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Demonstration of a Remotely Dual-Pumped Long-Reach PON for Flexible Deployment Bernhard Schrenk, Jose A. Lazaro, Dimitrios Klonidis, Francesc Bonada, Fabienne Saliou, Victor Polo, Eduardo T. Lopez, Quang T. Le, Philippe Chanclou, Liliana Costa, Antonio Teixeira, Sotiria Chatzi, Ioannis Tomkos, Giorgio M. Tosi Beleffi, Dmitri Leino, Risto Soila, Spiros Spirou, Guilhem de Valicourt, Romain Brenot, Christophe Kazmierski, and Josep Prat

Abstract—We propose and experimentally demonstrate a flexible wavelength division multiplexing/time division multiplexing network architecture for converged metro-access environment. Entire passiveness in the fiber plant is achieved with remote amplification in the signal distribution nodes along the metro ring and in the power splitters of the local access tree. We assist a traditional remote pumping scheme with a distributed pump provided by the optical network units and demonstrate that loss budgets beyond 30 dB can be supported. Data transmission of up to 10 Gb/s is evaluated in different deployment scenarios, reaching from a 78 km long reach rural to a dense 1:128 split/ urban configuration with field installed fibers, including also worst case resilience configurations. Manuscript received November 22, 2011; revised January 18, 2012; accepted January 19, 2012. Date of publication January 31, 2012; date of current version February 24, 2012. This work was supported in part by the European FP7 SARDANA, EURO-FOS, and FUTON Projects, and the Spanish MICINN/FEDER TEC2008-01887 project. B. Schrenk was with the Technical University of Catalonia, Barcelona 08034, Spain. He is now with the School of Electrical and Computer Engineering, National Technical University of Athens, 15773 Athens, Greece (e-mail: [email protected]). J. A. Lazaro, F. Bonada, V. Polo, E. T. Lopez, S. Chatzi, and J. Prat are with the Department of Signal Theory and Communication, Technical University of Catalonia, 08034 Barcelona, Spain (e-mail: [email protected]; [email protected]; [email protected]; [email protected]; [email protected]; [email protected]). D. Klonidis, and I. Tomkos are with Athens Information Technology, 19002 Athens, Greece (e-mail: [email protected]; [email protected]). F. Saliou and P. Chanclou are with Orange Labs, 22307 Lannion, France (e-mail: [email protected]; [email protected]). Q. T. Le was with Orange Labs, 22307 Lannion, France. He is now with the Technical University of Darmstadt, 64283 Darmstadt, Germany (e-mail: [email protected]). L. Costa and A. Teixeira are with the Instituto de Telecomunicações, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal (e-mail: [email protected]; [email protected]). G. M. Tosi Beleffi is with the Italian Ministry of Economic Development Communication Department ISCOM, 00144 Rome, Italy (e-mail: [email protected]). D. Leino and R. Soila are with Tellabs Oy, 02630 Espoo, Finland (e-mail: [email protected]; [email protected]). S. Spirou is with Intracom S.A. Telecom Solutions, 19002 Athens, Greece (e-mail: [email protected]). G. de Valicourt was with III–V Lab, a Joint Laboratory of “Alcatel-Lucent,” “Thales Research & Technology,” and “CEA LETI,” 91767 Palaiseau Cedex, France. He is now with WDM Dynamic Networks Department, Alcatel-Lucent Bell Laboratories, 91620 Nozay, France(e-mail: [email protected]). R. Brenot and C. Kazmierski are with III–V Lab, a Joint Laboratory of “Alcatel-Lucent,” “Thales Research & Technology,” and “CEA LETI,” 91767 Palaiseau Cedex, France (e-mail: [email protected]; [email protected]). 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.2012.2185781

Index Terms—Fiber to the home, optical access, optical fiber communication, optical pumping, passive optical network (PON).

