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JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 24, DECEMBER 15, 2011 3705

Demonstration of a 32 512 Split, 100 km Reach,2 32 10 Gb/s Hybrid DWDM-TDMA PON Using

Tunable External Cavity Lasers in the ONUsPeter Ossieur, Member, IEEE, Cleitus Antony, Member, IEEE, Alan Naughton, Student Member, IEEE,

Aisling M. Clarke, Member, IEEE, Heinz-Georg Krimmel, Xin Yin, Member, IEEE, Xing-Zhi Qiu, Member, IEEE,Colin Ford, Anna Borghesani, David Moodie, Alistair Poustie, Member, IEEE, Richard Wyatt, Bob Harmon,

Ian Lealman, Member, IEEE, Graeme Maxwell, Dave Rogers, David W. Smith, Sylvia Smolorz, Member, IEEE,Harald Rohde, Senior Member, IEEE, Derek Nesset, Member, IEEE, Russell P. Davey, and

Paul D. Townsend, Member, IEEE

Abstract—We report on a hybrid DWDM-TDMA optical accessnetwork that provides a route for integrating access and metro net-works into a single all-optical system. The greatest challenge inusing DWDM in optical access networks is to precisely align thewavelength of the customer transmitter (Tx) with a DWDM wave-length grid at low cost. Here, this was achieved using novel tun-able, external cavity lasers in the optical network units (ONUs) atthe customer’s end. To further support the upstream link, a 10 Gb/sburst mode receiver (BMRx) was developed and gain-stabilized er-bium-doped fiber amplifiers (EDFAs) were used in the network ex-periments. The experimental results show that 10 Gb/s bit ratescan be achieved both in the downstream and upstream (operatedin burst mode) direction over a reach of 100 km. Up to 32 50 GHzspaced downstream wavelengths and another 32 50 GHz spacedupstream wavelengths can be supported. A 512 split per wave-length was achieved: the network is then capable of distributinga symmetric 320 Gb/s capacity to 16384 customers. The proposedarchitecture is a potential candidate for future optical access net-works. Indeed it spreads the cost of the network equipment overa very large customer base, allows for node consolidation and in-tegration of metro and optical access networks into an all-opticalsystem.

Index Terms—Burst mode receiver (BMRx), dense wavelengthdivision multiplexing (DWDM), electroabsorption modulator(EAM), erbium doped fiber amplifier (EDFA), passive optical net-work (PON), semiconductor optical amplifier (SOA), time-divisionmultiple access (TDMA), tunable laser.

Manuscript received May 02, 2011; revised July 05, 2011, September 05,2011; accepted October 04, 2011. Date of publication October 25, 2011; dateof current version December 09, 2011. This work was supported by the ScienceFoundation Ireland under Grant 06/IN/I969 and the European Union under theFP6 project PIEMAN and FP7 Network of Excellence EuroFOS.

P. Ossieur, C. Antony, A. Naughton and A. M. Clarke are with the PhotonicsSystems Group, Tyndall National Institute and the Department of Physics, Uni-versity College Cork, Cork, Ireland (e-mail: [email protected]).

H. G. Krimmel is with Alcatel-Lucent, Bell Labs, Germany.X. Yin and X.Z. Qiu are with Ghent University/IMEC, Belgium.C. Ford, A. Borghesani, D. Moodie, A. Poustie, R. Wyatt, B. Harmon, I.

Lealman, G. Maxwell, D. Rogers, and D. W. Smith are with the Centre for In-tegrated Photonics, IP5 Ipswich, U.K.

S. Smolorz and H. Rohde are with Nokia Siemens Networks, Munich, Ger-many.

D. Nesset and R.P. Davey are with BT, Ipswich, IP5 3RE, U.K.P.D. Townsend is with the Photonics Systems Group, Tyndall National Insti-

tute and the Department of Physics, University College Cork, Ireland and alsowith the School of Engineering and Physical Sciences, Heriot-Watt University,Edinburgh EH14 4AS, U.K.

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JLT.2011.2173459

I. INTRODUCTION

T ODAY gigabit passive optical networks (PONs) are beingwidely deployed in response to the exploding demand

for high bandwidth [1]. Services such as IP high-definitionvideo delivery, voice-over-IP (VoIP), social networking andcloud computing will push the demand for bandwidth evenbeyond what is achievable with today’s gigabit PONs [2]. Newstandards supporting 10 Gb/s operation over these PONs arerapidly emerging [3]–[6]. However, these types of optical ac-cess networks which typically serve 32 customers over reachesranging from 20 km to 60 km (in the case of reach-extendedPONs) may not be the ultimate solution for network operatorswhich seek to radically reduce the cost of supplying broadbandservices [7].

Therefore, intensive research is directed to more advancedoptical access network architectures and their required photonicand electronic components that look beyond today’s 10 Gb/stime division multiplexed (TDM) PONs. All these new net-work concepts aim at either increasing the bandwidth availableto the customer (peak bandwidth as well as average sustain-able bandwidth), extending the physical reach of the network,increasing the number of customers served from a single net-work node (known as splitting ratio or split), or any combi-nation of these. Unlike metro and long haul transmission sys-tems, where high performance and maximum capacity are thedesign objectives, the overriding concern in optical access net-works is to achieve the aforementioned goals at low cost, be-cause of the relatively small number of customers sharing theaccess infrastructure.

A first logical step beyond today’s 10 Gb/s TDM-PONs isan increase of the line-rate towards 40 Gb/s. However, simplescaling of today’s 10 Gb/s non-return-to-zero modulationformat to 40 Gb/s runs into limitations such as high dispersionpenalties and severely constrained optical budgets. Whileadvanced modulation formats such as DQPSK (differentialquadrature phase shift keying) can improve the performancewith respect to chromatic dispersion, the complexity of therequired transmitters and receivers means that such solutionsare currently too costly for access applications. A possible al-ternative is a so-called stacked PON which combines different10 Gb/s PONs using coarse wavelength division multiplexing

0733-8724/$26.00 © 2011 IEEE

3706 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 29, NO. 24, DECEMBER 15, 2011

(CWDM), examples have been presented in [8]–[10]. This canbe viewed as a wavelength division multiplexed (WDM) PON.While this is an achievable scenario in the medium term, thereare obvious drawbacks to such a solution, for example CWDMmakes inefficient use of the available spectrum.

