Evolution from TDM-PONs to Next-Generation PONs

Evolution from TDM-PONs to Next-Generation PONs Ki-Man Choi, Jong-Hoon Lee, and Chang-Hee Lee Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, 7- Guseong-dong Yuseong-gu, Daejeon, 05-70, Korea chl@ee.kaist.ac.kr Abstract: An easy and efficient evolution method from a time-division multiplexing passive optical networks (TDM-PONs) to next-generation PONs is proposed and demonstrated. A single-type wavelength band combiner/splitter (WC) enables a simple and efficient evolution of TDM-PON maintaining the current PON infrastructure and wavelength plan. The feasibility of the proposed evolution architecture is shown by investigating the crosstalk effect between the legacy PON and a wavelength division multiplexing (WDM)-PON based on wavelength-locked Fabry-Perot laser diode (F-P LD) as a next-generation PON. Then, the crosstalk is negligibly small. I. Introduction ecently, network service providers have begun to deploy time division multiplexing passive optical networks (TDM-PONs) such as Broadband PON (B-PON), Ethernet PON (E-PON) or Gigabit PON (G-PON) []. Among them, the most advanced PON is G-PON that provides.5 Gb/s downstream bandwidth and.5 Gb/s upstream bandwidth. Since these bandwidths are shared by or 64 subscribers, the guaranteed bandwidth and quality of service (QoS) provided by these PONs might not be enough to satisfy the increasing bandwidth requirements of future video-centric services. For broadcast-video services in TDM-PON, international telecommunication union (ITU) specified an additional downstream band between 550 nm and 560 nm for video overlay []. However, TDM-PON with a video overlay is suitable as long as the broadcasting video is a dominant service. As video services are personalized (e.g., IP TV) and evolved to high definition quality, a high dedicated bandwidth is required to each customer. It is expected that the future access network needs to support more than 00 Mb/s dedicated bandwidth to each home []. Thus, the TDM-PON will eventually need to be upgraded to higher speed PON. ecently, the next-generation PON has been actively discussed for future access networks. What is needed in upgrading access network is a simple and efficient evolution path from TDM-PONs to next-generation PONs. Then, there should no change of outside plant (OSP) and current TDM-PON wavelength plan. In addition, evolution method needs to guarantee the user by user evolution and higher splitting ratio than the legacy PON. In this paper, an easy and efficient evolution method from TDM-PONs to next-generation PONs is proposed and demonstrated in section II. To add next-generation PON while maintaining the existing

fiber, optical power splitter, and wavelength plan of the current TDM-PON, a single-type -port wavelength band combiner/splitters (WCs) [4] are inserted at the central office (CO) and the remote node (N) in advance. With this configuration, there is no limitation in types of optical branching device (e.g., power splitter or arrayed waveguide grating (AWG)). This feature provides flexibility for next-generation access (NGA) architecture. Thus, a wavelength division multiplexing (WDM)-PON based on wavelengthlocked Fabry-Perot laser diode (F-P LD) [5] was assumed as a next-generation PON in this paper. The feasibility of the proposed evolution architecture is shown by investigating the crosstalk effect between the TDM-PON and the WDM-PON. In addition, it may be noted that video overlay can be also accommodated by the proposed scheme with a four-port WC and a slightly modified -port WC. Another evolution architecture from TDM-PON with a video overlay to next-generation PON is also demonstrated in section III. Finally, we will discuss several issues on the proposed evolution methods in section IV. II. Evolution architecture from the TDM-PON to the next-generation PON The proposed evolution architecture from the TDM-PON to next-generation PON is shown in Fig.. In this evolution architecture, we assumed TDM-PON can be either the B-PON, E-PON, or G- PON with a splitting ratio of and a transmission length of 0 km. For an optical source at the TDM- PON optical line termination (OLT), a DFB laser at 490 nm can be used. Either a DFB laser or an F-P LD can be used for an optical source at the optical network termination (ONT). The wavelength range of these lasers is specified by full-service access network (FSAN) and IEEE 80.. The downstream and upstream wavelength range is 480 nm ~ 500 nm and 60 nm ~ 60 nm, respectively. Then, the nextgeneration PON can then be added to provide future video-centric services at different wavelength bands. To evolve current TDM-PON maintaining TDM-PON up/downstream wavelength plan, a singletype -port WC was used to add/extract next-generation PON stream as shown in Fig.. The -port WC is a cascade of two edge-filters and a coarse WDM (CWDM)-filter. The edge-filter separates the long wavelength band (> 45 nm, eflect()) and the short wavelength band (< 60 nm, Pass (P)). An absorption peak at approximately 90 nm exists between those two bands of the deployed fiber. Therefore, this can be assigned as a guard band. The CWDM-filter selects TDM-PON downstream wavelength band (480 nm ~ 500 nm, eflect()) within a wider wavelength band. These types of filters are commercially available. The -port WC maintains the downstream and upstream path of the TDM- PON as and in Fig., respectively. In addition, it adds/extracts next-generation PON signals bidirectionally along the path. Following this, wavelength bands for the next-generation PON can be assigned in the range of 45 nm ~ 470 nm, or with a wavelength longer than 50 nm. Here, a 0 nm guard band is assumed for the CWDM-filter. The insertion loss of the -port WC is measured as nearly db in the 490 nm path and 0.75 db in the 0 nm path including the connectors. In addition, the insertion loss of the next-generation PON signal path is approximately 0.75 db including the connector loss.

