We demonstrate a reconfigurable all-optical packet processing system. The key device is a code-reconfigurable header decoder based on a fiber Bragg grating. The performance of the system is tested for different packet headers, and error-free operation is confirmed

Ibsen, M.

Petropoulos, P.

Richardson, D.J.

Tian, C.

Zhang, Z.

English

Chun Tian

Zhaowei Zhang

Morten Ibsen

Periklis Petropoulos

David J. Richardson

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A Reconfigurable Optical Header Recognition System for Optical Packet Routing Applications

Southampton (e-Prints Soton)

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 22, NOVEMBER 15, 2006 2395A Reconfigurable Optical Header RecognitionSystem for Optical Packet Routing ApplicationsChun Tian, Zhaowei Zhang, Morten Ibsen, Periklis Petropoulos, and David J. RichardsonAbstract—We demonstrate a reconfigurable all-optical packetprocessing system. The key device is a code-reconfigurable headerdecoder based on a fiber Bragg grating. The performance of thesystem is tested for different packet headers, and error-free oper-ation is confirmed.Index Terms—Fiber Bragg grating (FBG), optical code, opticalfiber communication, optical packet switching.I. INTRODUCTIONTHE adoption of optical packet processing technologypromises a route to more flexible optical networks of-fering potential advantages in terms of bandwidth utilizationefficiency and payload transparency amongst others. A criticalfunction in any optical packet-based network is the reliablegeneration and processing of headers. Most current schemesstill use electronic means for this purpose and the associatedspeed limitations restrict the header recognition and processingtimes. This limitation can be overcome by employing photonictechnologies in which the header recognition speed is deter-mined primarily by the propagation delay for light travelingthrough some relatively short photonic structure, e.g., a fibergrating, or a planar lightwave circuit [1], [2]. Optical solutionsare expected to employ an array of photonic processors tofacilitate parallel processing of headers, and conventionalbit-serial packet architectures, in which the packet header andpayload are allocated different time slots and are transmittedsequentially [1], [3], [4], can be employed.The codes we use in our proposed bit-serial configuration,have been extensively investigated for optical code-divisionmultiple-access (OCDMA) applications [5] and can be gen-erated and processed directly in the optical domain, thusguaranteeing ultrahigh-speed and minimum latency. Fig. 1illustrates the operating principle of bit-serial networking usingoptical codes [4]. The optical encoder converts a single shortoptical pulse into a code represented by a specific sequenceof individual pulses, and is normally designed, so that whena decoder with similar characteristics is assigned to it, then adistinct autocorrelation peak is produced via matched filtering.Conversely, for any unmatched codes or for the payload, onlyManuscript received July 5, 2006; revised September 21, 2006.The authors are with Optoelectronics Research Centre, University ofSouthampton, Southampton SO17 1BJ, U.K. (e-mail: cht@orc.soton.ac.uk;zhz@orc.soton.ac.uk; mi@orc.soton.ac.uk; pp@orc.soton.ac.uk; djr@orc.soton.ac.uk).Color versions of Figs. 2 and 3 are available at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2006.886148Fig. 1. Principle of header processing in a bit-serial packet routing system.a low intensity noise-like cross-correlation appears at the de-coder’s output. The autocorrelation peak can be easily detectedusing threshold detection and used to control the routing actionsof a suitable fast optical switch.Our system relies on superstructured fiber Bragg gratings(SSFBGs) to generate and process the optical codes. SSFBGs arecompact, polarization-insensitive devices, have the potentialof low cost and relatively straightforward fabrication. Thecodes are written within the SSFBGs by means of a spatialamplitude or phase envelope superimposed upon an otherwiseuniform FBG refractive index modulation profile. A matchedfilter for header recognition is formed by using an SSFBGwhich has the time-reversed profile of the encoding grating.We have previously demonstrated that it is possible to fabricateSSFBGs incorporating complex codes, including for example255 chip quaternary phase-shift-keyed sequences [3].However, despite the advantages offered by this technique,SSFBGs are normally passive devices carrying a fixed code.Thus, a different encoding/decoding SSFBG would be requiredto process each of the addresses used in the network. To en-hance the flexibility of switched networks, it is highly desirablethat header processing devices in the address bank array havethe capability to tune to a different optical header on demand.To this end, we have developed thermally tuned code-reconfig-urable devices based on uniform FBGs for optical code gener-ation and recognition [6]. The thermally induced phase shiftsalong the FBG can produce superstructure characteristics (andhence codes) similar to fixed-coded SSFBGs, for dynamic op-tical header recognition. We have recently shown that these de-vices are compatible with fixed-coded SSFBGs and have a rea-sonably fast reconfiguration time of less than 2 s [7].In this letter, we describe an experimental demonstration ofa reconfigurable optical header processing system based on acode-reconfigurable decoder. Preliminary results of this workwere presented in [8]. We show that a single decoding devicecan be electrically tuned to decode different codes, and we char-acterize the performance of the system.1041-1135/$20.00 © 2006 IEEEAuthorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on June 24, 2009 at 11:52 from IEEE Xplore.  