An alternative to increasing the line-rate while avoidingthe need to increase the modulation bandwidth of the opticalcomponents and electronics is orthogonal frequency divisionmultiplexing (OFDM) [11], [12]. Several groups have demon-strated very high capacities over PONs. For example in [13],polarization multiplexing and direct detection OFDM was usedto achieve 108 Gb/s downstream line-rate. Experiments in theupstream focus on how to combine the signals from multipleONUs into a single OFDM signal. In [14], 44 Gb/s/ upstreamtransmission line-rate was demonstrated using coherent detec-tion at the optical line terminal (OLT). A split (number of ONUssupported by a single OLT) of 32 was achieved over a distanceof 100 km. In [15], a record 1 Tb/s capacity was achieved over a32 split, 90 km reach PON. While OFDM allows for very highcapacity thanks to its high spectral efficiency, it may prove chal-lenging to reduce the cost of the required optical componentssuch as Mach-Zehnder modulators (MZMs), optical hybridsand low linewidth lasers down to the levels needed for accessapplications. Furthermore power hungry, high bandwidth D/Aand A/D converters are required, combined with complex andexpensive digital signal processing. Other advanced transmis-sion techniques being considered for optical access applicationsare optical code-division multiple access (OCDMA) [16], aswell as coherent PONs [17]–[20]. Coherent PONs benefitfrom the higher sensitivity of coherent receivers to increasereach and/or split. Similar to OFDM however, such techniquesstill require significant additional research and developmentespecially to meet the low-cost targets for PONs.

Pure wavelength division multiplexed (WDM) PONs areanother widely studied optical access technology [21]. In thisscheme, ONUs are assigned a downstream and upstream wave-length, which are aggregated using wavelength multiplexers(e.g., arrayed waveguide gratings—AWGs). Its advantagescompared to conventional TDM-PONs are reduced opticalpath loss (due to the low insertion loss of AWGs compared topower splitters) leading to higher splitting ratios and reaches,dedicated wavelength to the customer thus allowing signifi-cantly higher bandwidths, and protocol, modulation and bitratetransparency on a wavelength basis. Two disadvantages of thisnetwork architecture are that the cost of the OLT equipment isno longer shared across a large customer base and significantlevels of optical integration will be required to achieve thelow port densities typical of TDM-PONs. A wide variety ofarchitectures have been studied. The main focus is to achieveso-called colorless ONUs, meaning ONUs that use the samecomponents (modulator, receiver) regardless of the assignedwavelength. This reduces management, sparing and inventorycosts for the operator. Furthermore, the higher volumes maylead to additional cost savings due to the associated economiesof scale. Different methods have been studied to achieve color-less ONUs. The most obvious solution uses tunable lasers at theONU, which then need to be tuned to the appropriate channel ofthe AWG [22]–[24]. The major challenge is the requirement for

Fig. 1. High level system architecture of our proposed hybrid DWDM/TDMAPON.

a low-cost tunable laser technology. Rather than generating thewavelength locally at the customer end, another option (calledwavelength seeding or carrier distributed PONs) generatesunmodulated carriers at the OLT side, distributes them to theONUs where they are modulated and reflected back upstream[25], [26]. The advantage is that the wavelength generation canbe performed in the well-controlled environment of the OLT(in terms of temperature), avoiding the need for a tunable laserat the customer end. The disadvantage of this technique is thefact that when a carrier is sent down a fiber, a small fractionbackscatters (Rayleigh backscattering) and interferes with thereflected upstream signal. Additionally, any reflections of thecarrier due to e.g., splices or connectors can also interferewith the upstream signal. While several solutions have beenproposed such as for example the dual-feeder fiber scheme [27],usage of four-wave mixing in SOAs for frequency translation[28], Rayleigh backscattering and reflections currently stilllimit achievable splits and reaches.

Hybrid schemes combining two (or more) of the abovetechniques are possible [29], [30]. As outlined above, theuse of advanced modulation formats, coherent techniquesand OCDMA require components that currently still posechallenges in terms of cost for optical access applications.Therefore, we will focus on so-called hybrid DWDM-TDMAPONs [31]–[33], in which each DWDM wavelength is furthershared by a group of customers using TDM. All wavelengthsare 10 Gb/s, NRZ modulated, which has the advantage ofcombining the well-established TDM technique in PONs withthe well-established DWDM techniques from metro networks.A high-level view of the proposed network architecture whichconsists of a service node, a local exchange and the ONUs isshown in Fig. 1. Optical amplifiers are used to significantlyincrease the reach and split of the network, allowing a scenariowhereby optical access and metro networks converge. As thisallows to initially bypass local exchanges and ultimately toclose these down altogether, such a solution potentially allowsreduction of operational expenditure (OPEX) for the operator.Furthermore, intervening electronic switches and associatedequipment can be eliminated, thus potentially reducing capitalexpenditure (CAPEX) for the operator. The greatest chal-lenge from the perspective of the optoelectronic componentsthat comes with the introduction of DWDM in optical ac-cess networks is the need for the customer’s transmitter (Tx)wavelength to be precisely aligned with a predefined DWDMwavelength grid [34]. In this paper, we use a novel, potentially

OSSIEUR et al.: DEMONSTRATION OF A 32 512 SPLIT, 100 km REACH, 2 32 10 Gb/s HYBRID DWDM-TDMA 3707

low-cost tunable external cavity laser [35]. Its novel designallows the laser wavelength to be set once when the ONU isfirst brought into service. The output of this tunable laser is thenmodulated and boosted using an integrated electroabsorptionmodulator and semiconductor amplifier (EAM-SOA) [36]. Theintegration of the EAM together with the SOA enhances theprospect for low-cost manufacturing (which is mandatory foraccess applications) of the ONUs in high volumes.

Several hybrid DWDM-TDMA PONs have been reported inthe recent literature. Reference [31] discusses a WDM-TDMPON that supports up to 160 ONUs at bitrates of 1.25 Gb/sover reaches up to 22.5 km. A half-duplex, centralized opticalcarrier distribution based on tunable lasers was used. In [33]a hybrid DWDM-TDM using optical carrier distribution andlooped back transmission in the customer transmitter (Tx)is reported. However the upstream channels were operatedin continuous-mode rather than burst-mode, and thereforethis experiment did not reflect the actual performance of theupstream link. Furthermore, the optical carriers were generatedusing lasers in a local exchange which needs to be locatedat a relatively short distance from the customers. This is dis-advantegeous from a control and maintenance perspective. Acarrier-distributed hybrid DWDM-TDMA PON with a reach of135.1 km and symmetric 32 10 Gb/s capacity distributed to32 256 users was reported in [25], [26]. The achievable splitwas ultimately limited by the penalties introduced by the usedburst-mode receiver as well as Rayleigh backscattering despitethe use of a dual feeder fiber scheme. Reference [7] describesthe realization of a hybrid DWDM-TDMA PON with a reachup to 100 km. The downstream channels were operated at 10Gb/s and the upstream channel (operated in burst-mode), wasoperated at 2.5 Gb/s. Furthermore, a potentially costly O/E/Oconversion was required to map the single upstream wavelengthgenerated by the ONUs to a DWDM wavelength grid. Com-pared to an all-optical solution based on EDFAs, this O/E/Oconversion significantly limits the maximum allowable lossin the access part of the network, due to the lower sensitivityof even an APD-based receiver compared to a narrowbandoptically filtered and pre-amplified receiver. Finally, the hybridDWDM-TDMA PON architecture presented in [37] is based ona metro ring, to which TMDA-PON are attached via add/dropremote nodes. The full network demonstration will includeasymmetric 10 Gb/s downstream and 2.5 Gb/s upstream trans-mission. Up to 32 ONUs will be supported per wavelength.

