evolution of optical network

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Research: exploit technology cross-fertilization


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and WDM . D uring 2000, the volume of data traffic in the

USA surpassed the volume of voice traffic, highlighting the

dominant role that the Internet is playing in terms of ser-

vices. Forecasts indicate that growth in data traffic will con-

tinue at an exponential rate in the years to come.

One of the primary objectives of opti-cal research is therefore to pave theway for increased capacity and moreintelligence in optical networks.M eanwhile, as operators were investing to cope with Inter-

net growth ( although revenue was sti ll coming primari ly

from voice traffic) the capital crunch occurred. T his

increases the importance of another research objective:

mak e the technology less expensive to produce, i mple-

ment and operate.

To prepare for the future, innovation in the field of high

capacity transmission remains important. Long haul sys-

tems both terrestrial and submarine go through the well

known build and fill cycles. I ndustry-wise, we are in a fi ll

cycle. From a research perspective, it is important to pre-

pare for the next build cycle. T he next generation of opti-

cal transmission will be based on N x40 Gbit/s systems, pro-

viding capaciti es of several T bit/s. H owever, when con-

sidering transmission, one has to take into account the dis-

tance between regenerators because the Signal to N oiseRatio ( SNR) degrades with distance. Consequently, trans-

mission needs to be evaluated wi th respect to both capac-

ity and distance: For example, future transmission systems

might be identified as 10 Petabit/s*km networks, mean-

ing that they offer 10T bit/s transmission over 1000km, or

1 T bit/s over transoceanic distances.

A lcatel is strongly commi tted to N x 40 G bit/s systems.

R esearch embraces all the relevant fields, including

submari ne and terrestr ial systems, as well as a range of

enabling technologies. For example, A lcatel was one of

the first to demonstrate transmission at more than

10T bit/s [1]. A number of i nnovations made this demon-

stration possible: vestigial sideband filtering at the

receiver for narrow channel spacing, distributed Raman

amplifi cation to optimize the SNR , A lcatel T eralight

fiber to reduce fiber impairments, and polarization multi -

plexi ng to double the capacity.

Trends and evolution of optical networksand technologies

Introduction

O ver the past few years, opti cs has establi shed itself as

one of the basic communi cati on network technologies as

a result of the conjunction of several key technological

innovations ( optical fiber, semiconductor lasers, fi ber

amplifiers) and market needs. T hanks to the introduction

of Wavelength D ivision M ultiplexing ( WDM ) , optical

transmission now makes it possible to transmit enormous

amounts of information over almost unlimi ted distances.

A s far as transmission capacity is concerned, fiber has no

competi ti on. I n additi on, opti cs offers a number of

advantages in the field of network ing. Even though

recent cuts in capital expenditure ( capex) have slowed

down progress in this field, the fundamental trends in

telecommunications will inevitably bring optical tech-

nologies and networks back into the spotli ght.

O ptical communications is sti ll a very new industry. Fi bers

have only been widely installed over the past decade, and

mainly for long distance transmission. O ptical networking

is not really here yet. T he industry is young, and conse-

quently somewhat i mmature. H ence, research in optical

communication technology can actively contribute to

improving the technology in various industrial as well as fun-

damental areas, such as materials, devices, architectures

and protocols.T his arti cle exami nes the general trends in optical com-

munications and describes A lcatels main research di rec-

ti ons. Some of the key Alcatel research results are high-

lighted in other articles in this O ptics section of the

Alcatel Telecommun i cat ion s Revi ew.

More Bits to More Users

T he explosive growth in capacity is largely a result of mas-

sive use of the Internet. T he combination of an increasing

number of I nternet users and the introduction of new con-

tent-richer services with more picture and video content

has resulted i n the demand for capacity doubling every 6

to 9 months in some networks. Such growth faster than

was experienced in electronics has been possible thank s

to the combination of Time Division M ultiplexing ( T DM )

M. Erman

The fundamental trends in telecommunications

- more bandwidth hungry services, more

intelligent and easy to manage networks - will

inevitably bring optical technologies back

into the spotlight.

Alcatel Telecommunications Review - 3rd Quarter 2001 Trends and evolution of optical networks and technologies173

Optics


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detecting only a small percentage of the signal. I n col-

laboration with E uropean partners, A lcatel has demon-

strated an all-optical cross-connect and has tested it in

a real network [3].

