JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 8, APRIL 15, 2013 1323

Combined Self-Seeding and Carrier RemodulationScheme for WDM-PON

Ulysses Rondina Duarte, Rivael Strobel Penze, F. R. Pereira, F. F. Padela, Joao Batista Rosolem, andMurilo Araujo Romero

Abstract—In this work, we propose a novel self-seededWDM-PON topology. Unlike previous works, in our case, theself-seeding occurs in RSOAs located at the OLT, allowing up-stream transmission based on carrier remodulation at the ONT.Thus, a single wavelength per user can be employed for both trafficdirections while eliminating the need for an external broadbandlight source at the CO. A maximum bidirectional 2.5 dB powerpenalty for a BER of was obtained for an injection powerlevel of dBm into the ONT RSOAs. We also investigate thetolerance of downstream BER to downstream extinction ratio

and self-seeding power , as well as analyze theimpact of upstream extinction level , injection power

and levels on upstream BER performance, onsymmetric 1.25 Gbps OOK transmission over 20 km withoutDTLA and FFCI mechanisms. We have successfully demonstrateddownstream error free transmission (BER of ) for anas low as 3.5 dB.

Index Terms—Passive optical networks, reflective semicon-ductor optical amplifiers, self-seeding, wavelength divisionmultiplexing.

I. INTRODUCTION

I N a typical wavelength division multiplexing passive op-tical network (WDM-PON) system, each optical network

termination (ONT) requires its own upstream wavelength. Thisposes a barrier for commercial deployment because the simplestsolution, to use fixed wavelength transmitters at the customerpremises, creates a formidable challenge for the network oper-ator, regarding inventory administration and operational expen-diture (OPEX) costs.This issue has been identified since the very early days of

WDM-PON research. The answer is to develop colorless ONTs,in which the same basic optoelectronic hardware can be em-ployed to provide any wavelength required by a specific net-

Manuscript received November 14, 2012; revised January 23, 2013; acceptedFebruary 15, 2013. Date of publication February 21, 2013; date of current ver-sion March 08, 2013. This work was supported by the Fund for Technolog-ical Development of Telecommunications, of the Brazilian Ministry of Com-munications (FUNTTEL) and Financing Agency for Studies and Projects, ofthe Brazilian Ministry of Science and Technology (FINEP) under 100 GETHProject no. 01.09. 0629.00 (2623/09 - FINEP).U. R. Duarte, R. S. Penze, F. R. Pereira, F. F. Padela, and J. B. Rosolem are

with the Research and Development Center in Telecommunications (CPqD),Campinas, SP 13086-902, Brazil (e-mail: uduarte@cpqd.com.br).M. A. Romero is with University of Sao Paulo, Electrical and Computer

Engineering Department, Sao Carlos, SP 13566-590, Brazil (e-mail: murilo.romero@usp.br).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/JLT.2013.2248076

work client. For example, it is possible to use identical tunablelasers at each ONT, where each laser is then tuned to a specificand pre-assigned upstream wavelength. As a long-term alterna-tive, the use of wavelength tunable sources is very promising. Todate, however, the technology lacks full maturity to allow actualcommercial deployment. In particular, the cost of tunable lasersis still prohibitive as a subscriber equipment, although cost-re-lated issues are been actively tackled and progress is beingmade[1].In optical access networks high performance cannot be

achieved at the expense of cost-effectiveness. The under-standing that both features are simultaneously needed hasmotivated several research groups to study WDM-PON con-figurations which do not employ a coherent light source atthe ONT. Those colorless ONTs are often based on reflectivedevices, such as reflective semiconductor optical amplifiers(RSOAs) and the optical signal required for the upstreamcarrier, denominated optical seed, is generally provided by theoptical line terminal (OLT) at the central office (CO) [2]–[10].A more recent variation is the self-seeding scheme, in which

