previous Stokes.

92

The effect of the 1480 nm pump powers on the generation of a multi-

wavelength comb in the BEFL is also investigated. In the experiment, BP wavelength

and power are fixed at 1573 nm and 4.85 dBm, respectively. The length of SMF is set

at 25 km. Figure 4.10 demonstrates the BEFL output spectra at different 1480 nm pump

powers. It is obvious that both the number and power of the Brillouin Stokes increase

with the 1480 nm pump power. For six different pump powers, the number of generated

wavelengths increases from 1 to 10 lines and the powers of the corresponding

wavelengths increase with the increment of the pump powers. When the pump power is

greater than 80 mW, the preceding lower-order wavelengths become saturated and the

net increased power is transferred to the higher-order wavelengths. 16 Stokes lines are

generated at a pump power of 140 mW with ten lines exhibiting peak powers above -13

dBm. The highest peak power of -1.83 dBm was obtained at 1573.1 nm, which

corresponds to the wavelength of the BP. The 11th and subsequent lines have peak

powers below -40.5 dBm and this abrupt drop is caused by the laser system gain at

these wavelengths becoming approximately equal to the cavity loss. These results show

that the output of the BEFL depends on the pumping level of both the SMF and EDF

since the linear EDF gain contributes to most of the output power of the Brillouin

Stokes signal while the non-linear Brillouin gain contributes to the determination of the

operating wavelength. The total output power from the BEFL and hence the number of

simultaneous laser lines is shown to be limited by the available 1480 nm pump power.

The multi-line generation of BEFL also involves anti-Stokes generation as shown in

Figure 4.9 and Figure 4.10. The anti-Stokes signals arises due to the bidirectional

operation and four-wave mixing in the SMF. These anti-Stokes signals are more

obvious when the powers of 1480 nm and the BPs are increased. The generated comb

signal in the BEFL is stable in the room temperature. However, the large temperature

93

variations may affect the stability of the comb signal due to the length fluctuations of

the fibers in the same manner as the conventional silica-based EDF does.

-65

-55

-45

-35

-25

-15

-5

5

1572.3 1572.5 1572.8 1573 1573.3 1573.5 1573.8 1574 1574.3

Wavelength, nm

Pu

mp

po

we

r, d

Bm

40mW

60mW

80mW100mW

120mW

140mW

Figure 4.10 BEFL output spectra at different 1480 nm pump powers.

4.5 Summary

The generation of a multiwavelength comb is demonstrated between 1573 and

1574 nm using a 215 cm-long Bi-EDF and 25 km-long SMF. An optical comb was

produced by employing two 3-dB couplers joined in a reverse S-arrangement in the

resonator to capture a portion of the generated BEFL signal and re-inject it into SMF in

order to seed a cascaded line in the same direction as the first BEFL line. A laser comb

of more than 16 lines, including anti-Stokes was obtained using the BP and two 1480

nm pumps at powers of 4.85 dBm and 140 mW respectively. The anti-Stokes is visible

on the spectrum due to four wave mixing process among Stokes lines and also the BP.

10 lines of the Brillouin Stokes exhibited peak powers above -13 dBm and the BEFL

94

had a wavelength spacing of 0.09 nm. The power of 1480 nm pump and SMF length

exhibited a significant effect on the number of wavelengths and output power of the

generated wavelength comb.

95

References

[1] G. J. Cowle and D. Y. Stepanov, “Hybrid Brillouin/erbium fiber laser”, Opt.

Lett., vol. 21, pp. 1250-1252, 1996.

[2] G. J. Cowle, D. Y. Stepanov and Y. T. Chieng, “Brillouin/erbium fiber lasers”,

IEEE J. Lightwave Technology, vol. 15, pp. 1198-1204, 1997.

[3] S. Yamashita and G. J. Cowle, “Bidirectional IO-GHz optical comb generation

with an intracavity fiber DFB pumped Brillouin/erbium fiber laser”, Photonics

Technol. Letter, vol. 10, pp. 796-798, 1998.

[4] N. S. Shahabuddin, S. W. Harun, M. Z. Zulkifli, K. Thambiratnam and H.

Ahmad, "Bismuth-based Brillouin/erbium fiber laser", Journal of Modern

Optics, vol. 55, no. 8, pp. 1345-1351, 2008.

[5] G. J. Cowle and D. Y. Stepanov, “Multiple wavelength generation with

brillouin / erbium fiber lasers”, IEEE Photonics Technology Letters, vol. 8, pp.

1465–1467, 1996.

[6] X. S. Cheng, S. W. Harun and H. Ahmad, “Effects of output coupler reflectivity

on the performance of a linear cavity Brillouin/erbium fiber laser”, PRAMANA

Journal of Physics, vol. 68, no. 3, pp. 451–456, 2007.

[7] S. W. Harun, X. S. Cheng, P. Poopalan and H. Ahmad, “Self-excited brillouin–

erbium fiber laser for DWDM applications”, Optics & Laser Technology, vol.

39, pp. 94–97, 2007.

[8] A. Yeniay, J. Delavaux and J. Toulouse, “Spontaneous and Stimulated Brillouin

Scattering Gain Spectra in Optical Fibers”, Journal of Lightwave Technology,

vol. 20, no. 8, pp. 1425-1432, 2002.

96

[9] R. W. Tkach, A. R. Chraplyvy and R. M. Derosire, “Spontaneous Brillouin

Scattering For Single-Mode Optical-Fibre Characterisation”, Electronics

Letters, vol. 22, no. 19, pp. 1011-1013, 1986.

[10] S. Tanabe, N. Sugimoto, S. Ito and T. Hanada, “Broad-band 1.5 m emission of

Er3+ ions in bismuth-based oxide glasses for potential WDM amplifier,” Journal

of Luminescene, vol 87-89, pp 670-672, 2000.