To evolve the current TDM-PON, the next-generation PON can be either a TDM-PON supporting higher bandwidth than the current PON or a WDM-PON. Thus, we assumed the WDM-PON is the next-generation PON in this experiment. It may be noted that the proposed evolution method is independent of the architecture and optical source for WDM-PON. Thus, the feasibility of this evolution from a TDM-PON to the WDM-PON was verified using the available WDM-PON sources of the wavelength-locked F-P LD. In implementing the evolution architecture, as shown in Fig., each -port WC must be placed in advance at the CO and the N in order to provide in-service evolution without any disruption of the TDM-PON services. A WDM-PON OLT is added through port # of WC at CO for the next-generation PON OLT. An AWG for the WDM-PON is installed at the N (port # of WC ) maintaining an optical power splitter for the TDM-PON. Here, the number of WDM-PON users is independent of that of the TDM-PON. The WDM-PON signal can be added on (extracted from) the existing feeder fiber and extracted (added) at N by -port WC and WC, respectively. Thus, advanced services can be handled by the WDM-PON while the current services are handled by the TDM-PON on the same feeder fiber. A new distribution/drop fiber and a next-generation ONT is needed for a new user such as nextgeneration ONT. When an existing TDM-PON user wants to change to next-generation services, the connection of the corresponding distribution fiber need to be changed from the splitter to the AWG. In addition, a new next-generation ONT is required at the customers premises (ONT ). If a user requires current TDM-PON service and a next-generation service (ONT 9), both services can be provided through previously deployed distribution fiber by adding additional -port WCs at the N and the ONT. To demonstrate the evolution from the TDM-PON to the WDM-PON, two directly modulated DFB lasers at 0 nm were used for the ONT and the ONT 9. For the downstream signal, a directly modulated 490 nm DFB laser was used. The three DFBs were directily modultated with.5 Gb/s NZ data and output power of them was nearly dbm. For the next-generation PON, a 6-channel WDM- PON based on the wavelength-locked F-P LDs [6] was used. The next-generation OLT for the WDM- PON consisted of transmitters, receivers, an AWG, two broadband light sources (BLSs) for the C/Lband, and C/L-band WDM. The channel spacing for the AWG was 50 GHz, and the BLS was an erbiumdoped fiber amplifier (EDFA)-based amplified spontaneous emission (ASE). The F-P LDs inside the transmitters were TO-can packaged and the mode spacing was 0.6 nm. C-band (540.94 nm ~ 546.89 nm) was used for the upstream data and L-band (589.76 nm ~ 595.89 nm) was used for the downstream data. The injection powers of the C/L-band BLS were -8 dbm/0. nm and - dbm/0. nm, respectively. The measured optical spectra of the downstream and the upstream signal are shown in Fig. (a) and (b), respectively. The O/E interface of the WDM-PON accommodates 00-Base Ethernet packets with a data rate of 5 Mb/s. Thus, the performance was measured with the packet error rate (PE) instead of the biterror rate (BE). A variable optical attenuator (VOA) was inserted between -port WC and AWG at the N to measure the PEs of the WDM-PON. The measured PEs for 6-channel upstream data are

