A Metro-Access Integrated Network with All-Optical Virtual Private Network Function Using DPSK/ASK Modulation Format

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1 A Metro-Access Integrated Network with All-Optical Virtual Private Network Function Using DPSK/ASK Modulation Format Yue Tian* a, Lufeng Leng b, Yikai Su a a State Key Lab of Advanced Optical Communication Systems and Networks, Department of Electronic Engineering, Shanghai Jiao Tong University, Shanghai , China; b New York City College of Technology, City University of New York, 300 Jay Street, Brooklyn, NY 11201, USA Abstract All-optical virtual private network (VPN), which offers dedicated optical channels to connect users within a VPN group, is considered a promising approach to efficient internetworking with low latency and enhanced security implemented in the physical layer. On the other hand, time-division multiplexed (TDM) / wavelength-division multiplexed (WDM) network architecture based on a feeder-ring with access-tree topology, is considered a pragmatic migration scenario from current TDM-PONs to future WDM-PONs and a potential convergence scheme for access and metropolitan networks, due to its efficiently shared hardware and bandwidth resources. All-optical VPN internetworking in such a metro-access integrated structure is expected to cover a wider service area and therefore is highly desirable. In this paper, we present a TDM/WDM metro-access integrated network supporting all-optical VPN internetworking among ONUs in different sub- PONs based on orthogonal differential-phase-shift keying (DPSK) / amplitude-shift keying (ASK) modulation format. In each ONU, no laser but a single Mach-Zehnder modulator (MZM) is needed for the upstream and VPN signal generation, which is cost-effective. Experiments and simulations are performed to verify its feasibility as a potential solution to the future access service. Keywords: All-optical virtual private network, optical access network, passive optical network (PON), scalability. 1. INTRODUCTION All-optical virtual private network (VPN) is considered a promising approach to high throughput and enhanced security for access users. It has been demonstrated in conventional passive optical networks (PONs) [1, 2] and in two-stage PONs [3]. On the other hand, a time-division multiplexed (TDM)/wavelength- division multiplexed (WDM) network based on a feeder-ring with access-trees is considered a potential convergence scheme for access and metropolitan networks, due to efficiently shared bandwidth and hardware resources [4]. VPN service in such a metro-access integrated structure is expected to cover a wider area and therefore is highly desired. In this paper, we propose and demonstrate a metro-access integrated network enabling all-optical VPN functionality, which consists of a single-fiber ring with tree sub-pons attached to it. The proposed architecture features (1) orthogonal differential-phase-shift keying (DPSK) / amplitude-shift keying (ASK) modulation format for simultaneous downstream and VPN data transport, and bidirectional VPN communication; (2) centralized light sources at the optical line terminal (OLT); (3) a single Mach-Zehnder modulator (MZM) in each optical network unit (ONU) for both upstream [5] and VPN data encoding. Compared to the proposal in [6], this scheme eliminates the light sources and Mux/Demux in the ONU, which simplifies the structure. Furthermore, we investigate the performance of the DPSK multiplexing technique by experiment and its scalability by simulations. 2. PRINCIPLE AND ARCHITECTURE Fig. 1 shows the generic architecture of the proposed TDM/WDM metro-access integrated network. The sub-pons are time-division multiplexed, and in each sub-pon, ONUs are multiplexed in a WDM manner via an N N arrayed waveguide grating (AWG) in the remote node (RN). The downstream DPSK signals and upstream carriers are generated at the OLT, and terminated after traversing the ring once. They are allocated in different wavebands defined by the FSR Network Architectures, Management, and Applications VI, edited by Weisheng Hu, Shao-Kai Liu, Ken-ichi Sato, Lena Wosinska, Proc. of SPIE Vol. 7137, 71370O 2008 SPIE CCC code: X/08/$18 doi: / Proc. of SPIE Vol O-1

2 of the AWGs, as shown in Fig. 1, and are routed to the corresponding ONUs by the AWGs. Note that all the AWGs have the same FSR. Fig. 2~4 show the structure of a sub-pon including an RN and N ONUs with the traffic flows indicated. The ONUs at the same wavelength in different sub-pons can be grouped to form a VPN. on J6L OMU2LE nb2lge Fig. 1. Generic architecture of the metro-access integrated network. Downstream traffic RN To the right RN To N ONUs To N ONUs MZM PD DPSK Rx ONU Downstream Upstream λ Fig. 2. Structure of a sub-pon with downstream traffic indicated. Proc. of SPIE Vol O-2

3 10 pe ELOW UJ( OVfl2 OVfl2 Db2F< BGOLG woqnlsf OL'lC 2fL9UJ WOflI9f Fig. 3. Structure of a sub-pon with upstream traffic indicated. To the left RN Downstream traffic VPN signal RN With VPN signal Upstream carrier From the right RN To N ONUs VPN data MZM Before modulation PD DPSK Rx After modulation ONU Downstream Upstream λ Fig. 4. Structure of a sub-pon with VPN traffic indicated. Proc. of SPIE Vol O-3

