1306-km 20x124.8-Gb/s PM-64QAM Transmission over PSCF with Net SEDP 11,300 (b·km)/s/Hz using 1.15 samp/symb DAC

We demonstrated the transmission of a Nyquist-WDM signal based on PM-64QAM modulation in an EDFA-only submarine configuration composed of 54.4 km-long fiber spans: 20 channels at 124.8Gb/s were propagated over 1306 km of low-loss pure-silica-core fiber (PSCF). Thanks to an aggressive digital spectral shaping, we achieved a raw spectral efficiency (SE) of 10.4 b/s/Hz, corresponding to 8.67 b/s/Hz net SE when considering a 20% FEC overhead. Transmitter DACs are operated at a record-low 1.15 samples/symbol, enabled by the insertion of advanced anti-alias filters. The achieved SE-times-distance product was 11,327 (b·km)/(s·Hz), the highest reported so far for PM-64QAM. Combining the experimental results with the performance predictions obtained using an analytical model of nonlinear propagation in uncompensated coherent optical systems (the so-called “GN-model”), we show that PM-64QAM is a realistic option for ultra-high capacity systems in the 1,000 km range, carrying up 40 Tb/s in the C-band. ©2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.1660) Coherent communications; (060.4080) Modulation; (060.2360) Fiber optics links and subsystems. References and links 1. J. Renaudier, G. Charlet, O. Bertran Pardo, H. Mardoyan, P. Tran, M. Salsi, and S. Bigo, “Experimental analysis of 100Gb/s coherent PDM-QPSK long-haul transmission under constraints of typical terrestrial networks,” in Proceedings of ECOC 2008, paper Th.2.A.3. 2. M. Salsi, H. Mardoyan, P. Tran, C. Koebele, G. Charlet, and S. Bigo, “155x100 Gbit/s coherent PDM-QPSK transmission over 7,200 km,” in Proceedings of ECOC 2009, paper PD2.5. 3. 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Gnauck, “112-Gb/s polarization-multiplexed 16-QAM on a 25-GHz WDM grid,” in Proceedings of ECOC 2008, paper Th.3.E.5. 10. A. H. Gnauck, P. J. Winzer, C. R. Doerr, and L. L. Buhl, “10 × 112-Gb/s PDM 16-QAM transmission over 630 km of fiber with 6.2-b/s/Hz spectral efficiency,” in Proceedings of OFC 2009, paper PDPB8. 11. S. Yamanaka, T. Kobayashi, A. Sano, H. Masuda, E. Yoshida, Y. Miyamoto, T. Nakagawa, M. Nagatani, and H. Nosaka, “11 x 171 Gb/s PDM 16-QAM transmission over 1440 km with a spectral efficiency of 6.4 b/s/Hz using high-speed DAC,” in Proceedings of ECOC 2010, paper We.8.C.1. 12. M.-F. Huang, Y.-K. Huang, E. Ip, Y. Shao, and T. Wang, “WDM transmission of 152-Gb/s polarization multiplexed RZ-16QAM signals with 25-GHz channel spacing over 15×80-km of SSMF,” in Proceedings of OFC 2011, paper OThX2. 13. J.-X. Cai, H. G. Batshon, H. Zhang, C. R. Davidson, Y. Sun, M. Mazurczyk, D. G. Foursa, A. Pilipetskii, G. Mohs, and N. S. 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Nagatani, “45.2Tb/s C-band WDM transmission over 240km using 538Gb/s PDM-64QAM single carrier FDM signal with digital pilot tone,” in Proceedings of ECOC 2011, paper Th.13.C.6. 17. J. Yu, Z. Dong, H.-C. Chien, Y. Shao, and N. Chi, “7-Tb/s (7×1.284 Tb/s/ch) signal transmission over 320 km using PDM-64QAM modulation,” IEEE Photon. Technol. Lett. 24(4), 264–266 (2012). 18. A. Sano, T. Kobayashi, S. Yamanaka, A. Matsuura, H. Kawakami, Y. Miyamoto, K. Ishihara, and H. Masuda, “102.3-Tb/s (224 x 548-Gb/s) Cand extended L-band All-Raman transmission over 240 km using PDM64QAM single carrier FDM with digital pilot tone,” in Proceedings of OFC 2012, paper PDP5C.3. 19. O. Bertran-Pardo, J. Renaudier, H. Mardoyan, P. Tran, R. Rios-Muller, A. Konczykowska, J.