©1999 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEEMultiwavelength clock recovery is especially desirable in systems that use wavelength-division-multipleged technology. A multiwavelength clock-recovery device can greatly simplify costs by eliminating the need to have a separate regenerator for each wavelength. This letter discusses multiwavelength all-optical clock recovery using stimulated Brillouin scattering

Allen, Christopher Thomas

Demarest, Kenneth R.

Hui, Rongqing

Johnson, C.

Peddanarappagari, Kumar V.

Zhu, Benyuan

KU ScholarWorks

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 7, JULY 1999 895Multiwavelength All-Optical Clock RecoveryC. Johnson, K. Demarest, C. Allen, R. Hui,Senior Member, IEEE,K. V. Peddanarappagari, and B. ZhuAbstract—Multiwavelength clock recovery is especially desir-able in systems that use wavelength-division-multiplexed technol-ogy. A multiwavelength clock-recovery device can greatly simplifycosts by eliminating the need to have a separate regenerator foreach wavelength. This letter discusses multiwavelength all-opticalclock recovery using stimulated Brillouin scattering.Index Terms—Clock recovery, optical fiber communication, op-tical fiber device, optical nonlinearity, wavelength-division mul-tiplexing.I. INTRODUCTIONFUTURE broad-band integrated services digital networks(B-ISDN) may operate at terabit per second (Tb/s)data rates using time-division multiplexing (TDM) and/orwavelength-division multiplexing (WDM). These networkswill likely use all-optical 3-R regenerators, which reamplify,reshape, and retime the incoming data. The latter two ofthese operations would require all-optical clock-recoverydevices. Although multiwavelength amplification is alreadya reality with erbium-doped fiber amplifiers (EDFA’s),multiwavelength clock-recovery devices have not yet beendemonstrated in the literature.Previously reported all-optical clock-recovery devices in-clude mode-locked fiber ring lasers [1], mode-locked laserdiodes [2], self-pulsating laser diodes [3], and optical tanks [4].Most of these devices require prior knowledge of the bitrate.An exception is a Brillouin optical active filter, which is ableto operate without a prior knowledge of the bitrate [5]. To ourknowledge, there have been no reports of multiwavelengthclock recovery.In this letter, we present results showing dual-wavelengthall-optical clock recovery using a Brillouin active filter. Theremainder of this letter is divided into three sections. InSection II, we cover the theory of the device; in Section III,we discuss the results; and in Section IV, we summarize theadvantages of this device.II. THEORYOur device can be classified as a Brillouin active filter,which relies on the process of stimulated Brillouin scatter-ing (SBS). In SBS, a strong pump wave causes molecularvibrations, which result in a change in the index of refraction.Thus, an acoustic wave (Bragg grating) is formed. Part of apump wave will scatter off of the Bragg grating, resultingManuscript received February 22, 1999; revised March 29, 1999.The authors are with the Information and Telecommunication TechnologyCenter, University of Kansas, Lawrence, KS 66044 USA.Publisher Item Identifier S 1041-1135(99)05164-2.Fig. 1. Block diagram of the Brillouin active-filter clock-recovery device.in a downshifted wave (called a Stokes wave) traveling inthe opposite direction from the pump and the acoustic waves.The Stokes wave will experience exponential amplification ifthe pump wave is above a threshold that is dependent on thecharacteristics of the fiber. If an input signal is injected atthe Stokes wavelength, SBS can be used for amplification ofthat signal. The SBS-induced amplification has a narrow-gainbandwidth.This narrow bandwidth, nonlinear amplification processcan be exploited in a system to recover the clock of anincoming return-to-zero (RZ) optical data stream by selectivelyamplifying the strongest clock tones in the data spectrum,which are shifted from the carrier by the bitrate and multiplesof the bitrate. This filtering effect is caused by the strongesttones generating the most backscattered energy in an SBSprocess, thereby providing greater gain for tones of a signalat the Stokes wavelength. The RZ clock output is obtainedby effectively dumping some of the energy from the bit slotsthat contain ones into the bit slots that contain zeros. In thisway, the Brillouin active filter converts an RZ data stream withrandom ones and zeros into an RZ clock.There have been a variety of Brillouin optical active filtersreported in the literature [5], [6]. Our device, shown in Fig. 1,is an extension of these previously reported Brillouin activefilters. Here, an input optical RZ data stream is injected intoa directional coupler, producing counter-rotating waves in aloop that contains 20 km of dispersion-shifted fiber (DSF).The polarization controllers maintain the same polarizationstate for both the pump and Stokes waves. The clockwisepropagating signal is downshifted by the Brillouin shift ofthe DSF fiber ( 10.5 GHz) using a Mach–Zehnder modulator(MZM), biased to obtain double-sideband, suppressed-carriermodulation. The downshifted seed wave (Stokes wave) isamplified by the counterpropagating pump wave SBS, sincethe counterpropagating waves inside the DSF fiber are sepa-rated in frequency by exactly the Brillouin shift. However,1041–1135/99$10.00 1999 IEEE896 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 11, NO. 7, JULY 1999Fig. 2. 