the optical seed required for upstream transmission is providedby the RSOA itself. [11]. Specifically, the ASE output light fromeach RSOA is spectrally sliced by an arrayed waveguides grat-ings (AWG) at a remote node (RN) and injected back into theRSOA through a reflective optical path. The scheme uses iden-tical RSOAs at the ONTs and colorless operation is achievedbecause each AWG arm will give rise to a distinct wavelengthseed.In this paper, we explore the self-seeding technique to pro-

pose a novel WDM-PON topology. Unlike previous works, inour case, the self-seeding occurs in RSOAs located at the OLT,allowing upstream transmission based on carrier remodulationat the ONT. Thus, a single wavelength per user can be employedfor both traffic directions while eliminating the need for an ex-ternal broadband light source at the CO. In the following sec-tion, we discuss the literature on RSOA-based WDM-PON net-works, in order to place our work in the proper context. Nextthe experimental setup and results are described in analyzed indetail. Section VI concludes the paper.

II. WDM-PON TOPOLOGIES BASED ON RSOAS

The several investigations registered in the literature re-garding the use of RSOAs in WDM-PONs can be grouped intotwo main research lines. The first alternative is to employ dis-tinct wavelengths for upstream and downstream data [2]. In oneof those propositions both downstream and upstream carrierare created from a wavelength-specific optical seed, providedfor each RSOA in the network by means of narrowband AWG

0733-8724/$31.00 © 2013 IEEE

1324 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 8, APRIL 15, 2013

spectral slicing of the optical output coming from a broadbandlight source (BLS), usually the amplified spontaneous emission(ASE) arising from an erbium-doped fiber amplifier (EDFA)located at the CO.For upstream transmission, the optical seed is sent through

the optical link to the ONT, where it is amplified and modulatedwith subscriber data by the RSOA and sent back to OLT, in aloopback architecture. Colorless operation is achieved becauseeach identical RSOA receives a distinct seeding wavelength [2].However, the assignment of distinct wavelengths for upstreamand downstream signals leads to increased OPEX costs.The alternative approach consists of wavelength reuse at the

ONT side. In this scheme, a fraction of the optical power of thedownstream wavelength carriers are amplitude (re)modulatedwith subscriber data and sent back to the CO [3]. Downstreamdata is erased by operating the RSOA in deep saturation andcrosstalk between downstream and upstream data can be fur-ther minimized by using very unequal extinction ratios for theamplitude modulated traffic channels in each way, supported byproper electronic hardware for downstream remodulation [4].This remodulation scheme allows the sharing of a unique

wavelength for both data flows, enabling more efficient op-tical spectral usage. However, the use of narrowband ASE asseed light brings up several transmission impairments, suchas Rayleigh backscattering, crosstalk, ASE-ASE beat noise,chromatic dispersion (CD), and post filtering effect, which canbe severe in case of amplitude-remodulated transmission [5],[6]. It is possible to improve system performance by using thepre-spectrum slicing method [7], [8] in order to suppress therelative intensity noise (RIN) degradation caused by the postfiltering effect. Another approach is based on phase-encodedformats for downstream transmission, while upstream trafficis still based on the on-off keying (OOK) modulation [9].However, those solutions are not cost-effective, due to the useof customized broadband light sources, distributed feedbacklasers and sophisticated receivers.As a consequence, the first of the described approaches, the

centralized BLS topology, has attracted a great deal of interest inrecent years because it allows an easy and cost-effective man-agement of the optical apparatus for both the subscriber sideand CO plant, due to the sharing of a single BLS for the entiresystem [2]–[8], [10]. On the other hand, this reliance on a singleBLS creates a vulnerability issue since a BLS failure will shutdown the whole WDM-PON network. In addition, the central-ized scheme establishes a dependence of the upstream perfor-mance on the BLS seed characteristics, such as optical spectrumflatness, RIN, and power levels, limiting the upstream powerbudget and number of available channels.In this context, the self-seeding technique allows the network

designer to eliminate the need for a centralized BLS at the COfor upstream carrier generation [11]–[16]. The RIN is usuallylow enough to allow direct modulation of the RSOA (typicalRIN averaged values is about dB/Hz [12]) and stable op-eration of the self-seeding process can be assured by placinga Faraday rotator mirror (FRM) into the optical feedback path,thereby reducing the performance degradation caused by the po-larization dependent gain of the RSOA [13].

