(14IEEE POTENTIALS) The changing trends of optical communication

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0278-6648/14/$31.00?2014IEEE

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 today’s 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? 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 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 On–off 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 9010080706050403020100

Average International Internet Bandwidth

per Internet User in 2011 (Kb/s)

A f

r i c

A r a b

S t

a t e s

A s

i a &

P a c

i f i

I S

A m

e r i c

a s

o r

l d

E u r

o p

Fig. 2 The average Internet bandwidth per user (reproduced from the ITU data).

Global 9080706050G l o b a l I n t e r n e t B a n d w i d t h (T b /s )

403020100

Developing Developed

Year

Fig. 1 International bandwidth demand for the Internet (reproduced from the ITU data).

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the present and the future. These trends are:

toward transparency toward coherence

toward quantum systems

t oward every home access

t oward advanced wireless technologies.Toward transparency

Transparency 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.

Motivation

Transparency 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 technologies

The 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 coherence

Coherent 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 20 dB 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.

Motivation

With 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/O

O/E

E/O Total Electrical Fabric

Total Optical Fabric

Transparent Switching

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 technologies

The 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 systems

Significant 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.

Today’s 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. Motivation

In 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 technologies

The 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 access

In 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. J apan

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.

Motivation

There 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

JANuAry/FEbruAry 2014 31

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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 technologies

For 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 brou ght 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

Splitter Splitter

OLT

OXC OXC

OXC

OLT Transmitter/Receiver

Receiver/Transmitter

Transparent Optical Fiber

Fiber/Total Broadband Penetration in Houses (June 2011)

10%20%30%40%50%60%70%J a

p a n K o r e a S l o v a k R e p .S w e d e n N o r w a y D e n m a r k I c e l a n d H u n g a r y C z e c h R e p .P o r t u g a l N e t h e r l a n d s T u r k e y F i n l a n d I t a l y P o l a n d S p a i n F r a n c e A u s t r a l i a L u x e m b o u r g C a n a d a G e r m a n y S w i t z e r l a n d I r e l a n d A u s t r i a N e w ...G r e e c e B e l g i u m

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.)

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, on–off 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). Motivation

OFDM 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 technologies

The 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.

Conclusions

The 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.