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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.
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[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.
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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–
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39, pp. 94–97, 2007.
[8] A. Yeniay, J. Delavaux and J. Toulouse, “Spontaneous and Stimulated Brillouin
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96
[9] R. W. Tkach, A. R. Chraplyvy and R. M. Derosire, “Spontaneous Brillouin
Scattering For Single-Mode Optical-Fibre Characterisation”, Electronics
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[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.
100
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.
101
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.
104
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
107
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
109
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
111
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
112
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.
123
[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
127
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.
130
[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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