IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 11, JUNE 1, 2008
915
A Remotely Reconfigurable Remote Node for Next-Generation Access Networks Jong Hoon Lee, Ki-Man Choi, and Chang-Hee Lee, Senior Member, IEEE
Abstract—A remotely reconfigurable remote node (RN) for nextgeneration (NG) access networks is proposed and demonstrated. The RN is remotely reconfigured by instantaneous optical powering at the central office through feeder fiber and maintained in a passive state by employing optical latching switches (OLSs). The feasibility of the proposed RN is demonstrated by investigating the operating conditions of optical powering for the reconfiguration and its nonlinear effect both for time-division-multiplexing passive optical network (TDM-PON) as legacy services and wavelengthdivision-multiplexing passive optical network (WDM-PON) as NG services. The existence of OLSs for reconfigurability and crosstalk effect between TDM-PON and WDM-PON are negligible on transmission performance. Index Terms—New remote node (RN) configuration, next-generation (NG) access, wavelength-division-multiplexing passive optical network (WDM-PON), wavelength-locked Fabry–Pérot laser diode (F-P LD).
I. INTRODUCTION
R
ECENTLY, next-generation (NG) passive optical networks (PONs) have been actively discussed and some evolution methods maintaining both the wavelength plan and fiber infrastructure of the legacy PON have been proposed [1], [2]. These can be achieved by allocating specific wavelength band to specific services, i.e., wavelength band combiner/splitters (WCs) are installed at the central office (CO) and the remote node (RN) [1]. However, a field reconfiguration by craft-man at the RN increases operation cost and provisioning time and decreases the reliability, since the RN is usually located at the harsh environments. In this letter, we propose and demonstrate a new remotely reconfigurable RN to provide an efficient evolution path to NG services. It can reconfigure optical path remotely and maintain the RN in a passive state. The feasibility of the proposed RN is demonstrated by commercial products of a photovoltaic converter and optical latching switches (OLSs). II. OPERATION PRINCIPLE The network architecture with the proposed RN configuration for the evolution to NG access networks is shown in Fig. 1. Both a legacy time-division-multiplexing (TDM)-PON Manuscript received October 21, 2007; revised February 18, 2008. This work was supported by the Korea Ministry of Science and Technology under the National Research Laboratory and Brain Korea 21 Project. The authors are with the Department of Electrical Engineering and Computer Science, Korea Advanced Institute of Science and Technology, Daejeon 305701, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2008.921848
Fig. 1. Network architecture with a new remotely reconfigurable RN for the evolution to NG access networks.
and an NG-PON are accommodated in a single fiber infrastructure by using WCs at the CO and the RN [1]. Here we assume a wavelength-division-multiplexing (WDM)-PON as the NG-PON. The RN consists of branching devices: an optical power splitter (OPS) and an arrayed waveguide grating (AWG). It also has OLSs, a photovoltaic converter, and a control unit. A WC2 at the RN combines inputs of the OPS and the AWG to provide connection to the feeder fiber. Each output of the OPS and the AWG is combined with the corresponding distribution fiber through an OLS. For legacy services, the OLS is in a bar state to connect the OPS output to a legacy optical network termination (ONT) through the distribution fiber. The bar state is changed to the cross state to connect the AWG output to the legacy subscriber when legacy TDM-PON services are upgraded to WDM-PON services. The control unit selects an OLS to be switched to reconfigure the optical path of each legacy subscriber who wants to upgrade to the WDM-PON services. In order to provide electrical power for the reconfiguration of the RN, optical power from a high power laser is supplied at the CO through the feeder fiber and converted into electrical power by a photovoltaic converter at the RN. It should be noted that optical powering is needed only at the moment of reconfiguration, since the OLS maintains the changed state without energy. In this way, we can maintain the RN in a passive state except at the moment of switching. Therefore, we do not need to send truck roll for reconfiguration of the RN and keep the merits of PON infrastructure. It may be noted that a similar idea was proposed to realize a variable optical splitter [3]. III. EXPERIMENTAL RESULTS The experimental setup to demonstrate the feasibility of the proposed RN is shown in Fig. 2. A TDM-PON with a split ratio of 32 is assumed to be a legacy PON. For upstream and downstream signals, 1.25-Gb/s directly modulated DFBs
1041-1135/$25.00 © 2008 IEEE
916
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 20, NO. 11, JUNE 1, 2008
Fig. 4. Measured switching times of the OLSs as a function of optical input power at the photovoltaic converter.
Fig. 2. Experimental setup to demonstrate an evolution from a TDM-PON to a WDM-PON with the proposed reconfigurable RN.
