Experimental results are reported on an optical frequency synthesizer for use in dynamic dense wavelength-division-multiplexing networks, based on a tuneable laser in an optical injection phase-locked loop for rapid wavelength locking. The source combines high stability (50 dB), narrow linewidth (10 MHz), and fast wavelength switching (<10 ns)

Bayvel, P

Duser, M

Lovell, T

Puttnam, B

Renaud, CC

Seeds, AJ

Silva, CFC

English

UCL Discovery

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 3, MARCH 2004 903Nanosecond Channel-Switching Exact OpticalFrequency Synthesizer Using an Optical InjectionPhase-Locked Loop (OIPLL)Cyril C. Renaud, Michael Düser, Member, IEEE, Claudio F. C. Silva, B. Puttnam, T. Lovell,Polina Bayvel, Senior Member, IEEE, and Alwyn J. Seeds, Fellow, IEEEAbstract—Experimental results are reported on an opticalfrequency synthesizer for use in dynamic dense wavelength-di-vision-multiplexing networks, based on a tuneable laser in anoptical injection phase-locked loop for rapid wavelength locking.The source combines high stability ( 1-kHz channel frequencyerror over 5-K temperature change), with high output power( 2 dBm), wide tuning range (40 nm), high spurious suppression( 50 dB), narrow linewidth (10 MHz), and fast wavelengthswitching ( 10 ns).Index Terms—Optical frequency comb generator (OFCG), op-tical injection phase-locked loop (OIPLL), optical networks, tune-able laser.I. INTRODUCTIONDYNAMIC wavelength-routing has been proposed for op-tical burst- and packet-switched networks [1], [2], as wellas for optical switching matrices using passive routers [3]. Suchapplications require fast tuneable optical transmitters capableof switching between channels in microsecond to nanosecondtimescales to enable dynamic lightpath assignment in responseto variable traffic demands. The usable wavelength addressspace and spectral efficiency in these systems can be increasedby reducing the channel spacing below the 50-GHz ITU-gridstandard, requiring high wavelength stability. Wavelengthdrift in semiconductor lasers occurs as a result of temperaturechange ( 20 GHz/K), current fluctuation ( 1 GHz/mA), oraging mechanisms ( 2 GHz/year) [3]. Commercially availableetalon-based wavelength lockers could be used to compensatesuchimpairments,butarelimitedtoguardbandsof 3GHzandminimum channel spacings of 25 GHz [4]. The switching timesof fiber-based etalon stabilisers are restricted to millisecondtimescales (2 ms) [5], not compatible with the requirements ofoptical packet- and burst-switched networks operating on themicrosecond to nanosecond timescale. Recently, locked widelytuneable lasers have reached switching speeds of 50 ns (channelspacing of 100 GHz), with a frequency error 8 GHz [6].In this letter, we report experimental results for a novel rapidchannel-switching dense wavelength-division-multiplexing(DWDM) synthesizer system whose channel frequency isManuscript received October 7, 2003; revised November 6, 2003. This workwas supported by UK EPSRC, Marconi plc, and the Royal Society.The authors are with the Department of Electronic and Electrical Engi-neering, University College London (UCL), London WC1E 7JE, U.K. (e-mail:a.seeds@ee.ucl.ac.uk).Digital Object Identifier 10.1109/LPT.2004.823724Fig. 1. Experimental system of (a) OFCG and (b) OIPLL.determined by the supplied optical and microwave referencefrequencies [7], combining the advantages of a highly stableand widely tuneable system with fast wavelength switching.By using an atomic transition-locked optical reference, eachchannel can have an absolute frequency error 20 kHz [8],while the channel frequency spacing is determined by thestability of the microwave reference. We show that the systemcan hop between specified wavelengths despite 5-K lasersubmount temperature changes, the widest temperature rangefor an absolute frequency locking scheme yet reported. We alsodemonstrate the use of the scheme as a highly stable wave-lengthsourcesuitableforrapidswitchingindynamicultradense( 25-GHz channel spacing) wavelength-division-multiplexing(WDM) networks and switch fabrics, with electrical pulsegenerator limited total switching time 10 ns.II. EXPERIMENTAL SYSTEMFig. 1 shows the experimental system used for the switchingexperiment. The optical frequency comb generator (OFCG)usesanamplifiedfiberloopwithanembeddedphasemodulator.The loop, operated as a resonant cavity, multiplies the numberof equally spaced side-frequencies produced by the phasemodulator, driven by a microwave synthesizer ( GHz)and generates over 100 comb lines after recirculation [7].The referenced comb spectrum was delivered to a number ofchannel (slave) lasers in optical injection phase-locked loop(OIPLL) blocks, which were locked to selected lines. This isparticularly useful where a number of tuneable lasers needto share a common reference, e.g., in the edge routers ofoptical-burst switched networks [2].1041-1135/04$20.00 © 2004 IEEEAuthorized licensed use limited to: University College London. Downloaded on December 8, 2009 at 04:48 from IEEE Xplore.  