IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 12, JUNE 15, 2013 1125

Full-Duplex 4-PAM Transmission for CapacityUpgrade in Loop-Back PONs

Ioannis Lazarou, Stefanos Dris, Paraskevas Bakopoulos, Bernhard Schrenk, and Hercules Avramopoulos

Abstract— We report for the first time on the feasibility ofquaternary (4-ary) pulse amplitude modulation (4-PAM) forboth downstream (DS) and upstream (US) directions in a full-duplex passive optical network (PON) with wavelength reuse.The scheme is validated with a simple, low-cost, and colorlessoptical network unit (ONU) exploiting the all-optical carrierrecovery scheme. The ONU employs a high-finesse optical filterwith periodic frequency response for carrier extraction anddata erasure of the DS signal. A cascade of two semiconductoroptical amplifiers (SOAs) amplifies and modulates the recoveredcarrier with the 4-PAM US data-stream. The proposed schemeconstitutes a migration scenario towards low-cost TDM/WDMPONs with currently deployed hardware equipment, ensuringfull-duplex transmission up to 26 km, with higher spectralefficiencies and auspicious wavelength allocation.

Index Terms— 4-PAM, All-Optical carrier recovery, fiber tothe Home (FTTH), optical access networks, optical network unit(ONU), wavelength reuse, WDM PON.

I. INTRODUCTION

THE constant need for even greater connectivity and thegradual penetration of emerging applications such as

Ultra HD video, 3D TV and free viewpoint video, are push-ing network bandwidth requirements even further. Accordingto the latest forecast reports in 2016 the annual global IPtraffic is expected to surpass the zetabyte threshold, whereasglobal mobile data will grow three times faster than fixedIP traffic [1]. In order to cope with this demand, near-futureoptical access networks should be more flexible, bandwidth-competitive and must be able to offer enhanced operationspeeds and high spectral efficiencies to the end-user. Since amixed WDM/TDM solution would be the strongest candidatefor NG-PON2 according to the latest announcements of theFSAN community [2], the colorless operation and low costconstraints of the ONUs constitute an inevitable necessity.In this context, the use of reflective SOAs [3], as well ascascaded reflective electroabsorption modulators (EAMs) andSOAs have been reported as amplified solutions towardswavelength-agnostic US transmitters (TX), with small form

Manuscript received January 8, 2013; revised March 5, 2013 and April 18,2013; accepted April 19, 2013. Date of publication May 1, 2013; date ofcurrent version May 24, 2013. This work was supported in part by the GreekSecretariat for Research and Technology under Project PANDA 09�YN-71-839.

I. Lazarou, S. Dris, P. Bakopoulos, and H. Avramopoulos are withthe Photonics Communications Research Laboratory, National TechnicalUniversity of Athens, Athens 15780, Greece (e-mail: ilazarou@mail.ntua.gr;sdris@mail.ntua.gr; pbakop@mail.ntua.gr; hav@mail.ntua.gr).

B. Schrenk is with the Austrian Institute of Technology, Vienna 1220,Austria (e-mail: bernhard.schrenk@ait.ac.at)

Color versions of one or more of the figures in this letter are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LPT.2013.2260533

Fig. 1. Proposed ONU with all optical wavelength reuse scheme.

factor, low power consumption and reduced expenditures [4].Recently, wavelength reuse techniques in WDM–PONs havebeen proposed as an elegant way for further reducing the costand the complexity of the ONUs. By these means, a moreauspicious allocation of the wavelengths and thus a higherspectral efficiency of the optical channel can be achieved. Sofar the demonstrated wavelength-reuse schemes [5]–[10] donot favor the US in terms of spectral efficiency, employingsimple binary modulation. However, considering 4-PAM onboth DS and US as an alternative, the spectral efficiency andbit-rate can be doubled. Additionally, since 4-PAM offers thelowest implementation complexity of all advanced modulationformats and still increases the dispersion-limited distance [22],simple and low cost TX circuitry can be used at the ONU andOLT.

In this letter we demonstrate for the first time the feasibilityof 4-PAM full-duplex transmission with wavelength reuse inloop-back PONs. The proposed scenario incorporates low-drive TXs both at the OLT and the ONU, with reduced costand complexity. For simplicity reasons it is proven in anasymmetric 20/2 Gb/s WDM-PON, with the US limited onlyby the low electro-optical bandwidth of the SOA employed.Our results indicate successful transmission for DS and USup to 26 km without dispersion compensation, constituting anattractive solution for short-reach TDM/DWDM PONs withenhanced spectral efficiency and increased bit-rates.

