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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 22, Iss. 9 — May. 5, 2014
  • pp: 10455–10466
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Experimental demonstrations of dual polarization CO-OFDM using mid-span spectral inversion for nonlinearity compensation

Monir Morshed, Liang B. Du, Benjamin Foo, Mark D. Pelusi, Bill Corcoran, and Arthur J. Lowery  »View Author Affiliations


Optics Express, Vol. 22, Issue 9, pp. 10455-10466 (2014)
http://dx.doi.org/10.1364/OE.22.010455


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Abstract

We experimentally demonstrate fiber nonlinearity compensation in dual polarization coherent optical OFDM (DP CO-OFDM) systems using mid-span spectral inversion (MSSI). We use third-order nonlinearity between a pump and the signal in a highly nonlinear fiber (HNLF) for MSSI. Maximum launch powers at FEC threshold for two 10 × 80-km 16-QAM OFDM systems were increased by 6.4 dB at a 121-Gb/s data rate and 2.8 dB at 1.2 Tb/s. The experimental results are the first demonstration of using MSSI for nonlinearity compensation in any dual polarization coherent system. Simulations show that these increases could support a 22% increase in total transmission distance at 1.2-Tb/s system without increasing the number of inline amplifiers, by extending the fiber spans from 90 to 110 km. When spans of 80 km are used, simulations reveal that MSSI system performance shows less degradation with increasing transmission distance, and an overall transmission distance increase of more than 70% is expected using MSSI.

© 2014 Optical Society of America

1. Introduction

Previously, mid-span spectral inversion (MSSI) has been shown to be effective for nonlinearity compensation in intensity modulated systems [6

6. S. L. Jansen, D. van den Borne, P. M. Krummrich, S. Spalter, G. D. Khoe, and H. de Waardt, “Long-haul DWDM transmission systems employing optical phase conjugation,” IEEE J. Sel. Top. Quantum Electron. 12(4), 505–520 (2006). [CrossRef]

, 7

7. P. Minzioni, F. Alberti, and A. Schiffini, “Optimized link design for nonlinearity cancellation by optical phase conjugation,” IEEE Photon. Technol. Lett. 16(3), 813–815 (2004). [CrossRef]

] and differential phase-shift-keyed (DPSK) systems [8

8. S. L. Jansen, D. van den Borne, B. Spinnler, S. Calabrò, H. Suche, P. M. Krummrich, W. Sohler, G. D. Khoe, and H. de Waardt, “Optical phase conjugation for ultra long-haul phase-shift-keyed transmission,” J. Lightwave Technol. 24(1), 54–64 (2006). [CrossRef]

]. MSSI is scalable for WDM systems because multiple wavelengths can be spectrally inverted simultaneously using a single optical phase conjugation (OPC) module [9

9. S. L. Jansen, D. van den Borne, C. Climent, M. Serbay, C. J. Weiske, H. Suche, P. M. Krummrich, S. Spalter, S. Calabro, N. Hecker-Denschlag, P. Leisching, W. Rosenkranz, W. Sohler, G. D. Khoe, T. Koonen, and H. de Waardt, “10,200 km 22×2×10 Gbit/s RZ-DQPSK dense WDM transmission without inline dispersion compensation through optical phase conjugation,” in Optical Fiber Communication Conference (OFC/NFOEC)(2005), p. PDP 28.

]. Recently, both theoretical and simulation results for MSSI with CO-OFDM have been reported [10

10. V. Pechenkin and I. J. Fair, “Analysis of four-wave mixing suppression in fiber-optic OFDM transmission systems with an optical phase conjugation module,” Optical Communications and Networking, IEEE OSA Journal of 2(9), 701–710 (2010). [CrossRef]

, 11

11. X. Liu, F. Buchali, and R. W. Tkach, “Improving the nonlinear tolerance of polarization-division-multiplexed CO-OFDM in long-haul fiber transmission,” J. Lightwave Technol. 27(16), 3632–3640 (2009). [CrossRef]

]. Our group has previously reported the first experimental results for MSSI in single-polarization CO-OFDM systems [12

12. L. B. Du, M. M. Morshed, and A. J. Lowery, “604-Gb/s coherent optical OFDM over 800 km of S-SMF with mid-span spectral inversion,” in Opto-Electronics and Communications Conference (Busan, 2012), pp. 3B2–3. [CrossRef]

, 13

13. L. B. Du, M. M. Morshed, and A. J. Lowery, “Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation,” Opt. Express 20(18), 19921–19927 (2012). [CrossRef] [PubMed]

], which to the best of our knowledge represents the first use of MSSI for coherent communication systems. At the Opto-Electronics and Communications Conference 2013 (OECC 2013) [14

14. M. Morshed, L. B. Du, B. Foo, M. D. Pelusi, and A. J. Lowery, “Optical phase conjugation for nonlinearity compensation of 1.21-Tb/s pol-mux coherent optical OFDM,” in 18th Opto-Electronics and Communications Conference(Kyoto, Japan, 2013), pp. PD 3–4.

