Optical Fiber Technology 26 (2015) 126–134

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Optical Fiber Technology

www.elsevier .com/locate /yof te

WDM PONs based on colorless technology

http://dx.doi.org/10.1016/j.yofte.2015.08.0021068-5200/� 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author.E-mail address: fabienne.saliou@orange.com (F. Saliou).

Fabienne Saliou a,⇑, Gael Simon a, Philippe Chanclou a, Anna Pizzinat a, Huafeng Lin b, Enyu Zhou b,Zhiguang Xu b

aOrange Labs, 2 Avenue Pierre Marzin, 22300 Lannion, FrancebHuawei Technologies Co., Ltd., Bantian, Longgang District, Shenzhen 518129, PR China

a r t i c l e i n f o

Article history:Available online 28 August 2015

Keywords:Wavelength Division MultiplexingReflective Semiconductor Optical AmplifierColorless source

a b s t r a c t

Wavelength Division Multiplexing (WDM) Passive Optical Network (PON) is foreseen to be part of theNext Generation Passive Optical Networks. Business and mobile fronthaul networks already expressthe need to develop WDM PONs in the access segment. Fixed wavelength transceivers based on CoarseWDM are already available to respond to today’s market needs but Dense WDM technologies will beneeded and colorless technologies are essential to provide simple and cost-effective WDM PON systems.We propose in this paper to demonstrate the capabilities of a DWDM PON system prototype based onself-seeded RSOAs and designed to transmit CPRI over 60 km of fiber at 2.5 Gbit/s.

� 2015 Elsevier Inc. All rights reserved.

1. Introduction

The International Telecommunication Union ITU-T is finalizingthe upcoming standard for optical access networks on Next Gener-ation Passive Optical Networks 2. Several technological options arediscussed within this standard to be delivered under referencesG989.x: Time and Wavelength Division Multiplexing PONs andalso some point-to point WDM links as depicted on Fig. 1. Thisrepresents a first step towards the standardization of solutionsfor WDM in access networks.

Alternatively, WDM is already rolling out in optical access net-works with several use cases and a common objective: to avoiddeploying new fibers in the case of a lean fiber infrastructure.WDM is also foreseen to be the topology used particularly whensymmetrical bit rates and secured architectures are required. Inparticular, two use cases have been identified for WDM PONs:business Ethernet links as Fiber to The Office (FTTO) and mobilefronthaul links as Fiber To The Antenna (FTTA) as depicted in Fig. 2.

1.1. FTTO: Fiber to The Office

Optical fiber for enterprises (FTTO) is a decisive and essentialfactor in economic development of regions, aiming in a long termperspective to deliver attractive digital services to the companies.The FTTO offer is realized with a point to point topology, usingdual-fiber mediums and offering symmetrical bitrates from a few

Mbit/s to 10 Gbit/s. Bitrates requirements are proportional to theincrease of the service usage and type of service (File sharing, datasaving, videoconferencing, High Definition Voice over IP, Cloudcomputing, etc.). This point to point link connects an access Ether-net router placed at the enterprise site and an Edge Node router atthe central office. This offer is for offices, computer sites, call cen-ters, factories, medical centers, etc. . . Increased quality of service isguaranteed by FTTO offers (performance, service classes adapted toeach needs, commitments on the service availability and guaranteeof low recovery time). The evolution of this offer is going towardsthe optimization of the user’s connections to a central office: it isintended to densify the number of users per central and avoid sys-tematic connection to the nearest one. This will cause a large num-ber of fibers to be connected to a single central, but often fiberresource is not available between central offices. To reduce thisfiber need, it is possible to use passive multiplexing technologyoffering point to point connectivity and symmetrical bandwidthsuch as WDM PONs.

1.2. FTTA: Fiber to the Antenna with Mobile fronthaul links

Most of mobile antenna sites are already connected with opticalfibers to realize the backhaul of the Radio Access Network (RAN).Driven partly by a lack of space and climatic dispositions at theantenna sites, the evolution toward Centralized RAN (C-RAN) pro-poses to remove part of the RAN equipment from the antenna sitesto a common central office. This operation creates a new opticalaccess segment called ‘‘Mobile fronthaul” between the RemoteRadio Heads (RRH) placed at the antenna site and the Base BandUnit (BBU) placed at the Central office [1]. At the antenna site,

Fig. 1. Schematic view of the Next Generation PON2 standard.