II. PRINCIPLE OF WAVELENGTH REUSE

AND PROPOSED ONU

The wavelength reuse technique in PAM signals, which isused for loop-back-type optical access networks, is mainlybased on the process of erasing the data pattern of a DSand subsequently remodulating with the US data, on thesame wavelength. The common characteristic of all methodsaddressed in the literature is that a reduced extinction ratio(ER) of the DS is required, as a tradeoff between the receptionpenalties of both signals. This reduction of the ER can berealized by decreasing the electrical voltage that drives theoptical modulator and thus reducing its modulation-depth.

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1126 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 12, JUNE 15, 2013

Fig. 2. (a) Experimental setup and proof of concept ONU; (b) SOA’s gain spectrum at 200 mA (black solid line) versus the 100 GHz λs grid (dotted redline). PS: Phase shifter. VA: Variable attenuator. RN: Remote node. ODN: Optical distribution network. PPG: Pulse pattern generator.

Fig. 3. Normalized spectra of the: extracted carrier, DS and US.

In the time domain and during the photodetection process,this is practically manifested as an additional DC offset tothe zero-level of the DS signal. Likewise in the frequencydomain, the optical spectrum of a low ER M-ary PAM signalappears to have its data harmonics suppressed compared to theoptical carrier. Hence, on one hand this reduction in the RFswing relaxes the power requirements at the OLT TX, and onthe other hand error-free detection is still possible at the ONUreceiver (RX), despite the partial degradation in the DS signal’squality. Additionally, in the case where a low-drive opticalTX such as an EAM is used, no RF amplifiers are needed,further decreasing the power consumption demands at the OLTpremises. This compromise between the reception penaltiesof both DS and US signals can be outreached to a highdegree, by utilizing the all-optical wavelength reuse schemeat the ONU, which employs a tunable optical filter with high-finesse and a periodic frequency response. By these meansthe transmission of DS signals with ER>9 dB is enabled,while still maintaining acceptable US transmission quality andensuring the colorless operation of the ONU [11].

The proposed ONU configuration is based on the all-optical wavelength reuse scheme (Fig.1) and its fundamentalbuilding blocks are the DS Rx, a tunable high finesse opticalfilter with a periodic frequency response, and a dual-sectionSOA. The ONU Rx comprises a combination of a avalanchephotodiode (APD) and a transimpedance amplifier (TIA) forthe proper detection and amplification of the 4-PAM DS signal.The sharp optical filter is responsible for recovering the opticalcarrier and suppressing the data-pattern of the DS signal. Inprinciple, this operation can be implemented by any highfinesse filter (e.g. Fabry Perot (FP), Bragg Gratings), howeverfocusing on a more integrated and stable form of the ONU,micro-ring resonators [12] should be preferred.

The following cascade of the two SOAs can be treatedas stand-alone components or as an integrated multi-segmentSOA with individual electrodes [13]. In the first case, the lead-ing SOA amplifies the extracted carrier and further suppressesthe remaining DS pattern [5], whereas the subsequent SOAdirectly modulates it with the electrical four-level US pattern.Depending on the saturation conditions of the second SOA andthe electrical voltage-swing of the injected pattern, the resul-tant optical output can be purely 4-PAM, or alternatively, relyon quadrature phase shift keying (QPSK). Whilst the combina-tion of coherent detection and QPSK modulation undoubtedlyleads to higher loss budgets and less signal distortions duringtransmission [14], we have chosen 4-PAM for the US fortwo reasons: Firstly, by maintaining a pure PAM on both USand DS signals, compliance with the current PON standardsis preserved, and secondly the proposed scenario provides asimpler path for migrating low-cost WDM PONs, by aug-menting the spectral efficiency without a disruptive changein the transmitting and receiving subsystems. Also, an indi-rect advantage of incorporating higher order PAM encodinginstead of the simple 2-PAM for both transmission directions,is that the crucial parameter of the number of consecutiveidentical digits is advantageously reduced to�R/n�, where Rand n denote the Pseudorandom Binary Sequence (PRBS)and the PAM order, respectively. Similarly, in the case wherean integrated two-section SOA is used, the aforementionedfunctionalities are also preserved. The additional advantageof such a configuration is that of the optional modulationstage which can be offered by the leading SOA, in additionto the amplification stage. Therefore, the optical 4-PAM UScan be generated in two stages, by simply applying two-levelelectrical signals with unequal voltage swings to each section.Thus if the phase-matching between these sections is carefullyadjusted [15], enhanced bandwidth operation [16] and low-chirp modulated signals [17] are possible, yielding enhancedflexibility in the ONU TX.

