JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 23, NO. 1, JANUARY 2005 287

Error-Free Transmission for Incoherent Broad-BandOptical Communications Systems

Using Incoherent-to-CoherentWavelength Conversion

M. Menif, Student Member, IEEE, W. Mathlouthi, P. Lemieux, L. A. Rusch, Senior Member, IEEE, and M. Roy

Abstract—In this paper, a new method is presented to exploitthe cross-gain modulation mechanism in semiconductor opticalamplifiers to reduce the intensity noise of spectrum-sliced com-munications systems via conversion of the modulated incoherentoptical signal into a coherent signal before the detection process.Wavelength-division-multiplexed networks using broad-bandsources have performance limited by intensity noise; conversionfrom an incoherent to a coherent signal reduces the intensity noiseand enables the elimination of the bit-error-rate floor.

Index Terms—All-optical wavelength conversion, beat noise,broad-band source, cross-gain modulation (XGM), optical-code-division multiple access (OCDMA), semiconductor optical ampli-fiers (SOAs), spectrum-sliced wavelength-division multiplexing(SS-WDM).

I. INTRODUCTION

I NCOHERENT broad-band optical communication systemslike spectrum-sliced wavelength-division multiplexing

(SS-WDM) [1] and optical-code-division multiple access(OCDMA) [2] offer low-cost alternatives to conventionalwavelength-division-multiplexed (WDM) networks. These sys-tems avoid the need for multiple semiconductor lasers stabilizedat specific wavelengths and the potential exists for networks toshare a single source. The major weakness of systems usingbroad-band sources generated from spontaneous emission isthe intensity noise (or beat noise) generated by square-lawphotodetection [3]. As the bit rate increases, the detectedoptical noise becomes large, degrading network quality of ser-vice and limiting system performance. A bit-error-rate (BER)floor exists for SS-WDM due to signal-dependent noises, asopposed to coherent systems without such a floor. In order toavoid the BER floor, and/or accommodate a higher bit rate, a

Manuscript received March 23, 2004; revised August 12, 2004. This work wassupported in part by the National Sciences and Engineering Research Council(NSERC) under Grant CG029055 and PADCO FT076892. This material waspresented in part at IEEE International Conference on Communications (ICC2004), Paris, France.

M. Menif was with the Centre d’Optique, Photonique, et Laser (COPL), De-partment of Electrical and Computer Engineering, Université Laval, Québec,QC G1K 7P4, Canada. He is now with École Supérieure des Communications deTunis, 2083 El Ghazala Ariana, Tunisia (e-mail: mourad.mnif@supcom.rnu.tn).

W. Mathlouthi, P. Lemieux, L. A. Rusch, and M. Roy are with the Centred’Optique, Photonique, et Laser (COPL), Department of Electrical and Com-puter Engineering, Université Laval, Québec, QC G1K 7P4, Canada (e-mail:rusch@gel.ulaval.ca).

Digital Object Identifier 10.1109/JLT.2004.838817

wider optical bandwidth is required for SS-WDM, reducingthe spectral efficiency and increasing the effects of dispersion.Similarly for OCDMA, reduction of beat noise is essential toexploit the advantages of sharing the same spectral band byseveral users at the same time, that is, for increased networkcapacity, enhanced network flexibility, and ease of support foran increasing subscriber base.

The cross-gain modulation (XGM) mechanism in semicon-ductor optical amplifiers (SOAs) is used for the first time in orderto reduce the intensity noise via conversion of the modulated in-coherent optical signal into a coherent signal before the detectionprocess. A continuous-wave (CW) probe laser together with theincoherent modulated channel is injected into the SOA. The inco-herent optical signal modulates the gain of the SOA by depletingthe carriers. The CW laser encounters the modulated gain and isthus modulated by the inverted data carried initially by the inco-herent optical signal. The proposed scheme is also applicable tolast-mile systems using spectrum sliced sources that are in turnconverted to a coherent WDM network, thereby exploiting si-multaneous wavelength conversion of the broad-band source.

This proposal should be placed in context with other workusing SOAs, and in particular XGM, for wavelength conversionand for intensity noise mitigation. Here, previous work is catego-rized as 1) coherent-to-coherent conversion (predominantly forwavelength conversion) and 2) incoherent-to-incoherent con-version (predominantly for intensity noise mitigation). Whileno other work has addressed incoherent-to-coherent conversion,many aspects of previous work are relevant. For instance, theBER results attainable for any converted signal that is coherentis compared, whether the initial signal is coherent or incoherent.