O pti cal switching can, however, fi nd a place even in a

network that i s not fully transparent. I ndeed, an elec-

tronic switching matrix can be replaced by an optical

one. I n this case it does not provide a specifi c functi onal

advantage, but the expectation is that for large switch-

ing matrices, an optical implementation will be cheaper

than an electronic one. A n optical switching element i s

also bit rate i ndependent, which means that i t i s possi-

ble to upgrade ports from, say, 2.5 G bit/s per channel

to 10 G bit /s, or even 40 G bit /s, wi thout changing the

matri x. T his is not possible with an electronic version

since higher capaci ty requi res more processing power.

Whatever implementation is selected, such cross-con-

nects perform wavelength switching, and thus allow

wavelength servi ce ( end-to-end wavelength provi -

sioning, for instance) . Si gnaling, controlling and man-aging WD M network s have become hot research and

development topics.

Besides the introduction of an O ptical Channel ( O Ch) ,

which makes i t possible to treat each wavelength as a

separate logical channel, I nternet-based protocols,

such as M ulti P rotocol Wavelength Switching ( M PS)

and Generalized M ulti P rotocol Label Swi tching

( GM PLS) , are being introduced at the control layer. T he

basic dri ving forces are known: apply data-oriented pro-

tocols ( which proved so cost-effective for the I nternet)

to WD M networks and make dynami c establishment of

wavelength-based routing paths possible. A s data i s

becoming the dominant type of traffic, this trend

appears natural. Nevertheless, the required constraints

on Quality of Service ( Q oS) , restoration and protection

need to be carefully assessed.

T he impact of data i s even larger and more profound on

metropolitan network s. B ecause of the mix of dif ferent

formats I nternet Protocol ( I P ) , A synchronous Trans-

fer M ode ( AT M ) , G igabit E thernet, etc such networks

naturally have to evolve towards multi service network s.

O n the optical layer level, WD M is the most suitable tech-

nology, yet with even greater cost constraints. T rans-

parency, which is diffi cult to manage at the backbone

layer, might find an easier implementation in the

metropolitan area. A lcatel research is worki ng on a num-

ber of innovati ve solutions [4].T he ulti mate dream of an I P -over-optics approach

remains, however, an optical router. T his requires fast

opti cal switching fabri cs. A lcatel already has consid-

erable experience in optical packet switching, having

demonstrated the fi rst opti cal AT M switching demon-

strator some years ago as part of the E uropean AT M O S

and K E O P S programs. We have further refined these

ideas and have adapted the concept to tak e into

account the IP dimension. T he fi rst burst opti cal

router has been assembled; it exploits a number of inno-

vative approaches for both the optical elements and the

control layer. T his prototype has validated the feasibili ty

of implementing an all-optical burst router including

burst transmitters and receivers as well as high-speed

scheduling algorithms.

I t i s clear that opti cs can offer much more than just point-

to-point transmission. Wavelength service, network pro-

A s a result of these innovative technologies, A lcatel

achieved a record spectrum density of 1.28 bit /s/Hz. T his

parameter is important as it indicates the efficiency of

spectrum utilization and is therefore linked to the cost.

T he achieved efficiency is six t imes higher than for todays

commercial systems. In another experiment, N x40 Gbit/s

transmission was demonstrated in a submarine configu-

ration. T ransmission at 32x40Gbit/s ( in excess of 1Ter-

abit/s) was achieved over a distance of 2400 km using

amplification only, and no regeneration.

I n the case of ultra-long-haul t ransoceanic systems,

N x40Gbit /s systems will require regeneration. A lthough

one can implement this function using optoelectronic con-

version, this would be a step backwards compared wi th

the present situation in which one optical amplifier is used

to simultaneously amplify several ( in most cases all) wave-

length channels. A n optoelectronic regenerator is, by def-

inition, a single-channel device that mi ght jeopardize the

cost advantage opti cs has brought to transmission. T hus

research into opti cal regenerators is a key program. A lca-tel is investi gating several approaches based on semi-

conductor wavelength converters, in-line synchronous

modulation and saturable absorbers [2].