In this paper, we report on a hybrid DWDM-TDMA PONwith the following characteristics: symmetric 10 Gb/s bitrate,full upstream burst-mode operation, 100 km reach and supportof a 50 GHz DWDM wavelength grid [24]. A split of 512 perwavelength was demonstrated experimentally, hence the net-work can support up to 16384 users from a single service node.To the best of the author’s knowledge, these combined achieve-ments go beyond today’s state-of-the-art.

This paper is organized as follows. Section II describes thenetwork architecture, wavelength plan and optical budget. Thedeveloped component technologies to support the upstream linkare discussed in Section III. The constructed testbed and exper-imental results are presented in Section IV. Finally we end withthe conclusion in Section V.

Fig. 2. Wavelength plan with G.984.x and G.987.x bands indicated.

II. NETWORK CONFIGURATION

A. High Level System Architecture

Fig. 1 shows the generic concept of the proposed network ar-chitecture. A service node is connected to a local exchange usingtwo 90 km metro standard ITU-T G.652 single-mode (SMF)fibers: one for the downstream traffic and one for the upstreamtraffic. These metro fibers carry DWDM traffic on 32 down-stream and another 32 upstream wavelengths. Up to 32 TDMA-PONs are connected to the local exchange, each of which isassigned a unique upstream and a unique downstream wave-length. The TDMA-PONs feature single fiber operation with areach of 10 km, a split as high as 512 and use SMF. EDFAsare used in the service node and local exchange for both trans-mission directions to compensate for the insertion losses of thebackhaul fiber and especially the large splitting losses in theTDMA-PONs. It is important to note the cost of EDFAs is notnecessarily prohibitive for PON applications, in fact EDFAs arealready used today, for example as booster amplifier for GPONRF overlay [38]. In a typical network configuration, the servicenode would be a node of the inner core/metro network and thelocal exchange is the location where current copper broadbandand telephony equipment is sited. Therefore, we assume the ser-vice node and the local exchange (which now can be housed ina small cabinet in the street) are mains powered and hence canhouse the optical amplifiers as well as the DWDM multiplexers.The splitters and access fibers on the other hand may be locatedin unpowered street cabinets. The local exchange forms the con-nection between the two parts of the network, i.e., 1) from theservice node to the local exchange referred to as the metro sec-tion and 2) from the local exchange to the optical networks unitslocated on the subscriber’s premises referred to as the accesspart of the network.

B. Wavelength Plan

Fig. 2 shows the wavelength plan used in the proposednetwork: the 32 upstream and 32 downstream wavelengthsare placed in the C-band. Compared to currently deployedTDM-PONs, which use 1310 nm for upstream traffic (1270 nmin case of IEEE 802.3av 10GEPON and XGPON1) and1490 nm for downstream traffic (1577 nm for XGPON1), apure C-band scheme allows the use of EDFAs with low noisefigures (compared to for example SOAs which could be used forO-band amplification) and exploits the low attenuation windowof standard single-mode fiber. This choice maximizes the reach


Fig. 3. System design for the tunable approach.

and splitting factor of the network, which were consideredimportant design goals. Another alternative for the upstreamband is Raman amplification [39], [40]. However, Ramanamplification was considered to have a higher cost compared tousing C-band EDFAs (which already found widespread use inmetro and long-haul networks and hence are manufactured inrelatively large volumes). Some researchers have also consid-ered Praseodymium doped fiber amplifiers [41], however suchamplifiers have less gain than can be obtained from an EDFA,and hence were also found unsuitable to support the high split.As shown in Fig. 2, current PONs reserve the 1550–1560 nmband for broadcast video overlay, which is no longer possiblewith our proposed wavelength plan. The view we have takenis that IP video services will become prevalent and will thusbe transmitted in the downstream data payload. Packing all64 wavelengths together in the C-band necessitates the use of50 GHz spaced wavelengths (the alternative option, puttinge.g., the DS wavelengths in the L-band and using a 100 GHzwavelength grid was considered to be less cost-effective dueto the need for L-band EDFAs). An important consideration isthen the technology for wavelength aggregation in the servicenode and the local exchange. Here, we have chosen athermal50 GHz DWDM multiplexers, thus eliminating the need fortemperature stabilization (and associated power consumption).A 9 nm guard band is left between the upstream and down-stream bands, allowing the use of cost-effective diplexers tosplit and combine the upstream and downstream wavelengthsin the ONUs and the local exchange.

C. Network Design and Optical Budget

Fig. 3 shows the detailed network architecture; the networkinsertion losses are listed in Table I and the indicated powersare listed in Tables II and III. In the service node, the mul-tiplexed downstream wavelengths (each NRZ modulated with10 Gb/s data) are boosted using a DWDM-EDFA and launchedinto the metro fiber. After amplification in the local exchange,each wavelength is routed to its TDMA-PON using a DWDM

TABLE INETWORK INSERTION LOSSES

TABLE IIRELEVANT DOWNSTREAM OPTICAL POWER PER WAVELENGTH

TABLE IIIRELEVANT UPSTREAM OPTICAL POWER PER WAVELENGTH

multiplexer. One EDFA per TDMA-PON boosts the signal be-fore sending it to the ONUs. A single DWDM-EDFA (whosecost is shared by all users served by the network) is used priorto wavelength demultiplexing to relax the gain requirements on


the single wavelength EDFAs (whose cost is shared by all theusers on a single TDMA-PON). The alternative of using a singleDWDM-EDFA prior to demultiplexing would require a prohib-itively expensive EDFA with dBm (2 Watt) output satura-tion power to realize a dBm launched power at the inputto the TDMA-PON. Given the high launched power into the ac-cess fiber, standard linewidth broadening will be needed to avoidpenalties due to stimulated Brillouin scattering (SBS), see alsoSection IV.A. Other non-linearities such as four-wave mixingare not relevant here as only a single wavelength is launchedinto the access fiber. In the local exchange, diplexers are usedto combine the upstream and downstream wavelengths of theindividual TDMA-PONs. The optical distribution network ofthe TDMA-PON uses three stages of split. Using the insertionlosses from Table I, the maximum insertion loss of the accesssection (including the loss of the two diplexers) can be calcu-lated to be 36 dB. As the received power at the downstreamreceiver in the ONU is dBm, an avalanche photodiode(APD) is used to provide sufficient sensitivity. The downstreamlink and EDFA configuration were designed to achieve minimalOSNR impairments, this downstream link is therefore not thelimiting factor in achieving high split and long reach. Impor-tantly, the downstream OSNR is also sufficiently high that nar-rowband filtering is not required at the ONU. Instead, the fil-tering provided by the diplexer which is used to separate thedownstream and upstream wavelengths is sufficient to preventany significant penalties due to ASE-ASE (amplified sponta-neous emission) beat noise. This keeps the cost of the ONU lowand eliminates the need for a tunable filter which would be re-quired for a colorless ONU. Note further how the cost of theDWDM multiplexers is shared across all customers attached tothe network, keeping the cost per user minimal.