shown in Fig. 4. Here, a PE of 0-6 corresponds to a BE of 0-0, approximately. To investigate the crosstalk effect in the proposed evolution architecture, the BEs of both the upstream (0 nm) and the downstream (490 nm) TDM data were measured with and without WDM signals. The BEs for the 490 nm downstream data were measured only at ONT and ONT 9. The power penalty induced by the WDM-PON is negligible, as shown in Fig. 5(a). The BE curves of the 0 nm upstream data from only ONT and ONT 9 were also measured. The upstream also did not show crosstalk effect due to the coexistence of the TDM-PON and the WDM-PON, as shown in Fig. 5(b). In addition, the PE degradation for the upstream and downstream data from Tx 9 of the WDM-PON were investigated with and without TDM signals, as shown in Fig. 6. It was shown that the TDM-PON does not affect the PEs of the WDM-PON. It may be noted that the upstream signal of 0 nm is not in burst mode for this experiment. However, no performance degradation was estimated with a burst mode receiver, as the isolation of the WDM upstream signal at the TDM-PON OLT is measured with more than approximately 70 db, with 40 db from a BiDi and 0 db from the -port WC. The next generation access network can use both wavelength bands in a range of 45 nm ~ 470 nm and bands above 50 nm. Thus, the proposed evolution architecture can accommodate E-, S-, C-, and L-band for a next-generation PON. If a low water-peak fiber is used, it is possible to assign a wider wavelength range to the next-generation PON. III. Evolution architecture from the TDM-PON with a video overlay to the nextgeneration PON To accommodate video overlay services at 550 nm~ 560 nm, a CWDM-filter with two reflection bands (480 nm ~ 500 nm and 550 nm ~ 560 nm) may be needed inside the -port WC. However, we modified the -port WC with adding two additional CWDM-filters selecting wavelength band of 550 nm ~ 560 nm (CWDM-filter in Fig. 7) to provide video overlay path in -port WC. The block diagram of the modified -port WC adding and extracting different wavelength band signals (TDM-PON up/down stream, video signal, and WDM-PON stream) is shown in Fig. 7(a). The modified -port WC maintains the TDM-PON downstream and upstream path like and in Fig. 7(a). In addition, WDM-PON signal is added/extracted along the path. It adds/extracts video signal along the path 4 from port # of the modified -port WC. To evolve the TDM-PON with a video overlay, 4-port WC is also used to maintain TDM-PON up/down stream, video signal and to add/extract WDM-PON stream. The block diagram of 4-port WC is shown in Fig. 7(b). The 4-port WC is a cascade of two edge-filters, a CWDM-filter, and a CWDMfilter. Then, the video signal path is only different compared with the modified -port WC. 4-port WC adds/extracts video signal along the path 4 from port #4. Following this, wavelength bands for the WDM-PON can be assigned in the range of 45 nm ~ 470 nm, 50 nm ~ 540 nm, or with a wavelength longer than 570 nm. Here, a 0 nm guard band is assumed for the CWDM-filter.

