The authors demonstrate a procedure of generating picosecond optical pulses that are tunable over a wide wavelength range (65 nm) and have very high spectral purity side-mode suppression ratio [(SMSR)>60 dB]. The large tuning range is obtained by employing external injection into a gain-switched source containing two Fabry-Pe/spl acute/rot lasers. The use of a widely tunable Bragg grating at the output improves the SMSR such that it exceeds 60 dB over the entire tuning range

Anandarajah, Prince M.

Barry, Liam P.

Clarke, Aisling M.

English

Irish Universities

Generation of widely tunable picosecond pulses with large SMSR by externally injecting a gain-switched dual laser source

Name not available

2344 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004Generation of Widely Tunable Picosecond PulsesWith Large SMSR by Externally Injecting aGain-Switched Dual Laser SourceA. M. Clarke, P. M. Anandarajah, and L. P. BarryAbstract—The authors demonstrate a procedure of generatingpicosecond optical pulses that are tunable over a wide wavelengthrange (65 nm) and have very high spectral purity side-mode sup-pression ratio [(SMSR) 60 dB]. The large tuning range is ob-tained by employing external injection into a gain-switched sourcecontaining two Fabry–Pérot lasers. The use of a widely tunableBragg grating at the output improves the SMSR such that it ex-ceeds 60 dB over the entire tuning range.Index Terms—External injection, optical fiber communications,optical pulse generation, semiconductor laser, wavelength tunablesource.I. INTRODUCTIONAS THE demand for high-speed communications applica-tions such as wavelength-division multiplexing (WDM)and optical time-division multiplexing (OTDM) continues togrow, there will be an increasing need to develop optical pulsesources suitable for these systems. Current trends may resultin the operation of optical communication systems at line ratesof 40 Gb/s and beyond, thereby making it more likely that re-turn-to-zero coding be used for data transmission, as it is easierto compensate for dispersion and nonlinear effects in the fiber[1]. Furthermore, next-generation WDM systems that employdynamic provisioning with the use of wavelength tunability areattracting a lot of interest. Thus, the key requirements on pi-cosecond pulse sources to be used in high-speed communica-tions applications will include broad wavelength tuning range,a high side-mode suppression ratio (SMSR), variable repetitionrates, low timing jitter, and small frequency chirp [2]–[4].Picosecond pulse generation can be accomplished throughvarious methods, such as external modulation of a contin-uous-wave (CW) light signal [4], mode locking [5], and gainswitching [6]. Gain switching of a semiconductor laser diode isprobably one of the most reliable methods to generate opticalpulses, and by employing self seeding [7] of a gain-switchedFabry–Pérot (FP) laser, it is possible to obtain high-qualitywavelength tunable single-mode pulses which have low timingjitter and good spectral purity. Nonetheless, a major disadvan-tage with the self-seeded gain-switched (SSGS) scheme is thatthe length of the external cavity has to be continuously tuned soManuscript received February 13, 2004; revised June 15, 2004. This workwas supported by the Science Foundation Ireland Investigator Programme.The authors are with the Research Institute for Networks and Communica-tions Engineering, School of Electronic Engineering, Dublin City University,Dublin 9, Ireland (e-mail: liam.barry@eeng.dcu.ie).Digital Object Identifier 10.1109/LPT.2004.834523Fig. 1. Experimental setup for the generation of widely tunable externallyinjected gain-switched pulses.that the pulse repetition frequency is an integer multiple of thecavity round-trip frequency. An alternative technique entails ex-ternal injection of light from a CW source into a gain-switchedlaser [8]–[10]. No adjustment of the repetition frequency orexternal cavity length is required in this case. Thus, externalinjection provides a more stable operation, even though aCW tunable laser is commonly required. Recent work hasestablished that as the number of channels in a WDM systemusing SSGS pulse sources increases, the specifications on therequired SMSR due to cross-channel interference may becomevery stringent [3]. Thus, if externally injected gain-switchedlasers are to be used in future high-speed systems, it will benecessary to improve the SMSR of these sources to beyond30 dB.In this letter, we build on recent research we have under-taken which involved self-seeding of a gain-switched dual lasersource [11]. This article demonstrates the use of external in-jection into a gain-switched transmitter comprising of two FPlasers to generate picosecond pulses that are tunable over 65 nm,with SMSRs in excess of 60 dB over the entire tuning range.This is the largest tuning range and SMSR that has ever beenachieved for an optical pulse source based on gain-switchedlaser diodes.II. EXPERIMENTAL SETUPThe experimental setup is illustrated in Fig. 1. The lasersFP1 and FP2 are commercial 1.5- m InGaAsP devices, withthreshold currents of 19 and 26 mA, respectively. The laserswere chosen so that their gain profiles provided only a smalloverlap, which corresponds to the maximum wavelength of FP1,and the minimum wavelength of FP2, at which we can achieve1041-1135/04$20.00 © 2004 IEEECLARKE et al.: GENERATION OF WIDELY TUNABLE PICOSECOND PULSES WITH LARGE SMSR 2345suitable SMSRs using the external injection configuration. Con-tinuous wavelength tuning of the laser modes over the gain pro-files of the two devices can be achieved by temperature control-ling the diodes. The gain-switching process involves applying asinusoidal modulation signal (peak-to-peak current 200 mA;frequency 2.5 GHz) to both lasers, in addition to dc bias cur-rents of 15 and 26 mA for FP1 and FP2, respectively. The op-tical signal from both lasers is then coupled into fiber usinga GRIN lens fiber pigtail. External injection requires injectingCW light from a tunable external cavity laser (ECL) into oneof modes of the two FP lasers via an isolator, a 3-dB coupler,and a polarization controller (PC). The PC is varied in orderto ensure optimum coupling of the injected light from the ECLinto the selected FP laser cavity, which in turn optimizes theSMSR of the laser output. The output power of the CW sourceis set at 3 dBm, however, taking into account various losses,we estimate the injection level into the gain-switched sources tobe about 13 dBm. The resulting single-mode output obtainedafter external injection into one of the FP lasers, together withthe gain-switched signal from the FP laser that is not affectedby the external injection (because the signal injected from theECL does not lie within the gain curve of this FP diode), isthen passed through a tunable Bragg grating (TBG) filter. TheTBG has a bandwidth of 0.23 nm, a wavelength tuning range of1460–1575 nm, and an insertion loss of 5 dB. The filter is usedto eliminate the optical output from the gain-switched FP laserthat is not influenced by the external injection, and also to en-hance the SMSR of the generated pulses. The output pulses arecharacterized using a 50-GHz photodiode in conjunction with a50-GHz oscilloscope, and an optical spectrum analyzer (OSA)with a 0.05-nm (6 GHz) resolution.III. RESULTS AND DISCUSSIONThe optical spectrum of the dual wavelength signal from thegain-switched lasers, without external injection, is shown inFig. 