♦ The Evolution of Optical Systems: Optics
Everywhere
Rod C. Alferness, Herwig Kogelnik, and Thomas H. Wood
With the explosion of capacity demands driven by the Internet, optical networking
systems are experiencing tremendous growth and are providing increasingly high
transmission capacities. As importantly, with the advent of the optical amplifier and
wavelength division multiplexing (WDM), optics is playing a larger role in networking
and is extending further to the edge of the network. Once limited to long-haul
point-to-point systems, Lucent Technologies is now commercializing multipoint
metro WDM ring systems that include software-controlled optical wavelength
add/drop multiplexers and soon will offer large optical cross connects. These optical
network elements, together with network management software, will enable rapid
provisioning of wavelength services, as well as rapid network restoration. In addition, as the cost of optics is driven down and the demand for bandwidth to businesses and residential customers continues to grow, optical systems are extending
out from the network core and metro to access applications. The confluence of a
proliferation of broadband service applications and rapidly maturing optical technology are literally driving optical systems into all segments. Increasingly, optics is
literally everywhere.
Introduction
Early lightwave systems were intended primarily
for long-haul, uninterrupted optical transport between
distant points. The recent introduction of wavelength
division multiplexing (WDM) for capacity expansion
of point-to-point links has opened the way for evolution to optical layer networks. In these networks,
optical channels (defined on an optical fiber by their
wavelengths) provide the bandwidth units by which
multi-node transmission networks can be built. Such
networks, which require sophisticated optical network
elements that in turn require highly functional optical
components, including optical switching fabrics, are
now part of products or product plans of Lucent
Technologies’ Optical Networking Group. Beyond
backbone and metro transport optical networks, the
evolution of optics into access networks and enterprise
local area networks (LANs) appears highly viable.
Researchers and engineers at Bell Labs have
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played a critical role in the explosion and proliferation
of optics over the last 25 years. In this paper, we provide a brief overview of the evolving and expanding
role of optics in communication systems. We first discuss the growth of point-to-point transmission systems, then follow with the evolution of WDM
transport networks, and finish with a view of access
and LAN systems. The advances in optical systems and
higher-level network capabilities that provide our
focus have depended strongly on advances in optical
components and optical fiber technology. These
related topics are summarized in companion papers by
Brinkman et al.1 and Glass et al.2
High-Capacity Transmission Systems
The explosive growth of optical fiber transmission
technology parallels that of computer processing and
other key information technologies. These technolo-
Copyright 2000. Lucent Technologies Inc. All rights reserved.
gies are combining to meet the burgeoning global
demand for new information services including data,
Internet, and broadband services. Indeed, their rapid
advance has helped fuel this demand.
Lightwave technology is now advancing at a rate
exceeding a factor of 100 every ten years. This is evident from Figure 1, which depicts the transmission
capacity achieved per fiber as a function of year for the
last 20 years. Two trend lines are shown: one for commercial systems and one for system prototypes
demonstrated in research laboratories. Note the logarithmic scale on the vertical axis showing capacity in
bits per second. As in other information technologies,
“giga” (109) performance has been improving to “tera”
(1012) performance over a course of 15 years.
Long distance transmission entered the “tera era”
experimentally in 1996, when three research laboratories reported transmission capacities of 1 Tb/s per fiber
(discussed further below). To appreciate this staggering
transmission capacity, recall that a fiber is just a thin
strand of glass with a diameter comparable to that of a
human hair, and a terabit is, of course, a million
megabits. At the terabit-per-second rate, this implies
that one hair-thin fiber can support about 40 million
data connections at 28 kb/s, 20 million digital voice
telephony channels, or a half million compressed digital television channels.
To gain further perspective on the advances of
lightwave technology, recall the digital transmission
technologies that it replaces: twisted pair and coaxial
cable systems. Compared to these, fiber technology
offers economic advantages beyond increased capacity,
such as the small size and weight of fiber-optic cables
and the large spacing of optoelectronic regenerators.
The first digital transmission technology in the United
States was the T1 carrier system introduced in 1962. It
carried 24 digital voice channels over a twisted pair of
copper wires at a transmission rate of about 1.5 Mb/s.
