WDM-PON experiences in Korea [Invited]

Transcription

1 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 451 WDM-PON experiences in Korea [Invited] Chang-Hee Lee, 1, * Sang-Mook Lee, 1 Ki-Man Choi, 1 Jung-Hyung Moon, 1 Sil-Gu Mun, 1 Ki-Tae Jeong, 2 Jin Hee Kim, 2 and Byoungwhi Kim 3 1 Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Guseong-dong, Yuseong-gu, Daejeon, , South Korea 2 Network Infra Laboratory, KT Jeongmin-dong, Yuseong-gu, Daejeon, , South Korea 3 ETRI 161, Kajeong-dong, Yuseong-gu, Daejeon, , South Korea *Corresponding author: chl@ee.kaist.ac.kr Received January 5, 2007; revised March 13, 2007; accepted March 15, 2007; published April 20, 2007 Doc. ID WDM-PON activities in Korea including research and development were reviewed. The WDM-PON commercialized in Korea is aimed at delivering a 100 Mbits/ s dedicated symmetric bandwidth to each home, based on the prediction of a small statistical gain for future video-centric services. The developed 32-channel WDM-PON based on wavelength-locked Fabry Perot LDs was qualified by Korea Telecom (KT) and was deployed in KT s networks. Many studies have been conducted with the aim of enhancing the performance of the WDM-PON. High-speed data rate per channel, longer reach, narrow channel spacing, and a high split ratio with the help of subcarrier multiple access or time-division multiple access have been demonstrated. A protection method for feeder fiber fault and OSP fault-monitoring methods were also demonstrated. A gradual upgrade solution from the existing time-division multiplexing-pon to WDM-PON has also been proposed Optical Society of America OCIS codes: , Introduction From 2001 to 2004, Korea maintained the highest broadband service penetration rate in the world. However, the nation was ranked second in 2005 and fourth in This slip in rankings was not, however, due to a decrease in the penetration rate in Korea. It arose from a rapid increase of the penetration rate in other countries, such as Denmark, Holland, and Iceland. The number of broadband service subscribers in Korea has been growing continuously, as shown in Fig. 1. With an increase of broadband services, Korea is experiencing a change of traffic patterns from voice and data to video. Video services require not only high bandwidth but also high quality of service (QoS). These demands will be accelerated with the increasing popularity of wide-screen displays. For example, it is expected that high-definition (HD) quality video for widescreen displays will be a standard video format in the near future. A single HDTV channel encoded by MPEG 2 requires 20 Mbits/s. This is approximately 13 times the bandwidth compared with the national television system committee (NTSC) and five times that of standard (SD) quality video. Thus, future access networks should be prepared to deliver HD quality video services with high QoS. We also show the history of the revenue from the broadband subscriber [1]. It should be noted that there was no dip for the bandwidth or revenue during the bubble period of The Internet subscriber and revenue increased rapidly from 1999 to The rate of increase then began to saturate. However, the number of users requesting service in excess of 50 Mbits/s began to rapidly grow from This growth trend is expected to continue for more than 10 years. The Korean government announced a road map to deliver more than 50 Mb/s to 10 6 subscribers in 2010 [2]. The main services that will necessitate such a bandwidth will be HD quality video services while other services such as voice and data will be converged into the video services. Triple-play services, converged service of voice, data, and video, will play a key role in future access networks. This will, in turn, simplify network management and billing systems. Further simplification of the network will be accomplished by employing /07/ /$ Optical Society of America