2(a). It can clearly be seen that by combining the outputof the gain-switched lasers in the wavelength domain, thecomposite span of the laser profiles that could be used forseeding has been greatly increased. The peak of the spectrumfor FP1 is at 1524 nm, while the peak of the spectrum for FP2is at 1561 nm. As we can see from the composite spectra ofthe two gain-switched lasers, the spectra from the individualgain-switched devices overlap at about 16 dB down from thepeak of the spectra.Different longitudinal modes of each FP laser were selec-tively excited when the seeding wavelength from the ECL wastuned near the center of any desired mode. Fig. 2(b) displays theresulting spectral output before the optical filter showing goodSMSR for the seeded gain-switched diode. With the addition ofthe filter, the optical output from the unseeded FP laser is elimi-nated, and the SMSR of the output pulses is improved such thatit becomes almost impossible to detect the side-modes abovethe noise floor of the OSA. The resulting SMSR is around 60 dBfor the entire wavelength tuning range that can be achieved withthis setup. Examples of the temporal (nonaveraged) and spectralprofile of the output pulse (at 1520 nm) are shown in Fig. 2(c)Fig. 2. (a) Optical spectrum of dual gain-switched source, (b) spectrum beforethe filter at 1519.9 nm, (c) pulse at 1519.9 nm (with inset showing extinctionratio), and (d) spectrum after the filter at 1519.9 nm.and (d), and we can clearly see the excellent temporal and spec-tral purity of the pulse source. The pulse duration was about28 ps while the spectral width was approximately 20 GHz (thisspectral width is clearly not limited by the bandwidth, 29 GHz,of the optical filter), resulting in a time-bandwidth product of0.56 (slightly larger than that for transform-limited Gaussianpulses). The extinction ratio of the generated pulses was mea-sured to be 25 dB, and the timing jitter was estimated to be lessthan 1 ps. The timing jitter was measured to be 1 ps by usinghistogram analysis on an Agilent Digital Communications An-alyzer, however, given that 1 ps is the lower limit on this mea-surement, we conclude that the jitter is actually less than 1 ps.Fig. 3 illustrates the SMSR as a function of wavelength,across the tuning range of the pulse source. We obtain valuesgreater than 60 dB over the complete wavelength span. It is im-portant to note that the use of the filter in this setup is dependenton achieving a suitably high SMSR from the gain-switched ex-ternally injected laser before the filter (we have verified that thisvalue remains above 30 dB in our work). If this is not the casethen mode-partition-noise could seriously affect the temporalquality of the pulse source (from Fig. 2(c), this is clearly not the2346 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 10, OCTOBER 2004Fig. 3. SMSR of output pulses as a function of wavelength.Fig. 4. Pulsewidths of optical output pulses as a function of wavelength tuningrange.case). Fig. 4 shows the variation in pulsewidth over the tuningrange. The variation in pulsewidth around 1545 nm is due tothe external injection from the ECL changing from seedingFP1 to seeding FP2. Differences in various physical parameters(e.g., gain) of the two lasers are responsible for the variationin output pulsewidth. In addition, the output spectral widthfrom the higher wavelength laser (FP2) is slightly increased(from 20 to 29 GHz), and in this case, does become limitedby the bandwidth of the output filter. Our experimental resultsexhibited very stable operation even at the crossover sectionfrom operation with FP1 to FP2. This is achieved because thereis no overlap between the modes from the two different FPlasers, and thus, we never inject light into the same mode ofboth FP lasers at the same time.IV. CONCLUSIONThis experiment has demonstrated a simple and effective pro-cedure of generating widely tunable ( 65 nm) pulses with im-pressive SMSR ( 60 dB) by using external injection into asource consisting of two gain-switched FP lasers. Such a sourcecould play a vital part in ensuring the optimal performance ofhigh-speed hybrid WDM/OTDM optical communication net-works. It should also be noted that the tuning range could beexpanded further by introducing a third FP laser with an appro-priate spectral profile, and that by simultaneously injecting lightinto the FP lasers used, it may also be possible to develop a mul-tiwavelength pulse source.REFERENCES[1] R. Ludwig, U. Feiste, E. Dietrich, H. G. Weber, D. Breuer, M. Martin,and F. Küppers, “Experimental comparison of 40 Gbit/s RZ and NRZtransmission over standard single mode fiber,” Electron. Lett., vol. 35,pp. 2216–2218, 1999.[2] C.-K. Chan, K. L. Sherman, and M. Zirngibl, “A fast 100-channel wave-length-tunable transmitter for optical packet switching,” IEEE Photon.Technol. Lett., vol. 13, pp. 729–731, July 2001.[3] P. Anandarajah, L. P. Barry, and A. Kaszubowska, “Performance issuesassociated with WDM optical systems using self-seeded gain-switchedpulse sources due to mode partition noise,” IEEE Photon Technol. Lett.,vol. 14, pp. 1202–1204, Aug. 2002.[4] S. Kawanishi, “Ultrahigh-Speed optical time division-multiplexedtransmission technology based on optical signal processing,” IEEE J.Quantum Electron., vol. 34, pp. 2064–2079, Nov. 1998.[5] J. E. Bowers, P. A. Morton, A. Mar, and S. W. Corzine, “Actively mode-locked semiconductor lasers,” IEEE J. Quantum Electron., vol. 25, pp.1426–1439, June 1989.[6] K. Y. Lau, “Gain-switching of semiconductor injection lasers,” Appl.Phys. Lett., vol. 52, pp. 257–259, 1988.[7] D. Huhse, M. Schell, W. Utz, J. Kaessner, and D. Bimberg, “Dynamics ofsingle-mode formation in self-seeded Fabry–Pérot laser diodes,” IEEEPhoton. Technol. Lett., vol. 7, pp. 351–353, Apr. 1995.[8] L. P. Barry, J. Debeau, and R. Boittin, “40 nm tunable source of pi-cosecond pulses at 10 GHz by external injection into a gain switchedFP laser,” in Proc. Eur. Conf. Optical Communications, vol. 1, Florence,1994, pp. 555–558.[9] Y. Matsui, S. Kutsuzawa, S. Arahira, and Y. Ogawa, “Generation ofwavelength tunable gain-switched pulses from FP MQW lasers withexternal injection seeding,” IEEE Photon. Technol. Lett., vol. 9, pp.1087–1089, Aug. 1997.[10] M. Zhang, D. N. Wang, H. Li, W. Jin, and M. S. Demokan, “Tunable dualwavelength picosecond pulse generation by the use of two Fabry–Pérotlaser diodes in an external injection seeding scheme,” IEEE Photon.Technol. Lett., vol. 14, pp. 92–94, Jan. 2002.[11] P. Anandarajah, P. J. Maguire, A. Clarke, and L. P. Barry, “Self-seedingof a gain switched integrated dual laser source for the generation ofhighly wavelength tunable picosecond optical pulses,” IEEE Photon.Technol. Lett., vol. 16, pp. 629–631, Feb. 2004.

