IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005
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Orthogonal Label Switching Using Polarization-Shift-Keying Payload and Amplitude-Shift-Keying Label C. W. Chow and H. K. Tsang, Senior Member, IEEE
Abstract—We propose and demonstrate an orthogonal labeling scheme using amplitude-shift-keying (ASK) label and polarizationshift-keying (PolSK) payload. The PolSK/ASK orthogonal labeling scheme suffers less frequency chirp than differential-phase-shiftkeying/ASK labeling. Receiver sensitivity for payload can be improved by detecting the payload after the removal of ASK label. Performance analysis of the ASK label under different environments of polarization-dependent loss during transmission is also studied. Index Terms—Optical packet switching, optical label switching, orthogonal labeling.
I. INTRODUCTION
O
PTICAL orthogonal labeling [1]–[4] is an attractive approach for optical packet labeling. Orthogonal labeling has less precise timing control requirements when compared with bit-serial labeling [5]. It has a less stringent requirement on wavelength accuracy when compared with subcarrier multiplex (SCM) labeling [6] or optical carrier suppression and separation (OCSS) labeling [7]. When compared with optical code-based labeling [8], it does not need a large number of coders and decoders. Deploying polarization-shift keying (PolSK) [9] in orthogonal labeling [10] has been shown to reduce the excess frequency chirp, which causes degradation to the amplitude-shift keying (ASK) payload . In the PolSK/ASK labeling scheme, a low extinction ratio (ER) ASK payload is needed to provide sufficient optical power for the integrity of the PolSK label [10]. However, since the payload is left intact in the optical domain throughout the network, it may be a better choice to use the PolSK for the payload. In addition, receiver sensitivity for the payload would be improved by the removal of the ASK label before detection of the PolSK payload. II. PROPOSED ARCHITECTURE OF ORTHOGONAL LABELING AND EXPERIMENT
The setup for orthogonal labeling is shown in Fig. 1. Inside the source node, the ASK label was generated by modulating an electroabsorption modulator laser (EML) at 10 Gb/s, - . The EML pseudorandom binary sequence (PRBS) of Manuscript received June 16, 2005; revised July 15, 2005. This work was supported in part by the Research Grants Council of Hong Kong under Earmarked Grants CUHK4198/03E. The authors are with the Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong (e-mail:
[email protected];
[email protected]). Digital Object Identifier 10.1109/LPT.2005.857590
Fig. 1. Experimental setup of orthogonal labeling. EML: electroabsorption modulator. PM: phase modulator. EDFA: erbium-doped fiber amplifier. SMF: single-mode fiber. DCF: dispersion-compensating fiber. SOA: semiconductor optical amplifier. PD: photodiode. DFB: distributed feedback laser. FP: Fabry–Pérot laser diode. BERT: BER tester. LEU: label extraction unit. LRU: label removal unit. LIU: label insertion unit. (Inset: optical spectra of payload demodulation by injection-locking of FP-LD).
(wavelength nm) was biased at 33 mA. The signal was then directed to a phase modulator (PM) for 10-Gb/s - . The orthogonally PolSK payload modulation, PRBS of modulated signal (ASK label with PolSK payload) was then propagating in 40-km single-mode fiber (SMF) and 7.8-km dispersion-compensating fiber (DCF). At the intermediate node, 10% of the signal power was sent to the label extraction unit (LEU), where the label information was directly detected by a 10-Gb/s pin photodiode (PD). The electrical signal from the PD may be launched into a label processor [11] for label information retrieval. The remaining optical power went to the label removal unit (LRU), where it was amplified by an erbium-doped fiber amplifier (EDFA) to 5 dBm before being fed into a semiconductor optical amplifier (SOA). A tunable filter (3-dB linewidth of 1 nm) was used to remove the out-of-band amplified spontaneous emission (ASE) of the EDFA. The saturated gain in the SOA reduced the difference between the “1” and “0” levels of the ASK label, thus effectively erasing the label. At the label insertion unit (LIU), a new 10-Gb/s ASK label was encoded onto the PolSK payload via polarization-insensitive cross-gain modulation (XGM). The new label was originally produced by a distributed feedback nm) with input power of laser diode (LD) (wavelength 1.67 dBm. The SOA (polarization-dependent gain of 0.3 dB)
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 17, NO. 11, NOVEMBER 2005
Fig. 2. BER measurements for the ASK label and the PolSK payload at different nodes of the experiment (insets: eye diagrams corresponding to different measurement locations in Fig. 1). (50 ps/div.)
