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TRANSCRIPT
Ye Chen
Effect of Packetized Data on Gain Dynamics in Erbium-
doped Fiber Amplifiers Fed by Live Local Area Network
Traffic
Mémoire
présenté
à la faculté des études supérieures
de l’Université Laval
pour l’obtention
du grade de maître ès sciences (M. Sc.)
Département de génie électrique et de génie informatique
FACULTÉ DE SCIENCES ET DE GÉNIE
UNIVERSITÉ LAVAL
QUÉBEC
Mars 2000
© Ye Chen, 2000
2
Table of Contents
RESUMÉ… … … … … … … … … … … … … … … … … … … … … … … … … … … … .… … … … .4 ABSTRACT… … … … … … … … … … … … … … … … … … … … … … … … … … … .… … … … .5 ACKNOWLEDGMENTS… … … … … … … … … … … … … … … … … … … … … ..… … … … ..6 LIST OF FIGURES… … … … … … … … … … … … … … … … ...… … … … … … … ..… … … … 7 INTRODUCTION… … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … … ..9 CHAPTER 1. NETWORKING AND SELFSIMILARITY… … … … … … … … … … … … … … … … 11 1. Packet Switched Network… … … … … … … … … … … … … … … … … … … … … … ...11 2. Layered Networking Model----OSI Model… … … … … … … … … … … … … … … … .11 3. TCP/IP and Internet… … … … … … … … … … … … … … … … … … … … … … … … … 12 4. Ethernet… … … … … … … … … … … … … … … … … … … … … … … … … … … … … ..14 5. Optical Networking… … … … … … … … … … … … … … … … … … … … … … … … … 19 6. IP over WDM or DWDM… … … … … … … … … … … … … … … … … … … … … … ..28 7. Self-similar Nature of the Network Traffic… … … … … … … … … … … … … … … ..30 CHAPTER 2. ERBIUM-DOPED FIBER AMPLIFIER… … … … … … … … … … … … .… .33 Introduction… … … … … … … … … … … … … … … … … … … … … … … … … … … … … .33 1. Basic EDFA Configuration… … … … … … … … … … … … … … … … … … … … … ...35 2. Saleh and Sun’s Model… … … … … … … … … … … … … … … … … … … … … … … ..37 3. Bononi and Rusch’s Reservoir Model… … … … … … … … … … … … … … … … … ..39 4. Approximate Solutions… … … … … … … … … … … … … … … … … … … … … … .… .41 5. EDFA Cascade… … … … … … … … … … … … … … … … … … … … … … … … … … ..47 6. Gain-Clamped EDFA… … … … … … … … … … … … … … … … … … … … … … … … 48 7. Cross-Gain Modulation… … … … … … … … … … … … … … … … … … … … … … … .51 CHAPTER 3. EXPERIMENTAL SETUP… … … … … … … … … … … … … … ...… … … … .53 1. General Setup… … … … … … … … … … … … … … … … … … … … … … … … … … ...53 2. Ethernet Optical Transmitter… … … … … … … … … … … … … … … … … … … … … 54 3. 3 dB Coupler… … … … … … … … … … … … … … … … … … … … … … … … … … … 59 4. EDFA… … … … … … … … … ...… … … … … … … … … … … … … … … … … … … … 60 5. Circulator and Bragg Grating… … … … … … … … … … … … … … … … … … … … ..60 6. Photodetector… … … … … … … ...… … … … … … … … … … … … … … … … … … … 61 7. Data Acquisition Hardware and Software… … … … … … … … … … … … … … … ...64 CHAPTER 4. EXPERIMENTAL RESULTS… … … … … … … … … … … … .… … … … … … … … ..68 Introduction… … … … … … … … … … … … … … … … … … … … … … … … … … … … … .68 1. Time Response to Variable Data … … … … … … … … … … … … … … … … … … … ..69 2. Ethernet Data … … … … … … … … … … … … ..… … … … … … … … … … … … … … ..71 3. Measurements of Gain Swings Comparison with Bononi’s Artificial Traffic… … ..73 4. Gain Clamping… … … … … … … … … … … … … … … … … … … … … … … … … … ...75 5. Comparison with Results for Artificial Ethernet Data… … … … … … … … … … … ..79 6. Experimental Implementation… … … … … … … … … … … … … … … … … … … … ...80 CHAPTER 5. CONCLUSIONS… … … … … … … … … … … … … … … … … … … … … … ..81
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1. What was achieved… … … … … … … … … … … … … … … … … … … … … … … … ..81 2. Possible follow-up work… … … … … … … … … … … … … … … … … … … … … … ..82 REFERENCES… … … … … … … … … … … … … … … … … … … … … … … … … .… … .… ...83 APPENDIX I. The Ethernet Optical— Electrical Transmitter Circuit Scheme… … … .87 APPENDIX II. AUI on 10 Base-T Ethernet Hub… … … … … … … … … … … … … … … .89
4
RESUMÉ
La réponse des amplificateurs de fiber dopés à l’erbium (EDFAs) aux changements non-
périodiques des canaux d'entrée (dus au canal “add/drop”, à la reconfiguration de réseau, aux
coupes de fiber, ou le traffic en paquet) a été l'étude de beaucoup de recherche récente [1].
L'excursion du gain dans un amplificateur du aux changements dans les canaux d'entrée mène
possiblement à de larges oscillations du rapport signal/bruit optique de sortie (OSNR). Étant
donné la popularité extrême d'Internet, nous prévoyons que les futurs réseaux optiques devront
supporter les données Internet “de mode indigène”, c’est-à-dire des données qui sont transmises
par paquets et qui n’ont pas la modulation continue caractéristique du multiplexage temporel de
signaux. Pour cette raison, la recherche s'est concentrée sur la réponse des EDFAs à des données
qui se caractérisent par des périodes de modulation et de silence (périodes MARCHE-ARRÊT).
Pour la première fois, nous avons mesuré la réponse d'EDFAs aux données de type Ethernet à 10
Mb/s. De plus, nous avons vérifié des résultats théoriques où des données en paquet étaient
modelisées avec des caractéristiques autosimilaires. Nos mesures montrent une variation
substantielle de gain après cinq amplificateurs en cascade. Nous avons également asservi le gain
(boucle de “feedback”) pour stabiliser la puissance de sortie. La méthode s’est révélée
extrêmement pertinente contre les excursions de gain apparaissant lors de la transmission de
paquets Ethernet.
------------------------------ -------------------------------
Ye Chen Prof. Leslie Ann Rusch
Étudiant à la maîtrise Directrice de recherche
5
ABSTRACT
The response of erbium doped fiber amplifiers (EDFAs) to non-periodic changes in the input
channels (due to channel add/drop, network reconfiguration, fiber cuts, or packetized traffic) has
been the subject of much recent research [1]. Gain excursion in an amplifier due to changes in
the input channels leads to possibly wide swings in the output optical signal-to-noise ratio
(OSNR). Given the extreme popularity of the Internet, we anticipate that future optical networks
will be increasingly required to support “native mode” Internet data, that is, data that is
transmitted in packets do not have the continuous modulation that is characteristic of time
division multiplexing. For this reason, research has focused on the EDFA response to data that is
characterized by periods of modulation and silence (ON and OFF times).
For the first time we have measured the response of EDFAs to live Ethernet data at 10 Mb/s and
verified previous theoretical results where packetized data is modeled as having self-similar
characteristics. Our measurements show substantial output power variation after five amplifiers
in a cascade. We have also implemented gain clamping to combat the output power variation.
Gain clamping is extremely effective against output power variation due to packetized input.
6
Acknowledgements
During my two and half years’ Master study at Laval University, Dr. Leslie Ann Rusch kept on
guiding and encouraging me all the time, she is so kind and patient for my study and research
like a good friend or an elder sister. She helped me conquer lots of trouble when I first came to
Quebec. Though Quebec’s winters are severe, I always feel warm to be her student. To some
degree, I regard this work as my Christmas gift for her before she left for working in USA in
December 1999.
I am also obliged to express my special thanks to Dr. Jean-François Cliche who helped me
tremendously on the Ethernet transmitter and EDFAs’ PCBs. We worked together all day long
for a couple of weeks on circuits before I can start the measurements.
Thanks for Dr. Miroslav Karasek’s theoretical directions and experimental cooperation.
Special thanks for Dr. Bo Wen (Washington State University) who suggested me lots of good
idea on networking.
What’s more, I’d like to thank the group of Centre d'Optique, Photonique et Laser (COPL), for
the best research environment, friendly cooperation and brotherhood here.
7
List of Figures
Figure 1-1. The OSI model includes 7 layers that define network functionality… … … … … … ...12 Figure 1-2. Internet… … … … … … … … … … … … … … … … … … … … … … … … … … … … … … 12 Figure 1-3. TCP/IP and Internet architecture… … … … … … … … … … … … … … … … … … … ...13 Figure 1-4. TCP/IP suite and OSI layers… … … … … … … … … … … … … … … … … … … … .… ..14 Figure 1-5. The jam signal ensures all nodes recognize a collision… … … … … … … … … … … ...15 Figure 1-6 CSMA/CD Flow Chart… … … … … … … … … … … … … … … … … … … … … … ..… ...16 Figure 1-7. High-Speed Ethernet options and switching technologies offer more bandwidth to
users and workgroups… … … … … … … … … … … … … … … … … … … … … … … … .17 Figure 1-8. Gigabit Ethernet protocol stack… … … … … … … … … … … … … … … … … … … … ...18 Figure 1-9. End-to-End Wavelength Services… … … … … … … … … … … … … … … … … … … ...19 Figure 1-10. Wavelength Division Multiplexing… … … … … … … … … … … … … … … … … … ..19 Figure 1-11. Development Milestones… … … … … … … … … … … … … … … … … … … … … … ..21 Figure 1-12. Usage of Erbium-doped Fiber Amplifier (EDFA)… … … … … … … … … … … … ....23 Figure 1-13. DWDM Systems… … … … … … … … … … … … … … … … … … … … … … … … … ...24 Figure 1-14. ITU Channel Spacing… … … … … … … … … … … … … … … … … … … … … … … ....24 Figure 1-15. Unidirectional and Bi-directional DWDM… … … … … … … … … … … … … … … ....25 Figure 1-16. In-Fiber Bragg Grating Technology: Optical A/D Multiplexer… … … … … … … ....26 Figure 1-17. Key Functional Blocks for WDM Transport Systems… … … … … … … … … … … ..27 Figure 1-18. Network Bandwidth Prediction… … … … … … … … … … … … … … … … … … … .… 28 Figure 1-19. IP Transport Alternatives… … … … … … … … … … … … … … … … … … … … … … ..29 Figure 1-20. IP over DWDM… … … … … … … … … … … … … … … … … … … … … … … … … … .30 Figure 1-21. Pictorial Proof of Self-Similarity… … … … … … … … … … … … … … … … … … … . 31 Figure 2-1 (a) amplifiers in point-to-point transmission system (b) amplifiers in networks… … .34 Figure 2-2. Basic EDFA Configuration… … … … … … … … … … … … … … … … … … … … … … .35 Figure 2-3. Erbium Energy Level… … … … … … … … … … … … … … … … … … … … … … … … ..37 Figure 2-4. Reservoir model… … … … … … … … … … … … … … … … … … … … … … … … … … ..40 Figure 2-5. A simple WDM system… … … … … … … … … … … … … … … … … … … … … … … ...43 Figure 2-6. Add/Drop simulation using Reservoir Model … … … … … … … … … … … … … … … 44 Figure 2-7. Our Add/Drop Experiment… … … … … … … … … … … … … … … … … … … … … … ..44 Figure 2-8. Sun’s Results… … … … … … … … … … … … … … … … … … … … … … … … … … … ..45 Figure 2-9. Real case and approximations… … … … … … … … … … … … … … … … … … … … … 46 Figure 2-10. Some Fit Methods… … … … … … … … … … … … … … … … … … … … … … … … … .46 Figure 2-11. Gain-Clamped EDFA… … … … … … … … … … … … … … … … … … … … … … … … 48 Figure 2-12. A simple and practical Gain-clamped EDFA design… … … … … … … … … … … … 48 Figure 2-13. Gain versus average inversion and wavelength… … … … … … … … … … … … … … 49 Figure 2-14. Setup… … … … … … … … … … … … … … … … … … … … … … … … … … … … … .… 51 Figure 2-15. Cross-Gain Modulation… … … … … … … … … … … … … … … … … … … … … … … .51 Figure 3-1. Experiment Setup… … … … … … … … … … … … … … … … … … … … … … … … … ....53 Figure 3-2. Logical Structure of the Ethernet Optical Transmitter… … … … … … … … … … … ...54 Figure 3-3. AD96685BR ECL Comparator… … … … … … … … … … … … … … … … … … … … ..55 Figure 3-4. FMM3171VI Laser Driver… … … … … … … … … … … … … … … … … … … … … … ..56 Figure 3-5. FLD5F8LK DFB Laser… … … … … … … … … … … … … … … … … … … … … … … ...57 Figure 3-6. FLD5F8LK Laser’s Spectrum… … … … … … … … … … … … … … … … … … … ..… ..57 Figure 3-7. Transmission Performance in packet level … … … … … … … … … … … … … … … ....58
8
Figure 3-8. Laser’s Transient response in bit level… … … … … … … … … … … … … … … … … ..58 Figure 3-9. Overview of the Ethernet Transmitter… … … … … … … … … … … … … … … … … … 59 Figure 3-10. 3dB Coupler… … … … … … … … … … … … … … … … … … … … … … … … … … … .59 Figure 3-11. EDFA… … … … … … … … … … … … … … … … … … … … … … … … … … … … … … 60 Figure 3-12. EDFA ASE Character… … … … … … … … … … … … … … … … … … … … … … … .60 Figure 3-13. Optical Circulator and Bragg Grating… … … … … … … … … … … … … … … … … ..61 Figure 3-14. ANTEL ARX-GP amplified ultra high-speed photodetector… … … … … … … … … ..61 Figure 3-15. Data Acquisition Scheme… … … … … … … … … … … … … … … … … … … … … … ..62 Figure 3-16. BNC-2110 Accessory… … … … … … … … … … … … … … … … … … … … … … … ....63 Figure 3-17. Front Panel of BNC-2110… … … … … … … … … … … … … … … … … … … … … … .63 Figure 3-18. NI-DAQ driver for Windows… … … … … … … … … … … … … … … … … … … … ....65 Figure 3-19. Relationship between the Programming Environment, NI-DAQ, and the
Hardware… … … … … … … … … … … … … … … … … … … … … … … … … … … … ....66 Figure 3-20. LabVIEW DAQ Software Interface… … … … … … … … … … … … … … … … … … ..66 Figure 3-21. Read and Convert Program Interface… … … … … … … … … … … … … … … … ...… .67 Figure 4-1. Time evolution of photodetector voltage … … … … … … … … … … … … … … … ...… 69 Figure 4-2. Time response of the EDFAs Cascade.… … … … … … … … .… … … … … … … … … ..70 Figure 4-3. The effectiveness of gain clamping … … … … … … … … … … … … … … … … … ...… .70 Figure 4-4. Traffic Generation … … … … ...… … … … … … … … .… … … … … … … … … … … … ..71 Figure 4-5. PDF of the 1st EDFA .… … … … … … … … ...… … … … … … … … … … … … … … … ..73 Figure 4-6. PDF of the 3rd EDFA … … … … … … … … … … … ...… … … … … … … … … … ..… ..73 Figure 4-7. Output of the 5th EDFA … … … … … … … … … … … … … … … … … … … … … … … .74 Figure 4-8. PDF broadens along the EDFA cascade … … … … … … … … … … … … … … … … … 75 Figure 4-9. Time evolution of EDFA 3 and EDFA 5… … … … ..… … … … … … … … … … … … .76 Figure 4-10. Clamping also reduces the gain swing in the cascade… … … … … … … .… … .… … 77 Figure 4-11. Output of EDFA 1 under clamped and unclamped cases… … … … … … .… … … … 77 Figure 4-12. Probability density function of EDFA 3 and 5… … … … … … … … … … … … … … .78 Figure 4-13. Bononi’s Setup with artificial traffic… … … … … … … … … … … … … … … … … … 79 Figure 4-14. Bononi’s Results… … … … … … … … … … … … … … … … … … … … ..… … … … … .79
9
Introduction
Wavelength division multiplexing (WDM) technology employing erbium-doped fiber amplifiers
(EDFAs) provides a platform for significant improvement in network bandwidth capacity. WDM
will play a dominant role in backbone infrastructure supporting the next generation high-speed
networks. Fast signal power transients caused by cross-gain saturation effects pose a serious
limitation in amplified WDM transmission networks. Whereas most previous analysis on cross-
gain saturation in EDFAs focuses on circuit-switched scenarios, we address links carrying data
packets. When the number of WDM channels transmitted through a circuit-switching network
varies due to network reconfiguration or channel failure, channel addition/removal will tend to
perturb signals at the surviving channels that share all or part of the route. Although this
perturbation will generally be small in a single amplifier, it will grow rapidly along a cascade.
