trends and evolution of optical networks and technologies

Research: exploit technology cross-fertilization

and WDM. During 2000, the volume of data traffic in theUSA surpassed the volume of voice traffic, highlighting thedominant 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 the

way for increased capacity and more

intelligence in optical networks.Meanwhile, as operators were investing to cope with Inter-net growth (although revenue was still coming primarilyfrom voice traffic) the capital crunch occurred. Thisincreases the importance of another research objective:make the technology less expensive to produce, imple-ment and operate. To prepare for the future, innovation in the field of highcapacity transmission remains important. Long haul sys-tems – both terrestrial and submarine – go through the wellknown “build and fill” cycles. Industry-wise, we are in a “fill”cycle. From a research perspective, it is important to pre-pare for the next “build” cycle. The next generation of opti-cal transmission will be based on N x 40 Gbit/s systems, pro-viding capacities of several Tbit/s. However, when con-sidering transmission, one has to take into account the dis-tance between regenerators because the Signal to NoiseRatio (SNR) degrades with distance. Consequently, trans-mission needs to be evaluated with respect to both capac-ity and distance: For example, future transmission systemsmight be identified as “10 Petabit/s*km networks”, mean-ing that they offer 10 Tbit/s transmission over 1000 km, or1 Tbit/s over transoceanic distances.Alcatel is strongly committed to N x 40 Gbit/s systems.Research embraces all the relevant fields, includingsubmarine and terrestrial systems, as well as a range ofenabling technologies. For example, Alcatel was one ofthe first to demonstrate transmission at more than10 Tbit/s [1]. A number of innovations made this demon-stration possible: vestigial sideband filtering at thereceiver for narrow channel spacing, distributed Ramanamplification to optimize the SNR, Alcatel Teralightfiber to reduce fiber impairments, and polarization multi-plexing to double the capacity.

Trends and evolution of optical networksand technologies

Introduction

Over the past few years, optics has established itself asone of the basic communication network technologies asa result of the conjunction of several key technologicalinnovations (optical fiber, semiconductor lasers, fiberamplifiers) and market needs. Thanks to the introductionof Wavelength Division Multiplexing (WDM), opticaltransmission now makes it possible to transmit enormousamounts of information over almost unlimited distances.As far as transmission capacity is concerned, fiber has nocompetition. In addition, optics offers a number ofadvantages in the field of networking. Even thoughrecent cuts in capital expenditure (capex) have sloweddown progress in this field, the fundamental trends intelecommunications will inevitably bring optical tech-nologies and networks back into the spotlight. Optical communications is still a very new industry. Fibershave only been widely installed over the past decade, andmainly for long distance transmission. Optical networkingis not really here yet. The industry is young, and conse-quently somewhat immature. Hence, research in opticalcommunication technology can actively contribute toimproving the technology in various industrial as well as fun-damental areas, such as materials, devices, architecturesand protocols.This article examines the general trends in optical com-munications and describes Alcatel’s main research direc-tions. Some of the key Alcatel research results are high-lighted in other articles in this “ Optics ” section of theAlcatel Telecommunications Review.

More Bits to More Users

The explosive growth in capacity is largely a result of mas-sive use of the Internet. The combination of an increasingnumber of Internet users and the introduction of new con-tent-richer services with more picture and video contenthas resulted in the demand for capacity doubling every 6to 9 months in some networks. Such growth – faster thanwas experienced in electronics – has been possible thanksto the combination of Time Division Multiplexing (TDM)