was biased at 198 mA. The optical filters after the SOAs (3-dB linewidth of 1 nm) were used to remove the out-of-band ASE of the SOAs. The saturation powers of the SOAs in LRU and LIU are 8 and 9 dBm, respectively. The demodulation of the PolSK payload was performed by using injection-locking of a Fabry–Pérot laser diode (FP-LD) nm and longitudinal mode spacing (peak wavelength nm). It was biased at 24.4 mA. In injection-locking, the transverse-electric component of PolSK payload will be intensity clamped, amplified, and stabilized, whereas the transverse-magnetic component will be suppressed. III. RESULTS AND DISCUSSION Fig. 2 shows the bit-error-rate (BER) measurements for the ASK label and PolSK payload at different nodes of the experiment, with the corresponding eye diagrams. The payload power penalty was reduced by 1.2 dB (due to the removal of ASK label by the gain-saturated SOA) at the exit of the LRU. Label removal by the gain-saturated SOA may be applied at the egress node of the optical label switched network to improve the receiver margin of the payload. This is due to the increased eye opening after the gain-saturated SOA. After label swapping, the payload was preserved in the original wavelength, and had . The penalty a power penalty of about 1 dB at BER of may be due to the residual polarization-dependent gain of the SOAs and the polarization rotation in the gain-saturated SOA [12]. The injection-locked FP-LD successfully demodulated the PolSK payload. The state-of-polarization (SOP) of the demodulated payload was confined to a dot on the Poincaré sphere with degree-of-polarization (DOP) of 96%. ASK label measurements were carried out at the LEU and after the LIU. The new label after the intermediate node was extracted in the same way as the old label extraction. The new label has similar receiver margin when compared with the old label from the source node. Better receiver margin in the new label may be obtained by adding one or more SOAs to suppress further the old ASK label; however, this would also add noise and affect the subsystem performance [13]. The optical spectra of the free running FP-LD and the payload demodulated by injection-locking of FP-LD were measured by an optical spectrum analyzer (resolution bandwidth
Fig. 3. Suppression of ER of the input ASK label by gain-saturated SOA. Insets: dependence of ER of the newly added ASK label at different input power of (a) back-to-back NRZ signal (1554 nm) and (b) old label erased PolSK payload by XGM.
nm, as shown in insets of Fig. 1. Fig. 3 shows the suppression of ER of the ASK label by the gain-saturated SOA in the LRU. When the input power was 5 dBm, the label can be nearly removed by biasing the SOA at 180 mA. At higher input signal power, a lower bias may be applied to the SOA for label removal. The random birefringence of buried optical fiber typically causes only changes 2 –10 /day in the polarization angles of the propagating signals [14]. PolSK recovering and signal constellation tracking at receiver has been evaluated showing small penalties in practical situations [15]. However, in a packet switched network, packets can come from different network nodes traveling along different paths. There can be random polarization changes between packets. This will require further research to develop receivers capable of polarization tracking rapidly to cope with different packets. Polarization stabilization using injection-locking of FP-LD has been experimentally demonstrated with response time less than 17 ps [16]. Injection-locking may adapt to the varying power levels of different packets because of its intensity clamping characteristic. The injection-locked FP-LD may, therefore, be suitable for burst-mode receiving. Insets of Fig. 3(a) and (b) show the dependence of the ER of the newly added ASK label at different input power of back-to-back nonreturn-to-zero (NRZ) signal (1554 nm) and PolSK payload by XGM performed in LIU, respectively. It can be seen that the ER of the newly added ASK label increases with the power from the NRZ input when the power of PolSK payload is kept at 3.21 dBm. The ER decreases if the PolSK payload power is increased and the input NRZ power is kept at 1.67 dBm. Polarization-dependent loss (PDL) may introduce in-band crosstalk to the ASK label. Fig. 4(a) shows the simulation results of the influence of different ER of the ASK label by different magnitude of PDL during transmission in order to with different receiver sensitivities. Higher have a BER at received optical power of ASK label is needed to maintain when PDL increases. As the label only surthe BER at vives between neighboring nodes and may be short in length,
