JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 1 Long-Reach Passive Optical Networks Russell P. Davey, Daniel B. Grossman, Senior Member, IEEE, Michael Rasztovits-Wiech, David B. Payne, Derek Nesset, Member, IEEE, A. E. Kelly, Albert Rafel, Shamil Appathurai, and Sheng-Hui Yang, Member, IEEE

Abstract—This paper is a tutorial reviewing research and devel- opment performed over the last few years to extend the reach of passive optical networks using technology such as optical ampli- fiers. Index Terms—Communication systems, networks, optical am- plifiers, optical fiber communications.

I. INTRODUCTION

HE rapid growth of Internet access and services such as T IP video delivery and voice-over IP (VoIP) is accelerating Fig. 1. Typical configuration for B-PON, GE-PON, and G-PON. demand for broadband access. While most broadband services around the world are delivered via copper access networks, op- tical access technology has been commercially available for sev- eral years and is being deployed in volume in some countries [1]. Where optical access is deployed, passive optical networks (PONs) are often the technology of choice because the trans- mission fiber and the central office equipment can be shared by a large number of customers. Early PON deployments were based on B-PON systems as standardized in the ITU-T G.983 series. Fig. 2. Mid-span GPON extension. Currently being installed in Asian countries such as Japan are Ethernet PON (GE-PON) with gigabit transmission capability operation is made possible using wavelength division multi- that complies with IEEE 802.3ah. Meanwhile, operators in the plexing (WDM) with upstream wavelengths in the 1310 nm United States and Europe are now focusing on gigabit-capable region (1260–1360 nm) and downstream wavelengths in the G-PON systems as standardized in ITU-T G.984 series, with 1490 nm region (1480–1500 nm). Capacity is shared among typical bit rates of 2.5 Gbit/s downstream and 1.2 Gbit/s up- subscribers on the PON using a time-division multiple access stream [2]. (TDMA) protocol that assigns transmission time slots for each Fig. 1 shows the system configuration typical for B-PON, user. The maximum reach and split of a PON are determined GE-PON and G-PON. An optical line terminal (OLT) in the by both the PON protocol and the physical layer optical reach. central office is connected to several optical network units The G-PON protocol supports a maximum logical reach of 60 (ONU) via an optical distribution network (ODN) consisting of km and a maximum logical split of up to 128. In practice most optical fibers and passive optical splitters. The ODN is totally commercial systems conform to the class specification that passive, which is very attractive to an operator. Single-fiber allows a maximum optical loss budget of 28 dB: often this is used to deploy a split size of 32 and reach of up to 20 km. Manuscript received June 29, 2008; revised September 12, 2008. Current ver- Similarly GE-PON specifies a maximum distance of 10 or 20 sion published nulldate This work was supported in part by the European Com- mission’s FP6 Project MUSE. The work of D. B. Payne was supported in part km, reflecting the use of different laser types, and offers loss by the European Union through the Welsh Assembly Government. budgets of 20 and 24 dB excluding optical path penalty. In R. P. Davey, D. Nesset, and A. Rafel are with BT, Ipswich, IP5 3RE, U.K. calculating the achievable reach, the total loss must be within (e-mail: [email protected]). D. B. Grossman and S.-H. Yang are with Motorola Applied Research and the allowed loss budget, taking account of realistic fiber and Technology Center, Marlborough, MA 01752IEEE USA. Proof splitter losses. M. Rasztovits-Wiech is with Siemens IT Solutions and Services PSE, A-1210 The concept of increasing the reach and/or split of PONs via Vienna, Austria. D. B. Payne was with BT, Ipswich, IP5 3RE, U.K., and is now with intermediate equipment such as optical amplifiers has been of the [AUTHOR: PLEASE PROVIDE A COMPLETE MAILING AD- research interest since the 1990s [3], [4]. Recently research has DRESS.—ED.] Institute of Advanced Telecommunications, Swansea focused on extending the reach of G-PON and GE-PON via University, Swansea, U.K. A. E. Kelly is with Amphotonix Ltd, Glasgow, G20 0SP, U.K. midspan optical amplifiers [5] or transponders [6] as shown in S. Appathurai is with the [AUTHOR: PLEASE PROVIDE A COMPLETE Fig. 2. This concept has recently been standardized in ITU-T MAILING ADDRESS.—ED.] BT Design. Recommendation G.984.6. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. WebThe OLT is connected Version via a length of fiber known as the op- Digital Object Identifier 10.1109/JLT.2008.2006991 tical trunk line (OTL) to the active midspan extender equipment.

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This in turn is connected to the ODN and ONU. Note the in- deployed in an outdoor environment, there is the potential tention is for the OLT and ONU equipment to be essentially opportunity to close the smaller central offices altogether, unchanged compared to the traditional deployment configura- once all legacy (e.g., copper transmission) equipment has tion shown in Fig. 1. Placement of remote OLTs in the field is a been retired. Section IV will describe how network simpli- possible alternative to reach extension. In this scenario, an OLT fication of this kind is important to reduce end-to-end costs with a small number of PON ports and one or two backhaul ports in an environment where revenues do not increase in pro- is environmentally hardened and deployed in the same fashion portion to bandwidth. as a PON extender. The great advantage of reach extension in The next two sections will discuss development of two PON these applications is in the relative simplicity of the hardware extender technologies. Section II discusses a semiconductor deployed in the field. optical amplifier approach to extending the reach of a stan- Some deployments overlay cable television signals at dard G-PON. Section III describes PON extension applied 1550–1560 nm over the same fibers as G-PON (or B-PON or to a 10-Gbit/s, 100-km reach PON. Section IV discusses the GE-PON) using the enhancement band approach described in drivers for fiber access and simplifying the network architecture ITU-T Recommendation G.984.5. In these situations the cable through PON reach extension. television signals can be conveniently amplified using separate erbium doped fiber amplifiers. II. SEMICONDUCTOR OPTICALLY AMPLIFIED GPON Operators value greatly the passive nature of the access net- Semiconductor optical amplifiers (SOAs) are attractive can- work enabled by the PON architecture, and it is not the intention didates for GPON reach extension. They can provide high gain, of PON reach extension to move away from this. Nevertheless, low noise figure (NF), low polarization dependent loss, and fast having the option of an active midspan reach extender can pro- gain dynamics that are suitable for midspan PON signal ampli- vide several benefits, given as follows. fication. They can be designed to provide gain in the 1310-nm 1) Installing fiber cables represents a significant capital in- (O-band) and 1490-nm (S-band) windows used by GPON. The vestment and so PONs are often deployed in greenfield de- only alternative optical amplifier technology capable of oper- ployments, where cable installation costs (whether copper ating in the O- and S- bands are, respectively, praseodymium or fiber) are an inescapable fact. Often greenfield deploy- and thulium doped fluoride fiber amplifiers. These fibers are dif- ments can be located a long way from existing central of- ficult to work with [1], and the technology did not appear to fice buildings—potentially beyond the reach of G-PON or be ready for commercialization; however, recent work appears GE-PON. In this solution, one could build a new central promising [8]. Other advantages of SOAs relative to fiber ampli- office building or house PON OLT equipment in a street fiers in this application include their small size, high reliability, cabinet. Clearly, there are significant capital and opera- and low power consumption. tional costs associated with building a central office, nor is An important consideration for optical amplification of PONs it especially attractive to deploy a full OLT in a street cab- is burst mode operation in the upstream. In GPON, upstream inet. An attractive alternative could therefore be a simple bursts from different ONUs can have a dynamic range of up midspan PON extender box deployed in a street cabinet to 10 dB, due to differential fiber loss between the nearest and (or underground footway box). To offer benefits over the furthest ONU on the PON, and variation in input signal level. street-based OLT approach, the PON extender should be Fiber amplifiers exhibit a slow relaxation response to variations compact, low-power, and cost-effective and require min- in input signal level, which results in output signal distortion. imal configuration and management. As a result, the amplifier’s average output power varies slowly 2) A remote PON extender could give operators more flex- over many bit times at the beginning of each burst. The deci- ibility in deployments. When deploying PONs on long sion threshold circuit in the OLT cannot track receive power loops, the loss budget may not allow the operator to deploy variations on this scale, resulting in excessive BER. This oc- as great a split as they would on shorter loops. By using curs whether the amplifier’s control regime is constant gain or a PON extender, the operator has more possibilities to constant power, and whether its operating regime is linear or sat- deliver the same split regardless of geography. urated. As a result, a gain clamping scheme like that described 3) In areas of sparse take-up, a PON extender could be used in [8] is needed. Because SOAs have a fast relaxation response, to improve the PON utilization. The benefit would be im- they are able to operate in burst mode without need for addi- proved sharing of feeder fiberIEEE and OLT equipment. tional Proof control circuitry or out-of-band optical stabilizing signals. 4) A PON extender box could avoid the need for additional backhaul or metro network equipment to connect the OLT A. High-Gain SOAs for GPON Reach Extension in a minor central office to switching and routing equip- In this section we describe the development of a high-gain ment in major central offices. SOA module for use in GPON extension applications using the 5) A midspan PON extender could allow node consolidation restricted upstream wavelength range proposed in ITU-T Rec- by locating OLTs in a reduced number of major central of- ommendation G.984.5. We also describe the operation of the de- fices. PON extender boxes would then be housed at the vice in extending the loss budget of a commercial GPON system remaining minor central office locations. In the shorter to 54 dB. term this simplifies network operations sinceWeb the extender The system in thisVersion application is operating in the gain limit would require minimal configuration and management. In regime [9], where margin improvements are achieved by in- the longer term, provided that the PON extender can be creasing the SOA gain. The SOA is therefore designed to pro- DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 3

The OLT transmits at 1490 nm in the downstream with dBm output power. The ONU uses a DFB transmitter at 1310 nm in the upstream with mean output power during a burst of , , and dBm for ONU#1, ONU#2, and ONU#3, respectively. The three ONUs had slightly differing output powers due to device-to-device variability allowed within the G-PON standards. The OLT Rx is an APD device and the ONU uses a PIN. Optical filtering to 18 nm is provided in the upstream direc- tion via a 1310-nm CWDM add/drop multiplexer used to com- bine the upstream and downstream signals at the GPON ex- tender. No extra filtering was used in the downstream beyond that included in the ONU diplexer. At the OLT, VOAs were used to build out the attenuation of Fig. 3. Measured characteristics of developed high-gain SOA module. the fiber to typical values that might be observed in installed fiber cables. In the downstream the VOA was set to give a mean fiber loss of 0.30 dB/km at 1500 nm. In the upstream a mean loss duce dB gain over the 1300–1320 nm band. This is more of 0.46 dB/km was used to account for worst case losses over than is required in an SOA pre-amplified receiver where the the entire GPON upstream wavelength range of 1260–1360 nm. maximum sensitivity is achieved at gains dB. The noise Bidirectional, fast Ethernet trails (100 Mb/s) were configured figure is not a limiting factor and could be relaxed providing between the ONUs and the OLT. We measured Ethernet frame the ASE does not produce a DC offset that compromises the loss as the system parameters are adjusted. A system BER for burst-mode Rx performance. the link is inferred from this frame loss measurement. The SOA device we have developed is similar in cross section To assess the system performance with the new SOA device, to those reported in [10]: these high-gain variants have a mod- the number of lost Ethernet frames in a 10-min period was mea- ified active region composition to optimise the wavelength and sured as a function of the access network VOA setting. These the cavity length has been increased to 800 m. This device has measurements are converted into BER and the results can be been packaged hermetically as described in [10] using lensed seen in Fig. 5. fibers. The module performance at 200 mA is shown in Fig. 3. For all ONUs, a BER is obtained in both directions Between the wavelengths of 1300 and 1320 nm, a minimum of at access losses corresponding to 128-way split and 10 km of 27 dB of gain is available with noise figure and polarisation de- access fiber (so 60 km total fiber length). In the upstream, the pendent gain values less than 7 and 0.7 dB, respectively. The worst case ONU gives 1.5 dB margin and in the downstream a increase in noise figure compared with [10] is attributed to an minimum of 0.6 dB margin was recorded. In the downstream the increased level of ASE self saturation leading to a lower carrier “black box” sensitivity of the ONU receiver can be seen, which density at both the input and output of the device. At a drive is attributable to some combination of device variation and/or current of 250 mA and over the operating range of 0 C–70 C, coupling losses. the SOA and associated drive electronics has a calculated total power consumption of less than 2 W. B. GPON Reach Extender Prototype Fig. 4 shows the layout of the system experiment. The GPON OLT is connected to the GPON extender by a 50-km G.652 A prototype of an all-optical GPON Reach Extender backhaul fiber. The GPON extender consists of the newly de- (GPON-RE) was developed by a team at Motorola. To validate veloped high-gain 1300-nm SOA and a commercially available the system, bench top link measurements were conducted 1490-nm SOA for the downstream [11]. Both SOAs are con- at a Motorola facility in the US, and a field experiment was nected to the access network via a 4 4 optical splitter. The conducted at a BT facility in the UK. The measurements were SOAs were driven with injection currents of 240 and 500 mA also cross-checked against a numerical model developed by for the upstream and downstream SOAs, respectively. one of the authors. The purpose of this work was to show that To represent the access networkIEEE we used 10 km of G.652 GPON Proof reach extension using optical amplification is ready for fiber connected to one port of the 4 4 coupler followed by commercialization and deployment under field conditions, that a1 32 optical splitter. The 1 32 optical splitter is in turn a GPON-RE can be managed as a network element, and that connected to three GPON ONUs. Unused ports of the 4 4 are protection in the GPON-RE is a feasible approach to dual connected to additional 1 32 splitters on the downstream side parenting. and terminated to avoid reflections on the upstream side. Unused The GPON-RE prototype is shown in Fig. 6. It is compatible ports of the 1 32 splitters were not similarly terminated. with commercially available Motorola GPON ONUs and OLTs, A variable optical attenuator (VOAs) is used on the access and is managed by the Motorola AXSVision element manage- network side of each SOA to account for the excess losses in the ment system [12]. The GPON-RE is housed in a weatherproof fiber and splitters and the wavelength dependenceWeb of the fiber enclosure, which wasVersion adapted from a widely deployed Motorola loss. Losses of 17.2 dB and 6.8 dB are assumed for the 1 32 product. The enclosure contains two amplifier cards that can ei- and 4 4 couplers, respectively. ther operate independently or in a protected configuration. 4 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 4. System experiment for high-gain SOA GPON reach extension.

Fig. 5. BER versus total access loss for (a) upstream traffic and (b) downstream traffic.

Fig. 7. Experimental setup for laboratory bench experiments.

and trunk fibers. Commercially available SOAs from two ven- dors were used. The amplifier cards also include a microprocessor with DRAM, SRAM, flash, Ethernet and serial interfaces, control IEEEcircuitry Proof and drivers for SOA bias current and thermo- elec- tric coolers, and optical power monitors. Embedded software Fig. 6. Motorola GPON extender prototype (shown with enclosure open). included controls for SOA temperature and drive current, a protocol for coordinating fail-over between working and protection amplifier cards, an SNMP management agent, local It also contains two embedded ONUs for manager communi- control terminal and an IP/Ethernet protocol stack. cations and an Ethernet switch which supports communications The protection switching scheme provides redundancy amongst the other cards. The embedded ONUs are used to trans- for the amplifier cards, trunk fiber and entire OLT. In the dis- port network management traffic over Ethernet frames between tributed split arrangement shown in Fig. 8, it protects against the GPON-RE and the element management systemWeb (EMS). most failures that couldVersion affect all subscribers on a PON, and Each amplifier card contains two SOAs (one for each direc- also against catastrophic central office outages. In this scheme, tion), coupled through diplex filters to the bidirectional feeder faults are detected by optical power monitoring in the extender. DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 5

Fig. 8. Setup of amplified GPON reach extender field experiment.

The working and protection amplifier cards exchange messages to determine whether a fail-over is possible and likely to be fruitful. Switching takes advantage of the fact that SOAs are opaque when bias current is withdrawn. Thus, once the working and protection amplifier cards decide to fail over, the working amplifier’s SOAs are turned off, and the protection amplifier’s SOAs are turned on.

C. Experimental Setup The setup for the bench top link measurements is shown in Fig. 7. A test fixture containing a GPON OLT transceiver SFP was connected to an Agilent 81250A Bit Error Rate Tester (BERT). The transceiver was connected through a variable optical attenuator (VOA1) and 0–50 km of fiber (to emulate the OTL) to the input port of the GPON-RE. The output port of the GPON-RE was connected through 0–24 km of fiber and Fig. 9. GPON reach extender in footway joint box. a second VOA (VOA2) to a GPON ONU transceiver in a test fixture to emulate the ODN. The test fixture was also connected to the BERT. For upstream operation, the BERT was configured loss between OLT and ONU over a 2-min period as the system for burst mode operation. An Agilent 86100C digital sampling parameters were varied. oscilloscope was used to display the upstream and downstream In both the benchtop and field experiments, forward error cor- eye patterns. Optical spectrum was observed using an Agilent rection (FEC) was not enabled in either direction. All receiver 86146 optical spectrum analyzer. sensitivity measurements are referenced to a BER of . The setup for the field experiment is shown in Fig. 8. The OLTs and ONUs were located in a BT lab and the GPON-RE D. Results was installed in an underground footway joint box at a nearby 1) Benchtop Experiment: In order to cover a wider OSNR outside plant evaluation facility (see Fig. 9). The GPON-RE is range, BER measurements were made with only 24 km of trans- connected to the OLTs by two 50-kmIEEE trunk fibers, forming a mission Proof fiber. To justify this approach, the additional path penal- working and a protection path. VOAs were used to vary the ties of the 50-km OTL fiber/10-km ODN fiber configuration trunk loss on each path. The extender box was connected to were measured at higher OSNR in both directions and found a 2:4 splitter, which in turn was connected to a 1:32 splitter. to be less than 0.25 dB. In order to allow any SOA induced fiber The splitters were located in a splice enclosure in the footway transmission penalty to show up, the 24 km of fiber was posi- box. This splitter arrangement was connected back to the lab tioned after the SOA: so in the ODN for downstream and OTL by lengths of fiber. A VOA was used to vary distribution loss. for the upstream (referring to Figs. 2 and 7). The PON terminated on several ONUs. Typical BT installation Before conducting BER measurement, the GPON-RE was practices were used throughout. Motorola AXS 2200 and AXS characterized as a black box. In particular, extrinsic gain, NF, 1800 OLTs, and ONT1100GE ONUs were used. BidirectionalWeband amplified spontaneous Version emission (ASE) bandwidth were 400 Mb/s Ethernet trails were configured between an ONU and measured, to allowed estimation of signal and noise power and the working OLT. An Ethernet tester measured Ethernet frame correlation with simulation results. 6 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 12. Downstream BER performance normalized to Class fC Fig. 10. Downstream optical spectrum and (filtered) eye diagrams measured at specifications. GPON-RE output.

eyes shown in Fig. 10(b). BER flooring is observed when OSNR is less than 19 dB. This is readily described by a receiver model that takes into account ASE-ASE beat noise and ASE-enhanced shot noise introduced by optical amplification. The GPON PHY Recommendation requires a back-to-back ONU receiver sensitivity of better than dBm to achieve a Class (28 dB) ODN loss budget. Treating a receiver with this sensitivity as the worst expected case, we normalize the test results to this specification and replot BER versus ODN loss in Fig. 12. For a typical OTL loss of less than 20 dB, larger than 27-dB ODN loss can be supported, which represents a total loss budget of more than 19 dB than that of a nonextended GPON. Upstream Transmission: Upstream optical spectrum from one of the amplifier cards is shown in Fig. 13(a). The ASE spectral width is externally filtered to 35 nm by an optical bandpass filter. Measured small-signal gain and NF of the GPON-RE are 20.5 and 7.4 dB, respectively. Representative optical eyes are shown in Fig. 13(b). As in the downstream, pulse distortion is evident when SOA is driven into saturation; Fig. 11. Downstream receiver sensitivity versus OSNR (defined over 0.1-nm however the resultant penalty ( dB) is smaller. bandwidth). Burst-mode BER performance over 0-km ODN fiber/24-km OTL fiber is shown in Fig. 14. Sensitivity without amplification IEEEis ProofdBm, which is significantly better than the G.984.2 Downstream Transmission: Downstream optical spectrum specification of dBm for Class . The abrupt change from one of the amplifier cards is shown in Fig. 10(a). The in BER at very low input powers is characteristic of the deci- ASE spectrum is not externally filtered. Measured small-signal sion-threshold reset circuitry of the particular OLT receiver used gain and NF of the GPON-RE are 22.2 and 9.2 dB, respec- in our experiment. Therefore, a longer than typical preamble tively. BER performance over 0-km OTL fiber/24-km ODN was employed to better reveal the BER performance trend. We fiber is shown in Fig. 11. Sensitivity without amplification is note that the OLT receiver AGC circuitry which reacts to total dBm. ( ) power is partly responsible for BER flooring at We observe a 3-dB penalty that cannot be overcome by in- -dB OSNR. creasing OSNR. This is attributed to pulse distortionWeb caused by We normalize theVersion test results to the specification and re- the SOA when it is driven into saturation at higher OSNR (or, plot BER versus OTL loss in Fig. 15. Again, we observe that, for equivalently, higher input power), as evidenced by the optical a typical OTL loss of less than 20 dB, larger than 27-dB ODN DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 7

Fig. 15. Upstream burst-mode BER performance normalized to Class fC specifications. Fig. 13. Upstream optical spectrum and (filtered) eye diagrams measured at GPON-RE output.

