Multi-Wavelength Analysis of Active Galactic Nuclei
40G Optical TDMA Network with 2.5G Connections
1. Distributed Optical TDMA photonic
switch fabric based on gain-switched
distributed feedback semiconductor laser
diodes and electroabsorption modulators
by
Paul Gunning
A thesis submitted for the degree of Doctor of Philosophy.
Department of Electronic Systems Engineering
University of Essex
Monday, January 15th 2001
The candidate confirms that the work submitted is his own and the
appropriate credit has been given where reference has been made to the
work of others.
2. Abstract
Emerging computer environments will require interconnects with low-latency, high
data bandwidths, and fast reconfiguration to interconnect distributed computing,
storage and networking elements. This thesis describes the work that culminated
in the demonstration of a 40Gbit/s optical-TDMA LAN interconnect establishing
2.5Gbit/s interconnections with fast set-up between computer workstations using
single-mode optical fibre.
After some introductory material concerning the operation of optical transmis-
sion systems, wavelength chirped pulses from a gain-switched (GS) distributed feed-
back laser (DFB) semiconductor laser diode (SLD) are temporally compressed to 5ps
with a specially tailored step-chirped in-fibre bragg grating are described. Further
pulsewidth reduction obtained with non-linear fibre compression was investigated.
These pulses are then used within a 100Gbit/s packet self-routing photonic network
demonstrator. Electroabsorption (EA) modulators are introduced both for low-
and high- repetition rate modulation of a continuous wave (CW) optical source.
Other pulse source technologoes are considered including a fibre ring laser and mod-
ulation of a CW optical beam using EA modulators. The inherent timing jitter
intrinsic to the gain-switching process was reduced using coherent CW injection
whilst the resulting enhancement of the interpulse pedestal was removed by an EA
modulator acting as a synchronous temporal gate. EA modulator gating is then
extended to channel selection for optical time-division de-multiplexing when driven
with an electronic impulse generator synchronised to a network clock. An alterna-
tive, all-optical, channel selection scheme which used an integrated Mach-Zehnder
interferometer (IMZI) with the gating window produced directly from the optical
clock pulse will be described. These methods are used within two versions of a
40Gbit/s Optical TDMA network one based on polarisation-maintaining fibre and
containing the IMZI as a channel selection element. Another using the common
blown fibre infrastructure within a building with EA modulator channel selectors.
A star-topology, terabit/s interconnection fabric was outlined which included the
use of wavelength-division multiplexing to increase the aggreated bandwidth.
i
3. Acknowledgements
Kevin Smith magically set everyting in motion and his continuing guidance, support
kindness and advice was invaluable.
My day-to-day supervisor Julian Lucek was generous, considerate, and patient. Par-
ticularly for sharing his intuitive feel for the practical aspects of fibre optical com-
munication systems. His perception and insight was always timely and apposite.
My academic supervisor, Shamim Siddiqui, I thank for his patience and guidance.
I would like to express my gratitude to David Cotter who approved and supported
the PhD by way of a University of Essex research contract through BT Project
106: Ultrafast Networking. BT (through Project 106,) the University of Essex and
NATO also provided funding to travel to Italy, France, Scotland and New Mexico
to attend and participate in scientific meetings that were invaluable as background
to this thesis.
Dan Pitcher provided invaluable practical support and advice in the laboratory.
Keith Blow, Bob Manning, Alistair Poustie, and Paul Townsend were always helpful
and generous in sharing their knowledge, experience and wisdom.
It was a pleasure to work with, and learn from, Andrew Ellis on many occasions.
In addition Andrew also reviewed the first version of this thesis and provided many
excellent comments and suggestions.
Many other people at BT proved invaluable during the course of this research for
which I am extremely gratefull. These include: Dave Moodie, who provided all the
EA modulators used in this work; Raman Kashyap who provided the fibre bragg
gratings; Derek Nesset for sourcing and guidance with the IMZI; Doug Williams,
who provided much of the research fibre. Colin Ford packaged (and repaired) many
of the devices that were used (and abused.) Dominique Marcenac, John Collins,
Tony Kelly, Russell Davey, Monica Rocha, Jennifer Massicott, David Smith, Daniel
Pataca, Mohammed Shabeer, Paul Urquhart, Richard Wyatt and Terry Widdowson
deserve special mention.
ii
4. Elke Jahn and Niraj Agrawal from HHI Berlin kindly supplied the Integrated Mach-
Zehnder Interferometer used in this work. Vince Ruddy arranged my initial place-
ment at BT Laboratories.
Judy and Chris Chestnutt in Annesley, Great Bealings provided a quiet and stable
environment in which to write the thesis down the years. Marlies Janssen and
Andrew Ericsson were very kind and supportive.
My friends from Ballyfermot: Steven Kavanagh, Declan Kelly and Martin Smyth.
Some teachers including: Diarmuid O’ Donovan, Noel O’ Brien, and Oliver Murphy.
The Zecca family were very supportive.
Sweety-pie, Fatima, who showed me that when you look into the light, the light also
looks into you. Um abracos e beijos e amor.
Most importantly I was reared by my Aunt Nan and Aunt Kay. They indulged,
cajoled and supported me unconditionally through thick and thin, darkness and
light. This thesis is really a testament to their efforts and sacrifices.