The regenerator spacing of T1 systems was about 2 km
(6000 feet). The most advanced digital coaxial cable
transmission system introduced in the early 1980s was
the T4M system. It used a 0.375-in–diameter coaxial
cable and transmitted at a rate of 274 Mb/s with a
regenerator spacing of 1.6 km (1 mile). In contrast, the
regenerator spans of commercial fiber-based systems
Panel 1. Abbreviations, Acronyms, and Terms
ATM—asynchronous transfer mode
CATV—cable television
CPON—composite PON
CSMA/CD—carrier-sense multiple access/collision
detection
CWDM—coarse WDM
DARPA—Defense Advanced Research Projects
Agency
DFB—distributed feedback
DLC—digital loop carrier
DS—dispersion shifted
DSL—digital subscriber line
DWDM—dense WDM
EDFA—erbium-doped fiber amplifier
EMI—electromagnetic interference
ETDM—electronic time division multiplexing
FSAN—Full Service Access Network
FTTC—fiber to the curb
FTTH—fiber to the home
HFC—hybrid fiber-coax
IC—integrated circuit
InP—indium phosphide
LAN—local area network
MONET—Multi-Wavelength Optical Networking
NGLN—Next-Generation Lightwave Network
NTT—Nippon Telegraph and Telephone
Corporation
OC-N—optical carrier digital signal rate of
N 3 51.840 Mb/s in a SONET system
ONU—optical network unit
PON—passive optical network
QAM—quadrature amplitude modulation
RF—radio frequency
SDH—synchronous digital hierarchy
SM—single mode
SONET—synchronous optical network
T1—terrestrial (North American) facility for
transporting signals at the primary rate of
1.544 Mb/s (24 64-kb/s channels)
TDMA—time division multiple access
TDM—time division multiplexing
VCSEL—vertical-cavity surface-emitting laser
WDM—wavelength division multiplexing
are typically at least 50 km and reach to several hundred kilometers, as shown in Table I, an expansion of
an earlier published table.3
The dramatic increase in lightwave system capacity has had a strong impact on lowering the cost of
Bell Labs Technical Journal ◆ January–March 2000
189
1014
1013
Capacity per fiber (b/s)
1012
1011
1010
109
108
19
78
19
80
19
82
19
84
19
86
19
88
19
90
19
92
19
94
19
96
19
98
20
00
107
Year
Experimental
Single channel (ETDM)
Single channel (OTDM)
Multi-channel (WDM)
WDM + OTDM
WDM + POL
WDM + OTDM
WDM
Commercial
Single channel (ETDM)
Multi-channel (WDM)
ETDM – Electronic time division multiplexing
OTDM – Optical time division multiplexing
POL – Polarization division multiplexing
WDM – Wavelength division multiplexing
Figure 1.
Progress in lightwave transmission capacity.
long distance transmission. The Dixon-Clapp rule projects that the cost per voice channel reduces with the
square root of the systems capacity. Using this, we estimate from Figure 1 that the technology cost of trans-
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mitting one voice channel is decreasing by a factor of
10 every ten years. Consequently, distance is playing a
smaller and smaller role in the equation of telecom
economics. An Internet user, for example, will click a
Table I. Generations of terrestrial lightwave technologies.
Year
Fiber
type
Wavelength
WDM
channels
Bit rate/
channel
Bit rate/
fiber
FT3
1980
MM
0.82 µm
1
45 Mb/s
45 Mb/s
672
7 km
FT3C
1983
MM
0.82 µm
1
90 Mb/s
90 Mb/s
1,344
7 km
FTG-417
1985
SM
1.3 µm
1
417 Mb/s
417 Mb/s
6,048
50 km
FTG-1.7
1987
SM
1.3 µm
1
1.7 Gb/s
1.7 Gb/s
24,192
50 km
FTG-1.7 WDM
1989
SM
1.3/1.55 µm
2
1.7 Gb/s
3.4 Gb/s
48,384
50 km
FT-2000
1992
SM
1.3 µm
1
2.5 Gb/s
2.5 Gb/s
32,256
50 km
—
SM
1.3/1.55 µm
2
2.5 Gb/s
5 Gb/s
64,120
50 km
NGLN
1995
SM
1.55 µm
8
2.5 Gb/s
20 Gb/s
258,000
360 km
NGLN II
1997
SM
1.55 µm
16
2.5 Gb/s
40 Gb/s
516,000
360 km
WaveStar™ 400G 1999
SM
1.55 µm
80
40
2.5 Gb/s
10 Gb/s
200 Gb/s
400 Gb/s
2,580,000
5,160,000
640 km
640 km
System
FT-2000 WDM
Voice channels
per fiber
Regenerator
spans
MM – Multimode
NGLN – Next-Generation Lightwave Network
SM – Single mode
WDM – Wavelength division multiplexed
Web site regardless of its geographical distance. This
new paradigm was eloquently described in a recent
telecom advertisement in the United Kingdom:
“Geography is History.”
Let us now examine some milestones in optical
systems that underpin the progress recorded in
Figure 1 and Table I. Consider first the data near the
lower left corner of the figure. The lowest refers to the
FT3 lightwave technology introduced by the Bell
System in 1980.4 It had a digital transmission rate
of 45 Mb/s and a regenerator spacing of 7 km.