Although the self-seeding stability can be enhanced bymeans of polarization control techniques [14], a drawback forall self-injection topologies reported so far is the need of distinctwavelength bands for each direction of transmission (wherethe L-band is usually employed for the downstream). Thisexcessive spectral usage limits the system capacity, increasesOPEX costs and it is detrimental for widespread deployment.In this context, we propose a novel WDM-PON access

topology which combines features and benefits of both theself-seeding process and the remodulation scheme. The maingoal is to establish a network which uses a single-fiber andthe assignment of single wavelength band for both trafficdirections, while still using self-seeding to avoid the sharing ofa single and specific BLS source.Specifically, in the large majority of previous works, the

self-seeding process takes place at the ONT, in the customerpremises. One of the few exceptions is the very recent manu-script by Ma et al. [15], in which self-seeded RSOAs are usedto generate both the downstream and upstream carriers, at theOLT and ONT, respectively. Our configuration differs fromthose previous investigations because our WDM-PON systemis based on self-seeding at the OLT, only. In our case, upstreamtransmission relies on remodulation of the downstream modu-lated carrier, since the ONT is also RSOA-based, in such waythat the same downstream carrier wavelength can be amplified,remodulated and sent back to the CO. Compared to [15], asimplification of wavelength management is achieved becauseonly half of the RSOAs need to be self-seeded.In the next sections, we describe the measurements on the up-

stream bit error rate (BER) dependence to the injection powerlevels at the ONT RSOA and downstream extinction ratio onsymmetric 1.25 Gbps transmission over 20 km. A maximum 2.5dB power penalty is demonstrated for an injection power levelof dBm into the ONT RSOA without any support fromadditional carrier erasure mechanisms. The proposed topologyeliminates the dependence from a shared BLS at the OLT whileincreasing the tolerance to chromatic dispersion, improving theupstream power budget and sharing a single wavelength for up-stream and downstream transmission.To the best of our knowledge, the topology being detailed

here is the first experimental demonstration of combined self-seeding and carrier remodulation scheme. Compared to our pre-liminary investigation [17], we report, for the first time, theperformance evaluation of remodulated upstream signals in re-gard to upstream extinction ratio as well as to the differenceof respective downstream and upstream extinction ratios levels.Also, we characterize self-seeded downstream signals and eval-uate the downstream BER performance in regard to the down-stream extinction ratio levels (in the range of 5.5 dB to 3.0 dB)as well as investigate the impact of self-seeding power on down-stream performance. Finally, we measure the power penalties asa function of extinction ratio levels.

III. EXPERIMENTAL SETUP

Fig. 1 depicts the experimental arrangement, based onself-seeding and carrier remodulation, deployed in the 100 GHzWDM grid. A 20 km bidirectional feeder optical fiber (0.22

DUARTE et al.: COMBINED SELF-SEEDING AND CARRIER REMODULATION SCHEME 1325

Fig. 1. Experimental configuration deployed, based on self-seeding at the OLT and carrier remodulation at the ONT. In the inset, the OLT RSOA includes anEDFA for optical power boosting. The parameters studied in this work are also defined (see adjoining table).

dB/km of attenuation loss) connects the OLT to the subscriberONTs and three AWGs (5.5 dB of maximum insertion loss and0.5 nm of spectral width) are used in the proposed scheme. Incontrast to most previous works, in our setup, the self-seedingprocess takes place at the OLT, allowing the sharing of asingle FRM and a 10/90 power divider among all downstreamtransmitters. The self-seeding wavelength carriers are sentdownstream and remodulated at the ONTs for upstream trans-mission. During the measurements, the operating wavelengthof the RSOAs was set to 1550.91 nm. The 1.25 Gbps receiversare based on a PIN-TIA combination, in order to assure acost-effective configuration. Also, no temperature control wasemployed on the RSOAs.Both OLT and ONT transmitters are based on identical