Fig. 16. Operating envelopes for an extended GPON, normalized to G.984.2 specifications for a Class fC loss budget. Hashed area represents the admissible operating region.

was AC-coupled. The OLT decision threshold (as a percentage of eye opening) was determined by the same curve-fitting procedure. The GPON-RE measurements, along with OLT and Fig. 14. Upstream burst-mode receiver sensitivity versus OSNR (defined over ONU receiver models, were used to simulate the upstream and 0.1-nm bandwidth). downstream performance (respectively) of the extended GPON. IEEE TheProof operating envelopes of the amplified link are of par- ticular interest in engineering extended GPON deployments. loss can be supported, which represents a 19 dB greater total Therefore, we calculated the constant-BER contour over the loss budget than for that a nonextended GPON. ODN loss-OTL loss plane. The results, normalized to the 2) Link Model: A link model and numerical simulation G.984.2 specifications for the Class loss budget, are plotted were used to better understand the optical link characteristics in Fig. 16. Measured results are overlaid on the same plot. Loss of the extended GPON. The GPON-RE was characterized as values within the overlapping area (the hashed area) enclosed a black box. Signal gain, noise figure, and ASE bandwidth by contours lines are admissible. were measured as input power to the GPON-RE was varied. A good match is observed between simulation and measure- ONU and OLT receiver models were constructedWeb to closely ment results in bothVersion directions. However, a noticeable discrep- fit measured back-to-back BER curves. Typical APD and TIA ancy occurs in downstream when trunk loss is low. In this region parameters were used. The ONU receiver decision threshold pulse distortion at GPON-RE output is the dominant cause for 8 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 downstream sensitivity degradation but is not considered by our III. XL-PON PROTOTYPE: A 10-GBIT/S, 100-KM REACH PON model. The horizontal line describes the maximum OTL loss A. Introduction limit imposed by the embedded (management) ONU in the The extended GPON system described in the previous sec- GPON-RE. This ONU is coupled to the OTL through the 20% tion enables PON solutions to reach 128-way split with 60-km leg of an 80%-20% optical tap. This limit could be removed if reach for 2.5-Gbit/s downstream and 1.25-Gbit/s upstream pay- an alternative path for element manager communications were loads. This will enable a degree of node consolidation and net- available. work simplification. However to be more generally useful for The model enables us to analyze the impact of SOA parame- network architecture restructuring and reduce costs still further, ters (notably gain and NF) on operating envelopes and provide even longer reach and larger splits may be beneficial in the fu- guidance on SOA specifications. Since OTL and ODN losses ture. In this section we describe a prototype for next generation are asymmetric in typical applications, downstream transmis- optical access using PONs operating at 10 Gbit/s with a long sion tends to be power- and pulse distortion-limited, whereas reach (100 km) and high split (512-way per wavelength). In this upstream transmission tends to be more ASE-limited. There- paper, we refer to this prototype as XL-PON: in common with fore, increasing SOA gain is most effective for extending loss today’s commercially-available G-PON it uses a power-splitter budget in downstream, whereas reducing NF and limiting ASE based optical distribution network (ODN) and furthermore uses bandwidth are more effective in upstream. a very similar time division multiple access protocol to G-PON. 3) Field Experiment: The amplified GPON system was in- Practically, one has to take into account a maximum inser- stalled in the field experiment configuration of Fig. 4, initially tion loss of dB per 1:2 split for the optical power splitter without VOAs. Over a five-day period, a BER of or better and a variation of dB, which results in 25 – 35 dB inser- was observed in both directions. As of this writing, the amplifier tion loss for a 1:1024 splitter. The loss of a 100 km long optical has been operating in the field site for four and a half months. fiber is about 25 to 35 dB, mainly depending on whether the The only significant issue to date was traced to reflections in one 1.55- m range (C-band) or the 1.3- m range (O-band) is con- of the fibers. sidered. That means the typical loss of such an XL-PON can be To demonstrate protection switching, an IPTV HD video ser- about 60 dB for C-band operation or even more in O-band. vice was delivered across the system. The working trunk fiber To meet such high link budget demands at 10 Gbit/s, there is was disconnected and the video service switched over to the pro- no feasible solution to adapt present laser technology to achieve tection path and resumed playing in about 20 seconds. sufficiently high transmit power, and it is also not feasible to Measurements were taken with the field experiment config- increase receive sensitivity of photo-detectors. The only tech- uration, including the VOAs for BER versus input power anal- nological choice to realize an XL-PON for both extended ODN ysis. These are analyzed in [13]. loss budget and long-reach is optical amplification (OA). Op- tical amplification at each ONU is costly and so is not consid- E. Conclusions ered further here. Rather optical amplifiers should be used in the regions of the PON where their cost can be shared among mul- We have shown that a midspan reach extender based on SOAs tiple customers: at the OLT or a midspan amplifier where it is can increase the physical reach of a commercial G-PON system better optimized for signal to noise ratio(SNR). In this way op- to 60 km and the physical split ratio to 128:1. In this way, the tical amplifiers can be cost-effectively introduced and further- physical layer capabilities of G-PON can match the logical lim- more provide the opportunity to save component costs in the itations (reach and ONUs per PON) of the GPON transmission ONU. For example a high power laser transmitter can be re- convergence protocol. Furthermore we have successfully pro- placed by a low power type made from cheaper technology, and totyped and experimentally deployed a midspan GPON reach an avalanche photodetector (APD) may be replaced by a cheaper extender based on semiconductor optical amplifiers. BER per- PIN photodetector. formance was shown to be well within GPON objectives. We For the reasons discussed in the previous section, semicon- are confident that adequate robustness can be achieved with ex- ductor optical amplifiers (SOA) are an attractive option for pected improvements in the SOA devices and GPON-RE de- G-PON reach extension. For XL-PON there is as yet no agreed sign. In addition, we have demonstrated that the GPON-RE can standard wavelength plan and so we have some freedom to be deployed in an outdoor environment,IEEE that it can be managed choose Proof operating wavelengths. Given the challenging nature of as a network element, and that it can be protected in a the loss budget for XL-PON, the C-band ( to 1565 nm) configuration. and L-band ( to 1610 nm) spectral regions commonly We have established a model to analyze the amplified GPON used for metro and long-haul networks are attractive. The link, plotted operating envelopes, and correlated experimental fiber loss is low in these regions and optical components are and simulation results. While the model does not take into widely available. In particular, the Erbium doped fiber amplifier account SOA-induced pulse distortion and decision threshold (EDFA) is commercially available in the C-band and L-band adjustment mechanism of realistic OLT receivers, it is useful and has better noise performance than an SOA. It shall be noted for generating system engineering guidelines and defining SOA that an EDFA could in principle be deployed in a way that just specifications. Thus, with approval of ITU-T RecommendationWebthe erbium-doped fiberVersion is located in the outside fiber plant and G.984.6, GPON reach extension using optical amplification is the pump light (e.g., at 1480 nm) is provided from a remote now ready for commercialisation. location (e.g., an office node). Thus, electrical powering of DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 9

• no cross-gain modulation between up- and downstream channels; • optical amplifiers can include isolators, hence they are not sensitive to reflections; • different amplifier technologies can be used for upstream and downstream direction. In Fig. 18, the second approach is shown. Here a bidirectional optical amplifier (BOA) is located directly at the OLT’s inter- face towards the ODN, the S/R interface. The optical diplexer separating the up- and the downstream signals can already in- clude the optical bandpass filter (OBF, see Fig. 18) required Fig. 17. OLT architecture employing two optical amplifiers. Downstream and upstream signal are amplified by a separate device. to restrict the bandwidth and power of amplified spontaneous emission (ASE) generated in the optical amplifier in upstream direction. An OLT architecture equipped with a bidirectional amplifier include the following advantages: • simpler architecture, fewer components, and thus lower cost; • diplexer loss does not contribute to effective upstream noise figure; • a diplexer can already provide the upstream bandpass filter function; • beneficial for EDFAs, as it enables amplification of bursty upstream transmission (see below). Fig. 18. OLT architecture employing a bidirectional amplifier. Both down- stream and upstream signal are amplified in the same gain medium. B. Achievable Performance The presence of amplified spontaneous emission (ASE), the amplifiers situated in the outside plant could potentially which is generated in the OA, results in an electrical dc-current be avoided if this provides operational advantages. Instead in the optical receiver. If the optical bandwidth is wide, ASE of an amplifying fiber, also waveguide designs are possible: power may be significantly larger than the signal power, which erbium-doped waveguide amplifiers (EDWAs), described e.g., may impact particular receiver implementations. in [14], are more compact than EDFAs. Other types of fiber After photodetection of optically amplified signals, additional amplifiers, for other bands, are not yet viable for deployment beat-noise is generated: signal-ASE beat noise and ASE-ASE in telecommunication systems. beat noise. The corresponding beat-noise variance terms due to As an example for a single-sided OA, two different architec- ASE are [15]: tures of optical amplifiers are shown in Figs. 17 and 18, respec- tively. One approach (Fig. 17) uses separate optical amplifiers (1) for up- and for downstream signals, while the other approach (2) uses a bidirectional optical amplifier, characterized by ampli- fication of both up- and downstream signals in the same gain where is the noise variance of the signal-ASE beat medium. noise, is the photodiode responsitivity, is the optical input In Fig. 17 the OLT’s interface towards the ODN, the S/R in- power of the signal, is the spectral ASE density, is terface, is a single fiber interface, as usual for PONs. An op- the electrical bandwidth of the photoreceiver, is the tical diplexer separates the upstream and the downstream sig- noise variance of the ASE-ASE beat noise, and is the band- nals, which are carried on different wavelengths. The insertion width of an optical filter located between optical amplifier and loss of this diplexer has to be compensated by higher output the photoreceiver. power of the downstream OA andIEEE better noise figure of the up- If anProof optical signal arrives with an optical power level well stream OA. After upstream amplification an optical bandpass above the photodetector’s sensitivity limit, signal-ASE beat filter (OBF, see Fig. 17) is used to restrict the power of ampli- noise and ASE-ASE beat noise will dominate. Then there fied spontaneous emission (ASE) generated in the upstream OA are two possibilities to influence the amount of noise and its (see below for impact of the choice of the OBF bandwidth). associated bit-error-ratio (BER): the first is to keep the amount The advantages of the OLT architecture equipped with two of low by choosing an amplifier with low noise figure, amplifiers include: and the second is to reduce the optical bandwidth to keep • uUpstream and downstream channel can be located in in- the ASE-ASE beat noise term low. dependent bands; In the following we will evaluate the different conditions for • gain for up- and downstream can be set independently;Webthe different signal directionsVersion in the PON. • the upstream amplifier can be optimized for best noise 1) Downstream Transmission: In the downstream direction figure; optical amplifiers are employed as boosters at the OLT or as 10 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 19. Sensitivities at 10 Gbit/s with and without optical pre-amplification and various optical bandwidths. midspan amplifiers. Here usually beat noise terms are negligible bandwidth provides a sensitivity gain of 3.7 dB compared to 20 as both boosters and midspan amplifiers are operated at high to nm bandwidth (see Fig. 20). medium signal input values. Due to high ODN loss the pho- The tradeoff for an ONU laser is therefore: either lower power toreceiver sensitivity at the ONU limits the performance. At 10 or wavelength stabilization. Gbit/s and ODN insertion loss of 33 dB or higher, there is no 3) Forward Error Correction: Forward error correction can possibility to avoid costly avalanche photo-detector (APD) re- be employed to improve the loss budget of an optically ampli- ceivers at ONU, as the allowed transmit power level into the fied PON. For example the standard RS(255,239) FEC code em- optical fiber is limited by fiber non-linearities. ployed in GPON is able to correct a BER of to .In 2) Upstream Transmission: Upstream transmission and op- this way the receiver sensitivity is derived for a BER of , tical amplification is the more critical challenge in a XL-PON which is improved compared to , and the improvement is due to the high loss of the splitter, the bursty nature of the up- called FEC-gain. Under FEC, the sensitivity gain and thus loss stream signal, and the desire to keep ONU laser powers as low budget with 0.5 nm optical filtering compared to 20 nm band- as possible. The optical input power into the upstream pream- width is even further increased: 3.5 dB at 10 bit/s (see Fig. 3 and plifier will be very low, thus requiring a low noise figure. compare optical input power values for pre-amplified 20 nm and Next generation PONs may use either 10 Gbit/s upstream data 0.5 nm at ) and 5 dB at 2.5 Gbit/s (see Fig. 4). rates (symmetrical approach) or lower data rates of, for example, The reason for this phenomenon is the input power depen- 2.5 Gbit/s. Figs. 19 and 20 show the calculated BER perfor- dence of the signal-ASE beat noise (see (1)) which results in mance for 10 Gbit/s (Fig. 19) and 2.5 Gbit/s (Fig. 20), both op- flat BER curves. Therefore, the usage of FEC especially bene- tically pre-amplified [16]. An OA noise figure of 7 dB, C-band fits the upstream optical amplification. transmission, typical photodetector data, and a 6 dB extinction Related physical layer optical amplification experiments re- ratio of the transmit signal is assumed. Transmission penalties lated to 10 Gbit/s PONs were reported, e.g., in [17], [18]. and burst-mode penalties are neglected in this example. In both 4) Burst Mode Optical Amplification: The calculations cases typical performance of unamplified transmission using a shown above did not consider an additional penalty due to PIN or an APD (avalanche photodiode)IEEE detector is shown for upstream Proof burst-mode operation in a PON. Due to differential comparison. The performance curves for the optically ampli- loss in the ODN (due to different fiber length and splitter ports), fied system are shown for three different optical bandwidths, adjacent bursts may arrive with different optical power levels. 20 nm, 6 nm, and 0.5 nm. This corresponds to the upstream A maximum difference of 15 dB is usually assumed. This transmitter laser at the ONU being uncooled or simply cooled causes big challenges for the OLT optical receiver, but also may or wavelength-stabilized (by means of a thermo-electric cooler impact on upstream optical amplifier design. and a temperature control circuit), respectively. While amplification of burst-mode signals using semicon- At 10 Gbit/s and optical filtering with 0.5 ductor optical amplifiers is more straight-forward, EDFAs may nm bandwidth provides a sensitivity gain of 2 dB compared to show large gain transients if average signal power changes with 20 nm bandwidth (see Fig. 19 and compare opticalWeb input power relatively low speed, Version resulting in huge distortions of the signal. values for preamplified 20 nm and 0.5 nm at ). If an EDFA shall be used for upstream transmission there are At 2.5 Gbit/s and optical filtering with 0.5 nm DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 11

Fig. 20. Sensitivities at 2.5 Gbit/s with and without optical pre-amplification and various optical bandwidths. four relatively simple possibilities to enable burst mode opera- Recent results at 2.5 Gbit/s burst-mode reception were achieved tion: using commercial components [22], [23]. — Application of an additional continuous saturating signal There is already research work on integrated 10 Gbit/s with significantly higher power than the strongest up- BM-Rx [24]–[26]. Though those results are still far from the stream burst. targets for preamble length and guard time in mind at 10 Gbit/s — Inclusion of optical feedback before and after the Erbium upstream, the results give good confidence that suitable inte- fiber, resulting in an oscillation at a separate wavelength grated 2.5 Gbit/s burst-mode receivers can be made available which then serves as an internal saturating signal and thus soon after standardisation. establishes a constant gain for the upstream channel. Such 6) Burst Mode Transmitter: Burst-mode transmitters schemes are known as “gain clamped” EDFAs and de- (BM-Tx) are considered as the main cost challenge for a future scribed e.g., in [19]. high speed PON. As a burst-mode transmitter is required in — A bidirectional architecture (see Fig. 18) where the larger every customer unit (optical network termination, ONU), direct downstream signal saturates the EDFA which then pro- modulation is the best chance to keep costs low. A burst mode vides constant gain for the weak upstream. This scheme laser driver must provide two independent currents to the has additional advantages already described above, how- laser, a bias current, slightly above the laser threshold and a ever it requires that both upstream and downstream sig- proper modulation current. Both currents have to be applied nals are within the gain band of the EDFA. As isolators very accurately in order to provide a useful eye opening and cannot be used, reflections and back-scattering distortions sufficient extinction ratio which is important to achieve a good may occur. Practically this limits the achievable gain to performance after optical amplification. Further, the driving some 20 dB [20]. circuit has to maintain these optimum settings over the full — Active power compensation. As the saturating signal men- life-time and temperature range. The driving circuit has to tioned above wastes power, a dynamically controlled com- fully switch-off the laser, otherwise spontaneous emission of pensation signal can be used as described in [21]. The hundreds of ONUs would add up at the optical splitter and power level at the input is measured,IEEE and the power of the interfere Proof with the active burst. After going active the driving compensation signal (at different wavelength and filtered circuit has to re-establish a valid setting in few nanoseconds, out after the amplifier) is controlled in a way that the sum which is only possible by storing the values from the previous of the two signals is always constant, and thus the oper- burst. ating point of the EDFA is maintained. 5) Burst Mode Receiver: Burst-mode receivers (BM-Rx) are C. XL-PON Prototype considered as one of the key technical challenges in developing As a first step towards an evolution to a next generation op- a high data rate PON. Today, commercially available BM-Rxs tical access network, and to evaluate the technical feasibility of operate at 1.2 Gbit/s (GPON). Especially in XL-PONs a crit- such systems, a prototype of an XL-PON system was realized in ical requirement for the BM-Rx is to meet huge burst-dynamicsWebthe framework of theVersion European research project MUSE (Multi at very low adaptation and synchronisation time, important to Service Access Everywhere) [27]. The features of the prototype keep preamble length short and thus bandwidth efficiency high. were high data rates (10 Gbit/s in downstream and 2.5 Gbit/s 12 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

