Mo.3.D.5.pdf
Fiber Nonlinearity Compensation by Digital Backpropagation of an Entire 1.2-Tb/s Superchannel Using a Full-Field Spectrally-Sliced Receiver Nicolas K. Fontaine(1) , Xiang Liu(1) , S. Chandrasekhar(1) , Roland Ryf(1) , Sebastian Randel(1) , Peter Winzer(1) , R. Delbue(2) , P. Pupalaikis(2) , A. Sureka(2) (2)
(1) Bell Laboratories/Alcatel-Lucent, 791 Holmdel Rd., Holmdel, NJ 07733, USA LeCroy Corporation, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977, USA
[email protected]
Abstract We receive the full optical field of a 176-GHz wide, 1.2-Tb/s PDM-16QAM superchannel after 960-km TWRS using a spectrally-sliced coherent receiver. Simultaneous compensation of SPM and XPM with digital backpropagation enables 1 dB of Q-factor improvement. Introduction An optical superchannel consists of N closely spaced carriers which are treated as a single channel propagating through a link 1,2 . The carriers are spaced so closely, that optical demultiplexing of individual carriers is difficult, and that inter-carrier nonlinear interactions can be strong. Due to electronic limitations, the superchannel bandwidth is often much larger than the available coherent receiver bandwidth, which forces a separate measurement of each carrier using multiple phase-incoherent optical local oscillators (LOs). In this scheme, the phase-coherence among the carriers is lost upon detection, which limits the amount of electronic impairment mitigation, especially due to inter-carrier fiber nonlinear interactions. In wavelength-division multiplexed (WDM) systems, the inter-channel nonlinearities substantially reduce capacity 3,4 , and it has been predicted that inter-channel nonlinearity compensation via digital backpropagation (DBP) 5 can improve the signal Q-factor 3,6 . However, a recent experimental demonstration showed negligible improvement (∼0.2 dB) for 8ary quadrature amplitude modulation (QAM) 28GBaud WDM channels 7 . Superchannels, due to their close carrier spacing, are expected to be more susceptible to inter-carrier nonlinearities. Also, unlike WDM channels which are often routed and switched, the carriers within the superchannels are routed together enabling compensation of inter-carrier nonlinearities using DBP from the receiver to the transmitter. Spectrally-sliced coherent receivers can digitize the full optical field 8–10 well beyond electronic limitations and enable: 1) measurement of the entire superchannel and 2) compensation of impairments affecting the entire superchannel. Recent demonstrations have shown over ∼200GHz receiver bandwidths 9 . In this paper, we digitize the full optical field of a 1.2-Tb/s super-
channel with a 176-GHz spectrally-sliced coherent receiver and use DBP to mitigate nonlinear distortions resulting from self-phase modulation (SPM), inter-carrier cross-phase modulation (XPM), and possibly four-wave mixing (FWM). DBP of the entire superchannel handles XPM, SPM, and FWM simultaneously. Superchannel Generation and SpectrallySliced Receiver Fig. 1(a) shows the superchannel transmitter which comprises two polarization multiplexed nested Mach-Zehnder modulators (MZMs), also known as vector modulators, driven by a four-channel 60-GS/s digital-to-analog convertor ¯ outputs. (DAC) with data (D) and data-bar (D) Five independent external cavity lasers (ECLs) with linewidths of ∼100 kHz provide the five carriers. The three odd carriers (λ1 , λ3 , and λ5 ) are sent through the upper modulator which is driven by the D outputs and the two even carriers (λ2 and λ4 ) are sent to the lower modulator ¯ outputs. To generate which is driven by the D the drive signal waveforms, pseudo-random bit sequences (PRBS) of length 215 -1 are first encoded and mapped to PDM-16QAM symbols. To achieve close channel spacing, root raisedcosine (RRC) pulses with a roll-off factor of 0.1 were used to confine the optical spectra of the generated signals. After the modulation, the odd and even groups are combined with sufficient delay decorrelation. Fig. 1(b) depicts the spectrally-sliced receiver which measures the superchannel in many spectral slices against each LO line of a coherent optical frequency comb (OFC). These LO lines are precisely spaced by 34 GHz and have a constant phase and amplitude relationship. For simplicity, but without loss of generality, the LO comb spacing matches the superchannel subcarrier spacing. The OFC generator uses
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Fig. 1: (a) 5-carrier superchannel transmitter with PDM-16QAM modulation. (b) 5-slice spectrally-sliced coherent receiver. (c) Photograph of the polarization-diversity coherent receivers and LeCroy Labmaster 9 Zi.
