Tsia, KKM

Wong, KKY

Xu, X

Yuk, TTI

Zhang, C

HKU Scholars Hub

Title Stabilized Wide-band Wavelength Conversion Enabled by CW-Triggered SupercontinuumAuthor(s) Xu, X; Zhang, C; Yuk, TTI; Tsia, KKM; Wong, KKYCitation Photonics Technology Letters, 2012Issued Date 2012URL http://hdl.handle.net/10722/164104Rights Creative Commons: Attribution 3.0 Hong Kong License1886 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 20, OCTOBER 15, 2012Stabilized Wide-Band Wavelength ConversionEnabled by CW-Triggered SupercontinuumXing Xu, Student Member, IEEE, Chi Zhang, T. I. Yuk, Kevin K. Tsia, Member, IEEE,and Kenneth K. Y. Wong, Senior Member, IEEEAbstract— We experimentally demonstrate a wide-band wave-length converter enabled by continuous-wave (CW) triggeredsupercontinuum (SC) generated from a picosecond pulse source.In the presence of the trigger, the pulse-to-pulse SC stability con-siderably improved, leading to error-free wavelength conversionoperations spanning from 1510 to 1615 nm, which covers the S-,C-, and L-bands of the ITU grids. In addition, the CW-triggeredSC spectrum is significantly enhanced to over 300 nm measuredat 20 dB below the peak spectral power (from ∼1400 to 1700 nmand beyond). Our scheme thus provides a low pump power andwide-band solution for fiber-based optical wavelength conversion.With its better CW source, e.g., lower relative intensity noise andlonger coherence length, the present scheme has the potential tobe further applied to phase-modulated signals.Index Terms— Optical signal processing, supercontinuum (SC)generation, wavelength conversion.I. INTRODUCTIONSUPERCONTINUUM (SC), with its ultrabroadband spec-trum generated from narrow-band pulses, is the versatilelight source for a wide range of applications in both com-munication and bio-imaging, among which include ultrashortpulse generation, wavelength conversion, optical frequencymetrology, and optical coherence tomography (OCT) [1], [2].One effective scheme for ultrabroadband SC generation isbased on primarily soliton fission, which happens whenlaunching an intense pulsed pump into the nonlinear opticalfiber in its anomalous dispersion regime [3]. However, as beinginitiated by modulation instability (MI), another competingprocess with soliton fission, can amplify the input noise,which in turns serves as the precursor of soliton fission. Asa result, the soliton fission, and thus SC are initiated in arandom fashion. To its extreme case, the rogue soliton (arare but very intense event) [4] or the rogue void (a rarebut very weak event) can be formed [5]. It makes the SCinherently incoherent and exhibit large pulse-to-pulse intensityManuscript received July 10, 2012; revised August 25, 2012; acceptedSeptember 5, 2012. Date of publication September 10, 2012; date of currentversion September 26, 2012. This work was supported in part by grantsfrom the Research Grants Council of the Hong Kong Special Administra-tive Region, China, under Project HKU 7183/09E, Project HKU 7175/10E,and Project HKU 717911E, and the University Development Fund of TheUniversity of Hong Kong.The authors are with the Department of Electrical and ElectronicEngineering, University of Hong Kong, Hong Kong (e-mail: xuxing@eee.hku.hk; zhangchi@eee.hku.hk; tiyuk@eee.hku.hk; tsia@eee.hku.hk;kywong@eee.hku.hk).Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/LPT.2012.2218099Fig. 1. Principle of the CW-triggered SC.fluctuation - the pitfalls hampering the utility of SC in awide range of applications [6]. This problem is particularlysevere for wider input pulsewidth: picosecond or nanosecond[2]. Fortunately, it can be remedied by introducing a weakcontinuous-wave (CW) trigger together with the input pumppulse [7]. In this case, the MI is initiated by the controllableCW trigger, overwhelming the noise. Hence, the subsequentsoliton fission would become more deterministic, making theSC more stable and coherent [8], [9]. Comparing to someformer schemes which need