I. INTRODUCTION

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VER the last years, the internet explosion has lead to an enormous traffic growth. Optical backbone networks have advanced over the last decade in order to keep pace with this trend [1]. Lightwave systems have recently invaded the access segment, where traditional copper-based equipment is now replaced by its optical counterparts. Evidence has been provided that fiber to the home can be kept economically viable once the optical distribution plant is designed as fully passive optical network (PON) and shared among a high number of users, being also able to scale as core networks do. Time division multiplexing (TDM) is a simple solution to reach a shared passive infrastructure; however, it imposes stringent limits for the guaranteed data rate per user. On the other hand, wavelength division multiplexing (WDM) retains the granted data rate as in case of a dedicated fiber per user by introduction of virtual point-to-point links without power splitting. Data rates of up to 25 Gb/s have been demonstrated in combination with reflective modulators (REMs) at the optical network unit (ONU) [2]. Though the benefits of the WDM dimension are obvious, WDM-PONs with simple intensity modulation/direct detection-based ONUs have never been demonstrated in conjunction with high customer densities so far. A deployment of purely WDM-based access networks with a weakly shared fiber infrastructure would lead to a high number of separated networks to reach all the users in a certain geographical area. As a consequence, a large number of optical line terminals (OLTs) would be required, which is against the current trend of OLT consolidation. A mix of WDM and TDM toward a hybrid PON can trade-off the sustained data rate with a highly shared infrastructure. Recent efforts have demonstrated long-reach WDM/TDM PONs in-line with the trend of OLT consolidation and network aggregation, outlining also the matureness of colorless ONUs for their mass deployment [3], [4]. Nevertheless, the high optical loss budgets that are typically bound to such dense network architectures with extended reach require means of extra optical amplification. While the traditional way of placing electrically powered amplifiers in the optical distribution network violates the rule of being fully passive in the fiber plant, a remote pumping scheme requires careful dimensioning in case of long-reach networks.

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Fig. 1. Converged ring tree metro-access PON with passive OADNs as RNs.

In this paper, a resilient ring+tree PON with colorless ONUs is demonstrated in different network scenarios. The configurations under investigation are chosen from a 78 km OLT-ONU reach rural to an urban 1:128 split/ scenario with field-deployed fibers. Extended loss budgets of dB are proven to be compatible with a truly passive fiber plant by advancing traditional remote amplification schemes with controlled recycling of optical noise emission. This paper is organized as follows. Section II highlights the state of the art of metro-access networks and relates it with the proposed architecture. Section III describes the network subsystems and applied amplification scheme in detail. Section IV presents the signal evolution and transmission performance in different network scenarios. Finally, Section V concludes this paper. II. CONVERGED PASSIVE METRO-ACCESS NETWORK WITH REMOTELY PUMPED AMPLIFIERS The convergence of metro and access segments into a single PON emerges from the traffic collection of different TDM-PONs and the consolidation of the OLTs thereof toward a single point of presence. Instead of deploying a rather high number of OLTs in the field, dedicated to each of the TDM-PONs, passive optical add/drop nodes (OADN) relay the lower bandwidth traffic of these spur-like networks via a higher capacity WDM ring to a single central office (CO) (see Fig. 1). While ring-like metro distribution networks can additionally provide resiliency due to their specific topology, the exploitation of the wavelength multiplexing dimension allows us to extend the customer density of the PON by far. This can be easily achieved by dedicating different wavelengths to the trees of the network. However, to provide economic effectiveness, colored legacy PON customer premises equipment needs to be replaced by a colorless counterpart to retain the possibility for mass deployment of a unique ONU at all trees of the PON. Such colorless ONUs can either contain a tunable laser, or, as a more cost-effective solution nowadays, an REM that is remotely seeded from the CO. With the convergence of the network segments and the inherently introduced reach extension, the loss budget of the PON is significantly increased and the question for extra optical amplification between OLT and ONU is raised. Earlier approaches have introduced a bank of field-deployed erbium-doped fiber amplifiers (EDFA) at the conjunction point between metro trunk and access trees [3], [5]. As an alternative solution that avoids electrically powered and environmentally conditioned equipment in