Restrictions apply. 904 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 3, MARCH 2004Fig. 2. (a) Wavelength locking as a function of chip temperature measured by FPI. (b) Power of microwave signal generated in 1-kHz resolution bandwidth bybeating two OIPLL (or OIL) locked laser diodes (locked on two adjacent lines; 18-GHz spacing).In the OIPLL technique [9], fast and slow channel laser driftswere compensated by the operation of the optical injectionlocking (OIL) and the optical phase-locked loop (OPLL)paths respectively, the two locking techniques in combinationallowed increased OPLL loop delay time (27 ns), wide channellaser linewidths ( 10 MHz), and wide locking ranges simul-taneously, in contrast to previously implemented stand-alonetechniques [10], which only provided 2-GHz locking rangecompared with the 80 GHz of the OIPLL technique. Bothsample grating distributed Bragg reflector (SG-DBR) and superstructure grating (SSG)-DBR lasers were used as slave laserswith similar results.Wavelength monitoring was carried out using a preselectingoptical bandpass filter followed by an all-fiber scanningFabry–Pérot interferometer (FPI) tuned by a high-precision(1-mV resolution) digital-to-analogue converter, with theFPI output detected using a photodiode and digital storageoscilloscope. An oscilloscope trace was recorded for eachvoltage/wavelength setting before tuning the FPI by 1 pm( 125 MHz). Scanning the interferometer as the transmitteris, repetitively, switched between wavelengths allows profilesof wavelength and output power against time to be plotted.The wavelength range scanned was 1 nm, limited by thebandwidth of the optical filter.III. RESULTS AND DISCUSSIONA. Locking RangeFig. 2(a) shows the locking characteristics measured usingan FPI as the laser submount temperature is varied. Note thatfor this experiment, the loop control voltage was applied to thegain section only [Fig. 1(b)] since this allowed a tuning range of80 GHz as compared to 40 GHz for the phase section, and thiswill allow bigger temperature drift, which can be produced by alarge current pulse on the grating section. It could, however, in-duce change of up to 20% in the output power. The OIPLL-con-trolled laser remained locked (measurement precision is 1 GHz)over a 5-K temperature range.To provide a higher resolution measurement, we used twoOIPLL circuits locked to two adjacent lines from the comb gen-erator spaced 18 GHz apart. By heterodyning the output fromthe two lasers in a high-speed photodetector and analyzing theresultant signal with an electrical spectrum analyzer, we couldmeasure the frequency difference between the two channel out-puts. The coherence of the system leads to a heterodyne signalFig. 3. Optical spectrum of injected comb and locked laser output (0.1-nmOSA resolution).with low phase noise ( 75 dBc/Hz at 10-kHz offset), enablinga narrow spectrum analyzer resolution bandwidth to be used(1 kHz). Fig. 2(b) shows the measured power of the signal withthe spectrum analyzer span set to zero at 18-GHz center fre-quency, while tuning the temperature of one of the lasers. Asmall temperature change is enough to make the OIL systemlose lock, whereas, the OIPLL maintains the 18-GHz frequencydifference over a 5-K temperature range.B. Spurious Suppression Ratio (SSR)High SSR and low linewidths are prerequisites for lowcrosstalk values when reducing the channel spacing in ultra-dense WDM systems. Fig. 3 shows the spectrum of the lockedSG-DBR laser output. The SSR is greater than 50 dB over the12-nm measurement scan. One of the advantages of the schemeis that the linewidth of the slave laser is equal to that of themaster comb line 10 MHz. The output power of the laser,however, solely depends on the slave laser and was measured tobe 1.7 dBm at the system output for the SG-DBR laser usedin the experiment. Therefore, this source is suitable for DWDMsystems with 18-GHz channel spacing (the OFCG channelspacing) over a wavelength range of 12 nm (OFCG limited).C. Switching SpeedFig. 4 shows the result for switching between 1570 (0 mA onthe grating section) and 1529.88 nm (peak tuning current ap-plied to one grating section: 25 mA) at a frequency of 500 Hz(dwell time on each wavelength 1 ms). The measurement wasperformed with and without the phase-locked loop to see if, asexpected, the locking is limited by the OIL [8]. One can alsonote that to assess the stability of the locking, the Fabry–PérotAuthorized licensed use limited to: University College London. Downloaded on December 8, 2009 at 04:48 from IEEE Xplore.  