either precise time delay control[9] or complex feedback process [10], this method provides afairly simple and more effective solution to enhance both thecovering range and the pulse stability of the generated SC. Onthe other hand, the presence of the CW-trigger, locating withinthe MI gain bandwidth, could also accelerate the soliton fissionprocess [8], [9], [11]. To its extreme case, the rogue solitoncan be formed and the SC bandwidth can thus be drasticallyenhanced [7], [9].As a result, the CW-triggering scheme can be harnessed forrealizing stable (low-noise) and wide-band wavelength conver-sion. This is in contrast to some former experiments [12]–[14],in which, only limited range of wavelength conversion (tensof nanometer) could be achieved. In our previous work [15],we have shown some preliminary results which confirmed thefeasibility and advantages of our method. In the followingparts of this letter, we first analyze the pulse-to-pulse stabilityimprovement with the CW triggering, followed by a detaileddemonstration of a wavelength converter based on the CW-triggered SC scheme. We demonstrate, to the best of ourknowledge, for the first time that the stable SC can be restoredfrom the rogue void condition by adding the CW-trigger. Theinput pulsed pump was from a 10-Gb/s picosecond source withthe pulsewidth of 6.3 ps. During the experiment, the bandwidthof the SC spectrum was increased to over 300 nm measured1041–1135/$31.00 © 2012 IEEEXU et al.: STABILIZED WIDE-BAND WAVELENGTH CONVERSION 1887Fig. 2. Experimental setup. VOA: variable optical attenuator. PD: photodiode. (Other abbreviations are defined in the text.)at 20 dB below the peak spectral power (from ∼1400 to1700 nm and beyond), while error-free (less than 3.8-dB powerpenalty at the bit-error rate (BER) of 10−9) operations wereattained for over 100-nm spectrum range spanning S-, C- andL-band.II. PRINCIPLE AND EXPERIMENTAL SETUPThe principle of the CW-triggered picosecond SC is illus-trated in Fig. 1. The process is realized by bringing in anexternal weak CW trigger which is co-propagating with thepicosecond pulsed pump inside the fiber. Hence, the generatedSC spectrum can be significantly broadened if the wavelengthof the trigger locates at the MI gain peak [7]–[9]. As explainedpreviously in the introduction, the MI growth here is initiatedby the trigger rather than the noise, which leads to thestabilized soliton fission process during the SC generation. Asa result, in addition to the spectrum broadening, the pulse-to-pulse instability is also improved, which can be visualized bythe clear open eye-diagram comparing to the non-trigger case(right portion of Fig. 1).The experimental setup is shown in Fig. 2. The pulsedpump, located at 1554.77 nm, was generated from a mode-locked fiber laser (MLFL) and intensity-modulated by aMach-Zehnder modulator (MZM) driven by 10-GHz clocksignal. While the polarization controller (PC1) was utilizedto align the polarization of the pulse with the transmis-sion axis of the MZM. Two erbium-doped fiber amplifiers(EDFA) were applied to amplify the average pump power upto365 mW (∼ 10.43 W peak power) before the highly-nonlineardispersion-shifted fiber (HNL-DSF), which has the length L =150 m, zero dispersion wavelength (ZDW) λ0 = 1554.7 nmand nonlinear coefficient γ = 30W−1km−1. The opticalbandpass filter (OBPF), with 3-nm bandwidth between thetwo EDFAs, was used to minimize the amplified spontaneousemission (ASE) noise from the EDFA1. On the trigger branch,one CW tunable laser source (TLS), with the wavelength at1613 nm, was utilized. The power level of the trigger wasmore than 40-dB lower than the pump. After combining witha WDM coupler, the pulsed pump and the weak trigger werelaunched together into the HNL-DSF. At the fiber output,after removing the residual pump, the wavelength convertedpulsed signal was filtered out by a variable bandwidth tunablebandpass filter (VBTBPF).III. EXPERIMENTAL RESULTS AND DISCUSSIONThe