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the field, later work has proven the concept of remote amplification in these passive network nodes by distributing a pump together with the data signals from the CO [4], [6]. Though it can be deduced that in case of solely Raman-extended links WDM operation cannot be effectively supported, a combination of Raman amplification and remotely pumped rare-earth-doped fiber amplifiers allows us to effectively extend the reach and compatible loss budget of hybrid PONs [4]. Recently, a distributed pumping scheme assisted by bundled optical noise from the ONUs has been evaluated as well, validating that the high loss budget of strongly increased TDM splitting ratios can be supported [7]. A combined remote pumping scheme from the CO and the ONUs allows a flexible adaptation of resilient reach extension and high split in the tree, as it has been proven in brevity for different network configurations [8] and will be further analyzed in detail in the next sections. III. DUAL-PUMPING SCHEME AND SUBSYSTEM DESIGN The PON architecture contains only unpowered field components and was set up with a dual-fiber WDM ring that feeds several passive add/drop (A/D) modules, referred to as remote nodes (RNs), with the optical signals that are distributed at the local access trees (see Fig. 2). The deployment of the shared ring and tree feeder infrastructure as dual-fiber segment avoids Rayleigh backscattering (RB) effects since counterpropagating signals are physically separated. Two network nodes (RN1, RN2) have been distributed at the ring to assess normal and also resilient network operation in case of a worst case ring cut next to the CO, as illustrated in Fig. 2. The erbium-doped fiber (EDF) pumps required by the RNs can be supplied via OLT or distributed by the ONUs in order to optimize the efficiency of the network. Both RNs and tree splitters are solely based on fully passive, simple, and commercially available components, though the designs may appear as complex at first glance. Three scenarios have been evaluated, reaching from an urban with short reach and high tree split to a rural with long reach and low split. Respective configurations that have been analyzed are listed in Table I together with their tree split and ring ( ), feeder and drop reach. Before the performance of the proposed architecture in these scenarios is discussed, the particular subsystems of the PON are explained in detail. A. CO and Signal Generation The CO hosts several OLT subsystems which are, next to the conventional transceivers for down- and upstream, extended by the pump module that supplies the RNs. The downstream is modulated on (1560.61) nm for RN1 (RN2) via a Mach–Zehnder modulator with variable extinction ratio (ER), in order to investigate how the reduced ER impacts the downstream reception and the upstream transmission. This is of importance since a modulation scheme with wavelength reuse is targeted, so that just a single optical signal has to be delivered to the ONU for the full-duplex transmission of down- and upstream. Besides the fact that the RN design is greatly simplified compared to a dual-wavelength feed for the ONU [9], a more efficient use of optical carriers allows a higher customer density and, in turn, a more beneficial share of common infrastructure.

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Fig. 2. Fully passive PON with remotely pumped nodes supported by a distributed pump delivered by the ONUs and deployed fiber network in Lannion, France. Several network parameters for the scenarios under investigation, such as the fiber lengths – or the tree splitting ratio, are listed in Table I.

The optical carriers at and 1559.79 nm that are delivered to the second tree of the RNs, which are resulting from an optimized RN design as will be explained shortly, were used to account for gain saturation at the RN in usual operation conditions, also providing a beneficial gain stabilization, and left unmodulated. In a real scenario, these signals would carry data signals as it is the case for the former tree wavelength . The tree signals are, then, combined by a WDM multiplexer and boosted by a dual-stage EDFA. The latter holds also a dispersion compensating fiber (DCF) with variable length to assess the impact of residual dispersion and its induced pulse broadening along the single-mode fiber (SMF) spans on the transmission performance of down- and upstream. The OLT receiver for the upstream consists of a dual-stage gain-stabilized EDF preamplifier with a noise figure of 4.7 dB for its highly linear first stage, a DCF with variable dispersion, a longer, gain-stabilized second amplifier stage, a bandpass filter centered with respect to the upstream wavelength for emulating the WDM demultiplexer, and a PIN diode. No protection switch was implemented at the OLT since one-sided ring operation due to a worst case ring cut was considered. A set of ten wavelengths (see inset of Fig. 2) is injected at the OLT into the downstream ring with 3 dBm/ . Thanks to this relative low downstream launch, there is also no nonlinear distortion expected due to multichannel propagation [10]. While four signals are destined for the RNs as described earlier and distributed to their trees, the remaining six channels ( nm) are formed by an amplified spontaneous emission (ASE) sliced comb and act as ballast to account for further users loading the network. In ad-

dition, a pump at 1480 nm is emitted by a fiber Raman laser and transmitted along the upstream ring to supply the remote amplification stages at the RNs. The pump power was fixed to 0.7 W to evaluate the maximum possible reach and loss budget of the PON rather than adapting the launched pump toward a stronger one or extra pump fibers in case of having further RNs along the ring. Thanks to the remote pumping scheme via the upstream ring, the weak upstream signals benefit from Raman gain when passing from the ONU along the ring to the OLT receiver. In order to cope with a potential Raman pump depletion due to a multiple wavelength drop from the downstream ring and insertion into the upstream ring, the comb was reinjected in upstream direction via the ballast ONU. The injection power for these ballast channels was, thereby, adjusted by the attenuator toward the equivalent power of the upstream signals coming from the trees. In a similar manner, the tree signals at the RNs were reinjected into the upstream ring by simple dummy ONUs having the same power as the tree signals at that are passed toward the fully implemented ONUs. Although the dummy ONU does not imprint modulation on , the matched power level with respect to allows us to achieve the same effect of gain stabilization since the gain relaxation time of the EDFs is much larger than the inverse of the date rates used. B. Network Nodes and Remote Amplification Two RNs centered at 1550.92 nm (RN1) and 1560.2 nm (RN2) were deployed at the ring, whereas RN1 was located not farther than half the ring span and RN2 was used to investigate the worst case scenario of having a fiber cut directly

