-
28 IEEE POTENTIALS0278-6648/14/$31.002014IEEE
I n the early 1980s, optical communication emerged as a possible
means of practical communication. However, there were many
bottlenecks and short-comings. There was no optical amplifier at
that
time. Every node or repeater used to have reamplificaton,
reshaping, and retiming (3R) regeneration, and all the processing
was done in the electrical domain. The separation between two
adjacent repeaters was well within 20 km. The early trends of
optical communication until the end of 1980s are shown in Table 1,
which were very different from todays mainstream optical
communication.
Three decades later, there are scores of changing trends, which
gave optical communication a completely new shape. Be it in the
high-speed arena, the core or the access, or the optical burst
switching, optical communication has created a dominant position in
the market. The demand for bandwidth has been mono-
tonically increasing since the Internet arrived in 1989. In the
last eight to ten years it is skyrocketing, which was hardly
expected in the 1980s. All this has become possible through the
high-speed core optical networks supported by appropriate enabling
technologies at all critical junctures.
In developed countries, the demand for high-bandwidth
applications is increasing very fast. Due to free Web broadcasting
and various types of digital streaming, bandwidth demand has grown
exponen-tially in recent years. From Fig. 1, it is clear that the
trend of bandwidth demand is almost exponential across the world.
Since the arrival of the Internet, it is catching up to what Jakob
Nielsen predicted in 1998.
In developing countries, its growth rate is also very high. Of
course, developed countries are still a long way ahead as far as
the individual Internet bandwidth per user is concerned. Figure 2
shows how the Date of publication: 7 January 2014
Digital Object Identifier 10.1109/MPOT.2013.2279908
Sudhir K. routray
The changing trends of optical
communication
CAN STOCk PhOTO/ANTErOvIum
-
JANuAry/FEbruAry 2014 29
average Internet bandwidth per user is distributed in different
areas around the world. Europe is far ahead of others in this
regard. The global average bandwidth per user was almost 35 Kb/s in
2011, as per ITU (see Table 2 for all acronyms) statis-tics. The
rest of the world, except Europe, had a smaller average bandwidth
per user than this. The reason behind such a big gap is the
presence or absence of a large number of high-speed core optical
trans-port networks.
In this article, these significant changing trends of optical
com-munication are presented, which make it a tech-nology of the
future. The recent phase of growth is driven by user demand,
business values, and innovation, whereas the era until the 1990s
was the phase of
foundation building. Out of many such changes, the main five
trends are described here, which have made it an attractive and
accessible technology of
Table 1. Early trends of optical communications.
Year Breakthrough
1966 Charles K. Kao informs the Institution of Electrical
Engineers that fiber loss is fewer than 20 dB/km
1977 Early telephone service through optical fibers by
AT&T
1980 First transatlantic telecommunication (TAT) fiber-optic
communication cable TAT-8 deployment started
1987 EDFA was developed in the University of Southampton
1988 TAT-8 started service at 1.3 nm
Table 2. List of acronyms used.
1R Reamplification (only amplification without reshaping and
retiming)
2R Reamplification and reshaping
3R Re-amplification, reshaping, and retiming
AON All-optical network
AT&T American Telephone and Telegraph Company
BER Bit-error rate
CATV Cable television
CON Cognitive optical networking
DSL Digital subscriber line
DSP Digital signal processing
EDFA Erbium-doped fiber amplifier
EON Elastic optical network
EPON Ethernet passive optical network
FSO Free-space optics
FTTx Fiber to the x (x for curb/block/home, etc.)