Restrictions apply. RENAUD et al.: NANOSECOND CHANNEL-SWITCHING EXACT OPTICAL FREQUENCY SYNTHESIZER USING AN OIPLL 905Fig. 4. Channel-switching transient 1570–1532 nm (resolution 1 pm):wavelength stabilization is achieved within 10 ns of the onset of the electricaltrigger signal (a) with OIL only and (b) with OIPLL, (c) while the opticaloutput power stabilizes within 10 ns. (d) The OIPLL removes any wavelengthdrift over 250 ms.was only scanned between 1529.7 and 1529.9 nm. The newwavelength was acquired to the system measurement accuracyof 250 MHz, within 10 ns, whether the locking is providedby OIL only or OIPLL (Fig. 4(a) is for OIL only),limited by thespeed of the laser current controller (8-ns rise time). This con-firms that, as expected, the locking is almost instantaneous andlimited by OIL locking bandwidth [8] as the acquisition time isthe same whether the PLL path is used or not.The optical output power stabilized within 10 ns of the initialtrigger signal (Fig. 4(c), the initial trigger signal; Fig. 4(a), iscorrected for the length of the cables to the oscilloscope andthe laser diode). This also shows that there is no noticeabletemperature drift induced by the current pulse, thus, the loopcontrol voltage could have been applied on the phase sectionto reduce the output power variations. There was no measur-able long-term drift ( 250 MHz) for burst lengths up to 250 ms[Fig. 4(d)]. For burst lengths of 50-ms wavelength drift in ex-cess of 0.1 nm (12.5 GHz) was observed for free running op-eration, which cannot be tolerated in ultradense WDM systemswith channel spacings 25 GHz.IV. SUMMARY AND CONCLUSIONAll fiber-based OIPLL circuits were used with widely tune-able SSG-DBR and SG-DBR lasers in a first demonstration of arapidly tuneable source for dynamic switching in DWDM net-works and switch fabrics. Laser controller limited wavelengthswitching times of 10 ns were obtained, with long-termdrift 250 MHz (measurement limit). In high resolution staticmeasurements, we have shown that channel frequency errorsremained below the measurement limit of 1 kHz while the laserchips were temperature tuned by 5 K. Further improvementin the hold-in range is possible by applying control signalsto the reflector sections, ensuring mode stability. However,the achieved range is sufficient to compensate for thermaland aging frequency drifts over the lifetime of the device, en-abling the design of high spectral efficiency channel-switchingDWDM systems without guard bands, and with high SSR andlow linewidth.Whilethetotaltimetoacquirelockmeasuredfromthetriggerpulsewas10ns,thetimetoacquirelockoncethecorrectgratingsection current is reached is less than 2 ns, close to the valueexpected from OIL theory [9].The locking technique described here is expected to findapplication in burst- and packet-routed DWDM networks andwavelength-routed optical switch fabrics.REFERENCES[1] M. Düser and P. Bayvel, “Analysis of a dynamically wavelength-routedoptical network architectures,” J. Lightwave Technol., vol. 20, pp.574–585, Apr. 2002.[2] J. Gripp, M. Duelk, J. Simsarian, S. Chandrasekhar, P. Bernasconi, A.Bhardwaj, Y. Su, K. Sherman, L. Buhl, E. Laskowski, M. Cappuzzo,L. Stulz, M. Zirngibl, O. Laznicka, T. Link, R. Seitz, P. Mayer, and M.Berger, “Demonstration of a 1.2 Tb/s optical packet switch fabric [Gb/s] based on 40 Gb/s burst-mode clock-data-recovery, fast tunablelasers, and a high-performance AWG,” in Proc. 27th Eur. Conf.Optical Communications (ECOC 2001), Amsterdam, The Netherlands,2001, Postdeadline Paper PD.A.1.7, pp. 58–59.[3] O. A. Lavrova and D. J. Blumenthal, “Accelerated aging studies ofmulti-section tunable GCSR lasers for dense WDM applications,” J.Lightwave Technol., vol. 18, pp. 2196–2199, Dec. 2000.[4] D. Le-Guen, S. Lobo, F. Merlaud, L. Billes, and T. Georges, “25 GHzspacing DWDM soliton transmission over 2000 km of SMF with 25dB/span,” in Proc. 27th Eur. Conf. Optical Communications (ECOC2001), vol. 3, Amsterdam, The Netherlands, Oct. 2001, Paper We.F.1.5,pp. 244–245.[5] D. Sadot and E. Boimovich, “Tunable optical filters for dense DWDMnetworks,” IEEE Commun. Mag., vol. 36, pp. 50–55, Dec. 1998.[6] A. Dantcha, L. P. Barry, J. Murphy, J. Dunne, T. Mullane, and D. Mc-Donald, “BER performance in wavelength packet switched WDM sys-tems during Nano-second wavelength switching events,” in Proc. 29thEur. Conf. Optical Communications (ECOC 2003), vol. 3, Amsterdam,The Netherlands, Sept. 2003, Paper We4.P.146, pp. 848–849.[7] C. F. C. Silva and A. J. Seeds, “Terahertz span -Channel exactfrequency dense WDM source using comb generation and SG-DBR in-jection-locked laser filtering,” IEEE Photon. Technol. Lett., vol. 13, pp.370–372, Apr. 2001.[8] C. R. Schwarze and J. H. Rentz, “Frequency stabilized distributed feed-back laser diode system at 1.323 m using the modulated Zeeman ef-fect,” Rev. Sci. Instrum., vol. 70, pp. 3828–3831, 1999.[9] C. Bordonalli, C. Walton, and A. J. Seeds, “High-performance phaselocking of wide linewidth semiconductor lasers by combined use ofoptical injection locking and optical phase-lock loop,” J. LightwaveTechnol., vol. 17, pp. 328–342, Feb. 1999.[10] L. A. Johansson and A. J. Seeds, “Millimeter-wave modulated opticalsignal generation with high spectral purity and wide-locking bandwidthusingafiber-integratedopticalinjectionphase-lockloop,”IEEEPhoton.Technol. Lett., vol. 12, pp. 690–692, June 2000.Authorized licensed use limited to: University College London. Downloaded on December 8, 2009 at 04:48 from IEEE Xplore.  Restrictions apply. 