97

CHAPTER 5

ENHANCED MULTIWAVELENGTH BISMUTH-BASED BRILLOUIN

ERBIUM FIBER LASER

5.1 Introduction

Various approaches such as ring cavities and seed Brillouin signal feedback

systems have been studied in order to generate multiwavelength BEFL [1-4]. In the

previous chapter, a multiwavelength hybrid Brillouin/Erbium-doped fiber laser (BEFL)

in ring cavity has been demonstrated using a Bismuth-based Erbium-doped fiber (Bi-

EDF) and single-mode fiber (SMF) [4]. In this chapter, three linear cavity designs of

multiwavelength BEFL are demonstrated using a Bi-EDF and SMF. Polarization

maintaining fiber (PMF) is also used instead of the SMF as the nonlinear gain medium

as it has higher nonlinear characteristics. The effect of polarization on the BEFL lines

generation is investigated. In linear cavity BEFL system, the laser propagates through

the EDF twice per oscillation trip. The double-pass through the EDF reduces the

effective cavity loss and enhances the laser performance. The linear cavity BEFL

system does not require an extra feedback loop to generate a multiple wavelength

Stokes as in ring cavity [4]. The performance of the ring cavity and linear cavity is

compared in this chapter.

5.2 Linear Cavity Bismuth-Based Brillouin/Erbium Fiber Laser

The linear cavity BEFL system was setup as illustrated in Figure 5.1. The

system comprises of a Bi-EDF approximately 215 cm in length, a SMF approximately

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50 km in length, two 1480 nm pump diodes, wavelength division multiplexer (WDM)

couplers, a 3-dB coupler and two optical circulators. The Bi-EDF has an Erbium

concentration of 3,250 ppm and a cut-off wavelength of 1440 nm, with a pump

absorption of 83 dB/m at 1480 nm. The Bi-EDF was spliced to the SMF utilizing

angled cleaving to suppress the reflection from the index difference between the

Bismuth fiber and Silica fiber. The Bi-EDF was pumped bi-directionally using two

1480 nm lasers. An optical circulator, in which ports 3 are connected to port 1, is

employed at both ends of system to act as a reflector.

The Brillouin gain medium and Brillouin pump (BP) was provided by the SMF

and an external cavity tunable-laser source (TLS), respectively. The BP was coupled

into the SMF using a 3-dB coupler as shown in Figure 5.1. The first generated Stokes

signal propagated in the opposite direction of the BP signal and is passed into the

bidirectionally pumped Bi-EDF for effective amplification. This signal traveled to OC2

and re-circulated back into the Bi-EDF. The amplified Stokes signal traveled back into

the SMF and reached OC1. The signal is reflected back into the SMF through the OC1

and reached the 3-dB coupler for a complete one-trip oscillation. This oscillation

continued and when the intensity of the first Brillouin Stokes was higher than the

threshold value for Brillouin gain, the second Brillouin Stokes is generated and

oscillated in the cavity. This process continued and the cascaded Brillouin Stokes can

be generated as long as the total gain of the Brillouin and Bi-EDF media was equal to

the cavity loss. The line spacing is obtained at approximately 11 GHz, which is

equivalent to the Stokes shift in the SMF. The output of the linear cavity BEFL was

tapped from the 3-dB coupler and characterized by an optical spectrum analyzer (OSA)

with a resolution of 0.015 nm.

99

Figure 5.1 The linear cavity BEFL design which output is coupled out between the SMF and 1480/1550 WDM coupler.

In the experiment, it is the Brillouin gain spectrum that determines the operating

wavelength region of the BEFL. The Brillouin gain spectrum must coincide with the

peak of Erbium net gain to obtain the highest Stokes power and to enable the cascading

process for multiwavelength operation. The free-running spectrum of the BEFL as

illustrated in Figure 5.2, shows that the peak generated in the SMF at wavelength

around 1595 nm. The lasing gain region is determined by the cavity loss and the gain

spectrum of the Bi-EDFA. The gain spectrum of the Bi-EDFA covers the L-band

region from 1560 to 1600 nm. The free-running spectrum of the BEFL peaks at 1595

nm region where the difference between gain for Bi-EDF and cavity loss is highest.

The free-running BEFL also exhibits the highest lasing gain peak power of

approximately -10 dBm. The BEFL operating wavelength must be close or coincided to

the wavelength of the same resonator operating as a free running BEFL (without BP).

Therefore, in the experiment, the BP signal is launched into the SMF at a wavelength of

1593.5 nm, which is coincided to the free-running BEFL wavelength region.

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Figure 5.2 Free running spectrum of BFL (without Brillouin pump).

The impact of the 1480 nm pump power on the number of Stokes generated by

the BEFL is illustrated in Figure 5.3. The BP wavelength was set closely to the lasing

gain of the free running BEFL at 1593.5 nm. The BP power is set at 4 dBm. Below this

BP power, the BEFL system operated with the presence of the free running Bi-EDF

laser cavity modes within the cascaded Stokes bandwidth. Thus the experiment was not

continued for BP power lower than 4 dBm since the BEFL system worked under an

instability domain. As shown in Figure 5.3, the number of generated Stokes increased

as the 1480 nm pump power for each laser diode increased. This is attributed to the

increment of the Erbium gain around this wavelength range. This situation led to a

sufficient signal power for higher order Stokes signal to pump the SMF and kept the

continuity process of the multiple Stokes generation.

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Figure 5.3 BEFL output spectra at different 1480 nm pump power.

In this experiment, more than 20 Stokes lines were obtained at the maximum

pump power of 100 mW. The higher number of Stokes is expected at higher pump

power. The line spacing is approximately 0.09 nm in the wavelength domain and 11

GHz in the frequency domain and the 3-dB bandwidth of each line is about 0.02 nm,

limited by the OSA resolution of 0.015 nm. The multi-line generation of BEFL also

involves anti-Stokes generation as shown in Figure 5.3. The anti-Stokes signals arises

due to the bidirectional operation and four-wave mixing in the SMF [5]. These anti-

Stokes signals are more obvious when the powers of 1480 nm pumps and the Brillouin

pumps are increased.