iii
18. 5.15 Schematic of the path taken by the N wavelengths assigned to one
time slot through the interconnect. Key: AWG: Arrayed waveguide
grating; EAM: Elactroabsorption modulators; FD: fibre delay. 1)
At the AWG the wavelength channels are aligned within the time
slot; 2) The EAMs are located at the termination of a fibre spoke
and are subject to wavelength-dependent temporal skew; 3) the fibre
delays after the EAMs are adjusted appropriately to ensure temporal
alignment of the wavelength channels within the time slot at the
N × N coupler; 4) the second traversal of the fibre spoke towards the
WRITE section of the node induces wavelength-dependent temporal
skew; 5) the fibre delays are used once again to re-align the channels
prior to the EAM array. . . . . . . . . . . . . . . . . . . . . . . . . . 194
5.16 Power penalty arising from the finite rejection of adjacent wavelength
channels for unamplified, 10 and 16 channel systems. . . . . . . . . . 195
xvii
19. List of Tables
2.1 Classification and properties of normal and anomalously dispersive
optical fibre. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1 Specification of dispersion decreasing fibre. . . . . . . . . . . . . . . . 92
3.2 Sampling oscilloscope channel jitter measurements. . . . . . . . . . . 98
3.3 Classification and properties of the various pulse sources described in
this chapter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
4.1 Non-linear optical properties and figure of merit of several material
systems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154
5.1 Classification of switching speeds. . . . . . . . . . . . . . . . . . . . . 171
5.2 Optical fibre characteristics from ref. [8]. D, is the group delay disper-
sion; λo, is the zero-dispersion wavelength; θ, is the temperature; So, is
the dispersion slope. dλ/dθ|λ=λo , is the thermal coefficient term. NZ-
DSF: non-zero dispersion shifted fibre, LCF: large-core fibre, DFF:
dispersion-flattened fibre. . . . . . . . . . . . . . . . . . . . . . . . . . 192
xviii
20. Chapter 1
Introduction
1.1 Historical background
1.1.1 Information transmission
The digital communication age began when Samuel Morse invented both telegraphy
and morse code1
in 1835 [1]. At that time a good morse operator could transmit
10 bit/s of information. Western Union commercialised telegraphy in 1844 and laid
the first operational transatlantic telegraphic cable by 1866 [1]. Ten years later, on
March 10 1876 to be precise [2], in the attic of a boarding house in Boston, Mas-
sachusetts Alexander Graham Bell used twisted-pair copper wires to transmit the
words, “Mr. Watson, come here. I want you [2, 3]” to his colleague in the adjoining
room and so with the telephone laid the foundation of the present information age.
At the beginning of the sixties T H Maiman at the Hughes Research Laboratory
provided the first demonstration of a device that emitted coherent electromagnetic
radiation—the ruby laser. Several competing groups [4, 5, 6, 7] announced coherent
emission at 900nm from small, compact Gallium-Arsenide (GaAs) semiconductor
laser diodes at 77K within weeks of one another. The spectral purity and low-spatial
divergence of laser light held great promise for the transmission of information over
free-space point-to-point links. However the transmission distance was limited by
environmental conditions such as rain and fog.
By 1966, Kao and Hockham [8] suggested that thin, glass optical fibres could
provide a channel for transporting information using infrared light—including lasers,
but only if the then huge material losses ∼1000dB/km could be reduced to ∼20dB/km.
1
On 31st December 1997 morse code was discontinued as the global means of conveying distress
at sea!
1
21. Chapter 1 2 Introduction
In April 1970 one of the co-inventors of low-loss optical fibre, Donald Keck from
Corning, wrote in his notebook of his measurement on a 1m sample of optical fi-
bre: “Attenuation equals 16dB. Eureka! [9]” Later that year, at an IEE conference
in London, Corning announced the fabrication of an optical fibre with a loss of ∼
20dB/km at ∼900nm. So as the Seventies unfolded the possibility began to emerge
of using a modulated laser source to transmit information within an optical fibre.
By the latter half of the seventies this possibility became reality as several field trials
of optical fibre systems were deployed. In the UK one of the first trials ran from
BT Laboratories to Ipswich telephone exchange: 8 Mbit/s over 13km. In 1979 Miya
and co-workers [10] reported the fabrication of a single-mode optical fibre with a
loss of 0.2dB/km. The most significant advance in optical fibre transmission during
the eighties concerned the demonstration of lasing and amplification in single-mode,
Erbium-doped silica fibres, pumped by semiconductor lasers [11]. Fibre attenua-
tion could now be compensated by in-fibre optical gain element. Wavelength- and
time- division multiplexing technologies were then developed to increase the aggre-
gate data rate that could be supported by a single optical fibre. By February 1998
this had advanced sufficiently for Lucent Technlogies to announce a commercial 400
Gbit/s WDM system called Wavestar
TM
.
1.1.2 Information processing
In parallel with the developments in information transmission, remarkable advances
have been made in computer technology. The first computing engine design, al-
beit mechanical, is attributed to Charles Babbage and his Difference Engine in the
19th Century—although it wasn’t actually built until recently. The first electronic
computer was demonstrated by Mauchley and Eckert in 1946 [12]. They called it
the ENIAC and the logic gates were based on unreliable, bulky and power-hungry
triode valves. A recurring theme during the evolution of computer technology is
the reduction in physical size of the logic elements. Such an opportunity was pre-
sented in 1947 when John Bardeen and Walter Brattain invented the transistor [13].
Two years later, Maurice Wilkes at Cambridge University demonstrated EDSAC,
the world’s first general purpose stored-program computer. At about the same time
that T H Maiman was demonstrating the first laser, Fairchild Semiconductor pro-
duced the worlds first integrated circuit that comprised four transistors. By 1971
Intel had produced the first microprocessor, the 4004. The following year Chuck
Thacker at Xerox PARC started to design what is now widely recognised as the first
22. Chapter 1 3 Introduction
personal computer—the Alto [14]. As that decade came to an end personal com-
puters such as the Apple II were in some businesses and fewer homes. Then in 1981
the IBM PC was announced and it ushered in the era of a computer on every desk
and within many homes. It has now evolved into cheap PC-based, multi-processor
workstations.
1.1.3 Local- and wide- area networks
The forerunner of the Internet—ARPANET—began with just four nodes on the west
coast of the US in 1969. Interoperability was assured between the many different
proprietary protocols by Cerf and Kahn with their development of the TCP/IP in-
ternetworking protocol suite in 1974 [15]. In 1976 Xerox PARC introduced 3Mbit/s
Ethernet [16] which has evolved into the worlds most ubiquitous Local Area Network
(LAN) technology. The latest version—Gigabit Ethernet [17] is capable of switch-
ing and routing at 1Gbit/s allowing full-duplex interconnections at wire speed. A
10Gbit/s version is near completion and 100Gbit/s Ethernet and even Terabit/s
Ethernet are likely to follow. By 1983 the widespread adoption of TCP/IP allowed
many other wide-area networks such as the NFSNET and MILNET to form a net-
work that spanned the globe—the Internet [15]. The 1990s were notable for the
emergence of the Internet particularly the world-wide web (WWW) and the ex-
plosive growth of private intranets and extranets. The WWW has pervaded every
aspect of the work and home environments.