Multimode fibers were used and were operated at a
wavelength of 0.82 µm. The light sources were
gallium-arsenide–based semiconductor lasers. A speed
upgrade of this technology, the FT3C system, carried
90 Mb/s per fiber. It was deployed in the first major
lightwave installation of the United States—the
Northeast Corridor system, linking Washington, D.C.,
with New York in 1983 and New York with Boston
in 1984.3
A major shift of lightwave technology occurred
immediately after the initial Northeast Corridor
deployment. Research and development had prepared
the enabling component technologies for this breakthrough:1,2 The new lightwave generation used single-
mode (SM) fibers for higher capacity and switched the
operating wavelength to 1.3 µm and then to both
1.3 and 1.55 µm where the “low-loss” and “minimumloss” fiber windows occur. SM lasers were required to
match to the SM fibers; these had to be based on
indium phosphide (InP) substrates to deliver the new
wavelengths required. The FT Series G systems of
1985 operated at data rates of 417 Mb/s and then
1.7 Gb/s, and the lower loss allowed regenerator spacings of 50 km.3
Now let us switch our attention to the upper trend
line in Figure 1 showing the lightwave prototype system experiments demonstrated in research labs worldwide. The entries shown represent demonstrations
that were world records at the time; the research community calls them “hero experiments.” Many of the
entries represent Bell Labs accomplishments.
First, we focus our attention on the experiments
indicated by solid circles. These demonstrated increases
in fiber capacity by using high-speed electronic time
division multiplexing (ETDM) and the new SM fiber
technology operating at 1.55 µm to achieve large
regenerator spans. For this set of data, increases in
gigabit-per-second rates were accomplished by further
advances in the enabling technologies, including
Bell Labs Technical Journal ◆ January–March 2000
191
dispersion-shifted (DS) fibers, dispersion compensation
(in the later high-speed experiments), distributedfeedback (DFB) lasers providing spectral control, highspeed modulation, sensitive high-speed receivers, and
high-speed electronics. One example of these highspeed systems is the 1986 demonstration of 8 Gb/s
transmission over 68 km of SM fiber.5 In time, these
system experiments were pushed to 10 Gb/s, 20 Gb/s,
and, recently, 40 Gb/s. The trend for the solid circles
indicates that the single-channel bit rate of advanced
high-speed systems increases by about a factor of
10 within ten years. The major factor limiting this
advance has been the availability of high-speed electronics and high-speed integrated circuits (ICs).
Recognition of this limitation led to yet another revolution in lightwave technology: wavelength division
multiplexing.
Computers have a problem similar to that of lightwave systems: their processing power—pulled by
demand and pushed by advances in technology—
increases by a factor of 100 or more every ten years,
while the raw speed of the ICs that computers are
based on increases by only a factor of 10 or so. The
answer of computer designers has been the use of parallel architectures. The answer of the designers of
advanced lightwave systems is similar: the use of
many parallel high-speed channels carried by different
wavelengths. This is called wavelength division multiplexing (WDM). The use of WDM provides other advantages, such as the tolerance that WDM systems have of
the high dispersion present in the low-loss window of
embedded fibers. Together with newly developed
erbium-doped fiber amplifiers (EDFAs), WDM systems
have been responsible for yet another revolutionary
generation of lightwave systems, creating the “knee”
in the upper (research) trend line of Figure 1. This
latest generation has not only led to a dramatic
increase in fiber capacity but has enabled very large
regenerator spacings (up to 10,000 km in undersea
systems) and has opened a new dimension in networking: it added the dimension of wavelength to the
earlier dimensions of space and time.
WDM required the development of new enabling
technologies, including high-gain broadband optical
amplifiers, guided-wave wavelength filters and multi-
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plexers, and WDM laser sources. It also required new
systems and fiber concepts to counteract nonlinear
effects caused by the large optical power arising from
the presence of many channels in the fiber. Dispersionmanagement techniques were invented for this purpose, using system designs that avoid zero dispersion
locally but provide near-zero dispersion globally. New
“non-zero dispersion” fiber types were conceived for
the same purpose.
An early large-scale demonstration of the capability of WDM systems was the Roaring Creek Field
Trial6,7 (not shown in Figure 1), which was begun in
1989 and completed in 1991. It demonstrated transmission of four WDM channels at data rates of
1.7 Gb/s per channel over a system span of 840 km.
Thirteen EDFAs were used with a 70-km spacing
between them. A concurrent research experiment at
Bell Labs demonstrated transmission over eight channels at 2.5 Gb/s per channel. This was soon followed
by a 1993 hero experiment showing transmission of
eight WDM channels, each operating at 10 Gb/s over a
280-km span of dispersion-managed fiber.8 The bit
error rate achieved in this 80-Gb/s experiment was less
than 10-13. The entry for this experiment in Figure 1 is
the triangle at the knee of the upper trend line.
After 1993, rapid increases occurred in capacity
improvements of research systems, and we soon witnessed the first large-scale deployment of a commercial WDM system. This was the deployment of the
NGLN system, begun in 1995, in the long distance network of AT&T.