GPON Gigabit passive optical network
ICT Information and communication technology
IEE Institution of Electrical Engineers
IM/DD Intensity modulation/direct detection
IP Internet protocol
IPTV Internet protocol television
ITU International Telecommunication Union
LH Long haul
MIMO Multiple-input, multiple-output
OCED Organization for Economic Cooperation and Development
OEO/(O-E-O) Optical-electrical-optical
OFDM Orthogonal frequency division multiplexing
OFDMA Orthogonal frequency division multiple access
OLT Optical line terminal
ONU Optical networking unit
OOK Onoff keying
OTN Optical transport network
OXC Optical cross connect
PON Passive optical network (TDM-, WDM-, G-, E-, etc. are its
varieties)
QAM Quadrature amplitude modulation
ROADM Reconfigurable optical add/drop modulator
TAT Trans-Atlantic telecommunication
TDM Time division multiplexing
TON Transparent optical network
ULH Ultra-long haul
VoIP Voice over Internet protocol
WDM Wavelength division multiplexing
90100
80706050403020100
Average International Internet Bandwidthper Internet User in
2011 (Kb/s)
Afric
a
Arab
Sta
tes
Asia
& Pa
cific
CIS
Amer
icas
Wor
ld
Euro
pe
Fig. 2 The average Internet bandwidth per user (reproduced from
the ITU data).
Global9080706050
Glo
bal I
nter
net B
andw
idth
(Tb/
s)
403020100
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
DevelopingDeveloped
Year
Fig. 1 International bandwidth demand for the Internet
(reproduced from the ITU data).
-
30 IEEE POTENTIALS
the present and the future. These trends are:
toward transparencytoward coherencetoward quantum systems toward
every home access toward advanced wireless technologies.
Toward transparencyTransparency in optical communica-
tion means the absence of optical-elec-trical-optical
conversions in the interme-diate repeaters and nodes of the OTNs.
In other words, transparency is all-opti-cal communication without
any change to the electrical form of the signal along the transport
channel (see Fig. 3). Based on transparency, optical networks are
of three types. The first type is opaque, in which the 3R [or at
least reamplification and reshaping (2R)] processing is done at all
the repeaters. The second type is the translucent or the
semi-opaque, in which the 3R processing is done in some of the
repeaters, and at the rest, the pro-cessing may be just
reamplification (1R) or 2R. In the case of the third type, the
transparent optical networks, there is simply 1R processing (with a
few 2R processing). Exceptions are found in the case of very long
range communications, where a few (just one or two) intermedi-ate
nodes provide 3R processing to erase the accumulated errors,
nonlinearities, and noises.
It is very much certain that the optical transport networks will
be made as transparent as possible in the near future. However,
some of them may stay in the translucent form until the signal
processing in the optical domain becomes as flexible as in the
electrical domain. Whatever may be the case, in the ULH,
transparency is the first choice. Now, transparency made the OTNs
and all-optical networks synonymous. Having seen all of these
transformations, the ITU has changed its standards to include
transparency in its new versions. Most of the modern networks
deployed are transparent, whether local or metro-
politan or long haul or ULH. For instance, the fastest
communication service between Europe and the United States provided
by the Hibernia Network is a great example of a modern transparent
optical communication system. It takes record-low 65 ms for a
signal to travel from New York to London along the great-circle of
Hibernia. Transformation toward transparency is a bit slow for the
old optical networks deployed before 2000, due to the lack of
flexibility to handle the emerging traffic.
MotivationTransparency provides many opera-
tional advantages. In TONs, links are only provided with the
optical amplifi-ers, which are commonly known as 1R. There is no
need for any 3R. It saves costs and complexities. It can adapt to
the changes in the data rates and proto-cols. There is no need for
providing new fibers every now and then for increasing data rates.
It reduces the costs of data transmission (in terms of costs per
bit). The impairments that appear at the receivers due to the
absence of 3R can be removed by other new trends such as the
digital-signal-processing-based compensation meth-ods and optical
performance monitor-ing along the channel. Transparency in the
system gives great flexibility and the ability to grow unlike the
opaque and translucent systems. Latency is low, as all the
switching are done in the optical domain, and thus the ultra-fast
systems tend to be transparent.
