Abstract—All-optical wavelength conversion of differentialphase-shift keyed (DPSK) signals based on semiconductor opticalamplifier Mach–Zehnder interferometer (SOA-MZI) is simulatedand analyzed. The results show that, to obtain both good qualityof the converted signal and high conversion efficiency, phasedifference between the upper and lower arms of MZI should benear , which suggests strong cross-phase-modulation (XPM)and cross-gain-modulation (XGM) for optimized operation ofthis wavelength converter, while weak XPM and XGM will leadto sacrifice of conversion efficiency. The results also show largerwavelength up-conversion range, and suggest non-return-to-zero(NRZ) format for 10 Gb/s operation while return-to-zero (RZ)format for 40 Gb/s operation. Besides, short carrier lifetime ispreferred for high-speed applications and appropriate linewidthenhancement factor can be utilized to mitigate amplitude fluc-tuation of the converted signal if the carrier lifetime is not shortenough.
A LL-OPTICAL wavelength conversion is considered asa very important functionality for network transparency
and reliability. There have been a number of reports on wave-length conversions. However, most of them are of on-off keyed(OOK) signals. Recent studies have revealed that differentialphase-shift keying (DPSK) exhibits particularly better perfor-mance than conventional OOK in long-haul transmissions [1]and may find applications in future high-speed access networks[2] and optical label switching [3]. As a result, all-opticalwavelength conversion of DPSK signals would be of greatimportance in the near future.
All-optical wavelength conversion of DPSK signals hasrecently been demonstrated using four wave mixing (FWM)in either a semiconductor optical amplifier (SOA) [4], [5] or
Manuscript received February 21, 2009; revised June 18, 2009, August 17,2009. First published September 09, 2009; current version published November11, 2009. This work was supported in part by the Hi-Tech Research and De-velopment Program of China under Grant 2007AA03Z414 and in part by theNational Natural Science Foundation of China under Grant 60707005.
Digital Object Identifier 10.1109/JLT.2009.2031925
Fig. 1. Simulation setup of the DPSK wavelength converter based onSOA-MZI (Dashed rectangle gives the optional part).
a highly-nonlinear fiber (HNLF) [6]–[8], optical phase conju-gation in periodically poled lithium niobate (PPLN) [9] andsemiconductor optical amplifier Mach–Zehnder interferometer(SOA-MZI) [10], [11]. Among them, the last scheme has theadvantage of both integration capability and relatively highconversion efficiency with respect to others [4]–[9]. However,to the best of our knowledge, its conversion performance has notbeen theoretically investigated yet and only return-to-zero (RZ)formatted operation has been experimentally demonstrated.
In this paper, all-optical wavelength conversion of DPSK sig-nals based on SOA-MZI is investigated from both sensitivitypenalty and conversion efficiency, as sensitivity penalty is a di-rect embodiment of the quality of the converted signal, whileconversion efficiency is another important systematic param-eter related to the cascadabillity and power efficiency of a wave-length converter. Simulation setup and SOA model used in oursimulation are briefly introduced in Section 2. Section 3 devotesto the simulation results and analyses concerning operation prin-ciple, operation point selection, influences of SOA physical pa-rameters, different signal format and different operation speed.Conclusions are given in Section 4.
II. SIMULATION SETUP AND SOA MODEL
The whole setup considered in our simulation is shown inFig. 1. The setup comprises a DPSK transmitter, an all-opticalwavelength converter, and a single-ended DPSK receiver.LD1–3 are lasers for original DPSK signal, probe light andholding light (optional) respectively. They generate idealmonochromatic plane waves at wavelength of , andin the simulation. In the DPSK transmitter, monochromaticplane wave from LD1 is phase-modulated by the differentiallyencoded data to generate non-return-to-zero (NRZ) DPSK
HONG et al.: DYNAMIC ANALYSIS OF ALL-OPTICAL WAVELENGTH CONVERSION 5581
signal. The rising and falling time (10%–90%) of the electricalmodulating signal is , where is bit period. The optionalamplitude modulator (AM) driven by a synchronized clockis used as a pulse carver to generate RZ-DPSK signal. In thewavelength converter, the DPSK signal is firstly demodulatedinto two logically-inverted OOK signals by a one-bit delayedinterferometer (DI) and then injected into the two SOAs indifferent arms of the succeeding SOA-MZI. The probe lightfrom LD2 is injected into the common port of the SOA-MZI.Another optional AM driven by synchronized clock is placedafter LD2 for RZ operation of this wavelength converter. Inthe SOA-MZI, the probe is separated into two parts whichinteract with the two logically-inverted OOK signals throughcross-gain modulation (XGM) and cross-phase modulation(XPM) in the two SOAs respectively, and superimposed at theoutput port. The converted PSK signal is filtered out with anoptical bandpass filter (BPF) with center wavelength of . Itshould be noted that this wavelength conversion scheme doesnot preserve the differential encoding. However, this change indata encoding can be addressed by either pre-coding or postdetection data processing as described in [10]. In the DPSKreceiver, the converted PSK signal (or original DPSK signalin back-to-back case) is demodulated with another DI. Thenit is detected by a single-ended receiver to investigate theeye-diagrams and bit-error-rate (BER) performances.