Following the 1993 research experiments, a rapid
sequence of experiments worldwide has demonstrated
larger and larger capacities per fiber, and this trend is
still continuing.9–11 Note, for example, the 1994 experiment where 17 WDM channels, each having a data
rate of 20 Gb/s, were transmitted over 150 km of dispersion managed fiber.
In early 1996, three simultaneous announcements
reported breaking the terabit-per-second barrier,
launching lightwave transmission technology into the
“tera era.” These breakthroughs were demonstrated by
researchers at Fujitsu,12 Bell Labs,13 and NTT.14 It is
interesting to note the differences among the three
approaches, which all used WDM techniques. Fujitsu
used 150 km of conventional SM fiber with dispersion
compensation and demonstrated transmission of
55 WDM channels with data rates of 20 Gb/s each.
Bell Labs used 55 km of non-zero-dispersion fiber and
transmitted 25 WDM channels that were polarization
multiplexed to 50 independent channels with data
rates of 20 Gb/s each. NTT used 40 km of DS fiber and
transmitted 10 WDM channels at data rates of
100 Gb/s each. The latter data rates were obtained by
optical time division multiplexing.
Concluding our discussion of long distance transmission, we should add two remarks that relate to
fiber nonlinearities. The first has to do with soliton
transmission technology. Solitons are short pulses in
which fiber dispersion and fiber nonlinearities are balanced to maintain short pulse widths over very
long distances. Experimental soliton systems have set
several world records on another economic measure,
the bit-rate 3 distance product, which is a measure for
the number of regenerators required for a given transmission task. That is, the higher the bit rate and the
longer the distance between regenerators, the fewer
the number of fibers and regenerators that are needed.
This measure is particularly important for transoceanic
systems, which have spans up to 10,000 km. Solitons,
therefore, promise lower cost for future advanced
ultra-long distance transmission systems.
The second remark concerns the ultimate limit to
the capacity growth shown in Figure 1. We now know
that the loss window in silica fibers can be extended to
a maximum bandwidth of about 50 THz.1 Current systems use binary amplitude modulation and achieve
spectral efficiencies up to about 0.4 b/s/Hz, which
would theoretically provide transmission rates of
20 Tb/s per fiber if the entire low-loss window were
utilized. Shannon’s theorem15 tells us that spectral
efficiency can be increased at the expense of increased
signal power. For example, multilevel quadratureamplitude-modulation (QAM) systems that are used
in other lower-bit-rate technologies (for example,
wireless) achieve spectral efficiencies of about
4 b/s/Hz. In principle, such advanced coding techniques could enable transmission rates up to 200 Tb/s.
However, increasing the power levels in the fiber will
increase nonlinear effects, and the ultimate limit
imposed by these effects is yet to be fully understood.
Optical Networks
From the earliest days of fiber optics, researchers
envisioned optical networks that went beyond simple
point-to-point systems to include multipoint switched,
or at least configurable, networks. Many of the early
fiber optics researchers came from the radio-frequency
(RF) and microwave worlds and naturally viewed
frequency-division networks, including, for example,
channel-dropping filters, as obvious areas to pursue.
The concept of optical switching, the routing of optical
beams that could carry very high bandwidth signals,
was also a natural focal point. However, technology
was embryonic; primary efforts focused on optical
sources and detectors, the critical enablers for point-topoint links whose technical feasibility and economic
viability had to first be established. Nevertheless, a segment of early research efforts focused on the other key
optical technologies and components that would one
day be required to build optical networks—for example,
highly functional components like tunable wavelength
filters and couplers, optical switches, tunable lasers,
and star couplers.
Networks are fundamentally about sharing
resources. Optical networks could take various shapes.
The parameter around which optical networks might
be based would depend upon how optical sharing or
multiplexing of the transmission and distribution
evolved. While many early Bell Labs researchers were
predisposed to using frequency multiplexing to share
the transmission resource, time division multiplexing
(TDM)—driven by the digital revolution—was being
implemented in backbone transmission formats.
Electrical time-division multiplexing is well suited for
the rates required for voice circuits as well as the much
higher rates that would minimize the cost per bit for
long distance backbone transmission. As discussed in
the previous section, wavelength multiplexing would
have to wait until TDM rates became limited by transmission impairments and the optical amplifier made
WDM cost effective.
By analogy with TDM systems, the evolution
from simple point-to-point systems to multipoint
wavelength-based networks was rather apparent16
Bell Labs Technical Journal ◆ January–March 2000
193
Wavelength
multiplexer/
demultiplexer
Wavelength
multiplexer/
demultiplexer
WDM/point-to-point transport
• High-capacity transmission
Fixed WDM/multipoint network
• Fixed sharing between multiple nodes
• Passive access of wavelength channels
Photonic cross connect and WADM
Reconfigured WDM/multipoint network
• Automated connection provisioning
• Flexible adjustment of bandwidth
• Network self-healing/restoration
Fiber amplifier
Wavelength add/drop
WADM – Wavelength add/drop multiplexer
WDM – Wavelength division multiplexing
Wavelength cross connect
Figure 2.