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5.2.1 Employing 25 km SMF as Nonlinear Gain Medium

Figure 5.4 shows the BEFL spectrum when a shorter length of SMF of 25 km is

employed in the BEFL configuration of Figure 5.1. This figure shows the spectrum for

three BP wavelengths of 1568, 1569 and 1570 nm. The multiwavelength generation for

BEFL with shorter SMF occurs at smaller wavelength compared to that of using longer

SMF. This is attributed to the cavity loss, which is smaller in the cavity. Therefore the

operating wavelength shifted to a shorter wavelength which has a higher Erbium gain.

As shown in Figure 5.4, 7 Stokes lines were obtained with peak power above -8

dBm at the BP wavelength of 1569 nm. The 1480 nm pump power is fixed at 100 mW

in this experiment. The number of lines is smaller than the previous configuration due

to the lower Brillouin gain in this set-up.

Figure 5.4 The BEFL spectrum employing 25 km SMF as the nonlinear gain

medium.

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5.2.2 PMF as the Nonlinear Gain Medium in the Linear Cavity BEFL

The performance of the linear BEFL configuration (of Figure 5.1) using PMF as

the nonlinear gain medium is shown in Figure 5.5. The length of the PMF is 400 m.

The pump powers of BP and each 1480 nm laser diodes are fixed at 4 dBm and 100

mW, respectively. The figure shows the spectrum for three BP wavelengths of 1574,

1575 and 1576 nm. Four equally spaced lines are achieved with the BP wavelength of

1575 nm. The spacing between the lines is 0.09 nm. The number of lines is smaller than

two previous set-up due to the length of PMF used which is very short. However, if the

same length of fiber is used in the previous set-up (with SMF), no lasing will be

observed. The PMF has a much higher nonlinearity coefficient compared with the SMF

but PMF is not significant for BEFL.

Figure 5.5 The output spectrum of the BEFL using PMF as the linear medium.

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5.2.3 Comparison between Ring Cavity and Linear Cavity BEFL

The performance of the BEFL is also compared with the ring configuration [1]

as shown in Figure 5.6 using 50 km SMF. The pump powers of BP and each 1480 nm

laser diodes are fixed at 4 dBm and 100 mW, respectively. In the linear cavity BEFL

system, the laser propagates through the Bi-EDF twice for one oscillation trip. The first

shifted Stokes signal propagates in the opposite direction to the BP and is amplified by

the Bi-EDF before it is reflected back by the optical circulator OC2. The reflected

signal will be amplified again by the Bi-EDF before it goes through the SMF and

reaches the optical circulator OC1 to complete one oscillation. The oscillation

continues till the first Brillouin Stoke reaches the threshold required to generate a

second Stokes which is shifted further with a frequency of 11 GHz relative to the first

Stokes signal. The double-pass through the Bi-EDF reduces the effective cavity loss

and enhances the laser performance. Therefore, the number of Stokes and anti-Stokes

are increased with the linear cavity as shown in Figure 5.6(b). The operating

wavelength of the BEFL shifts from 1571 nm (for the ring configuration) to 1594 nm

for the linear configuration. This is also attributed to the net gain, which is higher in the

linear cavity compared with the ring cavity. This is due to the total cavity loss, which is

lower in the linear cavity. Compared with the ring cavity [4], the linear cavity system

does not require an extra feedback loop to generate a multiple wavelength Stokes.

There is also no optical isolator in the cavity to prevent injection locking of the BP

wavelength. The BP wavelength will not eliminate in the system and it is shown in the

spectrum as a line with the highest peak power as shown in Figure 5.6.

These results show that the deployment of linear cavity configuration will play

an important role in designing an efficient BEFL. A further improvement on the linear

cavity configuration will be proposed and discussed in the next section.

105

Figure 5.6 Comparison of the output spectrum of BEFL with 50 m SMF length in (a) ring configuration (b) linear configuration.

106

5.3 Enhanced Linear Cavity Bismuth-based Brillouin/Erbium Fiber Laser

In previous works, both ring and linear configuration have been demonstrated

using a Bi-EDF and SMF in both ring and linear configuration. The previous linear

cavity BEFL is seen to exhibit a lower threshold power to achieve a larger number of

Stokes and anti-Stokes compared to the ring configuration. A second linear cavity

BEFL design is constructed to allow the generation of higher number of Stokes and

anti-Stokes.

In this second design of a linear cavity BEFL, a 2x2 coupler is used to both

inject the BP and also to tap the output of the multiwavelength laser at the end of the

cavity, as opposed to previous designs which incorporates the coupler at the middle of

the cavity. Figure 5.7 shows the configuration of the proposed linear cavity BEFL. The

2x2 95/5 coupler, which is used to inject the BP and tap the output, is incorporated

between ports 3 and 1 of OC2. This is opposed to the previous linear cavity

configuration which incorporates the coupler in between the SMF and WDM coupler.

The BP is injected into the linear cavity via the coupler and then is amplified by

the bi-directionally pumped Bi-EDF. The amplified BP is then coupled into the SMF to

generate the first generated Stokes signal propagating in the opposite direction of the

BP signal. The first Stokes is amplified by the bi-directionally pumped Bi-EDF before

traveling to OC1 and being re-circulated back into the Bi-EDF and subsequently the

SMF to reach OC2. The signal is reflected back along the same path and reaches OC1

and the coupler to complete a round-trip oscillation. This oscillation continues and

when the intensity of the first Brillouin Stokes is higher than the threshold value for

Brillouin gain, the second Brillouin Stokes is generated and oscillates in the cavity.