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TABLE I PON CONFIGURATIONS UNDER INVESTIGATION AND THEIR PARAMETERS

Fig. 3. (a) RN design and (b) its transmission windows in case of RN1. (c) Electro-optical modulation response and small-signal gain spectrum of the RSOA.

at the West OLT port, meaning data transmission over the farthest distance from the East OLT side. The RNs provide A/D functionality and signal amplification. Their design is presented in Fig. 3(a). As the interconnection point between ring and tree, each RN drops a pair of wavelengths toward two trees by means of 200 GHz A/D thin-film dense wavelength division multiplexing (DWDM) filters. Fig. 3(b) shows the transfer characteristic for the passive RN, whose filters have a thermal drift of pm/ C and are, therefore, suitable for an implementation in outside fiber plants. Resiliency 50/50 couplers connected to these A/D filters ensure that downand upstream can be dropped and added to both ring directions, without incorporating an electrically driven switch at the RN. In case an interruption exists at the West ring side of the RN, communication can be kept via the East side, or vice versa [4]. Other wavelength pairs that do not match the response of the RN are forwarded to the next RN via the express (E) port of the A/D filters, experiencing pass-through losses of 0.55 and 1 dB at the down- and upstream ring port, respectively. The increased loss at the upstream ring is caused by the extra WDM couplers that are inserted to drop the EDF pump toward the amplification stages for down- and upstream. Each of these stages were composed by a 15 m long HE980 EDF with good pump conversion efficiency and a noise figure of 5 dB for typical operation conditions. For the sake of simplicity, no optimization of EDF length for different network reach and loss budgets has been performed. Thanks to the dual-wavelength drop from the ring, bidirectional amplification inside the two amplification stages with simultaneous gain stabilization can be provided. The continuous-mode (CM) downstream at stabilizes the gain of the counter-injected burst-mode (BM) upstream at , and vice versa for the second EDF stage [4].

Since down- and upstream of the same wavelength channel are amplified in different amplifiers, strong RB in the EDF is avoided at the same time [9]. The required wavelength separation and combination of and is achieved via 100 GHz DWDM filters and the circulators that are connected via the tree feeder to their pass-through (P) and reflective (R) ports. Note that in this proof of concept, simple WDM couplers and a 50/50 coupler are used at the upstream ring to drop the pump delivered from the ring. In a real scenario with more RNs and stronger pump supply, an additional power splitter would have to be inserted to distribute the pumps among the RNs by dropping just a fixed portion of it at each RN. To adapt for resilient operation, a dual-path drop using circulators and two different splitting ratios for East and West side would be required to provide the possibility of choosing different pump dropping ratios for the different ring directions. However, the latter may not be very practical since the complexity of the RN increases and, more important, the pump module at the CO needs to be overdimensioned to provide a relative high operational power for a rather short time of resilient network operation. For this reason, the option of supplying the RN with a distributed pump from the ONUs is investigated in this paper. C. Colorless ONU and Its Pump Seeding Capability The ONU is composed of an REM as upstream transmitter and a downstream receiver. REMs are commonly used as, or, in combination with active gain elements to provide additional gain and, as a consequence, a powerful upstream signal launch toward the OLT to cope with an increased PON loss budget. A commercial deployment of reflective semiconductor optical amplifier (RSOA)-based ONUs, operated in a hybrid WDM/ TDM-PON, has been recently reported [11]. According to the

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Fig. 4. Tree splitter design for (a) pump delivery to the RN and (b) reduced net splitting loss. (c) EDF pump delivered by the ONUs in case of the rural scenario.