-Y. Dupuy, F. Jorge, M. Riet, B. Duval, J. Godin, S. Randel, G. Charlet, and S. Bigo, “Transmission of 50-GHz-spaced singlecarrier channels at 516Gb/s over 600km,” in Proceedings of OFC 2013, paper OTh4E.2. 20. A. Nespola, S. Straullu, G. Bosco, A. Carena, J. Yanchao, P. Poggiolini, F. Forghieri, Y. Yamamoto, M. Hirano, T. Sasaki, J. Bauwelinck, and K. Verheyen, “1306-km 20x124.8-Gb/s PM-64QAM transmission over PSCF with Net SEDP 11,300 (b·km)/s/Hz using 1.15 samp/symb DAC,” in Proceedings of ECOC 2013, paper Th.2.D.1. 21. G. Bosco, V. Curri, A. Carena, P. Poggiolini, and F. Forghieri, “On the performance of nyquist-WDM terabit superchannels based on PM-BPSK, PM-QPSK, PM-8QAM or PM-16QAM Subcarriers,” J. Lightwave Technol. 29(1), 53–61 (2011). 22. R. Cigliutti, A. Nespola, D. Zeolla, G. Bosco, A. Carena, V. Curri, F. Forghieri, Y. Yamamoto, T. Sasaki, and P. Poggiolini, “16 × 125 Gb/s quasi-nyquist DAC-generated PM-16QAM transmission over 3590 km of PSCF,” IEEE Photon. Technol. Lett. 24(23), 2143–2146 (2012). 23. R. Schmogrow, M. Meyer, P. C. Schindler, A. Josten, S. Ben-Ezra, C. Koos, W. Freude, and J. Leuthold, “252 Gbit/s Real-Time Nyquist Pulse Generation by Reducing the Oversampling Factor to 1.33,” in Proceedings of OFC 2013, paper OTu2I.1. 24. A. Sano, T. Kobayashi, A. Matsuura, S. Yamamoto, S. Yamanaka, E. Yoshida, Y. Miyamoto, M. Matsui, M. Mizoguchi, and T. Mizuno, ”100 x 120-Gb/s PDM 64-QAM transmission over 160 km using linewidth-tolerant pilotless digital coherent detection,” in Proceedings of OFC 2010, paper PD2.4. 25. Y. Gao, A. P. T. Lau, C. Lu, J. Wu, Y. Li, K. Xu, W. Li, and J. Lin, “Low-complexity two-stage carrier phase estimation for 16-QAM systems using QPSK partitioning and maximum likelihood detection,” in Proceedings of OFC 2011, paper OMJ6. 26. X. Zhou, L. E. Nelson, P. Magill, R. Isaac, B. Zhu, D. W. Peckham, P. I. Borel, and K. Carlson, “High spectral efciency 400 Gb/s transmission using PDM time-domain hybrid 32–64 QAM and training-assisted carrier recovery,” J. Lightwave Technol. 31(7), 999–1005 (2013). 27. A. Carena, V. Curri, G. Bosco, P. Poggiolini, and F. Forghieri, “Modeling of the impact of nonlinear propagation effects in uncompensated optical coherent transmission links,” J. Lightwave Technol. 30(10), 1524–1539 (2012). 28. G. Bosco, R. Cigliutti, A. Nespola, A. Carena, V. Curri, F. Forghieri, Y. Yamamoto, T. Sasaki, J. Yanchao, and P. Poggiolini, “Experimental investigation of nonlinear interference accumulation in uncompensated links,” IEEE Photon. Technol. Lett. 24(14), 1230–1232 (2012). 29. E. Torrengo, R. Cigliutti, G. Bosco, A. Carena, V. Curri, P. Poggiolini, A. Nespola, D. Zeolla, and F. Forghieri, “Experimental validation of an analytical model for nonlinear propagation in uncompensated optical links,” in Proceedings of ECOC 2011, paper We.7.B.2. 30. A. J. Stark, Y.-T. Hsueh, T. F. Detwiler, M. M. Filer, S. Tibuleac, and S. E. Ralph, “System performance prediction with the Gaussian noise model in 100G PDM-QPSK coherent optical networks,” J. Lightwave Technol. 31(21), 3352–3360 (2013). 31. P. Poggiolini, “The GN model of non-linear propagation in uncompensated coherent optical systems,” J. Lightwave Technol. 30(24), 3857–3879 (2012). #199535 $15.00 USD Received 15 Oct 2013; accepted 27 Dec 2013; published 21 Jan 2014 (C) 2014 OSA 13 January 2014 | Vol. 22, No. 1 | DOI:10.1364/OE.22.001796 | OPTICS EXPRESS 1797


reach.To this
end, many of the recently published papers exploit high-order QAM modulation formats and resort the Nyquist WDM technique as an aggressive spectrum shaping solution to simultaneously reduce the bandwidth occupation and allow tighter channel spacing.