TABLE I XL-PON PARAMETERS.

tions were necessary, mainly to increase the number of possible ONUs to 512. For the upstream channel, forward error correc- tion (FEC) using the standard Reed–Solomon code RS(255,239) is implemented. For simplicity, OLT and ONU share the same mother-board design. The key-components are: — a commercial 10 Gbit/s transceiver module for down- stream where the transmit part is used at OLT and the APD-based receive part at ONU Fig. 21. XL-PON overview. — at ONU: a self-designed 2.5 Gbit/s burst mode transmitter module using a wavelength stabilized (1531.1 nm) direct modulated DFB laser with 6 dBm output power in upstream), high split (1:512), long reach of 100 km, and, in — at OLT: a self-designed 2.5 Gbit/s burst mode receiver addition dense wavelength division multiplexing (DWDM) in module using an APD photodiode. The design of this the metro (feeder, backhaul or trunk) part of the PON. The in- receiver is similar to the one used at the burst mode troduction DWDM technology in the metro part of the PON is transponder at MAP (see below) intended to share fibers in the metro-network (MN) part of this — 10GE interfaces on ONU and OLT, based on commercial XL-PON. It results in a hybrid time division multiplex (TDM) XFP-modules and WDM PON approach. — an FPGA (field programmable gate array) at both OLT and This XL-PON prototype uses a similar time division multiple ONU for processing the TC layer access protocol as GPON, and thus provides high bandwidth — an on-board controller efficiency. It further includes the functionalities of a real PON 2) Metro Access Point – MAP: Fig. 22 shows the inner con- system, like ranging and activation of ONUs, and real data trans- figuration of the MAP. By means of an optical drop filter a par- mission. ticular downstream channel (DSC) is selected out of the WDM Fig. 21 shows the configuration of the XL-PON. The network signal in the MN. The selected channel is forwarded to a com- consists of two parts: a metro network (MN) ring which is shared mercial Erbium-doped fiber amplifier (EDFA) acting as pre-am- among several central OLTs by means of DWDM, and several plifier which operates at constant output power. After passing a optical (passive) distribution networks (DN), each assigned to duplexer the DSC reaches a further EDFA (BEDFA) which is one OLT and serving up to 512 customer ONUs over up to 30 operated bidirectionally, i.e., it amplifies the DSC and the up- km of optical fiber. The DN is operated in duplex mode (bidi- stream channel (USC) in the same Erbium fiber. In this way, rectional transmission on a single fiber). as already discussed above, it is possible to amplify the bursty The proposed concept includes an additional network node upstream signal without distortion by an EDFA [28]. The down- called MAP (metro access point), serving as an interface be- stream output power at the BEDFA is controlled to a constant tween MN and DN, and providing the functions of a midspan power level of 18 dBm to achieve sufficient power at the ONU. optical amplifier among other things. As the MN is operated Both preamplifier and BEDFA are operated with a single pump- in WDM, simultaneous operation of multiple independent laser. XL-PONs on a single fiber ring is possible. Though imple- The DN side of the MAP includes a 1:8 splitter. This ensures menting WDM, the concept allowsIEEE for all ONU operating at eye-safe Proof power levels in the DN and avoids non-linear effects in the same upstream wavelength, in this case 1531.1 nm, due to the fiber. an upstream transponder in the MAP, as shown below. This is The USC bursts arrive at different power levels at the input of important to simplify inventory management. All ONUs are the BEDFA, mainly due to differences in fiber loss and due to able to receive all possible downstream wavelengths, in our splitter tolerance. We assume a range between and case channels in a 100 GHz grid around 1555 nm. . The BEDFA gain is set to 20 dB (almost equal Table I summarizes the key parameters of the XL-PON. for up- and downstream) by means of output power control of 1) OLT and ONU: OLT and ONU prototypes include TC both EDFAs, hence the bursty USC is amplified at fixed gain. layer (transmission convergence layer) functions which provide After amplification the USC is separated from the DSC in the the functionality of a real PON system. The TCWeb layer imple- duplexer, which providesVersion 3 nm optical bandwidth to the USC. mented is based on the GPON TC layer specification as stan- The USC signal now enters a burst-transponder stage which dardized in ITU-T Recommendation G.984.3. Small modifica- converts the incoming wavelength to the WDM channel used DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 13

Fig. 22. Metro access point (MAP).

Fig. 24. BER measurement on 10 Gbit/s downstream channel (upper) and 2.5 Gbit/s upstream channel (lower).

insertion loss of the variable optical attenuator. Without attenu- ator, the total loss between BEDFA and ONU was determined as 37 dB. For the physical layer tests a single ONU is connected to the DN. Due to testing with a single ONU, upstream bursts are sep- arated by a 10 s gap in order to provoke that the upstream burst Fig. 23. Measurement setup. transponder loses all its memory on amplitude and phase infor- mation of the preceding burst. The down-stream test signal is a GPON-type frame. A known for upstream transport in the MN. This transponder serves as test pattern is recognized inside the OLT’s electronics, and bit wavelength converter and thus allows using the same upstream errors are detected. wavelength (here 1531.1 nm) for all ONUs. Bit error ratio (BER) measurements where performed over The burst-transponder stage consists of an avalanche photo- the whole system using FPGA internal bit error counters of OLT diode, followed by a 2.5 Gbit/s burst-mode receiver (BMR). and ONU. In order to show the system margins the attenuation The BMR employs a specially adapted AC-coupled receiver in in the DN was increased using the variable optical attenuator. order to allow burst detection over loud-soft ratio of 15 dB even Fig. 24 shows the measured BER at OLT and ONU, respectively, under the condition of an un-coded upstream signal and short as obtained for two individual MAP [29]. In the upstream a bit burst overhead ( ns). The BMR is able to de- error ratio of , which after FEC results in a BER of tect and acquire the incoming bursts without any framing infor- much less than , was obtained at a minimum of 6 dB of mation derived from the DSC. A gap-filling function removes additional attenuation, resulting in an OSNR of 14.1 dB/0.1 nm the burst-shape of the signal. The BMR is followed by a fast at the burst-transponder input. In the upstream, the main reason clock-and-data recovery (CDR) stage and a directly modulated for the bit errors was identified as the beat-noise resulting from 2.5 Gbit/s DFB laser which re-transmits all bursts with equal the ASE (amplified spontaneous emission) contribution of the amplitude together with the gap-filling signal. The USC is in- BEDFA. serted into the MN by an optical add-filter. In the downstream direction there is no FEC, the margin for 3) Experimental Results: Here,IEEE we describe an experiment a ProofBER was determined as minimum 5.5 dB additional aimed to verify if all components deliver enough performance attenuation, which results in a very similar achievable budget to build a DN able to serve 512 users over 30 km. The sub- for upstream and downstream of at least 43.5 dB, which would systems of the XL-PON prototype are connected together in a be sufficient for a 1024-way split (35 dB) and 34 km of fiber measurement setup shown in Fig. 23. (8.5 dB). Between OLT and MAP 75 km of standard single-mode fiber End-to-end tests including the whole data path were also per- is inserted in both the upstream and downstream link. The DN formed i.e., from edge network interface to client interface and is formed by 25 km of standard single-mode fiber directly con- vice versa. The corresponding setup is shown in Fig. 25 [30]. nected to one of the MAP DN side ports, a variable optical at- In the downstream direction a data throughput of 9.0 Gbit/s, tenuator, and a 1:64 splitter. Together with the MAP-internalWebwhich is 92% of theVersion line rate, has been achieved using an Eth- 1:8 splitter the total split ratio is 1:512. Here 25 km instead of ernet frame length of 1465 Byte. This is about the theoretical the previously mentioned 30 km were used to accommodate the limit taking into account the overhead in a PON system. As a 14 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

IV. RIVERS FOR FTTP AND NETWORK SIMPLIFICATION

A. Commercial Drivers for Fiber Access

Future content rich services will continue to drive bandwidth growth in telecommunications networks. Increasing demand for these services will be a major driver of fiber into the access network, if ways to economically deploy FTTP can be found. Fiber deployment in access networks on a large scale will drive huge bandwidth growth throughout the network hierarchy; ac- cess, metro and core networks. Future bandwidth growth could see average (sustained) user Fig. 25. XL-PON data transmission test setup. bandwidths going beyond 10 Mb/s with peak rates requiring several hundred Mb/s to individual users. This will be driven particularly by increases in video content with higher defini- result of the present FPGA implementation and FPGA speed tion standards, eventually moving beyond 1080p resolution cou- the utilization rate goes down to 45% for Ethernet frames with pled with user behavioural changes such as increasing person- only 64 Bytes length. alization of programming material, distributed storage and con- In the upstream direction a throughput of about 2.1 Gbit/s tent systems such as peer to peer file sharing. These levels of could be measured. This is 84% of the line rate and also at the bandwidth demand could drive two or three orders of magni- theoretical maximum, considering the TDM structure of the up- tude growth in network bandwidth over the next ten years. DSL stream transmission consisting of gaps between bursts, a pre- and cable modem technologies with today’s hierarchical net- amble to detect the data bursts, and also FEC. work architectures will not be able to cope or scale economi- cally to meet these unprecedented demands. D. Conclusions on XL-PON There are a number of drivers for operators to install optical fiber in the access network, these include: We discussed aspects for optical amplification in next genera- meeting competitive threats tion GPON networks. Implementation aspects of Erbium doped reducing operational cost fiber amplifiers are summarized, also related to burst-mode op- meeting end user demand for new high bandwidth services eration. We evaluated the performance of the more critical up- staying internationally or regionally competitive stream transmission, and its dependence on optical bandpass fil- new revenue generation tering and forward error correction. Although narrowband fil- Once a fiber to the premises network is installed there is no tering after upstream amplification requires a more expensive competing network technology that can outperform it in terms wavelength stabilized transmitter at ONU, it significantly im- of technical capability. There may be performance differences, proves performance due to suppression of ASE-ASE beat-noise due to the choice of equipment and architecture, in terms of ser- terms, and thus allows for cost savings as a lower-power laser vice quality and economic viability, but the physical fiber in- can be used at ONU. This trade-off has to be considered. frastructure cannot be bettered by any currently known tech- We further introduced a full-functional prototype for a fu- nology. Because fiber can support all services, service provi- ture XL-PON network operating at 10 Gbit/s downstream and sion and service churn can be automated processes only rarely 2.5 Gbit/s upstream. This prototype includes DWDM in the requiring an end user or network visit. With properly engineered metro-network and consists of OLT, ONU, and an intermediate end user and service management systems the vast majority of (mid-span) stage, called MAP which contains optical ampli- service changes and provisions could in principle be configured fiers and a burst-mode transponder for 2.5 Gbit/s for upstream. remotely including churn from one service provider to another. All subsystems were realized with standard commercial com- Reduced fault rates and automated “plug& play” service provi- ponents. To our knowledge, for theIEEE first time such a system was sion couldProof lead to major reductions in operational costs. realized and tested. Test results showed the possibility to achieve As new high bandwidth requirements emerge, the demand 30 km reach in the distribution network at 1:512 split together for faster access speeds from end users will increase and their with several dB margin (corresponding to a 43 dB loss budget in impatience and dissatisfaction with slow networks could come the passive ODN). The experiments also showed that the proto- to the fore. The effect of low access speeds on user behaviour is type is capable for up to 100 km total reach. The prototype of the illustrated in Fig. 26 which shows the time users are prepared to new network element, MAP, which contains the optical ampli- wait for web pages to download. fier and an upstream burst-mode transponder showed a power If the delay is too long then users simply move to other web consumption of 20 W, making it suitable for mounting in re- sites. The problem of slow access speeds is further illustrated in motely powered street-cabinets. In summary, this XL-PONWeb pro- Fig. 27 which shows Version the time taken for large files to be down- totype fulfills all major requirements of a future next-generation loaded via various technologies and access speeds. For large PON. files such as video files or collections of high resolution images DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 15

Fig. 28. Example of end user bandwidth growth for FTTP users. Fig. 26. The ”Impatience Index”.

B. Bandwidth Growth Scenarios A major challenge arising from an FTTP future will be the huge level of bandwidth growth that this technology can support and deliver into the metro and core networks. By using service and usage scenarios, traffic models can be generated that make it possible to get a feel for the level of bandwidth growth that may arise in the future [31]. Results from an example service scenario for a FTTP future are shown in Fig. 28. It should be stressed that these models are scenarios to aid network strategy decisions and not forecasts for network plan and build purposes.

This scenario had a range of generic services split into two broad classes. The first class was real time services such as voice Fig. 27. Time to transmit files. and video telephony, video streaming of personalized video etc. The average streaming rate for videos services was allowed to increase with time due to reflect take up of HDTV and the in- etc. only FTTP comes close to delivering short enough transfer creasing quality/resolution of HDTV channels in the future. The delays to avoid severely testing the user’s impatience. other class of service was file transfer where the information to This demand for much faster access speeds (not necessarily be transmitted was pre-stored and could be treated as a data ob- huge increases in sustained bandwidth) is putting pressure on ject for transmission purposes. Examples are e-mail, transfer- operators and administrations to put high bandwidth next gen- ring photos, video clips etc. In the results shown in Fig. 28 the eration access networks in place. This certainly appears to be entertainment video was assumed to be streamed at the channel one of the drivers for some municipalities who are willing to rate. An alternative for any non-live video is to burst it through invest in basic fiber infrastructure for the benefit of their local the network as fast as possible: this minimizes file transfer delay, communities. The concept of staying internationally competi- maximizes customer experience and gets the traffic off the net- tive through deployment of the best telecommunications infra- work as fast as possible. structure is a major driver for Japan and Korea. Fig. 28 shows the average bandwidth of all FTTP users in the The last driver listed above is new revenue generation. This busy period rises to about 10 Mbit/s over 10 years. This average is an area of major uncertainty; although FTTP will certainly figure is the figure that drives network capacity build. The peak enable new high capacity servicesIEEE it is unclear whether there (burst) Proof bandwidth for individual users rises to 100 Mbit/s. These will be any significant revenue growth over and above traditional peak rate figures should drive the access technology line speeds. trends. The problem is that revenue generation derived directly The only technology to be able to offer such bandwidths and from new IT and bit transport services are often substitutional: connection speeds is fiber to the premises coupled with optical that is new service revenue simply displaces legacy service rev- networking in the core to deliver the high sustained bandwidth enue and net revenue growth remains relatively static. end to end. Historical analyses of revenue growth and bandwidth price decline suggest that there may not be sufficient revenue growth C. Bandwidth Growth : The Cost Challenge to sustain traditional network builds and that radically new ar- It is proving very difficult to make an economic business chitectures will be necessary to change the end to endWeb cost struc- case for mass market Version deployment of FTTP. The problem is not ture of networks and massively reduce the cost of bandwidth simply the cost of the FTTP access solution but also the back- provision [31]. haul/metro and core network build that will be needed to support 16 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 29. High bandwidth growth – eroding margins. Fig. 30. Physical channel capacity limits as a function of “carrier” frequency. the bandwidth growth that it enables. These costs are often ig- nored in comparative analyses of access solutions because usu- provided and FTTP is required and if price decline of network ally they are considered to be a common cost. However consid- equipment cannot be sufficient to meet the growth in bandwidth ering FTTP to be an access only problem is missing the point then the only other option is to find architectures that can remove because it is the problem of the total end to end cost growing the equipment from the network. This is the driver behind much beyond any potential revenue growth that ultimately will limit of the research into long reach PONs, as for example described FTTP deployment. The problem of the cost of meeting band- in this paper. width growth exceeding revenue growth is described in more D. Impact of Optical Physics on Future Network Architecture detail in [31] and arises from the simple macroeconomic ob- servation that revenue growth in the telecommunications sector The history of the development of communications tech- has been relatively static for many years and that the price of nology has seen an increasing exploitation of the electromag- bandwidth has declined only in line with equipment price de- netic spectrum whereby higher “carrier” frequencies have been clines (the bubble around the turn of the millennium distorted used to enable ever greater information bandwidths to be mod- these figure but the average trends have been fairly constant). ulated on to transmission channels. The current generation of When these two observations are combined in a simple analysis optical communications technology using optical fiber operates involving extrapolations of future broadband service demands in the nm to 1260 nm wavelength range, a “carrier” (similar to the service scenario analysis described above) then frequency range from 187 THz to 238 THz. the problem of revenues being outstripped by the cost of pro- It might be supposed that as technology progresses we could viding the network capacity becomes apparent, this is illustrated exploit higher and higher “carrier” frequencies indefinitely. in Fig. 29. However the photonic nature of electromagnetic radiation The problem cannot to be solved by normal equipment price means that the photon energy is also increasing in proportion declines: the potential future bandwidth growth is too large and to the carrier frequency. The problem with higher photon the projected revenue growths are too small in most broadband energies arises from the quantized nature of the universe and service scenarios utilizing FTTP capabilities . the uncertainty in the arrival time of photons at a detector. Conventional network architectures that rely on equipment This uncertainty produces a fundamental “quantum noise” price decline to remain viable as bandwidth grows will not scale in communications systems and becomes the limiting factor when the service capability and massive bandwidth growth en- to the information carrying capacity of a channel as higher abled by the mass rollout of FTTP is realized. These conven- frequencies are exploited. tional architectures do not fundamentallyIEEE change the economic AtProof low frequencies thermal noise dominates and is the funda- structure of networks: in particular they keep the backhaul and mental limit to channel capacity. As frequencies approach op- access networks separate and require multiplexing and aggrega- tical frequencies quantum noise becomes more significant and tion electronics at the local exchange site. The cost of the local at ultra-violet and beyond quantum noise begins to dominate, exchange and the corresponding backhaul networks can com- severely limiting channel capacity. The effect of quantum noise parable to the access network, when high bandwidth and low on channel capacity is illustrated in Fig. 30. contention service requirements are to be delivered. The channel capacity curve assumes that a constant per- Of course it could be that the high bandwidth service demand centage of the carrier frequency can be used for the information does not arise, either because it is not affordable or the network bandwidth that is modulated onto the carrier. Using this as- is not built to be capable of supporting them. IfWeb either is the sumption it can be seenVersion that the channel capacity peaks around case then optical access networks will not be required and the 5 ( nm wavelength). The roll-off in information argument becomes academic. However if the services are to be capacity beyond 3 nm wavelength is due to the dominance of DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 17