Fig. 2. Application of WDM PON for mobile and business enterprise networks.

F. Saliou et al. / Optical Fiber Technology 26 (2015) 126–134 127

for the operator Orange France, up to 5 RAN technologies exist (2Gand 3G both at 900 MHz and 2100 MHz) or will be deployed (4G at800 MHz, 1800 MHz and 2.6 GHz) and 3 sectors for each technol-ogy are required to cover all the area around the antenna site. Thisrepresents up to 15 point to point links to create between eachRRH and the BBU at the central office. In order to optimizethe use of fiber resources, a Wavelength Division Multiplexing(WDM) technology is foreseen to realize those multiple point topoint links. Mobile fronthaul links use Common public RadioInterface (CPRI) or Open Base Station Architecture Initiative(OBSAI) protocols with bitrates from 614.4 Mbit/s for CPRI1 to10.137 Gbit/s for CPRI8. The line bitrate depends on the RAN tech-nology, the frequency bandwidth of the radio carrier (10 MHz or20 MHz) and the Multiple Input Multiple Output (MIMO) option.

1.3. WDM in optical access networks today: CWDM Dual Fiber &CWDM single fiber

Wavelength Division Multiplexing technique is already used inaccess networks, particularly in a field trial of mobile fronthaul inOrange France using Coarse WDM technologies. The main advan-tage of this solution is that its infrastructure remains passive usingmultiplexers (MUX) and de-multiplexers (DMUX) for bidirectionaltransmissions. The CWDM technology, based on 20 nm spacingbetween channels, has been chosen to reduce the number of fibersto deploy and is, up to now, the only cost-effective WDM solutionsupporting bitrates at 2.5 Gbit/s and compliant with outdoor con-ditions (I-temp ranges [�40 �C;+85 �C] required at an antenna sitefor example). The CWDM grid offers up to 16 channels from1270 nm to 1610 nm, with a possibility to avoid the OH peak (high

insertion losses). In a mobile fronthaul application setup, 15CWDM links are dedicated to cover a full antenna site with 15RRH (2G, 3G and 4G) and the sixteenth link can be reserved forthe management of the optical link, as shown on Fig. 3. Indeed,the mobile operator owning the RRHs + BBUs can be different fromthe fixed operator owning the optical fiber. In this case, somedemarcation point needs to be defined to provide service levelagreements between multiple operators. An example of semi-passive monitoring is depicted on Fig. 3: using a passive loop backat the antenna site and a dedicated CWDM channel to realize asignal round trip monitoring from the central office.

Recent commercial products are available with a major techni-cal advance on the CWDM channel: evolution of the Small FormPluggable (SFP) transceiver from dual fiber to a single fiber output.Several techniques have been identified:

� Single Wavelength Single Fiber (SWSF): the same wavelength isused to transmit the downstream and upstream signals. A spe-cial technique is used to separate the signals before the receiver.Also, a specific product has been developed to enhance the per-formances of this transceiver: SWSF with Reflection ImmuneOperation (RIO) [2]. This SWSF transceiver can detect and lowerreflected signals and thus reduces the impact of back-reflectionsdue to connectors or Rayleigh back scattering in the fiber.

� Cooled Single Channel (CSC) [3]: the CWDM channel is sub-divided in two sub-bands of 6 nm wide. The transceiver at eachside of the network is chosen to emit in the ‘‘high” sub-band orthe ‘‘low” sub-band and vice versa for the other network termi-nation. Then, in order to maintain the laser in this sub-band, athermoelectric cooler is required.

Fig. 3. Mobile fronthaul links realized with CWDM dual fiber technologies and semi-passive monitoring of the fronthaul link.