C-band RSOAs. The main features of those devices are the lowdependence of the optical gain on the incoming light polariza-tion (PDG dB), small signal gain of 21 dB and saturationpower of 3 dBm. The devices were biased at 90 mA and 60mA at the OLT and the ONT sides, respectively. Regarding theOLT, the inset of Fig. 1 indicates that an additional EDFA anda couple of circulators were arranged in such way to emulatea high-gain and high-power RSOA, similar to the one used inprevious investigations [11]. This was necessary because theRSOAs available in our laboratory were not capable of providethe minimum optical seed power level needed to achieve astable self-seeding, assuring a maximum launched power atthe optical fiber (point B, Fig. 1) about dBm in 1550,91nm (typical values of launched power at feeder fiber are in therange of dBm [11] to dBm [13]). Thus, the maximumavailable injection power at RSOAs ONT is about dBm,

accounting for losses of 4.5 dB (20 km of SMF) plus 5.5 dB(AWG insertion loss at the RN) plus 0.6 dB (10/90 opticalcoupler at the ONT site).During the experiments, the RSOAs, at the OLT and ONT,

were directly modulated with data streams in the non return tozero (NRZ) format. In order to decorrelate the bit sequences,distinct pseudo random bit sequences (PRBS) were used,

and , respectively, for downstream and upstreamchannels. In the network configuration depicted in Fig. 1, thedownstream carriers are generated at the OLT by using theself-seeding technique. Next, the NRZ modulated carriers aresent downstream through 20 km of a feeder optical fiber, at atransmission rate of 1.25 Gbps, to reach the AWG at the RN.In the proposed topology, after reaching the RN, the optical

signals are then wavelength demultiplexed and addressed to itsown specific ONT. At the ONT side, we placed a 10/90 opticalpower divider in order to allow carrier remodulation. In otherwords, 10% of the downstream optical power is diverted to theONT receiver while the remaining 90% are pumped into theONT RSOA, remodulated with client data and sent back up-stream. Finally, at the OLT front-end, an optical circulator as-sures proper separation of downstream and upstream data.

IV. FEATURES OF THE SELF-SEEDED RSOA SIGNALS

As discussed in the introduction, the use of ASE-based onexternal seed light in the conventional remodulation scheme canpose a severe challenge for upstream transmission, since thosedata flows suffer from major penalties. Specifically, dependingon the AWGs filtering transfer functions, the impairment effectsrelated to linewidth of seed light, such as CD and post-filtering

1326 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 8, APRIL 15, 2013

Fig. 2. Comparison of the optical spectrum from self-seeded RSOA signalsand emulated narrowband ASE. The spectral resolution was set to 0.03 nm.

effect, could both limit the optical power budget and prevent theachievement of error free transmission.Keeping this problem in mind, we measured the optical spec-

trum of the RSOA output (at point B) under two conditions:self-seeded signal and spectrum sliced signal (obtained whenthe optical reflect path is disconnected, emulating the well knownarrowband spectrum from ASE-based on BLS). The goal wasto establish a spectral comparison of the seeded light from theOLT RSOA against the sliced signal usually employed as a seedfor remodulation purposes. Fig. 2 shows the results.According to the results, the linewidth of self-seeded RSOA

output (9.37 GHz) is about a sixth of that from spectrum slicedsignals (57.5 GHz), dictated by the spectral width of the AWGfilter (62.5 GHz). This reduction allows a tolerance increase onboth post-filtering and chromatic dispersion effects, eliminatingthe need for dispersion management.If polarization control issues are overcome, a narrower

linewidth (about 2.5 MHz) can be achieved by using a di-rectly modulated self-seeded Fabry-Pérot (FP) cavity, due toits natural coherent behavior, as reported by Presi et al. [18].However, when targeting high transmission rates and cost-ef-fectiveness in a long-term evolution, from 1.25 Gbps to 10Gbps, the directly modulated self-seeded FP is unable to ad-dress the challenge due to the relaxations oscillations, intrinsicto semiconductor lasers, which limits the bit rate transmissionto a few Gbps. On the other hand, the frequency response ofRSOA has a smooth rolloff with no relaxation oscillation peakand a classical low-pass behavior, which enables its operationat 10 Gbps supperted by proper electronic equalization tech-nique at receiver site while still keeping straightforward OOKmodulation.