All-optical wavelength converters for coherent optical com-munication can exploit SOAs for coherent-to-coherent conver-sion, either via the XGM [4]–[6] or the cross-phase modula-tion (XPM) [7]. The XGM type exploits the gain variation thatoccurs when an input signal changes the carrier density in anSOA. The XPM type is based on a Mach–Zehnder interferom-eter that uses refractive-index change. The advantages of thesewavelength conversion devices using SOAs include compact-ness, high conversion efficiency, and polarization insensitivity[8]. In [4], the authors demonstrate that a long SOA has supe-rior dynamic performance. In addition, they show that XGM-based converters degrade the extinction ratio when passing fromshorter wavelengths to longer ones, thus introducing a power

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penalty for the converted signal, especially when converters arecascaded [4], [9].

Incoherent-to-incoherent conversion exploits the SOA toreduce intensity noise and improve BER by modulating theSOA amplified spontaneous emission (ASE) [10]–[12]. Themodulated input light saturates the SOA as the input powerincreases, resulting in a decrease in the ASE power. Essentially,the SOA converts the modulated input signal to a modulatedASE signal that is the logical inverse. For SS-WDM systems,a single channel is filtered out by the receiver and sent to theSOA to modulate the ASE. Since the ASE is much larger inbandwidth than the SS-WDM channel, the signal-to-noise ratio(SNR), which is proportional to the optical bandwidth dividedby the electrical bandwidth, is improved. The spectral effi-ciency of the SS-WDM system is unaffected, as the bandwidthexpansion to improve signal-to-noise ratio (SNR) takes placeat the receiver and not the transmitter.1 Experimental resultsshowed better BER performance for the converted signal dueto the greater optical bandwidth at detection. This scheme wasalso used as a simple wavelength conversion device by taking aslice of the inverted ASE at the new desired wavelength [13].

The proposed use of SOAs for incoherent-to-coherent con-version relies on the complex XGM mechanism (described bythe traveling rate equations reported in Section IV). Ideally, theauthors would like to select the operation point of the SOA tomaximize the BER at reception. Failing that, they would maxi-mize the SNR or the output extinction ratio. The BER and SNRoptimizations require significant analysis and are outside thescope of this paper. Certain characteristics of the SOA, how-ever, can be easily verified experimentally or via simulation:for example, gain versus input power for a given wavelengthand injection current. These characteristics are sufficient for pre-dicting behavior of the output extinction ratio, which is thereforeadopted as a figure of merit. This ratio can be maximized in astraightforward manner via curves of gain versus input power,by seeking out the SOA operating point offering the greatestpossible gain excursion. The gain profile can be adjusted as afunction of the SOA operating point, both experimentally (withSOA hardware on hand) and via simulation for other SOAs re-ported in the literature.

The paper is organized as follows. In Section II, experimentalresults are presented that select the SOA operating point that op-timizes the output extinction ratio for the conversion process. In-coherent-to-coherent conversion and coherent-to-coherent con-version are considered. In Section III, experimental BER resultsare presented and the mechanisms for elimination of the BERfloor are discussed. In Section IV, a numerical model of the SOAand simulation results for an SOA optimized for this applicationare presented. Finally, the main findings of the paper are sum-marized in Section V.

II. EXPERIMENTAL OPTIMIZATION OF OPERATING POINT

As described in the introduction, we use the XGM mecha-nism in SOAs to reduce the intensity noise via conversion ofthe modulated incoherent optical signal into a coherent signal

1Contrast this approach with the proposal in this paper of incoherent-to-co-herent conversion where the photodetector receives coherent, nonthermal light.

Fig. 1. Experimental setup to find SOA operating point yielding greatest gainexcursion.

before the detection process. A CW probe beam, together withthe incoherent modulated channel, is injected into the SOA. TheCW laser is modulated by the gain that varies according to themodulated input signal. In this section, we seek the operatingparameters of the SOA, as follows:

1) CW wavelength;2) wavelength band of the incoherent signal;3) input power of the incoherent and coherent signals;4) injection current of the SOA;5) direction of propagation of the coherent probe signal.While ideally we wish to optimize BER performance, this

analysis is intractable given the complexity of the equationsdefining the gain dynamics of the SOA. We therefore choose in-stead to optimize the output extinction ratio for the converted co-herent signal. In this section, we present experimental validationthat the chosen operating point maximizes the output extinctionratio for the SOA available in our laboratory. A numerical SOAmodel will be used in Section V to simulate extinction-ratio im-provement that could be found for a SOA that was optimized forthis application.