In the upstream direction, the tunable laser output is exter-nally modulated using an EAM and boosted with an SOA. Anintegrated EAM-SOA is used, thus achieving high-speed modu-lation with low chirp at minimal cost. Details of the tunable lasercan be found in III.A; for more information on the integratedEAM-SOA the interested reader is referred to [26] and [36].The output power is deliberately constrained to a relatively lowvalue of dBm to keep the costs of the upstream transmitterlow. The SOA is turned off between bursts by quenching its biascurrent. This provides more than 50 dB off-state extinction, suf-ficient to avoid interference from the non-transmitting ONUs[42], [43]. In the local exchange, one EDFA per TDMA-PONamplifies the upstream signals prior to wavelength multiplexingand launching into the upstream metro fiber. As the ONU launchpower is limited and the loss of the access part of the networkis high (36 dB) due to the high split (512), the upstream chan-nels are OSNR limited and dictate the achievable split and reach.This is also the reason why the signal amplification is performedprior to wavelength multiplexing, even though this requires oneEDFA per TDMA-PON (rather than a single DWDM-EDFA).Without this feature, the additional insertion loss of the DWDMmultiplexer would further degrade the OSNR of the upstreamchannels severely limiting the achievable splitting factor (whichwas an important design goal). In the service node, the upstreamsignals are preamplified using an EDFA and demultiplexed to 32burst-mode receivers [44], [45]. More insight into the parame-

TABLE IVRELEVANT EDFA CHARACTERISTICS

ters that limit the optical budget can be obtained by calculatingthe achieved OSNR (in a 0.1 nm reference bandwidth) at theinput to the BMRx:

(1)

where the losses , the EDFA gainsand EDFA noise figures (expressed as linearquantities) are given in Tables I and IV respectively. isthe reference bandwidth (12.5 GHz) in which the OSNR is de-fined and is the optical carrier frequency. The approximationholds as the local exchange EDFA gain . As thegain of the local exchange EDFA is significantly largerthan the loss of the metro section, the OSNR is mainlydetermined by the access loss and local exchange EDFA noisefigure (for a given launched power from the ONU).Clearly, it is important to minimize this noise figure to maxi-mize the achievable split. Using the measured EDFA parametersfrom Table IV and the metro and access losses from Table I, anOSNR of 20.8 dB can be calculated at the input to the BMRx.The experimental results presented in Section IV demonstratethat this is sufficient to achieve a bit-error rate (BER) less than

. Hence, assuming Reed-Solomon RS(255,223) for-ward error correction (FEC), the network upstream channels canoperate error-free.

We have further assumed 11 dB differential access loss,mainly originating from splitter non-uniformity. Due to thisdifferential loss, successive packets arriving at the local ex-change can have power differences up to 11 dB. Loud packetsmay compress the EDFA gain and result in large transients.Therefore, all upstream EDFAs use electro-optical feedbackto mitigate these transients [46], the scheme is detailed inSubsection III.B.

As a large customer base is served from a single servicenode, an important consideration is the need for protectionswitching, especially of the metro link. Protection of the down-stream metro fibre can for example be achieved by adding anadditional standby EDFA and a coupler between both amplifierstages in the local exchange. Protection of the upstream metrofibre can be achieved by adding an additional EDFA in thelocal exchange as well as a coupler. This has already beendemonstrated in [26] and will not be considered further in thispaper.


III. COMPONENT TECHNOLOGIES

The upstream channels of the proposed network architecturepose two major challenges in terms of component technologies.First of all, a low cost laser is required at the ONU, which muststay tuned to its assigned DWDM channel. Further, to supportthe TDMA in the upstream, the transmitter, receiver and opticalamplifiers need to operate in burst-mode. The component devel-opments have therefore focussed upon technologies for the up-stream channels. Nevertheless, several components such as thetunable laser and the integrated EAM-SOA can also be used forthe downstream channels. In this section, we present the tunablelaser and give details about the gain stabilized EDFAs and theburst-mode receiver. Another important component in the up-stream transmitter is the integrated EAM-SOA which providesamplification and gating (between bursts) of the tunable laseroutput and high-speed modulation. For more information on thisintegrated EAM-SOA, as well as the specifications imposed onthe DWDM multiplexers, the interested reader is referred to [36]and [26] respectively.

A. Tunable Laser

The laser in the ONU which acts as the light source for theupstream transmission needs to tune to the 50 GHz ITU-T gridover the upstream part of the C-band (1530.0 nm–1542.8 nm).While fast (nanosecond scale) tuning speed is not required forthe proposed network architecture, an important requirement isthe need for “set-and-forget” tunability. This means that whenan ONU is first connected to the network and has tuned to itsassigned upstream wavelength, it must stay tuned to this wave-length even when cycling the electrical power of the ONU. Ad-ditional design parameters are sufficiently narrow linewidth toenable 10 Gb/s transmission over up to 100 km standard single-mode fiber, single-mode operation with sufficient side-modesuppression ratio ( dB), and an output power of dBm.These requirements need to be met within the low cost target forthe ONU. When considering different tunable laser technologieswith respect to their cost, one must consider both the manufac-turing, test and characterization costs, as well as the productionyield. Indeed, compared to traditional applications for tunablelasers in metro and long-haul networks, volumes for optical ac-cess will be several orders of magnitude larger, requiring po-tentially millions of manufactured devices per annum. Hence,yield, testing and characterization time become important con-siderations.