From photonic transport to photonic networks.
(see Figure 2). While WDM point-to-point systems
lier point. Once WDM transmission systems become
provide very large capacity between widely spaced
technically and economically feasible, by dropping
(300- to 600-km) end terminals, in many networks it
(and subsequently adding) only the necessary wave-
is necessary to drop some traffic at intermediate points
lengths, considerable cost can be saved while provid-
along the route between these major nodes. One
ing greater network connectivity. Wavelength
could drop all that traffic (all wavelengths), but that
add/drop multiplexers that can selectively drop a
requires putting expensive electronics on wavelength
desired wavelength while directly passing other wave-
channels that could instead be expressed through tak-
lengths without optoelectronic conversion are the
ing advantage of the lower cost of amplifiers at an ear-
required network elements.
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Bell Labs Technical Journal ◆ January–March 2000
Network
management
and control
Network element controller
Transport
interface
(multiwavelength)
λ1
Monitor
and adjust
λ1 → λi
Switch fabric
MUX
DMUX
λN → λj
λN
MUX
DMUX
Wavelength
adaptation
Non-compliant
Compliant
Client interface
Client interface
DMUX – Demultiplexer
MUX – Multiplexer
Fiber amplifier
Figure 3.
Configurable optical network elements.
Again by analogy to TDM networks, the next logical evolutionary step would be WDM rings in which
node-to-node bandwidth provisioning, as well as network protection, is based on wavelength channels (see
Figure 2). To provide greater connectivity and protection from multiple failures, simple WDM rings can be
interconnected to build a mesh of rings. Alternatively,
a network based upon a mesh of nodes interconnected
by wavelength-based paths or circuits through the net-
work can be used to provide continuity and restoration in case of fiber or equipment failures. This is again
the wavelength/optical analog of present electrical
TDM-based networks.
The optical network elements needed to build
flexible wavelength-channel-based optical transport
networks are wavelength add/drop multiplexers and
wavelength-based optical cross connects. These network elements are shown schematically in Figure 3.
Bell Labs Technical Journal ◆ January–March 2000
195
Optical
components
Optical systems
WDM networking
• Wavelength converters
• Robust WDM networks
• WDM cross-connect fabrics
• Integrated add/drop
• WDM cross connects
• Dynamic gain equalizers
• Ultra-wideband amplifiers
• WDM rings
• Wavelength monitors
• Tunable lasers/couplers
• WDM add/drops
• WDM routers
• WDM sources
• Point-to-point systems
• Amplifiers
• Fibers
WDM – Wavelength division multiplexing
Figure 4.
Optical components required to build robust, fully reconfigurable WDM networks.
Functionally, the two elements are quite similar, differing primarily in the number of input fibers that
need to be handled. Functionally, the role of each element is to provide, under network control, the ability
to connect any input wavelength (optical channel)
from an input fiber to any one of the output fibers or
to a drop channel. In addition, the element provides
power leveling, possibly wavelength monitoring and
connection verification. The network also is an entry
point for optical channels. Again, through network
control, an added optical channel can be switched to
any desired output port.
To build the network elements required for multipoint optical networks, optical devices with significant
optical functionality are required. Indeed, as shown in
Figure 4, a host of optical components, including
wavelength monitors, dynamic gain equalizers, optical
switch fabrics, and wavelength converters are required
to build robust, fully reconfigurable WDM networks.
Fortunately, the early vision that optical networks
would be dependent upon highly functional optical
components and subsystems drove, in turn, the early
concept and field of integrated optics. When low-loss
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fiber first suggested that fiber optic communications
might be feasible, the optical functional components
that existed (for example, wavelength filters) were
large bulk devices. Massive mounts and stable tables
were required to cascade such elements. Against this
backdrop—well before it was obvious that fiber-optic
communications would ever be technically or economically viable or commercially necessary—Bell Labs
researchers proposed the concept of integrated optics.
The idea was to build, on a single optical substrate,
optical waveguide circuits based on active and passive
waveguide functional components—with waveguide
interconnects between the components—achieving an
optical subsystem on a chip. Integrated optics was first
articulated and proposed in a number of papers in the
September 1969 Bell System Technical Journal.17
Several integrated optics materials systems
evolved. For circuits that included active devices, optical InP (the same technology used for lasers and
detectors) and titanium-diffused waveguides in the
electro-optically active lithium niobate substrate
have become the primary technologies.1,18 These
material systems provided an early technology plat-
form to build small integrated switch arrays. In
fact, lithium niobate switch arrays were used to
demonstrate an early multi-stage optical cross connect.19 Later, silica-on-silicon technology was developed. This has become the workhorse for passive
components such as wavelength demultiplexing
devices—including the waveguide grating router,20 a
key element in first-generation WDM point-to-point
systems and a building block for configurable wavelength add/drop multiplexers.