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

detecting only a small percentage of the signal. In col-laboration with European partners, Alcatel has demon-strated an all-optical cross-connect and has tested it ina real network [3]. Optical switching can, however, find a place even in anetwork that is not fully transparent. Indeed, an elec-tronic switching matrix can be replaced by an opticalone. In this case it does not provide a specific functionaladvantage, but the expectation is that for large switch-ing matrices, an optical implementation will be cheaperthan an electronic one. An optical switching element isalso bitrate independent, which means that it is possi-ble to upgrade ports from, say, 2.5 Gbit/s per channelto 10 Gbit/s, or even 40 Gbit/s, without changing thematrix. This is not possible with an electronic versionsince higher capacity requires more processing power.Whatever implementation is selected, such cross-con-nects perform wavelength switching, and thus allowwavelength service (end-to-end wavelength provi-sioning, for instance). Signaling, controlling and man-aging WDM networks have become hot research anddevelopment topics. Besides the introduction of an Optical Channel (OCh),which makes it possible to treat each wavelength as aseparate logical channel, Internet-based protocols,such as Multi Protocol Wavelength Switching (MPλS)and Generalized Multi Protocol Label Switching(GMPLS), are being introduced at the control layer. Thebasic driving forces are known: apply data-oriented pro-tocols (which proved so cost-effective for the Internet)to WDM networks and make dynamic establishment ofwavelength-based routing paths possible. As data isbecoming the dominant type of traffic, this trendappears natural. Nevertheless, the required constraintson Quality of Service (QoS), restoration and protectionneed to be carefully assessed.The impact of data is even larger and more profound onmetropolitan networks. Because of the mix of differentformats – Internet Protocol (IP), Asynchronous Trans-fer Mode (ATM), Gigabit Ethernet, etc – such networksnaturally have to evolve towards multiservice networks.On the optical layer level, WDM is the most suitable tech-nology, yet with even greater cost constraints. Trans-parency, which is difficult to manage at the backbonelayer, might find an easier implementation in themetropolitan area. Alcatel research is working on a num-ber of innovative solutions [4].The ultimate dream of an “IP-over-optics” approachremains, however, an optical router. This requires fastoptical switching fabrics. Alcatel already has consid-erable experience in optical packet switching, havingdemonstrated the first optical ATM switching demon-strator some years ago as part of the European ATMOSand KEOPS programs. We have further refined theseideas and have adapted the concept to take intoaccount the IP dimension. The first burst opticalrouter has been assembled; it exploits a number of inno-vative approaches for both the optical elements and thecontrol layer. This prototype has validated the feasibilityof implementing an all-optical burst router – includingburst transmitters and receivers – as well as high-speedscheduling algorithms.It is clear that optics can offer much more than just point-to-point transmission. Wavelength service, network pro-

As a result of these innovative technologies, Alcatelachieved a record spectrum density of 1.28 bit/s/Hz. Thisparameter is important as it indicates the efficiency ofspectrum utilization and is therefore linked to the cost.The achieved efficiency is six times higher than for today’scommercial systems. In another experiment, N x 40 Gbit/stransmission was demonstrated in a submarine configu-ration. Transmission at 32 x 40 Gbit/s (in excess of 1 Ter-abit/s) was achieved over a distance of 2400 km usingamplification only, and no regeneration. In the case of ultra-long-haul transoceanic systems,N x 40 Gbit/s systems will require regeneration. Althoughone can implement this function using optoelectronic con-version, this would be a step backwards compared withthe present situation in which one optical amplifier is usedto simultaneously amplify several (in most cases all) wave-length channels. An optoelectronic regenerator is, by def-inition, a single-channel device that might jeopardize thecost advantage optics has brought to transmission. Thusresearch into optical regenerators is a key program. Alca-tel is investigating several approaches based on semi-conductor wavelength converters, in-line synchronousmodulation and saturable absorbers [2].Optical transmission on long haul networks is only partof the picture. Fiber will inevitably be the transmissionmedium in metropolitan area networks, and is increasinglyextending its reach into access networks. Following thegeneralization of high speed Internet accesses (Asym-metric Digital Subscriber Line, ADSL; Very high speedDigital Subscriber Line, VDSL; etc), a need will soonemerge for high capacity transmission systems to the cus-tomer premises. As a result, photons are coming closerto the home! However, metropolitan and access networksraise a number of challenges other than purely trans-mission ones: protocols, multiservice capability and costare the dominant issues.

From Dumb Pipes to Intelligent Networks

If optical transmission – and WDM in particular – hasestablished an undisputed leadership, the use of pho-tonics and exploitation of the wavelength domain for net-working is still in its infancy. Nevertheless, it is tempt-ing to push further what photons can do in a network.The argument is simple. Consider a WDM network with80 wavelength channels of 10 Gbit/s on each fiber. Oneach of the network nodes, a cross-connect will have toswitch hundreds of 10 Gbit/s channels from input fibersto either drop channels or output fibers. Electronics isthe way to do it today. However, this requires atransceiver – which involves optoelectronic conversion– at both the input and output ports. These transceiversare the major cost element in a cross-connect. As most of the traffic in a node is transit traffic, replac-ing the electronic cross-connect by a fully transparentoptical cross-connect is the obvious “low cost” photonicalternative. It does, however, raise a number of issues,including the non-intrusive monitoring required to man-age all-optical networks. Several solutions are beinginvestigated within the Alcatel laboratories. These solu-tions are either based on additional control channels ormodulation, or the use of high speed electronic pro-cessing capable of assessing the quality of the signal while