V. PERFORMANCE EVALUATION

A. Self-Seeded Downstream Signals

Taking Fig. 1 as a reference, we monitored a set of points(A), (B), (C), (D), (E). Each one of those sites has a particularsignificance. At point (A) we measured and controlled the op-tical power levels reinjected into the OLT RSOA after the first

Fig. 3. Downstream BER measurements, for 3 distinct values of .

round-trip of the self-seeding process (RSOA-AWG-power di-vider-FRM-power divider-AWG-RSOA), namely .Our specific goal here is to evaluate the behavior of the

self-seeding process as a function of the injected optical powerlevel into the RSOA. After establishing the optical self-seedingcavity, our aim at point (B) is to measure and control the opticalpower effectively launched into the feeder fiber. Finally, points(C), (D) and (E) we monitor the optical power levels reachingan ONT RSOA, the ONT receiver (Rx DW) and the OLTreceiver (Rx UP), respectively (see Fig. 1).The first step was to evaluate our self-seeding arrangement,

when compared to the results provided by the literature. Fig. 3shows the BER measurement results as a function of the re-ceived power at point (D), the ONT receiver, for a few values ofthe self-seeding power ( dBm, dBm e dBmof optical peak power, measured over 0.2 nm of optical spec-trum resolution at point A). For power levels of at least

dBm/0.2 nm it is possible to achieve a satisfactory perfor-mance, with no BER floor. Those values of are consistentwith those registered in the literature, about dBm/0.2 nmfor a distinct topology [16]. In all the downstream analysis tofollow, was kept dBm/0.2 nm. It should be pointed theself-seeding performance could be enhanced with a fairly tighttemperature control (within a few degrees Celsius) as carriedout in previous works [11], [13].

B. Proof of Concept Measurements

1) Extinction Ratios Levels: In order to investigate thefeasibility of the proposed topology in the absence of carriererasure mechanisms, we conducted an initial set of proof ofconcept measurements. First, we investigated the results of theremodulation procedure as function of NRZ extinction ratio(ER) for the two traffic flows, upstream and downstream. In theabsence of carrier erasure mechanisms such as feed-forwardcurrent injection (FFCI) and decision threshold level adjust-ment (DTLA), the literature suggests that somewhat similarWDM-PON topologies, operating at 1.25 Gbps but using abroadband light source for wavelength seeding (instead of theself-seeding employed here) requires, at least, a 7 dB differencebetween the ER values for downstream and upstream signals(typically, 3 and 10 dB, respectively [10]). In addition, the

DUARTE et al.: COMBINED SELF-SEEDING AND CARRIER REMODULATION SCHEME 1327

Fig. 4. Experimental values of extinction ratio measured both for ER down-stream (self-seeding carrier) and upstream (carrier remodulation) in CWoperation.

injected power into the ONT RSOA is often kept belowdBm, in order to suppress the residual downstream extinctionratio on the upstream signal [10], avoiding a BER floor.On the other hand, it should be pointed out that the low(3 dB) required for downstream remodulation at ONT

site (in the absence of erasure mechanisms) could severelylimit the downstream performance in conventional narrowbandASE seed topology, causing a floor behavior at ONT receiveras well as preventing the achievement of error free region