The performance of the wavelength converter is analyzedbased on numerical simulation with a wideband multi-sectionmodel of SOA. It is similar to Connelly’s model described in[12], except that we use Agrawal’s material gain and sponta-neous emission rate, as well as linewidth enhancement factorapproximation for relating phase change with gain change [13].Considering the operation speed and physical mechanisms con-cerned within this paper, intra-band dynamics can be neglectedas it is generally considered that intra-band dynamics such asspectral hole burning and carrier heating are not significantfor applications using optical pulses longer than about 10 ps[14]. In a retarded frame, the time-domain wave propagationequations for optical signal [12], [15] and ASE [12], [13] canbe expressed as
(1)
(2)
It is assumed that the signal is traveling along forward direction.denotes electrical field and denotes optical power. Sub-
scripts and denote signal, probe and holding light withwavelength and , respectively. Subscripts and de-note the forward and backward traveling ASE. is the wave-length in th spectrum slice. is propagation coefficient in the
TABLE IPHYSICAL PARAMETERS OF THE SOA USED IN SIMULATION
amplifier waveguide. is confinement factor. is linewidthenhancement factor. is waveguide absorption loss. is mate-rial gain per unit length and is spontaneous emission rateper unit volume per unit energy. is the spontaneous cou-pling coefficient. is the width of each spectrum slice in thefrequency domain. is cross-sectional area of the active region.
is the Plank’s constant and is light speed in vacuum. Carrierdensity rate equation is adopted to describe electron-photon-in-teraction in SOA active region [12]
(3)
where is injected current. is electron charge. is volume ofthe active region. is carrier lifetime of SOA. andare stimulated emission rate for ASE and signal respectively andare given by [12]
(4)
where is the number of spectrum slices. The factor of “2” inthe first equation of (4) accounts for the fact that spontaneouslyemitted photons can exist in one of two mutually orthogonalpolarizations (TE or TM).
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Fig. 2. Static gain saturation behavior of the simulated SOA. (a) Wavelengthdependence of the gain and its saturation under 200 mA injection current.(b) Gain versus input power for different injection current.
III. SIMULATION AND ANALYSIS
The SOAs imbedded in the two arms of MZI are assumedto be strictly identical. They are bulk material devices with nopolarization dependence and zero residual reflection at the endfacets. Useful physical parameters used in the simulation arelisted in Table I. Static gain saturation behavior of the SOA canbe found in Fig. 2. It is shown that the SOA gives a peak small-signal gain of about 26 dB at 1560 nm when injection current is200 mA. It goes into deep saturation with input power of about0 dBm or higher, where the SOA-MZI based DPSK wavelengthconverter is operated.
A. Operation Principle
Firstly, we simulate the conversion process of NRZ-DPSKsignals to illustrate operation principle. In the simulation, bothSOAs are biased at 200 mA. The 10 Gb/s NRZ-DPSK signal atthe input of the converter is centered at 1550 nm and the inputpower is 5 dBm. The differentially encoded data is 1010 01101110 000. The CW probe light is at 1555 nm with input powerof 10 dBm. The waveforms observed at position A-E in Fig.1are shown in Fig. 3.
Fig. 3(a) gives the phase of the input DPSK signal (@A).Fig. 3(b) gives the power of the two logically inverted signalsobtained after demodulation by the first DI (@B1 & B2).Fig. 3(c) and (d) give the power and phase of the probe lightafter interaction with the demodulated signal in the two SOAs(@C1 and C2), respectively. Fig. 3(e) and (f) give the power andphase of the wavelength converted signal (@D), where it can befound that the obtained signal is somewhat RZ-like and the datacarried by its phase has been changed from differential form toits original form during the conversion process. Fig. 3(g) givesthe demodulated conversion signal at the destructive outputport of the second DI (@E).
Considering the importance of differential phase, we givethe differential phase eye-diagrams instead of the direct phaseeye-diagrams for the original signal and the wavelength con-verted signal in Fig. 3(h) and (i) respectively. These eye-dia-grams are obtained with pseudo-random data. Althoughnoise properties of the original DPSK signal are not taken intoconsideration in this work, complete phase regeneration of theconverted signal can be observed. It is shown in Fig. 3(i) that the
Fig. 3. Waveforms and eye-diagrams with NRZ-formatted operation at10 Gb/s. Both SOAs are biased at 200 mA. Original NRZ-DPSK signal is at1550 nm with input power of 5 dBm. The CW probe is at 1555 nm with inputpower of 10 dBm. SOA carrier lifetime is 250 ps. Linewidth enhancementfactor is 8. In (b)–(d), the blue solid line and red dash line represents upper andlower arm in the MZI. (h) and (i) are differential phase eye-diagram of originaland converted DPSK signal, respectively.
differential phase of the converted signal concentrates at “0” and“ ” scales. This regeneration characteristic agrees with thosereported in [16]–[18].
In the wavelength converter, SOA-MZI is operated inpush-pull mode, as it is driven by two logically inverted signals(demodulated from original DPSK signal). We analyze theoperation principle by using the transfer function of MZI. Theelectrical field of the probe light at the destructive output portof the SOA-MZI can be described as follows:
(5)
where is the input probe field. and arethe amplitude gain (phase) experienced in the upper and lowerarms of SOA-MZI, respectively.
If, and are complementaryowing to the logically-inverted modulating OOK signals, as canbe clearly seen in Fig. 3(c) and (d). In the following discussion,we assume that , which is generally satisfied.
From (5), probe output power can be deduced as
(6)
HONG et al.: DYNAMIC ANALYSIS OF ALL-OPTICAL WAVELENGTH CONVERSION 5583
The output power of wavelength-converted signal de-cided by (6) keeps constant in the middle of each bit slot, as
and in (6) are constantfrom the middle of one bit slot to the middle of another bit slotdue to the complementary nature of and ,if pattern effects of SOA is neglected for simplicity. Thisexplains why nearly constant amplitude can be obtained in themiddle of each bit slot, as illustrated in Fig. 3(e). However, atbit transitions, the gain (phase) in the upper and lower arms canbe very close to each other, that isnearly holds. At these points, output power decided by (6)returns to zero. So the converted DPSK signal is somewhatRZ-like.
It is worth noting that, at bit transitions, the phase of the con-verted signal can be deduced from (5) that
(7)
In (7), is a unit step func-
tion and is nearly constant. Thus, the convertedsignal will have a phase change whenor changes from positive to negative or viceversa.