O pti cal transmission on long haul network s is only part

of the picture. Fiber will inevitably be the transmission

medium in metropolitan area networks, and is increasingly

extending i ts reach into access networks. Following the

generali zation of high speed Internet accesses ( A sym-

metric D igital Subscriber Li ne, A DSL; Very high speed

D igital Subscriber Line, VD SL; etc) , a need will soon

emerge for high capacity transmission systems to the cus-

tomer premises. A s a result, photons are coming closer

to the home! However, metropoli tan and access networks

raise a number of challenges other than purely trans-

mission ones: protocols, multiservice capability and cost

are the dominant issues.

From Dumb Pipes to Intelligent Networks

I f optical transmission and WDM in parti cular has

established an undisputed leadership, the use of pho-

tonics and exploitation of the wavelength domain for net-

worki ng is sti ll i n i ts infancy. N evertheless, i t i s tempt-

ing to push further what photons can do in a network.

T he argument is simple. C onsider a WD M network wi th

80 wavelength channels of 10 Gbit /s on each fiber. O neach of the network nodes, a cross-connect wi ll have to

switch hundreds of 10 Gbit/s channels from i nput fi bers

to either drop channels or output fi bers. E lectronics is

the way to do i t today. H owever, this requir es a

transceiver which involves optoelectronic conversion

at both the input and output ports. T hese transceivers

are the major cost element in a cross-connect.

A s most of the traffic in a node is transit t raffic, replac-

ing the electronic cross-connect by a fully transparent

optical cross-connect is the obvious low cost photonic

alternative. I t does, however, raise a number of issues,

including the non-intrusive monitoring required to man-

age all-opti cal network s. Several soluti ons are being

investi gated within the Alcatel laboratories. T hese solu-

ti ons are either based on addit ional control channels or

modulation, or the use of high speed electronic pro-

cessing capable of assessing the quali ty of the signal while



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wider temperature range ( from 40 to +85C ) - an

attractive challenge for quantum mechanics specialists!

T he next move was to replace active fiber/laser alignment

by a passive technique. T his was achieved by rethi nk ing

the laser mounting process. Silicon motherboards have

been developed, which make it possible to use an auto-

matic self-aligning process for the laser and fiber, with the

help of indentation and appropriate structuring of the

laser chip. H owever, further innovations were needed at

the laser chip, such as the integration of a taper ( the equiv-

alent of an integrated lens) .

A ll thi s was necessary in order to develop the surface-

mountable plastic laser modules. Nothing would have been

possible without innovation in various fields of physics,

optics and processes.

What is coming next? One trend will clearly be to integrate

more functions, including both passive optical functions

and dedicated electronic interfaces. SiO 2/Si mother-

boards will play a key role in assembling the passive and

acti ve optical parts cost-effecti vely. T here are a numberof other interesting options for WD M components [6].

T hese trends indicate that the componentsindustry will evolve considerably overthe next few years to offer ever higherperformance and, even more impor-tant, greater functional integration atlower costs. T hus the industry will progressivelymature. R esearch and innovation are important factors

in mak ing this happen.

Innovation: the Art of Networking

So far, we have focussed on the near- and medium-term

evolution of opti cal technologies. H owever, optics ishere to stay for a long time, and dis-

ruptive technologies will inevitablyappear. Some may already be knocki ng at the door,for example, photonic bandgap materials which might

be used for opt ical f ibers, planar passive devices and

semiconductors. I n a world where i nnovation can hap-

pen in various places and environments, where a real

application might be diffi cult to detect at an early stage,

in other words, in a world of uncertainty, how should wemanage innovation? A lcatel believes that partnership is

the ri ght way to go; i t can take various forms.

Consider some examples in the optical field. M ulti -part-

ner projects national and international make i t pos-

sible to build effi cient multi disciplinary projects com-

bini ng the talents, experti se and vi sion from uni versi-

ti es and industry. I n E urope, A lcatel i s a major player

both wi thin national projects, such as BM BF i n Ger-

many and R N R T in France, as well as within i nterna-

ti onal projects, such as IST . M any of our advanced stud-

ies in the area of optical networking have been initi-

ated in this context , and proj ects such as O P E N

( transparent optical cross-connect) , K E O P S ( optical

packet switching) , M E PH I ST O ( management of all-

optical networks) and PE LI CA N ( field trial imple-

mentation of an all-optical network) were the first to

explore new, innovati ve options.

tection at the optical layer, wavelength routing and, even-

tually, a true I P -over-optics implementation are some

of the evolutionary steps that are at an advanced stage

within A lcatel R esearch.