. Typically, a minimum value of 6 dB isrequired for downstream signals in order to assure a reasonableperformance, even in the presence of FFCI and/or DTLAtechniques [3], [4].In this context, Fig. 4 shows the ER values extracted from

our oscilloscope at measurement points (B) and (E) for severalvalues of peak-to-peak (Vpp) applied to the RSOAs at the OLTand ONT referred to as and , respectively. The op-tical power levels and the injection power were keptat nm and dBm, respectively. We must alsopoint out the was obtained by keeping the self-seedingdownstream carrier in continuous-wave (CW) operation.Accordingly to our results, operating the OLT RSOA in satu-

ration (90 mA of DC bias) allowed a maximum value of ofabout 5.5 dB,measured at point (B), due the optical recirculatingfrom self-seeding process. Values reported in the literature arealso in a similar range, 5.5 to 6.0 dB [11]. On the other hand, itis possible to achieve values as high as 11 dB for the upstreamextinction ratio when a Vpp voltage of 2.2 volts is applied to theONT RSOA. As a result, a difference between andas high as 5.5 dB is established even in the best operating condi-tion for both OLT and ONT sides (i.e., 2.2 V). This value shouldguarantee full-duplex transmission if the level is kept lowerthan dBm, meaning that we could increase the upstreampower budget, while relaxing the value, without drasti-cally limiting the downstream performance.2) Impact of on Self-Seeded Downstream Transmis-

sion: Regarding the discussion in the previous section, in ourthird experiment we sought to assess the downstream systemperformance as a function of the downstream extinction ratio

Fig. 5. Downstream BER measurement results for several values of .

. The goal was to define the minimum value of forwhich it was still possible to achieve a BER of in the ab-sence of floor behavior. Fig. 5 shows those BER measurementresults for distinct values of .Analysis of Fig. 5 indicates that the combination of the self-

seeding and remodulation techniques allows a BER of inthe range of 5.5 dB to 3.5 dB, in this case with a maximum2.5 dB penalty for downstream signals. The correspondent eyediagrams are depicted in the Fig. 6. As the decreases, theneed for carrier erasure mechanisms is relaxed, due to deep gainsaturation in which RSOA is forced to operate. However, theeye diagram height also decreases as the downstream extinctionratio is diminished, establishing a lower limit to values forwhich it was still possible to avoid a BER floor at the ONT side.In a future work, we plan to incorporate additional features

at both transmission and detection sites, by using remodulationtechniques such as FFCI for both self-seed signals and ONTtransmitters and DTLA for OLT receivers. In fact, the extinctionratio of 5.5 dB for signals modulating the self-seeding carrier,should allow a satisfactory efficiency for the FFCI technique,almost completely erasing the downstream carrier. In addition,the use of DTLA allows more relaxed specifications concerningextinction ratios and injection powers. If both techniques areused simultaneously, an upper limit for the system performancecan be determined.3) Best-Case Upstream Performance: In order to identify

this performance upper-limit for upstream signals, the follow-upwas to measure the upstream BER when the downstream self-seeding carrier is operated in CW mode. In other words, the ex-tinction ratio downstream, , is made equal to zero dB. Thisscenario is actually a realistic best-case, because it correspondsto a complete erasing of the downstream wavelength, by meansof techniques such as FFCI and DTLA, as discussed above.Fig. 7 depicts the measured upstream BER for several distinct

values of self-seeding power, , as well as power injected intothe ONTRSOA, . The extinction ratio for the upstream data,

, was set to its maximum value, 11 dB. We registered avery small sensitivity variation, from dBm to dBmfor a BER of , as showed in Fig. 7(a). Therefore, therewas no significant difference on the upstream performance, re-garding the distinct sets of optical power levels we tested. Onthe basis of injected power, only, there is a 10 dB margin to

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Fig. 6. Downstream eye diagrams as a function of . The vertical scale was set to 142 W/div. The black mark refers to ground level.

Fig. 7. Upstream performance evaluation in the best-case scenario: (a) up-stream BER for several values of self-seeding power and injected powerinto the ONT RSOA ; and (b) upstream eye diagrams for some values of

keeping at dBm/0.2 nm.

increase the system reach to maybe about 60–70 km with thesame BER. This conclusion is supported by the open eye dia-grams observed in the Fig. 7(b). An experimental investigationon this regard is under way.