Fig. 31. Basic long reach access architecture – could reduce UK network to $ IHH nodes. quantum noise and is sufficiently great to produce an effective implicit that the end users connected to the access network via ceiling on information capacity as illustrated by the cumulative fiber must be sharing the total capacity of the fiber with other capacity curve. users. The only way to avoid this conclusion is by installing What is also interesting to observe is that today’s optical fiber dedicated point-to-point fiber between every pair of users in technology is only a couple of orders of magnitude from this the network, which evidently is not practical for networks of ceiling so the enormous capacity gains achieved when moving any size. from radio frequency communications to optical frequency We therefore conclude individual end users cannot have all communications will not be repeated as we move from optical the capacity of the optical fiber dedicated to them because there frequencies to the ultra-violet region of the spectrum. Although is no higher capacity transmission technology to multiplex the it is possible that new even higher speed technologies may access fiber capacity into. This is an important consideration emerge that exploits the higher frequencies up to the ultra-vi- when deciding on the access network architecture or topology olet region there is currently no sign of these technologies being for deployment as the issue for the optical access network is how researched anywhere in the world. multiplexing is going to be performed and what is the lowest The above few paragraphs may sound some what esoteric but end to end network cost that can be realized. All FTTH access they mean that as fiber penetrates to all parts of the network (ac- networks have the same future proofing capability, they all pro- cess, metro and core or backbone networks) the same intrinsic vide a share of the fiber bandwidth to the end user. Different transmission technology is beingIEEE used throughout and the same architectures Proof may achieve the sharing in differing ways and at fundamental capacity is available in the access network as in different points in the network, but they all fundamentally share the core network. This is the first time in the history of com- the fiber bandwidth among multiple customers. Some architec- munications networks that this has happened. In the past there tures can do this at lower costs and more flexibly than others has always been a higher capacity transmission technology for and it should be these economic and flexibility parameters that use in the core network that traffic from the access transmission are the major consideration when choosing the architecture for media could be multiplexed into. mass deployment. Multiplexing of traffic from the access network into core transmission systems will have to continue with optical net- E. A New Architectural Approach–Reach Access working if communications networks are to remainWeb practicable The conclusion fromVersion the discussions above are that the only and economically viable. Therefore, if the core technology is way to overcome the cost of bandwidth growth problem is to optical fiber and the access technology is also optical fiber it is radically simplify the architecture of the end to end network in 18 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 order to eliminate equipment port cards etc and hence reduce the REFERENCES cost of bandwidth much faster than equipment price declines can alone. Long reach access solutions achieve this simplifi- [1] , C. Lin, Ed., Broadband Optical Access Networks and Fiber-to-the- cation by eliminating separate backhaul networks and also the Home: Systems Technologies and Deployment Strategies. New York: electronic aggregation node at the local exchange or central of- Wiley[Author: Please provide the year of fice. An example of such an architecture is shown in Fig. 31 publication.--Ed.]. [2] A. Cauvin, A. Tofanelli, J. Lorentzen, J. Brannan, A. Templin, T. Park, which shows a simple single wavelength system Long Reach and K. Saito, “Common technical specification of the G-PON system Passive Optical Network (LR-PON). among major worldwide access carriers,” IEEE Commun. Mag., vol. Fig. 31 shows an amplified PON with 100 km reach, up to 44, no. 10, pp. 34–40, Oct. 2006. [3] ACTS Project AC050:Photonics Local Access Network (PLANET). 1000-way split (to share the backhaul fiber as much as possible [4] D. S. Forrester, A. M. Hill, R. A. Lobbett, and S. F. Carter, “39.81 and also to get maximum statistical gain) with a line speed of 10 Gbit/s 43.8 million way WDM broadcast network with 527 km range,” Gbit/s. The 10 Gbit/s line rate provides a sustained or average Electron. Lett., vol. 27, 1991. [5] K. Suzuki, Y. Foukada, D. Nesset, and R. Davey, “Amplified gigabit rate of 10 Mb/s per end user but more importantly enables end PON systems,” J. Optical Networking, vol. 6, no. 5, pp. 422–433, May users to burst for short durations up to Gigabit/s speeds, ulti- 2007. mately limited by the ONU end user port speed. This burst-mode [6] R. P. Davey, P. Healey, I. Hope, P. Watkinson, D. B. Payne, O. Marmur, J. Ruhmann, and Y. Zuiderveld, “DWDM reach extension of a GPON of operation allows large files to be transferred very quickly: to 135 km,” J. Lightw. Technol., vol. 24, no. 1, pp. 29–32, Jan. 2006. even feature length films could be transferred in a few 10s of [7] T. J. Whitley, “A review of recent system demonstrations incorporating seconds at such speeds. Operation in the 15xx nm wavelength 1.3-"m praseodymium-doped fluoride fiber amplifiers,” J. Lightw. Technol., vol. 13, no. 5, pp. 744–760, May 1995. region allows the use of midspan EDFAs at the local exchange [8] Y. Fukada, T. Nakanishi, K.-I. Suzuki, N. Yoshimoto, and M. Tsub- (or central office) site. These EDFAs could be deployed outside okawa, “Gain-clamp light auto level control (GCL-ALC) technique the exchange building thus allowing the possibility of closing for gain-controllable burst-mode PON amplifying repeater,” in Proc. OFC-NFOEC 2008, OThT5. the exchange building (once all legacy networks were closed [9] A. E. Kelly et al., IEEE Photon. Technol. Lett., vol. 18, no. 12, pp. down). Ultimately this architecture could simplify a network the 2674–2676, Dec. 2000. size of the United Kingdom from exchange buildings [10] Amphotonix Model OPA-18-N-S-FA. [11] R. Seifert, Gigabit Ethernet: Technology and Applications for High- to a few hundred. Speed LANs. Reading, MA: Addison-Wesley, 1998, p. 89. While this paper has focused on long reach TDMA PONs, it [12] Data Sheet, “Motorola AXS2200 GPON Optical Line Terminal” Mo- should be stressed that the long reach concept has been applied torola Inc., 2007. [13] D. Grossman, “Experience with a field-deployable amplified GPON,” to hybrid WDM/TDMA PONs [32] and to pure WDM-PONs in Proc. OFC-NFOEC 2008 Workshop, OSuF. dedicating a wavelength per customer [33]. As the PON and [14] K. Solehmainen et al., “Erbium doped waveguides fabricated with number of customers per fiber increase, resilience may at atomic layer deposition method,” IEEE Photon. Technol. Lett., vol. 16, no. 1, pp. 194–196, Jan. 2004. some point become a requirement. GPON already allows dual [15] G. P. Agrawal, Fiber Optic Communication Systems, 3rd ed. : , p. parenting to two separate headends as described in ITU-T 261[Author: Please provide the publisher Recommendation G.984.1. Protection of dual-parented PONs and year of publication.--Ed.]. requires not only that the PON can quickly divert traffic to a [16] M. Rasztovits-Wiech et al., “Optical amplification in 10 Gbit/s PONs,” second headend but also that the core network (particularly in Proc. NOC2008, Krems, Austria, Jul. 2008. the higher layers) acknowledge that the routing of frames and [17] D. Nesset et al., “Demonstration of 100 km reach amplified PONs with upstream bit-rates of 2.5 Gbit/s and 10 Gbit/s,” in Proc. ECOC 2004, packets through the network has to be altered to reflect the Stockholm, Sweden, Sep. 2004, We2.6.3. change of destination [34]. [18] D. Nesset et al., “10 Gbit/s bidirectional transmission in 1024-way split, 110 km reach, PON system using commercial transceiver mod- ules, super FEC and EDC,” in Proc. ECOC 2005, Glasgow, U.K., Sep. 2005, Tu1.3.1. [19] H. Choi et al., “High power gain-clamped EDFAs with flat gain,” in Proc. OFC 2001, Anaheim, CA, Mar. 2001, WDD28. [20] M. van Deventer and O. Konig, “Unimpaired transmission through a V. C ONCLUSION bidirectional erbium-doped fiber amplifier near lasing threshold,” IEEE Photon. Technol. Lett., vol. 7, no. 9, pp. 1078–1080, Sep. 1995. [21] G. Talli et al., “Integrated metro and access network: PIEMAN,” in Proc. NOC2007, Kista, Sweden, Jun. 2007, pp. 493–500. We have described potential benefits to network operators of [22] M. Rasztovits-Wiech et al., “10/2.5 Gbps demonstration in extra-large increasing the reach of commercially-available PONs recently PON prototype,” in Proc. ECOC 2007, 2007, We8.4.2. IEEE [23]Proof E. Hugues-Salas et al., “A 2.5 Gb/s edge-detecting burst-mode receiver been standardized in ITU-T Recommendation G.984.6. We have for GPON access networks,” in Proc. OFC 2007, OThK6. described a prototype of such a system using semiconductor op- [24] R. Davey et al., “Progress in IST project PIEMAN towards a 10 Gbit/s, tical amplifiers deployed in an underground footway box to ex- multi-wavelength long reach PON,” in Proc. BroadBand Europe Conf., Geneva, Switzerland, Dec. 11–14, 2006, pp. 1–6, Th3A3. tend the physical reach of G-PON to 60 km and 128-way split. [25] B. Thomsen et al., “10 Gb/s AC-coupled digital burst-mode optical Furthermore we have described a second prototype which ap- receiver,” in Proc. OFC 2007, OThK5. plies the concept of PON reach extension to a next generation [26] S. Nishihara et al., “A 10.3125-Gbit/s SiGe BiCMOS burst-mode 3R receiver for 10 G-EPON systems,” in Proc. OFC 2007, post deadline PON: achieving 10 Gbit/s, 100 km reach and 512-way split. Paper PDP8. Finally we have reviewed future directions of optical network [27] P. Vetter et al., “MUSE: Challenges to integrate the multi-disciplinary architectures and concluded that as bandwidths increaseWeb long field of BB accessVersion in one project,” in Proc. ECOC 2007, 2007, We8.4.2. [28] M. Rasztovits-Wiech et al., “Bidirectional EDFA for future extra large reach PONs will play an increasingly important role in future passive optical networks,” in Proc. ECOC 2006, Cannes, France, Sep. networks. 2006, Mo4.5.7. DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 19

[29] M. Rasztovits-Wiech et al., “10/2.5 Gbps demonstration in extra-large PONs with the objective of significantly reducing the end to end cost of FTTP PON prototype,” in Proc. ECOC 2007, 2007, We8.4.2. solutions. In September 2007 he took early retirement from BT. He is now [30] M. Rasztovits-Wiech et al., “Realization of an XL-PON prototype,” working part time as a freelance consultant and has recently joined the Insti- in Proc. BroadBand Europe Conf., Antwerp, Belgium, Dec. 2007, tute of Advanced Telecommunications, Swansea University, Swansea, U.K., to We3B2. continue work on future network architectures. [AUTHOR: PLEASE PRO- [31] D. B. Payne and R. P. Davey, “The future of fibre access systems,” BT VIDE YOUR EDUCATIONAL BACKGROUND.—ED.] Technol. J., vol. 20, no. 4, pp. 104–114, Oct. 2002. In 2005 he was awarded the Martlesham Medal for his contribution to optical [32] G. Talli and P. D. Townsend, “Hybrid DWDM-TDM long reach PON access networking. for next generation optical access,” J. Lightw. Technol., vol. 24, no. 7, pp. 2827–2834, Jul. 2006. [33] S. Lee, S. Mun, M. Kim, and C. H. Lee, “Demonstration of a long reach DWDM PON for consolidation of metro and access networks,” Derek Nesset (M’01) received the B.Sc. degree in physics and the M.Sc. degree J. Lightw. Technol., vol. 25, no. 1, pp. 271–276, Jan. 2007. in telecommunications engineering. [34] D. K. Hunter, Z. Lu, and T. H. Gilfedder, “Protection of long-reach He joined BT Labs, Ipswich, U.K., in 1989 and spent several years developing PON traffic through router database synchronization,” J. Optical Net- photonic components for fiber optic communication systems. Following this, he working, vol. 6, no. 5, pp. 535–549, May 2007. studied the applications of nonlinearities in semiconductor optical amplifiers to high-bit-rate fiber optic systems up to 100 Gbit/s. This included the first field Russell P. Davey received the honors degree in physics from Oxford University, demonstration of 40 Gbit/s transmission over BT’s fiber infrastructure. In 2000, Oxford, U.K., in 1989, the Ph.D. degree from Strathclyde University, Glasgow, he joined Marconi Communications to develop the world’s longest (3000 km) U.K., in 1992, and the M.Sc. degree in telecommunications engineering from commercially deployed and unregenerated 10-Gbit/s terrestrial DWDM trans- University College, London, U.K., in 2001. His doctoral work focused on mode- mission system. He returned to BT in 2003 to conduct research into the physical locked erbium fiber lasers. layer issues relating to enhanced PON systems. He has authored or coauthored He worked for Logica from 1992 to 1994, where his projects included soft- over 70 journal and conference publications and holds five patents. ware development for the European Space Agency’s Huygens probe to Titan. He joined BT, Ipswich, U.K., in 1994. He first performed research into 100-Gbit/s optical networks. He was then heavily involved in the first application of WDM to the BT network. Since 2001, he has managed BT’s optical research. His main A. E. Kelly previously worked at British Telecom Laboratories and Corning interest has been next-generation optical access, on which he has regularly pre- and was a cofounder of Kamelian Ltd and Amphotonix Ltd., Glasgow, U.K. sented at major international conferences and been involved in ITU-T standard- His current research is in the use of semiconductor optical amplifiers for PONs, ization. He now manages BT’s optical design team. optical burst switching, and ultrafast optical switching. He has published over 100 journal and conference papers on a range of optoelectronic devices and sys- tems and holds a number of patents.[Author: Please provide Daniel B. Grossman (SM’01) received the B.S. degree in computer science your educational background.--Ed.] from Worcester Polytechnic Institute, Worcester, MA, in 1979. He has had a series of roles at Codex Corporation and Motorola, Inc. since 1981, where he was involved in the areas of telecommunications and com- puter networking. He is presently a Fellow of the Technical Motorola Applied Albert Rafel received the degree “Enginyer en Telecomunicacions” and Research and Technology Center, Marlborough, MA, where he leads a team the Ph.D. degree in telecommunications from the Universitat Politècnica de working on next-generation PON architecture. Catalunya (UPC), Barcelona, Spain, in 1992 and 1999, respectively. He joined BT Labs, Ipswich, U.K., in April 2001, working on WDM networks, optical packet networks, and traffic modeling. Recently, he has been working on fiber access networks mainly looking at the protocols and resilience Michael Rasztovits-Wiech received the Ph.D. degree from Vienna University for next-generation PONs. During 2008, he was a BT delegate in FSAN and of Technology, Vienna, Austria, in 1996. ITU-T. He has participated in numerous European and UK collaborative He spent six years as a Research Assistant with Vienna University of Tech- projects being BT’s representative in the EU IST Project MUSEII-SPE. He nology, where he contributed to projects related to optical free-space inter-satel- has authored or coauthored several papers in international conferences and lite communication for the European Space Agency, and to the European ACTS journals. projects PHOTON and MOON, both related to optical fiber communication, in particular transmission issues in dense wavelength division multiplex optical networks. In 1998, he joined Siemens IT Solutions and Services PSE, Vienna, where he was involved in various optical fiber network projects in the access, Shamil Appathurai received the B.Eng (1st Class Honours) degree in elec- the metro and the long-haul domain. His tasks range from Researcher, Product tronic and electrical engineering and the Ph.D. degree from University College Developer, Systems Architect to Team-leader. He also contributed to the Euro- London (UCL), London,U.K., in 2000 and 2005, respectively. His doctoral work pean IST project MUSE. He is the author and coauthor of over 15 journal and was on nonlinear distortion and its suppression in high-speed WDM transmis- conference papers. sion systems. During the course of his academic work, he was awarded with studentships from the Royal Society and the Rank Prize. He joined BT Labs, Ipswich, U.K., in 2006, and has been working on the physical layer aspects fiber access net- Dave B. Payne joined BT Labs, Ipswich, U.K., in 1978, working on single- works and fiber sensors for network diagnostics. He is the coordinator for an mode fiber splicing and connectors. He subsequently moved into optical ac- EU collaborative project PIEMAN. He has authored or coauthored close to 20 IEEEpapers inProof international conferences and journals. cess networks, and part of this work was leading a team developing fused fiber couplers an essential component to many optical access architectures. He was Dr. Appathurai is a member of the IET (formerly known as the IEE). co-inventor of TPON, the first passive optical network, and wrote the first in- ternal paper on shared access networks in 1983. The early work on TPON was extended to amplified PONs, which culminated in 1991 in an experimental PON with 50-million-way-split, 500-km range carrying 16 2.5 Gb/s wavelengths. Sheng-Hui Yang (M’96) received the B.S. and M.S. degrees from National During the early 1990s, this work moved on to amplified PONs with more prac- Taiwan University, Taipei, in 1986 and 1990, respectively, and the Ph.D. de- tical splits of $ IHHH. In the later 1990s, he moved into business and traffic gree from the University of Maryland, College Park, in 1996, all in electrical modeling looking at drivers of bandwidth and the economic justification for engineering. large-scale deployment of optical access and core networks. In 1999, he took From 1996 to 2000, he was a Device Engineer working on high-power semi- over the Broadband Architecture & Optical Networks Unit and ran BT’s op- conductor lasers for industrial and telecom applications. From 2000 to 2007 tical research activities until the end of 2006. From 2007, he wasWeb the “Principle he was a Principal Engineer Version in several start-up companies making dense wave- Consultant on Optical Networks” in the BT Design organization at Adastral length-division multiplexing (DWDM) transmission systems or subsystems in- Park, Martlesham Heath, U.K. (formally BT Labs), working on extended-reach cluding 10 G/40 G DWDM transport equipment, polarization-mode dispersion 20 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

compensator, and optical performance monitor. He joined Motorola Applied Research and Technology Center (ARTC), Marlborough, MA in January 2008 as Distinguished Member of Technical Staff in Jan. 2008. Currently, he is ac- tively involved in passive optical network (PON) research which focuses on next-generation PON architecture, system engineering, and feasible transceiver technologies.