Our experimental setup is depicted in Fig. 1. A CW signalfrom a tunable distributed feedback (DFB) laser is injected intoa Kamelian optical preamplifier SOA, via an optical isolator, incounterpropagation. This laser will be extracted from the otherside of the SOA via the third port of an optical circulator. Thefirst port of the circulator is connected to the optical channel,whereas the second port is connected to the SOA. In two sepa-rate experiments, either a coherent DFB laser or an incoherentoptical signal is input to the external modulator. The incoherentsignal was obtained by slicing the ASE from an erbium-dopedfiber amplifier (EDFA) using a 1.2-nm tunable bandpass filter.

A higher extinction ratio on the converted channel can be ob-tained by maximizing its gain excursion, thereby increasing eyeopening. We compare the extinction ratio before and after con-version for two cases: 1) conversion of a modulated incoherentinput signal to a coherent output and 2) wavelength conversionof a modulated coherent input signal to a coherent output [17].The input signal is externally modulated with an extinction ratioof 11 dB. As the output extinction-ratio optimization is unaf-fected by bit rate, for convenience we optimize parameters at abit rate of 622 Mb/s, while later BER measurements are takenat 2.5 Gb/s. The incoherent signal is generated by slicing theASE from an EDFA using a 1.2-nm tunable bandpass filter.The EDFA offered maximum power in the incoherent signalaround the area of the 1550 nm. A center frequency of 1553 nmwas selected as it maximizes the extinction ratio at the outputof the Kamelian SOA, subject to maximizing the power in the

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Fig. 2. Measured extinction ratio of the converted signal as a function of theCW input power for a laser as modulated channel (solid line) and incoherentspectrum-sliced modulated channel (dashed line), respectively.

SS-WDM signal (about 6.86 dBm for a 1.2-nm optical band-width). The coherent input source (a DFB laser) was also placedat 1553 nm. For our experiment, the output coherent signal (aCW DFB laser introduced in counterpropagation) wavelengthwas adjusted to 1520 nm for convenience, as this parameter hadvery little effect on the gain excursion of the converted channel.

Results for the measured extinction ratio of the convertedsignal for coherent and incoherent cases are presented in Fig. 2.We obtain the maximum extinction ratio for either coherentor incoherent inputs around CW input power 15 dBm, witha minimum extinction ratio penalty of 1.5 and 4 dB, respec-tively, for coherent-to-coherent conversion and incoherent-to-coherent conversion. Note that although the extinction ratio isdegraded for the noncoherent signal, the BER can still mani-fest improvement given a coherent signal is now incident on thephotodetector.

III. BER MEASUREMENT

Having fixed the operating point to maximize the outputextinction ratio, we proceed to measure the BER for theincoherent-to-coherent conversion, as well as the baselineperformance for the coherent-to-coherent conversion. Ourexperimental setup is given in Fig. 3. Fig. 3(a) shows thecounterpropagation scheme requiring a circulator; Fig. 3(b)shows the copropagation scheme using a 2 1 coupler. As weassume the system is not power limited, we chose to place theattenuator at the receiver to exhibit the advantage of conversion.

A. Counterpropagation Experiments

In Fig. 4, we present BER measurement results for 2.5 Gb/s.Curve 1 shows the BER performance for the input DFB laserat 1553 nm with no SOA. No BER floor is present for the re-ceived powers examined. Curve 2 shows the BER performancefor the probe laser at 1520 nm after a coherent-to-coherent con-version exploiting the XGM in the SOA. Performance is con-sistent with results reported in [4] and [5]. The power penaltybetween curves 1 and 2 may be related to the length of the ac-tive region of the Kamelian optical preamplifier SOA [4]. In-deed, better performance in terms of modulation rate and drive

Fig. 3. Experimental setup for BER measurements for (a) the counter-propagating configuration and (b) copropagating configuration.

power budget are expected with longer SOAs that present highergains and thus higher spontaneous emission levels and extinc-tion ratios [5].