2 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 452 Fig. 1. History and prediction of broadband Internet subscribers in Korea. optical networks based on optical fiber. Fiber to the home (FTTH) based on a passive optical network (PON) is considered to be the ultimate goal of the access networks, since there are no limitations on both bandwidth and transmission length, contrary to copper-based networks. By making the network passive and optical, service providers can deliver higher bandwidth with considerably less OPEX and CAPEX [3]. There exist many different types of PON [4]. A point-to-multipoint architecture is preferred for PON to save optical fiber. Among the various types of PON, the wavelength division multiplexing (WDM)-PON is considered to be the most future-proof technology, since it can deliver an almost unlimited dedicated bandwidth with protocol transparency [4]. In this paper, we review the WDM-PON history in Korea, including research and development activities. In Section 2, we describe technology used for both field trials and commercial systems installed in Korea Telecom (KT) networks. We then describe the results of the field trials and provide a summary of deployment in Section 3. Research and development activities are reviewed in Section 4. A final summary is presented in Section Background and Technology for Field Trial and Commercial Systems 2.A. Background WDM-PON activities in Korea were started by a joint development project between KT and Novera Optics Korea Inc. in February The aim of the project was to develop an FTTH system based on a wavelength-locked Fabry Perot laser diode (F P LD), proposed by KAIST in 2000 [5]. At that time, the required bandwidth to be delivered to each home was estimated as 100 Mbits/s for both the upstream and the downstream. Details of this estimation are summarized in Table 1 [6]. The main service targeted by the project was HDTV based on Internet protocol (IP). The estimated maximum TV sets per home was three. Since there was no attainable MPEG 4 (H. 264) video CODEC at that time, it was assumed that HDTV signals were encoded by MPEG 2. By adding high-speed internet and some other services, such as a video phone, the authors arrived at a total of 73 Mbits/s downstream bandwidth. For the upstream, it was assumed that games on demand and education on demand would require symmetric bandwidth. It was also assumed that symmetric bandwidth would be required for high-speed Internet and the other services. The authors thereupon arrived at a figure of 53 Mbits/ s bandwidth for the upstream. By adding future upcoming services, the required bandwidth per home was determined to be

3 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 453 Table 1. Bandwidth Estimation Services Bandwidth Remarks Streaming video (HD) Total 60 M+ EoD and GoD, HD Broadcasting Video on demand (VoD) Home shopping 20 M/service 20 M/service 20 M/service class video clip require symmetric bandwidth Education on demand Picture in picture Overlay multicast (EoD), game on demand (GoD) FF/FR Internet 10 M PtP service requires symmetric bandwidth Video conference of video phone 2 M Requires symmetric bandwidth Remote control and sensing, etc. 1 M Requires symmetric bandwidth Total 73 M Downstream: 73 M Upstream: 53 M 100 Mbits/ s. Since statistical multiplexing gain for streaming video services was not anticipated, the bandwidth per home was assumed as dedicated bandwidth. The other motivation for the WDM-PON project was the business model of KT. KT thought that an analog video service would not provide competitiveness to the company, since there already existed 10 6 CATV subscribers at that time. Thus, they selected personalized HD quality video services based on IPTV. Another interesting video service was the networked personal video recorder (N-PVR). For this service, the PVR at home is moved to a central office (CO). The recorded contents are then streamed to the home upon the user s request. Under consideration of these bandwidth requirements, KT assessed many different technologies. At that time, Ethernet PON (E-PON) was the most advanced PON technology, providing 1 Gbit/ s Ethernet data for both the upstream and the downstream. The downstream data are broadcast to all users connected to the PON, while the upstream data are shared in the time domain with the help of multiple-access and ranging protocols. The estimated dedicated bandwidth was less than 30 Mbits/ s in the case of 32 subscribers being connected to the PON. Thus, it could not satisfy the requirement of 100 Mbits/ s per home, unless the subscribers per PON were reduced to eight. This approach was deemed a high-cost solution due to equipment costs and the need for a large amount of feeder fibers and related handling costs. KT also considered other technologies, including high-speed VDSL, hybrid fiber coax (HFC), and WDM-PON. Based on cost projection and future upgradability, KT selected WDM-PON, which provides virtual point-to-point connectivity through a dedicated wavelength [5,6]. This feature brings about many inherent advantages: unlimited bandwidth, protocol transparency, security, simplicity in electronics, etc. Also, the splitting ratio is not limited by the splitting loss of the remote node (RN). We show a typical configuration of a WDM-PON in Fig. 2. The network consists of an optical line termination (OLT), a feeder fiber, an RN, and optical network terminations (ONTs). An arrayed waveguide grating (AWG) can be used both at the OLT and at the RN for wavelength multiplexing and demultiplexing. Despite these advantages, there were unresolved issues that prevented commercialization of the WDM-PON. These included the high cost of wavelength specified optical sources, management and inventory problems associated with the sources, and wavelength matching between WDM devices (i.e., two AWGs) and the optical source with respect to environment change. These problems could be solved by using a low-cost ONT that operates the wavelength independently [4]. In other words, a single type of ONT can be used for every subscriber. This type of ONT is called a color-free ONT or colorless ONT. A temperature-independent AWG (i.e., athermal AWG) solves the wavelength-matching issue between two AWGs [7]. Within the joint development project, the wavelength-locked F P LD [5] was selected as the technology of the color-free ONT, since it was believed that the F P LD was the most economical optical source.