Tunable laser technologies can be broadly classified into threecategories: 1) DFB laser arrays, 2) monolithic multi-section de-signs and 3) external cavity lasers (ECL) [47]. Wavelength se-lection in DFB laser arrays can be achieved using for example acoupler (which results in large coupling losses) [48], or an ex-ternal micro-mechanical mirror [49]. A drawback of this deviceis that to ensure coverage of the required wavelength band, in-dividual DFB lasers may need to be specifically selected, whichbecomes impractical and costly for high volumes. Monolithictunable lasers are typically variants of the distributed Bragg re-flector (DBR) laser, such as for example the sample grating[50], super structure grating [51] and digital supermode DBRlasers [52]. Other types such as the modulated grating Y-branch

(MGY) laser are based on interferometric principles using inte-grated Mach-Zehnder modulators and Y-couplers [53]. Whilemonolithic tunable lasers may potentially achieve the lowestmanufacturing cost (provided sufficient yield from an individualwafer can be obtained), a drawback is the required characteriza-tion time. Indeed, wavelength tuning in these lasers is achievedby changing the refractive index of the different grating struc-tures through carrier injection. A (possibly complicated and dis-continuous) map of the lasing wavelength versus the various in-jection currents is then needed for each individual laser unit.Measuring such a parametric map can take a relatively long timeper device, especially over temperature, which is not practicalwhen one needs to manufacture devices in high volumes. Fi-nally, different types of tunable ECLs have been described [54].In its most simple form, an external cavity laser consists of again element (usually an SOA), a wavelength selective elementand mirrors (which may or may not be integrated into eitherthe gain element and the wavelength selective element). If re-quired, collimating optics are included in the cavity as well.In [55], a compact MEMS-tuned external cavity semiconductorlaser is presented. Tunability is achieved by rotating a mirrorusing a silicon MEMS actuator. While high performance andreliability were achieved, the laser only stays to its tuned wave-length as long as the bias voltage is maintained to the MEMSactuator. Using an intracavity electro-optical crystal, tunabilityover 50 GHz was achieved in [56]. However very high (kilo-volt) voltages are required for biasing the electro-optical crystalwhich makes this device impractical for use in FTTx applica-tions. An acousto-optic tunable filter was used in [57] to achievetunability over 132 nm and an (expensive) Fabry-Pérot etalonwas used to ensure single-mode operation. If such a device wereused in a hybrid DWDM/TDMA PON such as investigated here,the passbands of the etalon would need to be aligned exactlywith the passbands of the wavelength multiplexers in the localexchange (or hence to the 50 GHz ITU-T grid), which maybe difficult to achieve in practice. Furthermore, the acousto-optic filter requires an RF-drive and hence set-and-forget tun-ability cannot be achieved using this device. In [58], a liquid-crystal-based mirror was used as a wavelength selective ele-ment. While a very compact design was achieved, a Fabry-Pérotetalon was still required, which as mentioned already abovemakes this component impractical for hybrid DWDM/TDMAPONs. A promising device based on a vertical-cavity surface-emitting laser (VCSEL) is reported in [59]. Using a bulk-mi-cromachined, electrothermally actuated upper mirror, a tuningrange of 76 nm is achieved. Additional development will be re-quired to bring the costs for such a device down, especially asfor example the mirror and VCSEL are fabricated separately,and then require an active alignment procedure for assembly.

Although some interesting designs for tunable lasers havebeen presented, most designs today still do not meet the strin-gent cost targets for optical access applications. Therefore,a novel tunable laser was designed, with the primary goalsof minimizing of piecepart, assembly and characterizationcosts. Its design is based on a hybrid integrated ECL as shownschematically in Fig. 4. The gain element is a simple reflec-tive SOA (RSOA), which is placed inside a resonant cavitycreated by the rear facet of the RSOA and an external rear


Fig. 4. Schematic diagram of the tunable laser.

mirror. A thin film filter based on a dielectrically coated glasssubstrate placed inside the laser cavity provides the wavelengthselectivity required for single-mode operation. Tunability isachieved by rotating this thin film filter using a piezo electricmicromotor. As the angle of incidence on the thin film filter ischanged, the effective path length of the reflections inside thefilter changes, thus changing its peak transmission wavelength.The piezo electric micromotor only requires electrical drivewhen it is rotating the thin film filter. When the electrical driveis removed, it stays fixed at its last position and hence theONU wavelength remains fixed even when powering downthe ONU, thus achieving the set-and-forget tunability. Thusno (expensive) internal wavelength lockers nor any (compli-cated) network centric wavelength stabilisation is required. Afurther advantage of this thin film filter tuning is that it canbe made athermal through an appropriate choice of substrate.Its selectivity is critical to achieve single-mode operation withhigh SMSR. The filter featured a 35 layer design, achieving afull-width-half-maximum (FWHM) bandwidth of 0.35 nm with1.2 dB insertion loss at normal incidence (peak wavelength:1554 nm). This high selectivity is required to avoid the need foran additional etalon. The RSOA gain block should exhibit verylow output facet reflectivity to suppress gain ripple generatedby internal cavity modes. A curved waveguide was used tocombine ultra-low output facet reflectivity with normal cleavedbackfacet reflectivity. In addition, multilayer anti reflectionoptical coatings are used on the front facet. Finally, a taperedwaveguide mode expander is used to improve coupling to thefiber and provide a further reduction in output facet reflectivity.The overall facet reflectivity was less than . The RSOAused 4 quantum wells and is 2.7 mm long.

Throughout the entire laser design, care was taken to keep itsmanufacturing cost as low as possible. First of all, the hybridlaser design does not require time consuming and expensivetesting and parameter mapping, unlike the monolithic DBRs.The used piezo micromotor is based on the same technologydeveloped for mass-market application for mobile-phonecamera focussing. The RSOA gain block and external cavitycomponents are mounted on lithographically defined siliconsubmounts with the necessary precision alignment features,thus eventually enabling passive alignment of all the compo-nents. All the individual pieces (the SOA gain block, the filter(which can be manufactured on wafer scale) and the motor)are manufacturable with very high yield as these are simple

Fig. 5. (a) Assembly of tunable laser with piezo micro-motor, (b) Laser cavitywith SOA gain chip and rotating thin film filter.

Fig. 6. Tuning spectrum across upstream part of the C-band.

components. We therefore expect the yield to be much higherthan for example the current yields of wafer level multisectiontunable lasers. Two silicon submounts were used: one to mountthe RSOA, and another one to locate the piece parts that makeup the external cavity, including the collimating ball lens, tuningfilter and rear mirror. Due to the rigidity of the used siliconsubmounts, these offer a further advantage of minimizing theimpact of any mechanical vibrations on wavelength stability.Small fluctuations in the filter angle will only have a slighteffect on the laser wavelength (much less than the passband ofthe used AWGs), as the filter is selecting out one cavity moderather than determining the absolute frequency.