While if , XPM induced phase shift is not involvedand holds. In this case, (6) is reducedinto
(8)
and the output phase can be deduced from (5)
(9)
From (8), output power will also be constant in the middle ofeach bit slot (as is constant near this point) whilebe zero at bit transitions. However, peak power of the convertedpulse gets the minimum value as cosine function in (6) equals
. The definition of the step function in (9) is consistent withthat in (7). Equation (9) indicates that the converted signal willalso get a phase change when changesfrom positive to negative or vice versa, despite the absence ofXPM.
Consequently, in the SOA-MZI, intensity information in thelogically-inverted modulating OOK signals will be converted tophase information in the wavelength conversed signal, regard-less of the existence of XPM. The phase of the converted signalis decided by the signature of the gain/phase difference betweenthe two arms of SOA-MZI (or only gain difference if ),rather than the absolute phase-change induced by the demodu-lated signal. This indicates a step function relationship betweenthe phase of the converted signal and the differential phase ofthe original signal as described in [16].
Fig. 4 shows the bit error rate (BER) performance of theconverted DPSK signal, where the operation conditions are thesame as in Fig. 3. Back-to-back BER performance is also givenfor comparison. Electrical eye diagrams for the two cases aregiven as insets. The calculated power penalty at BER of
Fig. 4. BER performance for 10 Gb/s NRZ-DPSK signal with operation con-ditions specified in Fig. 3.
is only about 0.2 dB and no error floor is observed, which indi-cates good quality of the converted signal. It is worth noting thatthe relatively low conversion penalty can be partly attributedto NRZ-to-RZ conversion during the wavelength conversionprocess. The converted signal tends to get a relatively lowaverage power, comparing to the original signal (please seeFig. 3(b) with 3(g) for comparison).
In the simulation, all the BER values and eye-diagrams areobtained with PRBS length of . Single-ended detection isemployed in the DPSK receiver as it is quite adequate just forconversion performance evaluation purpose. We assume a fifth-order Bessel characteristic for the entire electronic circuitry withelectrical bandwidth of 0.75 Br, where Br is the bit rate, andno optical pre-amplification is involved. The photodiode has aresponsivity of 0.8 A/W and a single-sided thermal noise densityof 10 pA/Hz . As our major concern is the wavelength con-version performance that is limited by the physical nature of theSOAs, we have assumed that the original signal is clean and theconverted signal will not be transmitted along a fiber span andoptically amplified before receiving. While on the other hand,the SOAs are operated in deep saturation in our case. So theASE noise reaching the receiver will be quite small comparingto the signal. Moreover, an optical filter used to extract the wave-length converted signal makes received ASE noise even smaller.It is thus reasonable to neglect ASE-related noise in the receivermodel. The BER is calculated using an analytical method de-scribed in [19]. Both Threshold level and sampling time are op-timized during BER calculation and the total BER is averagedby a number of bits to account for pattern effects. The powerpenalty induced by the wavelength conversion can be decidedby degradation of the receiver sensitivity at BER of afterwavelength conversion.
B. Operation Point Selection
The conversion performance of the DPSK wavelength con-verter based on SOA-MZI is dependent on the operation pointsof the imbedded SOAs. Electrical injection current as well aspower and wavelength of the input DPSK signal and probe lightshould be properly selected for obtaining both good quality ofthe converted signal and high conversion efficiency. Wavelengthconversion of 10 Gb/s NRZ-DPSK signal is investigated in de-tail first.
Fig. 5(a) shows the sensitivity penalty (evaluated at BER of) and conversion efficiency (ratio of the average optical
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Fig. 5. Sensitivity penalty and conversion efficiency (a), �� �� � (b) as afunction of the probe input power for different injection current of SOA. OriginalNRZ-DPSK signal is at 1550 nm with input power of 5 dBm. The CW probe isat 1555 nm. SOA carrier lifetime is 250 ps. In (a), linewidth enhancement factoris 8.
power of converted DPSK signal and the original DPSK signal)as a function of the input power of probe light under different in-jection current of SOA. The wavelength of the signal and probelight are fixed at 1550 and 1555 nm respectively, and the inputpower of the original DPSK signal is 5 dBm. It is shown thatprobe power should be high enough for normal operation of thisDPSK wavelength converter (that is, BER of can be ob-tained), which can be attributed to strong pattern effects at lowerprobe powers (see the two inset eye-diagrams for comparison).
The dependence of conversion performance on probe powercan be analyzed using any one of the curves shown in Fig. 5(a).Generally speaking, there are two appropriate ranges of probepower where relatively low sensitivity penalty can be obtained.Firstly, high probe power can enhance the photon density inSOA active region and shorten the effective carrier lifetime[20], thus pattern effects can be alleviated and relatively lowsensitivity penalty can be expected. However, conversionefficiency is greatly sacrificed in this case due to the smallphase change obtained under deep saturation of the SOAs. Itcan be also found in Fig. 5(a) that sensitivity penalty can beeven negative at high probe powers. This negative penalty canalso be attributed to NRZ-to-RZ conversion during the wave-length conversion process. Secondly, if (absolutephase difference between upper and lower arms of MZI) isevaluated in the middle of each bit slot and averaged amongall bits, the change of as a function of probepower can be plotted as shown by the solid lines in Fig. 5(b).Comparing Fig. 5(b) with Fig. 5(a), it is found that reducedsensitivity penalty can be also obtained for probe powers where
approaches in the middle of each bit slot. Thiscan be explained by mitigation of pattern effects as the outputpower is related to cosine function of , whose slopeapproaches zero near (see (6) for reference). On the otherhand, the cosine function in (6) equals when
approaches in the middle of each bit slot, which means thehighest conversion efficiency will be obtained. Consideringboth the quality and efficiency of the converted signal, thissecond operation range can be defined as the optimal operationrange for this wavelength conversion scheme.