Power transients in the surviving channels can cause severe service impairment due to either
inadequate eye opening or the appearance of optical nonlinearities [1-3]. Several solutions to this
problem have been proposed and experimentally verified. These include fast pump control [4, 5],
fast link control by insertion of a control channel [6], and gain clamping by an all-optical
feedback loop [7, 8].
Recent traffic measurements from working packet networks (including Ethernet local area
networks, wide area networks, integrated services digital network, and variable bit rate video
over asynchronous transfer mode) have shown features of self-similarity: that is, realistic packet
network traffic looks the same when measured over time scales ranging from milliseconds to
seconds to minutes and hours [9-11]. It has been concluded that a superposition of many
ON/OFF sources or packet-trains with strictly alternating ON- and OFF-periods with infinite
variance produces aggregate network traffic that is self-similar. When such packet traffic is
directly transmitted on the WDM channels, as in the case of Internet Protocol (IP), long empty
slot intervals may give enough time to fiber amplifiers to reach gains greatly exceeding the
average values. This can in turn lead to significant variation in output power and optical signal-
to-noise ratio (OSNR). This effect accumulates along a cascade of fiber amplifiers in the same
way as the fast power transients in the circuit-switching scenario. The effect of WDM traffic
10
statistics on the output power and optical OSNR swings in a cascade of five erbium-doped fiber
amplifiers (EDFAs) of standard design has been theoretically investigated in [12, 13]. The results
of the simulations indicate that substantial power and OSNR swings occur at the output of a
cascade when highly-variable burst-mode traffic is transmitted. Power swings in excess of 9 dBm
and OSNR swings of more than 4 dB were observed. The stabilization effect of clamping the
gain of the first EDFA by all-optical feedback loop and letting the lasing power propagate
through the cascade of three EDFAs has been demonstrated experimentally in [14] for artificially
generated traffic with Pareto distribution.
In contrast to [14], this work demonstrates the effectiveness of gain clamping on the reduction of
output power spread in a gain clamped cascade of EDFAs fed by burst-mode live Local Area
Network traffic.
Recently, direct transmission of IP packet traffic over WDM channels is receiving increasing
attention. This solution avoids the costs of SONET/SDH (Synchronous Optical
Networking/Synchronous Digital Hierarchy) and significantly simplifies the network.
Chapter 1 introduces some basics about IP, Ethernet, optical networking and the self-similar
nature of the network traffic; Chapter 2 summarizes the basic principles of EDFA, presents some
approximation methods used for EDFA simulation and introduces the gain-clamped EDFA
principles; Chapter 3 deals with the experiments setup; Chapter 4 gives the results; Chapter 5 is
the conclusion.
Appendix I is the layout and schematics of the Ethernet Optical Transmitter; Appendix II
introduces the Ethernet Hub.
11
Chapter 1. Networking and Selfsimilarity
In this Chapter, we will introduce some ideas about networking and the selfsimilar nature of
packetized traffic.
1. Packet Switched Network
Packet switched networks, and computer networks in general, have become an intrinsic part of
the computer industry. With the fall in their cost and the increase in user-friendly software it is
now possible for people to connect many computers together to form a network with ease.
Computer networks come in all shapes and sizes and there are a lot of methods for connecting
two or more computers together. Each individual network is different, depending on its topology,
the transmission technology and the software running on it.
Packet switching differs from circuit switching in that whereas a circuit switching system treats a
connection as a continuous stream, packet switching breaks everything up into discrete, limited
size blocks of data. Each block is known as a packet. A common analogy is the idea of a
postcard in a postal network. To send a long message several postcards have to be written and
sent. Each postcard is treated independently and they may not arrive in the same order as they are
sent or follow the same route in getting to their destination.
2. Layered Networking Model: OSI model
For a complex, multivendor internetwork to operate, its devices must be able to communicate
with each other. The networking industry uses an Open System Interconnection (OSI) model that
provides guidelines for that communication.
Most communication environments separate the communication functions from application
processing. This separation of networking function is called layering. For the OSI model, shown
in Figure 1-1, seven numbered layers indicate different functions.
12
Figure 1-1. The OSI model includes 7 layers that define network functionality
3. TCP/IP and Internet
TCP and IP were developed by a Department of Defense (DOD) research project to connect a
number of different networks designed by different vendors into a network of networks (the
"Internet"). It was initially successful because it delivered a few basic services that everyone
needs (file transfer, electronic mail, remote logon) across a very large number of client and
server systems. Several computers in a small department can use TCP/IP (along with other
protocols) on a single LAN. The IP component provides routing from the department to the
enterprise network, then to regional networks, and finally to the global Internet with
multiservices (Figure 1-2).
Figure 1-2. Internet [35]
7
6
5
4
3
2
1
Networks processes to applications
Upper Layers
Lower Layers
Data Representation
Interhost Communication
End-to-End Connections
Addresses and Best Path
Access to Media
Binary Transmission
Application
Physical
Data Link
Network
Transport
Session
Presentation
13
The Internet is a system of linked networks that are worldwide in scope and facilitate data
communication services such as remote login, file transfer, electronic mail, the World Wide Web
and newsgroups.
With the meteoric rise in demand for connectivity, the Internet has become a communications
highway for millions of users. The Internet was initially restricted to military and academic
institutions, but now it is a full-fledged conduit for any and all forms of information and
commerce. Internet websites now provide personal, educational, political and economic
resources to every corner of the planet.
As with all other communications protocols, TCP/IP is composed of layers:
• IP - is responsible for moving packet of data from node to node. IP forwards each packet
based on a four-byte destination address (the IP number). The Internet authorities assign
ranges of numbers to different organizations. The organizations assign groups of their
numbers to departments. IP operates on gateway machines that move data from
department to organization to region and then around the world.
• TCP - is responsible for verifying the correct delivery of data from client to server. Data
can be lost in the intermediate network. TCP adds support to detect errors or lost data and
to trigger retransmission until the data is correctly and completely received (Figure 1-3).
Figure 1-3. TCP/IP and Internet Architecture
• Sockets - is a name given to the package of subroutines that provide access to TCP/IP on
most systems.
Application Services
Subnetworks
Internetworking
Service Provider Protocol TCP
IP
14
Figure 1-4. TCP/IP suite and OSI layers
TCP/IP’s layers do not correspond one-to-one with the OSI layers. You can overlay the TCP/IP
programs on this model to give you a rough idea of where all the TCP/IP layers reside.
Figure 1-4 shows the basic elements of the TCP/IP family of protocols. You can see that TCP/IP
is not involved in the bottom two layers of the OSI model (data link and physical) but begins in
the network layer, where the Internet Protocol (IP) resides. In the transport layer, the
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are involved.
Above this, the utilities and protocols that make up the rest of the TCP/IP suite are built using the
TCP or UDP and IP layers for their communications system.
Figure 1-4 shows that some of the upper-layer protocols depend on TCP (such as Telnet and
FTP), whereas some depend on UDP. Most upper-layer TCP/IP protocols use only one of the
two transport protocols (TCP or UDP), although a few, including DNS (Domain Name System)
can use both [15].
4. Ethernet
Ethernet is a contention based broadcast network designed to work over short distances (tens to
hundreds of meters). It is the most popular physical layer Local Area Network (LAN) technology
in use today. Other LAN types include Token Ring, Fast Ethernet, Fiber Distributed Data
Interface (FDDI), Asynchronous Transfer Mode (ATM) and LocalTalk.
Application
Physical
Data Link
Network
Transport
Session
Presentation
OSI GenericProtocol Stack
Telnet/FTPSMTP/NNTP
SNMP
TCP/UDP
IP
Packet Driver
Ethernet hardware
IP Protocol Stackon top of Ethernet
15
Ethernet is popular because it strikes a good balance between speed, cost and ease of installation.
These benefits combined with wide acceptance in the computer marketplace and the ability to
support virtually all popular network protocols, make Ethernet an ideal networking technology
for most computer users today. The Institute for Electrical and Electronic Engineers (IEEE)
defines the Ethernet standard as IEEE Standard 802.3. This standard defines rules for
configuring an Ethernet network as well as specifying how elements in an Ethernet network
interact with one another. By adhering to the IEEE standard, network equipment and network
protocols can communicate efficiently.
As shown in Figure 1-5, every station must sense the channel first whenever it is ready to send
the data. If the channel is idle (there is no signal on the channel), it will transmit data in the
medium and sense the channel at the same time. Since signals take a finite time to travel from
one end of an Ethernet system to the other, the first bits of a transmitted frame do not reach all
parts of the network simultaneously. Therefore, it is possible for two stations to sense that the
network is idle and to start transmitting their frames simultaneously so that this causes collisions.
Figure 1-5. The jam signal ensures all nodes recognize a collision
When a transmitting node recognizes a collision, it transmits a jam signal that causes the
collision to last long enough for all other nodes to recognize it. All transmitting nodes then stop
sending frames for a randomly selected time period, called the backoff time, before attempting to
retransmit. If subsequent attempts also result in collisions, the node tries to retransmit up to 16
times before giving up.
If the two backoff times are sufficiently different, one station will succeed the next time it tries to
transmit. The mean backoff time doubles with each consecutive collision up to the 10th retry,
A B C D
A B C D
JAM JAMJAMJAMJAM
Collision
16
thereby reducing the chance of collision in subsequent transmissions. From the 10th to the 16th
retry, the stations do not increase the backoff time any more but keep it constant [16, 17, 18].