Fig. 3. Measured switching characteristic of an OLS. (a) Electric gating pulse for generating an optical pulse at CO. (b) Electrical output of the photovoltaic converter. (c) Measured optical power for the cross state of OLS.
at 1310 and 1490 nm were used, respectively. For the NG PON, we used a 64-channel WDM-PON based on wavelength locked Fabry–Pérot (F-P) laser diodes (LDs) [4], [5]. Here, we assigned the first 32 channels for upgrading legacy subscribers and the others for new subscribers. A Raman fiber laser (SDL-RL30, SDL) with 1455-nm center wavelength was used at the CO to supply optical power to change the state of OLSs at the RN. Then, 1450-nm coarse WDM filters with an extended reflection band were used at the CO and the RN. The total insertion loss of the optical powering path from the CO to the RN was about 8 dB. The RN configuration was a simple case of Fig. 1. Here we have only two OLSs that were used for ONT 2 and ONT 9. The OLS state was preselected as a bar state to connect the OPS output to each TDM-PON ONT. The output of the photovoltaic converter drives the OLS, when an optical power is supplied from the CO. A photovoltaic converter (PPC-9LW, JDSU) has 25% conversion efficiency at 1480 nm. The OLS realized by a silicon MEMS technology has a micromirror with a bistable suspension characteristic which can be moved in and out to reconfigure the optical path and keeps the last selected state in power OFF. In order to investigate the switching characteristics of the OLS, we measured the switching time as a function of the input power to the photovoltaic converter. A typical switching characteristic of the OLS at 20-dBm input optical power is shown in Fig. 3. An optical pulse was generated with a 50-ms width at the CO by using an optical switch and a function generator [curve (a)]. An electrical output of the photovoltaic converter was directly applied to the OLS [curve (b)]. The measured optical power for the cross state of OLS was shown in curve (c).
The turn ON delay of 7.5 ms between curves (a) and (b) was induced by the optical switch used at the CO. The long tail of curve (b) can be explained as the discharge time of the capacitive load of the OLS. The cross state was maintained after the optical power was turned OFF, since the OLS has latching function. Measured switching times are shown in Fig. 4. The required minimum optical power for switching of one OLS was 10 dBm with the switching time of 42 ms. The switching time decreases as we increase the optical input power. The minimum switching time at a high power was measured as 5 ms. To drive two OLSs simultaneously, the minimum required power was 13.4 dBm with a switching time of 39 ms. The optical power difference between the case of driving one OLS and two OLSs was about 3.4 dB at the same switching time. The required minimum optical power at the CO was 18 dBm, since the insertion loss of the optical powering path was about 8 dB. The evolution from the TDM-PON to the WDM-PON with the proposed RN is demonstrated at ONT 2 and ONT 9. The WDM-PON OLT consists of transceiver modules with -band WDM, an AWG 1 with 50-GHz channel spacing, and -band [1], [5]. two broadband light sources (BLSs) for the The injection powers into the F-P LD were 21.5 dBm/0.2 nm and 12 dBm/0.2 nm at the -band and -band, respectively. To investigate the feasibility of evolution, we measured the transmission performance before and after reconfiguration. When the OLSs were in the bar state, the TDM signals (legacy OLT and ONT 1–32) and the 64 WDM channels showed error-free transmission. The legacy ONTs were replaced by WDM-PON ONTs at the subscriber side (ONT 2 and ONT 9) for NG services. In addition, the OLSs at the RN were reconfigured by transmitting an optical pulse at the CO. The transmission performances were measured using the packet-error rate (PER) because the implemented WDM-PON has an optical–electrical interface of 100-Base Ethernet (data Mb/s). A variable optical attenuator (VOA) was rate inserted between WC 2 and AWG 2 at the RN. Here, a PER approximately corresponds to a bit-error rate (BER) of . For comparison, we also measured PERs for ONT 2 of and ONT 9 by removing the OLSs (AWG output was directly connected to the NG ONT 2 and NG ONT 9). Under these conditions, no power penalty was observed, as shown in Fig. 5. It should be noted that the upstream PER is more severely affected by the VOA attenuation compared to the downstream PER, since both the -band BLS and wavelength locked signals experience the VOA attenuation. Thus, we focused our experiment on the upstream transmission performance. The
LEE et al.: REMOTELY RECONFIGURABLE RN FOR NG ACCESS NETWORKS
Fig. 5. Measured upstream PERs of channels 2 and 9 (a) with and without OLSs; upstream and downstream PERs of channel 33 (b) with and without TDM signals.