], we experimentally demonstrated MSSI for fiber nonlinearity compensation in a dual-polarization (DP) CO-OFDM system for the first time, at a rate of 1.2 Tb/s. This demonstration is, to the best of our knowledge, the first for dual-polarization MSSI for any coherent communication system. Subsequently, there have been demonstrations of Raman-enhanced MSSI [15

15. K. Solis-Trapala, T. Inoue, and S. Namiki, “Nearly-Ideal Optical Phase Conjugation based Nonlinear Compensation System,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. W3F.8. [CrossRef]

, 16

16. I. Phillips, M. Tan, M. F. Stephens, M. McCarthy, E. Giacoumidis, S. Sygletos, P. Rosa, S. Fabbri, S. T. Le, T. Kanesan, S. K. Turitsyn, N. J. Doran, P. Harper, and A. D. Ellis, “Exceeding the Nonlinear-Shannon Limit using Raman Laser Based Amplification and Optical Phase Conjugation,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.1. [CrossRef]

] and multiple phase-conjugation based coherent systems [17

17. H. Hu, R. M. Jopson, A. Gnauck, M. Dinu, S. Chandrasekhar, X. Liu, C. Xie, M. Montoliu, S. Randel, and C. McKinstrie, “Fiber Nonlinearity Compensation of an 8-channel WDM PDM-QPSK Signal using Multiple Phase Conjugations,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.2. [CrossRef]

] at the 2014 conference on Optical Fiber Communications (OFC).

In this paper, we elaborate on the results in [14

14. M. Morshed, L. B. Du, B. Foo, M. D. Pelusi, and A. J. Lowery, “Optical phase conjugation for nonlinearity compensation of 1.21-Tb/s pol-mux coherent optical OFDM,” in 18th Opto-Electronics and Communications Conference(Kyoto, Japan, 2013), pp. PD 3–4.

] and the MSSI module for DP signals, which is the key enabling system in this experiment [14

14. M. Morshed, L. B. Du, B. Foo, M. D. Pelusi, and A. J. Lowery, “Optical phase conjugation for nonlinearity compensation of 1.21-Tb/s pol-mux coherent optical OFDM,” in 18th Opto-Electronics and Communications Conference(Kyoto, Japan, 2013), pp. PD 3–4.

]. We experimentally demonstrate the benefit of MSSI for 121-Gb/s and 1.2-Tb/s DP CO-OFDM MSSI systems for fiber nonlinearity compensation, and give an overview of important design considerations. These results are the first for DP coherent systems. Additionally, we have simulated the system to define the benefits gained from the increase in maximum allowable power using MSSI. We also show the simulation results for improving the performance the OPC module itself.

2. System carrying 121 Gb/s

2.1 Experimental setup

A 20-GHz optical bandwidth complex Mach-Zehnder modulator (C-MZM) modulates the electrical OFDM signal onto the output of the ECL. The optical signal is then divided into two paths; one path is frequency shifted by 8 GHz using another C-MZM as a serrodyne modulator, then the shifted and un-shifted signals are combined to form a continuous 16-GHz wide channel, as shown in inset (i) of Fig. 1(a). The spectrum also shows the optical carrier at the center of each band. In order to maintain the optical carrier at the center, the two central subcarriers of the electrical OFDM signal were nulled. A delay of exactly 5 OFDM symbols (101.5 ns) produces an integer-OFDM-symbol delay between the shifted and un-shifted signals. The channel is then passed through a POLMUX emulator (Kylia PDME-00019), with a 19.8-ns delay between the two orthogonal polarizations to de-correlate them, to create a 121-Gb/s DP signal.

Figure 1(b) shows the receiver configuration, which is the same for the 121-Gb/s and 1.21-Tb/s systems. A wavelength selective switch (WSS, Finisar Waveshaper) is used to remove the out-of-band noise. The filtered signal is fed into the signal port of a coherent receiver, consisting of an optical hybrid (Kylia mint-2 × 8) and 25-GHz bandwidth balanced photodiodes. A second ECL, tuned to match the frequency of either the original signal for reference system without MSSI (193.1 THz) or the conjugate idler for the MSSI system (193.5 THz), is used as the local oscillator. A 40-GSample/s, 16-GHz bandwidth real-time sampling oscilloscope (Agilent DSO-X 92804A) digitizes the photo-detected signals. The signal is processed offline, with the equalizer comprising [13

13. L. B. Du, M. M. Morshed, and A. J. Lowery, “Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation,” Opt. Express 20(18), 19921–19927 (2012). [CrossRef] [PubMed]

]: a resampler; a frequency offset compensator; a butterfly OFDM 1-tap equalizer [2

2. S. L. Jansen, I. Morita, T. C. Schenk, and H. Tanaka, “Long-haul transmission of16×52.5 Gbits/s polarization-division-multiplexed OFDM enabled by MIMO processing (Invited),” J. Opt. Netw. 7(2), 173–182 (2008). [CrossRef]

]; and a blind symbol phase estimator [18

18. I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. 27(15), 3042–3049 (2009). [CrossRef]

, 19

19. Y. Xingwen, W. Shieh, and T. Yan, “Phase estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. 19(12), 919–921 (2007). [CrossRef]

]. In the system without MSSI, digital chromatic dispersion (CD) compensation precedes the 1-tap equalizer.