It may be noted that the CWDM-filter for the modified -port WC and 4-port WC is a commercially available CWDM filter that selects 540 nm ~ 560 nm instead of 550 nm ~ 560 nm of the video overlay standard wavelength. The measured insertion loss of the 4-port WC (including the connectors) is nearly db in TDM-PON downstream path and 0.8 db in the TDM-PON upstream path. In addition, the insertion loss of the WDM-PON signal path and video signal path are approximately 0.8 db. For the modified -port WC, the measured insertion loss in the TDM-PON upstream and video signal path is slightly increased to. db and.4 db, respectively. The evolution architecture from the TDM-PON with a video overlay to a WDM-PON is demonstrated as shown in Fig. 8. The experiment condition of TDM-PON is the same as Fig.. For the video overlay, DFB laser operating at 55. nm is used. Video signal consists of 5 channels of 56 QAM (445 MHz ~ 470 MHz) and an electrical noise source (594 MHz~ 80 MHz). A noise source is added to simulate more than 40 digital video channels. The optical modulation index (OMI) per a video channel is set to be 0.. The video signal is amplified by an EDFA to have output power of 8 dbm. There is no measurable effect of stimulated Brillouin scattering (SBS) after direct modulation of DFB. Here, the modulated video signal is combined to a single mode fiber (SMF) through a port #4 of a 4-port WC (WC at CO). A WDM-PON OLT can be added through port # of the WC at the CO for the next-generation PON. Here, the TDM-PON data and video services are provided to all TDM-PON users (ONT ~ ONT ) through port # of the modified -port WC (WC ) and the optical splitter at the N. At subscriber side, video services can be selected by CWDM-filter at 550 nm like as ONT 8. The added WDM-PON signal is extracted at the N through port # of the WC. The number of WDM-PON users is independent on that of the TDM-PON. When TDM-PON user wants to change to next-generation services, the connection of the corresponding distribution fiber need to be changed from the splitter to the AWG. In addition, a new next-generation ONT is required at the customers premise (ONT 4). The TDM-PON subscribers with and without video services can also use the WDM-PON services like as ONT 6 and ONT, respectively. Then, additional WCs (WC or WC between the splitter and ONTs) are needed at the N and in front of the ONTs. Thus, user by user evolution from a TDM-PON with a video overlay to a WDM-PON is feasible with maintaining the existing fiber, power splitter, and wavelength plan of TDM-PON including video stream. In addition, we believe the proposed evolution method is independent on system architecture, bit-rate, and optical source for WDM-PON. The wavelength of the WDM-PON based on the wavelength-locked F-P LDs was assigned at 59.96 nm ~55.8 nm (6 channel of C-band) for upstream and 578.4 nm ~ 584.47 nm (6 channel of L-band) for downstream, respectively. Then, the measured optical spectra of the downstream and the upstream from the experimental setup are shown in Fig. 9(a) and (b), respectively. To investigate the feasibility of evolution method from the TDM-PON with a video overlay to the WDM-PON, crosstalk effect was investigated. First of all, video signal performance of 5-channel 56 QAM is measured with the modulation error ratio (ME) at ONT 6 as shown in Fig. 0. The ME of 4