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formance analysis of the ASK label under different conditions of PDL during transmission is also studied and the degradation from the PDL may be partially mitigated by using FEC. REFERENCES [1] H. J. S. Dorren, M. T. Hill, Y. Liu, N. Calabretta, A. Srivatsa, F. M. Huijskens, H. de Waardt, and G. D. Khoe, “Optical packet switching and buffering by using all-optical signal processing methods,” J. Lightw. Technol., vol. 21, no. 1, pp. 2–12, Jan. 2003. [2] D. J. Blumenthal, “Optical packet switching,” in Proc. IEEE LEOS Annu. Meeting (LEOS 2004), vol. 2, Nov. 2004, pp. 910–912. [3] B. Meagher et al., “Design and implementation of ultra-low latency optical label switching for packet-switched WDM networks,” J. Lightw. Technol., vol. 18, no. 12, pp. 1978–1987, Dec. 2000. [4] T. Koonen, G. Morthier, J. Jennen, H. de Waardt, and P. Demeester, “Optical packet routing in IP-over-WDM networks deploying two-level optical labeling,” in Proc. Eur. Conf. Optical Communication (ECOC 2001), vol. 4, 2001, pp. 608–609. [5] D. J. Blumenthal, B.-E. Olsson, G. Rossi, T. E. Dimmick, L. Rau, M. Masanovic, O. Lavrova, R. Doshi, O. Jerphagnon, J. E. Bowers, V. Kaman, L. A. Coldren, and J. Barton, “All-optical label swapping networks and technologies,” J. Lightw. Technol., vol. 18, no. 12, pp. 2058–2075, Dec. 2000. [6] H. J. Lee, S. J. B. Yoo, V. K. Tsui, and S. K. H. Fong, “A simple all-optical label detection and swapping technique incorporating a fiber Bragg grating filter,” IEEE Photon. Technol. Lett., vol. 13, no. 6, pp. 635–637, Jun. 2001. [7] J. Yu and G. K. Chang, “A novel technique for optical label and payload generation and multiplexing using optical carrier suppression and separation,” IEEE Photon. Technol. Lett., vol. 16, no. 1, pp. 320–322, Jan. 2004. [8] N. Wada, W. Chujo, and K.-I. Kitayama, “1.28 Tbit/s (160 Gbit/s 8 wavelengths) throughput variable length packet switching using optical code based label switch,” in Proc. ECOC 2001, vol. 6, Oct. 2001, pp. 62–63. [9] S. Benedetto and P. Poggiolini, “Theory of polarization shift keying modulation,” IEEE Trans. Commun., vol. 40, no. 4, pp. 708–721, Apr. 1992. [10] C. W. Chow, C. S. Wong, and H. K. Tsang, “Optical packet labeling based on simultaneous polarization shift keying and amplitude shift keying,” Opt. Lett., vol. 29, pp. 1861–1863, 2004. [11] S. J. B. Yoo et al., “High-performance optical-label switching packet routers and smart edge routers for the next-generation Internet,” IEEE J. Sel. Areas Commun., vol. 21, no. 7, pp. 1041–1051, Sep. 2003. [12] B. F. Kennedy, S. Philippe, P. Landais, A. L. Bradley, and H. Soto, “Experimental investigation of polarization rotation in semiconductor optical amplifiers,” in Proc. Inst. Elect. Eng., Optoelectronics, vol. 151, Apr. 2004, pp. 114–118. [13] P. I. Kuindersma, G. P. J. M. Cuijpers, J. J. E. Reid, G. N. van den Hoven, and S. Walczyk, “An experimental analysis of the system performance of cascades of 1.3 m semiconductor optical amplifiers,” in Proc. ECOC’97, vol. 1, Sep. 1997, pp. 79–82. [14] G. Nicholson and D. J. Temple, “Polarization fluctuation measurements on installed single-mode optical fiber cables,” J. Lightw. Technol., vol. 7, no. 8, pp. 1197–1200, Aug. 1989. [15] S. Benedetto, R. Gaudino, and P. Poggiolini, “Polarization recovery in optical polarization shift-keying systems,” IEEE Trans. Commun., vol. 45, no. 10, pp. 1269–1279, Oct. 1997. [16] L. Y. Chan, W. H. Chung, P. K. A. Wai, H. Y. Tam, and M. S. Demokan, “All-optical stabilization of state of polarization of high speed pulse train using injection-locked laser diode,” Electron. Lett., vol. 38, pp. 1116–1118, Sep. 2002. [17] K. Azadet, E. F. Haratsch, H. Kim, F. Saibi, J. H. Saunders, M. Shaffer, L. Song, and M.-L. Yu, “Equalization and FEC techniques for optical transceivers,” IEEE J. Solid-State Circuits, vol. 37, no. 3, pp. 317–327, Mar. 2002. [18] H. C. Ji, J. H. Lee, and Y. C. Chung, “Effect of polarization dependent loss on polarization-shift-keying transmission system,” Proc. SPIE, Optical Components and Transmissions Systems, vol. 4906, pp. 313–318, 2002.
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Fig. 4. Simulation result (a) of the influence of different ER of ASK label to different magnitude of PDL during transmission in order to have a BER at 10 with different receiver sensitivities and (b) that relaxed by using FEC.
forward-error correction (FEC) [17] could be implemented to reduce the degradation caused by the PDL, as shown in Fig. 4(b). The simulation is based on the conditions shown in Fig. 1. By implementing FEC, when the orthogonally labeled signal (ASK label at 2 dB) suffers from PDL of about 0.35 dB (measured experimentally in the transmission link in Fig. 1), a smaller (from 25 to 40 dBm) received optical power is . It has been theoretineeded to achieve the same BER of cally and experimentally demonstrated that a 3-dB PDL could result in 1-dB power penalty in PolSK signal when FEC is not used [18]. IV. CONCLUSION An orthogonal labeling scheme using ASK label and PolSK payload is proposed and demonstrated. Wavelength conversion is not needed in label swapping. A power penalty of about 1 dB was measured in the PolSK payload during label at BER of dB) has similar receiver swapping. The new ASK label (ER margin when compared with the old label from the source node. Receiver sensitivity for the payload can be improved (1.2 dB) by detecting the payload after the removal of ASK label. Per-