The silicon submounts and laser cavity are shown in Fig. 5(a).Fig. 5(b) shows how the laser cavity is integrated with the piezomicromotor into the laser module. A thermoelectronic cooleris used to ensure that the laser wavelength does not drift withvarying environmental temperature. Spectral measurementswere carried out on the assembled prototype, the results areshown in Fig. 6. It can be seen that the laser can be tuned toany wavelength on the ITU-T 50 GHz grid across the entireupstream part of the C-band with a side mode suppressionratio between 25 dB and 35 dB. The output power variedbetween dBm to dBm across the tuning range, whichis sufficient for the integrated EAM-SOA to launch dBmupstream power from the ONU. Fig. 7 shows the detailedspectrum (0.020 nm resolution) of the tunable laser tuned to1536.22 nm: over 25 dB SMSR can be observed. Linewidthbroadening was used for suppression of stimulated brillouinscattering (SBS). Measurements using an optical spectrumanalyzer demonstrated that the lasing wavelength indeed re-turns to the “set” wavelength channel after cycling the bias


Fig. 7. Spectrum of the tunable laser output, �� nm.

Fig. 8. (a) Transients of uncompensated EDFA. (b) Gain stabilization activated,loud/soft � �� dB.

current of the RSOA gain element. This feature implies thatno continuous wavelength monitoring would be needed toensure that the wavelength remains locked to the chosen grid.Wavelength monitoring would only be required when a newONU is attached to the network and powered for the first time.

B. Gain Stabilized EDFA

The local exchange EDFA needed to support the upstreamtraffic in our hybrid DWDM/TDMA PON is subject to burstytraffic; the upstream service node EDFA is subject to both burstytraffic and a varying signal spectrum. Incoming packets with rel-atively high optical power will push the EDFA into its saturationregion. For packets with relatively low powers or gaps in the in-coming signal, the EDFA gain will recover resulting in slowlyrising transients. If after EDFA gain recovery a strong opticalpacket arrives at the EDFA, a strong (and potentially damaging)overshoot can be observed, after which the EDFA gain slowlydecays, as shown on Fig. 8(a). To mitigate these problems, a gainstabilization scheme as shown in Fig. 9(a) has been developed[26]. The scheme tries to keep the EDFA output power constantby closing a feedback loop around the EDFA that adds addi-tional power generated by an out-of-band auxiliary laser to theinput of the EDFA during soft packets or gaps, see also Fig. 9(b).As shown in Fig. 8(b), transients are suppressed when the feed-back loop is activated.

As mentioned in Section II.C, the noise figure of the localexchange EDFA determines the achievable split and should beas small as possible. Therefore, the auxiliary laser output was

Fig. 9. (a) Gain stabilization diagram. (b) Operation of the gain stabilization:� (uncomp.) is the output waveform in case the stabilization is switched off,� (comp.) is the output waveform with the stabilization switched on.

Fig. 10. Block diagram of the service node upstream EDFA.

coupled to the EDFA input using a low-loss 99%/1% coupler(insertion loss: 0.4 dB). The local exchange EDFA stabilizationscheme has been demonstrated to operate with less than 0.3 dBpower penalty. The service node DWDM EDFA has to operateunder significantly higher input power (as it needs to handleall 32 upstream wavelengths) and has to compensate both forthe insertion loss of dispersion compensating fiber (DCF) andthe service node DWDM multiplexer, and deliver sufficient op-tical power to the BMRx input. These requirements were metby using a two stage design with the DCF and an additionalgain flattening filter between both stages. The delay from theDCF makes it difficult to guarantee loop stability when closinga feedback loop across both EDFAs at once. Therefore it wasdecided to stabilize both EDFA stages separately as illustratedby Fig. 10 and to keep the DCF and gain flattening filter outof these loops. The compensation wavelength was set outsideof the upstream band at 1560 nm. CWDM diplexers were usedto tap the input and output of the EDFA. This reduces the lossin the path from the auxiliary laser output to the input of theEDFA, and hence reduces the required maximum power fromthe auxiliary laser. Indeed for proper operation of the stabilisa-tion loop, the maximum compensation power from the auxiliarylaser at the input of the EDFA should be at least as large as themaximum expected input signal to the EDFA (integrated overthe entire signal wavelength band). Fig. 11 shows the responseof the service node EDFA to a worst-case transition whereby 15‘ballast’ channels (modulated with 20 dB loud/soft ratio using aMach-Zehnder modulator) together with a ‘target’ channel (asgenerated by the local exchange EDFA in response to a signalwith 11 dB loud/soft ratio) are coupled into the service nodeEDFA. The small droop of the ballast channels is attributed to athermal transient in the MZM. Negligible distortion of the targetchannel is observed. The small residual overshoot at the start ofthe ‘target’ channel packets is a residual transient from the localexchange EDFA. This transient has been shown to result in anegligible 0.3 dB power penalty [23]. Note that in a real system,all individual wavelength channels will be asynchronous with


Fig. 11. Gain stabilization of the service node EDFA: upper trace shows oneof the input ballast channels, lower trace shows the target channel at the outputof the EDFA.

respect to each other, hence the transition shown in Fig. 11 isextremely unlikely to occur.

C. 10-Gb/s Burst Mode Receiver

The BMRx must recover the amplitude information frompackets that can have over 11 dB power difference from onepacket to the next. Therefore, it must adapt its gain and decisionthreshold during the preamble of each packet. Compared toimplementations intended for more conventional TDM-PONs[60]–[62], the long reach and use of optical amplifiers im-pose different and technically challenging requirements onthe BMRx. First of all, the utmost sensitivity is not needed,rather the BMRx needs to be optimized to work in an opticallypreamplified configuration using an EDFA in the service node.Secondly, due to the high number of optical network units(ONUs) connected to an optical line termination (OLT), hightraffic efficiency is required. Therefore, a minimum guard timeof 25.6 ns and a preamble of maximum 25.6 ns (similar tothe ITU-T GPON) were specified. These requirements canonly be met using a dc-coupled BMRx [64], or an edge-de-tecting BMRx [63]. The edge-detecting BMRx, despite beingan elegant solution, may introduce error bursts if an edge ismissed or wrongly detected. Therefore, we chose to implementa dc-coupled BMRx [44], [45], [65]. To further simplify theoperation of the BMRx in this system, two additional featureswere added. First of all, the BMRx automatically detects theend of a burst and subsequently generates its own reset pulse,which is used to erase all information (gain and extractedthreshold) from the previous burst. Automatic generation of therequired reset signal reduces the complexity of the interfacebetween the BMRx and the Medium Access Control (MAC)chip. Secondly, the BMRx detects and signals the start of aburst. This signal can be used to initiate the acquisition ofthe correct decision phase by a subsequent clock and datarecovery (CDR) chip. indicate it should start acquiring its cor-rect decision phase. Furthermore, this signal may aid rangingalgorithms to determine the fiber length between the ONUsand the central office at start-up or connection to the network.Fig. 12 shows the block diagram of the implemented BMRx. Itconsists of two chips: a burst-mode transimpedance amplifier

Fig. 12. BMRx block diagram (TH � Threshold extraction block).

