We demonstrate bidirectional transmission over 450 km of newly-developed dual-ring structured 12-core fiber with large effective area and low crosstalk. Inter-core crosstalk is suppressed by employing propagation-direction interleaving, and 409-Tb/s capacities are achieved for both directions.
©2013 Optical Society of America
Space division multiplexing (SDM) is a promising approach to increasing link capacity [1–7]. Recent transmission experiments demonstrated Pb/s-class capacity [5, 7] by using multi-core fibers (MCFs) and high-order quadrature amplitude modulation (QAM) format. However, the maximum distance of Pb/s-class MCF transmission was limited to 52 km , and thus increasing the transmission distance of such ultra-high capacity MCF transmission systems is still a challenge. In order to extend the attainable reach in Pb/s-class MCF transmission, reduction of nonlinear impairment and inter-core crosstalk (XT) is essential. Increasing the effective area (Aeff) of each core, however, generally results in increase in XT under the limitation on cladding diameter (Dc) extension due to mechanical factors.
In this paper, we demonstrate bidirectional transmission over newly-developed large Aeff and low XT 12-core MCF with total capacities of 409 Tb/s for both directions. Alternating the propagation direction between adjacent cores successfully reduces XT and achieves polarization-division multiplexed- (PDM-)32QAM signal transmission over 450 km, the longest in over-300 Tb/s transmission experiments.
2. 12-core fiber and propagation-direction interleaving
Figure 1 shows XT and normalized Aeff ( = N Aeff (MCF)/Aeff (SMF), where N is the number of cores) for recent MCF experiments. In our previous work , we employed a one-ring structure (ORS) 12-core fiber, which effectively suppressed XT because of the small number of adjacent cores. Aeff was 80 μm2 with Dc of 225 μm. In this experiment, we employed a novel dual-ring structure (DRS, Fig. 2) to reduce nonlinearity and XT. Compared with ORS, DRS can offer a larger core pitch for the same Dc. Consequently we achieved 30% extension in Aeff (105.8 μm2) and 7.7-dB XT suppression (Fig. 1) with only small Dc increase (230 μm).
In order to reduce XT further, the propagation-direction interleaving (PDI) is an attractive scheme. In this scheme, eastbound and westbound signals are accommodated in a single MCF, where neighboring cores are assigned to opposite directions, as shown in Fig. 2. This effectively suppresses the XT from the closest cores, the dominant component of total XT. We reported crosstalk suppression by using PDI in the signal propagation through a multi-core erbium-doped fiber amplifier (MC-EDFA) , and recently XT performance of this scheme has been reported in long-distance MCF transmission . The direction assignment of our DRS MCF is shown in Fig. 2; the number of the closest cores with the same direction can be reduced from 4 to 1, and thus the XT suppression can be expected by employing PDI. As in the case of interleaved bidirectional transmission over single-core fiber [10, 11], PDI is attractive for extending the attainable distance while maintaining high spatial spectral efficiency.
3. Experimental setup
Figure 3 shows the experimental setup. At the transmitter (Tx), 406 CW carriers (1526.83-1562.44, and 1567.95-1615.48 nm) with 25-GHz spacing were separately multiplexed into even-/odd-channel signals, and modulated by IQ-modulators (IQM) to create 20.16-Gbaud Nyquist-pulse-shaped 32QAM signals. We developed a 40.32-GS/s QAM signal generator  with high-speed 6-bit DACs based on InP HBT technology , where two DACs were operated at 2 × oversampling, and driven by 12-lane 40.32-Gb/s digital signals generated by an FPGA-based 48-lane multi-channel digital signal generator and a 48:12 multiplexer. The pattern length was 211-1. A roll-off factor of 0.1 was employed with pre-equalization of Tx frequency response. The even/odd signals were spectrum-shaped by 50G/25G interleaving filters (ILFs), multiplexed with 25-GHz spacing, and polarization-multiplexed by a PDM emulator with a 25-nsec delay. We used a tunable external-cavity laser (ECL) with ~60-kHz linewidth for the test channel; the remaining lasers were DFB lasers (linewidth ~2 MHz).