This process continues and thus cascaded Brillouin Stokes can be generated as long as

the total gain of the Brillouin and Bi-EDF medium is equal to the cavity loss. The line

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spacing is obtained at approximately 11 GHz, which is equivalent to the Stokes shift in

the SMF. The output of the linear cavity BEFL is tapped from the 95/5 coupler and

characterized by an OSA.

Figure 5.7 The linear cavity BEFL design with output tapped at the end of the cavity.

The operating wavelength of the BEFL is determined by the bi-directionally

pumped Bi-EDF gain spectrum which covers the L-band region from 1560 to 1600 nm

as well as the cavity loss. The free-running spectrum of the BEFL (without BP) with

reflectivity of 50%, 95% and 99% is shown Figure 5.8. The BEFL with 99%

reflectivity exhibits the highest lasing at wavelength around 1565 nm. The BEFL

spectrum with 50% reflectivity shows that the peak wavelength is generated at two

regions, which are around 1576 and 1596 nm. The BEFL spectrum for 95% reflectivity

has the lowest peak wavelength generated at approximately 1595 nm. The peak

wavelength is generated in the region where the difference between Bi-EDF’s gain and

cavity loss is largest. The free-running BEFL also exhibits a peak power of

approximately -30 dBm with bandwidth of approximately 3 nm. The coupler ratio

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affects the operating wavelength, power and number of lines of the output laser comb

[5].

Figure 5.8 Free running spectrum of the BEFL (without Brillouin Pump) with coupler ratios of 50/50, 5/95 and 1/99.

The chosen BEFL operating wavelength must be within or close to the

bandwidth of the free-running BEFL. Therefore, the BP is set within this region for

optimized operation. Figure 5.9 shows the multiwavelength generation with BP set

within the free-running bandwidth for different coupler ratios. As shown in Figure

5.9(c), when BP wavelength is set to be 1564 nm and 1/99 coupler is used, there are

only two Stokes and two anti-Stokes generated although it has the highest free-running

spectrum peak compared to that of BEFL with 50/50 and 5/95 coupler ratios. There are

21 Stokes lines generated by the BEFL using 50/50 coupler ratio at BP wavelength of

1572 nm as depicted in Figure 5.9(a). The BEFL with 5/95 coupler ratio generates 33

Stokes lines at BP wavelength of 1590 nm as shown in Figure 5.9(b). This BEFL

generates highest number of lines and has the best peak flatness. This is attributed to

the bandwidth of peak of the free-running spectrum which is broadest at this coupling

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ratio as shown in Figure 5.8. The broader bandwidth can accommodate more Stokes

and anti-Stokes lines.

-40

-35

-30

-25

-20

-15

-10

-5

1571 1572 1573 1574Wavelength (nm)

Po

we

r (d

Bm

)

BP : 1572 nm

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

1595 1596 1597 1598Wavelength (nm)

Po

wer

(d

Bm

)

BP : 1596 nm(a)

-55

-45

-35

-25

-15

-5

1589 1590 1591 1592 1593Wavelength (nm)

Po

wer

(d

Bm

)

BP : 1590 nm(b)

110

-45

-35

-25

-15

-5

5

15

1563.5 1564 1564.5 1565Wavelength (nm)

Po

wer

(d

Bm

)

BP : 1564 nm

(c)

Figure 5.9 BEFL spectrum for different coupler ratios ( (a) : 50/50, (b) : 5/95, (c) : 1/99 ).

The impact of the 1480 nm pump power on the number of Stokes generated by

the BEFL is depicted in Figure 5.10. The coupling ratio is fixed at 5/95 and therefore

the BP wavelength is set at 1590 nm which is close to the lasing bandwidth of the free

running BEFL. The BP power is fixed at 4 dBm. The BEFL system operates with the

presence of the free running Bi- EDF laser cavity modes within the cascaded Stokes

bandwidth at BP power less than 4 dBm. Therefore, the experiment is not continued for

BP powers below 4 dBm since the BEFL system is observed to be working under an

instable domain.

In the experiment, the pump power of each 1480 nm laser diode is varied from

40 to 100 mW. As shown in Figure 5.10, multiple Brillouin Stokes is obtained at pump

powers of 60 mW and above. Below pump powers of 60 mW, the erbium gain is very

low and cannot sufficiently compensate for the loss inside the laser cavity and thus no

Stokes are observed. The number of generated Stokes is observed to increase as the

pump power for each 1480 nm laser diode increases which is attributed to the

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increment of the erbium gain with pump power. This situation provides sufficient

signal power for higher order Stokes signal to pump the SMF and maintain the

cascading of the Stokes into multiple Stokes.

Figure 5.10 BEFL output spectra at different 1480 nm pump powers.

In this experiment, more than 30 Stokes lines are obtained at the maximum

pump power of 100 mW. However, a higher number of Stokes is expected at a higher

pump power. The line spacing is approximately 0.09 nm in the wavelength domain and

11 GHz in the frequency domain while the 3-dB bandwidth of each line is about 0.02

nm, limited by the OSA resolution of 0.015 nm. The BEFL also generates anti-Stokes

as shown in Figure 5.9, especially at the higher pump powers. The anti-Stokes signals

arise from the bidirectional operation and four-wave mixing in the SMF and they are

more obvious when the powers of the 1480 nm pumps and the BP are increased [5,6].

The number of lines obtained in this second BEFL design is higher as compared to the

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previous linear cavity BEFL configuration [7], which is only able to obtain

approximately 20 lines at the same 1480 nm pump and BP powers. In the previous

linear cavity BEFL configuration, the laser passes twice through the coupler located in

the middle of cavity for one oscillation trip. However, the proposed configuration

places the coupler between ports 3 and 1 of the optical circulator so as to allow only a

single pass of the laser through the coupler for one oscillation trip. The single pass of

the laser reduces the effective cavity loss and enhances the laser performance.

Therefore, the number of Stokes is increased to 33 lines in the proposed linear cavity

BEFL as shown in Figure 5.10 and Figure 5.11.