Restrictions apply.2396 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 22, NOVEMBER 15, 2006Fig. 2. Experimental setup of the reconfigurable packet routing system. Inset shows the spectrum of the transmitted signal.II. OPTICAL PACKET PROCESSING SYSTEMA. Experimental SetupThe experimental setup we used to demonstrate the operationof our scheme is shown in Fig. 2, and incorporated a packettransmitter and a reconfigurable packet processing node. Thepacket transmitter used a single continuous-wave (CW) lasersource (tuned to the wavelength of the encoding SSFBGs)which was split into two paths to generate the headers andpayload, respectively. For the header generation, the CWbeam was carved using an electroabsorption modulator intoa 10-GHz pulse train and then gated down to 78 MHz usinga LiNbO modulator. The generated 20-ps pulses were splitagain to generate two 400-ps-long headers (H1 and H2) fromtwo different 16-chip, 40 Gchip/s quaternary phase-shift-keyedfixed-coded SSFBGs. The codes used to represent the headerswere chosen from Family A Codes [9], which are designedfor four-phase level coding with near-optimum correlationproperties. Light in the second path was intensity modulatedby a LiNbO modulator with nonreturn-to-zero data sequencesat 10 Gb/s which represented the payloads for both types ofpackets (P1 and P2). The relative delay between each headerand associated payload was adjusted to allocate them sequentialtime slots before they were combined together.At the packet processing node, optical header recognition wasperformed to control the switching of the payload. The incomingpacket stream was split by a 3-dB fiber coupler. One part of thesignal was input to a LiNbO modulator which acted as an op-tical switch. The other part of the signal was injected into thecode-reconfigurable FBG. This comprised a uniform FBG uponwhich 15 thin electrical wires were coupled in a direction per-pendicular to the fiber axis. The wires were spatially separatedby 2.5 mm corresponding to a temporal separation of 25 ps.Dynamic assignment of a phase-shift-keyed code on the FBGsuperstructure could be achieved by appropriate control of thecurrent flowing through each of the wires due to the inducedlocalized heating [7]. A Peltier cooler attached to the fiber en-sured that the ambient temperature of the FBG remained stableto within 0.4 C, as required for reliable operation of the de-coder. The device was programmed to match either code of theSSFBG header generators. It should be noted that the wave-length-selective nature of the FBG means that no other filterFig. 3. Reflectivity spectra of the reconfigurable device when (a) no code (uni-form FBG), (b) code H1, and (c) H2 were configured to the FBG. (Solid lines:measured results. Dashed lines: simulation results).is required for header processing, even when more than onechannel is included in the system. The resulting autocorrelationpeak reflected off the FBG was detected by a photodetector andtriggered an electrical pulse generator, which generated an elec-trical pulse of a suitable width to drive the LiNbO switch. Bysimply changing the code configuration of the decoding device,we were able to detect either header H1 or H2 and switch outeither payload P1 or P2 accordingly.B. Experimental ResultsFig. 3 shows reflectivity spectra of the FBG when (a) no codeis assigned to it, and when it is set to either (b) code H1 or (c)code H2. The bandwidth of the device changes from 0.19 nm atthe 20-dB level when no phase shifts have been applied to it (cor-responding to the bandwidth of the uniform FBG) to 0.52 nmonce a code is configured on the FBG structure. This is deter-mined by the bandwidth of the 25-ps-long chips comprising thecodes. Simulation results are also shown in Fig. 3 which com-pares the experimental traces to the ideal spectra of the pro-grammed codes, showing good agreement between the two. TheAuthorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on June 24, 2009 at 11:52 from IEEE Xplore.  