a MZM to modulate an ECL with a 34-GHz tone to produce five lines. The LO lines are equalized using a wavelength selective switch (WSS), amplified to +28 dBm, and separated using a bulk-optics wavelength demultiplexer consisting of a fiber array with microlenses, a 180mm focal length collimating lens, and a 1200line/mm diffraction grating. Each demultiplexed line is sent to the LO port of a polarization diversified 90◦ optical hybrid. The superchannel is split into five copies which are directed to the signal ports of each optical hybrid. The hybrid outputs are input to balanced photodetectors whose outputs are recorded by 20 (4×5) synchronized 40-GS/s analog-to-digital converters (ADCs) (LeCroy LabMaster 9Zi, see Fig. 1(c)). These phase-diversified coherent receivers also act as optical filters with bandwidths of 40 GHz centered around each LO. Fig. 2(a) shows the waveform reconstruction DSP for a single polarization, which reproduces the full optical field in front of the 1×5 splitter. First, the digitized I and Q signals for each slice
are combined in quadrature. The resulting spectral slices are distorted due to the non-ideal frequency responses of the splitter, optical hybrid, photodiode, and ADCs. These distortions are characterized a-priori and a frequency domain correction, Hi (ω), is applied to each slice. Phase differences between the slices due to slow fiber path length variations are calculated by comparing the energy in the 6-GHz wide overlapping spectrum between adjacent spectral slices (real signals or ASE noise), and removed. The slices are shifted to their correct spectral location and summed together reproducing the waveform at the input to the spectrally-sliced receiver. The second polarization is processed similarly and the phase between the two polarizations is known since the polarization multiplexed hybrid is an integrated device. The superchannel is fully reconstructed, without a-priori knowledge of the input waveform, and is then sent to DBP DSP, followed by channel equalization, frequency and phase recovery, and bit-error counting. Fig. 2(b) shows the reconstructed spectrum
Fig. 2: (a) Waveform reconstruction DSP. (b) Reconstructed spectrum in front of the splitter.
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Fig. 3: Results of the nonlinear compensation of entire superchannel at (a) 6- and (b) 8-dBm launch powers. (c) Q2 factor vs. signal launch power for EDC only, single-carrier DBP, and entire superchannel DBP. (d) Constellation diagrams at 6-dBm launch power.
of the superchannel, with a fully coherent representation of the optical field across the entire superchannel bandwidth. The shaded regions indicate the 40-GHz bandwidth of each slice which also visualizes the 6-GHz overlap regions. Results The superchannel is transmitted through twelve 80-km spans of Truewave Reduced Slope (TWRS) fiber with D=4.65 ps/nm/km. After each span, the signal is amplified using an Erbium doped fiber amplifier (EDFA). After every fourth span, the out-of-band noise is removed with a WSS (see Fig. 1). The back-to-back transmitter obtains a bit-error ratio (BER) of 2 × 10−3 across the five carriers. Fig. 3(a,b) show the optimization of the DBP for 6-dBm and 8-dBm launch powers, respectively. At each power setting, we optimize the step size and the ratio between the compensating nonlinear phase shift (Φcomp ) and the overall nonlinear phase shift experienced by the signal (ΦNL ). At 6-dBm launch power, the optimal compensation parameters are 30 steps per span, with 60% ( |Φcomp /ΦNL |) of the nonlinear phaseshift compensated. At 8-dBm launch power, we used 30 steps per span, with 80% of the nonlinear phase-shift compensated. Fig. 3(c) shows the BER results of the central channel after 960-km transmission and expressed in terms of Q2 . The best Q2 obtained using electronic dispersion compensation is 6.4 dB at a span launch power of 4 dBm. Note that the typical Q2 threshold of soft-decision forwarderror correction (FEC) is ∼6.25 dB which corresponds to a BER of 2×10−2 11 . Next, we compensate for just the SPM of the central-carrier by digitally filtering out the four outer carriers
from the measured superchannel prior to the DBP [see inset of Fig. 3(c)]. The Q2 increases to 6.7 dB at a launch power of 5 dBm. Next, we perform DBP on the entire super channel (five-carriers) which treats SPM, XPM, and other nonlinearities simultaneously. The Q2 improves a further 0.7 dB to 7.4 dB at a launch power of 6 dBm. Additionally, at 8 dBm the Q2 improvement is 2.8 dB as compared to the EDC-only case at the same launch power. Fig. 3(d) shows constellation diagrams for the EDC, single-carrier DBP, and superchannel DBP compensation at 6-dBm launch power (a 1.46dB improvement with DBP). Finally, with full-field DBP all carriers received an average of 1.27-dB Q2 improvement at 6-dBm launch power. Conclusions We have used a spectrally-sliced receiver to receive the full optical field of a 1.2-Tb/s PDM16QAM superchannel occupying 176-GHz optical bandwidth. Digitizing the full optical field coherently across the superchannel bandwidth enabled mitigation of the nonlinear distortions that affect the entire superchannel including SPM, XPM, and FWM, which improved the optimal Q2 by 1 dB. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
S. Chandrasekhar et al., in ECOC, paper PD2.6 (2009). Y.-K. Huang et al., in OFC, paper OM3H.4 (2012). E. F. Mateo et al., Optics Express 18, 15,144 (2010). D. Rafique et al., Optics Express 19, 16,919 (2011). E. Ip et al., IEEE JLT 26, 3416 (2008). D. Rafique et al., Optics Express 21, 4174 (2013). E. Ip et al., in OFC, paper OM3A.6 (2012). C. Zhang et al., in ECOC, paper Th.10.A.5 (2010). N. K. Fontaine et al., in ECOC, paper Th.3.A.1 (2012). N. K. Fontaine et al., Nature Photonics 4, 248 (2010). L. Schmalen et al., IEEE PTL 24, 40 (2012).