The combined signal and pump wave is passed to a dual-polarization MSSI module [22

22. T. Hasegawa, K. Inoue, and K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization diversity technique,” IEEE Photon. Technol. Lett. 5(8), 947–949 (1993). [CrossRef]

], as recently used to demonstrate pre-compensation of fiber nonlinear effect in 80-Gb/s RZ-DPSK dual polarization signals [23

23. M. D. Pelusi, “Fiber looped phase conjugation of polarization multiplexed signals for pre-compensation of fiber nonlinearity effect,” Opt. Express 21(18), 21423–21432 (2013). [CrossRef] [PubMed]

]. Note that this polarization diverse configuration is insensitive to the polarization state of the input signal [22

22. T. Hasegawa, K. Inoue, and K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization diversity technique,” IEEE Photon. Technol. Lett. 5(8), 947–949 (1993). [CrossRef]

, 23

23. M. D. Pelusi, “Fiber looped phase conjugation of polarization multiplexed signals for pre-compensation of fiber nonlinearity effect,” Opt. Express 21(18), 21423–21432 (2013). [CrossRef] [PubMed]

]. The combined waves feed Port 1 of an optical circulator. Port 2 of the circulator connects to the common port of a polarization beam splitter (PBS). The two polarized (X Pol. and Y Pol.) ports of the PBS are interconnected via a polarization controller (PC), 1 km of highly nonlinear fiber (HNLF) and a 99%/1% coupler for power monitoring. After the insertion losses of the circulator, PBS and polarization controller, the pump and signal powers launched into the HNLF module from each direction are 12 dBm and 4 dBm respectively. The HNLF has a nonlinear coefficient, γ, of 11.5 W−1km−1, CD of 0.01 ps/nm/km at 1550 nm, CD slope of 0.02 ps/nm2/km, zero-dispersion-wavelength (ZDW) of 1549.120 nm and loss coefficient of 0.81 dB/km. Port 3 of the circulator connects to a 90%/10% coupler; the 10% port connects to an optical spectrum analyzer (OSA): the 90% port connects to a 200-GHz channel spacing demultiplexer (Siemens TransXpress), which selects the conjugated signal and removes the original signal, the pump and the out of band amplified spontaneous emission (ASE). The output of the demultiplexer is transmitted through the second half of the link.

Polarization controllers can be avoided entirely by using polarization maintaining fiber in the MSSI module, to ensure alignment of the pump wave to be 45° to the reference polarization axis of the polarization beam splitter and linear polarization states at input to the HNLF. However, our demonstration used non-PM HNLF, so required polarization controllers. PC3 is used to adjust the polarization of the travelling waves to be linearly polarized upon launch into the HNLF to maximize conversion efficiency (CE), which is continuously monitored at the OSA connected at Port 3 of the circulator, using a 90%/10% coupler. PC1 and PC2 can then be adjusted to reduce the pump power as measured at the 1% coupler output within the polarization diversity loop by 3 dB, which corresponds to an equal power split of the pump to both the clockwise (‘x’) and anti-clockwise (‘y’) arms of the polarization diverse MSSI module. Once set, the polarization of the pump wave in the polarization diversity loop was stable enough to gather consistent performance measurements.

By splitting the pump wave equally into the two branches of the loop, the CE of the orthogonally polarized counter-clock- and clockwise travelling signals should be similar [24

24. M. E. Marhic, G. Kalogerakis, and L. G. Kazovsky, “Gain reciprocity in fibre optical parametric amplifiers,” Electron. Lett. 42(9), 519–520 (2006). [CrossRef]

]. This is because CE is given by(γPpumpLeff)2, where Ppumpis pump power, γis nonlinear coefficient of HNLF, and Leffis its effective length. Therefore, in a counter propagating fiber loop scheme where both signals experience very similar loss and dispersion, we in fact expect almost equal CE between the two counter propagating signals.

Inset (i) of Fig. 1(c) shows an optical spectrum analyzer (OSA) trace (sensitivity: −75 dBm, resolution bandwidth: 0.1 nm) containing the original signal, pump and the OPC signal. This trace is taken after the circulator and before the demultiplexer filter in an 800-km MSSI system. The CE, defined here as the ratio of conjugate power to signal input power at circulator port 3, was about −20 dB.

Even if the CEs of the counter propagating signals are slightly different in practice, the true performance of the MSSI module is best revealed through rigorous testing through measuring the quality of the received signal at the end of the link.

2.2 Results for the 121-Gb/s system

Fig. 2 Back-to-back performance of the MSSI system for 121-Gb/s system. Launch power is measured at the output of the EDFA after the DCF.
Figure 2 shows the Q in a back-to-back system with our fiber spans for both X polarization (●) and Y polarization (○) versus signal input power measured at the output of the EDFA placed after the DCF. The Q was calculated from the counted bit error ratio (BER), using Q[dB]=20×log10(2erfc1(2BER)) [25

25. P. Wei-Ren, T. Tsuritani, and I. Morita, “Transmission of high-baud PDM-64QAM signals,” J. Lightwave Technol. 31(13), 2146–2162 (2013). [CrossRef]

, 26

26. G. P. Agrawal, Fiber-optic Communication Systems (Wiley and Sons, 2010).

].