High-Speed Ethernet Option
New applications can cause end users to experience delays and other troubles such as insufficient
bandwidth between end stations. In response to these problems, Ethernet networks have moved
forward with the availability of 100 Mbps technologies, such as:
• 100BaseFX--A 100 Mbps implementation of Ethernet over fiber-optic cable, the MAC layer
is compatible with the 802.3 MAC layer.
• 100BaseT4— A 100 Mbps implementation of Ethernet using 4-pair Category 3, 4, or 5
cabling, The MAC layer is compatible with the 802.3 MAC layer.
• 100BaseTX--A 100 Mbps implementation of Ethernet over Category 5 and Type 1 cabling.
The MAC layer is compatible with the 802.3 MAC layer.
• 100VG— Any LAN-The IEEE specification for 100Mbps implementation of Ethernet and
Token Ring over 4-pair UTP. The MAC layer is not compatible with the 802.3 MAC layer.
Figure 1-6. CSMA/CD Flow Chart
Increasing Ethernet bandwidth to 100 Mbps solves part of the bandwidth problem. Another part
of the solution is reducing the contention for the Ethernet media. One method of reducing
Station is ready to send
Wait according tobackoff strategy
(6)
Transmit succeed
Tranmit jam signal(5)
Transmit data & sensechannel
(4)
SenseChannel
(1)
New attempt
Channel Occupied(3)
No collision found
Channel available(2)
17
contention is built into Ethernet standard, namely the Carrier Sense Multiple Access with
Collision Detection (CSMA/CD) approach (Figure 1-6). A shared-media LAN must submit to
CSMA/CD so that no 2 users can simultaneously communicate over the shared LAN segment.
Switching also reduces contention for the media by creating multiple segments for desktop
devices and high-end applications (Figure 1-7). Switch in Figure 1-7 on the left splits the
Ethernet and thus reduces the number of users per shared segment. The switch makes multiple 10
Mbps or even 100 Mbps data pipes available. A limited number of users share a single 10 Mbps
or 100 bps segment. These users work in smaller collision domain with less contention from
other nodes. For users with high bandwidth needs and servers, one can provide a single dedicated
segment per user or server.
Figure 1-7. High-Speed Ethernet options and switching technologies offer more bandwidth to
users and workgroups
Coinciding with the rapid growth of high-speed Ethernet option is the deployment of ATM,
another high-speed technology.
Another high-speed technology is Gigabit Ethernet, or 1000BaseX. Gigabit Ethernet builds on
top of the Ethernet protocol but increases speed 10 times over fast Ethernet to 1000 Mbps, or 1
Gbps. This Media Access Control (MAC) and Physical Interface (PHY) standard, which was
approved on June 25, 1998, promises to be a dominant player in high-speed local-area network
(LAN) backbones and server connectivity. Customers will be able to leverage their existing
Ethernet knowledge base to manage and maintain Gigabit networks.
Switch
Router
18
It has been decided that Gigabit Ethernet will look identical to Ethernet from the data link layer
upward. However, to accommodate the increased speed from 100 Mbps fast Ethernet to 1 Gbps,
several changes need to be made to the physical interface. The challenges have been resolved by
merging two technologies: IEEE 802.3 Ethernet and ANSI X3T11 FibreChannel. Figure 1-8
shows how key components from each technology have been combined to form Gigabit
Ethernet.
Figure 1-8. Gigabit Ethernet protocol stack
The resulting standard takes advantage of existing high-speed physical interface technology of
FibreChannel while maintaining the IEEE 802.3 Ethernet frame format, backward compatibility
for installed media, and use of full or half duplex via CSMA/CD. It helps minimize the
complexity of resulting technology, produces a stable technology, and shortens development
time.
With the approval of 802.3z standard, Ethernet may gain a leading edge among LAN
technologies in pushing bandwidth speed [16].
5. Optical Networking
IEEE 802.2 LLC
IEEE 802.3CSMA/CD
IEEE 802.3 PhysicalLayer
IEEE 802.3 Ethernet
FC-4 Upper-LayerMapping
FC-3 CommonServices
FC-z Signalling
FC-1 Encode/Decode
FC-0 Interface andMedia
ANSI X3T11 FibreChannel
IEEE 802.3z GigabitEthernet
IEEE 802.2 LLC
CSMA/CD orFull-Duplex MAC
8B/10BEncode/Decode
Serializer/Deserializer
Connector
19
Optical networks provide the required bandwidth and flexibility to enable end-to-end wavelength
services (Figure 1-9) [17].
Figure 1-9. End-to-End Wavelength Services
Optical networking is generally defined to include the use of WDM (Figure1-10) to transport
multiple signals independently on individual fibers and the use of rearrangeable wavelength-
dependent routing to achieve increased network capacity and increased flexibility relative to
other approaches.
Figure 1-10. Wavelength Division Multiplexing
WDM has the following features [18]:
Optical GatewayENT MUX
Optical GatewayENT MUX
Long Haul
Regional
Access
END-TO-END
WAVELENGTH
SERVICES
Backbone
Regional
MUX
DEMUX
EDFA EDFA
1λ
2λ
........λ
Nλ
1λ
2λ
........λ
Nλ
20
• Multiplex multiple WDM channels from different users on the same fiber.
• The optical transmission spectrum is “carved-up” into a number of non-overlapping
wavelength (or frequency) bands, with each wavelength supporting a single communication
channel.
• The difference between electronics and optics is granularity (achieve rich connectivity
without a lot of optical connections. Electronics provide connectivity whereas optics provides
throughput).
Many factors are driving the need for optical networks. A few of the most important reasons for
migrating to the optical layer are described in this module.
• Fiber Capacity
The first implementation of what has emerged as the optical network began on routes that were
fiber limited. Providers needed more capacity between two sites, but higher bit rates or fiber
were not available. The only options in these situations were to install more fibers, which is an
expensive and labor-intensive chore, or place more time division multiplexed (TDM) signals on
the same fiber. WDM provided many virtual fibers on a single physical fiber. By transmitting
each signal at a different frequency, network providers could send many signals on one fiber just
as though they were each traveling on their own fiber.
• Restoration Capability
As network planners use more network elements to increase fiber capacity, a fiber cut can have
massive implications. In current electrical architectures, each network element performs its own
restoration. For a WDM system with many channels on a single fiber, a fiber cut would initiate
multiple failures, causing many independent systems to fail. By performing restoration in the
optical layer rather than the electrical layer, optical networks can perform protection switching
faster and more economically. Additionally, the optical layer can provide restoration in networks
that currently do not have a protection scheme. By implementing optical networks, providers can
add restoration capabilities to embedded asynchronous systems without first upgrading to an
electrical-protection scheme.
21
• Reduced Cost
In systems using only WDM, each location that demultiplexes signals will need an electrical
network element for each channel, even if no traffic is dropping at that site. By implementing an
optical network, only those wavelengths that add or drop traffic at a site need corresponding
electrical nodes. Other channels can simply pass through optically, which provides tremendous
cost savings in equipment and network management. In addition, performing space and
wavelength routing of traffic avoids the high cost of electronic cross-connects, and network
management is simplified.
• Wavelength Services
One of the great revenue-producing aspects of optical networks is the ability to resell bandwidth
rather than fiber. By maximizing capacity available on a fiber, service providers can improve
revenue by selling wavelengths, regardless of the data rate required. To customers, this service
provides the same bandwidth as a dedicated fiber.
The cornerstone of an optical network is the advanced optical technologies that perform the
necessary all-optical functions. Optical technologies continue to advance by ingenious
techniques and implementations to improve the performance and capabilities of the optical
network (see Figure 1-11) [17].
Figure 1-11. Development Milestones [17]
EDFA OpticalAmplifiers
(Flat Gain Supports WDM)
Space/Wavelength Optical Crossconnectsrequires WDM infrastructure prior to
widescale deployment
TunableLaser
WavelengthShifter
Optical Crossconnect(Space/Wavelength)
22
Early Technologies
As fiber optics came into use, network providers soon found that some improvements in
technology could greatly increase capacity and reduce cost in existing networks. These early
technologies eventually led to the optical network as it is today.
• Broadband WDM
The first incarnation of WDM was broadband WDM. In 1994, by using fused biconic tapered
couplers, two signals could be combined on the same fiber. Because of limitations in the
technology, the signal frequencies had to be widely separated, and systems typically used 1,310-
nm and 1,550-nm signals, providing 5 Gbps on one fiber. Although the performance did not
compare to today's technologies, the couplers provided twice the bandwidth out of the same
fiber, which was a large cost savings compared to installing new fiber.
• Optical Fiber Amplifiers
The second basic technology, and perhaps the most fundamental to today's optical networks, was
the erbium-doped fiber amplifier. By doping a small strand of fiber with a rare earth metal, such
as erbium, optical signals could be amplified without converting the signal back to an electrical
state. The amplifier provided enormous cost savings over electrical regenerators, especially in
long-haul networks.
Erbium-doped Fiber Amplifier is the Key Enabling Technology for Optical Networking, as we
know its advantages for long-haul telecommunication:
• No O/E, E/O conversion - Amplify the optical signals in an optical medium => No
electronic bottleneck associated with electronic repeaters (which are complex,
expensive and provide no transparency).
• Larger bandwidth than electronic repeaters (up to 100 Gbps).
• Insensitive to bit rates.
• Transparent to modulation formats.
23
• Simultaneous regeneration of multiple WDM signals.
• Low cost and high reliability.
• Low noise, high gain.
• Very well suited for loss compensation of passive components in an optical
transmission system.
• Potential to create a universal lightpipe between terminals.
• Potential for high bit rate applications through WDM.
Erbium-Doped Fiber Amplifier can be used as (Figure 1-12):
• Power amplifiers at the transmitter.
• Optical pre-amplifiers in high bit-rate receivers.
• In line amplifiers to compensate loss in optical networks.
Figure 1-12. Usage of Erbium-doped Fiber Amplifier
Curent Technologies
Systems deployed today use devices that perform similar functions to earlier devices but are
much more efficient and precise. In particular, flat-gain optical amplifiers have been the true
enablers for optical networks by allowing the combination of many wavelengths across a single
fiber.
Tx RxEDFA EDFAEDFAEDFASMF
Power amp In-line amp In-line amp pre-amp
24
• Dense Wavelength Division Multiplexing (DWDM)
Figure 1-13. DWDM Systems
Figure 1-14. ITU channel spacing
As optical filters and laser technology improved, the ability to combine more than two signal
wavelengths on a fiber became a reality. Dense Wavelength Division Multiplexing (DWDM)
combines multiple signals on the same fiber, ranging up to 40 or 80 channels. By implementing
DWDM systems and optical amplifiers, networks can provide a variety of bit rates (i.e., OC–48
or OC–192), and a multitude of channels over a single fiber (see Figure 1-13). The wavelengths
used are all in the range that optical amplifiers perform optimally, typically from about 1,530 nm
to 1,565 nm (see Figure 1-14).
OC-48 OC-48
OC-48OC-48
OC-192OC-192
DWDM
DWDM
EDFA EDFAEDFA
25
Two basic types of DWDM are implemented today: unidirectional and bi-directional DWDM
(see Figure 1-15). In a unidirectional system, all the wavelengths travel in the same direction on
the fiber, while in a bi-directional system the signals are split into separate bands, with both
bands traveling in different directions.
Figure 1-15. Unidirectional and Bi-directional DWDM
• Optical Amplifiers
The performance of optical amplifiers has improved significantly— with current amplifiers
providing significantly lower noise and flatter gain— which is essential to DWDM systems. The
total power of amplifiers also has steadily increased, with amplifiers approaching +20–dBm
outputs, which is many orders of magnitude more powerful than the first amplifiers.
• Narrowband Lasers
Without a narrow, stable, and coherent light source, none of the optical components would be of
any value in the optical network. Advanced lasers with narrow bandwidths provide the narrow
wavelength source that is the individual channel in optical networks. Typically, long-haul
applications use externally modulated lasers, while shorter applications can use integrated laser
technologies.
26
Depending on the system used, the laser may be part of the DWDM system or embedded in the
SONET network element. When the precision laser is embedded in the SONET network
element, the system is called an embedded system. When the precision laser is part of the WDM
equipment in a module called a transponder, it is considered an open system because any low-
cost laser transmitter on the SONET network element can be used as input.
• Fiber Bragg Gratings
Figure 1-16. In-Fiber Bragg Grating Technology: Optical A/D Multiplexer [17]
Commercially available fiber Bragg gratings have been important components for enabling
WDM and optical networks. A fiber Bragg grating is a small section of fiber that has been
modified to create periodic changes in the index of refraction. Depending on the space between
the changes, a certain frequency of light— the Bragg resonance wavelength— is reflected back,
while all other wavelengths pass through (see Figure 1-16). The wavelength-specific properties
of the grating make fiber Bragg gratings useful in implementing optical add/drop multiplexers.
Bragg gratings also are being developed to aid in dispersion compensation and signal filtering as
well.
27
• Thin Film Substrates
Another essential technology for optical networks is the thin film substrate. By coating a thin
glass or polymer substrate with a thin interference film of dielectric material, the substrate can be
made to pass through only a specific wavelength and reflect all others. By integrating several of
these components, many optical network devices are created, including multiplexers,
demultiplexers, and add/drop devices.
