Due to its relative low baud rate as well as simple and cost-efficient implementation, dual-carrier polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) is a promising candidate for the next generation optical systems and networks at 400Gb/s per channel. The co-polarized dual-pump scheme, based on four-wave mixing (FWM) in a 1-km high nonlinear fiber (HNLF), can realize the all-optical wavelength conversion (AOWC) of the dual-carrier PDM-16QAM signal with spectral non-inversion and polarization insensitivity. We first experimentally demonstrated AOWC of the 544-Gbit/s dual-carrier PDM-16QAM signal based on the co-polarized dual-pump scheme. We investigated the conversion efficiency (CE) and optical signal-to-noise ratio (OSNR) of the converted signal at different pump spacing and pump power. We measured that the OSNR penalty is 0.6 dB due to AOWC when the bit-error ratio (BER) and pump spacing is 2 x 10−2 and 200 GHz, respectively.
©2012 Optical Society of America
All-optical wavelength conversion (AOWC) will play an important role in the next generation optical systems and networks, in order to meet enhancing routing options and network properties such as re-configurability, non-blocking capability and wavelength reuse. Several schemes, including self-phase modulation (SPM) , cross-phase modulation (XPM) [2, 3], cross-gain modulation (XGM) and four-wave mixing (FWM) [4–10], can realize the technique of AOWC. FWM is one of the most promising schemes because it’s fully transparent to the bit rate and modulation format of the optical signal [4–10]. AOWC based on FWM in the high nonlinear fiber (HNLF) can be implemented using single- and multi-pump schemes. However, the single-pump scheme has the disadvantages of polarization sensitivity and spectral inversion, just as shown in Fig. 1(a) . A polarization-insensitive co-polarized dual-pump AOWC scheme was proposed in  based on FWM in the HNLF. This scheme has the advantage of spectral non-inversion, that is, the converted signals have the same spectral order as the original signals, just as shown in Fig. 1(b). It has been experimentally demonstrated that the co-polarized dual-pump scheme can be used to realize the polarization-insensitive AOWC of single- and multi-channel polarization-division-multiplexing return-to-zero quadrature phase shift keying (PDM-RZ-QPSK) and octal-phase shift keying (PDM-RZ-8PSK) signals [6–9]. In , AOWC of a 144-Gb/s single-carrier PDM-16QAM signal was experimentally demonstrated based on the co-polarized dual-pump scheme.
After 100G becoming commercial product, 400G is the potential bit rate for the next generation optical systems and networks based on historical, economic, technological, and bandwidth considerations [11–15]. As one of multi-level modulation formats, polarization-division-multiplexing 16-ary quadrature amplitude modulation (PDM-16QAM) is a promising candidate for meeting the rate of 400 Gb/s per channel. Several experiments with the rate of 400 Gb/s and beyond per channel have been reported based on PMD-16QAM [16, 17]. At the same time, multi-carrier modulation can be used to reduce the bandwidth requirements for optical and electrical components. Recently, many research groups have investigated and demonstrated the realization of 400G using dual-carrier PDM-16QAM . If dual-carrier PDM-16QAM modulation is adopted to achieve the line rate of 400G, the baud rate of each subcarrier is only ~30Gbaud, the same baud rate as 100G PDM-QPSK modulation. Thus, dual-carrier PMD-16QAM modulation can reuse the optical and electrical components developed for 100G PDM-QPSK modulation. As a result, dual-carrier PDM-16QAM modulation, requiring simply two optical carriers, is relatively simple and cost-efficient to commercially implement. However, no AOWC has been demonstrated for the 400G dual-carrier PDM-16QAM signal until now.
The generated 400G PDM-16QAM signal has two optical subcarriers with the fixed frequency order and frequency spacing in our experiment. Unlike AOWC of the single-carrier signal, a specific AOWC scheme is needed for the dual-carrier PMD-16QAM signal, in order to maintain the frequency order and frequency spacing of the two optical subcarriers after AOWC. In this paper, we first experimentally demonstrated AOWC of the 544-Gbit/s dual-carrier PDM-16QAM signal based on the co-polarized dual-pump scheme. The co-polarized dual-pump scheme, based on FWM in a 1-km HNLF, has the advantages of spectral non-inversion and polarization insensitivity. We investigated the conversion efficiency (CE) and optical signal-to-noise ratio (OSNR) of the converted signal at different pump spacing and pump power. We measured that the OSNR penalty is 0.6 dB due to AOWC when the bit-error ratio (BER) and pump spacing is 2 x 10−2 and 200 GHz, respectively. The BER of 2 x 10−2 is the forward-error-correction (FEC) threshold, assuming about 20% FEC overhead.