4 In an ONU, a part of the downstream DPSK signal is tapped for detection, and the rest is sent to the MZM ready for VPN data encoding before moving on to the next ONU. For upstream signal generation in each ONU, a coherent DPSK signal multiplexing technique is employed. Fig. 5 shows the principle of this technique, where an optical carrier passes two phase modulators consecutively. With half-bit time shift between Data1 and Data2, phase transitions (information) carried by them are multiplexed. As a result, a coherent DPSK signal with twice the bit rate is generated [5]. The technique can be extended to multiplex data streams from M consecutive ONUs with a time shift of bit-duration/m. In the end, the multiplexed upstream DPSK signal is demodulated by a high-speed DPSK receiver at the OLT, and data from each ONU is demultiplexed. Since both the downstream and upstream data pass through the same MZM in an ONU, they should be scheduled in different time slots to avoid interference. Node 1 MZM A Node 2 MZM B Bit error test Vπ Data1 PD Vπ Data2 MZDI (0.5bit delay) Phase modulation of data1 1 bit A 0 0 π π π π 0 0 π π Phase modulation of data2 π π 0 0 π π π π Multiplexed phases of data1 and data2 B π 0 π 00π 00 π π 0π π Demodulated signal after 0.5-bit delayed MZDI C C Data 1 Data 2 1 means π shift in phase Fig. 5. Structure of a sub-pon with VPN traffic indicated. In the time slots of downstream signal, the VPN data in ASK format can be simultaneously encoded on the counterpropagating downstream DPSK signal and upstream carrier (Fig. 4), so that the VPN signal can be transmitted in both directions and received by all the ONUs in the same VPN. The photodetector (PD) in the ONU is set up for bidirectional detection. To avoid contention, the traffic within the same VPN should be scheduled in a TDM manner. To meet the power budget and to maximize cascadability, a bidirectional Erbium-doped fiber amplifier can be inserted in each RN to compensate for the power losses. 3. EXPERIMENT We experimentally verify the feasibility of the proposed TDM/WDM metro-access integrated network with all-optical VPN function. The down-/upstream traffic at the wavelengths of nm and 1553 nm respectively are generated by using independent 5-Gb/s data streams with a pseudo-random bit sequence (PRBS) length of The VPN traffic is a 625-Mb/s data stream with a PRBS length of Firstly, we experimentally test the coherent DPSK signal multiplexing technique as shown in Fig. 5. Fig. 6(a) depicts the eye diagram and bit error rate (BER) measurements. The sensitivities for various misalignments between the two data streams ranging from -25 ps to +25 ps are measured as well, to investigate the technique s tolerance to the synchronization mismatch between nodes (Fig. 6(b)). Within ±10-ps misalignment, less than 1-dB penalty is observed. Proc. of SPIE Vol O-4

-4-5 O -6 LU -8-9 200 ps Data1 (single) Data2 (single) Data1 (Mux) Data2 (Mux) 200 ps (a) -10-20.0-19.0-18.0-17.0-16.0-15.0-14.

$5-30 Datal -. Data2 iii-rf (r\ -20-10 0 10 20 30 Misalignment (ps) Fig. 6.$ (b) Sensitivities for various misalignments between the two data streams.

7, where the upstream carrier is successively modulated by the MZMs in two different ONUs, and decoded by the DPSK receiver at

5 -4-5 O -6 LU ps Data1 (single) Data2 (single) Data1 (Mux) Data2 (Mux) 200 ps (a) Measured Received Power (dbm) (1) (I) E Datal -. Data2 iii-rf (r\ Misalignment (ps) Fig. 6. (a) BER performances and eye diagrams of the DPSK multiplexing technology. (b) Sensitivities for various misalignments between the two data streams. (b) Next, the performance of the upstream transmission is investigated in a setup depicted by Fig. 7, where the upstream carrier is successively modulated by the MZMs in two different ONUs, and decoded by the DPSK receiver at the OLT, which consists of a 100-ps-delay interferometer and a single-ended 9-GHz photodetector. Fig. 8 shows the BER performance and the eye diagram. Less than 1-dB power penalty is observed. R!f GLL EDEV V'JDI 2VJE LAJL/N D9 Fig. 7. Experimental setup for upstream transmission Proc. of SPIE Vol O-5

-4-5 Data1 b-to-b Data1 25-km Data2 b-to-b Data2 12.