At the turn-of-the-millenium information is truly an economic force: the timely
transmission and sharing of this information is now a valuable and exploitable
commodity. But at its foundation is the abilty to generate and disseminate such
information-rich content via fast processor chips, fast interconnects, and fast switch-
ing systems. It is widely appreciated (and endured) that WWW is an acronym for
“world wide wait” studies have concluded that the effective bandwidth available to
Internet users is a mere 40Kbit/s [18]!
To this end in an attempt to increase bandwidth and reduce latency new protocol
stacks that place IP directly on top of an optical layer and render the SONET/SDH
layer2
superfluous are emerging. Moreover network technology that was traditionally
implemented in software is now being performed with faster, dedicated hardware
with an attendent reduction in latency [19].
2
At the time of writing Cisco systems can provide 2.5Gbit/s optical interface cards running
their Dynamic Packet Transport technology which is an implemetation of ‘IP over Optics.’
23. Chapter 1 4 Introduction
1.2 Emerging trends and Limitations
1.2.1 Market drivers
Advances in computing technology for example rapidly increasing processor clock
speeds [20], allied with the push towards multi-processor computer platforms3
re-
quire ever-faster interconnection networks for high speed communication both within
and between computing machines or networking devices at various length scales that
span several hierarchies of interconnect:
• Intra-chip—Optics has a minimal impact because of the small dimensions typ-
ical on this scale which allow huge interconnect densities [21] Dimension on
the order of µm
• Inter-chip—of the order of mm’s
• MCM-to-MCM—Multichip module to multichip module cm’s
• PCB-to-PCB—Printed circuit board to printed circuit board distances up to
• Backplane to backplane over distances greater than say 20cm. Reconfiguration
likely
• Rack-to-rack—several metres
• System Area Networks—1m-to-1km
Desirable attributes for all these hierarchies include low latency, high bandwidth and
fast reconfiguration of the interconnection fabric. The applications at the so-called
‘bleeding-edge’ that drive these advances include [22]:
• Cryptography
• Nuclear weapons design
• Atmospheric dynamics simulations
• Fly-by-wire aircraft
• Synthethic aperture radar
• Molecular dynamics/Pharamaceuticals
3
At time of writing (March 1999) the Cray T3E-1200 can contain up to 2048, 600MHz processors
providing a peak performance of 2.5Teraflops.
24. Chapter 1 5 Introduction
• Oil exploration
• Synthethic theatres of war.
• Distributed interactive collaborative environments
1.2.2 Inter-chip: removal of the Von Neumann bottleneck
The application of advanced lithographic techniques that reduce the feature size on
processor and memory chips permit a ×4 increase in the number of transistors per
die every three years in-line with Moore’s law. There is no sign that this trend is
likely to stop. But whilst the clock speed of processors is increasing by 60% annu-
ally, memory speed is increasing at a mere 10% over the same interval [23]. This is
important because the Von Neumann architecture, that is predominant today and
which emerged during the forties, separates the processing unit from the memory
unit via an interconnect [24]. Consequently various trade-offs are carefully weighed
to amorotise the mismatch in latency and bandwidth between the processor and
memory. The use of a hierarchy of on- and off-chip caches [25] is one particular
example. But it relies on software compliers that make the best use of the caches
through temporal and spatial data locality in the processors references to mem-
ory [21]. Figure 1.1 graphically illustrates the bandwidth hierarchy of the memory
system (32 MBytes of 16Mbit DRAM) within an admittedly dated 100MHz com-
puter [26]. Most striking is that the bandwidth available within the memory is some
3000 times larger than that provided by the I/O pins—the bottleneck is mostly due
to the pins [27]. The most telling feature is how bandwidth is progressively squan-
dered as hierarchies of elements are crossed within the chip. The net effect is to
increase latency leaving the processor idle for several clock cycles. Moreover mod-
ern computer designs include advanced graphic engines for multimedia rendering
that compete with the processor for a share of the memory bandwidth which makes
matters worse still [28]. Taken together the memory latency, reduced bandwidth and
the faster clock speed serve to conspire against processor efficiency and represent
a large and growing gap between the processor and memory variously termed the
“von Neumann bottleneck” [29] or “memory wall” [30, 31].
However since it is now possible to include additional features and, by implica-
tion, functionality upon a single die a persuasive argument has emerged which con-
tends that if you cannot bring the memory bandwidth to the processor then why not
bring the processor to the memory? This idea for processor-memory integration has
various names: Computational RAM (CRAM) [26]; Intelligent RAM (IRAM) [23];
25. Chapter 1 6 Introduction
1TB/s100GB/s10GB/s1GB/s100MB/s
(c) Duncan Elliott 1998, used with permission
Figure 1.1: Typical memory bandwidth hierarchy within a 100MHz computer.
processor-in-memory (PIM) [32]. When implemented it has been conservatively es-
timated that the latency would be reduced by ×10, whilst the bandwidth available
between the memory and the processor would increase ×100 [23]. An additional
benefit that would accrue is that interactions between the PIM and off-chip inter-
connect would be reduced ×100 [33]. In addition a portion of the PIM chip could
be dedicated to re-configurable logic cells so that tailored functionality could be
incorporated after fabrication perhaps dynamically in-situ [32, 34] with attendent
benefits of scale and cost reductions. Once PIM chips become a commercial reality
then multi-PIM computers or networking elements that communicate to neighbour-
ing elements within a cabinet or across a network will emerge. This would shift
the bandwidth and latency bottlenecks onto the interconnection fabric that exists
”outside-the-box” and so low-latency, high bandwidth interconnections to service
this need will be at a premium.
1.2.3 SAN: System Area Networks
Working in the opposite direction it is now widely recognised that many of these
applications can be implemented more cost-effectively using low-cost, networks of
workstations (NOW) [35]. Scalability is possible by just adding more workstations.