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1.4. Evolution of optical access networks towards DWDM

With the advent of multiple sizes of cells on mobile antennasites (macro, micro, small cells), in the future, the number ofCWDM channels available might be unsufficient and a DenseWDM technique (DWDM) could be required. In order to re-usethe deployed WDM infrastructures, a first step towards DWDMin access networks could be to transmit several DWDM channelswithin a CWDM channel. When green-field areas are use cases,pure DWDM technologies can be foreseen. The main challenge todeploy DWDM technologies relies on the cost of DWDM transcei-vers. Fixed wavelength DWDM technologies are out of scope foroperational reasons: complex management of the products in thenetwork information system, require a lot of spare parts for thetechnician in the field, not outdoor compatible today (no I-tempproducts). Then, the key technology for DWDM in access is a costeffective colorless optical source: using the same device at eachuser termination will permit mass production and reduce the oper-ational costs compared to fixed wavelength items. Moreover, theexploitation of colorless transceiver would overcome inventoryproblems, which could burden the mobile network administration,limiting inventory and maintenance costs [4]. The wavelengthmanagement of the source also needs to be efficient with regardsto climatic changes and relies on simple technical tools to reducethe operational expenditure of the network. Many DWDM color-less technologies have been studied in research laboratories: Spec-trum slicing [5,6], Injection locked Fabry-Pérot [7,8], wavelengthreuse [9], Tunable Lasers [10], Reflective Semiconductor OpticalAmplifier (RSOA) self-seeding [11], etc. Only a few of thesetechnologies have been exploited recently to realize a commercialsystem and they are still limited to a prototype system: ADVAdeveloped a WDM PON system with transmissions at 1 Gbit/s baseon tunable lasers [10], Transmode proposed a 1.25 Gbit/s DWDMPON system based on Injection Locked Fraby-Pérot [12], Aeponyxrealized transmissions at 1.25 Gbit/s with a WDM PON technologyusing RSOA associated with Fiber Bragg Gratings [13] and finallyHuawei Technologies worked on self-seeded RSOA technologieswith a prototype system transmitting 1.25 Gbit/s [14].

2. Experimentation of a DWDM system prototype based on self-seeded RSOAs technologies

We report the evaluation of a new WDM PON prototype system[17]. This system is dedicated to mobile fronthaul applicationstransporting CPRI links and the DWDM technology adopted isbased on Self-seeded RSOAs.

2.1. Self-seeded based on RSOAs principle

Self-seeded sources based on Reflective Semiconductor OpticalAmplifier (RSOA) are investigated for optical access networks ascolorless sources with passive and automatic wavelength assign-ment [4]. Indeed, the use of colorless technologies will simplifythe introduction of Dense Wavelength Division Multiplexing(DWDM) in the access networks by reducing its impact on opera-tional activities: using the same device for every network termina-tion will permit mass production and reduce the number of sparingitems needed on the field. Moreover, an automatic assignment ofthe wavelength simplifies engineering rules using colorless deviceswith multiple channels infrastructure. The colorless operation ofself-seeded sources is depicted in Fig. 4.

At each termination of the network, the RSOA is directly modu-lated and emits a wide band source of ASE (Amplified SpontaneousEmission). The ASE is sent to a drop fiber, whose length corre-sponds to the distance between the network termination and theRemote Node (RN). The latter contains only passive devices: thewavelength multiplexer and a single partial mirror composed bya dissymmetric optical splitter and a mirror. Then, a simple con-nection of the drop fiber to one channel of the MUX permits to slicethe RSOA ASE wide band spectrum. This weak signal is sent partlyto a 90� Faraday Rotator Mirror (FRM) which reflects it back to theRSOA for re-amplification and reflection again. The RSOA and theFRM thus create a resonant cavity, where a laser is established afterseveral round trips. In order to maintain the same state of polariza-tion after one round trip in the cavity, a 45� Faraday Rotator can beinserted at the output of the RSOA [7]. Finally, at the other outputof the optical splitter, we obtain a strong and stable self-seededsignal to realize the transmission over several kilometers of feederfiber. The simple connection to one channel of the wavelengthmultiplexer selects the emission wavelength of each RSOA-basedtransmitter, and at the same time allows all the network termina-tion signals to be multiplexed in the same feeder fiber. Comparedto other colorless technologies, here, no additional externalseeding source or external mechanisms to control the wavelengthstability are required.