The surging demand for bandwidth can be handled through
increased trans-parency and the optimum use of band-width. It is
suitable for emerging IP ser-vices, such as VoIP, video on demand,
and digital streaming of different kinds. These services are very
much popular due to their low cost and good quality of services.
Transparency is the basis of the EONs of the future. EONs can save
a large amount of resources, and their lon-gevity is higher.
Enabling technologiesThe need for transparency was felt in
the early days of optical communication. However, at that time
there was no suit-able technology. The main enabling tech-nologies
of transparent optical communi-cation are effective and efficient
amplifiers (mainly EDFAs and Raman), multifaceted ROADMs,
monitoring and compensating methods, and the smart and reliable
architecture of the OXCs. The absence of 3R regeneration leads to
some degrada-tion in the signal quality. Now, the moni-toring and
compensation techniques take care of those issues effectively.
Recently proposed, OFDM-based optical commu-nications systems are
suitable for TONs and EONs of the future.
Toward coherenceCoherent detection is very popular in
wireless communication. It was also tried in optical
communication in the 1980s. In the early 1990s, the arrival of
EDFAs closed the doors for coherent detectors in optical
communication. However, it came back to the optical arena in the
new millennium with new hopes and paradigms. Coherent optical
systems provide up to 20dB extra gain over the IM/DD systems.
Furthermore, it is very efficient for high-performance and
high-data-rate systems. Coherent detectors created new application
areas for optical communication, which were impossible by the
common IM/DD detec-tors. These receivers facilitate the system
spectral efficiency to increase by several folds. The bit-error
rate of coherent sys-tems is significantly higher than IM/DD and
some other detection systems used in the optical domain. Coherence
in optical communication has come back to stay. Out of the two
commonly used coherent detection techniques, such as homodyne and
heterodyne, the latter type is widely used for optical
communication systems.
MotivationWith better qualities and high data
rates, coherent systems promise much to optical systems. In
recent years, when the quest for high spectral efficiency and high
data rates became intense, coherence was the solution for the
majority of such cases. Optical modulation formats became a popular
area of research to feed the coherent receivers. Ultrafast
transmission systems need smart detection techniques. It is not
possible to detect high-speed pulses using the IM/DD transceivers.
However, coherent detectors are able to detect multi-terabit/s
traffic. It also facili-tates the use of advanced modulation
Fig. 3 Opaque and transparent switching in the optical
networks.
Opaque Switching
O/E/O O/E/OO/E E/O
TotalElectrical
Fabric
TotalOpticalFabric
Transparent Switching
-
JANuAry/FEbruAry 2014 31
schemes such as OFDM in optical com-munication, which in return
provides sev-eral benefits such as spectral efficiency, better
quality of signal, and cost reduc-tion. Coherent systems are able
to accom-modate the digital signal processing sys-tems needed for
the compensation schemes and other improvements.
Enabling technologiesThe availability of the components
and good-quality lasers at the source make it possible to have
coherent detec-tors in optical communication systems. Developments
in signal processing have enabled the effective recovery of the
opti-cal signal at the receiving end with good quality. With the
emergence of optical monitoring and compensation schemes at both
the source and the destination, coherent systems achieved a new
high.
Toward quantum systemsSignificant developments in quantum
science led to the emergence of quan-tum communication. Quantum
commu-nication needs a medium for propaga-tion. Optical fiber is
perhaps the best medium available for the quantum prin-ciples to be
realized in communication. Research on both quantum communica-tion
and quantum computing are being carried out from optical
perspectives.
Todays supercomputers have partial optical processors. This
trend is changing, and by the year 2020, the majority of the
processing will be done in the optical domain. The initiatives for
a quantum computer may materialize in the optical domain. Recent
research in this area is quite impressive and indicates the
impor-tance of the optical systems in the future. Quantum
principles are the de-facto rules of processors at the small scale.
