rosstalk caused by switching events in fast tunable lasers in an optical label switching (OLS) system is investigated for the first time. A wavelength-division-multiplexed OLS system based on subcarrier multiplexed labels is presented which employs a 40-Gb/s duobinary payload and a 155-Mb/s label on a 40-GHz subcarrier. Degradation in system performance as the transmitters switch between different channels is then characterized in terms of the frequency drift of the tunable laser

Barry, Liam P.

Connolly, Eoin M.

Cotter, D.

Ellis, A.D.

Kaszubowska-Anandarajah, Aleksandra

Mishra, A.K.

Smyth, Frank

English

F. Smyth

E. Connolly

A.K. Mishra

A.D. Ellis

D. Cotter

A. Kaszubowska

L.P. Barry

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Effects of Crosstalk in WDM Optical Label Switching Networks Due to Wavelength Switching of a Tunable Laser

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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 20, OCTOBER 15, 2006 2177Effects of Crosstalk in WDM Optical Label SwitchingNetworks Due to Wavelength Switchingof a Tunable LaserF. Smyth, Student Member, IEEE, E. Connolly, Student Member, IEEE, A. K. Mishra, A. D. Ellis, D. Cotter,A. Kaszubowska, Member, IEEE, and L. P. Barry, Member, IEEEAbstract—Crosstalk caused by switching events in fast tunablelasers in an optical label switching (OLS) system is investigatedfor the first time. A wavelength-division-multiplexed OLS systembased on subcarrier multiplexed labels is presented which employsa 40-Gb/s duobinary payload and a 155-Mb/s label on a 40-GHzsubcarrier. Degradation in system performance as the transmittersswitch between different channels is then characterized in terms ofthe frequency drift of the tunable laser.Index Terms—Fast tunable laser, optical label switching (OLS),optical packet switching (OPS), subcarrier multiplexed (SCM)label.I. INTRODUCTIONOPTICAL packet switching (OPS) networks employingoptical label switching (OLS) techniques can offer highbandwidth efficiency, good scalability, and fast reconfigurability[1], [2]. In an OPS network, when electrical data packets fromthe client networks arrive at the “ingress” node, the Internetprotocol (IP) header is examined and a label and wavelengthare computed based on the destination of the packet. The packethas its newly computed label attached and is transmitted usinga fast tunable laser into the network on the appropriate opticalwavelength. At each node, a routing decision is made based onthe label, and by switching another fast tunable laser, the packetcan be shifted to a new wavelength using, for example, a semi-conductor optical amplifier (SOA)-based wavelength converter[3], [4]. On reaching the other side of the core network, thelabel is removed at the “egress” node and the original packet isrouted as normal using electrical routing hardware.Optical subcarrier multiplexed (SCM) labels are one of thepossible solutions for label coding [5]–[7]. The label data isplaced on a radio-frequency subcarrier and used to intensitymodulate the laser resulting in two sidebands containing thelabel information, centered on the optical carrier, which con-tains the payload data. The SCM label can be read by detectingthe entire multiwavelength signal and then downconverting thelabel [8], or by extracting the label using an optical bandpassManuscript received May 15, 2006; revised August 1, 2006. This work wassupported by Science Foundation Ireland through the Investigator Program andby the Centre for Telecommunications Value Chain Research (CTVR).F. Smyth, E. Connolly, A. Kaszubowska, and L. P. Barry are with the Re-search Institute for Networks and Communications Engineering, Dublin CityUniversity, Glasnevin, Dublin 9, Ireland (e-mail: smythf@eeng.dcu.ie).A. K. Mishra, A. D. Ellis, and D. Cotter are with the Tyndall National Insti-tute, “Lee Maltings”, Cork, Ireland (e-mail: arvind.mishra@ucc.ie).Digital Object Identifier 10.1109/LPT.2006.883888Fig. 1. Experimental setup employing two fast tunable lasers (FTLs).filter and detecting it directly [9]. The advantages of SCM labelsover the other methods are that they are very easily generatedand detection is simple. Also, there are no strict timing and syn-chronisation requirements. However, the technique is less spec-trally efficient than other methods and SCM labels suffer bothfrom dispersion-induced fading, and from intermodulation dis-tortion, which can lead to degradation of the labels.In this letter, we investigate interference in a two-channelOLS system caused by the wavelength drift of the