C. Bidirectional Transmission Measurements

Next, we departed from the best-case scenario to evaluatethe performance degradation suffered by the upstream signalswhen the extinction ratio downstream is increased. The valuesof and were kept at dBm, dBm/0.2nm and 11 dB, respectively, and Fig. 8 illustrates the measuredupstreamBER for some values of . Also, the correspondingdownstream BER from Fig. 5 are plotted together to allow abetter understanding of our results.As expected, there is a trade-off between upstream and down-

stream performances, regarding the best choice of the parameter. The value of 5.5 dB, which provided the best results

for downstream transmission, creates a BER floor at the OLT

Fig. 8. Upstream and downstream BER measurements for some values ofdownstream extinction ratio, .

receiver. In contrast, to set equal to 3.5 dB allows an up-stream performance very close to possible with complete carriererasing. On the other hand, a penalty of 2.5 dB is observed forthe downstream transmission (in comparison to the best case,

of 5.5 dB). An intermediate selection of around4.5 seems adequate, with moderate penalties compared to eachbest-case: around 1 dB downstream and 1.5 dB upstream, re-spectively, for a BER of . These results indicate we coulddecrease even further both and values without com-promising the system performance, while increasing the up-stream power margin.Thus, in order to investigate the maximum upstream power

budget offered by this WDM topology, we measured both up-stream and downstream power penalties for a BER offor some values of injection power, while keeping theat 11 dB (best upstream condition), as showed in the Fig. 9.The power penalties were calculated taking the best-case resultsfrom Figs. 5, 7 and 8 as reference.According to Fig. 9, it is possible to achieve an upstream in-

jection power margin of 10 dB (an increase of 7 dB in the powermargin, when compared to previous work [10]) with a max-imum bidirectional penalty of 2.5 dB, in the absence of down-stream floor behavior (see Fig. 5). For the maximum injectionpower available ( dBm), it is possible to achieve upstreamfree error region even for an as high as 5.0 dB. Fig. 10

DUARTE et al.: COMBINED SELF-SEEDING AND CARRIER REMODULATION SCHEME 1329

Fig. 9. Upstream power penalties as a function of downstream extinction ratiofor some values of . The downstream power penalty across the

range is also depicted. The dotted vertical line establishes the downstream BERfloor region (left side).

Fig. 10. Upstream eye diagrams as a function of . The vertical scale wasset to 131 W/div. The black mark refers to ground level.

shows the upstream eye diagram as a function of , keepingat dBm.

As a final result, in order to investigate the tolerance of up-stream performance to and the respective difference be-tween the ER values for downstream and upstream signals, wemeasured the upstream power penalties across the range,for an BER of and some values of , while keepingthe to an intermediary value ( dBm). Fig. 11 summa-rizes the above discussion. Defining a maximum power penaltyof 3 dB (not considering the BER floor region), the differenceof ER values is about 6 dB, which is consistent with results pre-viously reported in the literature.On a final note, it should be pointed out that in our topology

the self-seeding process takes place at the OLT. Therefore, com-pared tomostWDM-PON implementations, the use of a central-ized light source (CLS) is still retained. However, in our work,each OLT RSOA generates its own the downstream signal aswell as the seed light to its corresponding ONT. This scenarioallows immunity to the catastrophic failure of a single and spe-cific BLS, which can be an issue in the conventional RSOAremodulation scheme. Also, the characteristics of self-seededdownstream signals, such as low and ultra-narrowband

Fig. 11. Upstream power penalties as a function of downstream extinction ratiofor some values of . The downstream power penalty across therange is also depicted. The both dotted (left side) and dashed-dotted (right

side) vertical lines establish the downstream and upstream BER floor regions,respectively.

seed, improve the tolerance of data flows to chromatic disper-sion and ASE-ASE beat noise, relaxing the ONT remodula-tion requirements and increasing power budget without any sup-port from electronic hardware and/or additional optical erasuretechniques.