0 Measured Received Power (dbm) -14.0 Fig. 8.

both intensity modulated by the VPN data with an

10(1) illustrates the optical and electrical eye

$10(b) show the BER measurements for the EE1 v\jvj$

6 -4-5 Data1 b-to-b Data1 25-km Data2 b-to-b Data km O -6 LI.i Measured Received Power (dbm) Fig. 8. BER performances of the upstream data with de-modulated eye diagram inserted. Fig. 9 shows the setup for simultaneous transmission of VPN and downstream signals. The downstream DPSK signal and upstream carrier are both intensity modulated by the VPN data with an extinction ratio of ~4 db. Fig. 10(1) illustrates the optical and electrical eye diagrams of the orthogonal ASK/DPSK signals. Fig. 10(b) show the BER measurements for the VPN and downstream signals. EE1 v\jvj 2LtJE v\jvj 2LtJE EKL DE2K EE1 Fig. 9. Experimental setup for downstream and VPN transmission. Proc. of SPIE Vol O-6

-4 VPN b-to-b ' 0 Ui Db2k 1 E-3-1E 4 1E 5 1E6 1E7 1E8 1E9 1E-lO 1 E-1 1 1E-12 1E-13 1E-14-18 -5 O -6 w - -8

0 Measured Received Power (dbm) Measured Received Power (dbm) I --I- F - -' I Power (dbm) v--- 2-stage

(a) Eye diagrams of DPSK/ASK signal before and after filtering and after MZDI.

Since the multi-node DPSK multiplexing technique determines the scalability of the proposed network, we

adopted. Fig. 11 depicts the simulated BER performance of two-node, four-node and eight-node cases.

4 dbm, which is close to the experimental measurement. The sensitivity is degraded by 2.7 db for 4 2.

5 km, the penalty caused by transmission is less than 0.5 db with dispersion and power loss compensated for.

7 -4 VPN b-to-b ' 0 Ui Db2k 1 E-3-1E 4 1E 5 1E6 1E7 1E8 1E9 1E-lO 1 E-1 1 1E-12 1E-13 1E O -6 w - -8 (b) O.-19.( ( i-13i (-1-11 I.-10.O Measured Received Power (dbm) Measured Received Power (dbm) I --I- F - -' I Power (dbm) v--- 2-stage back-to-back -A stage transmission 4-stage back-to-back stage transmission stage back-to-back stage transmission VPN by PD1 VPN by PD2 Downstream b-to-b Downstream 25-km Fig. 10. (a) Eye diagrams of DPSK/ASK signal before and after filtering and after MZDI. (b) BER curves for VPN & downstream data. Since the multi-node DPSK multiplexing technique determines the scalability of the proposed network, we investigate its performance via numerical modelling in which the same parameters as in the experiment are adopted. Fig. 11 depicts the simulated BER performance of two-node, four-node and eight-node cases. The results show that for 2 5-Gb/s multiplexing, the sensitivity is dbm, which is close to the experimental measurement. The sensitivity is degraded by 2.7 db for Gb/s multiplexing, and by 6.3 db for Gb/s. For node spacing of 12.5 km, the penalty caused by transmission is less than 0.5 db with dispersion and power loss compensated for. Therefore, the proposed network consisting of typical sub-pons with 32 ONUs in each, can serve up to 256 ONUs in the eight-node case. In practice, avalanche photodetectors and forward error correction can be used to further improve the scalability. Fig. 11. (a) Eye diagrams of DPSK/ASK signal before and after filtering and after MZDI. (b) BER curves for VPN & downstream data. Proc. of SPIE Vol O-7

8 4. CONCLUSIONS We have proposed and demonstrated a TDM/WDM metro-access integrated network with all-optical VPN across different sub-pons. Its feasibility as a potential solution for future access services has been verified by error-free transmissions and simulation analysis. 5. ACKNOWLEDGEMENT This work was supported by the 863 High-Tech program (2006AA01Z255), and the Fok Ying Tung Fund (101067). REFERENCES [1] [2] [3] [4] [5] [6] A. V. Tran, C. J. Chae, and R. S. Tucker, Bandwidth-efficient PON system for broadband access and local customer internetworking, IEEE Photon. Technol. Lett., vol. 18, no. 5, pp , Mar. 1, Qiguang Zhao and Chun-Kit Chan, A Wavelength-Division-Multiplexed Passive Optical Network With Flexible Optical Network Unit Internetworking Capability, IEEE/OSA J. Lightw. Technol., vol. 25, no. 8, pp , Aug Yue Tian, Tong Ye, and Yikai Su, Demonstration and Scalability Analysis of All-Optical Virtual Private Network in Multiple Passive Optical Networks Using ASK/FSK Format, IEEE Photon. Technol. Lett., vol. 19, no. 20, pp , Oct. 15, Shing-Wa Wong, Wei-Tao Shaw, Ning Cheng, Chumming Qiao, and Leonid Kazovsky, Dynamic Wavelength Allocation in a Converged and Scalable Interface for Metro-Access Ring Integrated Networks, Proc. OFC 2008, OTuI6. Guo-Wei Lu, Kazi Sarwar Abedin and Tetsuya Miyazaki, Wideband All-Optical Phase Interleaving Technology Enabling Phase-Multiplexing of 3x10-Gb/s DPSK-WDM into 30-Gb/s DPSK using FWM in HNLF, Proc. ECOC 2007, PD 1.4. Yue Tian, Qingjiang Chang, and Yikai Su, "A Two-Stage Metro-Access Integrated Network Enabling All-Optical Virtual Private Network," Proc. ECOC 2008, Tu.1.F.3 Proc. of SPIE Vol O-8

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