wider temperature range (from –40 to +85°C) - anattractive challenge for quantum mechanics specialists! The next move was to replace active fiber/laser alignmentby a passive technique. This was achieved by rethinkingthe laser mounting process. Silicon motherboards havebeen developed, which make it possible to use an auto-matic self-aligning process for the laser and fiber, with thehelp of “indentation” and appropriate structuring of thelaser chip. However, further innovations were needed atthe laser chip, such as the integration of a taper (the equiv-alent of an integrated lens). All this was necessary in order to develop the surface-mountable plastic laser modules. Nothing would have beenpossible without innovation in various fields of physics,optics and processes. What is coming next? One trend will clearly be to integratemore functions, including both passive optical functionsand dedicated electronic interfaces. SiO2/Si mother-boards will play a key role in assembling the passive andactive optical parts cost-effectively. There are a numberof other interesting options for WDM components [6].These trends indicate that the components

industry will evolve considerably over

the next few years to offer ever higher

performance and, even more impor-

tant, greater functional integration at

lower costs. Thus the industry will progressivelymature. Research and innovation are important factorsin making this happen.

Innovation: the Art of Networking

So far, we have focussed on the near- and medium-termevolution of optical technologies. However, optics is

here to stay for a long time, and dis-

ruptive technologies will inevitably

appear. Some may already be knocking at the door, for example, photonic bandgap materials which mightbe used for optical fibers, planar passive devices andsemiconductors. In a world where innovation can hap-pen in various places and environments, where a realapplication might be difficult to detect at an early stage,in other words, in a world of uncertainty, how should wemanage innovation? Alcatel believes that partnership isthe right way to go; it can take various forms. Consider some examples in the optical field. Multi-part-ner projects – national and international – make it pos-sible to build efficient multidisciplinary projects com-bining the talents, expertise and vision from universi-ties and industry. In Europe, Alcatel is a major playerboth within national projects, such as BMBF in Ger-many and RNRT in France, as well as within interna-tional projects, such as IST. Many of our advanced stud-ies in the area of optical networking have been initi-ated in this context, and projects such as OPEN(transparent optical cross-connect), KEOPS (opticalpacket switching), MEPHISTO (management of all-optical networks) and PELICAN (field trial imple-mentation of an all-optical network) were the first toexplore new, innovative options.

tection at the optical layer, wavelength routing and, even-tually, a “true” IP-over-optics implementation are someof the evolutionary steps that are at an advanced stagewithin Alcatel Research.

Components: a Pace of Change

Components need to meet two sets of objectives: one con-cerns their performance and function, while the secondis linked to cost constraints. Both explain why optical com-ponents are at the heart of today’s communication sys-tems. Not only do they set the performance limits andfunctional constraints, but also, as they represent a sig-nificant and increasing proportion of the equipmentcost, they strongly impact the final system cost. The trends mentioned above for high speed transmissionand intelligent networks will materialize only if suitabletechnologies are available. The research highlightsincluded in this issue of the Alcatel Telecommunications

Review show where the challenges for optical compo-nents lie from a functional point of view. Transmissionat 40 Gbit/s requires high-speed modulation, detectionand associated electronics. Managing fiber impairments(chromatic dispersion, polarization mode dispersion,etc) requires dedicated passive components. DenseWDM requires multiplexers and demultiplexers forhigher channel counts and narrower channel spacings.Dedicated electronics, particularly the stages that inter-face directly with the optoelectronic chips, will beequally important for high-speed systems. Optical ampli-fication needs to be developed for new wavelength win-dows (after C and L, the next window will be S), whileat the same time the increasing number of channels willrequire more power [5]. Other functions become manda-tory when moving towards optical networks: opticalswitches of course, but also devices capable of monitoringthe QoS, and ultimately, optical regenerators. Alcatelresearch has achieved breakthroughs in all of these fields. As regards cost, one might think that this is more an indus-trial than a research issue. In fact, the cost of optoelec-tronic components has been reduced, and will continueto be reduced, through innovation. Oversimplifying, we can say that an optoelectronic deviceis made of a chip (front-end) packaged in a module (back-end). The short history of evolution of optoelectronicdevices was an alternation of breakthroughs in the front-end and back-end processes. The first important step was at the beginning of the 90swhen, for the first time, Alcatel demonstrated the feasi-bility of manufacturing full 2 inch InP wafers, each with15 000 lasers! This was made possible thanks to the devel-opment of strained quantum well lasers in the researchlaboratory. The technology proved capable of producinghigh-performance lasers which were uniform and repro-ducible. It also represented a breakthrough in the cost ofthe laser chip. However, the dominant cost then became the module,which was metallic, used a Peltier cooler and needed veryaccurate (manual) fiber/laser chip alignment. The first stepwas to eliminate the Peltier cooler and develop the so-called coax module, still using active alignment. Again thismade it necessary to go back to the laser chip anddevelop new laser structures that could operate over a