III. EXPERIMENTAL SETUP AND PROOF OF

CONCEPT ONU

Fig. 2(a) depicts the WDM-PON configuration setup, imple-mented for the evaluation of a proof-of-concept ONU. At theOLT side, the DS TX comprises a laser diode (LD) emittingat 1559.9 nm and an EAM for generating the optical 4-PAMsignal. The latter was electrically driven by a four-level data

LAZAROU et al.: FULL-DUPLEX 4-PAM TRANSMISSION FOR CAPACITY UPGRADE IN LOOP-BACK PONs 1127

Fig. 4. (a)-(b). Eye diagrams and BER curves of the DS for: i) B2B, ii) after 26 km with, and iii) without the simultaneous transmission of the US signal.(c)-(d) BER curves and eye diagrams of the US signal for: B2B and after 26 km transmission.

pattern with amplitude swing of only 0.75 Vpp. The electrical4-ary bit-stream was generated by a pulse pattern generator(PPG) through splitting, amplifying, decorrelating and com-bining two 27-1 PRBS bit streams, using a variable attenuatorand a delay-line at one of the outputs of the electrical splitter.The optimum biasing point of the EAM was realized usingthe method described in [18]. The optical injection power tothe optical distribution network (ODN) and the ER of theDS signal were 8.5 dBm and 9 dB, respectively. The OLTRX consisted of a two-stage Erbium Doped Fiber Amplifier(EDFA) with a 1nm optical bandpass filter (OBF) betweenits two stages, serving as a demultiplexing and ASE-filteringelement, and a commercial 10 GHz PIN diode. Due to thelack of electrical equipment, the detected 4-ary electrical datacoming from the PIN diode was captured with a 50 GS/s real-time oscilloscope for symbol-by-symbol signal decoding witha multilevel slicer implemented in DSP. No post-distortionor additional post-processing technique was applied to thereceived signal. The ODN employed a dual-feeder fiber linkwith 25.4 km of standard single mode fiber for the mitigationof Rayleigh back scattering (RB) stemming from the DS [21],and a drop fiber of 600 m length. The PON’s wavelengthdistributing element was emulated with a 1 nm OBF. The lossbudget defined between the OLT and the ONU for the seedand US signals, was fixed at 20 dB with the variable opticalattenuators (VOA) AN and AS respectively. For the DS eval-uation, no attenuation was applied with the VOA AN, due tothe low sensitivity Rx employed at the ONU.

At the ONU side, a 3 dB coupler served as a power-splitterof the received signal, with one of its outputs leading theDS data for detection. Owing to the absence of an APD anda linear TIA at the ONU Rx, the latter was implementedusing a commercial 11 GHz linear photoreceiver, with powersensitivity of -10 dBm. The FP placed at the second outputof the coupler was responsible for the extraction of the DScarrier and its subsequent modulation erasure. The intrinsiccharacteristics of the FP, namely finesse, free spectral rangeand total losses (filtering and insertion losses) were 1350,396 GHz and 4 dB respectively. Due to the lack of a linearSOA, the extracted carrier was amplified with a 20 dB gainEDFA with 6 dB noise figure, which in principle can emulatethe small-signal gain of a commercial SOA. The US data wasthen imprinted on the seed wavelength that is the extracted

DS carrier, by directly modulating a low-drive 1.5 mm longSOA with ∼1GHz modulation bandwidth. For this proofof concept ONU, the SOA was electrically fed with a 1Gbaud (2 Gb/s) 4-ary signal pattern of 1.5 Vpp which wasgenerated by decorrelating and combining the complementaryPRBS outputs of a PPG. To achieve a symmetrical 4-PAMeye diagram, the electrical driving signal was pre-distortedby means of adjusting the relevant amplitudes of the OOKtributaries so that the generated electrical levels result inequidistant gain figures in the SOA. Given that the SOA gainis also dependent on the power of the seed optical signal,the electrical levels were fine-tuned for different input opticalpower levels to maintain a symmetric 4-PAM eye diagram.The low-drive operation of the ONU TX was pursued in orderto comply with the needs for low power consumption, lowcomplexity and low cost ONUs, and was achieved through thereduced impedance of the SOA, which was driven without 50-Ohm RF impedance matching. It should be noted that in caseof a higher US data rate electrical reflections caused by theimpedance mismatch would severely degrade the US, however,for the given rate and the short RF feed line no degradation wasobserved. No electrical equalization circuit was used before theSOA for bandwidth enhancement. The ONU’s output powerwas measured to be −3 dBm and the measured signal-to-RBratio for the DS reception was >40 dB.