Fig. 3 Q versus launch power after 800 km with and without MSSI for 121-Gb/s system.
Figure 2 shows that the X and Y polarizations have very similar performance, demonstrating that the performance of MSSI in our system is polarization independent, able to effectively handle dual-polarization, coherent signals. It also shows that the optimum signal input power is 18 dBm, which gives the maximum back-to-back Q of 12.5 dB. For the transmission results shown in Fig. 3, the output power of the EDFA after the DCF was fixed at 18 dBm to maximize the transmission performance. The Q in a back-to-back configuration without MSSI is 15.2 dB. The 2.7-dB decrease in measured Q after the MSSI module, shown in Fig. 2, is likely to be due to nonlinear mixing products generated within the MSSI and ASE from the EDFA that is used to compensate for the low four-wave mixing conversion efficiency within the HNLF [27

27. M. Morshed, L. B. Du, and A. J. Lowery, “Performance limitation of coherent optical OFDM systems with non-ideal optical phase conjugation,” in IEEE Photonics Conference (IPC)(Burlingame, CA, 2012), pp. 394–395, TuU. [CrossRef]

, 28

28. M. Morshed, L. B. Du, and A. J. Lowery, “Mid-span spectral inversion for coherent optical OFDM systems: Fundamental limits to performance,” J. Lightwave Technol. 31(1), 58–66 (2013). [CrossRef]

].

3. System carrying 1.2-Tb/s

3.1 Experimental setup

The majority of the experimental setup for the 1.21-Tb/s system is the same as for the 121-Gb/s system. Only the differences are discussed below.

The output of a 193.7-THz ECL is fed into the frequency comb generator module. Two 40-GHz bandwidth phase modulators are used to generate 10 comb lines. The modulators haveVπ=6V, and are driven by a 16-GHz RF signal, that is split into two paths before amplification by SHF amplifiers to give peak-to-peak voltages of 7.10 V. These RF signals are fed into the modulators, to generate peak phase shifts of about0.6π. In order to flatten the output comb lines from the second modulator, the phase difference between the two RF paths is adjusted by a tunable RF delay. The 10 selected comb lines used for the OFDM super-channel have a 6-dB variation between the edge and center lines. These lines are selected and equalized in power by a WSS, as shown in inset (i) of Fig. 4(a).
Fig. 4 Experimental setup for the 1.21Tb/s system: (a) Comb generator; (b) Combining the signal and pump for Tb/s MSSI system; (c) Spectra. PM: Phase Modulator; WSS: Wavelength Selective Switch; PC: Polarization Controller; ECL: External Cavity Laser.
The tones are then amplified and fed through the C-MZM to modulate OFDM signal onto these tones. The modulated signal is then divided into two paths; one path is frequency shifted by 8 GHz and then combined with through paths to form a 20-band 160-GHz wide super-channel as shown in Fig. 4(c).

Some modifications were made to MSSI module also for the 1.2-Tb/s system, to increase the back-to-back performance with MSSI. Figure 4(b) shows the module that combines the signal and pump. The pump (ECL3, 193.1 THz) is amplified from 16 dBm to 33 dBm using a high power EDFA (Amonics AEDFA ـ33ـBـFA) run at maximum power. The signal is amplified to 19 dBm using another variable gain EDFA. A multiplexer with a 200-GHz channel spacing simultaneously combines the signal and pump, while reducing out-of-band ASE in each of these wavebands. The HNLF in the MSSI module is shortened to 45 m, to reduce the long-scale fluctuations of the ZDW which become significant in longer HNLFs. Such variations along the fiber occur in an unpredictable manner, which degrades the OPC gain and bandwidth [21

21. M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). [CrossRef]

]. Thus, a short HNLF is more suitable for broad band OPC [21

21. M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). [CrossRef]

]. However, the pump power needs to be increased in order to get reasonable conversion efficiency, and this is possible due to the higher SBS threshold of shorter HNLFs [29

29. S. Le Floch and P. Cambon, “Theoretical evaluation of the Brillouin threshold and the steady-state Brillouin equations in standard single-mode optical fibers,” J. Opt. Soc. Am. A 20(6), 1132–1137 (2003). [CrossRef] [PubMed]

].

The HNLF has a nonlinear coefficient, γ, of 11.5 W−1km−1, CD of −0.05 ps/nm/km at 1550 nm, CD slope of 0.02 ps/nm2/km, zero-dispersion-wavelength (ZDW) of 1552.82 nm and loss coefficient of 0.97 dB/km. Note that, the wavelength of the pump is adjusted to match with the ZDW of this HNLF. The pump and signal powers launched into the HNLF from each direction are 29 dBm and 15 dBm respectively.

Figure 4(c) shows the spectrum of 160-GHz OFDM super-channel comprising 20 OFDM bands each 8-GHz wide, measured with a high-resolution spectrophotometer (resolution 20 MHz) after the 50%/50% coupler and before the POLMUX emulator. Figure 4(d) is the spectrum after the circulator, measured with an optical spectrum analyzer (OSA) with a sensitivity of −80 dBm and resolution bandwidth of 0.1 nm.

3.1 Results for the 1.21-Tb/s system

Fig. 6 BER for 20 channels after 800 km at two launch powers, with and without MSSI for the 1.21-Tb/s system.
Figure 6(a) shows the BERs for all of the OFDM bands at a 5-dBm launch power, which is the optimal power for the system without MSSI.

The channels in the middle of the band have similar BERs; however, the roll-off of the 200-GHz multiplexers attenuates the edges of the MSSI signal, increasing the error rates. This could be avoided by using an optical filter with a wider passband.