tunable laserin the packet transmitter as it tunes between different outputwavelengths. These effects are examined and characterized asa function of the wavelength drift of the tunable laser.II. EXPERIMENTAL SETUPThe experimental setup is shown in Fig. 1. The two laser trans-mittersusedwerefast tunable lasersbasedonsampledgratingdis-tributed Bragg reflector technology [10] that can switch betweenevery -band channel on the 50-GHz spacing ITU grid.An SOA at the output of the lasers allowed the light to be“blanked” to prevent spurious output as the laser transitionsfrom one channel to another. The laser transmitters were bothmodulated with a data signal which consisted of 40-Gb/s duobi-nary baseband data combined with a 40-GHz subcarrier mod-ulated with 155-Mb/s data. The payload drive voltage to eachmodulator was approximately 0.9 V . The peak-to-peak driveamplitude of the subcarrier was less than 0.5 V . The use of theduobinary coding for the payload, previously used to increasethe spectral efficiency of a packet routing system [11], ensured1041-1135/$20.00 © 2006 IEEEAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 07:31 from IEEE Xplore.  Restrictions apply.2178 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 20, OCTOBER 15, 2006Fig. 2. Spectra at different stages of the system. (a) Two channels 100 GHzapart; (b) suppressed labels at one port of AMZI; (c) suppressed payload atsecond port of AMZI; (d) extracted label.minimal spectral overlap between payload and label. While theuse of independent modulators in series for label and payloadmodulation would require a compromised payload extinctionratio to avoid degradation of the label, the use of a single mod-ulator ensures that a subcarrier signal is always present, albeitsomewhat amplitude modulated by the payload data.Before coupling the two channels together, one of them waspassed through 1 km of optical fiber in order to decorrelate thedata patterns. This corresponded to six word lengths for the labeland 15 625 word lengths for the payload.Fig. 2(a) shows the input spectrum to the receiver, with bothlasers operating in continuous-wave mode. The signal waspre-amplified and the 194-THz channel was selected using a100-GHz Gaussian profiled arrayed waveguide grating (AWG)with a 3-dB bandwidth of 65 GHz. Another erbium-doped fiberamplifier was then used to boost the signal before filtering itwith an asymmetric Mach–Zehnder interferometer (AMZI)with an FSR of 85 GHz. The signals at the output ports of theAMZI are shown in Fig. 2(b) and (c). In Fig. 2(b), approxi-mately 30-dB suppression is shown on the extracted channel’slabels at the payload port, and in Fig. 2(c), a suppression ofapproximately 10 dB of the payload is shown at the labels port.In the label receiver, the input arm of a circulator was con-nected to a tunable fiber Bragg grating with a 3-dB reflectionbandwidth of approximately 12 GHz. This bandwidth is wideenough to ensure correct detection of the label even in the caseof wavelength drift of the tunable laser. The filter was tuned toreflect the left-hand label of the extracted channel. The reflectedlabel [Fig. 2(d)] was directly detected, electrically amplified, andlow-pass filtered. The output of the receiver was split electricallyand passed into the bit-error-rate (BER) tester and oscilloscope.III. RESULTS AND DISCUSSIONInitially we looked at the effects that the label and payloadhave on each other in a simple single-channel system. The pat-tern length of both the payload and label was . In Fig. 3(a)( and ), it is shown that the presence of the label barely af-fected the payload at all. However, the presence of the duobi-nary payload had a detrimental effect on the label introducingFig. 3. (a) BER of the 194-THz channel payload versus received power inone channel for four cases: single channel without label (); single labelledchannel (4); two static labelled channels ( ); two switching labelled chan-nels (). (b) BER of the 194-THz channel label versus received power in onechannel for four cases: single label without payload (); single channel labelwith payload present (4); two static channels ( ); two switching channels ().a power penalty of approximately 13 dB. This can be seen inFig. 