26. Chapter 1 7 Introduction
Commodity computer clusters are challenging conventional supercomputers in terms
of processing power but for a fraction of the cost. Beowulf from IBM is one working
example that uses conventional LAN technology to interconnect commodity PCs via
standard interface cards connected to the PCI bus4
.
That said the type of problems that can be addressed require a high ratio of
computation-to-communication because of the high latency overhead of conventional
LAN technologies and access via the PCI bus. Consequently problems or applica-
tions that require a low ratio of computation-to-communication are less successful.
To address this deficiency an emerging trend has been to scale-up high-performance
electronic interconnects typical of supercomputers to local-area network (LAN) di-
mensions. These so-called system area networks (SANs) span distances ranging
from 1m-1km—falls between that within a supercomputer cabinet and that of a
LAN [36, 23].
The most commercially successful SAN, to date, is produced by Myricom net-
works. The Myricom approach is centred on an 8-port electronic crossbar-based hub.
Each one of up to 8 hosts is attached to the hub by an electrical cable containing 18
separate twisted pairs (9 in each direction) that allows parallel bi-directional data
transfer of 9-bit words over distances of up to 25m at 1.28Gbit/s (160MByte/s) [37].
Specialised cards interface to the the memory bus within each workstation. The su-
percomputer supernet testbed (SST) envisages internetworking between Myrinet
switches to form a wide area network (WAN) that could interconnect supercom-
puters throughout the west coast of the US [38]. If the Myrinet approach is a
scaling-up of traditional electronic supercomputer fabric. An alternative approach
is to scale-down both developed and emerging technologies in optical networking
which have not been considered applicable over short distances (<1km) [39, 40].
The next section will argue why this alternative approach is now needed.
1.3 Electrical Problems and Optical Solutions
1.3.1 Physical limits of Electrical interconnects
Present-day computers rely on metallic interconnects for chip-chip interconnections.
However the bandwidth that they can provide is now coming up against hard phys-
ical limits. The main constraints include the increase in signal attenuation with
4
17 IBM netfinity servers containing 36 Pentium chips and running the Linux operating system
have equalled the performanace of a $5.5Million Cray T3T-900-AC64 in rendering a ray tracing,
image rendering a ray tracing program. The IBM Beowulf cluster cost only $150,000!
27. Chapter 1 8 Introduction
propagation length at frequencies above 1GHz due to the skin-effect resistance of
the metallic track and the nature of the dielectric substrate.
A metric has been proposed based on the aspect ratio–the ratio of propagation
length to cross-sectional area–of cu-based interconnects [41]. This maintains that for
a given length, an interconnect comprising many, small cross-section wires running
at low data rates is equivalent to a few, large cross-section wires running at high data
rates. Traditionally the problem of providing high bandwidth was addressed by the
former approach—spatially multiplexing a flat array of adjacent, parallel metallic
tracks at low signalling frequencies. But as computer clock rates increase so the
adverse effect of capacitative coupling between adjacent tracks leads to enhanced
crosstalk with a consequent loss of data integrity. Data skew between tracks requires
additional de-skewing circuitry and requires careful spatial routing of tracks within
the machine. Consequently there has been a gradual shift away from multiple,
narrow parallel tracks running at 100MHz towards a single serial track operating
at 1+ GHz. The benefits that follow from this approach include the greatly reduced
skew of a single track over several parallel tracks and the relaxation of track routing
constraints.
However as clock frequencies increase above 1GHz, the physical limitations inher-
ent in a metallic interconnection network become apparent. Figure 1.2 [42] illustrates
this in graphic form. In effect, each point-to-point link acts as an antenna that
serves as both a source and a sink of time-varying, radio frequency (RF) noise en-
ergy commonly called electromagnetic interference (EMI.) For example, RF noise
energy transmitted to, and received from, the surroundings can affect the decision at
receiver modules and induce phantom data events that cause incoherence between
memory registers. In addition, the fan-out and radiation-induced energy losses need
to be compensated by amplifiers to ensure an adequate signal-to-noise ratio at the
termination points of the system. But the introduction of amplifiers increases noise
and adds to the thermal load and power consumption of the system. Moreover,
thermal variations of the resistance cause variations in the signal phase that require
additional control elements. Nevertheless a pristine, untapped and properly termi-
nated single track with specialised dielectric substrate has been demonstrated to
9.6Gb/s over 0.5m. But this must be qualified by noting that improperly termi-
nated signal taps along the span of the interconnect would adversely impact signal
quality due to reflections from impedance mismatches. Optical Interconnects can
provide a solution to this problem.
28. Chapter 1 9 Introduction
10
10
10
10
10
10
13
12
11
9
8
10
10
10
7
6
frequency,Hz
distance, m
Transmission Line
Optics
Wire
1010
-2 -1
1 10
1
10
2
10
3
10
-3
Figure 1.2: Preferred interconnect technology: frequency-distance dependence.
1.3.2 Optical Interconnects emerge
It is easy to appreciate how interconnects have now become the dominant factor
in determining both the productivity and performance of computer technology [43].
Consequently data transfer rates within and between computing machines are sub-
ject to performance bottlenecks that expose the bandwidth limitations of electrical
interconnects and offer a compelling case for optical interconnects.
Amongst the compelling advantages of an opticall interconnect are [44]:
• High distance-bandwidth product: through much lower attenuation and greatly
reduced frequency dependent effects requiring fewer amplifiers.
• EMI immunity providing excellent isolation between data channels.
• High packing density through reduced weight and volume allow greater free-
dom to the system architect.
• Greatly enhanced bandwidth per track through the use of wave-, time-, or
spatial multiplexing can be used to extend the total bandwidth of the fibre
29. Chapter 1 10 Introduction
and not be hampered by the limited operational frequency of end-components:
Transmitter modulation and receiver demodulation.
It is this last point that is central to the use of optical fibre, namely the three
bandwidth-enhanced degrees of freedom: spatial, spectral and temporal that can be
provided by optical fibre. For example a number of optical fibres can be assembled
into a spatial multiplex. In turn the wide spectral bandwidth available within each
individual fibre can be utilised for wavelength multiplexing of several distinct data
channels. Each wavelength channel can, in turn, be time-multiplexed in the optical
domain into further independently modulated channels.