4 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 454 Fig. 2. WDM-PON configuration. 2.B. Wavelength-Locked F P LD It is well known that a F P laser shows multimode oscillation. The F P laser output shows a huge amount of mode partition noise when only a single mode is selected among the multimode output. This is an intrinsic feature in a multimode laser with a homogeneously broadened gain medium. Competition among lasing modes caused by fluctuation of spontaneous emission prevents steady oscillation of each mode in time. The semiconductor F P LD shows multimode oscillation as a common multimode laser, since the spontaneous emission is almost wavelength independent and the gain spectrum is very broad compared with the mode spacing. Thus, it is impossible to use a selected single-mode light from the F P LD for optical communications. Fortunately, it may be possible to generate a single longitudinal mode light by injecting a narrowband spontaneous emission into the F P LD. In this case, the injected spontaneous emission is amplified selectively when the injected light power is much higher than the spontaneous emission generated by the gain medium. The experimental setup to demonstrate this is shown in Fig. 3. It should be noted that the F P LD may be the most efficient light source for optical communications while also being cost effective. The broadband light source (BLS) that is amplified spontaneous emission (ASE) generated by a pumped erbium-doped fiber (EDF) is located at the receiver side. The AWG located between the BLS and the F P LD spectrally slices the broadband ASE to inject it into the F P LD. The bandwidth of the injected ASE is 0.4 nm, which is equal to the 3 db bandwidth of the AWG. We also used an antireflection (AR)-coated F P LD (as a cleaved back facet and 1% front facet reflectivity) with a cavity length of 600 m. The injected power into the F P LD (biased at 1.2 times the lasing threshold) is 16 dbm/ 0.2 nm. This setup represents the upstream data transmission configuration in a real system. The BLS and the receiver, including a wavelength division demultiplexer, can be located at a CO, while the F P LD is located at the customer s premises. The AWG is located at a remote node (RN). As shown in the measured spectrum, the output of the F P LD becomes a singlemode light after injection of a narrowband ASE. The linewidth of the F P LD output is 0.2 nm. To see the suppression of the mode-partition noise, we also show the measured relative intensity noise (RIN) before and after injection. The low-frequency RIN (solid curve) mainly caused by mode-partition noise was suppressed by 20 db with injection compared with a single-mode light of the free-running laser (dashed curve), as shown in Fig. 3(d). The RIN of the output light is 7 db less than that of the injected narrowband ASE (dotted curve). The side-mode suppression ratio depends on the injection power, bias current, and detuning between the lasing mode and injection wavelength [8]. We call this process wavelength locking instead of injection locking, since only the wavelength of the output light is locked to the injection wavelength without phase locking. The experimental results show a similar trend compared with previously reported findings [9]. To assess the possibility of data transmission with the wavelength-locked F P LD, we modulated the laser directly with a 155 Mbits/ s pseudorandom sequence of length The modulated signal was received by an optical receiver after transmission of a 20 km fiber. The measured eye diagrams in Fig. 3 with and without injection show

5 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 455 Fig. 3. (a) Schematic of wavelength-locked F P LD, (b) the output spectrum and eye diagram before wavelength locking, (c) those after wavelength locking, and (d) the measured RIN with (thick curve) and without (dashed curve). The dotted curve represents the RIN of the injected ASE. the effect of injection clearly. The completely closed eye without injection was opened with injection of the ASE. The measured bit error rate (BER) curve also showed errorfree transmission. It is predicted that the locking effect and efficiency will be the worst when the injection wavelength is located at the center of the adjacent modes. However, we can achieve error-free transmission with a 2.3 db penalty, as shown in Fig. 4. It is noted that the back-to-back BER curve is very similar to the best-case