Recent improvements in the net spectral efficiency are shown in Fig. 1, where the main record breaking experiments of the last five years, specifically based on coherent detection and single-carrier (non-OFDM) QAM modulation, have been considered [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].Trends in this figure show that any step towards high constellation order, from PM-QPSK up to PM-64QAM, generates an increase of the net SE up to 10b/s/Hz.Such progresses does not come for free, considering that the signal-to-noise ratio required for this higher cardinality format, combined with the impairments due to fiber nonlinearity, limits the transmission distance [21].Fig. 2 reports the distance vs. SE for the same record experiments of Fig. 1: contour lines represent constant SE-times-distance product (SEDP) levels.While PM-QPSK outperforms all other formats with regard to SEDP and it is the clear choice for transoceanic distance over 10,000 km, PM-64QAM together with PM-16QAM can play an important role in future flexible optical networks.In this scenario, where the system throughput may be adapted depending on the reach, PM-64QAM will be able to cover 1,000 km carrying up to 40 Tb/s over the C-band.High-order format generation and N-WDM spectral shaping can both be obtained with optical techniques [4,8], but current consensus strongly favors the use of transmitter (Tx) DSP with DACs.This powerful technology allows to generate arbitrary electrical signals to properly drive the optical modulators and obtain simultaneously an accurate high-order QAM together with the wanted N-WDM spectral shaping.The key component needed in this approach is a digital-to-analog converter (DAC) device, which is characterized by two main parameters: the sampling speed S DAC and the number of resolution bits N DAC .Typically, if S DAC increases, N DAC decreases.The higher is the order of the modulation format, the higher is the required value of N DAC , while S DAC limits the achievable symbol rate R s =S DAC /SpS, where SpS is the number of samples per symbol (oversampling factor).The achievable R s can be clearly increased by decreasing the oversampling factor: research efforts have been made to decrease the value of SpS without incurring in aliasing-like penalties.In [22] we reported a 1.5 SpS DAC-supported N-WDM PM-16QAM experiment delivering a raw SE of 7.81 b/s/Hz (net 6.48 b/s/Hz) over 3,590 km of PSCF in a submarine-system configuration.In this work we aimed at further drastically reducing the required DAC rate.At the same time, we wanted to increase the SE and constellation order to PM-64QAM, proving that this set-up could still achieve thousand-km reach in a submarine-system configuration, with very high SE.The DAC rate was decreased down to 1.15 SpS, a record-low for DSP-DAC generated N-WDM.The lowest previously reported rate was 1.33 SpS, obtained in a 100-km PM-64QAM singlechannel transmission at 252 Gb/s [23].Note than an even lower rate of 1 sample/symbol was reported in [24] but at such rate the DAC can intrinsically only generate conventional NRZ pulses, so that cascaded narrow optical filtering was needed in [24] to obtain N-WDM at the Tx output.In addition we achieved 20-channel N-WDM transmission at 124.8 Gb/s per channel (100 Gb/s net), over 1306 km (24x54.4km) at a raw SE of 10.4 b/s/Hz.This is the second-highest raw SE for PM-64QAM next to [18], which achieved 10.96 b/s/Hz by using a more complex transmitter structure that resorted to combining 8 lower-bit-rate PM-64QAM N-WDM subcarriers (at 68.5 Gb/s) into a single super-channel, with digital-pilot-tone assisted demodulation.Considering a FEC overhead of 20% and thus a net SE of 8.67 b/s/Hz, the resulting net SE-times-distance product (SEDP) was 11,327 (b•km)/(s•Hz).To the best of our knowledge, this is the highest SEDP reported so far for a PM-64QAM modulation.Gbaud spaced 12 GHz.


Experimental setup

A schematic view of the transmitter setup is shown in Fig. 3.An array of 20 DFB lasers, spaced 12 GHz, was divided into odd and even carriers and separately fed to two distinct nested Mach-Zehnder modulators (NMZM).A third NMZM, fed by an external cavity laser (ECL), with 100 kHz linewidth, was used to generate the channel under test: the corresponding DFB in the array of lasers is turned off.Each of NMZMs was driven by a pair of uncorrelated signals tak

from DAC output.DA
was a Tektronix 7122B with an analog bandwidth of 9.6 GHz, and with ten nominal resolution bits but lower Effective Number of Bit (ENOB).With reference to Fig. 3, two independent DAC output signals, called I p and Q p , were used to drive the NMZM for the channel under test.The four driving signals for the other two NMZMs were obtained by splitting each of the two logical complementary outputs of the DAC (I n and Q n ).Electrical delay lines were inserted in these paths for de-correlation of interfering channel patterns: EDL 1 and EDL 2 were respectively 4.8 and 4.4 ns).Each one of the in-phase and quadrature signals was created by mapping three independent (2 15 -1) PRBSs into a single 8-level PAM NRZ signal at 10.4 Gbaud, which was then digitally filtered to give it a square-root raised-cosine spectrum with electrical bandwidth equal to half the symbol rate and roll-off 0.05.Note that a 4 dB high-frequency digital pre-emphasis was also applied, to partially compensate for in-band electronics attenuation.