Curve 5 shows the BER performance for the SS-WDMwithout passing through the SOA, i.e., without conversion. Thecurve shows a BER floor at 15 dBm received power; that is,for high signal powers, the best attainable BER is . TheBER floor is determined by the ratio , where is theoptical bandwidth (1.2 nm in our case) and is the electricalbandwidth (0.7 times the bit rate). A wider optical filter wouldhave yielded a higher BER floor level. This illustrates themajor drawback to use of spectrum-sliced sources, where BERimprovement requires expansion of the optical bandwidth,hence degraded spectral efficiency.

Next, we present curve 3 for the incoherent-to-coherent con-version. The incoherent source is modulated and input to theSOA. The probe DFB laser at 1520 nm enters the SOA in coun-terpropagation and is routed to the BER tester. The curve forthis converted channel shows the elimination of the BER floor.In our experiments, we were able to achieve error-free trans-mission at a received power of 22 dBm. This result representsthe first proof of the capability to convert an optical incoherentsignal to an inverted optical coherent signal using the XGM inthe SOA [17].

We can compare curves 3 and 5 to assess the savings in spec-tral efficiency that can be attained with incoherent-to-coherentconversion. These curves are for the same optical bandwidth of1.2 nm. In order to attain a quality of service of BER inthe SS-WDM system, we must expand the optical bandwidth to3.6 nm. We assumed an extinction ratio of 10 dB and rectan-gular frequency response for optical and electrical filter shapes;an optical bandwidth of 3.6 nm will shift the BER floor in curve5 down enough to intersect curve 3 at BER [3]. Hence, ourconversion scheme enables more bandwidth-efficient systems.

In Fig. 5, we present the eye diagrams for a received powerof 17 dBm for 1) the SS-WDM externally modulated signal

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Fig. 4. BER measurements at 2.5-Gb/s transmission for various conversion schemes.

Fig. 5. Measured eye diagrams at receiver with (a) SS-WDM channel and(b) incoherent-to-coherent conversion.

before entering the SOA and 2) the modulated coherent signalafter conversion in the SOA via XGM. The coherent signal ex-periences intensity noise induced by the power fluctuations in

Fig. 6. Measured histograms of receiver samples for (a) SS-WDM channeland (b) incoherent-to-coherent conversion.

the modulated incoherent signal. The intensity variation, how-ever, is still much less severe than that of the broad-band source.

The experiments were run at 2.5 Gb/s, close to the maximumresponse time of the SOA. In comparing the eye diagrams inFig. 5, we see the rise-time slope deteriorating after conversion.In migrating to higher bit rates, this effect will become morepronounced and the primary source of errors. The transmissionrate for this intensity noise mitigation method is limited ulti-mately by the SOA dynamic response, a subject of continuingresearch, with results already presented at 10 Gb/s [8]. As men-tioned in [4] and [18], increasing the length of the SOA activeregion and the confinement factor can improve dynamic perfor-mance to achieve higher transmission rates.

In Fig. 6, we present histograms of the receiver samples for1) the SS-WDM system without conversion and 2) after con-version. The effects of greater eye opening are clear here: lessvariance (intensity noise) around the two modes at logical oneand zero, and steeper roll-off in the tails. The extinction ratiomeasures the distance between the modes but does not reflectthe reduced width in the two modes of the distribution. Hence,we achieve better BER with worse extinction ratio.

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Our optimality criterion was maximizing the extinction ratioat the output of the SOA, as instinctively this should leadto greater eye opening and reduced BER. Maximizing BERimprovement is our ultimate goal. Consider Fig. 5 where wepresent eye openings with and without conversion; in Fig. 6,we present the associated distributions of the received inten-sity. Let and be the mean and standard deviation of theintensity without conversion, and and with conversion.Qualitatively, we observe the following:

• without conversion: large intensity noise at logical one,medium intensity noise at logical zero, i.e., , andpoor eye opening;

• with conversion: medium intensity noise at logical oneand zero, i.e., , and improved eye opening.

In Fig. 6, we see that the extinction ratio after conversion is ac-tually worse . The eye opening is improved notbecause of greater extinction ratio, but rather due to reduced in-tensity noise: . When optimizing the outputextinction ratio, we assumed the intensity noise reduction wasindependent of the operating point. Thus, for a given standarddeviation (intensity noise) about the mean value for the outputsignal, increasing the extinction ratio would increase the eyeopening and improve the BER. In the future, we hope to chooseour operating point by effecting a joint optimization of bothextinction ratio and intensity noise to reach the greatest BERimprovement.