VI. CONCLUSION

In this work we experimentally demonstrated the configura-tion of a novel bidirectional WDM-PON topology combiningthe self-seeding technique and remodulation schemes on sym-metric 1.25 Gbps transmission over 20 km. We evaluated thebidirectional performance of both data flows, self-seeded down-stream and remodulated upstream, in regard to several param-eters, such as and . A maximum 3 dBpower penalty for a BER of was achieved, as well as anincrease of 7 dB on the injection power margin in the absence ofDTLA and FFCI mechanisms or additional optical techniques.Also, our solution can provide a more cost-effective configu-ration due to assignment of the same wavelength for both dataflowswhile excluding the sharing of external BLS. Further workis under way to expand the topology to a long reach scenario, byusing improved electronic hardware for both downstream gen-eration and upstream remodulation.

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Ulysses Rondina Duartewas born in Barretos, SP, Brazil, in 1985. He receivedthe B.S. degree in physics from the Federal University of Sao Carlos (UFSCar),Sao Carlos, SP, Brazil in 2007 and the M.Sc. degree in electric engineering fromUniversity of Sao Paulo (USP), Sao Carlos, SP, Brazil in 2011, where he is alsoworking towards the Ph.D. degree.He joined the Telecommunication Research and Development Center

(CPqD), Campinas, Brazil, in 2010 as a Telecommunication Researcher.He has been involved in the design and characterization of next generationpassive optical networks (NG-PON). He is currently the leading researcher inWDM-PON R&D at CPqD.

Rivael Strobel Penze was born in Bandeirantes, MS, Brazil, in 1973. He re-ceived the B.S. degree in electric engineering from the Federal University ofMato Grosso (1997), Cuiaba, MT, Brazil and the M.Sc. degree from the Camp-inas University, Campinas, SP, Brazil.Since 2002, he has been a researcher of CPqD Telecommunication Research

and Development Center, Campinas, Brazil. He has been involved in researchof telecommunications networks (optical access network), optical sensors andgeographic information system (GIS).

F. R. Pereira, biography not available at time of publication.

F. F. Padela, biography not available at time of publication.

Joao Batista Rosolem was born in Fartura, SP, Brazil, in 1963. He received theB.S., M.Sc., and Ph.D. degrees in electric engineering from the University ofSao Paulo, São Carlos, Brazil, in 1986, 1990, and 2005, respectively.Since 1990, he has been a researcher of CPqD Telecommunication Research

and Development Center, Campinas, Brazil. He has been involved in the designand characterization of optical system and research in optical amplifiers andoptical sensors. He has also been engaged in the development of trunk and ac-cess wavelength division multiplexing (WDM) transmission systems. He is theholder of three U.S. patents and seven additional patents are pending in Brazil.Dr. Rosolem is member of Optical Society of America (OSA) and author of

over 100 journal and conference papers.

Murilo Araujo Romero was born in Rio de Janeiro, RJ, Brazil, in 1965. Hereceived the B.S. and M.Sc. degrees in 1988 and 1991, both from the CatholicUniversity of Rio de Janeiro, Brazil. In 1995, he received the Ph.D. degree fromDrexel University, Philadelphia, PA, USA. His thesis work dealt with opticallycontrolled microwave devices and high-speed photodetectors for optically-feedphased array antennas.After his return to Brazil in 1995, he joined the University of Sao Paulo,

at Sao Carlos, as a faculty member. At the University of Sao Paulo (USP),he became an Associate Professor in 2001 and a Full Professor in 2008. Heis now the Head of the Electrical Engineering Department at EESC-USP andChair of the Electrical and Biomedical Engineering Committee of the BrazilianResearch Council (CNPq). His research interests span over a large variety oftopics in the microwave-photonics area, including microwave semiconductordevices and circuits, optical amplifiers, optical fibers, and optical networks.Samples of his research work can be found in 43 journal papers, including man-uscripts at IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES,IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, IEEE TRANSACTIONSON ELECTRON DEVICES, IEEE JOURNAL OF QUANTUM ELECTRONICS, IEEEPHOTONICS TECHNOLOGY LETTERS, IEEE/OSA JOURNAL OF LIGHTWAVETECHNOLOGY, IEEE SENSORS, Electronics Letters, Applied Optics and OpticsExpress, among other journals.