The filtered 8-level signals were then generated by the DACs, which operated at 11.96 GS/s, corresponding to 1.15 SpS.As a result, an alias signal replica was also generated, centered at 11.96 GHz (Fig. 4a).It was partially filtered out by the low-pass frequency response of the DAC, but, in order to further suppress it, a specifically-designed anti-alias (AA) filter, with steep cut-off, was interposed between the DAC and the modulators.The AA filter frequency response is shown in Fig 4a with a red dashed line.The spectrum of the NMZM driving signal, cleaned out of the signal replica, is reported in Fig. 4b.Not shown for clarity, the AA filter response goes back up to -20dB at about 15 GHz, but this causes no adverse effect because signal components at such frequencies are very low.

Preceding the AA filters, broadband RF amplifiers were used to obtain a peak to peak amplitude voltage of about 25% of the modulator V π , so as to operate them in linearity.All the modulated optical channels, so far single-polarization, were then coupled together and sent into a commercial PM emulator with a 10 ns polarization decorrelation delay.The transmission link, depicted in Fig. 5, consists of a re-circulating loop with two identical 54.44 km span of uncompensated PSCF.At 1550 n , the average fiber loss was 0.161 dB/km, chromatic dispersion 20.7 ps/nm/km and effective area 150 µm 2 .The total loss of each span was 9.7 dB, resulting from a combination of fiber loss and of the splicing loss between standard single-mode fiber and the large-effective-area PSCF.The loop made use of EDFAonly amplification.It included a spectrally-resolved gain equalizer (GEQ) and a loopsynchronous polarization scrambler (PS) to, respectively, compensate for the EDFA gain-tilt and ripples and effectively average the impact of polarization effects.The EDFA noise figure was 4.5 dB.At the receiver (Fig. 5), the WDM comb was sent into a tunable optical filter (TOF) with 30 GHz bandwidth to limit to total amount of power entering reaching photo-detectors.The filtered signal was subsequently fed to a standard coherent receiver, where the signal was mixed with the local oscillator, obtained from a tunable ECL of linewidth 100-kHz, different from the one used at the Tx.The four electrical outputs of the receiver front-end were digitized using a 50 GS/s real-time oscilloscope (Tektronix DPO71604).Finally, offline DSP was used to demodulate the resulting samples.

The DSP consisted of the following stages: a down-converting stage that lowers the sample-rate to 2 SpS; a chromatic dispersion compensation stage implemented using a pair of complex FIR filters; a frequency offset compensator; a first 2x2 complex MIMO equalizer with 21 taps that adjusts its coefficient through a multi-modulus CMA algorithm (initialized with a training sequence and then switched to blind radius-directed operation); a Maximum-Likelihood carrier phase-recovery (CPE) stage [25] and a inal decision-directed LMS 4x4 equalizer with 51 taps.


Results

The back-to-back performance in terms of BER vs. OSNR (defined over a 0.1-nm bandwidth), is shown in Fig. 6.We assumed to employ a soft-decision LDPC convolutional code with 20% overhead and layered decoding algorithm [26], corresponding to a FEC BER threshold equal to 2.7•10 -2 .The penalty between single-channel and WDM at FEC threshold is only 0.4 dB, confirming that DAC-enabled spectral shaping and the use of steep electrical anti-alias filters effectively curtails inter-channel linea

crosstal
, allowing an extremely tight channel spacing.The penalty in WDM conditions with respect to theory at FEC threshold amounts to 3.6 dB.It is due to a combination of electrical and optical component nonidealities, with main contributors the transmitter RF broadband amplifiers noise and electrical circuitry reflections due to impedance mismatch.In order to quantify the amount of penalty due to electrical components only, we tested the system in an electrical back-to-back configuration, connecting the AA filter output directly to the real-time oscilloscope.The measured BER was found to be equal to 1•10 -4 while the corresponding electrical SNR, verified by measuring EVM on the scattering diagram, was equal to 24.4 dB.Taking analytically into account such contribution, we derived the actual theoretical baseline for our system (shown in Fig. 6 as a green solid line) and we concluded that, at FEC threshold, the contribution of the electrical non idealities to the total penalty is 1 dB.Maximum reach for all the 20 channels at optimal power level of -6.5dBm (blue squares).Distances are quantized over a discrete number of spans of length equal to 54.44 km.