Components: a Pace of Change

Components need to meet two sets of objecti ves: one con-

cerns their performance and function, while the second

is linked to cost constraints. Both explain why optical com-

ponents are at the heart of todays communication sys-

tems. Not only do they set the performance limi ts and

functional constraints, but also, as they represent a sig-

nificant and increasing proportion of the equipment

cost, they strongly impact the final system cost.

T he trends mentioned above for high speed transmission

and intelli gent networks will materiali ze only i f suitable

technologies are available. T he research highlights

included in this issue of the Alcatel Telecommu ni cationsRevi ewshow where the challenges for optical compo-

nents lie from a functional point of view. T ransmission

at 40 G bit/s requires high-speed modulati on, detecti on

and associated electronics. M anaging fiber impairments

( chromatic dispersion, polarization mode di spersion,

etc) requires dedicated passive components. D ense

WD M requires multi plexers and demultiplexers for

higher channel counts and narrower channel spacings.

Dedicated electronics, parti cularly the stages that inter-

face directly with the optoelectronic chips, will be

equally important for high-speed systems. O pti cal ampli -

ficati on needs to be developed for new wavelength win-

dows ( after C and L, the next window will be S) , while

at the same time the i ncreasing number of channels will

require more power [5]. O ther functions become manda-

tory when moving towards optical networks: optical

switches of course, but also devices capable of monitoring

the Q oS, and ultimately, optical regenerators. A lcatel

research has achieved breakthroughs in all of these fields.

As regards cost, one might think that this is more an indus-

tr ial than a research issue. I n fact, the cost of optoelec-

tronic components has been reduced, and will continue

to be reduced, through innovation.

O versimpli fying, we can say that an optoelectronic device

is made of a chip ( front-end) packaged in a module ( back-

end) . T he short history of evolution of optoelectronic

devices was an alternati on of breakthroughs in the front-end and back-end processes.

T he first i mportant step was at the beginning of the 90s

when, for the first time, A lcatel demonstrated the feasi-

bili ty of manufacturing full 2 inch InP wafers, each with

15000 lasers! T his was made possible thanks to the devel-

opment of strained quantum well lasers in the research

laboratory. T he technology proved capable of producing

high-performance lasers which were uniform and repro-

ducible. I t also represented a breakthrough in the cost of

the laser chip.

However, the dominant cost then became the module,

which was metallic, used a Pelti er cooler and needed very

accurate ( manual) fiber/laser chip alignment. T he first step

was to eliminate the Pelti er cooler and develop the so-

called coax module, sti ll using active alignment. Again this

made i t necessary to go back to the laser chip and

develop new laser structures that could operate over a



5/8

research teams. Some other elements will require dis-

ruptive approaches that may not yet have been identif ied.

T his is where cooperati on with more academic centers of

excellence will play a determining role.O pen your eyes and let the light come in!

In the field of basic technologies, A lca-

tel i s also very committed to partner-

ships through international projects.

Proj ects on advanced topi cs such as

photonic bandgap materials, quantum

boxes and, more generally, nano-tech-

nologies, are areas of ongoing activity.

In specific fields, bi lateral cooperati on

can further help to achieve impressive

progress for the benefit of both parties.

With this in mind, A lcatel has launched

and is supporting a number of collab-

orations with major Universities and

Insti tutions worldwide. A s an example,

some remark able results have been

achieved in a bilateral program with

the Heinrich Hertz Institute, one of our key partners [7].

A nother such ini ti ative is the creation of O ptics Valley.

Located south of P aris, O pti cs Valley is an association of

major universities, engineering schools, small to mediumsize enterprises, and large corporations that are active in

the optics field ( seeFi gur e 1) . I t represents a unique pool

of sk i lls in both fundamental and applied sciences. A lca-

tel was one of the founders of O pti cs Valley and is the ref-

erence industrial partner working on optical communi-

cation. T he various participants in O ptics Valley are

expected to give birth to many promising university/indus-

try collaborations. A s an example, a prestigious CNRS lab-

oratory work ing on optics and nano-technologies ( LPN)

will be collocated with the Alcatel research laboratory in

M arcoussis. T he exchange of ideas and the collaboration

facilitated by the proximity of two large laboratories one

with an industrial culture and missions, the other with

more fundamental objecti ves wi ll certainly foster i nno-

vation. T hus, we believe that cooperation,

partnership and networking with other

centers of excellence are importantassets. A fter all, i t i s interconnection that providesintelligence to the human brain!