Figure 5.11 shows the output spectrum of the proposed BEFL configuration at

different BP wavelengths. As shown in the figure, the most number of Stokes is

obtained at a BP of 1590 nm. This region is the optimum as the difference between the

EDF gain and cavity loss is the largest, as compared to the other regions. As the BP

moves farther away from the lasing bandwidth of the free-running BEFL, less and less

Stokes lines are observed as the gain decreases and becomes less and less sufficient to

support the cascading process. If the BP is considerable far from the lasing bandwidth

of the free running BEFL, there is no gain to support the cascading process, and thus no

Stokes lines are observed. Additionally, the power of subsequent Stokes lines is

typically lower than that of the previous Stokes line as each subsequent Stokes is

generated with the energy of the previous Stokes, thus slightly reducing the Stokes

line’s power. The multiwavelength output of the BEFL is observed to be stable at room

temperature with only minor fluctuations observed coinciding with large temperature

variances.

113

Figure 5.11 BEFL output spectra at different BP pump wavelengths.

5.4 The Third Linear Cavity BEFL Design

A new configuration of the BEFL is proposed as shown in Figure 5.12, to obtain

a higher number of lines. In this configuration, the Bi-EDF is connected to the SMF

and is pumped bi-directionally using two 1480 nm lasers. A 25 km SMF instead of 50

km SMF is used as a non-linear gain medium and WDM coupler is used to combine the

pump and laser wavelengths. Two optical circulators, OC1 and OC2, that are used to

create a routing loop, are placed at both ends of the linear cavity to act as reflectors.

The coupler C1, which is located in OC1 loop is used to inject the signal BP from the

TLS. The coupler C2 is used to tap out the BEFL signal for the OSA.

The BP is injected into the linear cavity via C2 and is then amplified by the bi-

directionally pumped Bi-EDF. The amplified BP is then coupled into the SMF to

generate the first Stokes signal propagating in the opposite direction of the BP signal.

The Stokes signal is then amplified by the Bi-EDFA before being re-circulated by the

OC2 ring cavity back towards the SMF. The 1st Stokes then travels towards OC1 where

114

it is tapped by the coupler C1 for viewing at the OSA. As it travels, the 1st Stokes will

also generate the 2nd Stokes in the SMF, which will also travel towards OC1 and be re-

circulated into the system, much like the 1st Stokes. This generation process continues

as the incoming Stokes exceeds the threshold values for Brillouin gain, thereby

providing cascaded Brillouin Stokes. The number of Stokes generated depends on the

total gain of the Brillouin and the Bi-EDFA over the cavity loss. The output of the

linear cavity BEFL is tapped from the 5% port of C1 at OC1 and characterized by an

OSA with a resolution of 0.015 nm. The linewidth of the BP signal is 15 MHz, which is

measured using a heterodyne technique.

Figure 5.12 The linear cavity BEFL configuration.

The operating wavelength of the BEFL is determined by the free-running

spectrum of the BEFL and must be within the amplification band of the bi-directionally

pumped Bi-EDF, which covers the L-band region (1560 – 1600 nm). The free-running

spectrum of the BEFL, which is taken without BP at a pump power of 120 mW for the

two 1480 nm pumps is shown Figure 5.13.

115

Figure 5.13 Free-running spectrum of the BEFL (without BP). The power of 1480 nm pumps (P1 and P2) are fixed at 120 mW each.

In the experiment, C2 is a 99/1 coupler with the 99% output designated as Port

B. The peak wavelength is generated at the 1570 nm region, which is where the

difference between the Bi-EDF gain spectrum and cavity loss is the largest. The free-

running BEFL exhibits a peak power of approximately -30 dBm with bandwidth of

approximately 3 nm centered at 1570.5 nm. The chosen BEFL operating wavelength

must be within or as close as possible to the bandwidth of the free-running BEFL.

The effect of the coupling ratio of C2 on the number of Stokes and anti-Stokes

generated by the BEFL is depicted in Figure 5.14. The 1480 nm pump and BP powers

are fixed at 120 mW and 6 dBm, respectively. The BP wavelengths are optimized to

1570.7, 1570.3 and 1568.5 nm for the coupling ratios of 50/50, 80/20 and 99/1,

respectively. The coupling ratio of C2 controls the amount of BP power that is injected

into the cavity provides the reflectivity of the loop (OC2). A higher ratio at Port B

translates into a higher injected BP power and lower reflectivity of OC2. As shown in

Figure 5.14, an increment of the Port B ratio (50%, 80% and 99%) increases the

number of lines of the BEFL output, but reduces the peak power of these lines. The

116

reduction of the peak powers are due to the reflectivity of the OC2 loop, which

subsequently increases the cavity loss. This also has the effect of shifting the operating

wavelength of the BEFL travels to the shorter wavelength region as shown in Figure

5.14.

Figure 5.14 Output spectrum of BEFL at different C2 coupling ratios. The P1, P2 and BP powers are fixed at 120 mW, 120 mW and 6 dBm, respectively.

The impact of the 1480 nm pump power on the number of Stokes generated by

the BEFL is depicted in Figure 5.15. The BP is set at a wavelength of 1569.0 nm,

which is close to the lasing wavelength of the free-running BEFL and the BP power is

fixed at 6 dBm. Both the 1480 nm pump powers are varied from 60 and 120 mW.

Lower pump powers will not give any Stokes due to the low EDFA gain, and thus the

minimum pump power is at 60 mW. At a pump power combination of 60 and 100 mW,

the least number of lines are generated as shown in Figure 5.15. However, as the

combination pump power increases, the number of lines generated also increases. This

117

can be attributed to the increment of the erbium gain with increase in the pump power

as this situation provides sufficient signal power for higher order Stokes signal to pump

the SMF and maintain the cascading of the Stokes into multiple Stokes. As shown in

the figure, the highest number of lines is obtained at a pump power of 120 mW. The

number of lines is higher in Figure 5.15(d) as compared with that in Figure 5.15(c)

even though two pump power combinations are almost similar, due to gain

characteristics of the bi-directionally pumped Bi-EDF amplifier. A higher gain is

obtained if the signal is injected from the side with higher pump power.

