A state-of-the art of photonic telecommunication technology is reviewed and possible directions of future developments are outlined. In particular, the impact of nonlinear optical phenomena inherent to silica glass on the transmission performance of wavelength-division multiplexed optical signal through fibres is discussed. Also potential applications of nonlinear photonic devices for the purposes of optical signal processing that is foreseen in future all-optical networks are pointed out

Kowalewski, Marian

Marciniak, Marian

Biblioteka Cyfrowa Instytutu Łączności / National Institute of Telecomunications: Digital Library

Paper The present and futureof nonlinear optics applications in photonictelecommunication networksMarian Marciniak and Marian KowalewskiAbstract — A state-of-the art of photonic telecommunicationtechnology is reviewed and possible directions of future devel-opments are outlined. In particular, the impact of nonlinearoptical phenomena inherent to silica glass on the transmis-sion performance of wavelength-division multiplexed opticalsignal through fibres is discussed. Also potential applicationsof nonlinear photonic devices for the purposes of optical sig-nal processing that is foreseen in future all-optical networksare pointed out.Keywords — optical fibre communications, photonic networks,wavelength-division multiplexing, nonlinear optical phenomena.Optical technology and infrastructureRecent years have shown a rapid growth of demand forcapacity of telecommunication networks. It has inspiredmany laboratories to explore new techniques of more effi-cient utilisation of the huge bandwidth offered by opticalfibre links. One of the most promising and cost effec-tive ways to increase optical link throughput is a techniqueknown as Wavelength Division Multiplexing (WDM).Optical transparent transmission offers almost infinite op-tical bandwidth. This is especially attractive in view offuture information society needs for exchange of enormousinformation streams, resulting from a general use of multi-media and hypermedia services.Transparent optical networks that are actually being intro-duced on the basis of existing silica cable infrastructureoffer almost infinite transmission bandwidth [1]. This isof crucial importance in view of future information societyneeds for exchange of enormous information streams, re-sulting from a general use of multimedia and hypermediaservices.The transparent network technology is actually in a ma-ture state: a number of elements are already commerciallyavailable. Those are: tunable laser diodes and laser ar-rays as WDM sources based on a ITU 100 GHz opti-cal frequency grid, Arrayed-Waveguide Gratings (AWG) asmultiplexers and demultiplexers including optical add-drop(de)multiplexers, fibre Bragg gratings as filters, etc.Optically transparent technology is actually advancing veryfast. Although almost unlimited capacity is available, thefuture technology has to meet new demands especially inthe field of optical signal digital processing, including full3R (Reamplification, Reshaping, Retiming) regeneration.Moreover, a new concept of 4R regeneration [2] has re-cently appeared that reclaims also a proper regeneration ofthe optical spectrum of the aggregate signal in introducedrecently commercial transmission systems exploiting WDMtechnology.In a WDM system many information channels are transmit-ted through one fibre using different optical wavelengthsmodulated by independent data streams. This methodis analogous to Frequency Division Multiplexing (FDM)which is widely exploited in other communication systems,especially in radio broadcasting. Using WDM we can eas-ily increase the capacity of already existing fibre links thatis particularly significant in the areas where placing newcables is impossible or too expensive. One can also envi-sion the application of WDM in broadcast networks and/orin subscriber loop [3].The introduction of Erbium-Doped Fibre Amplifiers(EDFA) which have replaced electronic regenerators in fi-bre based transmission links in early 90 s resulted in opticaltransparency of the links. This was in contrary with elec-tronic regenerator based links. In those a combination ofelectronic logic circuit along with electro-optical and opto-electrical conversions of the digital signal transmitted hasbeen used in order to cope with signal distortion. In opticallinks the distortion results from physical limitations of thetransmission of light signals through fibres, namely fromfibre attenuation, dispersion, and nonlinear distortion.An enhancement of optical link exploitation enabled bythe EDFAs and WDM technologies have resulted in a dra-matic decrease of cost of transmission