db for all video channels can be achieved when the TDM-PON data and the WDM-PON data are transmitted simultaneously. It may be noted that a ME of 4.6 db corresponds to a BE of 0-9. Then, the constellation diagram of one video channel is clearly shown in inset of Fig. 0. When the video overlay and WDM-PON services are provided to ONT 6, the BE performance of the TDM-PON up/down stream do not show any power penalty due to the coexistence of video overlay and the WDM- PON signals as shown in Fig. (a) and (b), respectively. The performance for the 6-upstream channel of the WDM-PON is measured with PEs as shown in Fig.. Then, Upstream and downstream WDM signals from Tx 6 do not show crosstalk penalty from video overlay and TDM-PON signal as shown in Fig.. It comes from sufficient isolation among different signals at the modified -port and 4-port WCs. The dominant crosstalk is occurred from the video signal to the TDM-PON 0 nm x and the WDN-PON x at the CO. At this case, isolation is measured more than 70 db for 4-port WC. IV. Discussion and Conclusion To install WCs in advance, link margin is necessary to cover the insertion loss of the WCs. It is expected that the insertion loss of WCs can be reduced further by integrating these WCs as a single device. When the AWG channel spacing is increased to 00 GHz in WDM-PON, it may possible to achieve.5 Gb/s transmission with wavelength-locked F-P LDs for the NGA. This result will be published elsewhere. An easy and efficient evolution method from the TDM-PON to next-generation PON was proposed and demonstrated by using simple and commercially available WCs. In addition, another evolution method from a TDM-PON with a video overlay to next-generation PON was also demonstrated to provide high quality personalized video services. The proposed evolution methods enable user-by-user evolution maintaining the existing TDM-PON infrastructure and wavelength plan. Thus, it is possible to provide a smooth evolution from the current TDM-PON to NGA that requires high dedicated bandwidth per users by maintaining the installed infrastructures. V. eference []. M. Abrams, P. C. Becker, Y. Fujimoto, V. O Byrne, and D. Piehler, FTTP Deployments in the United States and Japan-Equipment Choices and Service Provider Imperatives, J. Lightw. Technol., vol., no., pp. 6-46, Jan. 005. []. ITU-T, Broadband optical access systems based on passive optical networks, ecommendation G. 984., 00. []. C. Ollivry, Why Fiber? Why Now? Montpellier, France, FTTH Council Europe, Nov. 4, 004. [4]. K.-M. Choi, S.-M. Lee, M.-H. Kim, and C.-H. Lee, An Efficient Evolution Method from TDM- PON to Next-Generation PON," IEEE Photon. Technol. Lett., vol. 9, no. 9, pp. 647-649, May. 006. [5]. H. D. Kim, S.-G. Kang, and C.-H. Lee, A low-cost WDM source with an ASE injected Fabry-Perot semiconductor laser, IEEE Photon. Technol. Lett., vol., no. 8, pp. 067-069, Aug. 000.

[6]. S.-M. Lee, S.-G. Mun, and C.-H. Lee, Consolidation of a Metro Network into an Access Network based on Long-reach DWDM-PON," Optical Fiber Communication Conf. (OFC 006), Anaheim, CA, Paper NWA, Mar. 006.

CO N ONTs ONT Legacy PON OLT Next-generation PON OLT 0 km WC WC Splitter....... ONT ONT 9 Tx9 x9 Tx x ONT Tx x... Tx6 x6 AWG C/L BLS AWG Tx x Next-generation ONT... Tx n x n Next-generation ONT n C/L WDM Wavelength band combiner/splitter (WC) Fig.. The proposed evolution architecture from the TDM-PON to the next-generation PON. Edge-filter Edge-filter C 60~60 nm C Legacy TDM-PON downstream P 480~500 nm P P Legacy TDM-PON upstream C CWDM-filter 45~470 nm, 50 nm Next-generation PON stream Edge-filter 60 nm : Pass (P), 45 nm : eflect () CWDM-filter 480~500 nm : eflect (), Others : Pass (P) Fig.. The block diagram of a -port wavelength band combiner/splitter (WC) that passes through the TDM-PON stream and adds/extracts next-generation PON signals. Power (dbm) 0-0 -0-0 -40-50 -60 Downstream of TDM-PON C-band BLS (a) Downstream of WDM-PON Power (dbm) 0-0 -0-0 -40-50 -60 Upstream of TDM-PON (b) Upstream of WDM-PON -70-70 -80 480 500 50 540 560 580 600 Wavelength (nm) -80 00 50 400 450 500 550 Wavelength (nm) Fig.. The measured optical spectra of downstream (a) and upstream (b) from the evolution architecture.