Figure 5.15 BEFL spectrum at different 1480 nm pump power combinations. (a) P1 = P2 = 120 mW, (b) P1 = P2 =105 mW, (c) P1 = 60 mW, P2 = 100 mW and (d) P1 = 105 mW, P2 = 60 mW.

118

Figure 5.16 shows the number of multi-wavelength lines as a function of BP

wavelength at different BP power. In the experiment, the BP wavelength is varied from

1568 to 1570 nm, which is close to the lasing bandwidth of the free-running BEFL and

the BP power is varied from 3 to 8 dBm. Both of the pump powers are fixed at 120

mW. Below a BP power of 3 dBm, the BEFL system operates with the presence of the

free-running Bi-EDF laser cavity modes within the cascaded Stokes bandwidth.

Therefore, the experiment is not continued for BP powers below this power since the

BEFL system is observed to be working under an instable domain. The optimum BP

wavelength is moving towards longer wavelength as the BP power reduces. The

maximum multi-wavelength line of 50 is obtained at BP wavelength of 1568.2 nm and

BP power of 5 dBm. As the BP moves farther away from this wavelength (1568.2 nm),

less and less Stokes and anti-Stokes lines are observed as the gain decreases and

becomes less and less sufficient to support the cascading process. If the BP is

considerable far from the lasing bandwidth of the free-running BEFL, there is no gain

to support the cascading process, and thus no Stokes line is observed. The number of

lines increases as the BP power increases from 3 to 8 dBm as is expected when the BP

power is increased as now more Stokes can be generated before the cascading process

stops.

119

Figure 5.16 Number of BEFL lines against BP wavelength at different injected BP power. Both the P1 and P2 pump powers are fixed at 120 mW.

Figure 5.17 shows the output spectrum of the multi-wavelength BEFL at BP

wavelength of 1568.2 nm and a BP power of 5 dBm. In this experiment, 50 multi-

wavelength lines are obtained at the maximum pump power of 120 mW. However, a

higher number of Stokes is expected at a higher pump power. The line spacing is

approximately 0.09 nm in the wavelength domain and 11 GHz in the frequency domain

while the 3-dB bandwidth of each line is about 0.02 nm, limited by the OSA resolution

of 0.015 nm. The BEFL also generates anti-Stokes as shown in Figure 5.14. The anti-

Stokes signals arise from the bi-directional operation and four-wave mixing in the SMF

and they are more obvious when the powers of the 1480 nm pumps and the BP are

increased. Additionally, the power of each subsequent Stokes lines is typically lower

than that of the previous Stokes line, as each subsequent Stokes is generated with the

energy of the previous Stokes, slightly reducing the Stokes line’s power. However,

some of the lines have a peak power, which is higher than the previous lines as shown

in Figure 5.17. This is attributed to other phenomenon such as four-wave mixing, which

120

will transfer energy from neighboring lines to this line. The multiwavelength output of

the BEFL is observed to be stable at room temperature with only minor fluctuations

observed coinciding with large temperature variances.

Figure 5.17 BEFL output spectra at BP wavelength of 1568.2 nm and BP power of 5 dBm.

The number of lines obtained in the proposed BEFL is higher as compared with

the previous ring cavity BEFL configuration [4]. The linear cavity BEFL allows the

lasing wavelength to pass the Bi-EDF gain twice per oscillation and thus increases the

net gain per oscillation. This allows the linear cavity BEFL to exhibit lower threshold

power and achieves a larger number of Stokes and anti-Stokes compared with the ring

configuration. The proposed BEFL using a short-length Bi-EDF gain medium will

allow for the development of compact BEFL devices.

121

5.5 Summary

In this chapter, efficient multiwavelength generation in linear cavity BEFLs

have been demonstrated. The linear cavity configuration exhibits better performance

compared to the ring cavity in the previous chapter. The linear cavity configuration

uses a pair of optical circulators at the input and output ends of the cavity. The

employment of PMF indicates that polarization does not play significant role in

producing BEFL. The best linear cavity among the three is designed to have a lower

cavity loss. The Brillouin pump is injected into one end of the cavity and the output is

tapped from another end of the cavity. The BEFL comb with a 2.15 m of Bi-EDF has a

wavelength spacing of 0.09 nm and operates in long-wavelength (L-) band region. A

stable output laser comb of 50 lines is obtained at a BP of 1568.2 nm and 5 dBm and

two 1480 nm pumps at 120 mW. The injected BP wavelength and power as well as the

1480 nm pump powers have a great effect on the number of lines and output power of

the BEFL. The employment of the linear gain medium, the Bi-EDFA, enables more

Stokes lines to be generated compared to the BEFL which employs only SMF as the

nonlinear gain medium [8,9]. This configuration is compact due to the use of the

significantly shorter Bi-EDF as the linear gain medium, and can be made more compact

by replacing the single-mode fiber with highly non-linear fibers such as holey fibers

[10].

122

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Configuration”, Fiber and Intergrated Optics, vol. 27, no. 1, pp. 35-40, 2008.

[8] M. R. Shirazi, S.W.Harun, K.Thambiratnam, M. Biglary and H. Ahmad, “New

Brillouin Fiber Laser Configuration With High Output Power”,Microwave and

Optical Tech. Letters, vol. 49, no. 11, pp. 2656-2658, 2007.

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[9] S. W. Harun, M. R. Shirazi and H. Ahmad,“Multiple Wavelength Brillouin

Fiber Laser from Injection of Intense Signal Light”, Laser Physics Letters, vol.

4, no. 9, pp. 678-680, 2007.