The new WDM PON system prototype permits to realize up to16 CPRI links on a single feeder fiber. The system is designed totransmit with line bitrates at 2.5 Gbit/s corresponding to CPRI3.An active box is placed at each side of the network termination:at the OLT, the line card processes the ‘‘black & white” CPRI signalsfrom the BBU to the DWDM sources. In a symmetric way at theuser side, the ONU processes the signals from the DWDM sourcesto the ‘‘black & white” links to RRH. Fig. 5 gives the full systemsetup description and Fig. 6 a detailed view of the Quad SmallForm-factor Pluggable (QSFP) and the Remote Node (RN).

Fig. 4. Description of the operation of the self-seeded sources.

Fig. 5. Full system setup.

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The ‘‘black & white” links are the CPRI links between the BBUand the RRH regularly based on dual fiber uncolored Multi-ModeFiber (MMF) or Single Mode Fiber (SMF) SFPs. When inserting thisWDM system in the fronthaul setup, those links are interruptedand brought to intermediate optical interfaces: MMF QSFPs atthe OLT (up to 4 dual-fiber links per QSFP) and MMF SFPs at theONU (4 dual-fiber links per ONU).

At the OLT, a high level of integration is provided. TheWDM PON OLT line card contains all the active and passiveelements:

- The Remote Node with the MUX and DMUX functions and theFaraday Rotator Mirror (FRM), as well as bidirectional operationfunction on a single feeder fiber thanks to a circulator. Fig. 6gives a detailed view of the Remote Node.

- The RSOA sources are packaged in QSFP modules by 4, also with4 Avalanche Photodiodes (APD) receivers.

The OLT line card and the ONU perform some processing of theCPRI signals as well as including FEC (Forward Error Codes).

At the ONU, the same devices are implemented to realize 4 CPRIlinks per ONU, except that the Remote Node is no more integrated:at the user side this passive device can be placed at a given dis-tance from the RRH (drop fiber up to 1 km or 5 km).

2.2. Characteristics of the MUX and DMUX

Recent research studies pointed out the impact of de-multiplexing a self-seeded signal with the same optical filter asthe one used as MUX and in the external cavity of the self-seeding laser [15]. Fig. 7 presents the spectral measurement ofthe 16 channels MUX and DMUX used in our experimental setup.

The shape of the MUX (in blue on the previous figure) isthe shape of a regular Gaussian Arrayed Wave Guide (AWG).However, the DeMUX (in black) transfer function is different and

Fig. 6. Detailed setup and connections between the QSFP and the Remote Node.

Fig. 7. Spectral transfer function of the MUX and DMUX used at each Remote Nodes.

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corresponds slightly to a shape designed to fit the original self-seeded laser spectral shape, which usually presents a spectrumslightly red shifted (to the high wavelengths) [16]. Indeed, lookingspectrally at each mode of the source independently (Free SpectralRange of a few kHz to several MHz depending on the cavity length),a gain competition exists between each mode in the cavity. Thisgain, for SOAs, is generally higher for the highest wavelengths. Thisexplains that the highest wavelengths (modes) are favored to reacha steady state. This effect also depends on the spectral shape of thefilter, since its transfer function relates the internal losses in thecavity according to the wavelength. Consequently, at the borderof the MUX, higher losses are experienced and the gain providedin the cavity is not sufficient to maintain an equal competitionbetween modes. Therefore, the red shift effect on the self-seededsource can be reduced using a Gaussian shape AWG as the multi-plexing device. Moreover, for this system, the �3 dB bandwidthof the DeMUX was designed to be always wider than the MUXone which permits to avoid cutting off the operating self-seededsignal and thus avoid penalties due to de-multiplexing as observedpreviously in [15].