10-Gb/s (27 1) pseudorandom optical input (500 ps/div).only the clock-related continuous-wave (CW) tones in theseed waveform have sufficient power to exceed the Brillouinthreshold, so the data portion of the seed spectrum is attenuatedrelative to the clock. Therefore, this device operates as anactive, optical filter. Since the same data stream is usedfor the pump and the Stokes seed, this device is bit-rateindependent.Multiwavelength clock recovery is possible from this devicefor the following reasons. First, the Brillouin gain bandwidthof the DSF fiber ( 20 MHz) is much narrower than thespectra of the data signals, thus allowing each seed signalto interact only with its own unshifted pump signal. Thisattribute is common to all single wavelength Brillouin ac-tive filters, but also makes multiwavelength clock recoverypossible if each channel’s clock tone power levels are allabove the Brillouin threshold. Second, the erbium-doped fiberamplifier (EDFA) in the loop of our device provides thegain necessary for multiwavelength operation for moderateinput powers; i.e., less than 15.5 dBm/channel. Althoughthe Brillouin shift varies inversely with the pump wave-length, multiwavelength clock recovery is possible for signalswithin a 3-nm bandwidth for the DSF fiber used in ourdevice.The data rates of each wavelength need not be the same.The only cost of adding each additional wavelength channelis that the input power to the Brillouin active filter or the gainof the EDFA in the loop must be increased by 3 dB for eachadditional wavelength.III. RESULTSFig. 2 shows a representative input waveform for eitherof the two channels. The input RZ pulses had pulsewidthsof 24 ps and repetition rates of 10 GHz. The input datastream was encoded with a 21 pseudorandom bit sequencegenerated by an HP70843A bit-error-rate tester (BERT). Thedata streams were then amplified to an average power of35 mW per channel using an EDFA and then injected intothe input port of the directional coupler. The coupling ratioof the directional coupler was 90 : 10. The clockwise Stokessignal was generated using a dual-electrode MZM, whichwas biased to produce a double-sideband suppressed-carrieroutput spectrum. The downshifted Stokes spectrum was thenamplified by a low-noise figure ( dB) EDFA withoutput power of 4 dBm per channel and then sent throughFig. 3. Recovered optical clock at 1557.4 nm (100 ps/div).Fig. 4. Recovered optical clock at 1556.5 nm (100 ps/div).the dispersion-shifted fiber to act as a seed for the SBSprocess.The output waveforms for the 1557.4 and 1556.5 nmchannels are shown in Figs. 3 and 4, respectively. The relativejitter of the 1557.4- and 1556.5-nm clocks were 1.2 and 1.3ps, respectively, where relative jitter is defined by. These waveforms clearly show that thedevice is capable of simultaneously extracting the clocks fromboth data streams.IV. CONCLUSIONIn this letter, we demonstrated a clock-recovery techniqueemploying an active filter using stimulated Brillouin scattering.The WDM compatibility of this clock-recovery techniquemakes it attractive for future optical networks. By using asingle device to recover the clocks of a number of opticalinputs, this technology could be integrated into future WDMnetworks, thereby eliminating the need for costly regeneratorsfor each wavelength.Although the total wavelength span of our clock-recoverydevice was roughly 3 nm, this span could be increased byeither of two methods. The first would be to use a fiberloop with a larger Brillouin-gain bandwidth, which could beaccomplished by introducing strain into the fiber [7]. Thesecond would be to replace the single modulator in theloop with multiple modulators and corresponding wavelengthfilters that provide different modulation shifts for differentwavelength bands. Using either of these techniques, wide-bandmultiwavelength clock recovery may be possible.JOHNSONet al.: MULTIWAVELENGTH ALL-OPTICAL CLOCK RECOVERY 897REFERENCES[1] A. Ellis, K. Smith, and D. Patrick, “All optical clock recovery atbit rates up to 40 Gbit/s,”Electron. Lett.,vol. 29, pp. 1741–1743,1993.[2] R. Ludwig, A. Ehrhardt, W. Pieper, E. John, N. Agrawal, H.-J. Ehrke,L. Küller, and H. Weber, “40 Gbit/s demultiplexing experiment with10 GHz all-optical clock recovery using a modelocked semiconductorlaser,” Electron. Lett.,vol. 32, no. 4, pp. 327–329, 1996.[3] P. Barnsley, H. Wickes, G. Wickens, and D. Spirit, “All-optical clockrecovery from 5 Gbit/s RZ data using a self-pulsating 1.56mlaser diode,”IEEE Photon. Technol. Lett.,vol. 3, pp. 942–945, Oct.1991.[4] M. Jinno and T. Matsumoto, “Optical tank circuits used for all-opticaltiming recovery,”IEEE J. Quantum Electron.,vol. 28, pp. 897–900, Apr.1992.[5] D. Butler, L. Douglas, J. Wey, M. Chbat, G. Burdge, and J. Goldhar,“Optical clock recovery from a data stream of an arbitrary bit rate byuse of stimulated Brillouin scattering,”Opt. Lett., vol. 20, no. 6, pp.560–562, 1995.[6] H. Kawakami, T. Mitamoto, T. Kataoka, and K. Hagimoto, “All-opticaltiming clock extraction using multiple wavelength pumped Brillouinamplifier,” IEICE Trans. Commun.,vol. E78-B, no. 5, pp. 694–701,1995.[7] T.-O. Tsun, A. Wada, T. Sakai, and R. Yamauchi, “Novel method ofusing white spectral probe signals to measure Brillouin gain spectra ofpure silica core fibers,”Electron. Lett.,vol. 28, no. 3, pp. 247–249, Jan.1992.

Multiwavelength all-optical clock recovery

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Multiwavelength all-optical clock recovery

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