A suitable combination of spatial-, wavelength and time-multiplexing can form
a very high capacity interconnect albeit limited by the constraints particular to
each degree of freedom i.e. chromatic dispersion in OTDM system or crosstalk in
a WDM system [?]. For example a range of 16 applied to each dimension gives
an aggregated capacity of over 40Tbit/s (= Modulating frequency of 10GHz x 16
fibres x 16 wavelength channels x 16 time channels.) Of course suitable multiplexing
transmitters and de-multiplexing receivers are required at the access-points of the
system.
But the main constraint to the use of optical interconnects has been down to
economics. Traditionally fibre optic component costs, when compared to their cop-
per brethren, were very much more expensive. Whilst silica optical fibre is cost
comparable to copper, the higher cost of end equipment such as transmitters and
receivers has rendered it viable for all but low-volume, high-margin telecommuni-
cation systems and supercomuters. However this is changing due to the economies
of scale that flow from the mass-production of advanced optical sources such as
distributed feedback (DFB) lasers and high-bandwidth optoelectronic receivers for
deployment in local and wide-area network technologies such as Gigabit Ethernet
and SONET/SDH. Consequently the distance over which optical interconnects com-
bined with advanced switching techniques are becoming economically attractive has
been shrinking continuously [45].
1.3.3 A practical demonstration: Optical Clock Distribu-
tion
Computing machines that contain two or more processors present the software en-
gineer with a concurrent programming environment that considerably lightens the
programming task. This concurrency is ultimately derived from a central clock
30. Chapter 1 11 Introduction
source based on a quartz crystal oscillator that generates a global timing reference.
The timing reference is fanned-out and electrically propagated across an interconnec-
tion network comprised of many copper- (or aluminium-) based point-to-point links
that terminate on the spatially dispersed timing modules located on every printed
circuit board, each of which contains one or several processors. The timing module
provides a local timing reference from which the event transitions for the hardware
registers originate. Consequently all local atomic data transition events that occur
within the hardware registers of the processors can be traced to a common source
and hence can be treated as being globally synchronous.
The excess time per clock cycle that remains after each register transition is
referred to as the clock margin. Insufficient clock margin can cause a register to
load or store data either before it has become valid or after it is no longer valid.
Both are manifest as data incoherence between the dispersed registers which if left
unchecked can lead to errors. The clock margin, then, serves to mitigate this effect
by providing timing slack for all global event transitions to occur and settle. But as
machines become physically larger the timing skew arising from the differences in
propagation delay between the dispersed point-to-point links increases and requires
careful design to manage the clock tolerances. These tolerances are now set to
become even tighter as the clock frequency of processors exceeds the 1GHz (sub
1ns) barrier. Moreover the proportion of timing jitter as a fraction of the clock
cycle period becomes more pronounced and places tight constraints on the design
of interconnections between modules.
For these reasons designers of advanced multiprocessor computing machines have
turned their attention towards optical interconnections for clock distribution [46].
Early attempts used free-space optics and weren’t practical propositions because
of the alignment tolerances and mechanical stability as well as clear line-of-sight
optical paths [47] that were required. In this context the benefits of optical fibre
are many. Optical fibre provides a noise-free clock conduit that neither generates
or is affected by RF interference. Its broad bandwidth (tens of THz) can support
high clock transmission rates per optical fibre strand. Silica based optical fibre, in
particular, has extremely low-attenuation and occupies 1/50th the area of a copper
equivalent. It is not constrained by line-of-sight and is mechanically stable
A less well-appreciated advantage of optical interconnects is related to the grow-
ing problem of thermal management and heat dissipation within a modern computer
system [48]. At the microscopic level the increased density of gates on each proces-
sor die adds to the heat flux of the system and this must be serviced at all levels
31. Chapter 1 12 Introduction
right up to the cabinet level. Modern multiprocessor systems must remove of the
order 10kW/cm2
of thermal flux. To put this into perspective a thermal flux of
100W/cm2
would be typical one mile from the blast centre of a 1 megaton nuclear
device [49]. At the very least this requires additional cooling elements to remove
the excess generated. To reduce this effect it is necessary to move the processing
elements further apart, in effect trading latency against thermal load. But if the
processing elements are moved apart then the data rate must be reduced because of
the physical limitations such as crosstalk and frequency dependent attenuation of
the Cu-based interconnects that have been outlined earlier.
Somewhat less appreciated is the real-estate constraints that compel manufac-
turers to keep the footprint and overall volume of a system tightly constrained in
line with standardised racking systems. The small cross-sectional area coupled with
its physical flexibility allows optical fibre to make full use of the 3rd dimension to
thread its way through the restricted passages and the confined spaces found within
these machines. The low expansion coefficient and refractive index variation are
particularly compelling reasons for choosing optical fibre. The fast rising edge of
an optical clock distribution system can provide a precise decision point to initi-
ate switching. In fact the viability of the optical approach for clock distribution
has been experimentally demonstrated in the laboratory [50] and found sufficiently
compelling for deployment within a commercial supercomputer system [46]. The
latter is shown in Figure 1.3 where Cray have implemented a laser clock distribu-
tion system for their T90 supercomputer. More recently, Cray have described a
Source: Carol Kleinfield, Cray Research
Figure 1.3: Laser source for clock distribution to module boards within CrayT90
supercomputer.
32. Chapter 1 13 Introduction
more advanced, yet potentially lower-cost, version of the clock distribution optics
based on low-cost polyimide waveguides [51].
1.4 Optical data distribution
The next logical step is to extend the use of optics from clock distribution to data
distribution within multi-processor computing systems. Architecturally, clock dis-
tribution is a fanned-out, unidirectional broadcast with a static configuration. In
contrast a data interconnect needs to support bi-directional operation between the
connected nodes. A facility for dynamic reconfiguration that allows any node to
exchange data with any other node is also required. The bandwidth requirements
are substantially higher than for clock distribution and this mandates some form of
multiplexing.