BER according to peak power of periodic opFig. 6. Power penalty at 10 tical powering with Raman fiber laser output.
upstream and downstream PERs of channel 33 with and without TDM signals were also shown as an example to demonstrate that no penalty was induced by crosstalk between TDM signals and WDM signals. We also investigated the nonlinearity induced penalty both for the TDM-PON and WDM-PON, since the optical powering (Raman fiber laser) is high enough to induce optical nonlinearity. To this end, we applied a periodic optical pulse at a 5-Hz repetition rate with 20% duty. Fig. 6 shows the highest power penalty of the TDM-PON and WDM-PON at a BER of (PER of for WDM-PON) according to the peak power of the Raman fiber laser. For the WDM-PON, we used eight channels (channel 25–channel 32) for this experiment. The TDM-PON signals (upstream and downstream) show a small penalty up to 26-dBm coupled power. The penalty then increases gradually as we increase the coupled power. For the WDM-PON signals, the worst penalty is less than 0.6 dB at 25 dBm. However, the penalty increased sharply, when the optical power increased further. When the optical power is above 26 dBm, it was difficult to measure the PER at some channels due to a severe degradation of the performance. This may be caused by Raman interaction among the high coupled power, the -band BLS, and the upstream signal. The downstream signals also have Raman interaction in the feeder fiber and contribute the penalty. Note that there was no stimulated Brillioun scattering (SBS) effect, since the linewidth of the Raman fiber laser was sufficiently wide. In addition, the measured isolation from the high power laser to the signal paths was more than 45 dB. It does not affect the transmission performance. IV. DISCUSSION AND CONCLUSION In order to check the bit-rate independency of the proposed architecture, we used directly modulated DFBs at 1.25 Gb/s
917
Fig. 7. Measured BER curves of 1547.68-nm WDM upstream (channel 46) and 1588.54-nm WDM downstream (channel 26) with DFB lasers.
for the WDM-PON. We only measured the BERs of the upstream at channel 46 (1547.68 nm) and of the downstream at channel 26 (1588.54 nm) due to availability of the DFB lasers. The measured BERs of the upstream and downstream data did not show any performance degradation due to the coexistence of low speed WDM-PON signals and TDM-PON signals, as shown in Fig. 7. Although the feasibility of the proposed RN was demonstrated without a control unit, selection of a specific OLS can be achieved in accordance with pre-established control functions. The control information can be encoded into a pulse train at the optical powering wavelength. Then, it is decoded with a control circuit at the RN to select a specific OLS. A new remotely reconfigurable RN for NG services was proposed and demonstrated. Instantaneous optical powering to the RN with a photovoltaic converter and OLSs can reconfigure the optical path remotely and maintain the RN in a passive state. The required minimum optical power for the reconfiguration was 18 dBm which can be reduced further by using an OLS that has low switching energy [6], while the maximum coupled power limited by optical nonlinearity was measured as 25 dBm. We believe that this can be easily achieved by using a high-power laser and a sufficiently wide dynamic range is provided. Thus, the proposed architecture can provide an efficient and cost-effective evolution path for NG services, while maintaining the RN in a passive state.
REFERENCES [1] 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. 19, no. 9, pp. 647–649, May 1, 2007. [2] R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for future optical access networks,” IEEE Commun. Mag., vol. 44, no. 10, pp. 50–56, Oct. 2006. [3] H. Ramanitra, P. Chanclou, Z. Belfqih, M. Moignard, H. L. Bras, and D. Schumacher, “Scalable and multi-service passive optical access infrastructure using variable optical splitters,” in OFC 2006, Anaheim, CA, Mar. 2006, Paper OFE2. [4] H. D. Kim, S.-G. Kang, and C.-H. Lee, “A low-cost WDM source with an ASE injected Fabry–Pérot semiconductor laser,” IEEE Photon. Technol. Lett., vol. 12, no. 8, pp. 1067–1069, Aug. 2000. [5] S.-M. Lee, M.-H. Kim, and C.-H. Lee, “Demonstration of a bidirectional 80-km-reach DWDM-PON with 8-Gb/s capacity,” IEEE Photon. Technol. Lett., vol. 19, no. 6, pp. 405–407, Mar. 15, 2007. [6] R. A. M. Receveur, C. R. Marxer, R. Woering, V. C. M. H. Larik, and N.-F. d. Rooij, “Laterally moving bistable MEMS DC switch for biomedical applications,” J. Microelectromech. Syst., vol. 14, no. 5, pp. 1089–1098, Oct. 2005.