Technologies on the Horizon
Figure 1-17. Key Functional Blocks for WDM Transport Systems
Key functions have been identified as requirements for the emerging optical network (Figure 1-
17). As component technologies advance, each of the functions required, such as tunable filters,
space switches, and wavelength converters, will become more cost effective and practical.
All of these technologies aim at reducing the network cost and provide valuable new services to
customers who are constantly demanding more bandwidth-intensive and flexible features from
their network providers.
In real system application, we have to pay attention to the following optical fiber non-linearities:
• Self-Phase Modulation
WDM Tx Source
EDFA
Demultiplexer/fixed fiber
EO
OO
Tunable Filter
Space Switch
Wavelength Converter
28
• Cross-Phase Modulation
• Modulation Instability
• Four wave mixing
• Stimulated Brillouin Scattering
• Stimulated Raman Scattering
6. IP over WDM or DWWM
Figure 1-18. Network Bandwidth Prediction [19]
The amount of traffic— data, voice, and multimedia— traveling over the Internet and other
networks grows at rates that are hard to quantify, but everyone agrees the increase is beyond
anyone’s wildest dreams (Figure 1-18).
Once all this traffic leaves a local network (or home or small business), it goes into the hands of
a carrier or service provider, and typically is sent over a fiber optic cabling infrastructure. Even
service providers too small to have their own fiber infrastructure use fiber built and run by
someone else. Fiber optic cabling moves lots of traffic quickly, but even these fat pipes feel the
bandwidth pinch. So rather than pay exorbitant amounts to lay new fiber cabling, providers rely
on technologies that increase the amount of data a single piece of fiber can handle.
29
In order to still guarantee a high transmission speed, broadband communications networks with
optical fibers are employed.
By means of the WDM technology the potential for the transport capacity up to 1000 Gbit/s and
more on a single fiber is given. The WDM technology involves new requirements for the
network elements, which are therefore partially in research & development state. Depending on
the functionality of the WDM network elements, different network structures can be generated,
e.g. “broadcast and select”-networks or wavelength-routed networks.
Figure 1-19. IP Transport alternatives
In comparison to ATM, SONET and B-ISDN, the lower equipment and operational cost of
WDM also attracts more attention (Figure1-19) [19].
IP
ATM
SONET/SDH
Optical
IP
ATM
Optical
IP
SONET/SDH
Optical
IP
Optical
B-ISDN
IP overATM
IP overSONET/SDH
IP overWDM
Lower Equipment Cost & Operational Cost
30
IP over DWDM networks is shown in Figure 1-20. Multitechnologies coexist within one
network. DWDM can organize several different technologies including SDH/SONET, ATM and
IP into one network [20].
Figure 1-20. IP over DWDM
7. Self-similar Nature of the packetized traffic
Self-similar process was considered in modeling cell traffic in modern communication networks,
in particular, telecommunication traffic in high-resolution Ethernet local area networks, wide-
area networks, and also for variable-bit-rate video traffic. This was motivated by experimentally
observed long-range dependence of traffic data and “burstiness” of traffic streams across an
extremely wide range of time scales [9, 10, 21].
A process is said to be self-similar when it exhibits correlation at all time lags. For instance
packet of network traffic looks the same when measured over time scales ranging from seconds
to minutes and hours, and it can be well modeled as having a heavy-tailed distribution. W.
Leland et al recorded Ethernet traffic of Bellcore for 27 hours [21]. Figure 1-21 shows plots of
the packet counts (number of packets/time unit) for 5 different time scales. Traffic “spikes” ride
on longer-term “ripples”, that in turn ride on still longer term “swells”. This graphic proof of the
self-similar nature of Ethernet traffic illustrates the statistical identity between Ethernet traffic on
one time scale and on a different time scale.
A self-similar process has a distribution that is heavy-tailed, i.e. its distribution follows a power
law asymptotically. That is,
∞→> − xxxXP as~][ α
EDFA
MU
XD
EM
UX
Traditional SONET ADMSONET ADM
IP
IP
IP
IP
ATM SONET ADMSONET ADM
SONET ADMSONET ADM
TxT
TxT
TxT
TxT
TraditionalSONET ADMSONET ADM
IP
IP
IP
IP
ATMSONET ADMSONET ADM
SONET ADMSONET ADM
RxT
RxT
RxT
RxTPhotonic Layer Protection
IP Recovery withoutPhotonic Layer Protection
31
where 0<α<2. One of the simplest heavy-tailed distributions is the Pareto distribution whose
probability density function is given by
where α is the shape parameter, and k is the location parameter. Its distribution function has the
following form
Figure 1-21 (a)-(e). Pictorial Proof of Self-Similarity: Ethernet Traffic on 5 different time scales
(Different gray levels are used to identify the same segments of traffic on the different time scales) [10]
Heavy-tailed distributions have a number of properties that are qualitatively different from
distributions more commonly encountered in network research, particularly, the exponential
distribution. If a = 2, the distribution has infinite variance, and if a = 1, the distribution has also
infinite mean. Thus, as a decreases, a large portion of the probability mass resides in the tail of
the distribution.
kxkxkxp ≥>= −− 0)( 1ααα
α
−=≤=
xkxXPxF 1][)(
32
The overall network utilization is given as ][][
][
offon
on
TETETE+
=ρ . When we know the utilization or
network load ρ and the mean of the ON periods ][ onTE , the offα parameter can be related to onα
by [13]
)1()1()1(
−−−−=
onon
onoff αραρ
αρα
Another quantitative measure of self-similarity is obtained by using Hurst parameter H [10],
which expresses the speed of decay of a time series’ autocorrelation function. As mentioned
earlier, a time series with long-range dependence has an autocorrelation of the form
Where 0<ß<1. Hence, when compared to the exponential decay exhibited by traditional traffic
models, the autocorrelation function of a long-range dependent process decays according to a
power-law. The Hurst parameters is related to ß via H=1- ß/2. So, for self-similar time series,
½<H<1. As H? 1, the degree of self-similarity increases. A test for self-similarity of a time
series can be reduced to the question of determining whether H significantly deviates from ½.
Traditional network traffic is modeled by Poisson processes, which become smoother as you
observe their behavior over longer time intervals. Self-similar renewal processes do not smooth
so that they are invariant with respect to the time period over which you observe them. Such
behavior has serious consequences when applied to traffic models since it indicates that you will
always get bursts of traffic at every time scale and you can never allocate enough queuing
resources to handle every situation [22].
In [12,13,14] the response of EDFAs to self–similar traffic was examined. This type of traffic
has a dramatic effect on gain transients and motivated the experimental work presented in later
chapters.
∞→− kkkr as~)( β
33
Chapter 2. Erbium-Doped Fiber Amplifier (EDFA)
Introduction
An alternative to optoelectronic regenerators has appeared in the last decade due to the discovery
of optical fiber amplifiers, notably the erbium-doped fiber amplifiers for the 1500 nm
communications band. These were first reported in 1987 to amplify optical signals directly and
thus make all-optical communication systems possible.
Erbium-doped fiber amplifiers are now widely used in long haul communication systems to
boost the signals. It is safe to say that, starting in 1989, erbium-doped fiber amplifiers were the
catalysts for an entirely new generation of high-capacity undersea and terrestrial fiber-optic links
and networks. EDFAs also reinvigorated the study of optical solitons for fiber-optic
transmission, since they now made practical the long distance transmission of solitons.
EDFA have been widely applied in optical networks as they have many unique properties which
are suited for optical communications [23, 24, 25], such as:
• They operate in the telecommunication wavelength band of 1530-1610 nm with high gain,
(Small signal gain > 40 dB), high output power (P > 100 mW) and low noise.
• High-power semiconductor laser diodes are practical sources to provide the light to pump
EDFAs.
• The EDFA is fiber compatible and can be spliced into transmission fibers with less than one
dB of insertion loss.
• The gain of EDFAs is unaffected by signal polarization.
• Because saturation occurs in EDFAs on such a slow time scale (~10 msec), EDFAs are
relatively immune to crosstalk among wavelength multiplexed channels or pulse distortion in
high-bit-rate systems.
34
EDFAs are developing so rapidly that they are soon expected to replace the optoelectronic
repeaters in many existing applications such as power amplifiers to boost transmitter power,
optical repeaters to amplify weak signals and optical preamplifiers to increase receiver
sensitivity.
Typical application of EDFA in point to point transmission systems and distribution systems is
shown in Figure 2-1.
Figure 2-1 (a) Amplifiers in point to point transmission system
(b) Amplifiers in networks
Tx RxEDFA EDFA EDFA
Power amp Repeater Preamp
(a)
(b)
Switch
EDFA
EDFA
EDFA
35
1. Basic EDFA Configuration
An EDFA consists of a short length of optical fiber (usually less than about 100 m) whose core
has been doped with less than 0.1 percent erbium, an optically active rare earth element that has
many unique intrinsic properties for optical amplification. For example, the erbium atom has a
metastable state with the remarkable long lifetime of 10 ms. This makes it possible to obtain an
optical gain which takes a long time to saturate.
Figure 2-2 shows the construction of a typical EDFA module. The erbium-doped fiber (EDF) is
compatible with conventional fiber and may be fusion spliced to other components. The pump
light is combined with the incoming signal by using a wavelength division multiplexer. Pump
light propagating along the EDF is depleted as erbium ions are raised to an excited state. As the
signal propagates in the EDF, it stimulates emission of light from the excited ions, thereby
amplifying the signal power.
Figure 2-2. Basic EDFA configuration
Currently an EDFA is constructed by fusion splicing discrete fiber-pigtailed components. It
mainly consists of an erbium-doped fiber, a wavelength division multiplexer and a pump light
source. In addition, a polarization insensitive optical isolator or an optical bandpass filter is
required to improve EDFA performance. The optical isolator is used to achieve stable amplifier
Signalout
EDFA
Isolator Filter
Signalin
Pump
WDM
36
operation (it prevents spurious oscillations). The filter is used to greatly reduce the amplified
spontaneous emission (ASE) and to protect the amplifier from saturation caused by ASE
accumulated in the in-line amplifier system. Care should be taken in applying isolators to EDFAs
in LANs. For example, we can’t use typical isolators in LANs with the star topology when the
signals are bi-directional. One solution is to keep the gain of EDFA small enough (around 15 dB)
to avoid using isolators.
In order to pump the erbium ions up to an upper energy level, there are several proper pump
wavelength bands. At present, 1480 nm and 980 nm high power laser diodes have proved to be
the two most efficient pump wavelengths.
In practice, there are three basic EDFA configurations. They are classified mainly according to
their pump light propagation direction. In forward pumping, the signal light copropagates with
the pump light whereas in backward pumping it counter-propagates relative to the pump light.
From the viewpoint of noise performance, forward may be more profitable [26]. On the other
hand, backward pumping may be effective for high-power output. In addition, one can have bi-
directional pumping with pump power traveling simultaneously in both directions.
37
2. Saleh and Sun’s Model
In this section we present a model of the amplification process of an EDFA, as described by
Saleh et al in [28]. As seen in Figure 2-3, the erbium ions in the EDFA can occupy one of these
states with increasing energy: the ground state ( 2/154I ), the metastable state ( 2/13
4I ), the excited
state ( 2/114I ).
Figure 2-3. Erbium Energy Level
First let’s introduce some symbols to describe a 2-level Er3+-doped fiber system (Figure 2-3.)
Table 2-1. EDFA parameters
the fraction of ions excited on metastable level N2 [0≤ N2≤1] the photon flux Qk [photons/s] the channels k, k=0… N metastable level lifetime τ [s] Er3+ ion density ρ [m-3] fiber core effective area A [m2] confinement factor of channel k Γk the emission cross-section of channel k σk
e [m2] the absorption cross-section of channel k σk
a [m2] σk
T=σke+σk
a the length of the Er3+ doped fiber L [m]
direction sign uk=l 11
0−
==
entering atentering at
zz L
38
Under the assumption of 980 nm pumping, the electrons of Er3+ions in the ground state are
excited to 4I11/2 level, from where they revert non-radioactively to 4I13/2 metastable level (lifetime
of the 4I11/2 level is about 1µs). In the case of 1480 nm pumping, electrons are excited directly to
the metastable level. To revert to thermal equilibrium distribution, the electrons on the 4I13/2 level
return to 4I15/2 ground level either spontaneously (spontaneous lifetime of the metastable level in
the Er3+-doped silica glass is about 10 ms), or they can be stimulated to do so by some (signal)
photons travelling down the Er3+-doped fiber. Photons generated by stimulated emission have
the same wavelength, phase and polarization as the incident photons. Energy of the incident
photons must, however, correspond to energy difference between the metastable and the ground
energy level. Spontaneously emitted photons have random wavelength, phase, and polarization
and propagate through the active fiber in both the positive and negative direction of the fiber
axis. They are amplified and result in an amplified spontaneous emission (ASE) –- the main
source of EDFA noise.
By neglecting the population of the 4I11/2 pump level or by assuming 1480 nm pumping scheme,
a two-level approximation of an Er3+-doped fiber can be used. When we further assume that the
Er3+ -doped fiber represents a homogeneously broadened gain medium, there is no excited state
absorption, no background loss, and no self-saturation by ASE; the behavior of an EDFA can be
described by the following two equations: the rate equation for the fraction of excited ions at the
metastable level N2, and by the propagation equation for photon flux Qk.