IV. RESULTS AND DISCUSSION

Fig. 3 illustrates the normalized spectra of the DS, theextracted carrier and the US signals, which are located at thesame optical frequency due to the wavelength reuse scheme.As can be seen, the data harmonics of the DS are compressedby 17 dB compared to the optical carrier due to the reducedmodulation ER. Moreover, owing to the ∼30 nm effectiveoptical bandwidth of the SOA (Fig. 2(b)) and the use of thetunable FP with a periodic frequency response, the ONU’scolorless operation is retained. The 1st column of Fig. 4(a)depicts the eye-diagrams of the 20 Gb/s DS at error-freereception, (that is zero errors with the acquired symbols), whilethe 2nd column those near the FEC-RS (255, 223) limit forthe following cases: Back-to-Back (B2B), after 26 km withoutand with the presence of the US. A comparative study ofthese cases was also evaluated in terms of Bit Error Rate

1128 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 25, NO. 12, JUNE 15, 2013

(BER) measurements, placing the VOA AN in front of theONU. The BER evaluation of the DS was estimated over2 · 106 bits and as shown in Fig. 4(b) detection at error ratesbelow the FEC limit was achieved in all instances, with the26 km-transmitted DS exhibiting a 3.6 dB power penalty atFEC level, compared to the B2B case. Also, as expected,due to the RS mitigation with dual feeder fibers at the ODNand the low ONU output power, the back-propagating USdoes not affect the quality of the DS reception. Given thatthe sensitivity of the employed ONU RX was -10 dBm, noadditional fixed attenuation was applied with the AN VOA atthe ONU input. For this reason the ONU received power atFig. 4(b), was adjusted to correspond to the performance of acommercial APD+TIA Rx with power sensitivity of -28 dBm.Thus, considering an extra power penalty of 4.8 dB due to4-PAM [22], the DS compatible loss budget was 30.5 dB forthe FEC and 32.8 dB for Turbo-FEC cases, leaving a powermargin of 18 dB and 20.3 dB respectively.

Similarly the US was evaluated with comparative BERmeasurements and eye diagrams over 2 · 105 bits, placing theVOA AS in front of the OLT (Fig. 4(c)-(d)). In both of theB2B and the 26 km-transmitted US, error floors were observedat the BER levels of 5·10−6, with the latter exhibiting a powerpenalty of 1.7 dB with respect to the former. Nevertheless, theBER performance of the US could be further improved, as anexchange for an additional power penalty at the BER curve ofthe DS, by a further decrease of its ER [5]. In this context,the compatible loss budget for the US was evaluated at 26 dBwith FEC and 30 dB if Turbo-FEC is employed, with powermargins of 16.7 dB and 20.7 dB correspondingly. However,one common drawback of WDM PONs with wavelength reuseis the limited compatible loss budget of the seed signal. Inour case this seed wavelength is delivered from the carrierextraction of the DS, and its loss budget was measured to be15.6 dB, limiting the overall system performance. The mainreason for this relies to the rather high optical power of 0 dBm,needed for saturating the long SOA and generating the 4-PAMUS in an efficient manner. Alternatively, employing a SOAwhich operates in the saturated regime with less input opticalpower could eventually increase the overall loss budget forthe seed signal. Also, it should be noted that even though ourproof-of-concept ONU was practically restricted to 1 Gbaudfor the US due to the SOA’s limited bandwidth, higher bitrates(>10 Gb/s) are feasible. Multi-section SOAs with a higherbandwidth as in [19] or sophisticated electronic equalizationat the OLT RX [20] can be used, in order to overcome thebandwidth limitation barrier.

V. CONCLUSION

In this letter we demonstrated for the first time the fea-sibility of 4-PAM generation for both DS and US, targetedfor loop-back PONs with wavelength reuse. The proposedcolorless ONU employs a tunable sharp filter with periodicfrequency response for all-optical carrier recovery and twocascaded SOAs for the 4-PAM US generation. The presentedscenario stands a simple and cost-effective solution, ensuringan upgrade for higher spectral efficiencies in PONs, withoutdisruptively affecting OLT’s currently deployed equipment.

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