research teams. Some other elements will require dis-ruptive approaches that may not yet have been identified. This is where cooperation with more academic centers ofexcellence will play a determining role.Open your eyes and let the light come in! ■

In the field of basic technologies, Alca-tel is also very committed to partner-ships through international projects.Projects on advanced topics such asphotonic bandgap materials, quantumboxes and, more generally, nano-tech-nologies, are areas of ongoing activity. In specific fields, bilateral cooperationcan further help to achieve impressiveprogress for the benefit of both parties.With this in mind, Alcatel has launchedand is supporting a number of collab-orations with major Universities andInstitutions worldwide. As an example,some remarkable results have beenachieved in a bilateral program withthe Heinrich Hertz Institute, one of our key partners [7]. Another such initiative is the creation of “Optics Valley”.Located south of Paris, Optics Valley is an association ofmajor universities, engineering schools, small to mediumsize enterprises, and large corporations that are active inthe optics field (see Figure 1). It represents a unique poolof skills in both fundamental and applied sciences. Alca-tel was one of the founders of Optics Valley and is the ref-erence industrial partner working on optical communi-cation. The various participants in Optics Valley areexpected to give birth to many promising university/indus-try collaborations. As an example, a prestigious CNRS lab-oratory working on optics and nano-technologies (LPN)will be collocated with the Alcatel research laboratory inMarcoussis. The exchange of ideas and the collaborationfacilitated by the proximity of two large laboratories – onewith an industrial culture and missions, the other withmore fundamental objectives – will certainly foster inno-vation. Thus, we believe that cooperation,

partnership and networking with other

centers of excellence are important

assets. After all, it is interconnection that providesintelligence to the human brain!

Conclusion

We have reviewed some of the trends in optical networksand technologies. Although optical telecommunicationsnow appears to be a well-established technology, it hasreally only been extensively used for transmission forabout ten years. Many challenges and opportunities areahead of us. The future is only partly predictable.Increased capacities are inevitable, even if the presenteconomic slowdown might change some of the mile-stones. The move to intelligent optical networks is alsoa strong move which will give added value to operators. Many of the advances needed to implement this vision arealready in the laboratory, as is illustrated by several arti-cles in this issue of the Alcatel Telecommunications

Review. These articles also demonstrate the strongcommitment and quality of the results of Alcatel’s


Thales Central Research

Orsay

Ecole Polytechnique

CNRS - “LULI, LOA”

OPTO +

CNRS-LPN

Z.I. Courtaboeuf :Picogiga...

Alcatel OpticsTerrestrial & submarine transmission

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: “From Transmissionto Processing: Challenges for New OptoelectronicDevices”, Alcatel Telecommunications Review, 3rd Quarter1998, pp 232–239.

3. J. L.Beylat, M. W. Chbat, A. Jourdan, P. A. Perrier: “FieldTrials of All-Optical Networking based on WavelengthConversion”, Alcatel Telecommunications Review, 3rdQuarter 1998, pp 218-224.

4. A. Jourdan, L. Tancevski, T. Pfeiffer: “How much opticsin future metropolitan networks?”, Alcatel Telecommunica-tions Review, 3rd Quarter 2001 (this issue), pp 219-221.

5. D. Bayart, L. Gasca, G. Gelly: “Cladding-pumped erbium-doped fiber amplifiers for WDM applications”, AlcatelTelecommunications Review, 3rd Quarter 2001 (thisissue), pp 179-180.

6. J. Jacquet, K. Satzke, I. Riant: “Low cost DWDMdevices”, Alcatel Telecommunications Review, 3rd Quarter2001 (this issue), pp 181-182.

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), pp 231-233.

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

trends and evolution of optical networks and technologies

Technology