[10] J. H. Lee, Z. Yusoff, W. Belardi, M. Ibsen, T. M. Monro and D. J. Richardson,

“Investigation of Brillouin effects in small-core holey optical fiber: lasing and

scattering”, Optics Letters, vol. 27, no. 11, pp. 927-929, 2002.

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CHAPTER 6

CONCLUSION AND FUTURE WORKS

6.1 Conclusion

In this dissertation, multiwavelength generation has been demonstrated utilizing

both linear and nonlinear gain medium. The stimulated Brillouin scattering effect plays

the main role in generating multiwavelength lines in the nonlinear gain medium, the

single mode fiber. The combination of nonlinear gain medium with the linear gain

medium, the Bismuth-based Erbium-doped fiber (Bi-EDF) allows more lines to be

generated.

The theoretical background behind the realization of Erbium-doped fiber

amplifier (EDFA) and Brillouin/Erbium-doped fiber laser (BEFL) has been presented

in chapter 2. The nonlinear effect which takes places within optical fiber is explained.

The stimulated Brillouin scattering (SBS) effect which occurs within the fiber

contributes to multiwavelength generation. Chapter 3 examines the characteristics and

performance of the single-pass and double-pass Bi-EDFA. The gain medium is the Bi-

EDF, a fiber with Bismuth trioxide as the host glass. The use of Bi2O3 allows high

erbium ions concentration to be doped without a significant concentration quenching

effect. The high refractive index of Bi2O3 broadens the emission spectrum of erbium

ions to achieve a broader gain profile compared to normal silica-based EDF. Bi-EDFA

exhibits better performance for amplification in extended L-band compared to silica-

based EDFA. Therefore it will play an important role in development of a compact

EDFA that operates in the L-band region.

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Efficient L-band EDFAs with high gain characteristics using a Bi-EDF in the

single-pass and double-pass configurations have been demonstrated. The double-pass

amplifier utilizes the double-propagation of the signal provided using an optical

circulator at the output end of the EDF and has obtained improved gain characteristics

as compared to an amplifier of single-pass configuration. This amplifier provides a gain

as high as 30 dB using a 215 cm Bi-EDF pumped by two 1480 nm pump signals

totaling 200 mW in power. In comparison to the single-pass configuration, this

amplifier has a gain enhancement of more than 9 dB from 1565 to 1600 nm wavelength

for small signal gain (-30dBm). For high input signal (0 dBm) within the wavelength of

1560 and 1605 nm, the gain is almost equal for both single-pass and double pass. The

single-pass shows higher gain at longer wavelength for high input signal because of

lower excited state absorption compared to that of double-pass. However, the double

pass amplifier suffers a high noise figure penalty at high input signal powers. Thus, the

single pass Bi-EDFA is preferred to be employed in multiwavelength BEFL system

because high noise figure degradation in double pass amplifier leads to low signal-to-

noise ratio (SNR). Multiwavelength signals will be buried in the noise or easily

experience intersysmbol interference or crosstalk because of low SNR.

Chapter 4 observes the SBS phenomenon in the SMF and investigates the

performance of a ring cavity BEFL. Backreflected Stokes and anti-Stokes with line

spacing of 0.09 nm are generated through the process called electrostriction. The ring

cavity BEFL which operates in the 1573 nm region employs both linear and nonlinear

gain from a Bi-EDF approximately 215 cm long and a SMF of various lengths to

generate an optical comb with a spacing of approximately 0.09 nm. Two 3-dB couplers

were used to form a looping arm in the system in order to produce cascaded Brillouin

Stokes waves as internal feedback for multiwavelength operation. A stable output laser

comb with 10 lines at more than -13 dBm was obtained with 4.85 dBm BP power and

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two 140 mW pumps at 1480 nm. 1480 nm pumps power and SMF length have a

significant effect on the number of wavelengths and on the output power of the

generated wavelength comb.

In chapter 5, linear cavity BEFLs are demonstrated to improve the performance

of BEFL. In this work, three designs have been proposed and demonstrated. In the first

linear cavity design, the generation of a multiwavelength comb at approximately 1594

nm using a Bi-EDF 215 cm in length and SMF 50 km in length is demonstrated. The

BP was coupled into the SMF using a 2x2 3-dB coupler located between the SMF and

the EDFA. The output of the linear cavity BEFL was tapped from the same 3-dB

coupler. An optical comb with a line spacing of 0.09 nm was produced by employing

two optical circulators to act as mirror at the output ends of the system. A laser comb of

more than 20 lines, including anti-Stokes was obtained using the BP and two 1480 nm

pumps at powers of 4 dBm and 100 mW respectively. Anti-Stokes is observed at

shorter wavelength because of four-wave mixing between the BP and the Stokes line.

Polarization maintaining fiber (PMF) is also employed in the linear cavity design as the

nonlinear gain medium instead of SMF because PMF has higher nonlinearity

coefficient. Less number of lines is achieved using 400 m PMF, because it is not

significant for a Brillouin gain medium as compared to the 50 km SMF as the nonlinear

gain medium. The number of lines is relatively higher for linear cavity configuration

compared with the ring configuration. The power of 1480 nm pump and effective

cavity loss in the cavity exhibited a great effect on the number of wavelengths and

output power of the generated wavelength comb. The linear cavity BEFL exhibits a

lower threshold power compared to the ring configuration.

In the second linear cavity BEFL design, the BEFL configuration employed a

2x2 coupler at the end of the linear cavity for increased Stokes line generation. This

linear cavity is able to generate up to 33 Stokes lines with a channel spacing of 0.09 nm

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at the 1590 nm region at a BP power of 4 dBm and 1480 nm pump power of 100 mW.