Interestingly, even before the technical viability of
the optical amplifier was demonstrated, researchers at
Bell Labs in the mid 1980s were exploring the possibilities of WDM-based LANs. These included star-based
networks that employed both direct and coherent
detection techniques. While this work did not result in
commercial products, it was important for several reasons. First, the network research provided the driver
and testing ground for critical WDM optical components, such as filters, tunable sources, and WDM multiplexers. In addition, this work sparked the formation
of the All-Optical Networking Consortium,21 a collaborative effort between Bell Labs (then part of AT&T),
the Massachusetts Institute of Technology Lincoln
Labs, and Digital Equipment Corporation (now
Compaq). The consortium demonstrated the technical
feasibility of local and metropolitan WDM networks
based upon passive routing of optical wavelengths.
The All-Optical Networking Consortium was followed by the Multi-Wavelength Optical Networking
(MONET) Program. The vision of the founders of
MONET was to demonstrate the feasibility of a
national level end-to-end wavelength routed network
based upon long-reach WDM transmission and wavelength cross connects and add/drops. The MONET
consortium, funded by the Defense Advanced
Research Projects Agency (DARPA), included AT&T,
Bellcore, Bell Atlantic, BellSouth, and Southwestern
Bell. 22 In its first phase, three testbeds—a local
exchange WDM ring testbed, an optical cross-connect
testbed, and a long distance WDM transmission testbed—were built in New Jersey. In an early demonstration, software-controlled provisioning of wavelength
circuits from the local exchange ring through the cross
connect and the long distance (2000 km) link and
back through another node on the WDM ring showed
the feasibility of wide-area optical networks.23 In the
second phase of MONET, a dual-WDM ring network
interconnected by a pair of optical cross connects was
built in Washington, D.C. The optical add/drop and
cross-connect network elements used to build this network, while prototypes, are fully operational in the
field. Completed in the fall of 1999, this network is
carrying live traffic between government agencies.24
Optical Access Networks
A trend in the evolution of optical networks is the
push of fiber from core applications out closer to the
end customer. This is happening as a result of two
forces: the continued drop in the cost of providing
optical links, generated by improved technologies and
ever-growing manufacturing volumes, and the
increased bandwidth demands from users, generated
by increased Internet use and applications such as
video-on-demand.
Large- and medium-sized business users can
often be connected to core networks using low- and
medium-speed synchronous-optical-network (SONET)/
synchronous-digital-hierarchy (SDH) TDM multiplexers. These can be deployed in rings to provide
protection, if needed. A variety of cost-effective TDM
multiplexers with tributaries having speeds
as low as T1 (1.5 Mb/s) and line rates between
OC-3 (155 Mb/s) and OC-48 (2.5 Gb/s) have been
developed. More recently, as costs have fallen and demands have increased, some economical WDM systems have been introduced for network access. These
make use of the protocol independence of WDM to
transport a variety of signals, such as OC-12 (622 Mb/s)
and Gigabit Ethernet (1 Gb/s), simultaneously on a
single fiber.
Another trend in the deployment of fiber for business use is the increasing application of fiber-based
links for interconnecting enterprise data routers and
switches in the LAN and campus environments. As
campus backbones have moved from 10 Mb/s to
100 Mb/s or Gigabit Ethernet rates, a variety of lowcost fiber interfaces have been developed that can haul
these broadband signals over distances exceeding the
roughly 100-m limit of copper twisted pairs. These are
Bell Labs Technical Journal ◆ January–March 2000
197
typically single-wavelength links operating bidirectionally on two separate fibers. One popular standard for
fiber-based Gigabit Ethernet uses 850-nm verticalcavity surface-emitting lasers (VCSELs) and multimode fiber running distances of about 250 m. Another
popular standard is based on 1310-nm edge-emitting
lasers running about 5 km on SM fiber. Continuing
this trend, a standard for 10-Gigabit Ethernet is
expected in 2002, with initial products in 2001.
At the other end of the enterprise network, copper
twisted pair reigns supreme (at least today) in connecting to individual desktops. Even Gigabit Ethernet-tothe-desk appears to be attainable with twisted pair at
lower cost than the most inexpensive fiber links today.
It remains to be seen whether some unique advantage
of fiber over copper (such as lower electromagnetic
interference [EMI]) or bandwidth demands exceeding
that of Gigabit Ethernet will finally drive the widespread deployment of fiber to the desk.
The challenge of bringing fiber close to residential
and small business customers is more difficult than for
larger enterprises because of the lower willingness to
pay and lower bandwidth demand. Nevertheless, this
is an important challenge for several reasons. First,
from the network-operator and equipment-builder
perspectives, the revenues possible from widespread
deployment of broadband technologies in this market
are enormous. Second, there is no issue more constraining the continued growth of the Internet than
the poor end-user performance seen over residential
dial-up modems, which are limited to 56-kb/s capacity. Clearly, there is tremendous pent-up demand
worldwide for broadband, always-on data connectivity
to residences and small businesses. The challenge is to
provide this in a cost-effective manner. Bringing fiber
close to the customer is an essential part of the solution to this problem.