With quantum computers, the conventional cryptography would fail in
seconds. Thus, quantum cryptography is the only suit-able option to
handle that problem. Even now, quantum cryptography is ahead of
others in this area. Having quantum com-puters around, the
communication would not depend on macro quantities such as current
and voltage; rather, only photons can manage the data
transmissions. In that situation, just a photon counter would serve
as the receiver. Of course, now ultrasensitive receivers are
similar to this but they need more than just one photon for proper
detection.
MotivationIn the case of quantum cryptogra-
phy, quantum laws help the sender and
receiver to communicate safely with their abilities to know
whether they are being spied or not. This is unique and accurate,
as any trial to get the informa-tion in the middle can be detected
by the change in the state of the photons by the sender and
receiver. In the future, when quantum computers arrive,
tradi-tional cryptography will be replaced by its quantum version.
This is the way to have a robust and reliable cryptosystem, which
can provide perfect data integrity. The researchers of quantum
computers and other high-speed computers see the principles of
quantum optics as the future of computing. Overall, quantum
principles are the limit of the extents to which the systems can be
pushed. This is also the way to explore the limits of communication
and computing.
Enabling technologiesThe main enabling technologies of
quantum optical applications are the availability of the good
photon genera-tors (i.e., high-precision lasers), accurate
receivers, such as the photon counters, and other high-quality
components. Advances in the quality of materials, high-grade
fibers, and high-precision sensors are instrumental in the
develop-ment of quantum systems. New varieties of quantum devices
and materials are being introduced to the field every year. Methods
of photon generation for opti-cal information processing have also
improved significantly.
Toward every home accessIn the 1960s, when the optical com-
munication perspectives were published, or in the 1980s, when
the fibers were deployed for communication, hardly anyone had
thought that it would some-day replace the popular copper wires of
that time. Even in the 1990s, no one thought that fiber could be
used for per-sonal communications in common houses. This was mainly
due to the high cost of optical communication over other access
technologies. That trend has changed. Now, fiber is readily
avail-able in access networks as fiber to the x (FTTx), with the x
. representing a curb, block, home, etc. DSLs and wireless
broadband technologies in the access area are the main rivals of
FTTx. However, the quality, ability, and fea-tures of fibers are
exemplary. FTTx is robust in quality, high data rates, and other
performance-related features. Many new varieties of the PONs are
being tested and implemented every
year around the world. The optical wire-less communication
technologies are also being researched for the implemen-tation of
the FSO communication sys-tems in the access area networks (i.e.,
end-user local networks).
Passive optical networks are the local-area networks that
emerge/terminate from the OLTs from/to the individual homes. As
shown in Fig. 4, the OXCs are connected to the OLTs, which then
con-nect to individual homes. The word pas-sive is used to denote
the absence of any active elements between the OLT and the final
access point, the ONU. The active element means mainly the
amplifiers. The lengths of the spans are chosen in such a way that
there is no need for any amplifi-cation between the links from the
OLTs and the ONUs. Splitters are used to sepa-rate the individual
links from a common link before reaching the ONUs. In some PON
systems, low-power amplifiers may be used to increase the
reach.
Today there are many varieties of PONs available such as time
division multiplexing PON, gigabit PON, ethernet PON, wavelength
division multiplexing PON, and OFDM-PON, among others. Each has its
own merits and limitations. In order to improve overall
perfor-mances, their hybrids are also being tried and implemented
in different parts around the world, where the bandwidth demand is
high. The penetration of opti-cal broadband in houses in different
countries (in the decreasing order of percentages) is shown in Fig.
5. Japan and South Korea lead the world in the optical-fiber
broadband penetration. The penetrations of optical fibers in the
access network are on the rise in devel-oping countries as
well.