∂∂ τ ρ
∂∂
N z tt
N z tA
uQ z t
zjj
j
N2 2
0
1( , ) ( , ) ( , )= − −
=∑ (2.1)
∂∂
ρ σ σQ z tz
u N z t Q z tkk k k
Tka
k( , )
( , ) ( , )= −Γ 2 (2.2)
where z is the linear displacement on the ions along the doped fiber and t is time. The index k in
the photon flux kQ refers to the channel number (ultimately the wavelength) of the signal. Other
quantities are as defined in Table 2-1. With a total of N channels, (2.1) and (2.2) represent a
system of N+1 partial differential equations describing the dynamics of an EDFA. Resolution of
the nonlinear partial differential equation is quite numerically intensive and a simple model was
proposed by Bononi and Rusch [28].
39
3. Bononi and Rusch’s Reservoir model
If we make some transformations to (2.1, 2.2), we can achieve Bononi and Rusch’s Reservoir
model:
Dividing both sides of (2.2) by Qk ≠ 0 , multiplying by dz and integrating from z=0 to L we
obtain
G t B r t Ak k k( ) ( )= − , k=0,......,N (2.3)
where
])()(
ln[0 tQ
tQQ
QuG in
k
outkL
k
kkk == ∫ ∂
(logarithmic gain) (2.4)
and A Lk k ka= ρ σΓ and B
Akk k
T
= Γσ are non-dimensional parameters.
Multiplying (2.1) by dz, integrating along the fiber axis from z=0 to L and substituting for Qkout
from (2.4), a single first order ordinary differential equation (ODE) is obtained, which describes
the time evolution of the length averaged metastable level population r(t)
∑=
−−+=N
j
AtrBinj
jjetQtr
trdtd
0
)( )1)(()(
)(τ
(2.5)
where r(t), the “reservoir”, represents the total number of excited ions in the amplifiers:
r t A N z t dzL
( ) ( , )= ∫ρ 20 max)(0 rtr << (rmax=ρAL is the total number of ions in the doped fiber).
When normalizing the state variable r(t) to rmax , a first order ODE for the normalized reservoir
x tL
N z t dz r trM
L( ) ( , ) ( )= =∫1
20 can be derived
40
∑=
−Γ−+−=N
j
txLinj a
kTkke
ALtQtx
txdtd
0
))(( )1()()(
)( σσρ
ρτ (2.6)
The reservoir model can be understood clearly by the following figure:
Figure 2-4. Reservoir model
Excited ions are represented by “water” in the reservoir. The pump fills the reservoir, except for
some leakage due to fluorescence. As signals enter from the left and exit to the right they draw
off ions from the reservoir and experience a wavelength dependent gain which varies with the
level of the reservoir.
Note that the set of 1+N coupled nonlinear partial differential equations in (2.1) and (2.2) have
been reduced to a single nonlinear ordinary differential equation in (2.5) vastly simplifying
simulation. The equation can be further simplified under certain conditions as described in the
next section.
41
4. Approximate Solutions
The Reservoir model is convenient to calculate the EDFA gain transient, which is caused by
1. network reconfiguration;
2. periodic addition/removal of channels or network failures;
3. sudden loss of many channels.
Numerical solution to the ODE (2.5) or (2.6) for an analysis of concatenated EDFAs may be
rather time consuming when packetized bursty traffic is considered. Further linearization of the
numerical problem is necessary to make the numerical calculations easier and faster.
Two separate approximations of the models presented have been proposed. Both approximations
lead to a exponential model for reservoir and gain, but with differing exponential time constants.
We develop these approximations here in order to compare and contrast results.
• Bononi & Rusch’s method [28]
We start from a small signal solution and use exponential approximation.
For small t, we use
))0()(())0()(( rtrBe krtrBk −≈− (2.7)
So the ODE solution of (2.5) yields
)1)(0(')0()( / ete errtr ττ −+ −+= (2.8)
where )0(' +r is the time derivative of the reservoir just after time t=0.
Suppose that the solution is of the form
etYeXtr τ/)( −+=
We force r(0), r(∞ ) and r’(0) to follow this, so
42
eterrrtr τ/)]()0([)()( −∞−+∞= (2.9)
where )0('
)0()(+
−∞=r
rreτ follows from these assumptions.
• Sun et al’s Approximation [27]
Start from the following:
])(exp[)( 000 tgLgLgQtQ nnninn
outn ∆+= (2.10)
where
=
)()(1
)(tQtQ
InL
tg inn
outn
n is the average exponential gain
We can easily get
)/exp(
)()0(
)()(τt
outn
outnout
noutn Q
QQtQ
−
∞∞= (2.11)
where
∑=
+=
N
iISi
outi
1
0
0
1
ττ , QiIS is each channel’s saturation photon flux
• Transforming Bononi’s solution to Sun’s
According to our Reservoir model, the gain is
kkk AtrBtG −= )()(
And the output is
)()()( tGnk
outk
ketQtQ =
The approximation is
eterrrtr τ/)]()0([)()( −∞−+∞≈ (2.12)
43
SSr (8 ) is the steady state of r.
We go further
)/exp(
/
)()0(
)()]()0([)(exp)(e
e
t
outk
outkout
kktSS
kSS
kink
outk Q
QQAerrBrBtQQ
ττ
−−
∞∞=−∞−+∞= (2.13)
We can see that reservoir approach gives the same form as Sun’s method; it just differs in τ.
In the next section we present simulation results for these two approximations which differ only
in the exponential time constant.
Figure 2-5. A simple WDM system
Figure 2-5 shows a simple WDM system. We will use our Reservoir model to simulate the
surviving channel (Channel 1)’s power excursion and the change of Reservoir when 7 channels
are added/dropped, and also when 4 channels are dropped.
Our simulation actually supposes that the EDFA has two input channels =1CHλ 1552.4 nm and
=2CHλ 1557.9 nm, with initial input powers =1CHP -2 dBm, =2CHP -2+10 )7(log10 dBm,
simulating the remaining 7 channels of an eight-channel system with –2 dBm/channel. EDFA is
pumped at 980 nm, =pumpP 18.4 dBm, L=35 m, t=10.5 ms. The absorption coefficients are
[0.257, 0.145, 0.125] 1m − and the intrinsic saturation powers are [0.440, 0.197, 0.214] mW at
[980, 1552.4, 1557.9] nm, respectively. The system is at equilibrium before t=0. At t=0 part of
the power on channel 2 is dropped, simulating the drop of a given number channels [28].
MU
XD
EM
UX
EDFA
CH1
CH2
CH1
CH2
CH3 CH3
CH8
CH7CH7
CH6CH6
CH5CH5
CH4CH4
CH8
44
Figure 2-6. Add/Drop simulation using Reservoir model
Figure 2-6 shows the dynamics of the Reservoir and output power excursion of channel 1
defined as ])0(
)([log10
1
110 +out
out
QtQ .
Figure 2-7. Our Add/Drop Experiment
Figure 2-7 is our Add/Drop experiment result measured by the photodetector, the vertical axis
stands for the output from the surviving channel 1 in dB scale. We can see that the responses of
the channel 1 to other channel’s add/drop have the same trends as our simulation, and it shows
that our reservoir model is correct to model the real add/drop situations.
0 20 40 60 80 100 120 140 160 180 1.1 1.2 1.3 1.4 1.5 x 10 14
Time (us)
Res
ervo
ir
7 ch add
7 ch drop
4 ch drop
0 20 40 60 80 100 120 140 160 180 -10 -5 0 5
10
Ch
1 P
ower
Exc
ursi
on (
dB)
Time (us)
7 ch add
7 ch drop 4 ch drop
0 20 40 60 80 100 120 140 160 180 200 -8
-7
-6
-5
-4
-3
-2
-1
0
1
PD
Out
put (
dBV
)
Time (10 us)
Channel Add Channel Add
Channel Drop Channel Drop
45
• Sun et al’s experimental results.
Figure 2-8. Sun’s results [29]
We can compare Figure 2-7 and Figure 2-8 (a) and see the same exponential rise (after a drop)
and exponential decay (after an add). Furthermore, in Figure 2-8 (b), Sun et al fit their data to an
exponential curve, but with the time constant determined numerically, not using equation (2.11).
In the next section we explain three simulations:
1). Bononi time constant--- equation (2.9)
2). Sun’s time constant---equation (2.11)
3). Numerical fit to the time constant
We compare the exact solution, exponential approximation and Sun’s method (Figure 2-9.), from
which we can see that Bononi’s exponential approximation is closer to the real case and hence is
better. It fits well in the small signal region, when t grows, it differs more from the real solution,
but as t tends to infinity, it converges to the real case again. When there is less change, the
difference between the exact solution and the approximation is smaller, but when the add/drop is
bigger, the difference is more evident.
46
Figure 2-9. Real case and approximations
• Our fit method
We can see that the EDFA gain dynamics has an exponential trend, which gives us some
intuition to make some exponential fit. But we can see it is not ideal in the small t region, so we
make a linear fit for small t, while putting an exponential fit for larger t, and it works well in both
7 and 4 channels data cases (Figure 2-10).
Figure 2-10. Some fit methods
0 20 40 60 80 100 120 140 160 180 2001.1
1.15
1.2
1.25
1.3
1.35
1.4x 10
14
Time (us)
7 channels data
Exact SolutionBononi (taue=93.49 us)Sun (taue=202.18 us)Exponential Fit (taue=63.88 us)Small Linear FitMiddle Expo Fit (taue=56.57 us)
0 20 40 60 80 100 120 140 160 180 2001.14
1.15
1.16
1.17
1.18
1.19
1.2
1.21
1.22
1.23
1.24x 10
14
Time (us)
4 channels data
Exact SolutionBononi (taue=63.04 us)Sun (taue=83.03 us)Exponential Fit (taue=53.03 us)Small Linear FitMiddle Expo Fit (taue=50.74 us)
0 20 40 60 80 100 120 140 160 180 2000
0.5
1
1.5
2
2.5
3x 10
17
Pow
er (p
hoto
ns)
Time (us)
1 Channel Drop
4 Channels Drop
7 Channels Drop
Exact Solution BononiSun
47
5. EDFA Cascade
A serious problem facing wavelength division multiplexed networks with fiber amplifier
cascades is transient cross saturation or gain dynamics of fiber amplifiers. Attention has been
focused primarily on circuit-switched scenarios. When the number of WDM channels
transmitted through a circuit-switching network varies, channel addition/removal will tend to
perturb signals at the surviving channels that share all or part of the route. Power transients in the
surviving channels can cause severe service impairment due to either inadequate eye opening or
the appearance of optical nonlinear ties [1].
Signal power excursions more serious than those induced by channel addition/removal in circuit
switched networks can arise when data on the WDM channels is highly variable in nature. Self-
similar traffic can lead to large variation in EDFA gain.
When self-similar packet traffic is directly transmitted in burst-mode on the WDM channels, as
in the case of Internet Protocol (IP) over WDM, long inter-burst idle intervals may give enough
time to fiber amplifiers to reach gains greatly exceeding the average values. This can in turn lead
to significant variation in output power and optical OSNR. This effect accumulates along a
cascade of fiber amplifiers in the same way as the fast power transients in the circuit-switching
scenario. The effect of WDM traffic statistics on the output power and OSNR swings in a
cascade of five EDFAs of standard design has been theoretically investigated in [31, 32]. The
results of the simulations indicate that substantial power and OSNR swings occur at the output of
a cascade when highly variable burst-mode traffic is transmitted. Power swings in excess of 9 dB
and OSNR swings of more than 4 dB were observed. The stabilization effect of clamping the
gain of the first EDFA by all-optical feedback loop and letting the lasing power propagate
through the cascade of six EDFAs has been studied in [33].
In order to prevent unacceptable error burst, the channel power should be maintained constant.
EDFAs for WDM systems do not have a very flat gain-wavelength figure (Figure 2-13), which
also tremendously varies due to saturation when the input power is large. In the design of
optically amplified links for WDM application, in which the number and the power level of the
input channels may vary randomly in time as in a packet switched scenario, it is thus important
48
to stabilize the EDFA’s gain profile. Gain-clamped EDFA can stabilize the average inversion
and thus clamp the gain to the desired level, which also motivates our measurements of cascades.
Our results show that output power and OSNR swings can be effectively suppressed if the first
amplifier of the cascade is gain clamped using an all-optical feedback loop and the lasing power
generated in the gain clamped EDFA propagates along the cascade.
6. Gain-Clamped EDFA
• Setup
Figure 2-11. Gain-clamped EDFA
Figure 2-11 shows a clear setup of a gain-clamped EDFA with a ring laser configuration. The
positive optical feedback leads to instability, and if the EDFA gain is initially larger than the
loop loss, the system starts oscillating at the wavelength selected by the bandpass filter centered
at a wavelength λ1.
Figure 2-12. A simple and practical gain-clamped EDFA design
Circulator plus Bragg grating form an optical bandpass filter as shown in Figure 2-11. But in
actual cases, both are very expensive, so an alternative simple and practical design is proposed
below (Figure 2-12), which can avoid bandpass filter and circulator.