wavelength reuse scheme, the REM is seeded by the downstream signal instead of using continuous-wave light injection. For this reason, a 50/50 coupler splits the incident downstream signal toward the detecting and transmitting optical subsystem of the ONU. Thanks to the simple intensity modulation format chosen for both, down- and upstream, low capital expenditures for the cost-sensitive user terminal equipment can be provided. Substantial crosstalk from the downstream into the upstream channel is avoided by choosing a reduced ER for the downstream and an additional downstream cancellation approach at the ONU. The latter recovers an optical carrier out of the downstream signal at the REM and requires just a simple electronic feed-forward path for the counter injection of the detected downstream data signal, similar as in [4]. Though orthogonal modulation formats with frequency shift keying in the downstream can be used alternatively without the need of additional electronics or a careful synchronization between the branches of the ONU [12], the optical ONU receiver is typically of a more complex nature. Depending on the network parameters on which light will be shed in Section IV, the REM is constituted by either an integrated version of semiconductor optical amplifier and reflective electro-absorption modulator (SOA/REAM) [13] with a high electro-optical modulation bandwidth of 12 GHz and a high dynamic ER beyond 13 dB, a chip-on-carrier RSOA with high optical gain [14], or a commercial off-the-shelf transistor outline (TO)-can RSOA. Note that in case of the RSOAs, a passive pre-distorting electrical equalizer circuit was incorporated in the RF microstrip feeding line in order to extend the 3 dB modulation bandwidth. Fig. 3(c) shows the e/o response of the chip-on-carrier RSOA with a bandwidth up to 7.2 GHz, and its fiber-to-fiber gain. The dynamic ER was 6.4 dB at 10 Gb/s. The main modification of the ONU resides in the detection branch. Instead of a conventional avalanche photodiode, a combination of SOA and PIN diode acts as receiver. Together with a broadband loop configuration that is laid over the SOA with the help of WDM multiplexers for the S- and C-band, the latter upgrades the ONU for generating a weak pump for remote EDFs: given that the SOA is able to provide gain around the 1480 nm pump waveband of conventional EDFs and assuming that a weak wavelength-selective feedback is provided at the fiber plant, this ONU configuration allows to seed a pump for the EDFs by controlled reamplification of ASE noise emitted by the SOA [7]. The SOA used in this study had a noise figure of 8.9

dB and its gain peak was centered at 1492 nm. As can be seen in Fig. 4(c), this fits well for the generation of chirped pumps at 1480 or 1490 nm. Moreover, the pump generation in the SOA as shared gain medium between EDF pump and downstream does not strongly depend on the injected downstream signal [7]. Although the ONU appears as fairly complex at first glance, an entirely photonically integrated design with uncooled SOAs [15] can lead to a more techno-economically viable and energy efficient introduction for next-generation PONs. It is noteworthy that the pump contribution of a single ONU is not strong enough to feed an EDF; however, a powerful pump can be provided by introducing means of multiplexing inside the seed loop. The key element to do so is the splitter of the tree, which is now explained in detail. D. Tree Splitter and Pump Seed Loop Besides the 1:M N power splitting stage (SPL) that is typical for TDM-PONs, the splitter holds the counterpart of the pump seed loop. The ASE seed in the S-band is separated from the C-band data signals via WDM filters at the drop-side splitter ports in order to redirect them with a circulator back to the ONU. The reshaping of the ASE emission is guaranteed by a bandpass filter prior to the circulator which is inserted as Y-branch multiplexing element . In this way, ASE is simultaneously recycled at multiple pump wavelengths to form a powerful pump that is distributively provided by different ONUs. Porting the wavelength selective element as WDM multiplexer to this side of the pump seed loop ensures also that a colorless ONU design is retained and that its SOA does not require a high output saturation power. A 80/20 coupler then extracts a big portion of this EDF pump either toward the feeder fiber of the tree [splitter type “R,” Fig. 4(a)] or to an EDF that is located at the splitter [type “U,” Fig. 4(b)]. While the latter enables high splits in combination with a dual TDM splitting stage as demonstrated in [7], the former can feed the EDFs at the RN with a sufficiently strong pump to guarantee resilient network operation even for RNs far from the OLT. This is especially interesting for an unlikely but still possible worst case ring cut close to the OLT. Otherwise, the pump module at the OLT has to be overprovisioned to maintain resilient operation over nearly the full ring length rather than over a much smaller length as in normal operation. Note that thanks to the dual tree feeder fiber and due to the dual pumping direction of the EDF inside the dual-splitting

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Fig. 5. Evolution of signal power, OSNR, and pump power levels along (a) the suburban and (b) the rural PON.