Although a constant linewidth enhancement factor is adoptedin this work as in many other literatures, it is actually dependenton wavelength and carrier density [21]. Fig. 5(b) also comparesthe change of for the cases of a fixedand a varied . The dependence of on carrier density andwavelength can be simulated by using (12) of [21]. To makethe results for a varied comparable with those for a fixed
, it is reasonable to assume that the varied equals 8at small-signal gain peak under injection current of 200 mA. It isshown that a fixed will over-estimate (under-es-timate conversion performance) at lower probe powers, espe-cially for large injection current, as given by (12) of [21]will be decreased when carrier density is depleted. However,the difference will be diminished at higher probe powers asphase change is related to gain change by linewidth enhance-ment factor, while gain change can be greatly suppressed bya high power CW light. Generally, the assumption of constantlinewidth enhancement factor is still quite reasonable within thescope of this work.
The injection current of SOA is changed from 200 mA downto 160 mA with a step of 20 mA to investigate the dependenceof conversion performance on SOA injection current. As
in the middle of each bit slot approaches at lower probepowers when injection current is decreased, the optimal op-eration range moves to lower probe powers accordingly. It isshown that conversion performance is significantly degraded byreducing SOA current of 40 mA, which corresponds to small-signal-gain reduction of about 10 dB. The decrease of conver-sion efficiency can be explained by the reduced average gain ofthe probe light, while the increase of sensitivity penalty is de-rived from stronger pattern effects under lower injection current[22]. The results above suggest relatively large injection currentof SOA. The best electrical eye diagram obtained in the optimalrange with SOA injection current of 200 mA is displayed in theright-hand-sided inset of Fig. 5(a).
Sensitivity penalty and conversion efficiency for differentinput powers of the original DPSK signal are given in Fig. 6,where SOA injection current is fixed at 200 mA. It can be seenthat larger input power of the original DPSK signal, whichmeans stronger modulating light in the SOAs, will bring theoptimal range of probe input power to higher values in orderto make approach in the middle of each bitslot. At the same time, the maximum conversion efficiency willbe reduced and obtained at higher probe powers accordinglyas approaches under deeper saturation of theSOAs. The lowest penalty obtained in the optimal range can beslightly reduced with larger input power of the DPSK signalas approaches with slightly weaker patterneffects. The best electrical eye diagrams obtained in the optimalrange for DPSK signal input power of 3 dBm and 7 dBm arealso given in the insets of Fig. 6, while that for input powerof 5 dBm has been already given in Fig. 5(a). At high probeinput powers, similar sensitivity penalty is observed while the
HONG et al.: DYNAMIC ANALYSIS OF ALL-OPTICAL WAVELENGTH CONVERSION 5585
Fig. 6. Sensitivity penalty and conversion efficiency as a function of the probeinput power for different input powers of the original DPSK signal. Both SOAsare biased at 200 mA. Original NRZ-DPSK signal is at 1550 nm. The CW probeis at 1555 nm. SOA carrier lifetime is 250 ps. Linewidth enhancement factor is 8.
conversion efficiency can be slightly elevated by increasing theinput power of the original DPSK signal as larger phase changecan be introduced.
The dependence of conversion performance on wavelengthallocation of the original DPSK signal and the probe light isalso investigated. The results are shown in Fig. 7(a), where SOAinjection current is 200 mA, the input powers of the originalDPSK signal and probe light are 5 dBm and 6 dBm, respectively.The wavelength of original DPSK signal should better locate atthe small signal gain peak of the SOA (1560 nm) or the shorterwavelength side nearby to induce large gain and phase change.We choose 1550 nm and 1556 nm for comparison in the simu-lation. The corresponding value of in the middle ofeach bit slot is evaluated as described previously and illustratedin Fig. 7(b). It can be expected that the optimal probe wave-length locates where approaches in the middle ofeach bit slot. As gain spectrum of SOA will be red-shifted whencarrier density is depleted by the signal light, [23] phase changeexperienced by the probe light will be increased for probe wave-length blue-shifted from this optimal value, while be decreasedin red-shift case. Larger deviation of from in themiddle of each bit slot eventually leads to increase of sensitivitypenalty and decrease of conversion efficiency. However, due tothe slower change of in the red-shift case as shownin Fig. 7(b), a much wider wavelength up-conversion range canbe observed. It is also shown in Fig. 7(a) that the shift of theoriginal DPSK signal wavelength toward small signal gain peakof SOA will move the whole wavelength conversion range tolonger wavelength. The conversion range obtained here agreeswith those reported in [10] where the up-conversion range is30 nm and down-conversion range is 5 nm (considering C-bandof 1530–1565 nm), although the probe wavelength instead ofthe signal wavelength is fixed in their experiment.
The case of a fixed is also compared with the case ofa varied in Fig. 7(b). The modeling of a varied is thesame as described previously. It is shown that the value of
will be over-estimated at shorter wavelength sidewhile under-estimated at longer wavelength side by excludingthe wavelength-dependence of and the discrepancy isrelatively larger at shorter wavelength side. Consequently, theestimated wavelength down-conversion range will be relatively
Fig. 7. Sensitivity penalty and conversion efficiency (a), �� � � � (b) asa function of probe wavelength for different DPSK signal wavelengths. BothSOAs are biased at 200 mA. The input power of the original NRZ-DPSK signaland CW probe is 5 dBm and 6 dBm, respectively. SOA carrier lifetime is 250 ps.Linewidth enhancement factor is 8.
larger if a varied is adopted. However, the general conver-sion performance will not be significantly influenced by usinga varied .
In this section, we actually investigate operation conditionsof this DPSK wavelength converter for case, whereXPM and XGM co-exist in the SOAs ( case will bebriefly discussed in the next section). In this case, the conver-sion mechanism is based on the gain/phase difference betweenthe upper and lower arms of MZI, no matter whether the differ-ence is large or small. However, to obtain both good quality ofthe converted signal and high conversion efficiency, phase dif-ference between the upper and lower arms of MZI should benear in the middle of each bit slot, which suggest strong XPMand XGM for optimized operation of this wavelength converter.Although weak XPM and XGM can be also used to obtain goodquality of the converted signal as in the case of very strong probelight, conversion efficiency is greatly sacrificed. The results alsoshow larger wavelength up-conversion range than down-conver-sion range due to gain spectrum shift of SOA with the depletionof carrier density.