6 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 456 Fig. 4. Best and worst BER of wavelength-locked F P LD. BER curve. In other words, there was a negligible dispersion penalty. These features imply that a single type of F P LD can be used for every ONT without temperature control. The gain bandwidth of the semiconductor active medium is typically broader than 50 nm. Thus, it is possible to use a single type of F P LD over an approximately equal wavelength range. It should be noted that the backscattering of the injected ASE due to Rayleigh scattering and/or reflection at splice or connection points will interfere with the upstream signal. This interference-induced penalty is negligibly small, since the band of the upstream signal (always broader than 0.1 nm) is very wide compared with the receiver bandwidth. The measured penalty was less than 0.1 db with a 30 db reflection coefficient. A schematic of the WDM-PON based on wavelength-locked F P LDs is presented in Fig. 5. The OLT consists of two BLSs, an AWG, and optical transceivers at the CO. A feeder fiber (transmission fiber) is connected to the OLT. At the other end of the feeder fiber, an AWG is located at the RN. Many branches of the distribution fiber connect the RN and subscribers. This is a point-to-multipoint architecture that supports point-to-point connectivity with two wavelengths for each subscriber. For upstream data transmission, A-band BLS output is transmitted through the feeder fiber and passed through the AWG at the RN. Spectrally sliced broadband light is then injected into F P LDs located at subscriber premises. Each F P LD is modulated by the upstream data. The outputs of the F P LDs are propagated in the upstream direction. These are multiplexed in the wavelength domain by the AWG at the RN. The multiplexed signals are transmitted to the OLT through the feeder fiber. These signals are demultiplexed by the AWG at the OLT and received by the optical receivers. There is another BLS for B-band and F P LDs at the CO for the downstream data transmission. The spectrally sliced B-band BLS by the AWG at the OLT is injected to the F P LDs. The remaining processes are similar to the upstream case. The multiplexed downstream signals are demultiplexed by the AWG at the RN after Fig. 5. Configuration of WDM-PON based on wavelength-locked F P LDs.

7 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 457 Table 2. WDM-PON Specifications Optical specifications OLT function OAM Bit rate per channel: 125 Mbits/s Maximum transmission length: 10 km Channel spacing: 100 Ghz Number of channels: 32 Upstream wavelength: nm Downstream wavelength: nm Layer 2 switching and forwarding QoS and packet classification IGMP snooping OAM EMS support (SNMP) Ethernet OAM Loop back Remote reset Surveillance Split ratio 32 ONT capacity 4 or 16 FES ports passing through the feeder fiber. The optical receiver at the customer premises receives the downstream data. The A-band wavelength and the B-band wavelength are separated by multiples of the AWG s free spectral range (FSR). In this manner, we consolidated multiplexing and demultiplexing in a single AWG. An athermal AWG, whose transmission wavelengths have a very small temperature dependency [7], was used to eliminate the wavelength drift induced by temperature change. It should be noted that the communication wavelength is determined by the output port of the AWG instead of the laser itself. The F P LD then operates like a wideband modulator with a gain. A diplexer is needed for a transceiver both at the OLT and the ONT, since only a single strand of fiber is used for communication. Note that a time-division multiplexing (TDM)-PON also uses an optical diplexer. To make the diplexer cost effective, a sufficient amount of guard band between the upstream and the downstream wavelength bands is needed. A conventional C band nm was selected for the upstream transmission, since the loss of the fiber is minimal at this band. We chose E band nm for the upstream to have a sufficient guard band. The ASE can be generated by a pumped doped fiber or a superluminescent diode (SLD). Since the F P LD made by a quantum well has a strong polarizationdependent gain, the polarization of the injected light should be matched with that of the F P LD output light. This can be accomplished with unpolarized injection light. A pumped EDF was used for the C-band BLS. Polarization-multiplexed SLDs were used for the E-band BLS. A WDM-PON with a 32-channel system (both for upstream and the downstream) was developed for FTTH application. It was targeted for triple-play services with HD-quality video. The specifications of the WDM-PON are listed in Table 2. The system consists of four shelves in a rack. Each shelf can accommodate ten OLTs. An OLT has an optics module that consists of two BLSs, an AWG, 32 transceivers, and an L2/L3 switch. So a shelf can accommodate 320 subscribers. In a shelf, we have a main controller unit (MCU) and an L3/L4 switch with a capacity of 8 20 Gbits/s. The developed system passed KT s qualification test in the first quarter of The WDM-PON system was subsequently installed in Kwangju for a field trial. 3. Field Trial To assess the performance of the developed WDM-PON system, a field trial was conducted under two different scenarios: an apartment complex and a residential area. The system was installed in a KT CO. The RNs were installed in a manhole for the residential users and in the basement of the building for the apartment users. Differ-

