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 (s.routray@ua.pt) 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.