C. Influence of SOA Physical Parameters
The physical parameters of the SOAs involved in the MZIhave direct effect on the induced gain and phase change, and fi-nally influence the signal quality and conversion efficiency ofthe converted signal. Among them, SOA carrier lifetime andlinewidth enhancement factor are most relevant to the phys-ical mechanism utilized to implement wavelength conversion ofDPSK signal based on SOA-MZI. Fig. 8(a) and (b) show sen-sitivity penalty and conversion efficiency as a function of car-rier lifetime and linewidth enhance factor, respectively. Eye di-agrams are also given to illustrate changes in the wavelengthconverted signal with different values of these parameters. The
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Fig. 8. Sensitivity penalty and conversion efficiency as a function of carrierlifetime (a) and linewidth enhance factor (b). The original DPSK signal is at1550 nm with input power of 5 dBm. The CW probe is at 1555 nm with inputpower of 6 dBm. In (a), SOA Linewidth enhancement factor is 8. SOA currentis changed with the variation of carrier lifetime to ensure the same small signalgain. In (b), SOA carrier lifetime is fixed at 250 ps and injection current is fixedat 200 mA.
original DPSK signal is centered at 1550 nm with input powerof 5 dBm. The probe light is centered at 1555 nm with inputpower of 6 dBm. SOA injection current is 200 mA for carrierlifetime of 250 ps. It is changed accordingly with the variationof carrier lifetime in order to ensure the same small signal gain.
Fig. 8(a) shows that increased sensitivity penalty and re-duced conversion efficiency will be observed with longercarrier lifetime of SOA. The reason can be attributed to largerpattern effects and smaller phase difference between the upperand lower arms of SOA-MZI when carrier density recovery iseven more incomplete. It is shown that, for operation speedof 10 Gb/s, the carrier lifetime of SOA should be no longerthan about 300 ps for sensitivity penalty less than 0.5 dB. Thisestimated up-limit of carrier lifetime is somewhat larger thanthe bit period due to reduction of effective lifetime for relativelyhigh probe powers, which is required by proper operation ofthe wavelength converter.
Fig. 8(b) shows that significant decrease of conversion effi-ciency can be observed when linewidth enhancement factor isreduced form 8 down to 0, as the cosine function in (6) equalsfrom about to . Despite of the significant decrease of con-version efficiency, the increase of sensitivity penalty is quitemoderate ( 1 dB). The results given in Fig. 8(b) reveal twoimportant facts. The first is: wavelength conversion of DPSKsignals can be obtained even with zero linewidth enhancementfactor, which means that phase modulation of the probe light isnot indispensable for obtaining phased-modulated signal whenSOA-MZI is driving by two logically inverted signals (push-pulloperation). This is consistent with the theoretical analysis for
(no XPM effect), as given previously in Section 3.1.
Fig. 9. Waveforms with RZ-formatted operation at 10 Gb/s. Both SOAs arebiased at 200 mA. Original RZ-DPSK signal is at 1550 nm with peak inputpower of 7 dBm. The probe is at 1555 nm with peak input power of 8 dBm.SOA carrier lifetime is 250 ps. Linewidth enhancement factor is 8. In (b)–(d),the blue solid line and red dash line represents upper and lower arm in the MZI.
The second is: the quality of the converted signal can be im-proved with larger linewidth enhancement factors, as cosinefunction in (6) can be utilized to mitigate fluctuations in the am-plitude of the converted signal when the phase difference be-tween the upper and lower arms of MZI approaches in themiddle of each bit slot, which is clearly illustrated by the eyediagrams shown in Fig. 8(b).
In practice, appropriate linewidth enhancement factor of theSOAs will lead to high conversion efficiency of this wavelengthconverter. However, zero linewidth enhancement factor mightbe also beneficial if the original DPSK signal has large ampli-tude noise [17]. Because translation from amplitude noise tophase noise through XPM can be avoided, although conversionefficiency has to be sacrificed. If carrier lifetime of the SOAs isnot short enough and significant pattern effects exist, non-zerolinewith enhancement factor might be more preferred as it canbe utilized to suppress the amplitude fluctuation of the convertedsignal.
D. Operation With RZ-Formatted Signals
Wavelength conversion of DPSK signals based on SOA-MZIcan also work with RZ format as long as the CW probe lightis replaced by a synchronized optical clock. For 10 Gb/s oper-ation, the corresponding waveforms at position A-E in Fig. 1are shown in Fig. 9. The signal and clock are both 33% RZ-for-matted signals with extinction ratio (ER) of 13 dB. The peakpower of input signal pulse and clock pulse are 7 dBm and8 dBm respectively.
In RZ case, pulse shape of the converted DPSK signal getslarger distortion comparing to NRZ case, as shown in Fig. 9(e).The pulses are forwardly-tilted due to dynamic gain saturationof the SOAs. Moreover, It is found that small pulse appears attheir leading edge when the demodulated data changes between“0” and “1”, which can be attributed to pattern-related signaturechange of gain (phase) difference between upper and lower armsof MZI in one bit period.
HONG et al.: DYNAMIC ANALYSIS OF ALL-OPTICAL WAVELENGTH CONVERSION 5587
Fig. 10. Sensitivity penalty and conversion efficiency as a function of the peakpower of the probe pulse for different peak powers of the signal pulse (a) andcomparison with NRZ-formatted operation (b). In the simulation, both SOAsare biased at 200 mA. Original DPSK signal is at 1550 nm. The probe is at1555 nm. SOA carrier lifetime is 250 ps, linewidth enhancement factor is 8.In Fig. 9(b), input power (peak power) of the original NRZ-DPSK (RZ-DPSK)signal is 5 dBm.