B. Copropagation Experiments

Finally, we examined the performance of incoherent-to-coherent conversion in copropagation, as seen in Fig. 3(b). Incurve 4 of Fig. 4, the probe laser at 1520 nm enters the SOAin copropagation with the incoherent signal. We observe againthe elimination of the BER floor; however, the performanceimprovement is lower than in counterpropagation. The dif-ference between these two measurements (copropagation andcounterpropagation) is the ASE power level accompanying theconverted channel. More ASE power is present with coprop-agation of the converted channel with the incoherent signal.This is due to the SOA emitting less ASE power from the sideaccepting more input power. Indeed, the ASE level in bothdirections (forward ASE+ and backward ASE-) depends onthe evolution of the carrier density along the active region.With a very low input power level, the distribution of carrierdensity is symmetric, and we get approximately the same ASEpower level in both directions. If we increase the input powerlevel, the maximum carrier density moves toward the injectingside and leads to more ASE power in the forward direction.For our case, the CW DFB laser is placed at 1520 nm with

15-dBm input power, and the average modulated input powerof the SS-WDM is 6.86 dBm with an input extinction ratioof 11 dB at 1553 nm. In both cases, the ASE+ power is greater,representing a small handicap for the copropagating convertedchannel versus counterpropagation.

IV. SIMULATION RESULTS FOR AN OPTIMIZED SOA

The Kamelian SOA used in our experiment was designedto have a flat-gain profile over a wide input bandwidth. For

incoherent-to-coherent conversion, we do not need this widebandwidth. Indeed, by having the gain concentrated over asmaller bandwidth, the gain could be higher and offer greatergain excursion to improve the output extinction ratio. Whileonly the Kamelian SOA was available for experimental results,we examine another SOA design to confirm that a greateroutput extinction ratio could be achieved. We simulate boththe Kamelian SOA and a smaller bandwidth SOA reportedin Connelly [16] (chosen due to the availability of simulationparameters and the wide acceptance of these simulation re-sults). We compare the output extinction ratio for each SOA asa function of both 1) input extinction ratio and 2) conversionwavelengths. Whereas the parameters from Connelly are pro-vided, we must infer these parameters from measurements ofthe Kamelian SOA, as described in the next section.

A. SOA Model

Many SOA models presented in the literature [14], [15] usesimplifying assumptions that do not permit accurate predictionand understanding of the XGM effect. We use the wide-bandmodel for a bulk In–InGaAsP SOA presented by Connelly in[16], which adequately accounts for internal parameters and dy-namic behavior and can be characterized using measurable ex-ternal variables such as output power and ASE.

This wide-band model is based on numerical solution of cou-pled equations for carrier-density rate and traveling-wave (orphoton rate) equations for forward- and backward-propagatingsignals and spectral components of ASE powers. In the ampli-fier, the spatially varying component of the field due to eachsignal placed at optical frequency ( to ), where

is the number of injected signals, can be decomposed intotwo complex traveling-wave equations for the signal fieldand , propagating in the positive and negative directions,respectively. Each signal obeys the complex traveling-waveequations [16]

(1)where is the signal propagation coefficient, the opticalconfinement factor, the material gain coefficient,

the carrier density, and the material loss coefficient. Itis assumed that the squared modulus of the amplitude of thetraveling wave is equal to the photon rate of the wave in thatdirection, so . On the other hand, andare defined as the spontaneous emission photon rates for a par-ticular polarization in a frequency spacing centeredon frequency traveling in the positive and negative direc-tions, respectively, where is defined by [16, eq. (31)], isa positive integer, and is the longitudinal mode frequencyspacing described by [16, eq. (33)]. Those spontaneous emis-sion photon rates observe the equation [16]

(2)where represents the spontaneously emittednoise coupled into or .

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The carrier density at acquires the form [16]

(3)

where is the bias current, the electric charge, the activeregion thickness, the active region length, the central ac-tive region width, the recombination rate term, thenumber of injected signals, and the number of spectral com-ponents of ASE powers.

For the steady-state solution, we used an iterative solutionbased on splitting the amplifier into a number of sections la-beled from to and assuming the steady-state numericalalgorithm described with further details in [16]. The time evolu-tion of the carrier-density rate depends only on current bias leveland the input fluxes. In other words, we adjust the carrier den-sity and then we compute the coefficients of the traveling-waveequations. Subsequently, the signal fields and noise photon den-sities are predicted.