Fig. 8. BER for all 20 channels at 24 spans (1306 Km).Inset: signal constellations (both polarizations) for channel #11.

WDM transmission experiments were then performed, carrying out a first set of measurements in order to optimize the launch power.In Fig. 7 we report the maximum reachable distance for the center channel (#11) at FEC threshold as a function of the transmitted power levels (red dots).At the optimum launch power, that was found to be -6.5 dBm per channel, channel #11 showed a maximum reach of 28 spans for a total of 1523 km.Having defined the optimal power per channel, we carried out the measurement of the maximum reach for all channels, shown in Fig. 7 as blue squares.All channels have quite similar performance with a max reached distance that goes form a minimum value of 1306 km (24 spans) to a max of 1632 km (30 spans).The differences observed are due to a residual unbalancing in channel power in the order of ±0.5 dB that results in different OSNR in each channel.

Fig. 8 shows the BER values measured for all 20 WDM channels at optimal power after 24 spans (1306 km): at this distance the BERs of all channels fall below the FEC threshold.In the same figure we also reported, in the inset, the signal constellations after transmission over 1306 km for the center channel (#11) and for both polarizations.


GN-model prediction

The experiment carried out was limited to 20 WDM channels, due to lack of DFB sources in the lab inventory: in this section we want to predict the potential performances of PM-64QAM when loading the en

re C-band using the
N-model for nonlinear propagation in uncompensated coherent optical systems [27], that has been recently experimentally validated by independent groups [14,[28][29][30].We first identified the parameters for this specific experimental setup and verified that the model is able to predict performances for the 20 channels scenario.In Fig. 9 we report the GN-model prediction for the 20 WDM case together with measurements taken for the center channel, whose performance is close to the average of all the WDM comb (see Figs. 7 and 8).As expected, we found a very good agreement between GN-model and measurements.

We then used the GN-model to predict the system performance under different conditions.First, we estimated the maximum reach when the full C-band is loaded: considering a channel spacing of 12 GHz, it means 400 channels for a otal net system throughput of 40 Tb/s (400 x 100 Gb/s).Black solid line in Fig. 9 shows that a distance of 1306 km could still be reached with an optimal power of -7.4 dBm.

This result was obtained assuming the same Tx/Rx performance of the experimental setup, that was affected by 3.6 dB of penalty with respect to theory (see Fig. 6).An optimized transceiver design together with device integration ill surely be able to deliver an improved back-to-back performance.Assuming a 1-dB OSNR improvement with respect to our Tx/Rx pair, the maximum reach would increase to 1632 Km (30 spans), as shown as the green solid line in Fig. 9.Such results clearly show that PM-64QAM, even considering adequate system margin in actual field deployment, can reach over 1,000 km offering a 40 Tb/s throughput in EDFA-only configurations based solely on C-band.


Conclusions

We have demonstrated the transmission of 20x124.8Gb/s Nyquist-WDM channels, based on PM-64QAM modulation with 12 GHz channel spacing, over 1306 km of PSCF, in an EDFA-only submarine-like set-up.All channels were below

C threshold
fter propagating over 24 spans of PSCF fiber, each 54.4 km long.The net spectral efficiency was 8.67 b/s/Hz and the spectral efficiency distance product was a record 11,327 (b•km)/(s•Hz) for PM-64QAM channels with throughput higher than 100 Gb/s.

Digital spectral shaping at the transmitter and the use of suitable anti-alias filters had a strong impact on the system performance: despite the DAC sampling speed was a record-low 1.15 SpS, the system was able to operate at very narrow channel spacing with negligible linear crosstalk between channels.

Our back-to-back results indicate that there is still margin for sensitivity improvement, which could be likely achieved by integrated transmitter electronics design.GN-model prediction clearly indicate with a proven degree of reliabi ity that Nyquist-WDM PM-64QAM is a realistic option for ultra-high capacity transmission up to a total throughput of 40 Tb/s in the C-band for 1,000 km range, in a submarine configuration based on EDFA only.

Fig. 1 .
1
Fig. 1.Net spectral efficiency evolution of main record transmission experiments based on coherent detection published in the last five years.


Fig. 2 .
2
Fig. 2. System reach vs. throughput trade-off in main record transmi