25 GHz spacing DWDM soliton transmission over 2000 km of SMF with 25 dB/span,” in

Accelerated aging studies of multi-section tunable GCSR lasers for dense

Analysis of a dynamically wavelength-routed optical network architectures,”

BER performance in wavelength packet switched WDM systems during Nano-second wavelength switching events,” in

Demonstration of a 1.2 Tb/s optical packet switch fabric [ Gb/s] based on 40 Gb/s burst-mode clock-data-recovery, fast tunable lasers, and a high-performance AWG,” in

Frequency stabilized distributed feedback laser diode system at 1.323 m using the modulated Zeeman effect,”

High-performance phase locking of wide linewidth semiconductor lasers by combined use of optical injection locking and optical phase-lock loop,”

Millimeter-wave modulated optical signal generation with high spectral purity and wide-locking bandwidth usingafiber-integratedopticalinjectionphase-lockloop,”IEEEPhoton.

Terahertz span -Channel exact frequency dense WDM source using comb generation and SG-DBR injection-locked laser filtering,”

Tunable optical filters for dense DWDM networks,”

Nanosecond channel-switching exact optical frequency synthesizer using an optical injection phase-locked loop (OIPLL)

Nanosecond channel-switching exact optical frequency synthesizer using an optical injection phase-locked loop (OIPLL)

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