MotivationThere are emerging applications,
which create special motivation for the PON technologies. The
applications, such as CATV and Internet protocol tele-vision need
fiber as the medium for proper quality of services. In compari-son
to the wireless broadband and DSL, the quality of the signal is
much better in the optical fibers. Now, in many cities around the
world, the access area net-works are optical due to their ability
to carry high-data-rate traffic. As men-tioned previously, data
streaming, social networking, and Web broadcasting are the major
areas where bandwidth demands are huge. For instance, for CATV and
video-on-demand applica-tions, the recommended bandwidth has
-
32 IEEE POTENTIALS
to be at least 2 Mb/s. This bandwidth can be provided by DSL
systems. However, the DSLs cannot guarantee the future changes as
the bandwidth demand increases every month. At the same time, PON
is quite reliable, easy to maintain, easy to install, and power
effi-cient. Clearly, fiber is the most suitable
answer for the emerging applications where huge bandwidth is
needed.
Enabling technologiesFor PON systems, the cost factor is
very important. The major obstacles for the PONs were the prices
of the ONUs and fibers because ONU prices are paid
by the customers at the beginning of the service. Initially, the
ONU prices were in thousands of U.S. dollars (or its equiva-lent in
other currencies). However, the availability of affordable
components and integrated photonic chips have brought the prices
down to under US$100 per unit.
Fig. 4 The optical fibers local networks for home access. (It
shows how the OXCs are connected with the OLTs and then to the
customer homes through the splitters.)
Backbone/Core Network
Local Network
All-Optical Switching
SplitterSplitter
OLT
OXC
OXC OXC OXC
OXC OXC
OXC
OLTTransmitter/
Receiver Receiver/TransmitterTransparent Optical Fiber
Fiber/Total Broadband Penetration in Houses (June 2011)
0%
10%
20%
30%
40%
50%
60%
70%
Japa
n
Kore
a
Slov
ak R
ep.
Swed
en
Norw
ay
Denm
ark
Icelan
d
Hung
ary
Czec
h Re
p.
Portu
gal
Neth
erlan
ds
Turk
ey
Finla
nd Italy
Polan
dSp
ain
Fran
ce
Austr
alia
Luxe
mbo
urg
Cana
da
Germ
any
Switz
erlan
d
Irelan
d
Austr
ia
New.
..
Gree
ce
Belgi
um
Fig. 5 Optical fibers in home access (as a percentage of total
households). (Courtesy of the Organization for Economic
Coopera-tion and Development Broadband Information Database.)
-
JANuAry/FEbruAry 2014 33
Toward advanced wireless technologies
Until the last few years, it was a common perception that
wireless com-munication and optical communication have different
trends in modulation, demodulation, and signal processing. This was
mainly due to the previous observa-tions of communication
processes. For example, onoff keying was very popular in optical
communication, which had little place in wireless communication.
The dis-appearance of the coherent receivers from the optical
communication in the 1990s also proved it for a decade.
In wireless communication, the usable spectrum is always scarce.
Thus, wireless spectral efficiency is welcome forever, which was
not the common case in optical communication until the last decade.
However, these odds are changing very quickly. Optical
commu-nication is readily following the trends that are effective
in wireless communi-cation. For instance, the popularity of the
OFDM and MIMO are tested for recent uses in the optical domain.
OFDM and orthogonal frequency divi-sion multiple access are quite
effective in the local area optical networks such as FTTH and PON.
OFDM is considered a main tool for elastic optical networks. Many
of these wireless technologies also reduce the consumption of
energy in the optical domain. Even cognitive optical networking is
being studied for probable uses in the future. The results obtained
from research are also impres-sive, and more emulation will follow
soon. Recently, FSO technologies are being tried in short-range and
indoor communications, though they are not very new (and were
experimented by Graham Bell a hundred years ago).
MotivationOFDM is used in wireless communica-
tion to mitigate the multipath fading effects from terrestrial
communications such as mobile and digital audio broad-casting. It
also facilitates high-data-rate communication through the large
constel-lations of quadrature amplitude modula-tion. In optical
communication, it can mitigate all types of dispersion effects,
which are very similar to the multipath fading of wireless
channels. In addition, it also provides the platform for high data
rate and high spectral efficiency. EONs can be implemented
effectively using OFDM. There is no effective alternative to
OFDM in the realization of transparency and elasticity in
optical networks. Similarly, MIMO-enabled optical systems can
provide a lot of advantages such as the mitigation of dispersion
and nonlin-earity related impairments.