CouplerCoupler 2
EDFA
Bragg Grating
Output
Input
3dBCoupler 2
3dBCoupler 3EDFA
Bragg Grating
Circulator
Input Output
Coupler 1 Coupler 2EDFA
BandpassFilter
InputOutput
49
• Analysis
We assume that the erbium ion has two energy levels and a homogeneously broadened gain
spectrum. We can get a new equation, taking into account the self-saturation induced by the
amplified spontaneous emission (ASE), so (2.5) is updated to
))(()()]([1)()(,,
trQtrtrGtQtrdtd
ASElpSj
jj −−−= ∑∈ τ
(2.14)
where Q r tASE ( ( )) is the ASE flux
When considering the optical feedback, we just let the laser input flux as a delayed and
attenuated version of the output flux, to which the ASE term Ql ASE, passed by the feedback filter
is added
)]([)]([)()( , lASElllll trQtrGtQtQ τττα −+−−= (2.15)
τ l is the loop propagation delay and 10 ≤≤α the loop attenuation, both at laser wavelength λl .
Equation (2.14) and (2.15) form the model of the gain-clamped EDFA. There are now 2
variables, the other is the laser flux Q tl ( ) .
In [30], Bononi et al analyzed these equations, and produced the curves shown in Figure 2-13.
Figure 2-13. Gain versus average inversion and wavelength [30]
50
According to equation (2.3), the gain in dB is
)(34.4log10)(log10 1010 jjjjArB ArBeArBe jj −=−=−
We plot the gain against wavelength, and inversion that is maxrrx = . For a fixed value of the
reservoir r, or the equivalent x, we have the well-known gain-wavelength profile. The variation
of x caused by the input power variation causes the undesirable profile changes. The gain has a
linear dependence on r.
The loop filter passes only wavelength lλ , i.e., chooses the straight line corresponding to the
laser gain shown on the surface (Figure 2-13). The horizontal contour line on the surface marks
the level corresponding to the loop loss at wavelength lλ (the loop loss at all other wavelengths
is infinity). The laser flux grows till its gain equals the loop loss, thus fixing the inversion and the
gain to a desired value. The desired inversion can be changed by either changing the loop loss for
fixed lλ (thus moving along the laser gain line), or by changing lλ for fixed loss (thus moving
along the loop loss contour).
The equilibrium point at the intersection of the loop loss contour and the laser gain line is stable.
Actually, if some channels are dropped, less reservoir ions are consumed, x tends to increase, and
so does the laser gain and the laser flux, which grows to consume the excess reservoir ions and
brings x back to its clamped value. If some channels are added, more reservoir ions are
consumed, x tends to decrease, and so does the laser gain and the laser flux, which costs less
reservoir ions and bring x back to its clamped level.
51
8. Cross-Gain Modulation
Figure 2-14. Setup
Gain-clamped EDFA is very effective in suppressing the surviving channel’s output excursion in
the Add/Drop scenario. In order to measure these effects, we assembled an experimental setup.
According to the setup of Figure 2-14, one channel is modulated by a square wave at 1 kHz to
simulate Add/Drop, and another probe channel is a CW channel that stands for the surviving
channel. We observe the CW channel’s output.
Figure 2-15. Cross gain modulation
1k Hzmodulated
signal1550nm
3dBCoupler 1
CW Laser1555.08nm
3dBCoupler 2
3dBCoupler 3
EDFA
Filter 1560 nm
PhotoDetector
Data Acquisition
Brag Grating 1555.08nm
Circulator
0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 2
4
6
8
10
Time (2.5 us)
PD
Out
put (
V)
Crossgain Modulation for Low Frequency(1 kHz) Unclamped Clamped
2 3 4 5 6 7 8 9 10 -6
10 -4
10 -2
10 0
PD
F
PD Output (V)
Unclamped Clamped
52
We see from Figure 2-15 that, the CW channel is also modulated, which is called cross-gain
modulation, while under gain-clamped circumstances, the amplitude of the modulation are
reduced greatly, from 4 V to 0.2 V. The lower plot is the Probability Density Function (PDF) of
the output; we will also discuss this later in Chapter 4.
The cross-gain modulation just occurs in the kHz range. When we repeat the experiment above
while raising the frequency up to hundreds of kHz, cross-gain modulation disappears.
53
Chapter 3. Experimental Setup
In this chapter we provide a detailed description of the experimental setup used to measure the
dynamic response of EDFAs to live Ethernet traffic that is self-similar in nature. In Chapter 4 we
will examine experimental results.
1. General Setup
Our experimental setup is shown in Figure 3-1. Electrical signal from the Ethernet Hub----
CenterCOM MR820TR for 10 Mbps (Appendix II) which carries all traffic of the LAN is
converted by an Ethernet transmitter to optical signal and fed into the gain-clamped (when the
toggle T is on) or unclamped (when the toggle T is off) EDFAs cascade. We used Corning Gain
Module EDFAs concatenated with five 20dB fiber attenuators that represent the loss of
transmission fibers.
3dB Coupler 1
CW Laser 1556.27nm
3dB Coupler 2
3dB Coupler 3
EDFA
Live Ethernet Data at 10 Mb/s
(Electrical Signals)
Ethernet Transmitter 1550nm
(Optical Signal) T
EDFA EDFA ATTN ATTN
Photo Detector
Data Acquisition
Bragg Grating 1556.27nm
Circulator
Bandpass Filter 1560 nm
Variable Attenuator
Figure 3-1. Experiment Setup
54
In order to monitor the effect of cross gain saturation in individual EDFAs on the bursty LAN
traffic, a CW signal at 1556.27nm is introduced at the input of the EDFA coupled via a 3dB
directional coupler. The Ethernet optical transmitter (1550nm) is used to represent several
channels of a WDM system. Therefore, the average power of this signal channel at the input
port of the EDFA was set to –13dBm and the power of the CW monitoring channel was set at
20dBm. Power fluctuations in the CW signal represent changes in the inversion level (or
Reservoir) and hence reflect gain variation of the EDFA. The power of the CW channel at the
output of the individual EDFAs along the cascade caused by bursty Ethernet traffic are
detected after filtering out this signal channel using an optical band pass filter (JDS-Fitel
CR2500 circulator with a Bragg grating at 1556.27 nm). The electrical signal from the ANTEL
ARX-GP photodetector is processed in a National Instrument’s PCI6110-E data acquisition
card inside a PC.
2. Ethernet Optical Transmitter
The block diagram of the Ethernet Optical Transmitter is depicted in Figure 3-2----Detailed
schematic diagram is in Appendix I.
EthernetDiffrential Input
Laser Driver
ECLComparator
Laser 1550nm
Figure 3-2. Logical Structure of the Ethernet Optical Transmitter
The ECL comparator first converts bipolar Ethernet signal to unipolar one and feeds an ECL
level signal to the Fujitsu laser driver FMM3171VI, then the driver drives the Fujitsu laser
FLD5F8LK. Ethernet electrical signal is thus converted to optical signal.
In the following we will introduce each component individually.
55
• ECL Comparator
Figure 3-3. AD96685BR ECL Comparator
The Analog Device’s ECL (Emitter-Coupling Logic) comparator AD96685BR is a single
ultrafast voltage comparator with 2.5 ns propagation that is a particularly important characteristic
of high-speed comparators. It is a measure of the difference in propagation delay under differing
overdrive conditions.
A fast, high precision differential input stage permits consistent propagation delay with a wide
variety of signals in the common-mode range from –2.5 V to +5 V. Outputs are complementary
digital signals fully compatible with ECL 10 K and 10 KH logic families. The outputs provide
sufficient drive current to directly drive transmission lines terminated in 50 O to –2 V. A level
sensitive latch input is included which permits tracking, track-hold, or sample-hold modes of
operation.
• Laser Driver
The FMM3171VI is a laser driver IC for up to 1.2 Gb/s optical transmission systems. The GaAs
MES-FET IC process allows for high-speed operation with low power consumption. There are
two data inputs options; a complementary input or a single-ended input using a selectable D-F/F.
The output duty ratio can be set by adjusting the input reference voltage (Vref). The differential
mark-density monitor (Mmk) output is proportional to the peak current output signal. The peak
current and bias current output are disabled by the shutdown terminal, which is ECL compatible.
56
Logic “high” causes the output to be disabled. The high speed turn-on and turn-off is
accomplished by a very low compute time constant. The amount of peak current and bias current
are monitored by the current flow at lp and lb terminal which must be connected to VSS. This
FMM3171VI is an excellent choice to be used as a laser driver for OC-3/OC-12/OC-24.
Figure 3-4. FMM3171VI Laser Driver
The laser driver’s features are listed in Table 3-1
Table 3-1 Laser Driver Features
ECL Compatible Data Input Modulation Current and Bias Current Monitor
Built in D-F/F (Optional) Duty Ratio Monitor
Complementary Data Input (Optional) Duty Ratio Control
Modulation Current: 0 to 70mA Single Power Supply: -5.2V
Bias Current: 0 to 70mA Separated Peak Current and Bias Current Outputs
Output Shutdown Function Small Package: SSOP-16
57
• Laser
Figure 3-5. FLD5F8LK DFB Laser
Fujitsu FLD5F8LK laser, a Quantum Well (MQW) DFB Laser is for use in long haul 156/622
Mb/s transmission systems. The uncooled, coaxial design offers a low cost/simplified design
alternative to cooled laser solutions. The module includes a DFB laser and monitor photodiode.
Central wavelength of the Fujitsu laser FLD5F8LK is at 1551.41 nm, the –30 dB line width is
0.5 nm, and output power is adjustable and at least 2 mW.
1547.19 nm to 1555.31nm
Figure 3-6. FLD5F8LK Laser’s Spectrum
58
-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1 x 10 -4
-7
-6
-5
-4
-3
-2
-1
0 x 10 -3
Time (s)
Out
put o
f the
Pho
tode
tect
or (V
)
Figure 3-7. Transmission Performance in packet level
Speed of the Ethernet Optical Transmitter has been tested by square wave input electrical signal.
The optical output power has been monitored by a fast PIN FET photodiode. Figure 3-7 shows
the detected signal, Figure 3-8 depicts the zoomed fall edge of the transient response.
Figure 3-8. Laser’s Transient response in bit level
-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5
x 10-8
-0.01
-0.009
-0.008
-0.007
-0.006
-0.005
-0.004
-0.003
-0.002
-0.001
0
(a)
Time (s)
Pho
to D
etec
tor O
utpu
t (V
)
-1.5 -1 -0.5 0 0.5 1 1.5 2
x 10-9
-0.01
-0.009
-0.008
-0.007
-0.006
-0.005
-0.004
-0.003
-0.002
-0.001
0
Pho
to D
etec
tor O
utpu
t (V
)
Time (s)
(b)
Bit level performance Zoom of (a)
59
We can see from Figure 3-8 that the fall time is 2.5 ns which is fast enough to capture the
ethernet traffic.
• Printed Circuit Board (PCB)
Photograph of the Ethernet Optical Transmitter is shown in Figure 3-9.
Figure 3-9. Overview of the Ethernet Transmitter
This Print Circuit Board (PCB), designed, assembled and tested by Ye Chen and Jean-François
Cliche, consists of 4 layers: Surfacemount parts layer, -5.2 V layer, +5 V layer and Ground
layer, under this case we can get very clean positive and negative power supplies, and avoid
ground loop. The assembly and debug instructions can be found in Appendix I.
3. 3 dB Coupler
Figure 3-10. 3dB Coupler
P1
P2
(P1+P2)/2
(P1+P2)/2
60
We use Fiberdyne Labs Inc.’s 50/50 single Window Coupler of FSC-022-250-022, 2×2, and
1550 nm. It will divide each of the 2 input in half (3dB) as output (Figure 3-10).
4. EDFA
The EDFA (Figure 3-11) is Corning Gain Module, which is a preamplifier with a small signal
gain of around 30 dB. Figure 3-12 is its spectrum of amplified spontaneous emission (ASE).
Figure 3-11. EDFA
Figure 3-12. EDFA amplified spontaneous emission (ASE) Character from 1514.86 nm to 1592.96nm
61
5. Circulator and Bragg Grating
The optical circulator is non-reciprocal device that redirects light from port to port sequentially
in one direction only.
Figure 3-13. Optical Circulator and Bragg Grating
From Figure 3-13, the light first input Port 1, and is then reflected by the Bragg grating at certain
wavelength from Port 2 and redirected to Port 3. So they together serve as a bandpass filter.
6. Photodetector
Figure 3-14. ANTEL ARX-GP amplified ultra high-speed photodetector
We use ANTEL ARX-GP amplified ultra high-speed photodetector system (Figure 3-14) to
receive the CW channel data, which is ideal for high-speed, low level light detection from 300-
1700 nm. It consists of a high-speed non–avalanche type PIN photodiode matched with a DC-
coupled microwave GaAs amplifier to provide state-of-the-art gain-bandwidth. The
Port 1
Port 2
Port 3
62
photodetector and amplifier circuitry are housed in a head with a flat window. Power of the
monitoring channel at the input of the optical bandpass filter is focused on the window of the
photodetector using a 10X microscope objective. Signal output is through a 50 Ω -matched SMA
connector on the detector head and a 1-m length of coaxial cable for connection to any 50 Ω
input impedance equipment. Bias voltages for the photodetector and amplifier are supplied from
a separate rechargeable battery-operated power supply.