2. Experimental setup
Figure 2 shows the experimental setup for AOWC of the 34-Gbaud dual-carrier PDM-16QAM signal based on the co-polarized dual-pump scheme. Inset (a) gives the detailed structure of the optical PDM-16QAM modulator. At the transmitter, there are four external cavity lasers (ECLs), each with the line-width less than 100 kHz and the output power of 14.5 dBm. Pump 1(P1) at 1561.38 nm and pump 2 (P2) at 1559.7 nm are respectively generated from ECL1 and ECL2, and then combined by a polarization-maintaining optical coupler (PM OC). The frequency spacing is 200 GHz between P1 and P2. After passing through EDFA1, the optical power of both P1 and P2 is 16 dBm.
As shown in inset (a) in Fig. 2, after passing through electrical amplifiers (EAs) and power couplers, two electrical 16QAM signals are generated from four 34-Gb/s binary signals with a word length of 215-1, and then used to drive the in-phase (I) and quadrature-phase (Q) ports of the I/Q modulator, respectively. For the generation of optical 16QAM signal, the two parallel Mach-Zehnder modulators (MZMs) in the I/Q modulator are both biased at the null point and driven at the full swing to achieve zero-chirp 0- and π-phase modulation. The phase difference between the upper and the lower branches of the I/Q modulator is controlled at π/2. The subsequent polarization multiplexing is realized by the polarization multiplexer, comprising a polarization beam splitter (PBS) to halve the signal into two branches, an optical delay line (DL) to provide a delay of 150 symbols, an optical attenuator to balance the power of the two branches and a polarization beam combiner (PBC) to recombine the signal. Two optical carriers at 1550.88 and 1551.28 nm are respectively generated from ECL3 and ECL4. The frequency spacing is 50 GHz between the two optical carriers. Two optical PDM-16QAM modulators are respectively used to modulate the two optical carriers, generating two 34-Gbaud optical PDM-16QAM signals. Then, an optical coupler (OC) is used to further generate the 34-Gbaud optical dual-carrier PDM-16QAM signal. After passing through EDFA2, the optical dual-carrier PDM-16QAM signal (S1 and S2) has the power of 1.7 dBm /sub-channel, i.e., a total power of 4.7 dBm.
The pumps (P1 and P2) and the optical dual-carrier PDM-16QAM signal (S1 and S2) are combined and then launched into the 1-km HNLF for AOWC. The HNLF in this experiment has an attenuation coefficient of 0.4 dB/km, a nonlinear coefficient of 10 W−1/km, a zero-dispersion wavelength of 1561 nm, and a dispersion slope of 0.02 ps/nm2/km. As shown in Fig. 3(a) , two groups of the converted signals are generated in two sides of the original signals (S1 and S2), one group at 1549.28 and 1549.79 nm (C1 and C2) and the other at 1552.48 and 1552.88 nm (C1’ and C2’), respectively. The optical power of the converted signals is −11.3 dBm/sub-channel, i.e., the power of each group of converted signals is −8.3 dBm. After passing through the programmable wavelength selective switch (WSS), the converted signals at 1549.28 and 1549.79 nm (C1 and C2) are reserved for further coherent detection and BER counting, just as shown in Fig. 3(b).
At the receiver, a tunable optical filter (TOF) with the 3-dB bandwidth of 0.5 nm is used to choose the desired sub-channel. An ECL with a line-width less than 100 kHz is used as the local oscillator (LO) source. A polarization-diversity 90 degree hybrid is used to realize the polarization- and phase-diversity coherent detection of the LO source and the received optical signal before balanced detection. The analog-to-digital conversion is realized in the digital scope with the sample rate of 80 GSa/s and the electrical bandwidth of 25 GHz.