Fig. 10(a) shows similar change of sensitivity penalty andconversion efficiency as a function of the peak power of theclock (probe) pulse, as well as move of the optimal probe powerrange to higher values with higher peak powers of the signalpulse with RZ-formatted operation. Fig. 10(b) displays compar-ison of RZ formatted operation with NRZ-formatted operationat 10 Gb/s, where the input power (peak power) of the originalNRZ-DPSK (RZ-DPSK) signal is 5 dBm. It is shown that con-version efficiency is generally higher with RZ-formatted opera-tion due to weaker saturation of SOA in RZ case. However, sen-sitivity penalty is much larger, which results from larger pulseshape distortion of the converted RZ-DPSK signals on one hand(as shown by the electrical eye diagrams given in Fig. 10(b)) andthe relatively low penalty with NRZ-DPSK signal due to the ac-companied NRZ-to-RZ conversion on the other hand. The re-sults highly suggest NRZ-formatted operation at 10 Gb/s.
To improve the conversion performance with RZ-formattedsignals, holding light injection is considered. It is worth notingthat the power of an in-band holding light should not be toolarge, otherwise conversion efficiency will be greatly sacrificed.In the simulation, holding light with wavelength of 1535 nm andinput power of 0 dBm, 3 dBm and 6 dBm is considered. Thepeak power of the input DPSK signal pulse is 7 dBm. Otheroperation conditions are the same as in Fig. 10(a). Simulationresults given in Fig. 11 show that holding light injection is veryefficient in improving the quality of the converted signal at rel-atively low probe powers due to the suppression of pattern ef-fects, which can be seen in the two inset eye-diagrams in Fig. 11.
Fig. 11. Sensitivity penalty and conversion efficiency as a function of the probepulse peak power with and without holding light. In the simulation, both SOAsare biased at 200 mA. Original DPSK signal is at 1550 nm with peak input powerof 7 dBm. The probe is at 1555 nm. The holding light is at 1535 nm. SOA carrierlifetime is 250 ps. Linewidth enhancement factor is 8.
Fig. 12. Sensitivity penalty and conversion efficiency as a function of probepower with NRZ and RZ formatted operation at 40 Gb/s. In the simulation, bothSOAs are biased at 500 mA. Original NRZ(RZ)-DPSK signal is at 1550 nm with(peak) input power of 7 dBm. The probe is at 1555 nm. SOA carrier lifetime is100 ps, linewidth enhancement factor is 8.
While for high probe powers, the major saturation effect will beinduced by the probe light itself and the effect of the holdinglight will be marginal.
E. Operation at 40 Gb/s
At operation speed of 40 Gb/s, pattern effects will be muchstronger than 10 Gb/s case for regular SOAs with carrier life-time in the range of 100 ps to 1 ns. Degradation of conversionperformance will be expected. Here, we try to demonstrate that40 Gb/s operation can be realized with moderate carrier lifetimeof SOA (100 ps), if the operation point of the SOAs is optimized.As the pulsewidth considered in this section is larger than 8 ps,while the characteristic time of SOA intra-band process is gen-erally less than 1 ps [14], an SOA model that only accounts forinter-band process can be still applied.
Sensitivity penalty and conversion efficiency as a function ofprobe power with NRZ and RZ formatted operation are shown inFig. 12. In the simulation, both SOAs give a peak small signalgain of 28 dB at 1555 nm when injection current is 500 mA.The original DPSK signal is at 1550 nm while the probe light isat 1555 nm. For NRZ case, the input power of original DPSKsignal is 7 dBm. For RZ case, the signal and clock are both 33%RZ-formatted signals with ER of 13 dB and the peak power ofthe original DPSK signal pulse is 7 dBm.
5588 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 24, DECEMBER 15, 2009
It can be found in Fig. 12 that, although carrier lifetime of100 ps is not short enough for 40 Gb/s operation, sensitivitypenalty of about 1.0–2.0 dB can still be obtained in the twoappropriate probe power ranges either for NRZ- or RZ-opera-tions: one locates at relatively lower probe powers (area1) andthe other locates at relatively higher probe powers (area2). Theeye diagrams obtained in each area with NRZ and RZ formattedsignals are also given as insets. Area1 is formed as phase differ-ence between the upper and lower arms of MZI approachesin the middle of each bit slot, where pattern effects can be miti-gated by cosine function to some extend. While in area2, strongprobe power leads to reduction of SOA effective carrier lifetimeso that pattern effects can be also mitigated. However, conver-sion efficiency is significantly reduced due to strong probe-in-duced SOA gain saturation. It is noted that the lowest sensitivitypenalty observed in these operation ranges is very similar tothose reported in [10] and [16].
In 40 Gb/s case, conversion efficiency of RZ-formatted signalis also higher than that of NRZ-formatted signal, as in 10 Gb/scase. However, their sensitivity penalty is very similar. So, thereare no strong evidences supporting a specific signal format (RZor NRZ) in 40 Gb/s case. Due to the larger conversion effi-ciency, lower average power of probe signal needed and rela-tively larger operation range (area1+area2), RZ format wouldbe slightly preferred.
IV. CONCLUSION
In this paper, All-optical wavelength conversion of DPSKsignal based on SOA-MZI is theoretically investigated for thefirst time, to the best of our knowledge. The operation principleis analyzed and operation point selection, influence of SOAphysical parameters, different signal format and operation speedare discussed in detail using the transfer function of MZI and awideband dynamic model of SOA.
Simulation and analysis reveal that the phase of the wave-length-converted DPSK signal is decided by the signature ofthe gain/phase (gain) difference between the two arms of SOA-MZI, rather than the absolute phase-change experienced by theprobe light. To obtain both good quality of the converted signaland high conversion efficiency, phase difference between theupper and lower arms of MZI should be near , which suggestsstrong XPM and XGM for optimized operation of this wave-length converter, while weak XPM and XGM will lead to sacri-fice of conversion efficiency. The results also show larger wave-length up-conversion range due to SOA gain spectrum shift. Itis also shown that SOA-MZI can be applied to wavelength con-version of both NRZ and RZ formatted DPSK signals. How-ever, which format is preferred depends on the operation speed.At 10 Gb/s, NRZ format is preferred for the lower sensitivitypenalty induced, while at 40 Gb/s, RZ format is slightly pre-ferred for the higher conversion efficiency and relatively largeroperation range. Besides, we show that holding light injectionis useful to improve the conversion performance with RZ-for-matted signals, especially for relatively low input powers of theprobe signal.