IEEE Proof Web Version JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 1 Long-Reach Passive Optical Networks Russell P. Davey, Daniel B. Grossman, Senior Member, IEEE, Michael Rasztovits-Wiech, David B. Payne, Derek Nesset, Member, IEEE, A. E. Kelly, Albert Rafel, Shamil Appathurai, and Sheng-Hui Yang, Member, IEEE

Abstract—This paper is a tutorial reviewing research and devel- opment performed over the last few years to extend the reach of passive optical networks using technology such as optical ampli- fiers. Index Terms—Communication systems, networks, optical am- plifiers, optical fiber communications.

I. INTRODUCTION

HE rapid growth of Internet access and services such as T IP video delivery and voice-over IP (VoIP) is accelerating Fig. 1. Typical configuration for B-PON, GE-PON, and G-PON. demand for broadband access. While most broadband services around the world are delivered via copper access networks, op- tical access technology has been commercially available for sev- eral years and is being deployed in volume in some countries [1]. Where optical access is deployed, passive optical networks (PONs) are often the technology of choice because the trans- mission fiber and the central office equipment can be shared by a large number of customers. Early PON deployments were based on B-PON systems as standardized in the ITU-T G.983 series. Fig. 2. Mid-span GPON extension. Currently being installed in Asian countries such as Japan are Ethernet PON (GE-PON) with gigabit transmission capability operation is made possible using wavelength division multi- that complies with IEEE 802.3ah. Meanwhile, operators in the plexing (WDM) with upstream wavelengths in the 1310 nm United States and Europe are now focusing on gigabit-capable region (1260–1360 nm) and downstream wavelengths in the G-PON systems as standardized in ITU-T G.984 series, with 1490 nm region (1480–1500 nm). Capacity is shared among typical bit rates of 2.5 Gbit/s downstream and 1.2 Gbit/s up- subscribers on the PON using a time-division multiple access stream [2]. (TDMA) protocol that assigns transmission time slots for each Fig. 1 shows the system configuration typical for B-PON, user. The maximum reach and split of a PON are determined GE-PON and G-PON. An optical line terminal (OLT) in the by both the PON protocol and the physical layer optical reach. central office is connected to several optical network units The G-PON protocol supports a maximum logical reach of 60 (ONU) via an optical distribution network (ODN) consisting of km and a maximum logical split of up to 128. In practice most optical fibers and passive optical splitters. The ODN is totally commercial systems conform to the class specification that passive, which is very attractive to an operator. Single-fiber allows a maximum optical loss budget of 28 dB: often this is used to deploy a split size of 32 and reach of up to 20 km. Manuscript received June 29, 2008; revised September 12, 2008. Current ver- Similarly GE-PON specifies a maximum distance of 10 or 20 sion published nulldate This work was supported in part by the European Com- mission’s FP6 Project MUSE. The work of D. B. Payne was supported in part km, reflecting the use of different laser types, and offers loss by the European Union through the Welsh Assembly Government. budgets of 20 and 24 dB excluding optical path penalty. In R. P. Davey, D. Nesset, and A. Rafel are with BT, Ipswich, IP5 3RE, U.K. calculating the achievable reach, the total loss must be within (e-mail: [email protected]). D. B. Grossman and S.-H. Yang are with Motorola Applied Research and the allowed loss budget, taking account of realistic fiber and Technology Center, Marlborough, MA 01752IEEE USA. Proof splitter losses. M. Rasztovits-Wiech is with Siemens IT Solutions and Services PSE, A-1210 The concept of increasing the reach and/or split of PONs via Vienna, Austria. D. B. Payne was with BT, Ipswich, IP5 3RE, U.K., and is now with intermediate equipment such as optical amplifiers has been of the [AUTHOR: PLEASE PROVIDE A COMPLETE MAILING AD- research interest since the 1990s [3], [4]. Recently research has DRESS.—ED.] Institute of Advanced Telecommunications, Swansea focused on extending the reach of G-PON and GE-PON via University, Swansea, U.K. A. E. Kelly is with Amphotonix Ltd, Glasgow, G20 0SP, U.K. midspan optical amplifiers [5] or transponders [6] as shown in S. Appathurai is with the [AUTHOR: PLEASE PROVIDE A COMPLETE Fig. 2. This concept has recently been standardized in ITU-T MAILING ADDRESS.—ED.] BT Design. Recommendation G.984.6. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. PrintThe OLT is connected Version via a length of fiber known as the op- Digital Object Identifier 10.1109/JLT.2008.2006991 tical trunk line (OTL) to the active midspan extender equipment.

0733-8724/$25.00 © 2009 IEEE 2 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

This in turn is connected to the ODN and ONU. Note the in- deployed in an outdoor environment, there is the potential tention is for the OLT and ONU equipment to be essentially opportunity to close the smaller central offices altogether, unchanged compared to the traditional deployment configura- once all legacy (e.g., copper transmission) equipment has tion shown in Fig. 1. Placement of remote OLTs in the field is a been retired. Section IV will describe how network simpli- possible alternative to reach extension. In this scenario, an OLT fication of this kind is important to reduce end-to-end costs with a small number of PON ports and one or two backhaul ports in an environment where revenues do not increase in pro- is environmentally hardened and deployed in the same fashion portion to bandwidth. as a PON extender. The great advantage of reach extension in The next two sections will discuss development of two PON these applications is in the relative simplicity of the hardware extender technologies. Section II discusses a semiconductor deployed in the field. optical amplifier approach to extending the reach of a stan- Some deployments overlay cable television signals at dard G-PON. Section III describes PON extension applied 1550–1560 nm over the same fibers as G-PON (or B-PON or to a 10-Gbit/s, 100-km reach PON. Section IV discusses the GE-PON) using the enhancement band approach described in drivers for fiber access and simplifying the network architecture ITU-T Recommendation G.984.5. In these situations the cable through PON reach extension. television signals can be conveniently amplified using separate erbium doped fiber amplifiers. II. SEMICONDUCTOR OPTICALLY AMPLIFIED GPON Operators value greatly the passive nature of the access net- Semiconductor optical amplifiers (SOAs) are attractive can- work enabled by the PON architecture, and it is not the intention didates for GPON reach extension. They can provide high gain, of PON reach extension to move away from this. Nevertheless, low noise figure (NF), low polarization dependent loss, and fast having the option of an active midspan reach extender can pro- gain dynamics that are suitable for midspan PON signal ampli- vide several benefits, given as follows. fication. They can be designed to provide gain in the 1310-nm 1) Installing fiber cables represents a significant capital in- (O-band) and 1490-nm (S-band) windows used by GPON. The vestment and so PONs are often deployed in greenfield de- only alternative optical amplifier technology capable of oper- ployments, where cable installation costs (whether copper ating in the O- and S- bands are, respectively, praseodymium or fiber) are an inescapable fact. Often greenfield deploy- and thulium doped fluoride fiber amplifiers. These fibers are dif- ments can be located a long way from existing central of- ficult to work with [1], and the technology did not appear to fice buildings—potentially beyond the reach of G-PON or be ready for commercialization; however, recent work appears GE-PON. In this solution, one could build a new central promising [8]. Other advantages of SOAs relative to fiber ampli- office building or house PON OLT equipment in a street fiers in this application include their small size, high reliability, cabinet. Clearly, there are significant capital and opera- and low power consumption. tional costs associated with building a central office, nor is An important consideration for optical amplification of PONs it especially attractive to deploy a full OLT in a street cab- is burst mode operation in the upstream. In GPON, upstream inet. An attractive alternative could therefore be a simple bursts from different ONUs can have a dynamic range of up midspan PON extender box deployed in a street cabinet to 10 dB, due to differential fiber loss between the nearest and (or underground footway box). To offer benefits over the furthest ONU on the PON, and variation in input signal level. street-based OLT approach, the PON extender should be Fiber amplifiers exhibit a slow relaxation response to variations compact, low-power, and cost-effective and require min- in input signal level, which results in output signal distortion. imal configuration and management. As a result, the amplifier’s average output power varies slowly 2) A remote PON extender could give operators more flex- over many bit times at the beginning of each burst. The deci- ibility in deployments. When deploying PONs on long sion threshold circuit in the OLT cannot track receive power loops, the loss budget may not allow the operator to deploy variations on this scale, resulting in excessive BER. This oc- as great a split as they would on shorter loops. By using curs whether the amplifier’s control regime is constant gain or a PON extender, the operator has more possibilities to constant power, and whether its operating regime is linear or sat- deliver the same split regardless of geography. urated. As a result, a gain clamping scheme like that described 3) In areas of sparse take-up, a PON extender could be used in [8] is needed. Because SOAs have a fast relaxation response, to improve the PON utilization. The benefit would be im- they are able to operate in burst mode without need for addi- proved sharing of feeder fiberIEEE and OLT equipment. tional Proof control circuitry or out-of-band optical stabilizing signals. 4) A PON extender box could avoid the need for additional backhaul or metro network equipment to connect the OLT A. High-Gain SOAs for GPON Reach Extension in a minor central office to switching and routing equip- In this section we describe the development of a high-gain ment in major central offices. SOA module for use in GPON extension applications using the 5) A midspan PON extender could allow node consolidation restricted upstream wavelength range proposed in ITU-T Rec- by locating OLTs in a reduced number of major central of- ommendation G.984.5. We also describe the operation of the de- fices. PON extender boxes would then be housed at the vice in extending the loss budget of a commercial GPON system remaining minor central office locations. In the shorter to 54 dB. term this simplifies network operations sincePrint the extender The system in thisVersion application is operating in the gain limit would require minimal configuration and management. In regime [9], where margin improvements are achieved by in- the longer term, provided that the PON extender can be creasing the SOA gain. The SOA is therefore designed to pro- DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 3

The OLT transmits at 1490 nm in the downstream with dBm output power. The ONU uses a DFB transmitter at 1310 nm in the upstream with mean output power during a burst of , , and dBm for ONU#1, ONU#2, and ONU#3, respectively. The three ONUs had slightly differing output powers due to device-to-device variability allowed within the G-PON standards. The OLT Rx is an APD device and the ONU uses a PIN. Optical filtering to 18 nm is provided in the upstream direc- tion via a 1310-nm CWDM add/drop multiplexer used to com- bine the upstream and downstream signals at the GPON ex- tender. No extra filtering was used in the downstream beyond that included in the ONU diplexer. At the OLT, VOAs were used to build out the attenuation of Fig. 3. Measured characteristics of developed high-gain SOA module. the fiber to typical values that might be observed in installed fiber cables. In the downstream the VOA was set to give a mean fiber loss of 0.30 dB/km at 1500 nm. In the upstream a mean loss duce dB gain over the 1300–1320 nm band. This is more of 0.46 dB/km was used to account for worst case losses over than is required in an SOA pre-amplified receiver where the the entire GPON upstream wavelength range of 1260–1360 nm. maximum sensitivity is achieved at gains dB. The noise Bidirectional, fast Ethernet trails (100 Mb/s) were configured figure is not a limiting factor and could be relaxed providing between the ONUs and the OLT. We measured Ethernet frame the ASE does not produce a DC offset that compromises the loss as the system parameters are adjusted. A system BER for burst-mode Rx performance. the link is inferred from this frame loss measurement. The SOA device we have developed is similar in cross section To assess the system performance with the new SOA device, to those reported in [10]: these high-gain variants have a mod- the number of lost Ethernet frames in a 10-min period was mea- ified active region composition to optimise the wavelength and sured as a function of the access network VOA setting. These the cavity length has been increased to 800 m. This device has measurements are converted into BER and the results can be been packaged hermetically as described in [10] using lensed seen in Fig. 5. fibers. The module performance at 200 mA is shown in Fig. 3. For all ONUs, a BER is obtained in both directions Between the wavelengths of 1300 and 1320 nm, a minimum of at access losses corresponding to 128-way split and 10 km of 27 dB of gain is available with noise figure and polarisation de- access fiber (so 60 km total fiber length). In the upstream, the pendent gain values less than 7 and 0.7 dB, respectively. The worst case ONU gives 1.5 dB margin and in the downstream a increase in noise figure compared with [10] is attributed to an minimum of 0.6 dB margin was recorded. In the downstream the increased level of ASE self saturation leading to a lower carrier “black box” sensitivity of the ONU receiver can be seen, which density at both the input and output of the device. At a drive is attributable to some combination of device variation and/or current of 250 mA and over the operating range of 0 C–70 C, coupling losses. the SOA and associated drive electronics has a calculated total power consumption of less than 2 W. B. GPON Reach Extender Prototype Fig. 4 shows the layout of the system experiment. The GPON OLT is connected to the GPON extender by a 50-km G.652 A prototype of an all-optical GPON Reach Extender backhaul fiber. The GPON extender consists of the newly de- (GPON-RE) was developed by a team at Motorola. To validate veloped high-gain 1300-nm SOA and a commercially available the system, bench top link measurements were conducted 1490-nm SOA for the downstream [11]. Both SOAs are con- at a Motorola facility in the US, and a field experiment was nected to the access network via a 4 4 optical splitter. The conducted at a BT facility in the UK. The measurements were SOAs were driven with injection currents of 240 and 500 mA also cross-checked against a numerical model developed by for the upstream and downstream SOAs, respectively. one of the authors. The purpose of this work was to show that To represent the access networkIEEE we used 10 km of G.652 GPON Proof reach extension using optical amplification is ready for fiber connected to one port of the 4 4 coupler followed by commercialization and deployment under field conditions, that a1 32 optical splitter. The 1 32 optical splitter is in turn a GPON-RE can be managed as a network element, and that connected to three GPON ONUs. Unused ports of the 4 4 are protection in the GPON-RE is a feasible approach to dual connected to additional 1 32 splitters on the downstream side parenting. and terminated to avoid reflections on the upstream side. Unused The GPON-RE prototype is shown in Fig. 6. It is compatible ports of the 1 32 splitters were not similarly terminated. with commercially available Motorola GPON ONUs and OLTs, A variable optical attenuator (VOAs) is used on the access and is managed by the Motorola AXSVision element manage- network side of each SOA to account for the excess losses in the ment system [12]. The GPON-RE is housed in a weatherproof fiber and splitters and the wavelength dependencePrint of the fiber enclosure, which wasVersion adapted from a widely deployed Motorola loss. Losses of 17.2 dB and 6.8 dB are assumed for the 1 32 product. The enclosure contains two amplifier cards that can ei- and 4 4 couplers, respectively. ther operate independently or in a protected configuration. 4 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 4. System experiment for high-gain SOA GPON reach extension.

Fig. 5. BER versus total access loss for (a) upstream traffic and (b) downstream traffic.

Fig. 7. Experimental setup for laboratory bench experiments.

and trunk fibers. Commercially available SOAs from two ven- dors were used. The amplifier cards also include a microprocessor with DRAM, SRAM, flash, Ethernet and serial interfaces, control IEEEcircuitry Proof and drivers for SOA bias current and thermo- elec- tric coolers, and optical power monitors. Embedded software Fig. 6. Motorola GPON extender prototype (shown with enclosure open). included controls for SOA temperature and drive current, a protocol for coordinating fail-over between working and protection amplifier cards, an SNMP management agent, local It also contains two embedded ONUs for manager communi- control terminal and an IP/Ethernet protocol stack. cations and an Ethernet switch which supports communications The protection switching scheme provides redundancy amongst the other cards. The embedded ONUs are used to trans- for the amplifier cards, trunk fiber and entire OLT. In the dis- port network management traffic over Ethernet frames between tributed split arrangement shown in Fig. 8, it protects against the GPON-RE and the element management systemPrint (EMS). most failures that couldVersion affect all subscribers on a PON, and Each amplifier card contains two SOAs (one for each direc- also against catastrophic central office outages. In this scheme, tion), coupled through diplex filters to the bidirectional feeder faults are detected by optical power monitoring in the extender. DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 5

Fig. 8. Setup of amplified GPON reach extender field experiment.

The working and protection amplifier cards exchange messages to determine whether a fail-over is possible and likely to be fruitful. Switching takes advantage of the fact that SOAs are opaque when bias current is withdrawn. Thus, once the working and protection amplifier cards decide to fail over, the working amplifier’s SOAs are turned off, and the protection amplifier’s SOAs are turned on.

C. Experimental Setup The setup for the bench top link measurements is shown in Fig. 7. A test fixture containing a GPON OLT transceiver SFP was connected to an Agilent 81250A Bit Error Rate Tester (BERT). The transceiver was connected through a variable optical attenuator (VOA1) and 0–50 km of fiber (to emulate the OTL) to the input port of the GPON-RE. The output port of the GPON-RE was connected through 0–24 km of fiber and Fig. 9. GPON reach extender in footway joint box. a second VOA (VOA2) to a GPON ONU transceiver in a test fixture to emulate the ODN. The test fixture was also connected to the BERT. For upstream operation, the BERT was configured loss between OLT and ONU over a 2-min period as the system for burst mode operation. An Agilent 86100C digital sampling parameters were varied. oscilloscope was used to display the upstream and downstream In both the benchtop and field experiments, forward error cor- eye patterns. Optical spectrum was observed using an Agilent rection (FEC) was not enabled in either direction. All receiver 86146 optical spectrum analyzer. sensitivity measurements are referenced to a BER of . The setup for the field experiment is shown in Fig. 8. The OLTs and ONUs were located in a BT lab and the GPON-RE D. Results was installed in an underground footway joint box at a nearby 1) Benchtop Experiment: In order to cover a wider OSNR outside plant evaluation facility (see Fig. 9). The GPON-RE is range, BER measurements were made with only 24 km of trans- connected to the OLTs by two 50-kmIEEE trunk fibers, forming a mission Proof fiber. To justify this approach, the additional path penal- working and a protection path. VOAs were used to vary the ties of the 50-km OTL fiber/10-km ODN fiber configuration trunk loss on each path. The extender box was connected to were measured at higher OSNR in both directions and found a 2:4 splitter, which in turn was connected to a 1:32 splitter. to be less than 0.25 dB. In order to allow any SOA induced fiber The splitters were located in a splice enclosure in the footway transmission penalty to show up, the 24 km of fiber was posi- box. This splitter arrangement was connected back to the lab tioned after the SOA: so in the ODN for downstream and OTL by lengths of fiber. A VOA was used to vary distribution loss. for the upstream (referring to Figs. 2 and 7). The PON terminated on several ONUs. Typical BT installation Before conducting BER measurement, the GPON-RE was practices were used throughout. Motorola AXS 2200 and AXS characterized as a black box. In particular, extrinsic gain, NF, 1800 OLTs, and ONT1100GE ONUs were used. BidirectionalPrintand amplified spontaneous Version emission (ASE) bandwidth were 400 Mb/s Ethernet trails were configured between an ONU and measured, to allowed estimation of signal and noise power and the working OLT. An Ethernet tester measured Ethernet frame correlation with simulation results. 6 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 12. Downstream BER performance normalized to Class fC Fig. 10. Downstream optical spectrum and (filtered) eye diagrams measured at specifications. GPON-RE output.

eyes shown in Fig. 10(b). BER flooring is observed when OSNR is less than 19 dB. This is readily described by a receiver model that takes into account ASE-ASE beat noise and ASE-enhanced shot noise introduced by optical amplification. The GPON PHY Recommendation requires a back-to-back ONU receiver sensitivity of better than dBm to achieve a Class (28 dB) ODN loss budget. Treating a receiver with this sensitivity as the worst expected case, we normalize the test results to this specification and replot BER versus ODN loss in Fig. 12. For a typical OTL loss of less than 20 dB, larger than 27-dB ODN loss can be supported, which represents a total loss budget of more than 19 dB than that of a nonextended GPON. Upstream Transmission: Upstream optical spectrum from one of the amplifier cards is shown in Fig. 13(a). The ASE spectral width is externally filtered to 35 nm by an optical bandpass filter. Measured small-signal gain and NF of the GPON-RE are 20.5 and 7.4 dB, respectively. Representative optical eyes are shown in Fig. 13(b). As in the downstream, pulse distortion is evident when SOA is driven into saturation; Fig. 11. Downstream receiver sensitivity versus OSNR (defined over 0.1-nm however the resultant penalty ( dB) is smaller. bandwidth). Burst-mode BER performance over 0-km ODN fiber/24-km OTL fiber is shown in Fig. 14. Sensitivity without amplification IEEEis ProofdBm, which is significantly better than the G.984.2 Downstream Transmission: Downstream optical spectrum specification of dBm for Class . The abrupt change from one of the amplifier cards is shown in Fig. 10(a). The in BER at very low input powers is characteristic of the deci- ASE spectrum is not externally filtered. Measured small-signal sion-threshold reset circuitry of the particular OLT receiver used gain and NF of the GPON-RE are 22.2 and 9.2 dB, respec- in our experiment. Therefore, a longer than typical preamble tively. BER performance over 0-km OTL fiber/24-km ODN was employed to better reveal the BER performance trend. We fiber is shown in Fig. 11. Sensitivity without amplification is note that the OLT receiver AGC circuitry which reacts to total dBm. ( ) power is partly responsible for BER flooring at We observe a 3-dB penalty that cannot be overcome by in- -dB OSNR. creasing OSNR. This is attributed to pulse distortionPrint caused by We normalize theVersion test results to the specification and re- the SOA when it is driven into saturation at higher OSNR (or, plot BER versus OTL loss in Fig. 15. Again, we observe that, for equivalently, higher input power), as evidenced by the optical a typical OTL loss of less than 20 dB, larger than 27-dB ODN DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 7

Fig. 15. Upstream burst-mode BER performance normalized to Class fC specifications. Fig. 13. Upstream optical spectrum and (filtered) eye diagrams measured at GPON-RE output.