B. SOA Characterization

In order to model the Kamelian SOA, we characterize it viastatic measurements. This characterization takes into accountthe output power level and the ASE profile in order to approxi-mate the material gain coefficient . We model thematerial gain as having a Lorentzian line shape and fit this withexperimental measurements

(4)

where the gain constant is , the carrier density at the trans-parency is 1.2 m , the 3-dB bandwidth of the lineargain coefficient is , and isthe spectral shift, where is the peak wavelength transparencyand is a constant characterizing the gain-peak shift. To accu-rately model the small-signal ( 15-dBm input power) gain ofthe Kamelian SOA over the 1490- to 1600-nm wavelength rangeshown in Fig. 8(a), we varied , , and in (4). For the Con-nelly SOA, we used 122.5 nm, m ,and 1639 nm.

In Fig. 7, the modeled and measured Kamelian SOA outputspectra are presented; the input signal power is 15 dBm at1550 nm, and the bias current is 130 mA. The difference be-tween the predicted and experimental signal gain is less than0.2 dB; however, this model is less precise for the ASE powerlevel. Indeed, the difference between predicted and measuredASE reaches 5 dB in the region of 1450 nm. In fact, we do notfit the ASE profile, but we use the spontaneously emitted noiseexpressed by

(5)

where is the gain coefficient as defined by [16, eq. (16)].

Fig. 7. Predicted and measured Kamelian SOA output power spectrum as afunction of wavelength; the input signal at 1550 nm, �15 dBm, and the biascurrent is 130 mA.

As mentioned previously, we believe a less spectrally wideSOA could offer greater gain and thus improved output extinc-tion ratio. To verify this hypothesis, we simulate two other SOAdesigns based on the widely accepted model of Connelly [16].The output extinction ratio is influenced by the small-signal gainand the saturation power. We therefore compare the Kamelianwith the Connelly SOA when the SOAs have equal small-signalgain and when the two SOAs have equal saturation power. Wewill see in the next section that the greatest gain excursion isachieved for the case of equal saturation power due to the sig-nificant increase in small-signal gain for this case.

Fig. 8(a) and (b) shows the predicted and measured fiber-to-fiber signal gain as a function of wavelength and input power,respectively. In Fig. 8(a), the gain is shown for different wave-lengths while keeping constant the input power at 15 dBm;the current bias is 65 mA (Connelly with equal small-signalgain), 125 mA (Kamelian), or 130 mA (Connelly with equalsaturation power). The dashed and dotted lines show thesimulated Connelly SOA using the material gain coefficient

and parameters in [16, Table 1]. The solid lineand asterisks represent the simulated and measured KamelianSOA. Note the peaked response of the Connelly SOAs com-pared with the Kamelian SOA wide, flat response.

Fig. 8(b) shows the signal gain observed at 1535 nm fordifferent input power levels. We see clearly the property ofequal small-signal gain on the dotted Connelly SOA curve andapproximately equal saturation power for the dashed ConnellySOA curve. Note the saturation power is 4.23 dBm for theKamelian with 130 mA as the bias current compared with4.58 dBm for the Connelly SOA using parameters in [16] and125-mA bias current.

C. Comparison of Three SOA Designs

For our experiment, we consider one input extinction ratio,one conversion wavelength, and one input power; in our simu-lations, we consider three input extinction ratios and the gamutof conversion wavelengths. We will compare output extinctionratios for the Kamelian versus the equal-small-signal-gain Con-nelly SOA [Fig. 9(a)] and Kamelian versus equal-saturation-

MENIF et al.: ERROR-FREE TRANSMISSION 293

(a)

(b)

Fig. 8. Predicted (markers) and measured (line) fiber-to-fiber signal gain:(a) as a function of wavelength with �15-dBm input power and (b) as afunction of input power for 1535-nm signal wavelength.

power Connelly SOA [Fig. 9(b)]. Recall that in experimentalmeasurements, we reached the maximum output extinction ratioon the converted signal for both DFB laser and SS-WDM inputs,with a CW input power of 15 dBm at 1520 nm, a modulatedinput power of 6 dBm at 1553 nm, and an input extinctionratio of 11 dB.