For the digital signal processing (DSP), the electrical polarization recovery is achieved using a three-stage blind equalization scheme . Firstly, the clock is extracted using the “square and filter” method, and then the digital signal is re-sampled at twice of the baud rate based on the recovered clock. Secondly, a T/2-spaced time-domain finite impulse response (FIR) filter is used for the compensation of chromatic dispersion (CD), where the filter coefficients are calculated from the known fiber CD transfer function using the frequency-domain truncation method. Thirdly, two complex-valued, 13-tap, T/2-spaced adaptive FIR filters are used to retrieve the modulus of the 16QAM signal. The two adaptive FIR filters are based on the classic constant modulus algorithm (CMA) and followed by three-stage CMA, to realize multi-modulus recovery and polarization de-multiplexing. The carrier recovery is performed in the subsequent step, where the 4-th power is used to estimate the frequency offset between the LO source and the received optical signal. The phase recovery is obtained by feed-forward and Least-Mean-Square (LMS) algorithms for offset compensation. Finally, differential decoding is used for BER counting after decision. In this experiment, BER is counted over 5.76 x 106 bits (20 data sets, and each data set contains 288000 bits).
3. Experimental results
Figure 4 shows OSNR and CE of one sub-channel (C1) versus the pump spacing for the 34-Gbaud PDM-16QAM converted signal. Both the signal wavelength and P2 at 1561.38nm are fixed when the wavelength of P1 is changed for OSNR measurement. Inset (a) gives the optical spectrum for the 300-GHz pump spacing with CE of −12-dB, while inset (b) the 400-GHz pump spacing with CE of −14dB. We can see that both CE and OSNR decrease with the pump spacing. It’s because the increase of pump spacing will introduce an increasing mismatched phase between interacting waves in the 1-km HNLF.
Figure 5 shows OSNR of one sub-channel (C1) versus the pump power for the 34-Gbaud PDM-16QAM converted signal, corresponding to 200-GHz pump spacing. Here, the power of P1 and P2 is increased simultaneously. Both the signal wavelength and signal power are fixed when the power of P1 is changed for OSNR measurement. We can see that OSNR initially increases with the pump power until the pump power reaches 14 dBm. The OSNR saturation is mainly due to the low stimulated Brillouin scattering (SBS) threshold of the 1-km HLNF used for AOWC.
Figure 6 shows BER versus OSNR of one sub-channel (C1) for the 544-Gb/s dual-carrier PDM-16QAM original signal (S1) and converted signal (C1), respectively. Insets (a) and (b) give the constellations of the original signal at 45-dB OSNR and converted signal at 34-dB OSNR, respectively. The converted signal corresponds to 200-GHz pump spacing. The required OSNR at the BER of 2 x 10–2 is 23 and 23.6 dB for the original signal and converted signal, respectively. The OSNR penalty is thus 0.6 dB due to AOWC. Furthermore, we can see from insets (a) and (b) that the constellation of the converted signal is worse than that of the original signal. It’s because that the OSNR of the converted signal is smaller than that of the original signal. The BER of the converted signal (C2) is identical to that of C1, so we do not show the BER curve of this converted sub-channel.
We first experimentally demonstrated AOWC of the 544-Gbit/s dual-carrier PDM-16QAM signal based on the co-polarized dual-pump scheme. The co-polarized dual-pump scheme, based on FWM in a 1-km HNLF, has the advantages of spectral non-inversion and polarization insensitivity. We investigated the CE and OSNR of the converted signal at different pump spacing and pump power. We measured that the OSNR penalty is 0.6 dB due to AOWC when the BER and pump spacing is 2 x 10−2 and 200 GHz, respectively.
References and links
1. P. V. Mamyshev, “All-optical data regeneration based on self-phase modulation effect,” in Proceedings of 24th European Conference and Exhibition on Optical Communication (ECOC), 1998, pp. 475–476.