bandwidth. More-over, WDM offers an orthogonality between wavelengthand time, so they can be processed independently and si-multaneously. However, significant research challenges stillremain to realize the huge potential offered by optical net-working.WDM transmission is actually being introduced in commer-cial fibre telecommunication systems. Transmitting severalwavelengths (16, 32, 64 or more) and amplifying the aggre-gate signal in optical amplifiers results in an increase of thetotal optical power, what causes nonlinear interactions veryeffective. The transmission performance of the system canbe then seriously degenerated, mainly via nonlinear signaldistortion and nonlinear crosstalk (power transfer betweendifferent wavelength channels). Thus, there exists a greatneed to minimize the nonlinear distortion and to optimizethe transmission parameters. On the other hand there is anurgent necessity to establish new international standards forWDM nonlinear systems.JOURNAL OF TELECOMMUNICATIONSAND INFORMATION TECHNOLOGY 1-2/2000 3Marian Marciniak and Marian KowalewskiNonlinear limitationsAnalogous features of a silica fibre: attenuation, dispersion,and nonlinear distortion result in distortion, crosstalk andnoise of the transmitted optical signal. Therefore, a digitalsignal can be transmitted successfully only at certain dis-tance of the fibre link, this distance is called transparentlength.Silica glass exhibits only small value of nonlinear coeffi-cient. However, due to small spot-size of modal beamsin the core of the fibre and high power at the output ofoptical sources and amplifiers, going up to 100 mW, andalso extremely long distances of propagation which is ofthe order of hundreds or thousands of kilometres, nonlin-ear effects are accumulated along fibre link and can causea significant change of the optical signal in positive or neg-ative way. Moreover, the fibre nonlinearity is believed todecrease signal distortion caused by Polarisation Mode Dis-persion, which is fibre intrinsic defect that is due to lack ofideal symmetry of practical fibres.At present state of technology nonlinear effects affect thetransmission system performance in great scale. This is dueto combining high level of optical power at the output ofmodern lasers and amplifiers with extremely small cross-section of guiding core of the fibres.In spite of its merits the WDM technique is not free fromlimitations. The most characteristic and essential problemfor multichannel optical systems, beside attenuation anddispersion, is interchannel crosstalk [4]. One can distin-guish crosstalk caused by nonlinear interactions betweenthe light in different channels or between the light and thefibre material. In spite of the intrinsically small values ofthe nonlinearity coefficients in fused silica, the nonlineareffects in optical fibres can be observed at low power lev-els. This is possible because of important characteristicsof single-mode fibres, a very small optical beam spot-size,and extremely low attenuation.In WDM systems a nonlinear interplay between many dif-ferent spectral components of the aggregate signal causesinterchannel crosstalk. The nonlinear phenomena involvedare Self-Phase Modulation (SPM), Cross-Phase Modula-tion (XPM), Four-Wave Mixing (FWM), Stimulated Ra-man Scattering (SRS), and Stimulated Brillouin Scattering(SBS).System requirementsA number of functions in the optical domain can be real-ized only via nonlinear effects. This is an inherent featureof optics in contrary to electronics: two electrons interactstrongly via electrostatic and magnetic forces even in thevacuum, while two photons do not at all. Actually this wasthe reason successful application of photonic transmissionin fibre links. Unfortunately, this is also the reason why itis so difficult to realise processing of light by light. Thiscan be done only via nonlinear interaction between lightand matter or between the beams themselves.Core networksCore networks represent the backbone of the informationsuperhighway. Optical cable infrastructure is well devel-oped in many countries and no substantial investment isneeded to upgrade the transmission capacity. The mostpromising one is WDM technology, which does not needto install faster electronics.Broadband access networksThe expected introduction of optical transparency to sub-scriber loop will allow taking advantage also from WDMtechnology. A combination of various signals (i.e. ana-logue or digital television, interactive broadband services)could be transmitted simultaneously. New ways of provid-ing access are emerging based upon the need for interactivebroadband services. This means a need for a mass deploy-ment of fibre in