Conclusion

We have reviewed some of the trends in optical networks

and technologies. A lthough opti cal telecommuni cati ons

now appears to be a well-established technology, it hasreally only been extensively used for transmission for

about ten years. M any challenges and opportuni ti es are

ahead of us. T he future is only part ly predictable.

I ncreased capaciti es are inevitable, even if the present

economic slowdown might change some of the mile-

stones. T he move to intelli gent opti cal network s is also

a strong move which wil l give added value to operators.

M any of the advances needed to implement this vision are

already in the laboratory, as is illustrated by several arti-

cles in this issue of the Alcatel Telecommun i cat ion s

Revi ew. T hese arti cles also demonstrate the strong

commitment and quali ty of the results of A lcatels


Thales Central Research

Orsay

Ecole Polytechnique

CNRS - LULI, LOA

OPTO+

CNRS-LPN

Z.I . Courtaboeuf :Picogiga...

Alcatel OpticsTerrestrial & submarinetransmission

Alcatel R&I

University Paris-SudIEF

SupelecIOTA

Fig. 1 Some of the key participants in Optics Valley

References

1. S. Bigo, W. Idler, A. Scavennec, L. Du Mouza: Road toultra-high-capacity transmission, Alcatel Telecommunica-

tions Review, 3rd Quarter 2001 (this issue), pp 177-178.2. F. Brillouet, F. Devaux, M. Renaud: FromTransmission

to Processing: Challenges for New Optoelectronic

Devices, Alcatel Telecommunications Review, 3rd Quarter1998, pp232239.

3. J . L.Beylat, M. W. Chbat, A. J ourdan, P. A. Perrier: FieldTrials of All-Optical Networking based on WavelengthConversion,Alcatel Telecommunications Review, 3rd

Quarter 1998, pp218-224.4. A. Jourdan, L. Tancevski, T. Pfeiffer: How much optics

in future metropolitan networks?,Alcatel Telecommunica-tions Review, 3rd Quarter 2001 (this issue), pp219-221.

5. D. Bayart, L. Gasca, G. Gelly: Cladding-pumped erbium-doped fiber amplifiers for WDM applications, AlcatelTelecommunications Review, 3rd Quarter 2001 (thisissue), pp179-180.

6. J . J acquet, K. Satzke, I . Riant: Low cost DWDMdevices,Alcatel Telecommunications Review, 3rd Quarter2001 (this issue), pp181-182.

7. F. Devaux, O. Leclerc, B. Lavigne, P. Brindel, H. P. Nolt-ing, B. Sartorius: Alcatel-HHI collaboration on all-opti-cal 3R regeneration, Alcatel Telecommunications Review,

3rd Quarter 2001 (this issue), pp231-233.

Marko Erman is Senior Research & InnovationDirector and Member of the Optics Group Board.He is based in Marcoussis, France.


6/8Alcatel Telecommunications Review - 3rd Quarter 2001 Road to ultra-high capacity transmission177

S. Bigo, W. Idler, A. Scavennec, L. Du Mouza

Road to ultra-high-capacity transmission

Than ks ar e du e to Laur ent Du

Mouza, who is now work i ng wi th

Alcatel Submar i ne Network s

Div i sion i n Vi l lar ceaux , for hi s

helpful con tr i but i ons to thi s ar t i c le.

Introduction

O ver the past few years, the capac-

i ty of opti cal t ransmi ssion systems

has been doubling faster than the

already impressive M oores law pre-

di ctions for electr onics. T heincrease in the number of channels

through Dense Wavelength D i vi -

sion Mult ip lex ing ( D WDM )

accounts for most of this evolu-

ti on. H owever, i n order to achieve

even higher capaci ti es, the channel

bitrate will have to be increased

( the next step is 40 G bit /s in com-

mercial pr oducts) , t he channel

spacing must be reduced and the

total ex ploi ted opti cal bandwidt h has to be enlarged

( see F i gu r e 1) . A ll t hree approaches will be inves-

tigated simultaneously to realize higher spectral

effi ciency ( expressed in bit /s/Hz) and greater total

throughput.