stage, twice the number of ONUs can participate for a given set of pump wavelengths, e.g., four ONUs in case of the demonstrated scheme with two pumps at 1480 and 1490 nm. A larger number of pump wavelengths can be used to decrease the required saturation output power of the SOA at the ONU. Note that due to the unavailability of SOAs, the pump contributions from other ONUs were emulated with laser diodes, whose delivered pumps were adjusted to the equivalent power level. For splitter “U,” a 15 m long HE980 EDF was inserted between the two splitting stages with ratios 1:M and 1:N. To account further for ASE accumulation over M EDFs at the first 1:M splitting stage , an extra ASE source loads noise via the splitter to emulate the equivalent optical signal-to-noise ratio (OSNR) degradation for the upstream. For this reason, the power of this ASE source was adjusted to the equivalent noise background that would be present in case that M EDFs are used at the splitter “U.” IV. SIGNAL EVOLUTION AND TRANSMISSION PERFORMANCE The flexibility of the PON architecture to adapt itself to different deployment scenarios was evaluated in three network configurations, whose parameters are listed in Table I. Next to a suburban network with extended reach and medium tree split, a long-reach rural and short-reach urban scenario with lower and high split was investigated in addition. All scenarios are subject to an extended loss budget beyond 30 dB, corresponding to gigabit-capable PON (GPON) class C or 10 G PON extended class. A. Suburban (Extended-Urban) Scenario In the suburban scenario, several RNs along the 50 km ring are pumped from the OLT while the distributed ONU pump reduces the net 1:64 splitting loss in the tree. The REM at the ONU relies on an SOA/REAM for full-duplex 10 Gb/s transmission of a pseudorandom bit sequence (PRBS) of length . The overall loss budget of the PON, defined between the OLT and the ONU, was 31.2 dB. The evolution for signal and pump for the farthest RN2 at 50 km is shown in Fig. 5(a). The OLT pump arrives at its ring

TABLE II DELIVERED PUMPS [DBM] FOR RNS AND SPLITTERS

port (Q in Fig. 2) with 15.2 dBm (see Table II), which is strong enough to feed its two EDFs even in case of resilient ring operation after a worst case ring cut directly next to the OLT. A net gain of 8.8 dB is provided at the RN for down- and upstream. The extracted ONU pumps of 7.9 dBm per pump wavelength (i.e., ONU) at the splitter port [X in Fig. 4(b)] are fed into the EDF inside the splitter with a total multiplexed power of 13.9 dBm. The net splitting loss is, thereby, reduced from to 3.9 dB for a 1:64 split, which ensures together with the amplification at the farthest RN2 an ONU input (J) of dBm and an OSNR of 36.2 dB. Note that this relative high input power for the ONU was chosen due to the high chip coupling losses at the SOA/REAM without mode-size converter at its input chip facet and the balanced 50/50 coupler . The upstream is launched with dBm from the ONU (Q) and is delivered with dBm and an OSNR of 24.3 dB to the OLT receiver (V). Despite ASE loading at the first tree splitter stage , in order to account for noise accumulation at the splitter type “U,” there is no additional degradation due to the already low transmitted upstream OSNR of 25.5 dB, which can be kept at this acceptable value thanks to the gain achieved in the tree splitter that avoids a high, concentrated loss element. Raman gain has been experienced at the last upstream ring spool (U) despite the presence of eight ballast channels, so that the insertion losses of this ring segment is compensated and an extra gain of 0.9 dB is provided. The bit error rate (BER) performance for the ONU connected to RN2 is presented in Fig. 6(a)–(c), whereas the better performance for ONUs attached to the closer RN1 is omitted for

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Fig. 6. BER measurements in CM and BM for the suburban scenario. Half-duplex transmission of (a) upstream and (b) downstream, and (c) full-duplex transmission.

the sake of brevity. Half- and full-duplex operation as well as CM and BM traffic has been analyzed. Dispersion management has been applied at the OLT due to the chirped upstream. For this reason, DCFs with fixed dispersion have been included at the booster and preamplifier, leaving a certain amount of residual dispersion . An optimized dispersion management for a PON with multiple RNs can be implemented by means of adaptive dispersion compensation at the wavelength basis, such as electronic dispersion compensation at the receivers. As will be proven shortly, this is especially interesting for the chirped upstream transmission, while for the downstream, a fixed dispersion compensator adjusted to the average reach among the users can be used. For the sake of simplicity, this was not implemented in the experiments, and the transmission performance for different values of is reported instead. Half-duplex CM upstream operation at 10 Gb/s can be established at a low BER of and partial dispersion compensation ( ps/nm). A fully uncompensated link with ps/nm suffers from severe pulse broadening, causing a high BER above . Additional BM measurements are from interest to assess the influence of gain transients caused by the amplification stages of RN and tree splitters on the transmission performance. Upstream packets without specific frame structure and containing entirely payload data have been generated according to a GPON-compatible 125 s long frame with a duty cycle of 1:4, meaning a packet length (i.e., transmission gate) of 31.5 s. The guard time between the gate of the REM bias and the slightly shorter data packet was ns at the begin and the end of the burst. No degradation was observed between CM and BM traffic if just a single ONU is connected to the tree splitter, though no sophisticated means of gain stabilization have been applied at the remote EDFAs in the RN and the tree splitter. Instead, the gain clamping in the EDFs via the second tree wavelength and the downstream itself in the RN and the splitter, respectively, ensure that negligible overshoots of only 0.2 dB are introduced. Moreover, if both, the electro-absorption modulator and the SOA sections are gated according to the BM, an excess gain is experienced in the EDF of the splitter, which improves the BM reception with respect to CM by dB at a low BER of . With a second, 6 dB louder ONU at the second tree splitting stage that emits packets directly before the weak