access.ATM networksMultimedia service networks based on fibre core networksand ATM technology can provide the broadband commu-nications platform needed by business and residential usersfor integrated services with of voice, data and visual in-formation transmission. This should provide high-qualityperformance and economic advantages of ATM networks.The future transparent photonic networkIt is generally believed that opticalisation which is a termstanding for keeping the signal in optical domain thusavoiding its conversion to electronic domain in the wholetransparent network should result in a much more completeexploitation of the combined huge low-loss fibre windowand optical amplifier bandwidth, which is actually estimatedto be equal to ten terahertz.All optical networks offer new possibilities for high band-width applications. New techniques will be demonstratedfor optical switching and network management for complexoptical networks. WDM systems allow upgrading of thebackbone optical network. This theme explores the currentstate of research and future developments of optical net-work technology and applications. End-to-end broadbandtransparent transmission is an essential condition to pro-vide reliable broadband services. The demand for Internetis a driving force leading rapidly towards WDM. However,WDM is an analogue technology that suffers from analoguedistortions of the signal. On the other hand, special kindof optical pulses exploring silica glass nonlinearity calledsolitons have a digital nature (they exist or do not). Soli-tons are then more compatible with digital transmission. Acombination of both technologies appears as a very promis-ing one. Solitons are better for synchronous (SDH) digital4JOURNAL OF TELECOMMUNICATIONSAND INFORMATION TECHNOLOGY1-2/2000The present and future of nonlinear optics applications in photonic telecommunication networkssystems, while a passive WDM network is better suited forATM.WDM is a technique compatible with the idea of all-opticalnetworks, where one can create transparent optical pathsconnecting successive network nodes by switching opti-cal channels organized at the different light wavelengths.Wavelength converters (λC) are developed in order to profitfrom another degree of freedom of transparent network,which is the signal wavelength. This allows a realisationof wavelength routing functions.Transparency is very attractive also from user point of view:he/she sends his/her own data streams and the transparentnetwork transmits them regardless of their format, bitrateetc. A functional model of a transparent passive networkconsists of an optical telecommunication cloud throughwhich clients send and receive their messages of variouskinds.Reduced cost of bandwidth enabled by the optical amplifierand WDM technologies have resulted in a dramatic reduc-tion of cost of transmission bandwidth. Moreover, WDMoffers an orthogonality between wavelength and time, sothey can be processed independently and simultaneously[5]. However, significant research challenges still remainto realize the huge potential offered by optical networking.References[1] M. Marciniak, „Transparency of optical networks: How to manageit?”, in International Conference on Transparent Optical NetworksICTON’99, Conference Proceedings, Kielce, Poland, June 1999, In-vited Paper Th.B.2, pp. 85–88.[2] L. Thyle´n, „Some aspects of photonics and electronics in communi-cations and interconnects”, in 1st International Conference on Trans-parent Optical Networks ICTON’99, Opening Lecture, Kielce, Poland,June 1999.[3] Y. Namihira, „Relationship between nonlinear effective area and modefield diameter for dispersion shifted fibres”, Electron. Lett., vol. 30,no. 3, pp. 262–264, 1994.[4] G. P. Agrawal, Fiber-optic communication systems. New York:Willley-Interscience Publication, 1992.[5] R. C. Alferness, „Research challenges in optical networking”, inECOC’98, Madrid, Spain, Sept. 1998.Marian MarciniakNational Institute of Telecommunications, 1 Szachowa Str.,04-894 Warsaw, Poland, e-mail: M.Marciniak@itl.waw.plMarian KowalewskiNational Institute of Telecommunications, 1 Sza-chowa Str., 04-894 Warsaw, Poland, e-mail:M.Kowalewski@itl.waw.plJOURNAL OF TELECOMMUNICATIONSAND INFORMATION TECHNOLOGY 1-2/2000 5

The present and future of nonlinear optics applications in photonic telecommunication networks, Journal of Telecommunications and Information Technology, 2000, nr 1-2

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The present and future of nonlinear optics applications in photonic telecommunication networks, Journal of Telecommunications and Information Technology, 2000, nr 1-2

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