Fiber is already widely deployed in residential telephony loop plant via digital loop carrier (DLC) systems. These systems use fiber to connect a central
office to a remote terminal, which performs optical-toelectrical signal conversion and delivers service to several thousand living units by transferring the optical
signals onto twisted pairs. Although originally engineered for voice service, DLCs can be upgraded to
198
Bell Labs Technical Journal ◆ January–March 2000
higher capacities by using one of the various forms of
digital-subscriber-line (DSL) technology on the twisted
pairs to the living units and by using the large bandwidth of optical fiber to connect the DLC units to the
central office.
In order to achieve even higher bandwidths, fiber
can be economically pushed closer to the customer
through the use of a passive optical network (PON).25
In a PON, a single fiber connects a central office to a
passive optical splitter, which distributes the signal to
about 16 optical network units (ONUs). The ONUs
perform optical-to-electrical conversion and deliver
service to living units. If an ONU is located at a home,
this is described as a fiber-to-the-home (FTTH) system; if
the ONU is shared over several (about 4 to 32) living
units, this is termed a fiber-to-the-curb (FTTC) system. A
PON is more cost-effective than running a fiber to
every ONU because the cost of the central-office optoelectronics and the cost of the fiber between the central office and the passive splitter are shared over
multiple users.
A variety of PON systems have been demonstrated
in the laboratory. Since many users share the fiber, an
important question is how the different users should
access the fiber bandwidth. The simplest approach is
for all users to operate in the same wavelength band
and share the bandwidth in timeslots. In the downstream (central office to ONU) direction, this is simply
TDM; in the upstream direction, this is termed time
division multiple access (TDMA). Unlike the LAN environment, where users are physically close to each
other, collisions generated by randomly transmitting
users cannot be easily detected. Thus, unlike the
carrier-sense multiple-access/collision-detection
(CSMA/CD) system used in Ethernet LANs, TDMA
PONs typically use a protocol that grants upstream
transmitters the uncontested right to transmit in a particular timeslot.
Higher data throughputs can be achieved without
introducing complex protocols if the PON separates
user traffic on the basis of wavelength. In a WDM
PON, the wavelength-independent optical splitter is
replaced with a wavelength-dependent router, and
each ONU communicates with the central office on its
own wavelength, eliminating any bandwidth sharing.
However, the cost of WDM components (particularly
the unshared laser in the ONU) is still too high for this
approach to be practical. A way to circumvent this
problem by replacing the ONU laser with an optical
modulator has been proposed.26 In this “loopback”
system, a single-frequency laser in the central office
illuminates the ONU modulators, allowing them to
send their upstream data.
A number of network operators have joined
together to form the Full Service Access Network
(FSAN) consortium.27,28 By specifying a common
access platform that will be purchased by a large number of network operators, the FSAN hopes to drive
down the cost of PON systems. The FSAN network is a
coarse WDM (CWDM) PON that uses the 1.5-µm
band for downstream transmissions and the 1.3-µm
band for upstream transmissions. Baseband signaling
at 155 Mb/s is used in each direction, with a TDMA
protocol implemented for upstream transmission.
Voice and data services are encapsulated into asynchronous transfer mode (ATM) cells. The FSAN work
has led to a number of trials and small-scale deployments of FTTC and FTTH systems.
The FSAN proposal provides a useful standard for
comparing the feasibility of various higher-bandwidth
PON architectures. One study compared various alternatives assuming an FSAN-like fiber network having a
demand of 155 Mb/s per ONU downstream and
10 Mb/s per ONU upstream.29 WDM PONs were
found incapable of providing this level of service using
reasonable commercially available components.
Loopback systems were unable to attain the required
power budget and placed unreasonable demands on
the WDM router. The most cost-effective WDM PON
architecture was judged to be a composite PON (CPON),
which operates as a dense-WDM (DWDM) PON with
a separate wavelength for each ONU (in the 1.5-µm
band) in the downstream direction and as a TDMA
PON (with all ONUs sharing the entire 1.3-µm band)
in the upstream direction.
In addition to this work on deploying fiber closer
to the customer in telephone networks, there has been
a parallel deployment of fiber in the cable television
(CATV) industry. Pre-fiber CATV networks typically
employed very long runs of coaxial cable, with as
many as 100 RF amplifiers in series to make up for the
cable and tap losses. These long runs of amplifiers led
to limited service bandwidths (limiting the systems to
carrying only 20 to 30 television channels), poor reliability, and poor picture quality at the end of the cascade. By carrying the CATV signal optically to a fiber
node that does the optical-to-electrical conversion for
only 500 to 2000 living units, the amplifier cascades
could be reduced to only 3 to 5 amplifiers, providing
much better capacity, reliability, and picture quality.30
Since the CATV analog signal is quite fragile (unlike
the baseband digital signals commonly found in telephony), special optics must be engineered for this
application. Nevertheless, these hybrid fiber-coax
(HFC) networks have been widely deployed for broadcast video applications.