Enabling technologies for hi gh capacity, point-to-point,

long distance transmi ssion include:

L ow-noise, hi gh-power, wi de bandwi dth and gain-

flattened optical amplifiers.

O pti mi zed opti cal fiber and associ ated dispersi on

management techniques for future-proof infras-

tructures and transmission which is tolerant to

propagation effects.

H igh-speed electronics and opto-electronics for the

transmitter and receiver equipment.

Polarization M ode D ispersion ( PM D ) miti gation. D ispersion compensating modules compatible wit h

large optical bandwidths.

Fast optical processing for 2R-3R regeneration, pro-

viding efficient but lower cost regeneration compared

with back to back transmi t/receive equipment.

Very h igh bitrate equipment ( 60 G bit/s and

beyond) , based on a combinati on of E lectroni c

T ime Div ision Mult iplexing ( ET DM ) and Opt ical

T ime D ivision M ultiplexi ng ( T D M ) techniques.

N ew generati on of forward error correcti on

techniques to facilitate noise tolerant trans-

mission.

O ptimi zed modulation format for high spectral effi-

ci ency ( approaching 1 bit/s/H z) .

O pti cal filters with well defined amplit ude and

phase shapes for ul tra-narrow ( narrower t han the

WD M channel bandwidt h) fi ltering.

A mong recent advances in the fi eld of ultra-high

capacity transmission and ultra-high-bitrate com-

ponents are: a record terrestri al 10 T bit/s

( 128 x 2 x 40 G bit/s) transmi ssion system wit h a

spectr al effi ciency of 1.28 bi t/s/Hz, which has never

been achieved before; and recent progress in the

fi eld of 40 G bit /s optoelectronic and electronic com-

ponents, showing that a new generati on of compo-

nents is feasible for the practical implementation of

40 G bi t/s transmi ssion systems.

Record 10 Tbit/ s Terrestrial

Transmission Experiment

T he principle of the 10 T bit/s transmission experi ment

( see F igu r e 2) i llustrates the combined use of severalof the key technologies cited above to achieve an unsur-

passed spectral effi ciency of 1.28 bit /s/Hz. Low noise

and high amplification are achieved by using an opti-

mum combination of Erbium-doped and distributed

Raman amplification over the C and L bands. T he chan-

nel bitrate i s 42.7 G bit /s, obtained by the use of SiG e

E T D M technologies. T his includes the actual infor-

mation bi trate of 40 G bit /s plus a 7% overhead for the

Forward Error Correcting Code ( FE CC ) . FE CC pro-

vides a very efficient way of converting the 10-4 Bit

E rror Rate ( BE R ) at the link output to virtually error-

free data transmission for the actual information.

T he innovat ive channel spectral allocati on is based on

the association of alternati ve 75 and 50 G Hz channel

spacings ( each channel being electronically coded at

42.7 G bit/s) , Vestigial Side Band ( VSB ) filtering and

Polarization D ivi sion M ultiplexi ng ( PD M ) . T hese tech-

Channelrate R

R'>R

Optical Spectrum

Increase bitrate

per channel

Decrease channelspacing

Increase totalbandwidth

(a)

(b)

(c)

bandwidth Btot

Upgraded configuration

Btot

Btot

B'tot>Btot

TxTerminals

RxTerminals

Fig. 1 System upgrading to achieve ultra-high capacity transmission willfollow three directions: (a) increase the channel bitrate,(b) decrea-se the channel spacing, and (c) increase the optical bandwidth


7/8Alcatel Telecommunications Review - 3rd Quarter 2001 Trends and evolution of optical networks and technologies178

ti on ratio. H i gher bandwidths and lower dri ving

voltages can be achieved wit h the more compact I nP -

based E lectro-A bsorption M odulators ( E A M ) . M od-

ulators of thi s type are presently being studi ed in R & I

to im prove their chi rp and ext i ncti on rati o ( see

F i gu r e 3) .