Early attempts at increasing the transmission capacity with optoelectonics used
spatial divivion multiplexing techniques (SDM) utilising multiple fibre ribbon cables.
For example Kaede et al. [52] demonstrated 12×14 Megabit/s in 1990. Since then
research has focussed on low-cost, high-volume versions comprising data-modulated
vertical cavity, surface emitting laser (VCSEL) array transmitters and metal-semiconductor-
metal (MSM) receiver arrays with the interconnection fabric provided by polyimide
ribbon cable. A commercially mature implementation of this technology was devel-
oped during the POLO-2 initiative providing for two contradirectional 10×1Gbit/s
interconnections with an aggregated bandwidth of 20Gbit/s [53].
Yet one single-mode fibre can provide isolation of several independently modu-
lated channels via wavelength division multiplexing. The data transmitted at each
separate wavelength is independent of its neighbours so in effect it forms a virtual rib-
bon cable [54]. All wavelengths are subject to identical environmental effects but do
suffer from deterministic wavelength-dependent data skew. However over extended
distances recent suggestions [55] utilise adaptive electronic bit-skew compensation
and demonstrations [56, 57] have underlined the potential of this approach.
1.5 Shared-media Interconnects
A generic switching fabric is shown in Figure 1.4. Every node contains a transmit-
ter (T) and a receiver (R) to interconvert data between the optical domain within
the fabric and the electrical domain within the attached workstations. In a pho-
tonic packet switching network the route followed by optical packets between the
33. Chapter 1 14 Introduction
node1
nodeN
R
T
R
T
R
T
R
T
node3
node2
Photonic
Switching
Fabric
Figure 1.4: Photonic Switching fabric: T: Transmitter; R: Receiver.
source and destination nodes might not be explicitly prescribed. Instead reliance
is placed upon statistical multiplexing which assumes that the bursty nature of the
traffic originating from the nodes after aggregation within the fabric is averaged and
smoothed-out with time. However recent measurements on real networks contradicts
this assumption: traffic aggregation within a network tends to be self-similar on all
time scales and across different length scales from LANs [59] to WANS [60, 61, 62].
Should the aggregated demand for bandwidth exceed that which the fabric can sup-
port then buffering must be provided lest packets be discarded. But buffering can
introduce variable packet latency and out-of-order delivery. Both are undesirable
for real-time multimedia applications such as video.
It would be more useful to establish dedicated connections between nodes across
the photonic fabric and where each connection provides bandwidth in excess of
that generated by, or acceptable to, a node. Now when a connection is established
bandwidth is guaranteed and buffering is unnecessary thus allowing sustained data
transfer without the latency that arises from packet segmentation/reassembly and
buffering—the path between source and destination nodes is explicitly prescribed
and the latency is deterministically defined. However to establish a connection
the underlying network topology becomes critical. It can take several geometric
forms including Bus, Star, Ring, Tree, Mesh, Cubes, Hypercubes [36, 63]. Ideally
the chosen topology should allow connections to be established on-demand and
independently of other traffic within the photonic fabric i.e. be non-blocking.
A shared medium interconnection fabric can allow several nodes to interconnect
34. Chapter 1 15 Introduction
independently and concurrently through a sequential assignment of time (or wave-
length) slots, one per node. Data from the write section of a node is time- (or
wavelength-) multiplexed onto the shared optical fibre. The use of tunable time
slot selectors (or wavelength filters) at the receive section of each node allows the
channel of interest to be chosen for reception. The selector must have temporal
(or wavelength) agility to allow synchronisation (in the case of a TDMA network,)
or wavelength stability (within a WDMA network.) Shared medium interconnects
have been usefully classified as [64, 65]
1. Fixed-Transmitter(s), Fixed-Receiver(s) (FT-FR)
2. Tunable-Transmitter(s), Fixed-Receiver(s) (TT-FR)
3. Fixed-Transmitter(s), Tunable-Receiver(s) (FT-TR)
4. Tunable-Transmitter(s), Tunable-Receiver(s) (TT-TR)
The latter two, FT-TR and TT-TR, provide a broadcast and select network where
the transmission from one node can be received by one, many or all other node(s).
The TT-TR approach, though, suffers from an inablilty to broadcast efficiently as
well as the possibility of blocking.
To date most research into single-hop, shared-medium networks has been fo-
cussed on WDM implementations. Examples would include LAMBDANET [66]
from Bellcore which used a FT-FR configuration. Two versions were demonstrated:
18 wavelengths × 1.5 Gbit/s and 16 wavelengths × 2 Gbit/s. At the receive section
of a node the aggregated wavelength channels were spatially separated with each
allocated a separate receiver. The channel of interest was selected electronically.
Rainbow from IBM [67] was a FT-TR network with a broadcast star topology sup-
porting 32 workstations @ 200Mbit/s per wavelength channel. Signaling to effect
channel allocation was distributed to all nodes using a dedicated wavelength channel
that required an additional FT-FR pair within each node. In contrast, there have
been few demonstrations of TDM based interconnects. This is sightly surprising
since clock distribution and recovery is a common requirement of all systems. Barry
et al [68, 69] demonstrated a FT-TR, star-based, implementation that used a non-
linear optical loop mirror (NOLM) for channel gating in a lone receiver. The lack
of additional independent receivers limited the functionality to broadcast-only—bi-
directional transmission was not possible. Bi-directionality is necessary to properly
demonstrate a network. In this thesis the steps that led to the construction of such
a system will be reported.