Although it is demonstrated by simulation that 40 Gb/s op-eration can be realized with moderate carrier lifetime of SOA(100 ps). The result that carrier lifetime of SOA should be no
longer than about 300 ps for operation speed of 10 Gb/s, indi-cates that short carrier lifetime is still preferred for high-speedoperation. It is also shown that although phase modulation of theprobe light is not indispensable when SOA-MZI is driving bytwo logically inverted signals, appropriate linewidth enhance-ment factor is useful to obtain phase difference between theupper and lower arms of MZI, and thereby mitigate amplitudefluctuation of the converted signal pulse if the carrier lifetime isnot short enough to avoid pattern effects.
ACKNOWLEDGMENT
The authors would like to thank Prof. Y. Yu with WuhanNational Lab for Optoelectronics, Huazhong University of Sci-ence and Technology for helping us with language issue of themanuscript.
REFERENCES
[1] H. Gnauck and P. J. Winzer, “Optical phase-shift-keyed transmission,”J. Lightw. Technol., vol. 23, no. 1, pp. 115–130, Jan. 2005.
[2] W. Hung, C. K. Chan, L. K. Chen, and F. Tong, “An optical net-work unit for WDM access networks with downstream DPSK and up-stream re-modulated OOK data using injection-locked FP laser,” IEEEPhoton. Technol. Lett., vol. 15, no. 10, pp. 1476–1478, Oct. 2003.
[3] N. Chi, L. Xu, J. Zhang, P. V. Holm-Nielsen, C. Peucheret, C.Mikkelsen, H. Ou, J. Seoane, and P. Jeppesen, “Orthogonal opticallabeling based on a 40 Gbit/s DPSK payload and a 2.5 Gbit/s IM label,”in Technical Digest of 2004 Optic Fiber Commun. Conf. (OFC’2004),,Paper FO6.
[4] Z. Li, Y. Dong, J. Mo, Y. Wang, and C. Lu, “Cascaded all-optical wave-length conversion for RZ-DPSK signal based on four-wave mixing insemiconductor optical amplifier,” IEEE Photonics Technol. Lett., vol.16, no. 7, pp. 1685–1687, Jul. 2004.
[5] H. Wen, H. Jiang, X. Zheng, H. Zhang, and Y. Guo, “Performanceenhancement of multiwavelength conversion of RZ-DPSK based onfour-wave mixing in semiconductor optical amplifier,” IEEE PhotonicsTechnol. Lett., vol. 19, no. 18, pp. 1377–1379, Sep. 2007.
[6] P. Devgan, R. Tang, V. S. Grigoryan, and P. Kumar, “Highly effi-cient multichannel wavelength conversion of DPSK signals,” J. Lightw.Technol., vol. 24, no. 10, pp. 3677–3682, Oct. 2006.
[7] P. A. Anderson, T. Tokle, Y. Geng, C. Peucheret, and P. Jeppesen,“Wavelength conversion of a 40 Gb/s RZ-DPSK signal usingfour-wave-mixing in a dispersion-flattened highly nonlinear pho-tonic crystal fiber,” IEEE Photon. Technol. Lett., vol. 17, no. 9, pp.1908–1910, Sep. 2005.
[8] M. P. Fok, C. Shu, and D. J. Blumenthal, “Dual-pump four-wavemixing in bismuth-oxide highly nonlinear fiber for wideband DPSKwavelength conversion,” in Technical Digest of 2007 Optic FiberCommun. Conf. (OFC’2007), Paper JThA52.
[9] S. L. Jansen, D. van den Borne, B. Spinnler, S. Calabro, H. Suche, P.M. Krummrich, W. Sohler, G. D. Khoe, and H. de Waardt, “Opticalphase conjugation for ultra-long-haul phase shift keyed transmission,”J. Lightw. Technol., vol. 24, no. 1, pp. 54–64, Jan. 2006.
[10] Sartorius, C. Bornholdt, J. Slovak, M. Schlak, Ch. Schmidt, (1),A. Marculescu, P. Vorreau, S. Tsadka, W. Freude, and J. Leuthold,“All-optical DPSK wavelength converter based on MZI with inte-grated SOAs and phase shifters,” in Technical Digest of 2007 OpticFiber Commun. Conf. (OFC’2006), Paper OWS6.
[11] M. P. Fok’, J. A. Summers, M. L. Ma§anovie, C. Shul, and D. J. Blu-menthal, “Tunable DPSK wavelength converter using an SOA-MZImonolithically integrated with a sampled-grating distributed bragg re-flector,” in CLEO/Europe and IQEC 2007 Conf. Digest, Paper CI1_5.
[12] M. J. Connelly, “Wideband semiconductor optical amplifier steadystatenumerical model,” IEEE J. Quantum Electron. Letters, vol. 37, no. 3,pp. 439–447, Mar. 2001.
[13] G. P. Agrawal and N. K. Dutta, Semiconductor Lasers. New York:Van Nostrand Reinhold, 1993.
[14] A. Mecozzi and J. Mork, “Saturation induced by picosecond pulses insemiconductor optical amplifiers,” J. Opt. Soc. Amer. B, vol. 14, no. 4,pp. 761–770, Apr. 1997.
HONG et al.: DYNAMIC ANALYSIS OF ALL-OPTICAL WAVELENGTH CONVERSION 5589
[15] J. Park and X. Li, “Theoretical and numerical analysis of superlumines-cent diodes,” J. Lightw. Technol., vol. 24, no. 6, pp. 2473–2480, Jun.2006.