Fig. 16. Operating envelopes for an extended GPON, normalized to G.984.2 specifications for a Class fC loss budget. Hashed area represents the admissible operating region.

was AC-coupled. The OLT decision threshold (as a percentage of eye opening) was determined by the same curve-fitting procedure. The GPON-RE measurements, along with OLT and Fig. 14. Upstream burst-mode receiver sensitivity versus OSNR (defined over ONU receiver models, were used to simulate the upstream and 0.1-nm bandwidth). downstream performance (respectively) of the extended GPON. IEEE TheProof operating envelopes of the amplified link are of par- ticular interest in engineering extended GPON deployments. loss can be supported, which represents a 19 dB greater total Therefore, we calculated the constant-BER contour over the loss budget than for that a nonextended GPON. ODN loss-OTL loss plane. The results, normalized to the 2) Link Model: A link model and numerical simulation G.984.2 specifications for the Class loss budget, are plotted were used to better understand the optical link characteristics in Fig. 16. Measured results are overlaid on the same plot. Loss of the extended GPON. The GPON-RE was characterized as values within the overlapping area (the hashed area) enclosed a black box. Signal gain, noise figure, and ASE bandwidth by contours lines are admissible. were measured as input power to the GPON-RE was varied. A good match is observed between simulation and measure- ONU and OLT receiver models were constructedPrint to closely ment results in bothVersion directions. However, a noticeable discrep- fit measured back-to-back BER curves. Typical APD and TIA ancy occurs in downstream when trunk loss is low. In this region parameters were used. The ONU receiver decision threshold pulse distortion at GPON-RE output is the dominant cause for 8 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 downstream sensitivity degradation but is not considered by our III. XL-PON PROTOTYPE: A 10-GBIT/S, 100-KM REACH PON model. The horizontal line describes the maximum OTL loss A. Introduction limit imposed by the embedded (management) ONU in the The extended GPON system described in the previous sec- GPON-RE. This ONU is coupled to the OTL through the 20% tion enables PON solutions to reach 128-way split with 60-km leg of an 80%-20% optical tap. This limit could be removed if reach for 2.5-Gbit/s downstream and 1.25-Gbit/s upstream pay- an alternative path for element manager communications were loads. This will enable a degree of node consolidation and net- available. work simplification. However to be more generally useful for The model enables us to analyze the impact of SOA parame- network architecture restructuring and reduce costs still further, ters (notably gain and NF) on operating envelopes and provide even longer reach and larger splits may be beneficial in the fu- guidance on SOA specifications. Since OTL and ODN losses ture. In this section we describe a prototype for next generation are asymmetric in typical applications, downstream transmis- optical access using PONs operating at 10 Gbit/s with a long sion tends to be power- and pulse distortion-limited, whereas reach (100 km) and high split (512-way per wavelength). In this upstream transmission tends to be more ASE-limited. There- paper, we refer to this prototype as XL-PON: in common with fore, increasing SOA gain is most effective for extending loss today’s commercially-available G-PON it uses a power-splitter budget in downstream, whereas reducing NF and limiting ASE based optical distribution network (ODN) and furthermore uses bandwidth are more effective in upstream. a very similar time division multiple access protocol to G-PON. 3) Field Experiment: The amplified GPON system was in- Practically, one has to take into account a maximum inser- stalled in the field experiment configuration of Fig. 4, initially tion loss of dB per 1:2 split for the optical power splitter without VOAs. Over a five-day period, a BER of or better and a variation of dB, which results in 25 – 35 dB inser- was observed in both directions. As of this writing, the amplifier tion loss for a 1:1024 splitter. The loss of a 100 km long optical has been operating in the field site for four and a half months. fiber is about 25 to 35 dB, mainly depending on whether the The only significant issue to date was traced to reflections in one 1.55- m range (C-band) or the 1.3- m range (O-band) is con- of the fibers. sidered. That means the typical loss of such an XL-PON can be To demonstrate protection switching, an IPTV HD video ser- about 60 dB for C-band operation or even more in O-band. vice was delivered across the system. The working trunk fiber To meet such high link budget demands at 10 Gbit/s, there is was disconnected and the video service switched over to the pro- no feasible solution to adapt present laser technology to achieve tection path and resumed playing in about 20 seconds. sufficiently high transmit power, and it is also not feasible to Measurements were taken with the field experiment config- increase receive sensitivity of photo-detectors. The only tech- uration, including the VOAs for BER versus input power anal- nological choice to realize an XL-PON for both extended ODN ysis. These are analyzed in [13]. loss budget and long-reach is optical amplification (OA). Op- tical amplification at each ONU is costly and so is not consid- E. Conclusions ered further here. Rather optical amplifiers should be used in the regions of the PON where their cost can be shared among mul- We have shown that a midspan reach extender based on SOAs tiple customers: at the OLT or a midspan amplifier where it is can increase the physical reach of a commercial G-PON system better optimized for signal to noise ratio(SNR). In this way op- to 60 km and the physical split ratio to 128:1. In this way, the tical amplifiers can be cost-effectively introduced and further- physical layer capabilities of G-PON can match the logical lim- more provide the opportunity to save component costs in the itations (reach and ONUs per PON) of the GPON transmission ONU. For example a high power laser transmitter can be re- convergence protocol. Furthermore we have successfully pro- placed by a low power type made from cheaper technology, and totyped and experimentally deployed a midspan GPON reach an avalanche photodetector (APD) may be replaced by a cheaper extender based on semiconductor optical amplifiers. BER per- PIN photodetector. formance was shown to be well within GPON objectives. We For the reasons discussed in the previous section, semicon- are confident that adequate robustness can be achieved with ex- ductor optical amplifiers (SOA) are an attractive option for pected improvements in the SOA devices and GPON-RE de- G-PON reach extension. For XL-PON there is as yet no agreed sign. In addition, we have demonstrated that the GPON-RE can standard wavelength plan and so we have some freedom to be deployed in an outdoor environment,IEEE that it can be managed choose Proof operating wavelengths. Given the challenging nature of as a network element, and that it can be protected in a the loss budget for XL-PON, the C-band ( to 1565 nm) configuration. and L-band ( to 1610 nm) spectral regions commonly We have established a model to analyze the amplified GPON used for metro and long-haul networks are attractive. The link, plotted operating envelopes, and correlated experimental fiber loss is low in these regions and optical components are and simulation results. While the model does not take into widely available. In particular, the Erbium doped fiber amplifier account SOA-induced pulse distortion and decision threshold (EDFA) is commercially available in the C-band and L-band adjustment mechanism of realistic OLT receivers, it is useful and has better noise performance than an SOA. It shall be noted for generating system engineering guidelines and defining SOA that an EDFA could in principle be deployed in a way that just specifications. Thus, with approval of ITU-T RecommendationPrintthe erbium-doped fiberVersion is located in the outside fiber plant and G.984.6, GPON reach extension using optical amplification is the pump light (e.g., at 1480 nm) is provided from a remote now ready for commercialisation. location (e.g., an office node). Thus, electrical powering of DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 9

• no cross-gain modulation between up- and downstream channels; • optical amplifiers can include isolators, hence they are not sensitive to reflections; • different amplifier technologies can be used for upstream and downstream direction. In Fig. 18, the second approach is shown. Here a bidirectional optical amplifier (BOA) is located directly at the OLT’s inter- face towards the ODN, the S/R interface. The optical diplexer separating the up- and the downstream signals can already in- clude the optical bandpass filter (OBF, see Fig. 18) required Fig. 17. OLT architecture employing two optical amplifiers. Downstream and upstream signal are amplified by a separate device. to restrict the bandwidth and power of amplified spontaneous emission (ASE) generated in the optical amplifier in upstream direction. An OLT architecture equipped with a bidirectional amplifier include the following advantages: • simpler architecture, fewer components, and thus lower cost; • diplexer loss does not contribute to effective upstream noise figure; • a diplexer can already provide the upstream bandpass filter function; • beneficial for EDFAs, as it enables amplification of bursty upstream transmission (see below). Fig. 18. OLT architecture employing a bidirectional amplifier. Both down- stream and upstream signal are amplified in the same gain medium. B. Achievable Performance The presence of amplified spontaneous emission (ASE), the amplifiers situated in the outside plant could potentially which is generated in the OA, results in an electrical dc-current be avoided if this provides operational advantages. Instead in the optical receiver. If the optical bandwidth is wide, ASE of an amplifying fiber, also waveguide designs are possible: power may be significantly larger than the signal power, which erbium-doped waveguide amplifiers (EDWAs), described e.g., may impact particular receiver implementations. in [14], are more compact than EDFAs. Other types of fiber After photodetection of optically amplified signals, additional amplifiers, for other bands, are not yet viable for deployment beat-noise is generated: signal-ASE beat noise and ASE-ASE in telecommunication systems. beat noise. The corresponding beat-noise variance terms due to As an example for a single-sided OA, two different architec- ASE are [15]: tures of optical amplifiers are shown in Figs. 17 and 18, respec- tively. One approach (Fig. 17) uses separate optical amplifiers (1) for up- and for downstream signals, while the other approach (2) uses a bidirectional optical amplifier, characterized by ampli- fication of both up- and downstream signals in the same gain where is the noise variance of the signal-ASE beat medium. noise, is the photodiode responsitivity, is the optical input In Fig. 17 the OLT’s interface towards the ODN, the S/R in- power of the signal, is the spectral ASE density, is terface, is a single fiber interface, as usual for PONs. An op- the electrical bandwidth of the photoreceiver, is the tical diplexer separates the upstream and the downstream sig- noise variance of the ASE-ASE beat noise, and is the band- nals, which are carried on different wavelengths. The insertion width of an optical filter located between optical amplifier and loss of this diplexer has to be compensated by higher output the photoreceiver. power of the downstream OA andIEEE better noise figure of the up- If anProof optical signal arrives with an optical power level well stream OA. After upstream amplification an optical bandpass above the photodetector’s sensitivity limit, signal-ASE beat filter (OBF, see Fig. 17) is used to restrict the power of ampli- noise and ASE-ASE beat noise will dominate. Then there fied spontaneous emission (ASE) generated in the upstream OA are two possibilities to influence the amount of noise and its (see below for impact of the choice of the OBF bandwidth). associated bit-error-ratio (BER): the first is to keep the amount The advantages of the OLT architecture equipped with two of low by choosing an amplifier with low noise figure, amplifiers include: and the second is to reduce the optical bandwidth to keep • uUpstream and downstream channel can be located in in- the ASE-ASE beat noise term low. dependent bands; In the following we will evaluate the different conditions for • gain for up- and downstream can be set independently;Printthe different signal directionsVersion in the PON. • the upstream amplifier can be optimized for best noise 1) Downstream Transmission: In the downstream direction figure; optical amplifiers are employed as boosters at the OLT or as 10 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 19. Sensitivities at 10 Gbit/s with and without optical pre-amplification and various optical bandwidths. midspan amplifiers. Here usually beat noise terms are negligible bandwidth provides a sensitivity gain of 3.7 dB compared to 20 as both boosters and midspan amplifiers are operated at high to nm bandwidth (see Fig. 20). medium signal input values. Due to high ODN loss the pho- The tradeoff for an ONU laser is therefore: either lower power toreceiver sensitivity at the ONU limits the performance. At 10 or wavelength stabilization. Gbit/s and ODN insertion loss of 33 dB or higher, there is no 3) Forward Error Correction: Forward error correction can possibility to avoid costly avalanche photo-detector (APD) re- be employed to improve the loss budget of an optically ampli- ceivers at ONU, as the allowed transmit power level into the fied PON. For example the standard RS(255,239) FEC code em- optical fiber is limited by fiber non-linearities. ployed in GPON is able to correct a BER of to .In 2) Upstream Transmission: Upstream transmission and op- this way the receiver sensitivity is derived for a BER of , tical amplification is the more critical challenge in a XL-PON which is improved compared to , and the improvement is due to the high loss of the splitter, the bursty nature of the up- called FEC-gain. Under FEC, the sensitivity gain and thus loss stream signal, and the desire to keep ONU laser powers as low budget with 0.5 nm optical filtering compared to 20 nm band- as possible. The optical input power into the upstream pream- width is even further increased: 3.5 dB at 10 bit/s (see Fig. 3 and plifier will be very low, thus requiring a low noise figure. compare optical input power values for pre-amplified 20 nm and Next generation PONs may use either 10 Gbit/s upstream data 0.5 nm at ) and 5 dB at 2.5 Gbit/s (see Fig. 4). rates (symmetrical approach) or lower data rates of, for example, The reason for this phenomenon is the input power depen- 2.5 Gbit/s. Figs. 19 and 20 show the calculated BER perfor- dence of the signal-ASE beat noise (see (1)) which results in mance for 10 Gbit/s (Fig. 19) and 2.5 Gbit/s (Fig. 20), both op- flat BER curves. Therefore, the usage of FEC especially bene- tically pre-amplified [16]. An OA noise figure of 7 dB, C-band fits the upstream optical amplification. transmission, typical photodetector data, and a 6 dB extinction Related physical layer optical amplification experiments re- ratio of the transmit signal is assumed. Transmission penalties lated to 10 Gbit/s PONs were reported, e.g., in [17], [18]. and burst-mode penalties are neglected in this example. In both 4) Burst Mode Optical Amplification: The calculations cases typical performance of unamplified transmission using a shown above did not consider an additional penalty due to PIN or an APD (avalanche photodiode)IEEE detector is shown for upstream Proof burst-mode operation in a PON. Due to differential comparison. The performance curves for the optically ampli- loss in the ODN (due to different fiber length and splitter ports), fied system are shown for three different optical bandwidths, adjacent bursts may arrive with different optical power levels. 20 nm, 6 nm, and 0.5 nm. This corresponds to the upstream A maximum difference of 15 dB is usually assumed. This transmitter laser at the ONU being uncooled or simply cooled causes big challenges for the OLT optical receiver, but also may or wavelength-stabilized (by means of a thermo-electric cooler impact on upstream optical amplifier design. and a temperature control circuit), respectively. While amplification of burst-mode signals using semicon- At 10 Gbit/s and optical filtering with 0.5 ductor optical amplifiers is more straight-forward, EDFAs may nm bandwidth provides a sensitivity gain of 2 dB compared to show large gain transients if average signal power changes with 20 nm bandwidth (see Fig. 19 and compare opticalPrint input power relatively low speed, Version resulting in huge distortions of the signal. values for preamplified 20 nm and 0.5 nm at ). If an EDFA shall be used for upstream transmission there are At 2.5 Gbit/s and optical filtering with 0.5 nm DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 11

Fig. 20. Sensitivities at 2.5 Gbit/s with and without optical pre-amplification and various optical bandwidths. four relatively simple possibilities to enable burst mode opera- Recent results at 2.5 Gbit/s burst-mode reception were achieved tion: using commercial components [22], [23]. — Application of an additional continuous saturating signal There is already research work on integrated 10 Gbit/s with significantly higher power than the strongest up- BM-Rx [24]–[26]. Though those results are still far from the stream burst. targets for preamble length and guard time in mind at 10 Gbit/s — Inclusion of optical feedback before and after the Erbium upstream, the results give good confidence that suitable inte- fiber, resulting in an oscillation at a separate wavelength grated 2.5 Gbit/s burst-mode receivers can be made available which then serves as an internal saturating signal and thus soon after standardisation. establishes a constant gain for the upstream channel. Such 6) Burst Mode Transmitter: Burst-mode transmitters schemes are known as “gain clamped” EDFAs and de- (BM-Tx) are considered as the main cost challenge for a future scribed e.g., in [19]. high speed PON. As a burst-mode transmitter is required in — A bidirectional architecture (see Fig. 18) where the larger every customer unit (optical network termination, ONU), direct downstream signal saturates the EDFA which then pro- modulation is the best chance to keep costs low. A burst mode vides constant gain for the weak upstream. This scheme laser driver must provide two independent currents to the has additional advantages already described above, how- laser, a bias current, slightly above the laser threshold and a ever it requires that both upstream and downstream sig- proper modulation current. Both currents have to be applied nals are within the gain band of the EDFA. As isolators very accurately in order to provide a useful eye opening and cannot be used, reflections and back-scattering distortions sufficient extinction ratio which is important to achieve a good may occur. Practically this limits the achievable gain to performance after optical amplification. Further, the driving some 20 dB [20]. circuit has to maintain these optimum settings over the full — Active power compensation. As the saturating signal men- life-time and temperature range. The driving circuit has to tioned above wastes power, a dynamically controlled com- fully switch-off the laser, otherwise spontaneous emission of pensation signal can be used as described in [21]. The hundreds of ONUs would add up at the optical splitter and power level at the input is measured,IEEE and the power of the interfere Proof with the active burst. After going active the driving compensation signal (at different wavelength and filtered circuit has to re-establish a valid setting in few nanoseconds, out after the amplifier) is controlled in a way that the sum which is only possible by storing the values from the previous of the two signals is always constant, and thus the oper- burst. ating point of the EDFA is maintained. 5) Burst Mode Receiver: Burst-mode receivers (BM-Rx) are C. XL-PON Prototype considered as one of the key technical challenges in developing As a first step towards an evolution to a next generation op- a high data rate PON. Today, commercially available BM-Rxs tical access network, and to evaluate the technical feasibility of operate at 1.2 Gbit/s (GPON). Especially in XL-PONs a crit- such systems, a prototype of an XL-PON system was realized in ical requirement for the BM-Rx is to meet huge burst-dynamicsPrintthe framework of theVersion European research project MUSE (Multi at very low adaptation and synchronisation time, important to Service Access Everywhere) [27]. The features of the prototype keep preamble length short and thus bandwidth efficiency high. were high data rates (10 Gbit/s in downstream and 2.5 Gbit/s 12 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