The transmission line consisted of a 50-km MCF with 12-fold re-circulating loops operated synchronously. 406 wavelength-division multiplexed (WDM) channel signals were divided into 4 copies, each of which was gated by 4 load switches, divided into 3 copies, and loaded into each re-circulating loop. The signal into each loop was sufficiently de-correlated by changing the delay (D1-D12) before the loop. The signal at the input of each loop was first amplified by an EDFA that consisted of C- and extended L- (L+-)band EDFAs with parallel configuration; we used a C-band MC-EDFA and L+-band discrete EDFA. MC-EDFA configuration, shown in Fig. 4(a), utilized the outer cores of a dual 7-core EDF, where PDI is employed to suppress XT in the MC-EDF to below −54 dB . Each core was pumped bi-directionally with a 980-nm band forward pumping and a 1480-nm band backward pumping. A gain flattening filter (GFF) was inserted at the output of each core. Fiber-type fan-in/fan-out (FI/FO) devices were used to splice the dual 7-core EDF and the pump/signal WDM couplers. The gain and noise figure (NF) of the MC-EDFA is shown in Fig. 4(b). The core-averaged gain of 13 dB and NF of 5.6 dB were achieved across the entire C-band. The gain and NF of L+-band discrete EDFA were around 13 dB and 6 dB, respectively . After amplification, the signal was fed into each core of the MCF through a FI/FO device, where MCF and small diameter fibers were aligned with two ferrules and a split sleeve, as shown in Fig. 5. The FI/FO device achieved physical contact connection with an insertion loss of 0.3-1.1 dB and a return loss of more than 55 dB. The average signal power at FI input was −5 dBm/ch. The average loss and chromatic dispersion coefficients were 0.186 dB/km and 19.6 ps/nm/km at 1550 nm, respectively. After the propagation through each core, the signal was split by FO. We also utilized backward-pumped distributed Raman amplification (DRA)  with pumping wavelength from 1422 to 1505 nm; we used 6 pumping wavelengths for each core with average total pumping power of 1.3 W (typical pumping wavelengths were 1422/1435/1450/1460/1480/1505 nm)The span loss including FI/FO was 10.1-12.1 dB at 1550 nm, and the DRA on-off gains of all cores were about 6-10 dB. In addition to PDI, we utilized the core-to-core signal rotation scheme  to equalize the transmission performance caused by the small difference in attenuation and XT values between the cores of the inner- and outer-rings. In this experiment, we configured four groups (Core1,6,9 and Core2,5,10 for eastbound direction, Core 3,8,11, and Core4,7,12 for westbound direction) as shown in Fig. 3; in each group, the output signals from one core were sent to the re-circulating loop input of the next core, in cyclic fashion. At the third loop for each group (Core9-12), dynamic gain equalizers (DGE) were used to equalize the residual gain ripple in the C-band.
At the receiver (Rx) side, a 1x12 optical switch selected one of the received signals, which was wavelength-demultiplexed by using a 50-GHz optical tunable filter (OTF) and detected by a polarization-diversity intradyne Rx. We used a free-running ECL with a linewidth of ~70 kHz as the local oscillator. In this experiment, demodulation was post-processed offline using the algorithm described in , where Hamming-window based linear-phase filters with cutoff frequency of 10.48 GHz were used for low pass filtering. In order to avoid loop-induced polarization effects, the Tx operated a polarization scrambler (PS) in synchronization with the load switches. The Q factor was calculated by averaging the bit error ratio of demodulated signals for 6 different polarization states (2 Mbit data were used for each measurement). The resulting net spectral efficiency per core was 6.72 b/s/Hz assuming 20% FEC overhead, and thus the 12-core aggregate capacity after subtracting FEC overhead was 409 Tb/s + 409 Tb/s.
4. Experimental results
Figure 6 shows the measured XT as a function of wavelength for the 50-km MCF. This plot shows core-averaged XT including FI/FO. In unidirectional transmission, the XT was −40.3 dB at the wavelength of 1613 nm. By using PDI, on the other hand, the worst XT was successfully reduced to −44.3 dB. Figure 7 shows the measured Q-factors as a function of distance in single-channel transmission. In this measurement, Core4 was tested and core-to-core rotation was not used. When neighboring cores (Core-2,3,5,6) were excited with the same propagation direction, rapid Q-factor degradation caused by XT was triggered as the transmission distance increased. A 0.6-dB Q-penalty was observed at 500 km. However, by using PDI, this penalty was successfully suppressed to 0.1 dB since only Core3 propagated in the same direction.
Next, we demonstrated 12-core 406-channel WDM/SDM transmission. The received optical spectra after 450-km transmission are shown in Fig. 8, together with the high resolution spectra as the inset. The measured Q-factor performance after 450-km MCF transmission is shown in Fig. 9(a). Q-factors of all 406 channels for 12 cores were confirmed to be better than 5.80 dB, which exceeds the Q-limit (5.70 dB, dashed line) of the LDPC convolutional codes using layered decoding algorithm with 20% overhead, where the constraint length of 10032 and the maximum number of iterations of 12 were assumed to obtain the net coding gain of 11.5 dB . The recorded constellation diagrams for X- and Y- polarizations are shown in Fig. 9(b). We confirmed that the noise statistics can be well approximated by an additive Gaussian distribution indicating the effectiveness of the soft-decision FEC.
We demonstrated bidirectional transmission over a novel large Aeff and low XT 12-core DRS fiber with total capacities of 409 Tb/s for both directions. Moreover, we utilized the PDI scheme to reduce XT. These novel techniques successfully reduced XT by 11.7 dB compared with the previous 12-core fiber transmission , and achieved 450-km transmission of 406-channel 20.16-Gbaud PDM-32QAM signals, which is about 9-fold extension of the attainable distance in over-300-Tb/s transmission experiments.
Part of this research uses results from research commissioned by the National Institute of Information and Communications Technology (NICT) of Japan.
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