3(b) ( and ). Of this label penalty, 11.4 dB were attrib-utable to the power distribution between the payload (93%) andlabel (7%). The remaining 1.6-dB penalty was attributed to theaggregate modulation of the bias position of the 40-GHz subcar-rier by the payload data (averaged over 258 bits). Owing to thelow label signal amplitude, this pattern-dependent penalty in-creased by 5 dB when the payload pattern length was increasedto . To overcome this limitation, it may be feasible to useManchester encoding on the payload as this ensures that thereare never more than two zeros in a row [12].We then demonstrated the performance (BER versus receivedpower) of the payload and label, respectively, in three differentcircumstances. The first case is with just a single labelledchannel. The second case is with two channels 100 GHz apartbut without any switching of the fast tunable laser. The finalcase is when the channel being measured is static and the otheris switching from the adjacent channel to a distant channelevery 260 ns. The laser output is blanked for the first 60 ns ofthis time. The 60 ns ensures that after coming out of blanking,the laser is always within 12.5 GHz of its target frequency forall channel combinations. A wavelength locking circuit is thenactivated which holds the output within 2.5 GHz of the targetchannel after a maximum delay of 200 ns after blanking.It can be seen that for the payload (Fig. 3(a); , , ), bysimply having two channels adjacent to each other, a powerpenalty of approximately 0.5 dB is introduced due to crosstalkfrom the label of the adjacent channel. This could be reducedor even eliminated by optimizing the profile of the AWG whichis used to select the channels. Further degradation due to theswitching of the adjacent channel is negligible. This is the ex-pected behavior because of the wide spacing between the pay-loads in comparison to any drift of the tunable laser that mayoccur. For the label (Fig. 3(b); , , ), again the presence of anadjacent channel introduces very little penalty. However, whenone of the lasers is switching, an error floor aroundis introduced due to the label of the switching laser, enteringAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 07:31 from IEEE Xplore.  Restrictions apply.SMYTH et al.: EFFECTS OF CROSSTALK IN WDM OLS NETWORKS DUE TO WAVELENGTH SWITCHING 2179Fig. 4. Time-resolved BER measurements for the extracted label ( ) and in-stantaneous frequency of the switching laser’s label. The received power was ap-proximately 28.5 dBm (a) laser at frequency 192.1 THz; (b) laser blanked for60 ns; (c) laser settling into frequency 193.94 THz for 200 ns; (d) laser blankedfor 60 ns; (e) laser at frequency 192.1 THz for 200 ns; (f) laser blanked for 60 ns;(g) laser settling into frequency 193.94 THz for 200 ns.the filter passband of the measured label. This is caused by thewavelength drift of the tunable laser around its target wave-length. The main cause of this drift after blanking is attributed tothe wavelength locking circuit. When this is disabled, althoughtuning takes longer, the wavelength drift is greatly reduced.This effect is further illustrated in Fig. 4, which showstime-resolved BER measurements (8-ns gating interval) witha received power of 28.5 dBm along with the instantaneousfrequency of the label of the switching laser (extracted fromcomplimentary outputs of the 85-GHz AMZI biased at quadra-ture for the target frequency). It can clearly be seen that theswitching events in the tunable laser correspond to the timeintervals when there are significant errors on the received labeldata. While the laser is blanked and also while it sits at thedistant channel, the error rate drops considerably. However, asthe laser tunes towards and begins settling on the 193.94-THzchannel, the error rate increases dramatically.While the results presented in this letter are specific to ourexperiment, there are a number of ways in which the perfor-mance of our system and other similar systems can be improved.Clearly the error floor could be minimized by increasing theblanking time and/or channel spacing. However, both of thesesolutions will reduce network throughput.More attractive solutions would include optimized channeland label filter profiles, the use of single sideband subcarriergeneration to eliminate interference between labels of adjacentchannels, and optimization of the transient laser dynamics in-cluding operation of wavelength-locking technique during theexisting blanking period.IV. CONCLUSIONWe have looked at the interference between adjacent wave-length channels in an optical label switched system that em-ploys SCM labels and a duobinary payload. Such a system of-fers good spectral efficiency, but requires coding to reduce thesensitivity to pattern length. To date, most of the literature onOLS has looked primarily at single-channel systems. We haveshown error-free transmission for both payload and label in atwo-channel system with 0.4-b/s/Hz spectral efficiency. In ad-dition, we have shown that in WDM systems, severe interfer-ence between channels can be caused by wavelength switchingeffects in tunable lasers that would be used in optical packettransmitters.ACKNOWLEDGMENTThe authors would like to thank the Centre for Integrated Pho-tonics for the loan of the diplexer.REFERENCES[1] S. J. B. Yoo, “Optical-packet switching and optical-label switchingtechnologies for the next generation optical internet,” in Proc. OpticalFiber Communications Conf. (OFC 2003), 2003, vol. 2, pp. 797–798.