Figure 6(b) shows the BER of the OFDM bands at a power of 8 dBm, where nonlinearity dominates the system’s performance. All but three of the edge subcarriers of the MSSI system have BERs better than the 7%-overhead hard-decision FEC limit of 3.8 × 10−3. When the BER is averaged over all of the subcarriers, the MSSI beneficially decreases the BER from 1.0 × 10−2 to 3.0 × 10−3. Pairwise coding or similar techniques could be used to balance the error rates across the channels [33

33. Y. Hong, A. J. Lowery, and E. Viterbo, “Sensitivity improvement and carrier power reduction in direct-detection optical OFDM systems by subcarrier pairing,” Opt. Express 20(2), 1635–1648 (2012). [CrossRef] [PubMed]

].

4. Simulation results and discussion

Fig. 7 (a) Q versus launch power with and without MSSI for single polarization 160-GHz OFDM super channel with different span lengths (10 × 80 km, 10 × 90 km and 10 × 110 km). (b) Optimum performance and improvement versus span length. HLNF length, L = 45m, OPC conversion efficiency, CE = −24 dB, and number of spans = 10.
For sake of simplicity, the simulated system is a single polarization system. However, in a system that is limited by fiber nonlinear impairment rather than cross polarization nonlinear effects, this is able to demonstrate the practical benefit of using MSSI with a 160-GHz wide dual polarization 16-QAM OFDM super channel. In our simulations, we have used the combined insertion loss inside MSSI module as a simulation parameter, which comprises losses of a Siemens TransXpress Mux, Circulator, 99%/1% coupler, 90%/10% coupler and Siemens TransXpress Demux. Peak Q in the simulation and experiment was made similar by tuning the total combined insertion loss (IL) between 9 and 12 dB. Figure 7(a) shows the simulation results for reference system (▲) and system with MSSI (■) with 12-dB IL after 10 × 80-km transmission. Compared with the results shown in Fig. 5, a similar relation in peak Q between experiment and simulation was obtained. All other parameters in simulation were set as described in the experimental setup in Section 3.1.

Figure 7(a) also shows the simulation results of Q versus launch power with increased span lengths (10 × 90-km: ; and 10 × 110-km: ▲) in a 160-GHz OFDM super-channel for the reference system without MSSI. The purple curve with squares (■) shows the 10 × 110-km results with MSSI. The gray dashed line (–) shows the FEC limit for BER ˂ 3.8 × 10−3. The results show that maximum span length of the reference system is about 90 km, beyond which performance degrades below the FEC limit. On the other hand, the MSSI system sustains its performance over the FEC limit up to span lengths of 110 km. At launch powers of 8 dBm, the MSSI system clears the FEC limit, while the 10 × 110-km system without MSSI is well below this limit for all launch powers. Therefore, the overall transmission distance could be increased from 10 × 90 km to 10 × 110 km; a reach improvement of nearly 22% without increasing the number of inline amplifiers. Figure 7(b) shows the maximum Q value for different span lengths from 80 km to 110 km for both systems (Reference system: ▬; MSSI system: ▬) and the corresponding improvement with MSSI (▲). This shows that effect of nonlinearity mitigation due to MSSI increases with longer spans. This is due to the increased maximum launch power with MSSI. With span lengths of 90 km, MSSI shows about 1.2 dB improvement compared with the reference system.

Fig. 10 Improvement in maximum Q performance versus OPC conversion efficiency and HNLF length.
Figure 10 shows Q improvement for two different lengths of HNLF, 45 m (∆) and 22.5 m, (■) versus CE. The insertion loss in both of these cases is 12 dB. The HNLF nonlinear coefficient was kept as in Section 3.1. The result shows that maximum Q performance could be improved by about 0.7 dB by improving the CE from −24 dB to −18 dB using a shorter HNLF. Numerical results shows better performance with shorter HNLF at the same conversion efficiency, which agrees with the analytical prediction of [28

28. M. Morshed, L. B. Du, and A. J. Lowery, “Mid-span spectral inversion for coherent optical OFDM systems: Fundamental limits to performance,” J. Lightwave Technol. 31(1), 58–66 (2013). [CrossRef]

]. Shorter HNLF is also preferable to minimize the effect of the variation in zero dispersion wavelength along the HNLF. This helps maintain a uniform gain over a wide OPC bandwidth and a higher threshold for SBS.

5. Conclusions

We have experimentally demonstrated the first use of MSSI for fiber nonlinearity compensation for DP CO-OFDM systems. Polarization independence of the MSSI module was confirmed by observing very similar performance between counter-propagating X and Y polarization signals. MSSI increases the maximum permissible launch power by 2.8 dB for a 1.21-Tb/s 16-QAM DP CO-OFDM system and 6.4 dB at 121 Gb/s. Our simulations show that this increase in maximum viable launch power could support about a 22% increase in total transmission distance without increasing the number of inline amplifiers, by extending each fiber span from 90 km to 110 km, for 1.21-Tb/s system. For a fixed length of span, the MSSI system shows less degradation in optimum performance with increasing number of spans (i.e. increased transmission distances), and supports more than 70% increase in overall transmission distance when 80-km spans are used. As for optimization of the MSSI module itself, simulations show that reducing total insertion loss inside MSSI module is very important in achieving higher maximum Q performance as is using shorter HNLF and increasing the OPC conversion efficiency.

Acknowledgments

This research was conducted by the Australian Research Council Centre of Excellence for Ultrahigh bandwidth Devices for Optical Systems, CUDOS (Project number CE110001018). We should like to thank VPIphotonics.com for the use of VPItransmissionMakerTM.