TABLE III OPTICAL BUDGETS AND POWER MARGINS FOR DOWN- AND UPSTREAM

ONU inside the TDM frame [see inset of Fig. 6(a)], a penalty arises. However, the transmission can be still maintained with a Reed–Solomon (255,239) forward error correction (FEC). The power margin at the FEC threshold, defined as the difference between delivered power at the OLT receiver and the reception sensitivity, is as large as 17.4 dB (see Table III). Note that a simple PIN diode was used as upstream detector and that the intelligence of a more sophisticated BM receiver was partially implemented by manual decision threshold adaptation. The 10 Gb/s downstream [see Fig. 6(b)] is not degraded by the BM upstream amplification in the common EDF since it arrives strong and sets the population inversion constant. No severe distortion has been experienced for the CM downstream envelope, as it is evidenced in Fig. 6(b). A penalty of just 0.7 dB is caused at a BER of . There has been no dispersion penalty experienced for the downstream thanks to its unchirped transmitter. The power margin is 5.2 dB. When applying full-duplex transmission through the electrical feed-forward downstream cancellation, the upstream performance worsens with increased downstream ER [see Fig. 6(c)]. The optimum downstream ER, for which the BER is minimized for both data streams, has been found with dB, for which a BER of below the FEC level can be obtained for full-duplex CM-down- and upstream transmission. A further optimization of the downstream ER can be performed, e.g., by balancing the power margins for the reception. However, this study is beyond the scope of this paper. B. Rural Scenario A long-reach rural scenario with RNs at 50 and 75 km has been further investigated. The main interest is the analysis

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Fig. 7. BER measurements for (a) the downstream and (b) the upstream of the rural long-reach PON and (c) the urban PON scenario.

of chirped transmission over long reach and high loss budget (32.2 dB) with a high gain and high e/o bandwidth RSOA [see Fig. 3(c)]. In the demonstrated resilient case, RN1 at 50 km is pumped by the OLT while RN2 at 75 km receives its pump from the ONUs. It is worth to recall that this avoids overprovisioning of the OLT pump for the unlikely resilient case. The evolution for the signal and the pumps is presented in Fig. 5(b). RN1 at 50 km receives the OLT pump with 16.1 dBm (see Table II), so that a strong downstream of dBm can be provided to the ONU (J) for a split of 1:16 (yellow line ). The upstream OSNR can be maintained at 31.1 dB. For RN2 at 75 km, the pump from the OLT is too weak to provide the required gain for the given optical budget. However, the ONUs of one single tree are able to deliver 12.7 dBm to the RN [see Fig. 4(c)], and thus, the downstream arrives with dBm to the ONU [see Fig. 5(b), green line ], which is still acceptable for the RSOA as it is evidenced in Fig. 3(c). The upstream is delivered with dBm and an OSNR dB to the OLT (V). As for the suburban scenario, Raman gain is experienced in the upstream ring close to the OLT. Though the relative high RSOA gain contributes positively to the power budget, type-II RB becomes a severe problem as the signal-to-RB ratio falls below 20 dB. The BER for CM at 10 and 2.5 Gb/s are depicted in Fig. 7(a) and (b). Downstream reception for the farther RN2 at 10 Gb/s can be established for a split of 1:8 at a BER of and a split of 1:16 with FEC. The power margins are dB. The penalty of the based ONU receiver with respect to an APD is 1.6 dB at a low BER of . This penalty is attributed to the unfiltered ASE background caused by the SOA. The upstream can be received at 2.5 Gb/s at a BER of without dispersion management for RN1 at 50 km, though an error floor builds up below that BER level for a fully uncompensated link ( ps/nm). This proves the impact of the RSOA chirp already for lower data rates over the extended reach of approximately half the ring length. For RN2 at 75 km, the performance of the 2.5 Gb/s transmission is dominated by the type-II RB effects in the drop fiber in case of a compensated link, as mentioned earlier. For this reason, FEC is already required for the receivers that are dedicated to the upstream wavelengths