power stability improvement was quantified by thepulse-to-pulse amplitude histograms (Fig. 3). The filtered SC(a)(b)Fig. 3. Amplitude histograms for sliced SC @ 1566.82 nm without(red circle) and with (blue diamond) trigger. Insets: real-time pulse traces(a) without and (b) with trigger.pulse traces (1000 pulses) were obtained from a real-timeoscilloscope (4 GHz, 20 GS/s) as shown in the insets of Fig. 3.From the figure of the non-trigger case (Fig. 3, red circle), itshows a clear left-skewed long-tail distribution, which resem-bles the occurrence of the optical rogue voids [4]. Indeed, thereexists some rare and weak pulses in the pulse train as shownin the inset of Fig. 3. We note that this rare suppression is incontrast to our previous demonstration in which right-skewedextreme value statistic was observed [7]. Such difference ispartly attributed to that the histograms in the two cases wereobtained in different wavelength ranges of the SC. It has beenshown that the temporal statistic of the SC shows dramaticvariation across the whole SC spectrum [16], [17]. Anotherfactor affecting the SC’s power statistics is the pump power.Specifically, when the pump power exceeds a threshold beyondwhich soliton fission occurs during the SC generation, thepower of the SC pulses (the longer-wavelength side of the SC)exhibit a left-skewed statistic (associated with rogue voids).In contrast, below this threshold, a right-skewed statistic (asignature of the existence of rogue waves) can be observed[5]. In our experiment, the pump power was measured tobe higher than the threshold power, which was ∼280 mW(∼8-W peak power). Therefore, the generated SC here showsthe left-skewed statistic. Based on this result as well as ourprevious works [7], [8], we observe that stimulating the SCwith a CW-trigger suppresses rogue waves (rogue voids) whenthe pump power is lower (higher) than the threshold. This1888 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 20, OCTOBER 15, 2012Fig. 4. (a) Pump spectrum before the fiber input (OSA resolution bandwidth:0.02 nm). (b) SC spectra: without (red solid line) and with (blue dottedline) trigger. Insets: eye-diagrams @ 1516.4 nm (OSA resolution bandwidth:0.06 nm).is because both rogue waves and rogue voids are caused byspontaneous MI (i.e. noise), which is now overwhelmed bythe controlled CW-trigger. Moreover, the temporal stabilitycould also depend on a multitude of parameters, such aspump wavelength, power, pulse width and so on. Furtherextensive investigation, which takes all these parameters intoaccount and studies how the interplay among them results indifferent temporal statistics, should be carried out in orderto establish a more complete understanding of the entireSC dynamics.When the CW trigger was introduced, the amplitude sta-tistics was changed into an clear quasi-Gaussian distribution(Fig. 3, blue diamond) with ∼75% reduction in the standarddeviation comparing to non-trigger case. Such improvement inpulse-to-pulse stability is also readily visualized from the real-time pulse traces (Fig. 3, insets). The small pulse amplitudevariation in the triggered case can be attributed mainly tothe relative intensity noise (RIN) and the limited coherencelength (∼30 m) of our CW laser source. The former onecould certainly contaminate the SC temporal stability, andthe latter one degrades the phase coherence of the pulses [7].Thus, a CW trigger with low RIN and longer coherence length(comparable to fiber length, in our experiment ∼150 m) arefavorable, especially for the potential applications of wave-length conversion or multicasting of phase-modulated pulsesignals.Results shown above to some extent illustrates the pulsestability enhancement with the CW trigger. In order to geta complete picture, we further investigated the wavelengthconversion performance based on the CW-triggered SC. TheFig. 