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S. L. Jansen, D. van den Borne, C. Climent, M. Serbay, C. J. Weiske, H. Suche, P. M. Krummrich, S. Spalter, S. Calabro, N. Hecker-Denschlag, P. Leisching, W. Rosenkranz, W. Sohler, G. D. Khoe, T. Koonen, and H. de Waardt, “10,200 km 22×2×10 Gbit/s RZ-DQPSK dense WDM transmission without inline dispersion compensation through optical phase conjugation,” in Optical Fiber Communication Conference (OFC/NFOEC)(2005), p. PDP 28.

10.

V. Pechenkin and I. J. Fair, “Analysis of four-wave mixing suppression in fiber-optic OFDM transmission systems with an optical phase conjugation module,” Optical Communications and Networking, IEEE OSA Journal of 2(9), 701–710 (2010). [CrossRef]

11.

X. Liu, F. Buchali, and R. W. Tkach, “Improving the nonlinear tolerance of polarization-division-multiplexed CO-OFDM in long-haul fiber transmission,” J. Lightwave Technol. 27(16), 3632–3640 (2009). [CrossRef]

12.

L. B. Du, M. M. Morshed, and A. J. Lowery, “604-Gb/s coherent optical OFDM over 800 km of S-SMF with mid-span spectral inversion,” in Opto-Electronics and Communications Conference (Busan, 2012), pp. 3B2–3. [CrossRef]

13.

L. B. Du, M. M. Morshed, and A. J. Lowery, “Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation,” Opt. Express 20(18), 19921–19927 (2012). [CrossRef] [PubMed]

14.

M. Morshed, L. B. Du, B. Foo, M. D. Pelusi, and A. J. Lowery, “Optical phase conjugation for nonlinearity compensation of 1.21-Tb/s pol-mux coherent optical OFDM,” in 18th Opto-Electronics and Communications Conference(Kyoto, Japan, 2013), pp. PD 3–4.

15.

K. Solis-Trapala, T. Inoue, and S. Namiki, “Nearly-Ideal Optical Phase Conjugation based Nonlinear Compensation System,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. W3F.8. [CrossRef]

16.

I. Phillips, M. Tan, M. F. Stephens, M. McCarthy, E. Giacoumidis, S. Sygletos, P. Rosa, S. Fabbri, S. T. Le, T. Kanesan, S. K. Turitsyn, N. J. Doran, P. Harper, and A. D. Ellis, “Exceeding the Nonlinear-Shannon Limit using Raman Laser Based Amplification and Optical Phase Conjugation,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.1. [CrossRef]

17.

H. Hu, R. M. Jopson, A. Gnauck, M. Dinu, S. Chandrasekhar, X. Liu, C. Xie, M. Montoliu, S. Randel, and C. McKinstrie, “Fiber Nonlinearity Compensation of an 8-channel WDM PDM-QPSK Signal using Multiple Phase Conjugations,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.2. [CrossRef]

18.

I. Fatadin, D. Ives, and S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. 27(15), 3042–3049 (2009). [CrossRef]

19.

Y. Xingwen, W. Shieh, and T. Yan, “Phase estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. 19(12), 919–921 (2007). [CrossRef]

20.

P. Minzioni, I. Cristiani, V. Degiorgio, L. Marazzi, M. Martinelli, C. Langrock, and M. M. Fejer, “Experimental demonstration of nonlinearity and dispersion compensation in an embedded link by optical phase conjugation,” IEEE Photon. Technol. Lett. 18(9), 995–997 (2006). [CrossRef]

21.

M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). [CrossRef]

22.

T. Hasegawa, K. Inoue, and K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization diversity technique,” IEEE Photon. Technol. Lett. 5(8), 947–949 (1993). [CrossRef]

23.

M. D. Pelusi, “Fiber looped phase conjugation of polarization multiplexed signals for pre-compensation of fiber nonlinearity effect,” Opt. Express 21(18), 21423–21432 (2013). [CrossRef] [PubMed]

24.

M. E. Marhic, G. Kalogerakis, and L. G. Kazovsky, “Gain reciprocity in fibre optical parametric amplifiers,” Electron. Lett. 42(9), 519–520 (2006). [CrossRef]

25.

P. Wei-Ren, T. Tsuritani, and I. Morita, “Transmission of high-baud PDM-64QAM signals,” J. Lightwave Technol. 31(13), 2146–2162 (2013). [CrossRef]

26.

G. P. Agrawal, Fiber-optic Communication Systems (Wiley and Sons, 2010).

27.

M. Morshed, L. B. Du, and A. J. Lowery, “Performance limitation of coherent optical OFDM systems with non-ideal optical phase conjugation,” in IEEE Photonics Conference (IPC)(Burlingame, CA, 2012), pp. 394–395, TuU. [CrossRef]

28.

M. Morshed, L. B. Du, and A. J. Lowery, “Mid-span spectral inversion for coherent optical OFDM systems: Fundamental limits to performance,” J. Lightwave Technol. 31(1), 58–66 (2013). [CrossRef]

29.

S. Le Floch and P. Cambon, “Theoretical evaluation of the Brillouin threshold and the steady-state Brillouin equations in standard single-mode optical fibers,” J. Opt. Soc. Am. A 20(6), 1132–1137 (2003). [CrossRef] [PubMed]

30.

M. Morshed, A. J. Lowery, and L. B. Du, “Improving performance of optical phase conjugation by splitting the nonlinear element,” Opt. Express 21(4), 4567–4577 (2013). [CrossRef] [PubMed]

31.