2.3. Characterization of the RSOAs

The RSOAs used at both side of the system are designed to emitan ASE in the C-band. According to the ASE measurementperformed on the RSOA1 from the QSFP1, presented in Fig. 8(b),

its gain peak is centered at 1550 nm. The RSOA presents also highripple peaks with amplitude up to 10 dB. We measured the RSOA1gain according to the input power injected in the RSOA. Results arepresented on Fig. 8(a) for a laser injected at different wavelength:1530 nm, 1550 nm and 1570 nm. Then, the low signal RSOA gain isrespectively of 16.5 dB, 18 dB, and 20 dB. This variation of gainaccording to the input wavelength and the spectral position of rip-ple peaks may induce some variation of the output power of theself-seeded sources, according to the position of the MUX channel.This could also have an impact on the performances of the trans-mission. In a previous study [18], it has been showed that with asimilar RSOA, multiple channel operations were realized success-fully (BER < 10�9) but transmissions performances varied for eachchannel, with penalties up to 4 dB between the best and worstchannel. With this system, we expect to obtain similar behaviorand penalties, comparing each channels of the system, and we willpresent results obtained on the best channel available (CH11).

2.4. Laser spectrums and optical power measurements

Associating the RSOA1 with the remote node presentedpreviously, we realized the external cavity sources based onself-seeded RSOAs. To realize the transmissions using the fullsystem, 4 QSFPs, each ones containing 4 RSOAs, are connected tothe 16 channels of the MUX. However, in order to compare theself-seeded channel performances over several channels, we used

Fig. 8. (a) RSOA1 gain measurement for various input power injected (left figure); (b) RSOA1 ASE spectrum.

Fig. 9. Optical spectrum of the self-seeded sources for each channel.

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the same RSOA ‘‘RSOA1” connected alternately to each channel ofthe MUX: from Channel 1 ‘‘CH1” to ‘‘CH16”. Fig. 9 presents theoptical spectrums measured for each channels when no drop fiberis inserted in the cavity. Some spectrum samples were also mea-sured for CH1, CH9 and CH16 with 1 km of drop fiber inserted inthe cavity.

The spectral shape of the self-seeded source follows the MUX’sone: a Gaussian shape with a peak wavelength centered accordingto the MUX channel center. As discussed in the previous sectionrelated to the RSOA Gain, we observe a variation of the outputpower for each channel of the MUX. This is related to the variationof the RSOA gain and the spectral alignment or misalignment of theripple peak and the MUX channel. When 1 km of fiber is added inthe cavity, we observe no variation of the peak wavelength; onlythe output power is reduced due to higher insertion losses in thecavity.

Fig. 10. Upstream transmission of CPRI3 on CH11 in back to back conditions,varying the drop fiber length.

2.5. Transmission performances

The optical budget (OB) is measured from the common port ofthe Remote Node (the ONU MUX/DMUX) output, to the commonport of the OLT MUX/DMUX. Using a CPRI protocol tester, the sys-tem performances cannot be measured in terms of Bit Error Ratio(BER), but in terms of number of received Frames, with regardsto 15000 frames transmitted.

First, we measured the back to back performances of theupstream transmission on the eleventh channel and for differentlengths of the drop fiber (equivalent to the distance between theRRH at the antenna site and the remote node place in a streetcabinet). Results are presented on Fig. 10 for drop fibers of a fewmeters (cavity close to 0 km), 1 km, 2 km and 5 km.

For short cavities, when the Remote Node is co-located with theRRH site, an optical budget of 18 dB is achieved without any loss of

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frame. When the cavity length is increased, the optical budget isreduced to 16 dB, 15 dB and 13 dB respectively for 1 km, 2 kmand 5 km of drop fiber. This can be related to reduction of the out-put power of the self-seeded signal when the cavity length andconsequently the losses increase. Also adding more fiber in the cav-ity brings more chromatic dispersion linked to the multiple roundtrips of the self-seeded signal in the cavity and thus increases thenoise in the cavity and degrades the eye diagram of the self-seeded output.

Inserting several length of feeder fiber (20 km, 40 km and60 km) in the experimental setup permits to obtain the transmis-sion results in Fig. 11(a) and (b) respectively for the upstreamand the downstream CPRI3 signals. The impact of chromatic dis-persion, restricting the optical budget, is clearly observable whenthe optical fiber length is increased. An upstream optical budgetof 12 dB is achieved with 60 km of feeder fiber whereas 14 dBcould be achieved in the downstream. The downstream opticalbudget is generally about 1.5 dB higher than the upstream one:the high level of integration provided at the OLT using splicesinstead of connectors reduces the losses and risks of misconnectioninside the cavity.