8 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 458 Fig. 6. Configuration for field trial of WDM-PON. ent drop fiber installation methods were used. For the residential users, aerial fiber was employed whereas air-blown fiber was used for the apartment complex [10]. The feeder fiber was installed inside an underground duct for both cases. The configuration for the field trial is shown in Fig. 6. The average distance from the CO to the residential users was 7.8 km and that for the apartment users was 8.8 km. The maximum loss from the CO to the ONT was 10 db for the apartment user and 12 db for the residential user. There were seven optical connectors along the transmission path from the OLT to an ONT. The connectors were a combination of square connector/physical contact (SC/PC) and square connector/angled physical contact (SC/APC). The difference in backreflection between the SC/PC and the SC/APC was 30 db. However, this did not impact the transmission performance owing to the very wide linewidth of the upstream signal due to spectrally sliced ASE injection. The services offered for this field trial were IP-TV, HD-VoD, SD-VoD, N-PVR, EoD, and Internet. To offer these services, the OLT of the WDM-PON has an L2/L3 switch with IGMP support. At the customer s home, an IP set-top-box (STB) and PC are connected to the ONT. The IPTV contents were generated by broadcast TV channels received by satellite STBs. The received channels were encoded into a MPEG2 video stream and encapsulated into an IP packet with a multicasting IP address. One HD channel and 20 SD channels were available for the IPTV service. The IGMP was used to deliver IPTV to IP-STB at the home. After a successful field trial, KT installed the FTTH system in the Kwangju area. To date, the total number of homes in the area covered by the WDM-PON is Among them, 1000 homes have been connected. In addition to the FTTH system, a fiber-to-the-pole (FTTP) system has been developed by Novera Optics Korea. This system has 16 channels in both directions. The bit rate per channel is 125 Mbits/s. The FTTP system has an Ethernet switch on the pole to share bandwidth among 24 or 16 homes. The configuration of the installed system is shown in Fig. 7. The total number of systems installed by KT for this application is This implies that a total of 100,000 homes is currently served by the FTTP system. 4. Research and Development Activities Since the announcement of a joint development project by KT, a commercial FTTH system and optical modules have been developed mainly by LG/Nortel and Novera Optics Korea, respectively. At the same time many groups, including KAIST, Samsung, and ETRI, have pursued WDM-PON research, including development of related devices.

9 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 459 Fig. 7. Configuration of the installed FTTP system. 4.A. System-Related Activities Samsung published research results for WDM-PON based on a wavelength-locked F P LD [11]. They reported on a 32-channel system at a data rate of 125 Mbits/s per channel with 100 GHz channel spacing. They used F P LDs developed by themselves. In an effort to increase the capacity of the WDM-PON, a WDM-PON at 50 GHz channel spacing was demonstrated by KAIST [12]. The number of subscribers in a single PON can be further increased by using hybrid approaches, i.e., a single wavelength can be shared by employing time-division multiple access (TDMA) [13] or subcarrier multiple access (SCMA) [14]. A hybrid WDM TDM-PON that can serve 128 subscribers at data rates of 1.25 Gbits/s downstream and 622 Mbits/s upstream was demonstrated by cascading 1 16 AWG and 1 8 splitters [13]. Each of sixteen 100 GHz spaced WDM channels was shared by eight subscribers in the time domain. However, this requires a media access controller and ranging, as in a conventional TDM-PON. Another approach was proposed to maintain dedicated point-to-point connectivity. To do this, a wavelength was shared by using SCMA in the rf domain [14]. The optical beat interference noise among multiple subscribers in a WDM channel was mitigated via forward error correction (FEC). When the channel spacing is 100 GHz, the total users in a single PON can be increased up to 192 with broadband BLSs. Color-free operation of ONTs was reported in both cases. To simplify the network configuration, downstream data can be remodulated at the ONT [15]. The remodulated signal can then be used as an upstream signal. A reflective semiconductor optical amplifier (RSOA) operated in gain saturation regime was proposed [16] instead of F P LD [17]. The RSOA employed in the ONTs was designed to have a low saturation input power at a high saturation gain. ETRI demonstrated bidirectional transmission of a 1.25 Gbits/ s upstream over 20 km of fiber with 2.5 Gbits/ s downstream data. However, the upstream transmission performance is sensitive not only to the injection power into the RSOA but also to the extinction ratio (ER) of the downstream optical signal. In addition, interference between the backscattered downstream signal and upstream signal degrades transmission performance seriously, since the wavelength of the upstream signal is exactly the same as that of the downstream signal. The ER of the injected downstream signal can be reduced by detuning the downstream wavelength from the transmission peak of the filter [18]. The penalty due to backscattering induced interference was suppressed by changing the upstream frequency with a subcarrier modulation method [19]. The proposed hybrid WDM SCM-PON is shown in Fig. 8. A directly modulated distributed feedback (DFB) LD with 622 Mbits/s data was remodulated with 100 Mbits/s upstream data superimposed in a 900 MHz subcarrier. The RSOA was used as a modulator and amplifier for the upstream. In an effort to consolidate a metronetwork and an access network for simplification of the overall network and reduction of CAPEX and OPEX, a long-reach DWDM-PON was demonstrated [20]. A channel spacing of 50 GHz was used to accommodate more