TABLE I XL-PON PARAMETERS.

tions were necessary, mainly to increase the number of possible ONUs to 512. For the upstream channel, forward error correc- tion (FEC) using the standard Reed–Solomon code RS(255,239) is implemented. For simplicity, OLT and ONU share the same mother-board design. The key-components are: — a commercial 10 Gbit/s transceiver module for down- stream where the transmit part is used at OLT and the APD-based receive part at ONU Fig. 21. XL-PON overview. — at ONU: a self-designed 2.5 Gbit/s burst mode transmitter module using a wavelength stabilized (1531.1 nm) direct modulated DFB laser with 6 dBm output power in upstream), high split (1:512), long reach of 100 km, and, in — at OLT: a self-designed 2.5 Gbit/s burst mode receiver addition dense wavelength division multiplexing (DWDM) in module using an APD photodiode. The design of this the metro (feeder, backhaul or trunk) part of the PON. The in- receiver is similar to the one used at the burst mode troduction DWDM technology in the metro part of the PON is transponder at MAP (see below) intended to share fibers in the metro-network (MN) part of this — 10GE interfaces on ONU and OLT, based on commercial XL-PON. It results in a hybrid time division multiplex (TDM) XFP-modules and WDM PON approach. — an FPGA (field programmable gate array) at both OLT and This XL-PON prototype uses a similar time division multiple ONU for processing the TC layer access protocol as GPON, and thus provides high bandwidth — an on-board controller efficiency. It further includes the functionalities of a real PON 2) Metro Access Point – MAP: Fig. 22 shows the inner con- system, like ranging and activation of ONUs, and real data trans- figuration of the MAP. By means of an optical drop filter a par- mission. ticular downstream channel (DSC) is selected out of the WDM Fig. 21 shows the configuration of the XL-PON. The network signal in the MN. The selected channel is forwarded to a com- consists of two parts: a metro network (MN) ring which is shared mercial Erbium-doped fiber amplifier (EDFA) acting as pre-am- among several central OLTs by means of DWDM, and several plifier which operates at constant output power. After passing a optical (passive) distribution networks (DN), each assigned to duplexer the DSC reaches a further EDFA (BEDFA) which is one OLT and serving up to 512 customer ONUs over up to 30 operated bidirectionally, i.e., it amplifies the DSC and the up- km of optical fiber. The DN is operated in duplex mode (bidi- stream channel (USC) in the same Erbium fiber. In this way, rectional transmission on a single fiber). as already discussed above, it is possible to amplify the bursty The proposed concept includes an additional network node upstream signal without distortion by an EDFA [28]. The down- called MAP (metro access point), serving as an interface be- stream output power at the BEDFA is controlled to a constant tween MN and DN, and providing the functions of a midspan power level of 18 dBm to achieve sufficient power at the ONU. optical amplifier among other things. As the MN is operated Both preamplifier and BEDFA are operated with a single pump- in WDM, simultaneous operation of multiple independent laser. XL-PONs on a single fiber ring is possible. Though imple- The DN side of the MAP includes a 1:8 splitter. This ensures menting WDM, the concept allowsIEEE for all ONU operating at eye-safe Proof power levels in the DN and avoids non-linear effects in the same upstream wavelength, in this case 1531.1 nm, due to the fiber. an upstream transponder in the MAP, as shown below. This is The USC bursts arrive at different power levels at the input of important to simplify inventory management. All ONUs are the BEDFA, mainly due to differences in fiber loss and due to able to receive all possible downstream wavelengths, in our splitter tolerance. We assume a range between and case channels in a 100 GHz grid around 1555 nm. . The BEDFA gain is set to 20 dB (almost equal Table I summarizes the key parameters of the XL-PON. for up- and downstream) by means of output power control of 1) OLT and ONU: OLT and ONU prototypes include TC both EDFAs, hence the bursty USC is amplified at fixed gain. layer (transmission convergence layer) functions which provide After amplification the USC is separated from the DSC in the the functionality of a real PON system. The TCPrint layer imple- duplexer, which providesVersion 3 nm optical bandwidth to the USC. mented is based on the GPON TC layer specification as stan- The USC signal now enters a burst-transponder stage which dardized in ITU-T Recommendation G.984.3. Small modifica- converts the incoming wavelength to the WDM channel used DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 13

Fig. 22. Metro access point (MAP).

Fig. 24. BER measurement on 10 Gbit/s downstream channel (upper) and 2.5 Gbit/s upstream channel (lower).

insertion loss of the variable optical attenuator. Without attenu- ator, the total loss between BEDFA and ONU was determined as 37 dB. For the physical layer tests a single ONU is connected to the DN. Due to testing with a single ONU, upstream bursts are sep- arated by a 10 s gap in order to provoke that the upstream burst Fig. 23. Measurement setup. transponder loses all its memory on amplitude and phase infor- mation of the preceding burst. The down-stream test signal is a GPON-type frame. A known for upstream transport in the MN. This transponder serves as test pattern is recognized inside the OLT’s electronics, and bit wavelength converter and thus allows using the same upstream errors are detected. wavelength (here 1531.1 nm) for all ONUs. Bit error ratio (BER) measurements where performed over The burst-transponder stage consists of an avalanche photo- the whole system using FPGA internal bit error counters of OLT diode, followed by a 2.5 Gbit/s burst-mode receiver (BMR). and ONU. In order to show the system margins the attenuation The BMR employs a specially adapted AC-coupled receiver in in the DN was increased using the variable optical attenuator. order to allow burst detection over loud-soft ratio of 15 dB even Fig. 24 shows the measured BER at OLT and ONU, respectively, under the condition of an un-coded upstream signal and short as obtained for two individual MAP [29]. In the upstream a bit burst overhead ( ns). The BMR is able to de- error ratio of , which after FEC results in a BER of tect and acquire the incoming bursts without any framing infor- much less than , was obtained at a minimum of 6 dB of mation derived from the DSC. A gap-filling function removes additional attenuation, resulting in an OSNR of 14.1 dB/0.1 nm the burst-shape of the signal. The BMR is followed by a fast at the burst-transponder input. In the upstream, the main reason clock-and-data recovery (CDR) stage and a directly modulated for the bit errors was identified as the beat-noise resulting from 2.5 Gbit/s DFB laser which re-transmits all bursts with equal the ASE (amplified spontaneous emission) contribution of the amplitude together with the gap-filling signal. The USC is in- BEDFA. serted into the MN by an optical add-filter. In the downstream direction there is no FEC, the margin for 3) Experimental Results: Here,IEEE we describe an experiment a ProofBER was determined as minimum 5.5 dB additional aimed to verify if all components deliver enough performance attenuation, which results in a very similar achievable budget to build a DN able to serve 512 users over 30 km. The sub- for upstream and downstream of at least 43.5 dB, which would systems of the XL-PON prototype are connected together in a be sufficient for a 1024-way split (35 dB) and 34 km of fiber measurement setup shown in Fig. 23. (8.5 dB). Between OLT and MAP 75 km of standard single-mode fiber End-to-end tests including the whole data path were also per- is inserted in both the upstream and downstream link. The DN formed i.e., from edge network interface to client interface and is formed by 25 km of standard single-mode fiber directly con- vice versa. The corresponding setup is shown in Fig. 25 [30]. nected to one of the MAP DN side ports, a variable optical at- In the downstream direction a data throughput of 9.0 Gbit/s, tenuator, and a 1:64 splitter. Together with the MAP-internalPrintwhich is 92% of theVersion line rate, has been achieved using an Eth- 1:8 splitter the total split ratio is 1:512. Here 25 km instead of ernet frame length of 1465 Byte. This is about the theoretical the previously mentioned 30 km were used to accommodate the limit taking into account the overhead in a PON system. As a 14 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

IV. RIVERS FOR FTTP AND NETWORK SIMPLIFICATION

A. Commercial Drivers for Fiber Access

Future content rich services will continue to drive bandwidth growth in telecommunications networks. Increasing demand for these services will be a major driver of fiber into the access network, if ways to economically deploy FTTP can be found. Fiber deployment in access networks on a large scale will drive huge bandwidth growth throughout the network hierarchy; ac- cess, metro and core networks. Future bandwidth growth could see average (sustained) user Fig. 25. XL-PON data transmission test setup. bandwidths going beyond 10 Mb/s with peak rates requiring several hundred Mb/s to individual users. This will be driven particularly by increases in video content with higher defini- result of the present FPGA implementation and FPGA speed tion standards, eventually moving beyond 1080p resolution cou- the utilization rate goes down to 45% for Ethernet frames with pled with user behavioural changes such as increasing person- only 64 Bytes length. alization of programming material, distributed storage and con- In the upstream direction a throughput of about 2.1 Gbit/s tent systems such as peer to peer file sharing. These levels of could be measured. This is 84% of the line rate and also at the bandwidth demand could drive two or three orders of magni- theoretical maximum, considering the TDM structure of the up- tude growth in network bandwidth over the next ten years. DSL stream transmission consisting of gaps between bursts, a pre- and cable modem technologies with today’s hierarchical net- amble to detect the data bursts, and also FEC. work architectures will not be able to cope or scale economi- cally to meet these unprecedented demands. D. Conclusions on XL-PON There are a number of drivers for operators to install optical fiber in the access network, these include: We discussed aspects for optical amplification in next genera- meeting competitive threats tion GPON networks. Implementation aspects of Erbium doped reducing operational cost fiber amplifiers are summarized, also related to burst-mode op- meeting end user demand for new high bandwidth services eration. We evaluated the performance of the more critical up- staying internationally or regionally competitive stream transmission, and its dependence on optical bandpass fil- new revenue generation tering and forward error correction. Although narrowband fil- Once a fiber to the premises network is installed there is no tering after upstream amplification requires a more expensive competing network technology that can outperform it in terms wavelength stabilized transmitter at ONU, it significantly im- of technical capability. There may be performance differences, proves performance due to suppression of ASE-ASE beat-noise due to the choice of equipment and architecture, in terms of ser- terms, and thus allows for cost savings as a lower-power laser vice quality and economic viability, but the physical fiber in- can be used at ONU. This trade-off has to be considered. frastructure cannot be bettered by any currently known tech- We further introduced a full-functional prototype for a fu- nology. Because fiber can support all services, service provi- ture XL-PON network operating at 10 Gbit/s downstream and sion and service churn can be automated processes only rarely 2.5 Gbit/s upstream. This prototype includes DWDM in the requiring an end user or network visit. With properly engineered metro-network and consists of OLT, ONU, and an intermediate end user and service management systems the vast majority of (mid-span) stage, called MAP which contains optical ampli- service changes and provisions could in principle be configured fiers and a burst-mode transponder for 2.5 Gbit/s for upstream. remotely including churn from one service provider to another. All subsystems were realized with standard commercial com- Reduced fault rates and automated “plug& play” service provi- ponents. To our knowledge, for theIEEE first time such a system was sion couldProof lead to major reductions in operational costs. realized and tested. Test results showed the possibility to achieve As new high bandwidth requirements emerge, the demand 30 km reach in the distribution network at 1:512 split together for faster access speeds from end users will increase and their with several dB margin (corresponding to a 43 dB loss budget in impatience and dissatisfaction with slow networks could come the passive ODN). The experiments also showed that the proto- to the fore. The effect of low access speeds on user behaviour is type is capable for up to 100 km total reach. The prototype of the illustrated in Fig. 26 which shows the time users are prepared to new network element, MAP, which contains the optical ampli- wait for web pages to download. fier and an upstream burst-mode transponder showed a power If the delay is too long then users simply move to other web consumption of 20 W, making it suitable for mounting in re- sites. The problem of slow access speeds is further illustrated in motely powered street-cabinets. In summary, this XL-PONPrint pro- Fig. 27 which shows Version the time taken for large files to be down- totype fulfills all major requirements of a future next-generation loaded via various technologies and access speeds. For large PON. files such as video files or collections of high resolution images DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 15

Fig. 28. Example of end user bandwidth growth for FTTP users. Fig. 26. The ”Impatience Index”.

B. Bandwidth Growth Scenarios A major challenge arising from an FTTP future will be the huge level of bandwidth growth that this technology can support and deliver into the metro and core networks. By using service and usage scenarios, traffic models can be generated that make it possible to get a feel for the level of bandwidth growth that may arise in the future [31]. Results from an example service scenario for a FTTP future are shown in Fig. 28. It should be stressed that these models are scenarios to aid network strategy decisions and not forecasts for network plan and build purposes.

This scenario had a range of generic services split into two broad classes. The first class was real time services such as voice Fig. 27. Time to transmit files. and video telephony, video streaming of personalized video etc. The average streaming rate for videos services was allowed to increase with time due to reflect take up of HDTV and the in- etc. only FTTP comes close to delivering short enough transfer creasing quality/resolution of HDTV channels in the future. The delays to avoid severely testing the user’s impatience. other class of service was file transfer where the information to This demand for much faster access speeds (not necessarily be transmitted was pre-stored and could be treated as a data ob- huge increases in sustained bandwidth) is putting pressure on ject for transmission purposes. Examples are e-mail, transfer- operators and administrations to put high bandwidth next gen- ring photos, video clips etc. In the results shown in Fig. 28 the eration access networks in place. This certainly appears to be entertainment video was assumed to be streamed at the channel one of the drivers for some municipalities who are willing to rate. An alternative for any non-live video is to burst it through invest in basic fiber infrastructure for the benefit of their local the network as fast as possible: this minimizes file transfer delay, communities. The concept of staying internationally competi- maximizes customer experience and gets the traffic off the net- tive through deployment of the best telecommunications infra- work as fast as possible. structure is a major driver for Japan and Korea. Fig. 28 shows the average bandwidth of all FTTP users in the The last driver listed above is new revenue generation. This busy period rises to about 10 Mbit/s over 10 years. This average is an area of major uncertainty; although FTTP will certainly figure is the figure that drives network capacity build. The peak enable new high capacity servicesIEEE it is unclear whether there (burst) Proof bandwidth for individual users rises to 100 Mbit/s. These will be any significant revenue growth over and above traditional peak rate figures should drive the access technology line speeds. trends. The problem is that revenue generation derived directly The only technology to be able to offer such bandwidths and from new IT and bit transport services are often substitutional: connection speeds is fiber to the premises coupled with optical that is new service revenue simply displaces legacy service rev- networking in the core to deliver the high sustained bandwidth enue and net revenue growth remains relatively static. end to end. Historical analyses of revenue growth and bandwidth price decline suggest that there may not be sufficient revenue growth C. Bandwidth Growth : The Cost Challenge to sustain traditional network builds and that radically new ar- It is proving very difficult to make an economic business chitectures will be necessary to change the end to endPrint cost struc- case for mass market Version deployment of FTTP. The problem is not ture of networks and massively reduce the cost of bandwidth simply the cost of the FTTP access solution but also the back- provision [31]. haul/metro and core network build that will be needed to support 16 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

Fig. 29. High bandwidth growth – eroding margins. Fig. 30. Physical channel capacity limits as a function of “carrier” frequency. the bandwidth growth that it enables. These costs are often ig- nored in comparative analyses of access solutions because usu- provided and FTTP is required and if price decline of network ally they are considered to be a common cost. However consid- equipment cannot be sufficient to meet the growth in bandwidth ering FTTP to be an access only problem is missing the point then the only other option is to find architectures that can remove because it is the problem of the total end to end cost growing the equipment from the network. This is the driver behind much beyond any potential revenue growth that ultimately will limit of the research into long reach PONs, as for example described FTTP deployment. The problem of the cost of meeting band- in this paper. width growth exceeding revenue growth is described in more D. Impact of Optical Physics on Future Network Architecture detail in [31] and arises from the simple macroeconomic ob- servation that revenue growth in the telecommunications sector The history of the development of communications tech- has been relatively static for many years and that the price of nology has seen an increasing exploitation of the electromag- bandwidth has declined only in line with equipment price de- netic spectrum whereby higher “carrier” frequencies have been clines (the bubble around the turn of the millennium distorted used to enable ever greater information bandwidths to be mod- these figure but the average trends have been fairly constant). ulated on to transmission channels. The current generation of When these two observations are combined in a simple analysis optical communications technology using optical fiber operates involving extrapolations of future broadband service demands in the nm to 1260 nm wavelength range, a “carrier” (similar to the service scenario analysis described above) then frequency range from 187 THz to 238 THz. the problem of revenues being outstripped by the cost of pro- It might be supposed that as technology progresses we could viding the network capacity becomes apparent, this is illustrated exploit higher and higher “carrier” frequencies indefinitely. in Fig. 29. However the photonic nature of electromagnetic radiation The problem cannot to be solved by normal equipment price means that the photon energy is also increasing in proportion declines: the potential future bandwidth growth is too large and to the carrier frequency. The problem with higher photon the projected revenue growths are too small in most broadband energies arises from the quantized nature of the universe and service scenarios utilizing FTTP capabilities . the uncertainty in the arrival time of photons at a detector. Conventional network architectures that rely on equipment This uncertainty produces a fundamental “quantum noise” price decline to remain viable as bandwidth grows will not scale in communications systems and becomes the limiting factor when the service capability and massive bandwidth growth en- to the information carrying capacity of a channel as higher abled by the mass rollout of FTTP is realized. These conven- frequencies are exploited. tional architectures do not fundamentallyIEEE change the economic AtProof low frequencies thermal noise dominates and is the funda- structure of networks: in particular they keep the backhaul and mental limit to channel capacity. As frequencies approach op- access networks separate and require multiplexing and aggrega- tical frequencies quantum noise becomes more significant and tion electronics at the local exchange site. The cost of the local at ultra-violet and beyond quantum noise begins to dominate, exchange and the corresponding backhaul networks can com- severely limiting channel capacity. The effect of quantum noise parable to the access network, when high bandwidth and low on channel capacity is illustrated in Fig. 30. contention service requirements are to be delivered. The channel capacity curve assumes that a constant per- Of course it could be that the high bandwidth service demand centage of the carrier frequency can be used for the information does not arise, either because it is not affordable or the network bandwidth that is modulated onto the carrier. Using this as- is not built to be capable of supporting them. IfPrint either is the sumption it can be seenVersion that the channel capacity peaks around case then optical access networks will not be required and the 5 ( nm wavelength). The roll-off in information argument becomes academic. However if the services are to be capacity beyond 3 nm wavelength is due to the dominance of DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 17