When longer distances between the antenna site and theremote node are required, the length of the cavity can be increasedup to 5 km and CPRI3 error free transmissions are realized withfeeder fibers up to 40 km for cavities of 1 km and 2 km and a max-imum reach of 20 km realized a cavity of 5 km. Results are pre-sented on Fig. 12, presenting the optical budget realized forseveral feeder fiber lengths, with a cavity length of 1 km Fig. 12(a) and 2 km Fig. 12(b).

Fig. 11. (a) Upstream and (b) Downstream transmissions of CPRI3 o

Fig. 12. Upstream transmission results with longer drop and fe

2.6. Coexistence of the DWDM PON with legacy TDM PONs

Finally, with more than 11 dB of optical budget available for anyconfiguration tested previously, this WDM system can be insertedin wavelength overlay over legacy TDM PONs: GPON and XGPON1,as shown in Fig. 13. A Coexistent Element (CEx) designed forNGPON2 standard is placed after each PON technology OLTs torealize the wavelength overlay and create the coexistence over asingle feeder fiber. Then the TDM Remote Node consists of an opti-cal splitter 1–4 or 1–8. The DWDM PON system based on self-seeded RSOAs was then inserted at each side of network: beforethe CEx (DWDM OLT) and after the RN (DWDM Remote Node).One ONU was connected per PONs technology (see Fig. 14).

The GPON and XGPON1 downstream transmission links presentedthe following parameters measured at OLT multiplexer output:

- GPON: traffic bandwidth 59.2 Mbit/s; Mean optical power:2.0 dBm; Wavelength: 1490.8 nm.

- XGPON1: traffic bandwidth 78.9 Mbit/s; Mean optical power:2.3 dBm; Wavelength: 1576.5 nm.

- In the other way, the GPON and XGPON1 upstream transmis-sion links presented the following parameters:

- GPON: traffic bandwidth 59.2 Mbit/s; Wavelength: 1311 nm.- XGPON1: traffic bandwidth 493 Mbit/s; Wavelength: 1271 nm.

The following figure presents the optical spectrums of thedownstream (a) and upstream (b) signals:

The drop fiber of the self-seeded system at ONU side is first setto 0 km (a few meters cavity). The feeder fiber length is 20 km, and

n CH11 for a few meters cavity, varying the feeder fiber length.

eder fibers: (a) for 1 km drop fiber; (b) for 2 km drop fiber.

Fig. 13. Experimental setup with coexisting GPON, XGPON1 and DWDM PON systems.

Fig. 14. Optical spectrums of the GPON, XGPON1 and DWDM Self-seeded systems for: (a) Downstream signals and (b) Upstream signals.

F. Saliou et al. / Optical Fiber Technology 26 (2015) 126–134 133

the remote node consists in a 1–8 splitter. Thus, the optical powerof the self-seeded upstream signal is measured at �22 dBm atcommon port of the coexistence multiplexer. At the same point,the downstream self-seeded optical power was �2.7 dBm. Errorsfree transmissions are realized both for the legacy PONs (GPONand XGPON1), or the DWDM Self-Seeded system. Thus, theDWDM PON system is able to co-exist with legacy PON systemssuch as GPON and XGPON1. Error free transmissions are alsorealized with a drop fiber of 1 km, 20 km of feeder fiber and asplitter 1–4.

3. Conclusions

After the ascension of fixed wavelength transceivers for opticalnetworks, colorless technologies in DWDM PONs are essential tomaintain a cost-effective access network. We demonstrated in thispaper the error free transmission of CPRI3 with a DWDM PONsystem based on self-seeded RSOAs. This colorless technologydelivers an automatic assignment of the DWDM wavelength, by asimple connection to a multiplexer. Up to 16 mobile fronthaullinks can be realized with this system in order to decrease thenumber of horizontal fibers to deploy from a central office to theantenna sites. Finally, this system prototype is compatible, interms of optical budget performance, with the coexistence inwavelength overlay with legacy PONs such as GPON andXGPON1.

Acknowledgments

The authors would like to thank Huawei technologies for theirhelp and support with the DWDM system prototype. This workwas also supported by the French research program ANR projectLAMPION under grant agreement ANR-13-INFR-0002.

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