of RN2. However, reception is then still possible without dispersion management at the OLT receiver ( ps/nm), leaving power margins of dB. If the data rate is increased to 10 Gb/s, the high chirp of the RSOA becomes the main limitation for the transmission performance and makes dispersion compensation indispensable to reach the FEC level. Electronic dispersion compensation cointegrated with the photoreceiver is, therefore, an attractive option. Still, in case of an overcompensated link , the BER improves due to the chirp, leaving a power margin of dB for the 10 Gb/s upstream over a loss budget of 32.2 dB and a reach of 78 km. This proves that RSOAs are promising candidates for their deployment in next-generation PONs. C. Urban Scenario With Field-Deployed Fibers The PON architecture was further evaluated in combination with field-deployed ring fiber in an urban fiber network in Lannion, France (see inset of Fig. 2). The main interest in this scenario is to evaluate the maturity of commercial off-the-shelf components for the ONU together with the proposed remote pumping scheme. RN1 and RN2 have been deployed at a reach of 5 and 23 km, respectively. With the given excess loss of the 18 km ring fiber, the ring budget was 10.2 dB. The chosen tree split of 1:128 reflects a dense, urban territory and was realized by a tree splitter of type “U” with an intermediate EDF after the first 1:8 splitting stage. A commercial TO-can RSOA with a small signal gain of 14 dB and a pre-equalized modulation bandwidth of 2.5 GHz was used for 2.5 Gb/s upstream transmission. The 10 Gb/s downstream was launched with 6 dBm/ (A) as one of ten channels, six of them as ballast emulated by SFP modules. No dispersion management was implemented. The OLT pump reaches RN2 at 23 km (Q) with a power of 16.3 dBm and, despite the high split in the tree, the downstream is delivered with dBm to the ONU (J). Though the small signal gain of the commercial RSOA is rather low with 14 dB, the upstream arrives with a power of dBm and an OSNR of 23.7 dB at the OLT receiver (V). Fig. 7(c) shows the BER measurements at RN2. Downstream reception can be established at a low BER of and a small power margin of 0.4 dB without FEC. In case of the upstream, patterning effects

SCHRENK et al.: REMOTELY DUAL-PUMPED LONG-REACH PON

from the gain dynamics of the RSOA has been experienced for the longer PRBS. However, the caused error floor is well below the FEC threshold, leaving a power margin of more than 10 dB for the reception (see Table III). This proofs that commercially available low-cost devices are suitable for extended loss budgets as high as 34.2 dB. V. CONCLUSION A flexible WDM/TDM PON architecture for converged metro-access environment with fully passive fiber plant has been demonstrated in different deployment scenarios. A mixed pumping scheme from OLT and ONUs for remote amplification in the PON enables transmission at high rates of 10 Gb/s over loss budgets beyond 30 dB for ten WDM channels in combination with REMs in the colorless ONUs, reaching from a 78 km long reach rural to a dense 1:128 split/ urban configuration. The developed pumping scheme relaxes the resource provisioning for resilient PON operation in case of a ring cut, by simply recycling optical noise that is naturally generated by the ONUs. ACKNOWLEDGMENT The authors gratefully acknowledge the support of the ImagineLab and Keopsys. REFERENCES [1] A. Saleh and J. Simmons, “Technology and architecture to enable the explosive growth of the internet,” IEEE Commun. Mag., vol. 49, no. 1, pp. 126–132, Jan. 2011. [2] K. Y. Cho, B. S. Choi, Y. Takushima, and Y. C. Chung, “25.78-Gb/s operation of RSOA for next-generation optical access networks,” IEEE Photon. Technol. Lett., vol. 23, no. 8, pp. 495–497, Apr. 2011. [3] P. Ossieur, C. Antony, A. M. Clarke, A. Naughton, H.-G. Krimmel, Y. Chang, C. Ford, A. Borghesani, D. G. Moodie, A. Poustie, R. Wyatt, B. Harmon, I. Lealman, G. Maxwell, D. Rogers, D. W. Smith, D. Nesset, R. P. Davey, and P. D. Townsend, “A 135-km 8192-Split carrier distributed DWDM-TDMA PON with 2 32 10 Gb/s capacity,” J. Lightw. Technol., vol. 29, no. 4, pp. 463–474, Feb. 2011. [4] B. Schrenk, F. Bonada, J. A. Lazaro, and J. Prat, “Remotely pumped long-reach hybrid PON with wavelength reuse in RSOA-based ONUs,” J. Lightw. Technol., vol. 29, no. 5, pp. 635–641, Mar. 2011.

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