10 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 460 Fig. 8. Architecture of hybrid WDM and/or SCM-PON. users in a single PON. The lasing wavelength of the F P LD was matched to the injection wavelength of the spectrum-sliced BLS in order to have a higher link budget and to reduce the required injection power. While transmitting 100 Base-x Ethernet packets data rate=125 Mbits/s as the upstream and the downstream data, 70 km transmission without an optical amplifier was demonstrated [20]. It may be noted that the bit rate transparency of the WDM-PON can be used to support lifeline communication for an emergency voice communication channel. The transceiver can be operated at a voice bit rate to reduce the power consumption. A conventional coin-type battery can be used for the lifeline communication [21]. Since a WDM-PON uses a wavelength selective device in a transmission path, e.g., AWG, it is not well suited for the delivery of broadcasting services. However, there have been some efforts to solve this problem. The delivery of broadcast video signals by using the cyclic property of the AWG was demonstrated [22]. The downstream data were transmitted with multichannel DFB LDs at the 1550 nm band, while the upstream data were transmitted with LEDs at the 1310 nm band. A DFB LD operating at 1530 nm was used for broadcasting the signals of more than 70 digital video channels. To use a single DFB LD for the broadcast signal, an optical power splitter and an N N cyclic AWG were used. In a different approach, a subcarrier multiplexed quadrative amplitude modulation (QAM) data and baseband digital signal were combined to generate a downstream signal for the subscriber. A DFB LD was then used for transmission of the downstream signal. At the ONT, a baseband receiver and an analog receiver were used, and the remodulation scheme with a RSOA was used for the upstream data transmission [23]. A low-noise BLS can be used to generate a multiple wavelength broadcast signal with an external modulator. The periodic property of the AWG can then be used to transmit the broadcast signal at an unused band [24]. For IP video services that require large bandwidth and high QoS, service availability is one of the critical concerns for the service provider. To increase service availability, protection from fiber faults and a fault localization method to reduce the recovery time are necessary. The wavelength-locked F P LD has been used as a tunable optical time-domain reflectometry (OTDR) with a tunable filter instead of a tunable laser [25]. A low power BLS of 3 dbm (total power) and a thermally tunable filter can be used to realize a low-cost OTDR [26]. Another fault-localization scheme entailing monitoring of the status of the upstream signals was proposed [27]. When a failure is detected at an upstream channel, the corresponding downstream light source is switched to transmit OTDR pulses instead of data. A smooth evolution scheme from the installed TDM-PON to WDM-PON was also proposed and demonstrated [28]. The TDM-PON can be broadband PON (B-PON), E-PON, or gigabit PON (G-PON). The basic assumption for these TDM-PONs is that the upstream and the downstream wavelength bands follow the standard wavelength bands defined by ITU-T. The upstream and the downstream wavelength bands are nm and nm, respectively. The WDM-PON and existing TDM- PON share a feeder fiber by adding a three-port wavelength combiner splitter (WC)