Restrictions apply.TIAN et al.: RECONFIGURABLE OPTICAL HEADER RECOGNITION SYSTEM FOR OPTICAL PACKET ROUTING 2397Fig. 4. Oscilloscope traces for (a) packet stream at Point A (2 ns/div),(b), (c) output of header recognition at Point B (1 ns/div), and (d), (e) droppedpayload at Point C (2 ns/div).Fig. 5. PER test results.figure also shows that there is a slight shift in the central wave-length of the FBG after application of a code, which is due tothe thermally induced change in the Bragg condition.Fig. 4(a) shows the generated packet stream at Point A, inwhich two different packets are transmitted alternately. Eachpacket has a length of 6.4 ns, which comprises the 0.4-ns header,the 5.2-ns payload, and 0.4-ns guard bands both before and afterthe header. Fig. 4(b) and (c) shows the output of the reconfig-urable decoder at Point B, when the reconfigurable device isprogrammed to decode header H1 or H2, respectively. The cor-responding header is decoded correctly as denoted by the dis-tinct autocorrelation peak, which is detected by the thresholddetector which controls the switch. The extinction of the corre-lation pulses is 5.5 dB (detailed measurements of the correlationproperties of the device can be found in [7]). Fig. 4(d) and (e)show the corresponding dropped packets at Point C.To evaluate the system performance of our system, we mea-sured the packet error-rate (PER) as determined at Point B of theexperimental setup. For comparison, we tested the back-to-backcase (which contained no encoding), the one code autocorrela-tion case (which contained only one code), and the cases whenthe payloads were switched OFF and ON, respectively. As can beseen in Fig. 5, when there is no data payload in the packet, thepower penalty due to the encoding process is 3 dB. Error-freeperformance is also achieved when the payload is switched ON,however, the remaining power of the cross-correlation signal be-tween the FBG code and the payload reaches the receiver, re-sulting in an additional power penalty of 7 dB.III. CONCLUSIONWe have demonstrated the operation of an all-optical packetprocessing system based on optical header recognition throughmatched filtering in a dynamically reconfigurable FBG device.The decoding device used in our experiments incorporated16-chip codes at a rate of 40 Gchip/s. Electrical adjustment ofthe heat-inducing electrodes allowed tuning of the decoder tomatch different codes/headers within a time of 2 s. The recon-figurable decoder can easily be scaled to accommodate evenlonger codes by using a longer uniform grating, and mountingmore electrodes upon it. Faster (subsecond) response timesshould be possible by using a combination of thinner gratingfibers and electrodes. Note that such tunable code recognitiondevices are also of significant interest for use in reconfigurableOCDMA systems where code tuning speed is a lesser concern.REFERENCES[1] K. Kitayama and N. Wada, “Photonic IP routing,” IEEE Photon.Technol. Lett., vol. 11, no. 12, pp. 1689–1691, Dec. 1999.[2] A. Srivatsa, H. de Waardt, M. T. Hill, G. D. Khoe, and H. J. S. Dorren,“All-optical serial header processing based on two-pulse correlation,”Electron. Lett., vol. 37, pp. 234–235, 2001.[3] P. C. Teh, B. C. Thomsen, M. Ibsen, and D. J. Richardson, “400 G/bits4 Gigapackets/s multiwavelength optical packet router based on super-structure fiber Bragg gratings,” presented at the Optoelectronics andCommunications Conf. (OECC 2002), Yokohama, Japan, 2002, Post-deadline paper PD-1-1.[4] B. C. Thomsen, P. C. Teh, M. Ibsen, J. H. Lee, and D. J. Richardson,“A multi-hop optical packet switching demonstration employing all-optical grating based header generation and recognition,” presented atthe ECOC 2002, Copenhagen, Denmark, 2002, Paper 5.5.3.[5] P. C. Teh, P. Petropoulos, M. Ibsen, and D. J. Richardson, “A com-parative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstruc-tured fiber Bragg gratings,” J. Lightw. Technol., vol. 19, no. 9, pp.1352–1365, Sep. 2001.[6] M. R. Mokhtar, M. Ibsen, P. C. Teh, and D. J. Richardson, “Recon-figurable multilevel phase-shift keying encoder-decoder for all-opticalnetworks,” IEEE Photon. Technol. Lett., vol. 15, no. 3, pp. 431–433,Mar. 2003.[7] Z. Zhang, C. Tian, M. R. Mokhtar, P. Petropoulos, D. J. Richardson,and M. Ibsen, “Rapidly reconfigurable optical phase encoder-decodersbased on fiber Bragg gratings,” IEEE Photon. Technol. Lett., vol. 18,no. 11, pp. 1216–1218, Jun. 1, 2006.[8] C. Tian, Z. Zhang, M. Ibsen, M. R. Mokhtar, P. Petropoulos, and D.J. Richardson, “Reconfigurable all-optical packet switching based onfiber Bragg gratings,” presented at the OFC 2006, Anaheim, CA, 2006,Paper OFJ4.[9] S. Boztas, R. Hammons, and P. Y. Kumar, “4-phase sequences withnear-optimum correlation properties,” IEEE Trans. Inf. Theory, vol. 38,no. 3, pp. 1101–1113, May 1992.Authorized licensed use limited to: UNIVERSITY OF SOUTHAMPTON. Downloaded on June 24, 2009 at 11:52 from IEEE Xplore.  Restrictions apply.

A comparative study of the performance of seven- and 63-chip optical codedivision multiple-access encoders and decoders based on superstructured ﬁber Bragg gratings,”

A multi-hop optical packet switching demonstration employing alloptical grating based header generation and recognition,” presented at the ECOC

All-optical serial header processing based on two-pulse correlation,”

Authorized licensed use limited to:

Rapidly reconﬁgurable optical phase encoder-decoders based on ﬁber Bragg gratings,”

Reconﬁgurable all-optical packet switching based on ﬁber Bragg gratings,” presented at the OFC

Reconﬁgurable multilevel phase-shift keying encoder-decoder for all-optical networks,”

A reconfigurable optical header recognition system for optical packet routing applications

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A reconfigurable optical header recognition system for optical packet routing applications

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