35. Chapter 1 16 Introduction
1.6 Thesis Outline
This thesis describes the key ideas and components that were used to construct
a 40Gbit/s optical-TDMA interconection fabric which was used to interconnect
high-specification Unix workstations. The thesis will deliberately constrain itself
to the “optical plumbing” of this single-stage (single-hop) distributed switching fab-
ric. Chapter 2 which follows will develop the technical background that underpins
the thesis as well as outlining some of the common ideas within modern optical
communications systems. Chapter 3 will describe the various optical pulse source
technologies that were considered for the transmitter and will justify why one—based
on a combination of a gain-switched DFB laser diode and an electroabsorption mod-
ulation was chosen. Chapter 4 describes two separate demultiplexing techniques one
based on traditional optoelectronic clock recovery that utilised an electroabsorption
modulator, the other based on all-optical clock recovery that used an integrated
Mach-Zehnder inteferometer. Chapter 5 will describe how the work of Chapter 3
and Chapter 4 was synthesised and extended to construct a 40Gbit/s optical-TDMA
interconnection fabric that used the common optical fibre infrastructure within a
building. It will also outline an enhancement that uses wavelength-division multi-
plexing to increase the aggregate bandwidth. The work will be reviewed and an
attempt made to put it into context in Chapter ??. A list of patents, publications
and references generated during the course of the work described in the thesis will
be given in Appendix A. Finally Appendix B includes a copy of the patent that
arose from some of the work described in Chapter 3 as well as a small selection of
the peer-reviewed publications that were generated.
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43. Chapter 2
Background material
Claude Shannon described a generalised model of a point-to-point telecommunica-
tions link [1] shown in Figure 2.1. A network is usually composed of many point-
Receiver
Signal Received
Signal
Information
Source Destination
Noise
Source
Message Message
Transmitter
Figure 2.1: Shannon’s generalised communication network.
to-point links since it is uneconomic to establish a dedicated, one-to-one connection
between every user and so rationalisation is desirable to share connections. This
introduces concepts such as multiplexing, routing and switching. These functions
are presently implemented with electronics however research is being undertaken
to implement them optically. The desire is to relegate electronic processing to the
periphery of a network and replacing it with simple, but fast, all-optical techniques
within the network to route and convey information across a room, building, city or
even between continents. The traditional telephone network is circuit switched—a
one-to-one physical path is established between source and destination whether or
not information is being conveyed. But this is now being replaced by packet-switched
24
44. Chapter 2 25 Background material
networks based predominantly on the IP protocol. Packet switched networks seg-
ment information into packets that can be aggregated using statistical multiplexing
over many point-to-point links.
Electronics still offers a cost-advantage over optics but it can be anticipated
that this advantage will erode and be supplanted by optics as the demand for high-
bandwidth transmission and switching increases. This trend is reflected in the es-
tablishment of certain bodies such as the optical internetworking forum which sees
router vendors like Cisco, Juniper and Avici sharing the floor with telecommunica-
tions companies like Nortel and Lucent. The Japanese OITDA1
recently produced
a roadmap [2] which outlined the likely evolution of optical networks. Amongst
its forecasts for the year 2010 were: A transmission rate of 100 Mbit/s will be re-
quired within the home; 5 Tbit/s will be required for backbone network systems;
100 Gbit/s for LANs; and 600 Gbit/s for computer backplanes. It is uncertain if
electronics provide this, yet optics certainly can.
2.1 Transmitter
2.1.1 Optical pulse sources
Semiconductor-based devices are the first choice as optical transmitters because they
are compact, consume little power, have no moving parts and are a mature and re-
liable technology. A useful historical review of Semiconductor Lasers is given by
Holonyak [3] in which he credits John Bardeen 2
and his invention of the transistor
as being the starting point. The first theoretical proposition of the use of semicon-
ductors as coherent light sources was derived by John Von Neumann in a note to
Edward Teller in 1953 [4, 5].
During the the autumn of 1962 several groups in the United States demonstrated
stimuated emission from homojunction GaAs [6, 7, 8] and Ga(As1−xPx) [9] material
systems. The devices were essentially forward-biased p-n junctions where above a
critical carrier population (threshold current) population inversion leading to excess
optical gain. A coherent oscillator resulted when a resonant cavity was formed by
cleaving along the natural lattice planes of the material structure. These devices
supported osciilations at several cavity modes each corresponding to a separate wave-
length. Many improvements to the structure of devices was made in the intervening
1
Optoelectronic Industry and Technology Development Association
2
the co-inverntor of the transistor and the only person to win the Nobel prize for Physics
twice—for the Transitor and the BCS theory of superconductivity.
45. Chapter 2 26 Background material
years. Most notable was the development of band-gap engineering [3, 10] which
exploited quantum-size effects. The quantum-well3
superlattices that resulted arti-
ficially modified the bulk properties of the materials and produced devices towards
longer wavelengths where optical fibre loss was much lower.
For high speed (≥ 10GHz) TDM-based photonic networks short duration optical
pulses (<10ps) are required at a single wavelength. Data can be imparted onto the
pulses by subsequent external modulation. For semiconductor materials the most
important characteristic of modulation is given by the relaxation frequency, fr. The
bigger, fr, the shorter the pulse duration possible. This is expressed in terms of
some of the fundamental properties of the laser in Equation 2.1 [11],
fr =
1
2π
AP0
τp
(2.1)
where, A, is the differential optical gain; P0, is the average photon density within
the laser cavity; and, τp, is the photon lifetime. There are several techniques of
short optical pulse generation in semiconductor structures. Lau [12] provides a
comprehensive and clear exposition of these techniques in semiconductor lasers, as
do White [13] and Mamyshev [14]. The main techniques are:
1. Mode-locking or phase locking whereby a mechanism within the laser cavity
causes longitudinal cavity modes to interact and become highly correlated.
This process forms a super-modal short optical pulse with a repetition rate
that is inversely proportional to the cavity length. The pulsewidth attainable,
∆τ, is given by Equation 2.2
∆τ =
1
(2M + 1)∆ν
(2.2)
where, ∆ν is the frequency separation between cavity modes; M, is the number
of cavity modes supported within the gain bandwidth of the device. Several
variants of mode-locking are possible. Anecdotally it produces the best pulses
amongst the other varients. However most implementations depend on an
external diffraction grating which can suffer from mechanical instabilities, as
the repetition rate is lowered so the the external cavity must be lengthened:
(a) Active mode-locking: is achieved by actively modulating the gain or loss
of the laser cavity at a frequency equal to the frequency spacing between
3
A quantum-well is formed if a thin slice of a low band gap material, such as InGaAs, is
sandwiched between two layers of a high bandgap material, AlGaAs/GaAs for example.