Fig. 31. Basic long reach access architecture – could reduce UK network to $ IHH nodes. quantum noise and is sufficiently great to produce an effective implicit that the end users connected to the access network via ceiling on information capacity as illustrated by the cumulative fiber must be sharing the total capacity of the fiber with other capacity curve. users. The only way to avoid this conclusion is by installing What is also interesting to observe is that today’s optical fiber dedicated point-to-point fiber between every pair of users in technology is only a couple of orders of magnitude from this the network, which evidently is not practical for networks of ceiling so the enormous capacity gains achieved when moving any size. from radio frequency communications to optical frequency We therefore conclude individual end users cannot have all communications will not be repeated as we move from optical the capacity of the optical fiber dedicated to them because there frequencies to the ultra-violet region of the spectrum. Although is no higher capacity transmission technology to multiplex the it is possible that new even higher speed technologies may access fiber capacity into. This is an important consideration emerge that exploits the higher frequencies up to the ultra-vi- when deciding on the access network architecture or topology olet region there is currently no sign of these technologies being for deployment as the issue for the optical access network is how researched anywhere in the world. multiplexing is going to be performed and what is the lowest The above few paragraphs may sound some what esoteric but end to end network cost that can be realized. All FTTH access they mean that as fiber penetrates to all parts of the network (ac- networks have the same future proofing capability, they all pro- cess, metro and core or backbone networks) the same intrinsic vide a share of the fiber bandwidth to the end user. Different transmission technology is beingIEEE used throughout and the same architectures Proof may achieve the sharing in differing ways and at fundamental capacity is available in the access network as in different points in the network, but they all fundamentally share the core network. This is the first time in the history of com- the fiber bandwidth among multiple customers. Some architec- munications networks that this has happened. In the past there tures can do this at lower costs and more flexibly than others has always been a higher capacity transmission technology for and it should be these economic and flexibility parameters that use in the core network that traffic from the access transmission are the major consideration when choosing the architecture for media could be multiplexed into. mass deployment. Multiplexing of traffic from the access network into core transmission systems will have to continue with optical net- E. A New Architectural Approach–Reach Access working if communications networks are to remainPrint practicable The conclusion fromVersion the discussions above are that the only and economically viable. Therefore, if the core technology is way to overcome the cost of bandwidth growth problem is to optical fiber and the access technology is also optical fiber it is radically simplify the architecture of the end to end network in 18 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009 order to eliminate equipment port cards etc and hence reduce the REFERENCES cost of bandwidth much faster than equipment price declines can alone. Long reach access solutions achieve this simplifi- [1] , C. Lin, Ed., Broadband Optical Access Networks and Fiber-to-the- cation by eliminating separate backhaul networks and also the Home: Systems Technologies and Deployment Strategies. New York: electronic aggregation node at the local exchange or central of- Wiley[Author: Please provide the year of fice. An example of such an architecture is shown in Fig. 31 publication.--Ed.]. [2] A. Cauvin, A. Tofanelli, J. Lorentzen, J. Brannan, A. Templin, T. Park, which shows a simple single wavelength system Long Reach and K. Saito, “Common technical specification of the G-PON system Passive Optical Network (LR-PON). among major worldwide access carriers,” IEEE Commun. Mag., vol. Fig. 31 shows an amplified PON with 100 km reach, up to 44, no. 10, pp. 34–40, Oct. 2006. [3] ACTS Project AC050:Photonics Local Access Network (PLANET). 1000-way split (to share the backhaul fiber as much as possible [4] D. S. Forrester, A. M. Hill, R. A. Lobbett, and S. F. Carter, “39.81 and also to get maximum statistical gain) with a line speed of 10 Gbit/s 43.8 million way WDM broadcast network with 527 km range,” Gbit/s. The 10 Gbit/s line rate provides a sustained or average Electron. Lett., vol. 27, 1991. [5] K. Suzuki, Y. Foukada, D. Nesset, and R. Davey, “Amplified gigabit rate of 10 Mb/s per end user but more importantly enables end PON systems,” J. Optical Networking, vol. 6, no. 5, pp. 422–433, May users to burst for short durations up to Gigabit/s speeds, ulti- 2007. mately limited by the ONU end user port speed. This burst-mode [6] R. P. Davey, P. Healey, I. Hope, P. Watkinson, D. B. Payne, O. Marmur, J. Ruhmann, and Y. Zuiderveld, “DWDM reach extension of a GPON of operation allows large files to be transferred very quickly: to 135 km,” J. Lightw. Technol., vol. 24, no. 1, pp. 29–32, Jan. 2006. even feature length films could be transferred in a few 10s of [7] T. J. Whitley, “A review of recent system demonstrations incorporating seconds at such speeds. Operation in the 15xx nm wavelength 1.3-"m praseodymium-doped fluoride fiber amplifiers,” J. Lightw. Technol., vol. 13, no. 5, pp. 744–760, May 1995. region allows the use of midspan EDFAs at the local exchange [8] Y. Fukada, T. Nakanishi, K.-I. Suzuki, N. Yoshimoto, and M. Tsub- (or central office) site. These EDFAs could be deployed outside okawa, “Gain-clamp light auto level control (GCL-ALC) technique the exchange building thus allowing the possibility of closing for gain-controllable burst-mode PON amplifying repeater,” in Proc. OFC-NFOEC 2008, OThT5. the exchange building (once all legacy networks were closed [9] A. E. Kelly et al., IEEE Photon. Technol. Lett., vol. 18, no. 12, pp. down). Ultimately this architecture could simplify a network the 2674–2676, Dec. 2000. size of the United Kingdom from exchange buildings [10] Amphotonix Model OPA-18-N-S-FA. [11] R. Seifert, Gigabit Ethernet: Technology and Applications for High- to a few hundred. Speed LANs. Reading, MA: Addison-Wesley, 1998, p. 89. While this paper has focused on long reach TDMA PONs, it [12] Data Sheet, “Motorola AXS2200 GPON Optical Line Terminal” Mo- should be stressed that the long reach concept has been applied torola Inc., 2007. [13] D. Grossman, “Experience with a field-deployable amplified GPON,” to hybrid WDM/TDMA PONs [32] and to pure WDM-PONs in Proc. OFC-NFOEC 2008 Workshop, OSuF. dedicating a wavelength per customer [33]. As the PON and [14] K. Solehmainen et al., “Erbium doped waveguides fabricated with number of customers per fiber increase, resilience may at atomic layer deposition method,” IEEE Photon. Technol. Lett., vol. 16, no. 1, pp. 194–196, Jan. 2004. some point become a requirement. GPON already allows dual [15] G. P. Agrawal, Fiber Optic Communication Systems, 3rd ed. : , p. parenting to two separate headends as described in ITU-T 261[Author: Please provide the publisher Recommendation G.984.1. Protection of dual-parented PONs and year of publication.--Ed.]. requires not only that the PON can quickly divert traffic to a [16] M. Rasztovits-Wiech et al., “Optical amplification in 10 Gbit/s PONs,” second headend but also that the core network (particularly in Proc. NOC2008, Krems, Austria, Jul. 2008. the higher layers) acknowledge that the routing of frames and [17] D. Nesset et al., “Demonstration of 100 km reach amplified PONs with upstream bit-rates of 2.5 Gbit/s and 10 Gbit/s,” in Proc. ECOC 2004, packets through the network has to be altered to reflect the Stockholm, Sweden, Sep. 2004, We2.6.3. change of destination [34]. [18] D. Nesset et al., “10 Gbit/s bidirectional transmission in 1024-way split, 110 km reach, PON system using commercial transceiver mod- ules, super FEC and EDC,” in Proc. ECOC 2005, Glasgow, U.K., Sep. 2005, Tu1.3.1. [19] H. Choi et al., “High power gain-clamped EDFAs with flat gain,” in Proc. OFC 2001, Anaheim, CA, Mar. 2001, WDD28. [20] M. van Deventer and O. Konig, “Unimpaired transmission through a V. C ONCLUSION bidirectional erbium-doped fiber amplifier near lasing threshold,” IEEE Photon. Technol. Lett., vol. 7, no. 9, pp. 1078–1080, Sep. 1995. [21] G. Talli et al., “Integrated metro and access network: PIEMAN,” in Proc. NOC2007, Kista, Sweden, Jun. 2007, pp. 493–500. We have described potential benefits to network operators of [22] M. Rasztovits-Wiech et al., “10/2.5 Gbps demonstration in extra-large increasing the reach of commercially-available PONs recently PON prototype,” in Proc. ECOC 2007, 2007, We8.4.2. IEEE [23]Proof E. Hugues-Salas et al., “A 2.5 Gb/s edge-detecting burst-mode receiver been standardized in ITU-T Recommendation G.984.6. We have for GPON access networks,” in Proc. OFC 2007, OThK6. described a prototype of such a system using semiconductor op- [24] R. Davey et al., “Progress in IST project PIEMAN towards a 10 Gbit/s, tical amplifiers deployed in an underground footway box to ex- multi-wavelength long reach PON,” in Proc. BroadBand Europe Conf., Geneva, Switzerland, Dec. 11–14, 2006, pp. 1–6, Th3A3. tend the physical reach of G-PON to 60 km and 128-way split. [25] B. Thomsen et al., “10 Gb/s AC-coupled digital burst-mode optical Furthermore we have described a second prototype which ap- receiver,” in Proc. OFC 2007, OThK5. plies the concept of PON reach extension to a next generation [26] S. Nishihara et al., “A 10.3125-Gbit/s SiGe BiCMOS burst-mode 3R receiver for 10 G-EPON systems,” in Proc. OFC 2007, post deadline PON: achieving 10 Gbit/s, 100 km reach and 512-way split. Paper PDP8. Finally we have reviewed future directions of optical network [27] P. Vetter et al., “MUSE: Challenges to integrate the multi-disciplinary architectures and concluded that as bandwidths increasePrint long field of BB accessVersion in one project,” in Proc. ECOC 2007, 2007, We8.4.2. [28] M. Rasztovits-Wiech et al., “Bidirectional EDFA for future extra large reach PONs will play an increasingly important role in future passive optical networks,” in Proc. ECOC 2006, Cannes, France, Sep. networks. 2006, Mo4.5.7. DAVEY et al.: LONG-REACH PASSIVE OPTICAL NETWORKS 19

[29] M. Rasztovits-Wiech et al., “10/2.5 Gbps demonstration in extra-large PONs with the objective of significantly reducing the end to end cost of FTTP PON prototype,” in Proc. ECOC 2007, 2007, We8.4.2. solutions. In September 2007 he took early retirement from BT. He is now [30] M. Rasztovits-Wiech et al., “Realization of an XL-PON prototype,” working part time as a freelance consultant and has recently joined the Insti- in Proc. BroadBand Europe Conf., Antwerp, Belgium, Dec. 2007, tute of Advanced Telecommunications, Swansea University, Swansea, U.K., to We3B2. continue work on future network architectures. [AUTHOR: PLEASE PRO- [31] D. B. Payne and R. P. Davey, “The future of fibre access systems,” BT VIDE YOUR EDUCATIONAL BACKGROUND.—ED.] Technol. J., vol. 20, no. 4, pp. 104–114, Oct. 2002. In 2005 he was awarded the Martlesham Medal for his contribution to optical [32] G. Talli and P. D. Townsend, “Hybrid DWDM-TDM long reach PON access networking. for next generation optical access,” J. Lightw. Technol., vol. 24, no. 7, pp. 2827–2834, Jul. 2006. [33] S. Lee, S. Mun, M. Kim, and C. H. Lee, “Demonstration of a long reach DWDM PON for consolidation of metro and access networks,” Derek Nesset (M’01) received the B.Sc. degree in physics and the M.Sc. degree J. Lightw. Technol., vol. 25, no. 1, pp. 271–276, Jan. 2007. in telecommunications engineering. [34] D. K. Hunter, Z. Lu, and T. H. Gilfedder, “Protection of long-reach He joined BT Labs, Ipswich, U.K., in 1989 and spent several years developing PON traffic through router database synchronization,” J. Optical Net- photonic components for fiber optic communication systems. Following this, he working, vol. 6, no. 5, pp. 535–549, May 2007. studied the applications of nonlinearities in semiconductor optical amplifiers to high-bit-rate fiber optic systems up to 100 Gbit/s. This included the first field Russell P. Davey received the honors degree in physics from Oxford University, demonstration of 40 Gbit/s transmission over BT’s fiber infrastructure. In 2000, Oxford, U.K., in 1989, the Ph.D. degree from Strathclyde University, Glasgow, he joined Marconi Communications to develop the world’s longest (3000 km) U.K., in 1992, and the M.Sc. degree in telecommunications engineering from commercially deployed and unregenerated 10-Gbit/s terrestrial DWDM trans- University College, London, U.K., in 2001. His doctoral work focused on mode- mission system. He returned to BT in 2003 to conduct research into the physical locked erbium fiber lasers. layer issues relating to enhanced PON systems. He has authored or coauthored He worked for Logica from 1992 to 1994, where his projects included soft- over 70 journal and conference publications and holds five patents. ware development for the European Space Agency’s Huygens probe to Titan. He joined BT, Ipswich, U.K., in 1994. He first performed research into 100-Gbit/s optical networks. He was then heavily involved in the first application of WDM to the BT network. Since 2001, he has managed BT’s optical research. His main A. E. Kelly previously worked at British Telecom Laboratories and Corning interest has been next-generation optical access, on which he has regularly pre- and was a cofounder of Kamelian Ltd and Amphotonix Ltd., Glasgow, U.K. sented at major international conferences and been involved in ITU-T standard- His current research is in the use of semiconductor optical amplifiers for PONs, ization. He now manages BT’s optical design team. optical burst switching, and ultrafast optical switching. He has published over 100 journal and conference papers on a range of optoelectronic devices and sys- tems and holds a number of patents.[Author: Please provide Daniel B. Grossman (SM’01) received the B.S. degree in computer science your educational background.--Ed.] from Worcester Polytechnic Institute, Worcester, MA, in 1979. He has had a series of roles at Codex Corporation and Motorola, Inc. since 1981, where he was involved in the areas of telecommunications and com- puter networking. He is presently a Fellow of the Technical Motorola Applied Albert Rafel received the degree “Enginyer en Telecomunicacions” and Research and Technology Center, Marlborough, MA, where he leads a team the Ph.D. degree in telecommunications from the Universitat Politècnica de working on next-generation PON architecture. Catalunya (UPC), Barcelona, Spain, in 1992 and 1999, respectively. He joined BT Labs, Ipswich, U.K., in April 2001, working on WDM networks, optical packet networks, and traffic modeling. Recently, he has been working on fiber access networks mainly looking at the protocols and resilience Michael Rasztovits-Wiech received the Ph.D. degree from Vienna University for next-generation PONs. During 2008, he was a BT delegate in FSAN and of Technology, Vienna, Austria, in 1996. ITU-T. He has participated in numerous European and UK collaborative He spent six years as a Research Assistant with Vienna University of Tech- projects being BT’s representative in the EU IST Project MUSEII-SPE. He nology, where he contributed to projects related to optical free-space inter-satel- has authored or coauthored several papers in international conferences and lite communication for the European Space Agency, and to the European ACTS journals. projects PHOTON and MOON, both related to optical fiber communication, in particular transmission issues in dense wavelength division multiplex optical networks. In 1998, he joined Siemens IT Solutions and Services PSE, Vienna, where he was involved in various optical fiber network projects in the access, Shamil Appathurai received the B.Eng (1st Class Honours) degree in elec- the metro and the long-haul domain. His tasks range from Researcher, Product tronic and electrical engineering and the Ph.D. degree from University College Developer, Systems Architect to Team-leader. He also contributed to the Euro- London (UCL), London,U.K., in 2000 and 2005, respectively. His doctoral work pean IST project MUSE. He is the author and coauthor of over 15 journal and was on nonlinear distortion and its suppression in high-speed WDM transmis- conference papers. sion systems. During the course of his academic work, he was awarded with studentships from the Royal Society and the Rank Prize. He joined BT Labs, Ipswich, U.K., in 2006, and has been working on the physical layer aspects fiber access net- Dave B. Payne joined BT Labs, Ipswich, U.K., in 1978, working on single- works and fiber sensors for network diagnostics. He is the coordinator for an mode fiber splicing and connectors. He subsequently moved into optical ac- EU collaborative project PIEMAN. He has authored or coauthored close to 20 IEEEpapers inProof international conferences and journals. cess networks, and part of this work was leading a team developing fused fiber couplers an essential component to many optical access architectures. He was Dr. Appathurai is a member of the IET (formerly known as the IEE). co-inventor of TPON, the first passive optical network, and wrote the first in- ternal paper on shared access networks in 1983. The early work on TPON was extended to amplified PONs, which culminated in 1991 in an experimental PON with 50-million-way-split, 500-km range carrying 16 2.5 Gb/s wavelengths. Sheng-Hui Yang (M’96) received the B.S. and M.S. degrees from National During the early 1990s, this work moved on to amplified PONs with more prac- Taiwan University, Taipei, in 1986 and 1990, respectively, and the Ph.D. de- tical splits of $ IHHH. In the later 1990s, he moved into business and traffic gree from the University of Maryland, College Park, in 1996, all in electrical modeling looking at drivers of bandwidth and the economic justification for engineering. large-scale deployment of optical access and core networks. In 1999, he took From 1996 to 2000, he was a Device Engineer working on high-power semi- over the Broadband Architecture & Optical Networks Unit and ran BT’s op- conductor lasers for industrial and telecom applications. From 2000 to 2007 tical research activities until the end of 2006. From 2007, he wasPrint the “Principle he was a Principal Engineer Version in several start-up companies making dense wave- Consultant on Optical Networks” in the BT Design organization at Adastral length-division multiplexing (DWDM) transmission systems or subsystems in- Park, Martlesham Heath, U.K. (formally BT Labs), working on extended-reach cluding 10 G/40 G DWDM transport equipment, polarization-mode dispersion 20 JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 27, NO. 1, JANUARY 1 2009

compensator, and optical performance monitor. He joined Motorola Applied Research and Technology Center (ARTC), Marlborough, MA in January 2008 as Distinguished Member of Technical Staff in Jan. 2008. Currently, he is ac- tively involved in passive optical network (PON) research which focuses on next-generation PON architecture, system engineering, and feasible transceiver technologies.

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