In order to become providers of data services via
cable modems, CATV companies now need to reengineer their plant from being a one-way network,
capable of only downstream broadcast, to a two-way
network, capable of targeting data services to individual customers. One way to accomplish this while producing a plant that is more robust against noise ingress
is to push the fiber closer to the customer in either an
FTTC or an FTTH arrangement.31,32 These networks
can provide a full set of services (analog CATV, videoon-demand, high speed Internet access via cable
modem, and telephony) in a single easy-to-maintain
network with good upgradability.
Another approach33 for providing increased bidirectional bandwidth in CATV networks combines a
broadcast analog downstream signal (amplified and
split over a large number of receivers using high-power
1.5-µm EDFAs) with a set of eight DWDM channels,
each carrying up to 1 Gb/s of digital data in QAM subcarrier format. At a remote node, the individual
DWDM wavelengths are split apart and combined with
a fraction of the analog signal. This combined signal is
sent to eight separate fiber nodes. Each fiber node thus
receives the broadcast analog signal along with a digital
signal dedicated to its customers only. DWDM can also
be used in the upstream direction, if necessary, to
increase upstream capacity. This approach can be used
by itself or in conjunction with a program of pushing
the fiber closer to the living unit.
Bell Labs Technical Journal ◆ January–March 2000
199
Some comment is appropriate on the technologies
that have supported, and will continue to support, the
continued push of fiber towards the customer. In
access applications, cost is key. This means very lowcost optical components. In the enterprise data world,
low-cost 850-nm VCSELs have been key components.
For the generally longer-distance telephony networks,
lower-cost lasers, particularly in the form of uncooled
1.3-µm Fabry-Perot lasers are key components.
Analog-grade CATV systems have progressed from
1.3-µm DFB lasers to 1.5-µm externally modulated
systems with high-power EDFAs, which have a very
low cost per milliwatt. In order for DWDM systems to
further penetrate access applications, the cost of
DWDM components will need to fall significantly. In
addition, DWDM components (sources, filters, and
routers) will need to operate at stable wavelengths
over the large temperature ranges required for access
applications. Finally, for a DWDM PON to really make
sense, a very low-cost, single-frequency laser for the
ONU is required. This laser needs to be tunable over
the entire wavelength range of the system so that network operators can install any ONU on any drop fiber
from the router.
We have seen that the last two decades have provided us with many examples of how more powerful
networks can be achieved by pushing fiber closer to
end users. While the ultimate goal of fiber-to-the-desk
or FTTH may be sometime off, the trend toward
deeper fiber penetration, driven by increased user
demands and dropping costs, continues.
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(Manuscript approved May 2000)
ROD C. ALFERNESS is Chief Technical Officer of Lucent’s
Optical Networking Group in Holmdel,
New Jersey. He joined Bell Labs after receiving a Ph.D. in physics from the University of
Michigan in Ann Arbor, where his research
concerned optical propagation of volume
holograms. His early research at Bell Labs centered on
novel waveguide electro-optic devices and circuits—
including switch/modulators, polarization controllers,
and tunable filters—and their applications in highcapacity lightwave transmission and switching systems.
Later, as head of the Photonic Circuits and Photonic
Networks Research departments, he engaged in research
in photonic integrated circuits in InP, photonic switching systems, and WDM optical networks. Dr. Alferness
has authored five book chapters and over 100 papers;
he also holds more than 30 patents. A fellow of both
the Optical Society and the IEEE Lasers and ElectroOptics Society, he is currently editor-in-chief of the
Journal of Lightwave Technology.
HERWIG KOGELNIK is adjunct director of Photonics
Systems Research within Bell Labs in
Holmdel, New Jersey. He received both the
Dipl. Ing. and doctor of technology degrees
from the Technische Hochschule Wien in
Vienna, Austria, and the Ph.D. degree from
Oxford University in England. Dr. Kogelnik is a fellow
of both the IEEE and the Optical Society of America
and is an honorary fellow of St. Peter’s College in
Oxford, England; he is a member of both the National
Academy of Engineering and the National Academy of
Sciences. His current responsibilities include research
in photonic systems.
THOMAS H. WOOD is a senior manager in the Advanced
Technology Department of Lucent Technologies in Holmdel, New Jersey. He holds
a Sc.B. degree in physics from Brown University in Providence, Rhode Island, and both
M.S. and Ph.D. degrees in physics from the
University of Illinois in Urbana. Dr. Wood is currently
responsible for optical networking technology strategy
and assessment for Lucent’s Optical Networking Group.
He is a fellow of the Optical Society of America. ◆
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