ReceiversI nG aAs PI N photodiodes offer very large bandwidths

and high responsivi ti es. I n addit ion, they can wit h-

stand high optical input powers and often exhibit very

low polari zati on sensi ti vi ti es. N evertheless, consid-

erable work is in progress to define structures that

can meet t he requir ements of real systems, parti cu-

larly in terms of reliability.

ElectronicsSiG e bipolar and GaA s P-HE M T ( H igh E lectron

M obili ty T ransistor) technologies ( provided by exter-

nal foundries) are currently used for multi plex-

ing/demultiplexing and modulator driver applica-

ti ons, respecti vely. F or example, a 40 G bit/s G aAs

H EM T driver provided a high output voltage ( >6 V) ,based on a 2-stage structure with the preamplifier fol-

lowed by the main amplifier-combiner using dis-

tri buted amplifi ers. O n the other hand, I nP integrated

circuits are expected to be used for specific func-

ti ons, such as a 40 G bit /s deci sion fli p-flop at the

transmi tter and receiver si des, to provi de impr oved

performance margins.

Conclusion

T erabit/s capacity has been demonstrated. C ompared

with commercially-available WDM systems, thi s has

been achieved by increasing the channel rate to

40 G bit /s, by increasing the number of WD M channels

to 256 to achieve an unprecedented spectral effi-

ciency, and using the full bandwidth of the C-band and

niques help to reduce detr imental crosstalk between

channels to below an acceptable level. I n additi on, the

signal is transmi tted on the new Alcatel TeraL ight fiber,

which has been shown to be the optimum fiber design

for cost-effective ultra-high-bit rate t ransmission and to

be future proof because of its compatibility with higher

channel bi trates.

Optoelectronic and ElectronicComponents for 40 Gbit/ sApplications

M any development s have been necessary to achieve

a transmi ssion rate of 40 G bit /s per channel. O pto-

electronic devices and very fast integrated circuits

are now available, although their high speed capa-

bilities often remain far from what is required. New

generati ons of components are being developed for

the practi cal i mplementati on of 40 G bit /s systems.

R & I and the A lcatel Business D i visi ons are acti ve in

these fields, either through in-house developments

or through collaborations with external suppliers.

TransmittersL i thi um N iobate electro-optic modulators are cur-

rently the lowest bandwidt h components wit hin the

transmi tt er. M oreover, they require a high drive

voltage of more than 5 V pp to achieve a high ext i nc-

100 kmTeralight

42.7 Gbit/s (FECC) equipment- SiGe Technology

Distributed Raman amplification +C/L band EDFA

Dispersion management- 100 kmTeralight(+8ps/nm/km) DCF

"Vestigal Side-Band" optical filtering- 1.28 bit/ s/Hz

- 10.2 Tbit/ s

Left-sidefiltering

Wavelength (nm)UncorrectedBER

SNR(in0.1.nm)

Right-sidefiltering

Left andright-sidefiltering

C

L

C

L

40 Gbit/ s+FEC

40 Gbit/ s+FEC

Polar.mux

DCF DCF

DCF DCF

C

L

Tx1

Tx2

RX

1

128

1

128

C

L

C

L

1525

50GHz75GHz 75GHz

352025352025

0-6

0-5

0-4

0-3

1545 1565 1585 1605

Fig. 2 Experimental setup for demonstrating terrestrial transmission with a record spectral efficiency of 1.28 bit/s/Hz

Fig. 3 InP-based electro-absorption modulator

DCF: Dispersion Compensating Fiber

Alcatel Telecommunications Review - 3rd Quarter 2001 Road to ultra-high capacity transmission178


8/8Alcatel Telecommunications Review - 3rd Quarter 2001 Road to ultra-high capacity transmission179

Sbastien Bigo is head of the WDM transmis-sion group within Alcatel Research & Innovation

in Marcoussis, France.

L-band optical amplifiers. Further improvements in

the t ransmi ssion performance and cost-effecti ve-

ness of such large capaci ty systems partly depend on

the progress of electronics and opto-electronics.

Wilfried Idler is in charge of N x40 Gbit/stransmission at the Alcatel Research & Innova-tion center in Stuttgart, Germany.

Andr Scavennec is Deputy Director of Opto+based in Marcoussis, France.

evolution of optical network

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