7. Data Acquisition Hardwares
The Data acquisition hardware setup is shown in Figure 3-15. The output electrical signal from
the photodetector is fed by a coaxial cable terminated with BNC connector to BNC-2110
Accessory board. The BNC-2110 is connected to the PCI-6110E with a shielded cable. Data is
finally transferred to the computer hard drive.
Figure 3-15. Data Acquisition Scheme
• PCI-6110E Data Acquisition Card
For acquisition of the time variable CW channel output power we use the National Instrument’s
PCI-6110E data acquisition hardware. The 6110 E board is a completely Plug and Play,
( NI PCI-6110E)
63
multifunction analog, digital, and timing I/O board for PCI bus computers. The 6110 E board
features a 12-bit ADC per channel with four simultaneously sampling analog inputs, 16-bit
DACs with voltage outputs, eight lines of TTL-compatible digital I/O, and two 24-bit
counter/timers for timing I/O. The 6110 E board is also easily software-configured and
calibrated.
• BNC-2110 Accessory
Figure 3-16. BNC-2110 Accessory
Figure 3-17. Front Panel of BNC-2110
The BNC-2110 (Figure 3-16, Figure 3-17) is a desktop and DIN rail-mountable BNC adapter we
can connect directly to data acquisition (DAQ) devices. The BNC-2110 includes 15 BNC
connectors and a digital screw terminal with 30 pins.
64
The BNC-2110 has a 68-pin I/O connector that connects to the PCI-60XXE and PCI-61XXE
DAQ devices. The BNC-2110 is ideal for simplifying connections between our measurement
apparatus and our DAQ device in laboratory, test, and production environments.
BNC-2110 Connector Signal Description
Front Panel BNC
Connectors
Signal Description
ACH<0..7> Differential Analog Channels 0~7
DAC0OUT Digital to Analog Converter 0 Output----This pin supplies the voltage output of analog output channel 0.
DAC1OUT Digital to Analog Converter 1 Output----This pin supplies the voltage output of analog output channel 1.
EXTREF External Reference----This is the external reference input for analog output circuitry.
PFI0/TRIG1
Programmable Function
Input 0/Trigger 1----As an input, this is either one of the PFIs or the source for the hardware analog trigger.
CTR0OUT Counter 0 Output----This is the output from the General-Purpose Counter 0 Output.
USER1 User-Defined 1----Connected to USER1 digital screw terminal block. This connector allows you to modify a signal depending on your application
USER2 User-Defined 2----Connected to USER2 digital screw terminal block. This connector allows you to modify a signal depending on your application
65
8. Data Acquisition Softwares
We use National Instrument’s NI-DAQ driver (Figure 3-18) for Windows and LabVIEW as
acquisition software.
Figure 3-18. NI-DAQ driver for Windows
NI-DAQ also internally addresses many of the complex issues between the computer and the
DAQ hardware such as programming interrupts and DMA controllers. NI-DAQ maintains a
consistent software interface among its different versions so that we can change platforms with
minimal modifications to our code. Whether we are using conventional programming languages
or National Instruments application software, our application uses the NI-DAQ driver software,
as illustrated in Figure 3-19.
66
Figure 3-19. Relationship between the Programming Environment, NI-DAQ and the Hardware
Data is acquired and stored in a binary format using software shown in Figure 3-20, and then it is
converted to ASCII format with a software shown in Figure 3-21. A short Matlab program has
been developed to calculate the Probability Density Function (PDF).
Figure 3-20. LabVIEW DAQ Software Interface
10000.00Scan Rate
10000Buffer Size
5000
# of Scans toWrite at a Time
0Scan Backlog
0
# scans written to file
500000
Max # of scans to write to file
- 1 . 0 0 E + 0
I n t e r c h a n n e l D e l a y ( s e c s )
( -1 :hardw a re defau l t )
H a rdw a re Set t ings
1 . 0 0
high l imit
-1.00
low l imit
0
inpu t l im i t s (no change )
1Device
from your PCStart Date
10:23:01 AMStart Time
STOP
Photo Detector OutputFile Header Text
Output InformationAcquisition Settings
0Channels
ConventionalProgrammingEnvironment
ComponentWorks,LabVIEW,
LabWindows/CVI, orVirtualBench
NI-DAQDriver Software
DAQ orSCXI Hardware
Personal Computeror Workstation
67
The LabVIEW software (Figure 3-20) first acquires the data and stores them in binary format,
then we use another program followed (Figure 3-21) to read and convert them to ASCII format.
Figure 3-21. Read and Convert Program Interface
1000
Block Size toRead at a Time
4 N u m b e r o f C h a n n e l s
in File
1650
0
binary data (I16)
0
# of Scans Readfrom File so far
10 ,00
upper input l imi ts
-10,00
lower input l imi ts
10 ,00
range
bipolar
polar i ty
0 ,50
gain
DC
coup l i ng
di f ferent ia l
i npu t mode
4 , 8 8 2 8 1 E - 3
sca le mul t ip l ie r
0 ,00
sca le o f f se t
0
c h a n n e l0
g r o u p c h a n n e l s e t t i n g s
0 ,80570
0
voltage data
0,00Scan Rate
2,00E-6 I n t e r c h a n n e l D e l a y ( s e c s )
HARDWARE CONFIGURATION FROM FILE
0
Channel L is t in F i le
from your fileStart Date
from your fileStart Time
0
-0
1 0 0 00
0
1
2
3
4
5
6
7
LOGGED DATA
Your header text will appear here.File Header Text
STOP
NEXTBLOCK
mus t be l ess t han t he
amount o f da ta in f i l e
68
Chapter 4 Experimental Results
Introduction
In this chapter we will present measurements of the gain swings in EDFAs handling live
Ethernet data that is self-similar nature.
We will first present results for the time domain and show how long intervals of ON times (when
data is being transferred on the network) and OFF times (when the network is idle) lead to
transients in the EDFA gain. The activity or utilization factor is the percentage of time the
network is not idle. We will examine the response for a single amplifier characterized by the
exponential rise and fall we discussed in chapter 2. Cascades of EDFAs give rise to undershoots
and overshoots in the gain time response (hence gain swings). Finally we will see how clamping
reduces these gain transients significantly.
Next we discuss how the network was manipulated in order to vary the ON and OFF times of the
network, as characterized by the network activity factor. Ethernet data is self-similar, so we
expect that long ON times and long idle times to occur throughout the observation interval,
leading to variation in the gain. We will measure the gain throughout the observation interval
and plot the frequency of occurrence of each gain value, that is a histogram that approximates the
probability density function (PDF) of the gain.
The measurements of the estimated PDF are presented for different network activity factors in a
single EDFA and for a cascade of amplifiers. We will see that extent of gain variation varies
little with activity factor, however the gain variation broadens as we traverse the cascade. Gain
clamping is used to mitigate the gain variation and we examine the time response and gain
variation for single amplifiers as well as cascades.
Finally we compare our results for live Ethernet data with measurements using synthetic data to
approximate a self-similar source, and discuss some challenges we had in implementing the
experiment and the solutions we found.
69
1. Time response to variable data
This section presents the time domain evolution of a single EDFA and along a cascade of EDFA.
• Single Amplifier
Figure 4-1 Time evolution of photodetector voltage corresponding to fluctuations of the monitoring channel power at 1556.27nm recorded at the output port of directional coupler no.3
(a) one second sample, (b) zoomed part of (a)
Figure 4-1 shows the 1st EDFA time domain response when the utilization factor equals 76.5%.
This gain variation is reflected in power fluctuations of the monitoring channel. The whole
sample of 1 second is displayed in Figure 4-1(a), and a zoomed interval from 53 to 59 ms is
shown in Figure 4-1 (b). Three packets of 1.1 millisecond duration and an interval of 2.31ms of
no traffic are shown in Figure 4-1 (b). The 2.31ms long interval of no traffic gives the amplifier
enough time to recover the population inversion of the metastable energy level and the EDFA
gain. The simulations using reservoir model and experiment with periodic input in chapter 2 also
indicate the response for a single amplifier featured by the exponential rise and fall.
• Cascade of EDFAs
Figure 4-2 shows the time evolution of EDFA No. 3 (Figure 4-2 (a)), and EDFA No. 5 (Figure 4-
2 (b)). From their corresponding zooms, we see that overshoots and undershoots appear after the
3rd EDFA, which is identical to Sun et al’s simulations [1]. This warns us that gain swings will
spread along the EDFAs cascade and thus harm the WDM system if we take no measure to deal
with those transients.
0,0 0,2 0,4 0,6 0,8 1,0
-0,16
-0,14
-0,12
-0,10
-0,08
U pd ,
V
time, sec
53 54 55 56 57 58 59
-0,16
-0,14
-0,12
-0,10
-0,08
no traffic
traffic ON
U
pd , V
time, ms
(a) (b)
70
Figure 4-2. Time response of the EDFAs Cascade
Gain clamping greatly reduces these gain transients as shown in Figure 4-3. We will discuss
clamping in greater detail in a later section.
Figure 4-3. The effectiveness of gain clamping
0 200 400 600 800 1000 -0.24 -0.22 -0.2
-0.18 -0.16 -0.14 -0.12 -0.1
-0.08
PD
Out
put (
V)
Time (ms)
(b). EDFA No. 5 Time Response (utilization=72.5%)
900 910 920 930 940 950 960 -0.24 -0.22 -0.2
-0.18 -0.16 -0.14 -0.12 -0.1
-0.08 undershoot
overshoot P
D O
utpu
t (V
)
Time (ms)
zoom of (b)
0 200 400 600 800 1000 -0.2
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08
PD
Out
put (
V)
Time (ms)
(a). EDFA No. 3 Time Response (utilization=77%)
80 90 100 110 120 130 140 150 160 -0.2
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08 undershoot
overshoot
zoom of (a)
PD
Out
put (
V)
Time (ms)
0 100 200 300 400 500 600 700 800 900 1000 -0.2
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08
-0.06
Time (ms)
PD
Out
put (
V)
(a). EDFA No. 3 Time Response (utilization=77%) Unclamped Clamped
0 100 200 300 400 500 600 700 800 900 1000 -0.24 -0.22 -0.2
-0.18 -0.16 -0.14 -0.12 -0.1
-0.08
Time (ms)
PD
Out
put (
V)
(b). EDFA No. 5 Time Response (utilization=72.5%) Unclamped Clamped
71
2. Ethernet Data
As follows from the theoretical studies [31, 32, 33], the effect of cross-gain modulation depends
on the statistical features of the packetized traffic. The overall network
utilization][][
][
offon
on
TETETE+
=ρ is used to describe the statistical features of the traffic. Traffic
with different utilization factor has been generated to evaluate the effect of traffic statistics.
We generate variable Ethernet network traffic by:
(i). Playing movie files in Centre d'Optique, Photonique et Laser (COPL)’s Ethernet LAN
environment. According to Figure 4-4, we play a movie with the name of the Red Valley in
MPEG format on the station Saphir while the movie file is located in station Beryl. This can
generate approximately 15% of network utilization. If we play this file simultaneously on
stations Citrine, Prague and Saphir, they can bring the traffic up to 45%~60% load.
Figure 4-4. Traffic Generation
(ii). Copying files among these LAN stations, downloading big files from some websites such as
http://www.oracle.com. This kind of method can generate huge load of traffic, while these
kinds of traffic are not stable and predictable.
Ethernet
Beryl
Vidocq (Server)
Saphir
CenterCOM MR820TR (Hub)
Ethernet Uplink toInternet
Citrine
Prague
Video Source File
72
From [31-34], we know that the self-similar traffic will impact the gain’s PDF (i.e. the frequency
of occurrence of each gain value that is a histogram that approximates the probability density
function of the gain) of EDFAs cascade by the following manner.
• When the first EDFA is unclamped, the gain has a broad PDF that has 2 edges with a middle
mild peak between them. The PDF is constrained in the range between a maximum (reached
when the modulated signal is always OFF) and a minimum (modulated signal always ON).
The edge peaks (ideally Dirac delta function) correspond to the finite probability that the
probe power takes on the steady state maximum and minimum values. For smaller values of
a (higher variability), such edge peaks become higher. The right peak (corresponding to
Signal OFF) will decrease when the network load increases, the left peak (corresponding to
Signal ON) will increase. If network utilization value ρ is different from 0.5 the PDF loses its
symmetry, the double peaks will change their relative position when the utilization factor
varies. The PDF will broaden along the cascade until it saturates.
• When the first EDFA is clamped, the PDF has a bell shape. Clamping has the effect of
significantly narrowing the PDF’s width. In the unclamped case, the PDF of the clamped
case does not change much with variable network load in comparison to unclamped case.
3. Measurements of gain swings
In this section we demonstrate how a cascade of EDFAs respond to live Ethernet traffic with
different activity factors statistically.
• Effect of different activity factors---single EDFA
Figure 4-5 shows the PDF of the output of the 1st EDFA under different utilization factors if the
1st EDFA is not clamped.