Fig. 13. Principle of fast threshold extraction inside the BMRx (TH �

Threshold extraction).

(BMTIA) featuring fast gain switching (between high andlow gain) and locking, followed by a burst-mode limitingamplifier (BMLA) with fast decision threshold extraction. Fastextraction of the decision threshold results in uncertainty on theextracted threshold, both due to noise as well as imperfectionsof the threshold extraction circuitry. To break this trade-off,the signal is amplified with respect to a reference in multiple(limiting amplifier) stages prior to the final data decision. Asshown by Fig. 13, this results in successive improvement ofthe relative accuracy with which the threshold is extracted.The actual amplitude recovery is performed by the limitingaction of each successive amplifier, whereby the signal swing islimited to well defined clamping levels as indicated on Fig. 13.For strong input signals, this limiting already happens in thefirst stage, for the weakest signal the limiting happens in thelast stage. Each time, the signal is amplified with respect to areference (extracted using peak detectors). For weak signals,this reference is sightly below half the peak signal swing dueto the peak detector action, and otherwise it is equal to halfthe peak signal swing. More details about the operation of theBMLA can be found in [45]. The automatic reset detectionis performed by the BMLA, and a copy of the reset pulse isprovided to the BMTIA to reset its gain setting to its initial(high) state. Its principle is based on the observation that thetime corresponding to the maximum number of consecutive0 s (which is limited to 72, hence corresponding to 7.2 nstime duration) is significantly less than the minimum length ofthe guard time (25.6 ns). Hence the automatic reset detectioncircuitry operates by monitoring the output of the BMLA forgaps that last longer than 8 ns, and generating a pulse wheneversuch a gap is detected. The interested reader is referred to [45]for more details on the automatic reset detection. Fig. 14 showsthe principle of the activity detection circuit. A differentiatoris used to remove slowly varying components (due to slow


Fig. 14. Principle of fast activity detection.

Fig. 15. (a) Top trace: optical input, bottom trace: BMLA output; (b) Tracesfrom top to bottom: output of BMTIA, output of BMLA, Reset signal, Burstdetected signal.

residual transients from the gain stabilized EDFAs for example)from the input signal, retaining only information about thetransitions in the signal. Next this signal is low-pass filteredwith a 170 MHz filter (a compromise between reaction speedon one hand and noise suppression on the other hand), andprovided to a comparator. Whenever at least 20 consecutive 1 sreach the activity detection circuit, the comparator state willswitch. This is latched using a Set/Reset flip-flop, whose outputis then the “Burst Detected” signal. The state of the flip-flop isreset using the reset signal from the BMLA.

Both chips were fabricated using a SiGe:C 0.25 m tech-nology; the chip sizes are 1.2 1.8 mm and 2.3 2.5 mmrespectively. Fig. 16 shows the micrographs of both chips.Fig. 15(a) shows the output of the BMLA when receivingpackets with a loud/soft ratio of 12 dB, demonstrating theamplitude recovery. Fig. 15(b) shows the relationship betweenthe BMTIA output, BMLA output, reset signal and “burstdetected” signal. Within 12 ns after the end of the packet, theBMLA automatically generates a 10 ns reset signal. This signalresets the BMTIA to its high gain state and erases the thresholdsettings in the BMLA. Within 1 ns after the arrival of the newpacket, a rising edge on the “burst-detected” signal indicatesthe arrival of the new packet. The BMTIA switches to low

Fig. 16. Micrograph of (a) BMTIA mounted on ceramic substrate and (b)BMLA.

gain, and within less than 23.8 ns valid data can be seen on theBMLA outputs.

IV. TESTBED AND EXPERIMENTAL RESULTS

A. Experimental Setup

As it was impractical to completely implement the networkas shown in Fig. 3 an equivalent testbed was constructed thatfully emulates the proposed network architecture. Fig. 17 showsthe experimental setup. Attenuation was added to the metro andaccess sections to emulate end-of-life fiber (0.3 dB/km) andworst-case splitting loss. The 50 GHz spaced DWDM operationwas achieved by placing a target channel (on which bit-errorrates (BERs) were measured) between 100 GHz spaced bal-last channels (which served to load the EDFAs with the cor-rectly specified power levels as set out in Section II). The down-stream ballast channels were generated using a bank of 16 DFBlasers. Both the ballast and target channels employed standardlinewidth broadening for SBS suppression of stimulated bril-louin scattering. The downstream target channel was generatedby modulating and boosting the output of a tunable laser withan EAM and an SOA. The downstream ballast channels weregenerated using a second bank of 16 DFB lasers and were mod-ulated with a Mach-Zehnder modulator. The power of the ballastchannels was set 3 dB above the target channel power to emulatethe full load of 32 wavelengths. The upstream channel was gen-erated by modulating the output of the external cavity tunablelaser using an integrated SOA-EAM. For these experiments, anintegrated reflective SOA-EAM was used as no transmissionmode device was available. This has no impact on the presentedresults, however obviously in a real system a transmission modedevice would be used. Burst mode operation fully equivalent tohaving different ONUs active on the TDMA-PON was achievedby generating soft and loud packets by driving the ONU SOAwith an arbitrary waveform generator. In the local exchange,a tunable optical filter (3 dB bandwidth: 0.4 nm) emulated theWDM multiplexer. The upstream ballast channels were gener-ated by carving loud and soft packets (20 dB loud/soft ratio)out of the combined DFB laser outputs using an MZM. Thepower of the loud ballast packets was set 3 dB above the targetchannel’s loud packets, again to emulate a fully loaded networkwith 32 wavelength channels.


Fig. 17. Experimental setup for the tunable approach.

Fig. 18. Downstream spectra (a) at the output of the service node, (b) at theinput of the APD-Rx.

Fig. 19. (a) BER curves of the DS channel (1560.66 nm), (b) System marginand dispersion penalty for representative DS wavelengths.

B. Experimental Results

For all results, all the downstream and upstream channelswere always activated: we observed no penalties due to crosstalkbetween the upstream and downstream channels. All spectra andOSNR numbers were measured with 0.1 nm resolution band-width. Fig. 18(a) shows the spectrum of the downstream signalbefore the metro fiber. Fig. 18(b) shows the spectrum at the inputto the APD-Rx. High OSNR ( dB) can be seen. To eval-uate the downstream, BERs were measured versus the poweron the APD-Rx by adjusting the downstream attenuator (DSatt. in Fig. 17). The results for the 1560.65 nm channel bothfor back-to-back and the transmission fibers included are shownin Fig. 19(a). Of the 101.1 km downstream transmission fibers,

Fig. 20. (a) Burst-mode signalling scheme, (b) Target channel packet wave-forms.