Packet Loss Error ate.e-0 0 -.E-0 0-0 -.E-0 0-4.E-04 0-5.E-05 0-6.E-06 0-7.E-07 Ch. Ch. Ch. Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.0 Ch. Ch. Ch. Ch.4 Ch.5 Ch.6-4 -40-9 -8-7 -6-5 -4 - eceived Power (dbm) Fig. 4. The measured upstream PEs in a WDM-PON based on a wavelength-locked F-P LD. TDM downstream to ONT 9 without WDM signals with WDM signals TDM upstream from ONT without WDM signals with WDM signals TDM downstream to ONT without WDM signals with WDM signals (a) TDM upstream from ONT 9 without WDM signals with WDM signals (b) Fig. 5. The measured BE curves of 490 nm TDM downstream (a) and 0 nm TDM upstream (b) with and without WDM signals..e-0 0 -.E-0 0 - WDM upstream without TDM WDM upstream with TDM 0 -.E-0.E-04 0-4 0-5.E-05 0-6.E-06 0-7.E-07 WDM downstream without TDM WDM downstream with TDM -44-4 -40-8 eceived Power ( dbm) Fig. 6. The measured WDM upstream and downstream PEs of Tx 9 with and without TDM signals.

TDM-PON downstream 480 ~ 500 nm Video signal 550 ~ 560 nm Edge-filter C P Edge-filter P C TDM-PON upstream 60 ~ 60 nm WDM-PON stream 45 ~ 470 nm, 50 ~ 540 nm, >570 nm CWDM-filter 4 P C CWDM-filter C P P C CWDM-filter (a) TDM-PON downstream 480 ~ 500 nm Edge-filter C P Edge-filter P C TDM-PON upstream 60 ~ 60 nm WDM-PON stream 45 ~ 470 nm, 50 ~ 540 nm, >570 nm P C CWDM-filter Video signal 550 ~ 560 nm P C CWDM-filter 4 4 (b) 4 z Edge-filter - <60 nm : Pass (P) - >45 nm : eflect () CWDM-filter 480~500 nm : eflect () Others : Pass (P) CWDM-filter 550~560 nm : eflect () Others : Pass (P) Fig. 7. The block diagram of a modified -port wavelength band combiner/splitter (WC) (WC ) (a) and a 4-port WC (WC ) (b) adding and extracting three different wavelength signals (TDM-PON up/down stream, video signal, and WDM-PON stream). CO N ONT Tx 4 TDM- PON OLT Video overlay 0 km 4 WC WC Splitter...... WC ONT 4 550 nm CWDM WC 4 x 4 ONT 8 Analog x ONT 6 Analog x Tx 6 x 6 VOA ONT WDM-PON OLT WC WC Tx x Tx x... AWG C/L BLS AWG Tx x Next-generation ONT... Tx n x n C/L WDM Tx n x n Next-generation ONT n Fig. 8. The experimental setup to demonstrate an evolution method from a TDM-PON with a video overlay to a WDM-PON

0-0 -0-0 -0-0 Power (dbm) -0-40 -50-60 Power (dbm) -40-50 -60-70 -80-70 -90-80 485 505 55 545 565 585 Wavelength (nm) -00 00 50 400 450 500 550 Wavelength (nm) Fig. 9. The measured optical spectra of downstream (a) and upstream (b) from the experimental setup. 7 5 Ch. of 445 MHz Ch. of 45 MHz Ch. of 457 MHz Ch.4 of 46 MHz Ch.5 of 469 MHz 9 7 5 - - - -0-9 -8-7 -6-5 -4 - - - -0 eceived Power ( dbm) Fig. 0. The measured ME of 5-channel 56 QAM for video signals. (a) (b) Fig.. The measured BE curves of 0 nm TDM upstream (a) and 490 nm TDM downstream (b) with and without video and WDM signals.

.E-0 Ch. Packet Error ate.e-0.e-0.e-04.e-05.e-06 Ch. Ch. Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.0 Ch. Ch. Ch. Ch.4 Ch.5.E-07 0 9 8 7 6 5 Additional attenuation w/ 0 km Ch.6 4 Fig.. The measured PEs of upstream in WDM-PON based on the wavelength-locked F-P LDs. Packet Error ate.e-0.e-0.e-0.e-04.e-05.e-06 WDM downstream without video and TDM signal WDM downstream with video and TDM signal WDM upwnstream without video and TDM signal WDM upwnstream with video and TDM signal.e-07 0 9 8 7 6 5 4 Additional attenuation w/ 0 km Fig.. The measured downstream and upstream PEs of Tx 6 with and without video and TDM signal.