46. Chapter 2 27 Background material
longitudinal modes, so that each mode is driven by the modulational
sidebands of its neighbours. Section 3.5.3.4 of Chapter 3 provides an
example of such a device.
(b) Passive mode-locking: here the same effect is achieved using a passive
intra- or extra- cavity saturable absorber.
(c) Self-pulsating laser diodes: are comprised of two sections. One section,
the gain region, is strongly forward biased; whilst the other section, the
absorption region, is weakly forward biased. Under appropriate bias con-
ditions the amount of optical attenuation and feedback within the cavity
can produce a regular train of optical pulses.
(d) Colliding pulse mode-locking: if the saturable absorber region is placed
centrally within the laser cavity then two pulses can propagate simulta-
neously. The pulses collide in the central region and produce a train of
optical pulses at twice the repetition rate of conventionally mode-locked
lasers.
2. Gain-switching: in gain-switching an initial electrical current spike is termi-
nated, preventing a second optical relaxation oscillation from occuring, to
produce a single light pulse. Of course, if the current spike is repeated at
regular intervals a train of light pulses is generated. In a distributed feed-
back (DFB) laser where periodic perturbations within the gain region assured
single-mode operation. Chapter 3 will reveal some of the problems associated
with this device. For example they suffer undesirable effects such as timing
jitter and interpulse pedestal. Chapter 3 will outline some methods to reduce
these effects.
3. Q-switching: this involves increasing the loss of the laser cavity to suppress
lasing whilst simultaneously pumping the laser with carriers. Eventually when
a sizable gain-inversion is obtained the cavity loss is suddenly removed and a
short, intense Q-switched pulse emerges.
4. Electroabsorption modulation: This is an attractive technique particularly for
high speed (>10GHz) applications. It can be used to modulate the output
from a continuous wave (CW) source to produce a train of optical pulses.
The main drawback stems from the static insertion loss and the necessity of
discarding some of the power in the modulation process. Nevertheless it is a
very attractive technology. The use of electroabsorption modulators as pulse
47. Chapter 2 28 Background material
sources (Chapter 3) and de-multiplexers (Chapter 4) will be considered in this
thesis.
2.1.2 External modulation
The techniques described in the last subsection produce an optical pulse sequence
consisting entirely of ‘1’s at the base rate B. External modulation serves to gate
these optical pulses with a time-dependent electrical data signal for transmission
to a remote receiver. In direct detection systems the data is represented by the
presence or absence of light within a time-interval, 1/B.
The electrical field within an optical pulse can be expressed in terms of a time-
dependent vector, E(r, t) given by Equation 2.3 [15]
E(r, t) = E P exp[−ı(k · r − ωct − δ)] (2.3)
where, E is the peak electric field amplitude, P is the polarisation matrix vector, k
is the propagation vector, r is the range vector, ωc is the carrier angular frequency
(≈ 1014
Hz), δ is the carrier phase and t, as usual, represents time. Causality as
represented by the Kramers-Kronig relations [16] dictates that any change in the
imaginary refractive index, nimag, begets a change in the real refractive index, nreal
and vice versa4
. The linewidth-broadening (or linewidth-enhancement) factor, α, is
given by Equation 2.4 [17]
α ≡
∆nreal
∆nimag
(2.4)
where ∆nreal, is the change in the real refractive index inducing a phase change,
and ∆nimag is the change to the imaginary refractive index inducing an absorptive
change.
In an InGaAsP/InP electroabsorption modulator where α is small the application
of a reverse-bias electric field increases the absorption (decreases E in Equation 2.3.)
Consequently the application of a time-varying electrical signal, s(t), opens a time-
varying optical gate or window,
E (1 − mas(t)) P exp[−ı(k · r − ωct − δ)] (2.5)
where ma ≤ 1, is the amplitude modulation index of the device. In contrast, for
LiNBO3, where α is large, the application of an external electical data signal induces
4
Figures 1(a)–(c) of Toll [16] provide a crystal clear exposition and a very intuitive explanation
of the Kramers-Kronig relations based on the principle of classical causality—namely that an event
cannot precede its cause.
48. Chapter 2 29 Background material
a change to the refractive index of the material via the Pockels effect. This modulates
the optical path length inducing a phase change to the coherent optical field within
the material. This can be converted to an amplitude change when placed in one (or
both) arms of a Mach-Zehnder interferometer [19]. It can be represented thus
E P exp[−ı(k · r − ωct − δ − mδs(t)] (2.6)
where, mδ, is the phase modulation index of the material and is, ideally, an exact
multiple of π/2 in the balanced Mach-Zehnder geometry described in Chapter 4.
In many cases non-linear effects cause the quantitites in Equation 2.3 to inter-
act. For example, direct electrical modulation of a laser modulates both the the
amplitude and phase of the emitted optical field. The non-monotonic change of the
carrier frequecy is called chirping which leads to power penalties in transmission
systems [20]. Gain-switching which was mentioned in the previous subsection is a
particular variation of direct modulation (the data stream applied is, essentially, a
continuous sequence of ‘1’s.) The, α factor in this case provides a useful index for
the wavelength chirp of the device [17, 18].
2.1.3 Multiplexing
One method that can be used to increase the quantity of information carried between
a source and a remote destination is to increase the data capacity of the intervening
transmission medium. The most obvious technique is to install several more optical
fibres to carry additional, but separate time-division multiplexed systems shown in
Figure 2.1. However the cost of installing more optical fibre may be economically
prohibitive. So in many cases it is preferable to upgrade the transmission and
receiving equipment at both the source and destination of the system, especially
given that a single optical fibre has an estimated 40THz of bandwidth available
in the near-infrared wavelength region. Three techniques are possible: Upgrading
of the TDM link by increasing the transmitter and receiver bit-rates; Wavelength
Division Multiplexing (WDM); Optical Time Division Multiplexing (OTDM).
2.1.3.1 Time-division multiplexing
A typical TDM system is shown in Figure 2.2. Here the optical data rate transmitted
over the optical fibre, the line rate, is equivalent to the electrical signal rate or
base-rate. A high-speed electronic multiplexer (MUX) is required to electrically
combine the data from several information sources (ISn) before application to an