20 km were dispersion compensated using dispersion compen-sating fiber (DCF, ps/nm). A dispersion penalty of 0.5 dBwas measured. The system margin was calculated by subtractingthe power required for a BER of from the worst-caseONU input power ( dBm). A system margin of 3 dB wasachieved at a BER of . The result for representative down-stream wavelengths is shown in Fig. 19(b): a worst-case 2.8 dBsystem margin is achieved; maximum 0.8 dB dispersion penaltycan be seen.

The upstream target channel was evaluated in burst-modeusing an alternating series of 20 loud and 20 soft packets witha length of 1 s, separated by 25.6 ns guard time as shown onFig. 20(a). This packet sequence maximally stresses both thelocal exchange upstream EDFA and the BMRx. Indeed gaintransients in an EDFA have time constants that range from themicrosecond to the millisecond range. Each packet consisted ofa 23.8 ns overhead needed for adjusting the BMRx gain anddecision threshold for the incoming packet and PRBSdata. The maximum loud/soft ratio was 11 dB (equal to theworst-case differential loss of the TDMA-PONs); the achievedupstream transmitter extinction ratio (ER) was 9.5 dB. To stress


Fig. 21. (a) Eye at ONU output, (b) Upstream spectrum launched into the metrofiber.

Fig. 22. BER at BM-Rx output vs. OSNR.

the service node BM-EDFA, the upstream ballast channels alsotransported packets, which had a length of 20 s and were asyn-chronous with respect to the target channel. The loud/soft ratioon the ballast channels was dB, corresponding to the worst-case situation (for the service node EDFA) during which loudONUs on all ballast channels stop transmitting and a dark periodon all ballast channels starts. Fig. 21(a) shows the measured eyeat the output of the ONU: a clear open eye with 9.5 dB extinctionratio can be seen. Of the 100 km upstream transmission fibers,40 km were dispersion compensated using DCF ( ps/nm).The dispersion penalty of the SOA-EAM had a maximum of0.7 dB across the upstream wavelength band. Fig. 21(b) showsthe spectrum of the signal launched into the metro fiber in thecase where the target channel power is equal to a soft packet;the ballast channels are as described above. First, the upstreamtarget channel was set to 1535.4 nm. The local exchange up-stream EDFA gain was measured to be 36 dB; 21.9 dB OSNRwas achieved at the local exchange upstream EDFA output. Thenet (including gain flattening filter and DCF losses) service nodeEDFA gain was 15 dB. The OSNR achieved at the BMRx inputfor worst-case losses was 21.0 dB. The additional 0.9 dB degra-dation is due to the service node upstream EDFA. Next we mea-sured the BER (counting errors in the worst-case soft packetjust after the loud packet, see Fig. 20(a)) versus OSNR by ad-justing the SOA current for the soft packets. The results areshown in Fig. 22. The shape of the BER curve can be under-stood by noting that the input signal to the BMRx not only ex-hibits a varying OSNR, but also a varying signal level, as theOSNR is increased by increasing the launched power from the

Fig. 23. (a) Achieved upstream OSNR at input of BM-Rx (512-split),(b) Achieved upstream BER at 512 split.

ONU (and keeping the same losses in the network). This cor-responds to the true network operation, whereby the signal thatarrives at the service node from an ONU with small access loss,will both have larger signal swing as well as higher OSNR.Hence, the input signal swing increases together with increasingOSNR. For smaller signal swings (corresponding to an OSNRless than 22.5 dB), the threshold extraction on the BMRx ex-tracts a decision threshold that actually lies below halfway theeye opening, thus reducing the BER as expected for a receiveroperating in a signal-ASE beat noise limited regime (where thenoise on the 1 s is larger than the noise on the 0 s). For signalswings with an OSNR higher than 22.5 dB, the BMRx extracts athreshold that lies approximately at the midpoint of the eye, ex-plaining the jump in the BER curve. To estimate the burst-modepenalty, the BER versus OSNR of an equivalent continuousmode receiver with the same 3 dB bandwidth (9 GHz), thermalnoise (1.4 A ) and photodiode responsivity (0.7 A/W) as theBMRx front-end for 9.5 dB extinction ratio is also shown. Thedecision threshold was set at 50% of the eye, corresponding tothe BMRx operation [45]. At a BER of , the BMRx re-quires 26.2 dB OSNR, while the equivalent continuous modereceiver requires 22.0 dB: 4.2 dB burst mode penalty is thus in-curred. If a BMRx were available that sets its decision thresholdat the optimum (for a signal degraded by ASE-signal beat noise)value of 40%, a 3 dB lower OSNR would be required for aBER of . Additional reduction in required OSNR can beobtained by optimizing the electrical bandwidth of the BMRx.Finally, the BMRx output BER for the worst-case OSNR of21.0 dB is . This implies that canbe obtained using the Reed-Solomon (255,223) FEC algorithm(standardized in IEEE 10G-EPON) [3]. Next, operation at rep-resentative upstream wavelengths was verified. Fig. 23(a) showsthe OSNR achieved at the BMRx input for worst-case splittingand fiber losses and Fig. 23(b) shows the corresponding BERs.The 0.5 dB lower achieved OSNR at longer wavelengths is dueto the fact that the local exchange EDFA gain is 3 dB lowerthan at its peak wavelength. The BER remains below the FECthreshold of across the full upstream band.

V. CONCLUSION

We have presented the results of transmission experimentsover an extended hybrid DWDM-TDMA PON, supported bynovel external cavity, tunable lasers in the ONUs. Very high split(32 512), reach (100 km) and capacity (2 32 10 Gb/s)


were demonstrated. The upstream channels were further sup-ported by gain-stabilized EDFAs and a dc-coupled 10 Gb/s burstmode receiver which featured automatic reset generation. Theauthors believe the network concept is a good candidate for fu-ture optical access networks. The proposed architecture sharesthe cost of equipment across a very large customer base, and po-tentially allows the integration of the access and metro networksinto a single, all-optical system. This brings potential cost-sav-ings in terms of both OPEX and CAPEX for the network oper-ator. While the use of TDMA allows direct re-use of deployedoptical distribution PON networks, further research is requiredto address the question of how an operator can migrate from ex-isting PON deployments to our proposed network architecture.Further additional research (for example as already briefly dis-cussed in [26], regarding the implications for the MAC layer,as well as the true performance of FEC in presence of burstmode traffic) and standardization will be required to turn thisproof-of-principle into a mature technology that meets the costtarget for any optical access application.

ACKNOWLEDGMENT

STMicroelectronics is acknowledged for the fabrication ofthe BMRx chipset.

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Author biographies not included at author request due to space constraints.

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