[16] P. Vorreau, A. Marculescu, J. Wang, G. Böttger, B. Sartorius, C. Born-holdt, J. Slovak, M. Schlak, C. Schmidt, S. Tsadka, W. Freude, andJ. Leuthold, “Cascadability and regenerative properties of SOA all-op-tical DPSK wavelength converters,” IEEE Photon. Technol. Lett., vol.18, no. 18, pp. 1970–1972, Sep. 2006.
[17] J. Wang, Y. Jiao, R. Bonk, W. Freude, and J. Leuthold, “Regen-erative properties of bulk and quantum dot SOA based all-opticalMach–Zehnder interferometer DPSK wavelength converters,” pre-sented at the Int. Conf. Photon. Switching, Oct. 16–18, 2006.
[18] R. Elschner, A. M. de Melo, C.-A. Bunge, and K. Petermann, “Noisesuppression properties of an interferometer based regenerator fordifferential phase-shift keying data,” Opt. Lett., vol. 32, no. 2, pp.112–114, Dec. 2007.
[19] M. Pauer, P. J. Winzer, and W. R. Leeb, “Bit error probability reduc-tion in direct detection optical receivers using RZ coding,” J. Lightw.Technol., vol. 19, no. 9, pp. 1255–1262, Sep. 2001.
[20] N. Storkfelt, B. Mikkelsen, D. S. Olesen, M. Yamaguchi, and K. E.Stubkjaer, “Measurement of carrier lifetime and linewidth enhance-ment factor for 1.5-m ridge-waeguide laser amplifier,” IEEE Photon.Technol. Lett., vol. 3, no. 7, pp. 632–634, Dec. 1991.
[21] L. Occhi, L. Schares, and G. Guekos, “Phase modeling based on thealpha factor in bulk semiconductor optical amplifiers,” IEEE J. Sel.Topics Quantum Electron., vol. 9, no. 3, pp. 788–797, Mar. 2003.
[22] R. J. Manning, D. A. O. Davies, and J. K. Lucek, “Recovery rates insemiconductor laser amplifiers: Optical and electrical bias dependen-cies,” Electron. Lett., vol. 30, no. 15, pp. 1233–1235, Jul. 1994.
[23] K. Obermann, S. Kindt, D. Breuer, and K. Petermann, “Performanceanalysis of wavelength converters based on cross-gain modulation insemiconductor-optical amplifiers,” J. Lightw. Technol., vol. 16, no. 1,pp. 78–85, Jan. 1998.
Wei Hong received the M.S. degree in science in1996 from the Department of Physics, HuazhongUniversity of Science and Technology (HUST),Wuhan, China, and the Ph.D. degree in physicalelectronics in 2003 from Department of Optoelec-tronic Engineering, HUST.
She is currently with Wuhan National Lab for Op-toelectronics and the School of Optoelectronics Sci-ence and Engineering, HUST, as an Associate Pro-fessor. Her working area is semiconductor optoelec-tronic devices and all-optical signal processing.
Minghao Li received the M.S. degree from theDepartment of Electronics and Information En-gineering, Huazhong University of Science andTechnology (HUST), in 2005. He is currently pur-suing the Ph.D. degree in optoelectronic informationengineering at the School of Optoelectronic Scienceand Engineering and the Wuhan National Laboratoryfor Optoelectronics, HUST.
His research focuses on high-speed all-opticalsignal processing, 2R regeneration, and advancedmodulation format.
Xinliang Zhang received the Ph.D. degree in phys-ical electronics from Huazhong University of Scienceand Technology, Wuhan, China, in 2001.
He is currently with Wuhan National Laboratoryfor Optoelectronics and the School of OptoelectronicScience and Engineering, HUST, as a Professor. Heis the author or coauthor of more than 70 journaland conference papers. His current research interestsinclude all-optical signal processing and relatedcomponents.
Junqiang Sun received the Ph.D. degree in elec-tronic physics and optoelectronics from HuazhongUniversity of Science and Technology (HUST),Wuhan, China, in 1994.
From September 2000 to September 2001, he wasa Research Associate at the Department of Electricaland Electronic Engineering, Hong Kong Universityof Science and Technology. From June 2005 toDecember 2005, he was a Research Fellow at theSchool of Information Technology and Engineering,University of Ottawa, Canada. He is now a Professor
at Wuhan National Laboratory for Optoelectronics and the School of Opto-electronic Science and Engineering, HUST. He has authored and co-authoredover 100 papers in refereed journals and conference proceedings. His currentresearch interests include all-optical signal processing, all-optical wavelengthconversion, fiber lasers and amplifiers, photonic generation of microwavesignals, and optical network technologies.
Dexiu Huang was born in Hunan Province, China,in October 1937. He graduated from the Departmentof Radio Engineering, Huazhong Institute of Tech-nology (now Huazhong University of Science andTechnology), Wuhan, China, in 1963.
Since then, he has been with the same universityas Assistant Professor, Lecturer, Associate Professor,and Professor in 1963, 1978, 1986, and 1990, respec-tively. Prior to 1972, he was engaged in research onsemiconductor devices and passive devices in radioengineering. From 1972 to 1981, he performed re-
search on solid-state lasers and applications. From 1981 to 1983, he was a Vis-iting Scientist with the Oregon Graduate Center, focusing on semiconductoroptoelectronic devices. Since then, he has been in the field of optical communi-cation performing research on semiconductor optoelectronics devices and somepassive devices. He is currently a Professor and the Dean of the College of Infor-mation Science and Engineering and the Associate Director of the Wuhan Na-tional Laboratory for Optoelectronics. He is the author of five books, namely,Semiconductor Optoelectronics (Univ. Electronic Science and Technology ofChina Press, 1994), Semiconductor Lasers and Their Applications (National De-fense Industry Press, 2001), Introduction of Information Science (Chinese Elec-trical Power Press, 2001) and Fiber Optics (National Defense Industry Press,and Fiber Technology and Applications, 1995). He has authored and co-au-thored more than 200 papers.