We can see that ?=50% is the double peaks’ balance point, the double peaks will be undulating
when network utilization varies. The right peak (corresponding to Signal OFF) decreases with
increasing network load, the left peak (corresponding to Signal ON), on the contrary, increases.
73
Figure 4-5. PDF of the 1st EDFA
The single EDFA’s experimental result agrees with former theoretical study.
• Effect of different activity factors---cascade of EDFAs
Figure 4-6. PDF of the 3rd EDFA
-22 -21 -20 -19 -18 -17 -16 -15 10 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0 1st EDFA
PD
F
Voltage (dBV)
Utilization=6% Utilization=49% Utilization=77%
-22 -21 -20 -19 -18 -17 -16 -15 -14 10 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0
Voltage (dBV)
PD
F
the third EDFA
Utilization=23% Utilization=51% Utilization=77%
74
The third EDFA’s output PDF is shown in Figure 4-6, we see they also have two peaks
corresponding to ON/OFF stages. We also see gain swings taking place in this EDFA, so the
PDFs are broader than that of EDFA No. 1. We can find still broader PDFs after EDFA No. 5 in
Figure 4-7 in which we plot both its PDFs and time domain response.
Figure 4-7. Output of the 5th EDFA
The single EDFA’s responses at different positions to different network loads of live Ethernet
traffic are investigated. They are in accordance with [31-34].
From a statistical point of view, when network utilization varies, the monitoring channel is also
changing dramatically, and this indicates the system of WDM will suffer nonnegligible gain
swings, and this kind of harm will accumulate and worsen along the cascade as we will see in
next section.
• Gain swing in cascade
In this section we fix the activity factor ? and focus our attention on how the PDF of the gain is
broadening along the cascade of EDFAs.
• Broadening along the cascade
-21 -20 -19 -18 -17 -16 -15 -14 -13 -12 -1110-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Utilization=4% Utilization=51%Utilization=82%
PD
F
PD Output (dBV)
The 5th EDFA
OvershootsUndershoots
0 1 2 3 4 5 6 7 8-0.24
-0.22
-0.2
-0.18
-0.16
-0.14
-0.12
-0.1
-0.08
Undershoots
Overshoots
Undershoots
Overshoots
(a) (b)
Time (ms)
PD
Out
put (
V)
Overshoots & Undershoots
75
We fix the network utilization factor to 74%, and measure the outputs along the cascade and plot
their PDFs as in Figure 4-8. The PDF is broadening and its width increases along the chain,
although when the gain saturates the PDF width tends to saturate as well.
Figure 4-8. PDF broadens along the EDFA cascade
4. Gain clamping
As we see from the results above, we must keep the gain constant in order to avoid the gain
swing along the cascade and thus protect our WDM systems. Gain clamping really does the very
work of gain keeping according to our results in this section.
• Time Response
The lasing wavelength was set at las? =1562.2 nm where the EDFA gain is approximately equal
to the signal channel wavelength. With signal and monitoring channel powers at the input port of
EDFA no. 1 equal to SP = –13 dBm (average power with square wave, 500Hz and 100% depth
modulation) and CWP = –20 dBm (CW), pump power of the first EDFA and the loss of the ring
laser resonator have been adjusted to obtain a gain of 23dB and lasing power at the output port of
-22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 10 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
10 0
Voltage (dBV)
PD
F Broadening along the cascade (utilization =74%)
EDFA No. 1 EDFA No. 3 EDFA No. 5
76
directional coupler no. 3 of 7mW. Power fluctuations of the monitoring channel due to cross gain
saturation effects in EDFA have been effectively suppressed. However, intervals between
packets give rise to relaxation oscillations of the ring laser. Such oscillations have been observed
when all-optical gain clamping has been used to protect surviving channels against add/drop
crosgain saturation effects [36]. Relaxation oscillations of the lasing power propagate and are
amplified along the cascade. Figures 4-9 (a), (b) show short parts of one second long records of
the photodetector voltage corresponding to monitoring channel power fluctuations after EDFA
No. 3 and No.5, respectively. Amplitude of pdU (t) fluctuations is 0.013 V after EDFA No.3 and
0.022 V after EDFA No. 5. Monitoring channel power fluctuations after the gain clamped EDFA
no. 1 were very low and the amplitude of pdU (t) almost comparable to the noise of our detection
and data acquisition system.
Figure 4-9: Time evolution of photodetector voltage corresponding to fluctuations of the
monitoring channel power at 1556.27nm recorded at the output port of: (a) EDFA No. 3, (d)
EDFA No. 5 (gain clamped cascade)
• Gain Swings
Figure 4-10 shows the gain swing will be greatly reduced if we clamp the 1st EDFA. We see
from this figure that clamping has the effect of reducing by more than 4 times (on a dB scale) the
width of the PDF at 510 − .
178 179 180 181 -0,090
-0,085
-0,080
-0,075
U pd ,
V
time, ms
158 159 160 -0,115
-0,110
-0,105
-0,100
-0,095
-0,090
U pd ,
V
time, ms (a) (b)
77
Figure 4-10 shows that gain clamping is also valid in reducing the gain swing along the cascade.
Although under the clamping case, the PDF also broadens a little but not so much as the
unclamped case.
Figure 4-10. Clamping also reduces the gain swing in the cascade
Figure 4-11. Output of EDFA 1 under clamped and unclamped cases
Figure 4-11 shows both time evolution and PDF of the first EDFA under clamped and unclamped
case, and we see that the gain swing in single EDFA is reduced. In the clamped case the PDF has a bell
shape. The PDF of the clamped case does not change much with variable network load.
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1 1.5
2 2.5
3 3.5
4 4.5
5 5.5
6
Clamped
Unclamped
Time Evolution of the PD Ouput with Sampling rate of 4 MS/s
Time (s)
Inve
rse
Am
plifi
ed P
D O
utpu
t (V
)
Utilization=16.43% Utilization=62.61%
2 4 6 8 10 12 14 16 10 -7
10 -6
10 -5
10 -4
10 -3
10 -2
10 -1
Voltage (dBV)
PD
F
Clamped
Unclamped
PDF of the PD Output Utilization=16.43% Utilization=62.61%
-23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 10 -6
10 -4
10 -2
10 0
Voltage (dBV)
PD
F Unclamped----Utilization 74%
1st EDFA 3rd EDFA 5th EDFA
-23 -22 -21 -20 -19 -18 -17 -16 -15 -14 -13 -12 10 -6
10 -4
10 -2
10 0
Voltage (dBV)
PD
F
Clamped 3rd EDFA 5th EDFA
78
Figure 4-12: Probability density function of )(U pd t : (a) after EDFA no. 3, and no. 5; (b) after EDFA no.
5 – effect of lasing power
Figure 4-12 (a) shows the PDF of pdU (t) recorded at the output of EDFA No. 3 and 5. The PDF
width was 1.7dB and 2.3dB after EDFA no. 3 and no. 5, respectively; almost four times narrower
than in the case of unclamped cascade.
The stabilizing effect of the lasing power generated in the gain clamped EDFA no. 1 and
propagated along the cascade has also been investigated. The pump power of the first amplifier
and the loss of the variable attenuator has been readjusted to obtain 5mW and 9mW of lasing
power at the output of directional coupler no. 3. Amplitude of monitoring channel power
fluctuations decreased with increasing lasing power. The positive effect is demonstrated in Fig.
5b where the PDF of pdU (t) recorder after EDFA No. 5 is plotted for the three values of lasing
power lasoutP = 5, 7, and 9 mW at the output of directional coupler No. 3. The width of the PDF
at the level of 310 − was 1.9, 2.3, and 2.7dB for lasoutP = 5, 7, and 9 mW, respectively.
-22 -21 -20 -19 -18 -17 -161E-6
1E-5
1E-4
1E-3
0,01
0,1
1
EDFA no. 3
PD
F
20*log(-Upd), dBV
-22 -21 -20 -19 -18 -17 -161E-6
1E-5
1E-4
1E-3
0,01
0,1
1
EDFA no. 5
-20 -19 -18 -17 -16 -151E-6
1E-5
1E-4
1E-3
0,01
0,1
1
Plasout=9mW
PD
F
20*log(-Upd), dBV
-20 -19 -18 -17 -16 -151E-6
1E-5
1E-4
1E-3
0,01
0,1
1
Plasout=7mW
-20 -19 -18 -17 -16 -151E-6
1E-5
1E-4
1E-3
0,01
0,1
1
Plasout=5mW
(a) (b)
79
5. Comparison with results for artificial Ethernet data
Figure 4-13. Bononi’s Setup with artificial traffic [34]
We can compare our result with Bononi’s [34]. Figure 4-13 is Bononi’s Setup. They used
artificial and repeated traffic with a Pareto distribution to feed the EDFA, measured the
fluctuations in CW signal by a digital Oscilloscope.
Figure 4-14. Bononi’s Results [34]
Figure 4-14 is similar to our results mentioned above. The simulation result is squared at the
upper right part of the PDF figure. And our experiments also show that Pareto distribution is the
80
good choice to model self-similar network traffic, because by simulation and artificial traffic
experimental results mentioned in earlier sections, they predicted the PDF shape of the EDFA’s
response by supposing that the traffic has a Pareto distribution.
6. Experimental implementation
In order to decrease the digitization error due to weak CW signal fluctuations after EDFA No. 1,
we can put an inverse electronic amplifier after the PD and before the data acquisition card, or
get the value without inverse electronic amplifier for every 10~20 points’ mean, or even use the
data acquisition card’s maximum sampling rate of 5 MS/s.
If we do not have any band-pass filter, we can also use Figure 2-12’s quick setup.
81
Chapter 5. Conclusions
1. What was achieved
EDFA gain dynamics in WDM systems was studied in this work. For the first time we have
measured the response of EDFAs fed by live Ethernet data at 10 Mb/s and verified previous
theoretical results where packetized data is modeled as having self-similar characteristics.
We built an Ethernet transmitter, captured the network traffic from the LAN Ethernet by the
transmitter and thus converted the traffic from electrical level to optical signal. 5 EDFAs were
finely adjusted and linked together. Data acquisition system is composed of the PCI-6110E data
acquisition card and LabVIEW program.
Analysis of the data acquired by the data acquisition system showed, in the unclamped case,
substantial gain variation after 5 EDFA in a cascade caused by gain fluctuation in burst-mode
packet switched network. The implementation of gain clamping is extremely effective to combat
the gain variation.
Our experimental results demonstrate that substantial swings in EDFA output power can be
induced by crossgain saturation when packetized bursty traffic is transmitted through a cascade
of fiber amplifiers. Amplitude of these swings grows along the cascade in the same way as
surviving channel fluctuations when channels are dropped/added in a WDM circuit-switching
scenario. Although our experimental setup allowed us to transmit the packetized traffic over one
channel, only numerical simulations [33] predict that even in 8 channel WDM system with
bursty traffic the probability of long empty slots is rather high and can give rise to large
fluctuations of EDFA gain.
The stabilizing effect of clamping the gain of the first amplifier in the cascade by a ring laser
configuration and propagating the lasing power through the cascade has been confirmed. The
width of the PDF of the monitoring channel power at the output of the cascade of five amplifiers
82
has been reduced four times compared with the unclamped cascade when 7mW of lasing power
were launched in the cascade.
2. Possible follow up work
In our experiment mentioned above, we used only one real channel input. The next step is to feed
the EDFA cascade with 3 live traffic, that is, we plug 3 Ethernet transmitters to 3 different LANs
thus capturing their traffic and input the amplifiers, which is more similar to the WDM case.
WE could also try the router-to-router traffic instead of the host-to-host traffic of Ethernet.
83
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89
Appendix II AUI on 10 Base-T Ethernet Hub
• A hub is a common connection point for devices in a network. Hubs are commonly used to
connect segments of a LAN. A hub contains multiple ports. When a packet arrives at one
port, it is copied to the other ports so that all segments of the LAN can see all packets. A
passive hub serves simply as a conduit for the data, enabling it to go from one device (or
segment) to another. So-called intelligent hubs include additional features that enable an
administrator to monitor the traffic passing through the hub and to configure each port in the
hub. Intelligent hubs are also called manageable hubs. A third type of hub, called a
switching hub, actually reads the destination address of each packet and then forwards the
packet to the correct port.
• AUI, short for Attachment Unit Interface, is the portion of the Ethernet standard that specifies
how a cable is to be connected to an Ethernet card. AUI specifies a coaxial cable connected
to a transceiver that plugs into a 15-pin socket on the network interface card (NIC). We tap
off the Ethernet traffic from this DB15 port and drive the laser, the pin assignment is as
followed:
1. not used
2. "C+" : Collision + (output from MAU)
A signal to indicate that multiple stations are contending for access to the transmission medium.
3. "T+" : Transmit + (input to MAU)
This line is intended to operate into terminated transmission lines.
4. not used
5. "R+" : Receive + (output from MAU)
90
a data input sourced by the MAU.
6. GND : GROUND
7. not used
8. not used
9. "C-" : Collision - (output from MAU)
10. "T-" : Transmit - (input to MAU)
11. not used
12. "R-" : Receive - (output from MAU)
13. +12VF : +12Vdc Power (fused)
fused +12Vdc sourced by the DTE
14. not used
15. not used
• Category 5 cable pinout and connection