V. Pechenkin and I. J. Fair, “On four-wave mixing suppression in dispersion-managed fiber-optic OFDM systems with an optical phase conjugation module,” J. Lightwave Technol. 29(11), 1678–1690 (2011). [CrossRef]

32.

M. Nazarathy, J. Khurgin, R. Weidenfeld, Y. Meiman, P. Cho, R. Noe, I. Shpantzer, and V. Karagodsky, “Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links,” Opt. Express 16(20), 15777–15810 (2008). [CrossRef] [PubMed]

33.

Y. Hong, A. J. Lowery, and E. Viterbo, “Sensitivity improvement and carrier power reduction in direct-detection optical OFDM systems by subcarrier pairing,” Opt. Express 20(2), 1635–1648 (2012). [CrossRef] [PubMed]

OCIS Codes
(060.4080) Fiber optics and optical communications : Modulation
(060.4510) Fiber optics and optical communications : Optical communications
(070.4340) Fourier optics and signal processing : Nonlinear optical signal processing
(070.5040) Fourier optics and signal processing : Phase conjugation
(190.4380) Nonlinear optics : Nonlinear optics, four-wave mixing

ToC Category:
Optical Communications

History
Original Manuscript: February 28, 2014
Revised Manuscript: April 9, 2014
Manuscript Accepted: April 12, 2014
Published: April 23, 2014

Citation
Monir Morshed, Liang B. Du, Benjamin Foo, Mark D. Pelusi, Bill Corcoran, and Arthur J. Lowery, "Experimental demonstrations of dual polarization CO-OFDM using mid-span spectral inversion for nonlinearity compensation," Opt. Express 22, 10455-10466 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-9-10455