The number of lines obtained in this second BEFL design is higher as compared to the

previous linear cavity BEFL configuration, which is only able to obtain approximately

20 lines at the same 1480 nm pump and BP powers. In the first linear cavity BEFL

configuration, the laser passes twice through the coupler located in the middle of cavity

for one oscillation trip. This second linear cavity places the coupler between ports 3 and

1 of the optical circulator so as to allow only a single pass of the laser through the

coupler for one oscillation trip. The single pass of the laser reduces the effective cavity

loss and enhances the laser performance. Therefore, the number of Stokes is increased

to 33 lines. The number of Stokes lines obtained depends on the 1480 nm pump power

and operating wavelength region, which must be as close as possible to the lasing

bandwidth of the free-running BEFL. The Stokes could be obtained as long as the

pump power exceeds its threshold value, in this case, 60 mW. Below pump powers of

60 mW, the erbium gain is very low and cannot sufficiently compensate for the loss

inside the laser cavity.

The cavity loss has big contribution to the performance of the BEFL. Thus,

third linear cavity BEFL is designed in such to have a lower cavity loss compared with

the two previous linear cavity designs. The configuration uses a pair of optical

circulators at the input and output ends of the cavity to form a resonator for multi-

wavelength generation in conjunction with optical couplers to inject the BP and to tap

the output at the two ends. The BEFL has generated an optical comb with a

wavelength spacing of 0.09 nm and operates in long-wavelength (L-) band region. A

stable output laser comb of 50 lines is obtained at a BP of 1568.2 nm and 5 dBm and

two 1480 nm pumps at 120 mW. The injected BP wavelength and power as well as the

1480 nm pump powers have a great effect on the number of lines and output power of

the BEFL.

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6.2 Future Works

The BEFL performed is suitable for a compact multiwavelength BEFL due to

the use of the significantly shorter Bi-EDF as the linear gain medium. Furthermore,

further reductions in size can be obtained if the SMF is replaced with highly non-linear

fibers such as photonic crystal fiber (PCF) and Chalcogenide fiber. A PCF with a core

diameter of 1.6 mm would require less than 100 m to obtain the SBS effect desired.

Highly nonlinear Bismuth-oxide fiber as well as Bismuth-based PCF are

promising candidates for multiwavelength generation in BEFL. Besides Bismuth glass,

Chalcogenide glass is also attractive for nonlinear all-optical signal processing because

of its large Kerr nonlinearity.

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LIST OF PUBLICATIONS

Journals

[1] N. S. Shahabuddin, S. W. Harun, M. Z. Zulkifli, K. Thambiratnam and H.

Ahmad, “Bismuth-based Brillouin/Erbium Fiber Laser”, Journal of Modern

Optics, vol. 55, no. 8, p pp. 1345-1351, 2008.

[2] H. Ahmad, N. S. Shahabuddin, A. A. Rahman, K. Thambiratnam and S. W.

Harun, “SOA-based multi-wavelength source”, Journal of Modern Optics, vol.

55, no. 14, pp. 2179-2185, 2008.

[3] S. W. Harun, H. Ahmad, N. S. Shahabuddin, K. Dimyati and Z. Jusoh, “An

enhanced Bismuth-Based Brillouin/Erbium Fiber Laser with linear cavity

configuration”, Fiber and Integrated Optics, vol. 27, no. 1, pp. 35-40, 2008.

[4] S. W. Harun, M. C. Paul, M. Pal, A. Dhar, R. Sen, S. Das, S. K. Bhadra and N.

S. Shahabuddin, “An efficient and flat-gain Erbium-doped fiber amplifier in the

region of 1550 nm to 1590 nm”, Optoelectronics and Advanced Materials-

Rapid Communications, vol. 2, no. 8, pp. 455 – 458, 2008.

[5] M. R. Shirazi, N. S. Shahabuddin, S. N. Aziz, K. Thambiratnam, S. W. Harun

and H. Ahmad, “A linear cavity Brillouin fiber laser with multiple wavelengths

output”, Laser Physics Letters, vol. 5, no. 5, pp. 361-363, 2008.

[6] N. S. Shahabuddin, S. W. Harun, M. R. Shirazi, and H. Ahmad “A Linear

Cavity Brillouin/Bismuth-Based Erbium-Doped Fiber Laser with Enhanced

Characteristics”, Laser Physics, vol. 18, no. 11, pp. 1344, 2008.

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[7] S. W. Harun, S. N. Aziz, N. Tamchek, N. S. Shahabuddin and H. Ahmad,

“Brillouin fibre laser with 20 m-long photonics crystal fiber”, Electronics

Letters, vol. 44, no. 18, pp. 1065-1066, 2008.

[8] S. Shahi, S. W. Harun, N. S. Shahabuddin, M. R. Shirazi and H. Ahmad,

“Multi-wavelength generation using a bismuth-based EDF and Brillouin effect

in a linear cavity configuration”, Optics & Laser Technology, vol. 41, no. 2, pp.

198-201, 2009.

[9] H. Ahmad, N. S. Shahabuddin and S. W. Harun, “Multi-wavelength sources

based on SOA and loop mirror”, Optoelectronics and Advanced Materials-

Rapid Communications, vol. 3, no. 1, pp. 1-3, 2009.

Conferences

[1] N. S. Shahabuddin, S. W. Harun, M. Z. Zulkifli and H. Ahmad,“Bismuth-Based

Brillouin/Erbium Fiber Laser”, Mathematical and Physical Science Graduates

Congress, 2007, University of Malaya.

[2] N. S. Shahabuddin, S. W. Harun, M. Z. Zulkifli, Z. Jusoh and H. Ahmad, “A

Bismuth-Based Brillouin/Erbium Fiber Laser with Linear Cavity

Configuration”, PERFIK, 2007, Kuala Terengganu.

[3] N. S. Shahabuddin, H. Ahmad, A. A. Rahman, K. Thambiratnam and S. W.

Harun, “SOA-based Multi-Wavelength Source Using Sagnac Loop Mirror”,

Mathematical and Physical Science Graduates Congress, 2008, National

University of Singapore.

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