[2] D. J. Blumenthal, “Photonic packet switching and optical label swap-ping,” Opt. Netw. Mag., pp. 1–12, Nov./Dec. 2003.[3] D. Nesset, T. Kelly, and D. Marcenac, “All-optical wavelength conver-sion using SOA nonlinearities,” IEEE Commun. Mag., vol. 36, no. 12,pp. 56–61, Dec. 1998.[4] D. J. Blumenthal, “Photonic packet and all-optical label switchingtechnologies and techniques,” in Proc. Optical Fiber CommunicationsConf. (OFC 2002), 2002, pp. 282–284.[5] A. Martinez et al., “Experimental demonstration of subcarrier multi-plexed optical label swapping featuring 20 GB/s payload speed and622 Mb/s header conveyed @ 18.3 GHz,” in Proc. 31st Eur. Conf. Op-tical Communications (ECOC 2005), 2005, vol. 4, pp. 959–960.[6] M. Y. Jeon, Z. Pan, J. Cao, and S. J. B. Yoo, “BER performance of alloptical subcarrier label swapping with 2R regeneration,” IEEE Photon.Technol. Lett., vol. 16, no. 1, pp. 323–235, Jan. 2004.[7] D. J. Blumental, A. Carena, L. Rau, V. Curri, and S. Humphries, “All-optical label swapping with wavelength conversion for WDM-IP net-works with subcarrier multiplexed addressing,” IEEE Photon. Technol.Lett., vol. 11, no. 11, pp. 1497–1450, Nov. 1999.[8] B. Meagher, G. K. Chang, G. Ellinas, Y. M. Lin, W. Xin, T. F. Chen,X. Yang, A. Chowdhury, J. Young, S. J. Yoo, C. Lee, M. Z. Iqbal, T.Robe, H. Dai, Y. J. Chen, and W. I. Way, “Design and implementationof ultra-low latency optical label switching for packet-switched WDMnetworks,” J. Lightw. Technol., vol. 18, no. 12, pp. 1978–1987, Dec.2000.[9] H. J. Lee, S. J. B. Yoo, V. K. Tsui, and S. K. H. Fong, “A simple all-optical label detection and Swapping technique incorporating a fiberBragg grating filter,” IEEE Photon. Technol. Lett., vol. 13, no. 6, pp.635–637, Jun. 2001.[10] J. Buus, M.-C. Amann, and D. J. Blumenthal, “Widely tunable mono-lithic laser diodes,” in Tunable Laser Diodes and Related OpticalSources. Hoboken, NJ: Wiley, 2005.[11] M. Duelk et al., “Fast packet routing in a 2.5 Tb/s optical switch fabricwith 40 Gbit/s duobinary signals at 0.8 b/s/Hz spectral efficiency,” inProc. Optical Fiber Communications Conf. (OFC 2003), 2003, vol. 3,pp. PD8 1–PD8 3.[12] J. Zhang, N. Chi, P. V. Holm-Nielsen, C. Peucheret, and P. Jeppesen,“Performance of Manchester-coded payload in an optical FSK la-belling scheme,” IEEE Photon. Technol. Lett., vol. 15, no. 8, pp.1174–1176, Aug. 2003.Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. 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Effects of crosstalk in WDM optical label switching networks due to wavelength switching of a tunable laser

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A simple alloptical label detection and Swapping technique incorporating a ﬁber Bragg grating ﬁlter,”

All-optical wavelength conversion using SOA nonlinearities,”

Alloptical label swapping with wavelength conversion for WDM-IP networks with subcarrier multiplexed addressing,”

BER performance of all optical subcarrier label swapping with 2R regeneration,”

Design and implementation of ultra-low latency optical label switching for packet-switched WDM networks,”

Experimental demonstration of subcarrier multiplexed optical label swapping featuring 20 GB/s payload speed and

Fast packet routing in a 2.5 Tb/s optical switch fabric with 40 Gbit/s duobinary signals at 0.8 b/s/Hz spectral efﬁciency,” in

Optical-packet switching and optical-label switching technologies for the next generation optical internet,” in

Performance of Manchester-coded payload in an optical FSK labelling scheme,”

Photonic packet and all-optical label switching technologies and techniques,” in

Photonic packet switching and optical label swapping,”

Widely tunable monolithic laser diodes,” in Tunable Laser Diodes and Related Optical Sources.

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Effects of crosstalk in WDM optical label switching networks due to wavelength switching of a tunable laser

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