However, the biggest motivation for following the wireless
trends is the cost savings. These technologies can save a
significant amount of money. The self-organizing and other smart
approaches of the wireless networks are also demanded in optical
networks. Despite fundamental differences in the opera-tions, both
are growing very fast.
Enabling technologiesThe main enabling technologies for
these developments are the availability of the components and
advanced signal processing. Optical OFDM systems are expensive and
complex. However, now integrated chips overcome these obsta-cles.
Similarly, the implementation has become quite easy through digital
signal processing techniques.
ConclusionsThe recent trends in optical communi-
cations are changing very quickly. It is quite amusing to see
that the core of every large communication network car-ries huge
traffic every now and then, which was very much unrealistic 20
years ago. This would not have been possible without optical
fibers. With the changes in the demand and availability of the new
technologies, new frontiers are being added to the main fiber-optic
technolo-gies. Now, there are so many emerging technologies in this
list, such as the visi-ble-light communication, wireless-optical
communications, all-optical computing, intelligent and
automated-optical net-working, and software-defined optical
networking. Furthermore, there are sev-eral new initiatives in the
optical field out-side of telecommunication. In the future, it will
be more advanced and diversified with new applications and trends.
One
day, it may be possible that the whole static communication
network will be purely optical.
Read more about it A. Morea, F. Leplingard, T. Zami, N. Brogard,
C. Simonneau, B. Lavigne, L. Lorcy, and D. Bayart, New
trans-mission systems enabling transparent network perspectives,
Compt. Rend. Physiq., vol. 9, nos. 910, pp. 9851001, Nov. 2008.
R. Ramaswamy and K. Shivrajan, Optical Networks: A Practical
Perspec-tive, 3rd ed. Burlington, MA: Morgan-Kaufman, 2009.
G. P. Agrawal, Lightwave Technol-ogy: Telecommunication Systems,
4th ed. New York: Wiley, 2005. E. M. Ip and J. M. Kahn, Fiber
im-pairment compensation using coherent detection and digital
signal processing, J. Lightwave Technol., vol. 28, no. 4, pp.
502519, 2010. G. Li, Recent advances in coher-ent optical
communication, Adv. Opt. Photon., vol. 1, no. 2, pp. 279307,
2009.
W. Shieh and I. Djordjevic, OFDM for Optical Communications.
Burling-ton, MA: Academic, 2010. G. Zhang, M. D. Leenheer, A.
Morea, and B. Mukherjee, A survey on OFDM-based elastic optical
network-ing, IEEE Commun. Surveys Tuts., vol. 15, no. 1, pp. 6587,
2013. A. N. Pinto . J. Almeida, N. A. Silva, N. J. Muga, and L. M.
Martins, Optical quantum communications: An experimental approach,
in Proc. SPIE Int. Conf. Applications Optics Photonics, 2011, vol.
8001, p. 8. B. Skubic, E. de Betou, T. Ayhan, and S. Dahlfort,
Energy efficient next-generation optical access networks, IEEE
Commun. Mag., vol. 50, no. 1, pp. 122127, 2012.
L. C. Andrews, R. L. Phillips, and C. Y. Hopen, Laser Beam
Scintillation with Applications. Bellingham, WA: SPIE, 2001.
About the authorSudhir K. Routray ([email protected]) is
a Graduate Student Member of the IEEE Portugal Section. He has a
bachelors degree in electrical engineering from Utkal University,
India, and masters degree in communication engineering from
Sheffield University, United Kingdom. He is currently a Ph.D.
student in opt ical communicat ion at the University of Aveiro,
Portugal.
With the changes in the demand and availability of the new
technologies, new frontiers are being added
to the main fiber-optic technologies.