5. SC spectra after WDM couplers without (red solid line) andwith (blue dotted line) trigger, and after VBTBPF (blue solid line). (a) and(b) C-band: 1536.15 and 1543.68 nm. (c) L-band: 1566.82, 1592.8 and1611 nm (OSA resolution bandwidth: 0.06 nm).10-GHz clock train was substituted with 231-1 pseudorandombinary sequence (PRBS) in Fig. 2. For the pump, its spectrumbefore the fiber input is shown in Fig. 4(a), and after theHNL-DSF output, the generation of supercontinuum is readilyobvious as in Fig. 4(b). The dip around 1560 nm was due tothe residual pump after the WDM coupler. Clearly observedfrom Fig. 4(b), the original spectrum of the SC was greatlybroadened with the weak CW trigger, especially at some wave-length regions: 1380 nm to 1470 nm, 1630 nm to 1700 nm andbeyond, which had significant enhancement. The largest powerenhancement, occurred around 1400 nm and 1700 nm, was ashigh as 40-dB comparing to the non-trigger case. In order toinvestigate the quality of the wavelength conversion, differentWDM couplers were used to slice out the SC spectrum fromS-band (1510 nm) to L-band (1615 nm), which covered over100 nm. The insets of Fig. 4(b) show the wavelength convertedsignal at 1516.4 nm originated from pulsed pump, and thebandwidth of the VBTBPF was 3.2 nm, and the time-domainpulse width of the wavelength converted signal was 13 ps(Fig. 4(b)). The improvement with trigger is obvious as shownin the eye-diagrams.In addition to 1516.4 nm, several more wavelengths(C-band: 1536.15 and 1543.68 nm; L-band: 1566.82, 1592.8and 1611 nm) were measured across nearly 100-nm spectrumwhile keeping the same filter bandwidth. With the exception ofS-band, all the recorded eye-diagrams and their correspondingBER at 10−9 are shown in Figs. 5 and 6. Stability improvementcan be observed from the eye-diagrams for all measuredwavelengths. Error-free operations were achieved with lessthan or equal to 3.8-dB power penalty. With proper filteringand detection equipment, it will also be interesting to furtherexplore the red-shifted (around 1700 nm) and blue-shiftedXU et al.: STABILIZED WIDE-BAND WAVELENGTH CONVERSION 1889Fig. 6. BER curves and their corresponding power penalties.(around 1400 nm) region of the triggered SC spectrum. Thispart together with different modulation formats will be thefocus of our future work.IV. CONCLUSIONWe have demonstrated the pulse-to-pulse power stabilityimprovement as well as the bandwidth enhancement of theCW-triggered SC scheme and utilized it for realizing wide-band and error-free wavelength conversion. Specifically, weshowed, to the best of our knowledge, for the first time thatthe rogue void condition of the SC can be transformed to themore stable (quasi-Gaussian) temporal statistics by introducingthe CW trigger. In addition, the SC spectrum was significantlyenhanced to nearly tripled bandwidth (wider than 300 nm). Inthe wavelength conversion based on the CW-triggering schemeclear and widely open eye-diagrams have been obtained forall wavelength points, from 1510 to 1615 nm. While lessthan or equal to 3.8-dB power penalty were achieved forC-band and L-band signals. The proposed scheme providesa low pump power and wide covering range solution forwavelength conversion. With better CW trigger source, thescheme could be further extended to wavelength conversionusing the phase-modulated signals.ACKNOWLEDGMENTThe authors would like to thank Sumitomo (SEI), Osaka,Japan, for providing the HNL-DSF.REFERENCES[1] R. R. Alfano, The Supercontinuum Laser Source: Fundamentals withUpdated References, 2nd ed. New York: Springer-Verlag, 2005.[2] J. M. Dudley, G. Genty, and S. Coen, “Supercontinuum generation inphotonic crystal fiber,” Rev. Mod. Phys., vol. 78, no. 4, pp. 1135–1184,2006.[3] J. 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Stabilized Wide-band Wavelength Conversion Enabled by CW-Triggered Supercontinuum

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Stabilized Wide-band Wavelength Conversion Enabled by CW-Triggered Supercontinuum

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