11 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 461 Fig. 9. (a) Structure of the polarization-independent high-power SLD, and (b) its cross section. The cavity length is 1500 m, and W 1 and W 2 are 1 and 7.5 m, respectively. at both the OLT and the RN. The WDM-PON signal is combined with the TDM-PON signal by the WC at the OLT. They are then separated by a WC located at the RN before splitting. The optical power splitter is connected after the WC. Since the WDM signal does not go through the TDM-PON optical network unit (ONU) and/or optical network terminations (ONTs), and vice versa, it is possible to design ONU and/or ONTs that are independent of each other. Recently, KT formed a consortium to develop a WDM-PON with a capacity of more than 20 Gbits/ s (1.25 Gbits/ s times 16). Three different technologies, a wavelengthlocked F P LD, remodulation of downstream data, and a coherent seeding method, were investigated by this consortium. The first scheme uses F P LDs at both OLT and ONT with BLSs, as described in Sections 2 and 3. The last two schemes use RSOA at the ONT. The remodulation scheme uses DFB LDs at the OLT 16. The coherent seeding scheme uses RSOA or DFB LD for downstream data. However, an extra array of DFB LDs is required for seed light [29]. 4.B. Device-Related Activities As explained in Subsection 2.B, the BLS is one of the key elements for a wavelengthlocked F P LD. A polarization-independent high-power BLS based on a semiconductor was demonstrated with an output power of 150 mw and a spectral bandwidth of 40 nm [30]. The structure and the cross section of the SLD are shown in Fig. 9. It consists of a 30 m straight stripe section, a 200 m bent waveguide, and a 1270 m angle-flared gain section. Polarization insensitivity and high power were achieved by incorporating a 0.16% tensile-strained active layer and an angle-flared waveguide structure. As a different approach, a new BLS having lower noise compared with ASE was proposed [31]. It was realized by using two mutually injected F P LDs. A low noise of 135 db/ Hz was reported. Color-free operation of DWDM-PON by injecting the low-noise BLS instead of an EDFA-based BLS was demonstrated [32]. To improve the performance of the wavelength-locked F P LD, two kinds of F P LDs having modified structure were proposed [33,34]. One is an F P LD with two- Fig. 10. (a) Schematic epistructure of the WBG-LD, and (b) its gain curves.

12 Vol. 6, No. 5 / May 2007 / JOURNAL OF OPTICAL NETWORKING 462 Fig. 11. (a) Structure of the wavelength tunable ECL, and (b) cross-sectional view along line A. Fig. 12. Superimposed output lasing spectra of the ECL by thermally tuning the WBG. segmented contacts along the cavity for inhomogeneous current injection [33]. By controlling the injected currents into the double contact F P LD, a lasing envelope is fixed regardless of temperature variation. It can then be used as a temperatureindependent WDM source. The other is an uncooled C-band wideband gain laser diode (WBG-LD) [34]. The epistructure and gain curves of the WBG-LD are shown in Fig. 10. The asymmetric multiple quantum well (MQW) structure and low antireflection coating widen the gain spectrum. A tunable light source was also demonstrated as an independently self-lasing light source for the WDM-PON [35]. The structure of the wavelength-tunable external cavity laser (ECL) is shown in Fig. 11. The ECL was constructed by placing a wavelength Bragg grating section in front of the LD on a planar light wave circuit. Heating of the wavelength Bragg grating tuned lasing coarsely. A phase-control heater section was inserted between the LD and the WBG for fine wavelength tuning and stable singlemode lasing. As shown in Fig. 12, the lasing wavelength was tuned beyond 26 nm. 5. Summary We reviewed WDM-PON activities in Korea including research and development. The WDM-PON commercialized in Korea targeted FTTH for future video services with HD quality. The target was to deliver 100 Mbits/ s dedicated symmetric bandwidth to each home, since a small statistical gain for video-centric services was anticipated. A 32-channel WDM-PON with 100 GHz spacing was realized based on wavelengthlocked F P LDs. It was qualified by KT and deployed in KT s networks. To cope with areas uncovered by VDSL, KT also deployed a 16-channel WDM-PON with 200 GHz channel spacing in a FTTP configuration. Many studies have been undertaken in order to enhance the performance of the WDM-PON. A high-speed data rate per channel, longer reach, narrow channel spacing, and a high split ratio with the help of SCMA or TDMA have been demonstrated. A protection method for feeder fiber faults and outside plant (OSP) fault monitoring

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