2. W. Wang, H. N. Poulsen, L. Rau, H. F. Chou, J. E. Bowers, and D. J. Blumenthal, “Raman-enhanced regenerative ultrafast all-optical fiber XPM wavelength converter,” J. Lightwave Technol. 23(3), 1105–1115 (2005). [CrossRef]
3. J. Yu, X. Zheng, C. Peucheret, A. Clausen, H. Poulsen, and P. Jeppesen, “All-optical wavelength conversion of short pulses and NRZ signals based on a nonlinear optical loop mirror,” J. Lightwave Technol. 18(7), 1007–1017 (2000). [CrossRef]
4. J. Ma, J. Yu, C. Yu, and Z. Zhou, “Reducing polarization sensitivity for all-optical wavelength conversion of the optical packets based on FWM in HNL-DSF using co-polarized pump scheme,” Opt. Commun. 260(2), 522–527 (2006). [CrossRef]
5. J. Ma, J. Yu, C. Yu, Z. Jia, X. Sang, Z. Zhou, T. Wang, and G. K. Chang, “Wavelength conversion based on four-wave mixing in high-nonlinear dispersion shifted fiber using a dual-pump configuration,” J. Lightwave Technol. 24(7), 2851–2858 (2006). [CrossRef]
6. Y. Xie, S. Gao, and S. He, “All-optical wavelength conversion and multicasting for polarization-multiplexed signal using angled pumps in a silicon waveguide,” Opt. Lett. 37(11), 1898–1900 (2012). [CrossRef] [PubMed]
7. J. Lu, L. Chen, Z. Dong, Z. Cao, and S. Wen, “Polarization insensitive wavelength conversion based on orthogonal pump four-wave mixing for polarization multiplexing signal in high-nonlinear fiber,” J. Lightwave Technol. 27(24), 5767–5774 (2009). [CrossRef]
8. M. F. Huang, J. Yu, and G. K. Chang, “Polarization insensitive wavelength conversion for 4x112Gbit/s polarization multiplexing RZ-QPSK signals,” Opt. Express 16(26), 21161–21169 (2008). [CrossRef] [PubMed]
9. J. Yu and M. Huang, “Wavelength conversion based on copolarized pumps generated by optical carrier suppression,” IEEE Photon. Technol. Lett. 21(6), 392–394 (2009). [CrossRef]
10. M. Huang, J. Yu, Y. K. Huang, E. Ip, and G. K. Chang, “Wavelength converter for polarization-multiplexed 100-G transmission with multilevel modulation using a bismuth oxide-based nonlinear fiber,” IEEE Photon. Technol. Lett. 22(24), 1832–1834 (2010). [CrossRef]
11. C. Cole, “Beyond 100G client optics,” IEEE Commun. Mag. 50(2), s58–s66 (2012). [CrossRef]
12. P. J. Winzer, “Beyond 100G ethernet,” IEEE Commun. Mag. 48(7), 26–30 (2010). [CrossRef]
13. D. van den Borne, V. Sleiffer, M. S. Alfiad, and S. L. Jansen, “Towards 400G and beyond: How to design the next generation of ultra-high capacity transmission systems,” in Proceedings of 16th OptoElectronics and Communications Conference (OECC), 2011, pp. 429–432.
14. M. W. Chbat and S. Spalter, “From 100G to 1000G: Is there a straight road ahead?” in Proceedings of 36th European Conference and Exhibition on Optical Communication (ECOC), 2010, pp. 1–16.
15. M. Camera, B.-E. Olsson, and G. Bruno, “Beyond 100Gbit/s: System implications towards 400G and 1T,” in Proceedings of 36th European Conference and Exhibition on Optical Communication (ECOC), 2010, pp. 1–15.
16. P. J. Winzer, A. H. Gnauck, S. Chandrasekhar, S. Draving, J. Evangelista, and B. Zhu, “Generation and 1,200-km transmission of 448-Gb/s ETDM 56-Gbaud PDM 16-QAM using a single I/Q modulator,” in Proceedings of 36th European Conference and Exhibition on Optical Communication (ECOC), 2010, pp. 1–3.
17. J. Yu, Z. Dong, X. Tang, W. Jian, Y. Xia, S. Shi, S. Fan, and G. Chang, “Generation of 432Gb/s single-carrier optical signal by format conversion from QPSK to 16QAM,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference(OFC/NFOEC), 2011, pp. 1–3.
18. V. A. J. M. Sleiffer, D. van den Borne, V. Veljanovski, M. Kuschnerov, M. Hirano, Y. Yamamoto, T. Sasaki, S. L. Jansen, and H. de Waardt, “Transmission of 448-Gb/s dual-carrier POLMUX-16QAM over 1230 km with 5 flexi-grid ROADM passes,” in Proceedings of Optical Fiber Communication Conference and Exposition and the National Fiber Optic Engineers Conference(OFC/NFOEC), 2012, pp. 1–3.