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References

  1. W. Shieh, H. Bao, Y. Tang, “Coherent optical OFDM: theory and design,” Opt. Express 16(2), 841–859 (2008). [CrossRef] [PubMed]
  2. S. L. Jansen, I. Morita, T. C. Schenk, H. Tanaka, “Long-haul transmission of16×52.5 Gbits/s polarization-division-multiplexed OFDM enabled by MIMO processing (Invited),” J. Opt. Netw. 7(2), 173–182 (2008). [CrossRef]
  3. R.-J. Essiambre, “Exploring capacity limits of fibre-optic communication systems,” in 34th European Conference on Optical Communication, ECOC(2008), p. We.1.E.1. [CrossRef]
  4. A. D. Ellis, Z. Jian, D. Cotter, “Approaching the non-linear Shannon limit,” J. Lightwave Technol. 28(4), 423–433 (2010). [CrossRef]
  5. A. J. Lowery, “Fiber nonlinearity pre- and post-compensation for long-haul optical links using OFDM,” Opt. Express 15(20), 12965–12970 (2007). [CrossRef] [PubMed]
  6. S. L. Jansen, D. van den Borne, P. M. Krummrich, S. Spalter, G. D. Khoe, H. de Waardt, “Long-haul DWDM transmission systems employing optical phase conjugation,” IEEE J. Sel. Top. Quantum Electron. 12(4), 505–520 (2006). [CrossRef]
  7. P. Minzioni, F. Alberti, A. Schiffini, “Optimized link design for nonlinearity cancellation by optical phase conjugation,” IEEE Photon. Technol. Lett. 16(3), 813–815 (2004). [CrossRef]
  8. S. L. Jansen, D. van den Borne, B. Spinnler, S. Calabrò, H. Suche, P. M. Krummrich, W. Sohler, G. D. Khoe, H. de Waardt, “Optical phase conjugation for ultra long-haul phase-shift-keyed transmission,” J. Lightwave Technol. 24(1), 54–64 (2006). [CrossRef]
  9. S. L. Jansen, D. van den Borne, C. Climent, M. Serbay, C. J. Weiske, H. Suche, P. M. Krummrich, S. Spalter, S. Calabro, N. Hecker-Denschlag, P. Leisching, W. Rosenkranz, W. Sohler, G. D. Khoe, T. Koonen, H. de Waardt, “10,200 km 22×2×10 Gbit/s RZ-DQPSK dense WDM transmission without inline dispersion compensation through optical phase conjugation,” in Optical Fiber Communication Conference (OFC/NFOEC)(2005), p. PDP 28.
  10. V. Pechenkin, I. J. Fair, “Analysis of four-wave mixing suppression in fiber-optic OFDM transmission systems with an optical phase conjugation module,” Optical Communications and Networking, IEEE OSA Journal of 2(9), 701–710 (2010). [CrossRef]
  11. X. Liu, F. Buchali, R. W. Tkach, “Improving the nonlinear tolerance of polarization-division-multiplexed CO-OFDM in long-haul fiber transmission,” J. Lightwave Technol. 27(16), 3632–3640 (2009). [CrossRef]
  12. L. B. Du, M. M. Morshed, and A. J. Lowery, “604-Gb/s coherent optical OFDM over 800 km of S-SMF with mid-span spectral inversion,” in Opto-Electronics and Communications Conference (Busan, 2012), pp. 3B2–3. [CrossRef]
  13. L. B. Du, M. M. Morshed, A. J. Lowery, “Fiber nonlinearity compensation for OFDM super-channels using optical phase conjugation,” Opt. Express 20(18), 19921–19927 (2012). [CrossRef] [PubMed]
  14. M. Morshed, L. B. Du, B. Foo, M. D. Pelusi, A. J. Lowery, “Optical phase conjugation for nonlinearity compensation of 1.21-Tb/s pol-mux coherent optical OFDM,” in 18th Opto-Electronics and Communications Conference(Kyoto, Japan, 2013), pp. PD 3–4.
  15. K. Solis-Trapala, T. Inoue, and S. Namiki, “Nearly-Ideal Optical Phase Conjugation based Nonlinear Compensation System,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. W3F.8. [CrossRef]
  16. I. Phillips, M. Tan, M. F. Stephens, M. McCarthy, E. Giacoumidis, S. Sygletos, P. Rosa, S. Fabbri, S. T. Le, T. Kanesan, S. K. Turitsyn, N. J. Doran, P. Harper, and A. D. Ellis, “Exceeding the Nonlinear-Shannon Limit using Raman Laser Based Amplification and Optical Phase Conjugation,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.1. [CrossRef]
  17. H. Hu, R. M. Jopson, A. Gnauck, M. Dinu, S. Chandrasekhar, X. Liu, C. Xie, M. Montoliu, S. Randel, and C. McKinstrie, “Fiber Nonlinearity Compensation of an 8-channel WDM PDM-QPSK Signal using Multiple Phase Conjugations,” in Optical Fiber Communication Conference(Optical Society of America, San Francisco, California, 2014), p. M3C.2. [CrossRef]
  18. I. Fatadin, D. Ives, S. J. Savory, “Blind equalization and carrier phase recovery in a 16-QAM optical coherent system,” J. Lightwave Technol. 27(15), 3042–3049 (2009). [CrossRef]
  19. Y. Xingwen, W. Shieh, T. Yan, “Phase estimation for coherent optical OFDM,” IEEE Photon. Technol. Lett. 19(12), 919–921 (2007). [CrossRef]
  20. P. Minzioni, I. Cristiani, V. Degiorgio, L. Marazzi, M. Martinelli, C. Langrock, M. M. Fejer, “Experimental demonstration of nonlinearity and dispersion compensation in an embedded link by optical phase conjugation,” IEEE Photon. Technol. Lett. 18(9), 995–997 (2006). [CrossRef]
  21. M. Karlsson, “Four-wave mixing in fibers with randomly varying zero-dispersion wavelength,” J. Opt. Soc. Am. B 15(8), 2269–2275 (1998). [CrossRef]
  22. T. Hasegawa, K. Inoue, K. Oda, “Polarization independent frequency conversion by fiber four-wave mixing with a polarization diversity technique,” IEEE Photon. Technol. Lett. 5(8), 947–949 (1993). [CrossRef]
  23. M. D. Pelusi, “Fiber looped phase conjugation of polarization multiplexed signals for pre-compensation of fiber nonlinearity effect,” Opt. Express 21(18), 21423–21432 (2013). [CrossRef] [PubMed]
  24. M. E. Marhic, G. Kalogerakis, L. G. Kazovsky, “Gain reciprocity in fibre optical parametric amplifiers,” Electron. Lett. 42(9), 519–520 (2006). [CrossRef]
  25. P. Wei-Ren, T. Tsuritani, I. Morita, “Transmission of high-baud PDM-64QAM signals,” J. Lightwave Technol. 31(13), 2146–2162 (2013). [CrossRef]
  26. G. P. Agrawal, Fiber-optic Communication Systems (Wiley and Sons, 2010).
  27. M. Morshed, L. B. Du, A. J. Lowery, “Performance limitation of coherent optical OFDM systems with non-ideal optical phase conjugation,” in IEEE Photonics Conference (IPC)(Burlingame, CA, 2012), pp. 394–395, TuU. [CrossRef]
  28. M. Morshed, L. B. Du, A. J. Lowery, “Mid-span spectral inversion for coherent optical OFDM systems: Fundamental limits to performance,” J. Lightwave Technol. 31(1), 58–66 (2013). [CrossRef]
  29. S. Le Floch, P. Cambon, “Theoretical evaluation of the Brillouin threshold and the steady-state Brillouin equations in standard single-mode optical fibers,” J. Opt. Soc. Am. A 20(6), 1132–1137 (2003). [CrossRef] [PubMed]
  30. M. Morshed, A. J. Lowery, L. B. Du, “Improving performance of optical phase conjugation by splitting the nonlinear element,” Opt. Express 21(4), 4567–4577 (2013). [CrossRef] [PubMed]
  31. V. Pechenkin, I. J. Fair, “On four-wave mixing suppression in dispersion-managed fiber-optic OFDM systems with an optical phase conjugation module,” J. Lightwave Technol. 29(11), 1678–1690 (2011). [CrossRef]
  32. M. Nazarathy, J. Khurgin, R. Weidenfeld, Y. Meiman, P. Cho, R. Noe, I. Shpantzer, V. Karagodsky, “Phased-array cancellation of nonlinear FWM in coherent OFDM dispersive multi-span links,” Opt. Express 16(20), 15777–15810 (2008). [CrossRef] [PubMed]
  33. Y. Hong, A. J. Lowery, E. Viterbo, “Sensitivity improvement and carrier power reduction in direct-detection optical OFDM systems by subcarrier pairing,” Opt. Express 20(2), 1635–1648 (2012). [CrossRef] [PubMed]

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