In Fig. 9, we present the output extinction ratio for the SOAsunder comparison as a function of the wavelength of the probeCW laser signal. There are six curves, three for each SOA. Thethree curves correspond to input extinction ratios of 15, 10, and5 dB (represented by the solid, dashed, and dotted curves, respec-tively) for the input-modulated broad-band source. The threecurves for the Kamelian SOA show little sensitivity to the wave-length of the probe CW signal, while the Connelly SOA (bold

Fig. 9. Output extinction ratio of the converted CW signal as a function ofits wavelength for two different SOAs and for three different input extinctionratios. Input-modulated broad-band signal at 1553 nm and �5-dBm inputpower; maximum power of the CW channel is�15 dBm. (a) Equal small-signalgain. (b) Equal saturation power.

lines) shows clearer maxima for wavelengths in the vicinity of1515 nm. When the small-signal gain is equal [Fig. 9(a)], wesee no improvement in the output extinction ratio.

For SOAs with equal saturation power [Fig. 9(b)], in all casesthe Connelly SOA shows greater output extinction ratio thanthe Kamelian SOA due to the narrower spectrum that enablesthe difference in small-signal gain. We see that the maximumoutput extinction ratio is within 0.5 dB of the input extinctionratio, meaning little penalty in the conversion process. In all thesimulations, the input-modulated broad-band signal is placedat 1553 nm with 5 dBm as input power, the bias current is130 mA, and the power of the CW channel is 15 dBm.

V. CONCLUSION

For the first time, conversion of an optical incoherent signalto an inverted optical coherent signal using the XGM in anSOA is presented. This process can be useful for the all-opticalconversion of broad-band signals for 1) SS-WDM, 2) last-milesystems of SS-WDM joining coherent WDM networks, and 3)OCDMA systems such as fast-frequency-hopped CDMA. Theexperimental setup in this paper demonstrated the elimination of

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the BER floor. An SOA with a more peaked gain profile wouldhave given even better performance by increasing the outputSNR. Future work will examine the efficiency of conversion fordifferent SOA gain profiles, optical bandwidths, and bit rates.

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M. Menif (S’99) was born in Tunis, Tunisia, on April 23, 1973. He received theB.S.E.E., M.S.E.E., and D.E.A. degrees from the École Nationale d’Ingénieursde Tunis, Tunis, and the Ph.D. degree in electrical engineering from the Univer-sité Laval, Québec, QC, Canada, in the field of optical fiber communications in2002.

He is currently a Maitre-Assistant at the École Supérieure des Communica-tions de Tunis, Tunisia. His research interests include transient gain analysis oferbium-doped fiber amplifiers (EDFAs) fed by high-speed self-similar traffic forwavelength-division-multiplexing long-haul and access networks.

W. Mathlouthi received the B.S.E.E., M.S.E.E.,and D.E.A. degrees from the École Nationale d’In-génieurs de Tunis, Tunis. He is currently workingtoward the Ph.D. degree in electrical engineering atthe Université Laval, Québec, QC, Canada.

His research interests include semiconductoroptical amplifiers, spectrum-sliced wavelength-di-vision-multiplexing, fiber Bragg gratings, op-tical-code-division multiple access, and wirelessoptical communications.

P. Lemieux received the B.S. degree in engineeringphysics from Université Laval, Québec, QC, Canada,in 2004. He is currently working toward the Master’sdegree in optical telecommunications at the Centred’Optique, Photonique et Laser, Department of Elec-trical and Computer Engineering, Université Laval.

His research interests include semiconductoroptical amplifiers, spectrum-sliced wavelength-di-vision-multiplexing, fiber Bragg gratings, op-tical-code-division multiple access, and wirelessoptical communications.

L. A. Rusch (S’91–M’94–SM’00) received theB.S.E.E. (honors) degree from the California Insti-tute of Technology, Pasadena, in 1980 and the M.A.and Ph.D. degrees in electrical engineering fromPrinceton University, Princeton, NJ, in 1992 and1994, respectively.

She was previously the manager of a groupresearching new wireless technologies at Intel Corp.from 2001 to 2002. She is currently a Full Professorin the Department of Electrical and Computer En-gineering at Université Laval, Québec, QC, Canada,

performing research in wireless and optical communications. Her researchinterests include optical-code-division multiple access using noncoherentsources for metropolitan area networks; semiconductor and erbium-dopedoptical amplifiers and their dynamics; and in wireless communications, highperformance, reduced complexity receivers for ultra-wide-band systemsemploying code-division multiple access.

M. Roy received the B.S. degree in engineering physics from Université Laval,Québec, QC, Canada, in 2004. He is currently working toward the Master’sdegree at University of Toronto, ON, Canada.