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40 nm tunable source of picosecond pulses at 10 GHz by external injection into a gain switched FP laser,” in

A fast100-channel wavelength-tunable transmitter for optical packet switching,”

Actively modelocked semiconductor lasers,”

D.Huhse,M.Schell,W.Utz,J.Kaessner,andD.Bimberg,“Dynamicsof single-mode formation in self-seeded Fabry–Pérot laser diodes,”

Gain-switching of semiconductor injection lasers,”

Generation of wavelength tunable gain-switched pulses from FP MQW lasers with external injection seeding,”

M.Zhang,D.N.Wang,H.Li,W.Jin,andM.S.Demokan,“Tunabledual wavelength picosecond pulse generation by the use of two Fabry–Pérot laser diodes in an external injection seeding scheme,”

Performance issues associated with WDM optical systems using self-seeded gain-switched pulse sources due to mode partition noise,”

Self-seeding of a gain switched integrated dual laser source for the generation of highly wavelength tunable picosecond optical pulses,”

Ultrahigh-Speed optical time division-multiplexed transmission technology based on optical signal processing,”

A.M. Clarke

P.M. Anandarajah

L.P. Barry

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Generation of Widely Tunable Picosecond Pulses With Large SMSR by Externally Injecting a Gain-Switched Dual Laser Source

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Generation of widely tunable picosecond pulses with large SMSR by externally injecting a gain-switched dual laser source

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