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We confirm the feasibility of 100-Tbit/s-class trans-oceanic SDM transmission. Using seven-core fiber spans with seven-core full C-band EDFAs, 7 × 264-channel quasi-Nyquist-WDM 60-Gbit/s PDM-QPSK signals are transmitted over 6,370 km.
©2013 Optical Society of America
1. Introduction
Space-division multiplexing (SDM) in multi-core fibers (MCFs) has recently been demonstrated to have the potential to dramatically increase the transmission capacity which can be obtained in a single-core fiber [1–6]. Using 10.1-km MCF with 19 cores, which is the largest number of cores in MCFs reported so far, a capacity of 305 Tbit/s was demonstrated [1]. The first demonstration of 1 Pbit/s transmission has been achieved in a 52-km MCF with 12 cores [2]. In this experiment, the aggregate spectral efficiency of 91.4 bit/s/Hz has been achieved by using a multi-level modulation format and wavelength-division multiplexing of Nyquist-pulse-shaped signals with rectangular-shaped spectra in the C- and extended L-bands.
Although the total capacity could be dramatically raised by increasing the number of cores of MCF, the crosstalk between cores would be enhanced, resulting in the limitation of the transmission distance [2]. Seven cores in a single fiber seem to be suitable for long-haul applications because the core-to-core crosstalk can be suppressed [4–6]. Indeed, there have been reports on long-haul SDM transmission experiments using seven-core fibers [5,6]. A record distance of MCF-based SDM transmission was reported using a seven-core fiber with crosstalk of less than −50 dB [6]. Using a seven-core Erbium-doped fiber amplifier (MC-EDFA) as an optical repeater, 6,160-km transmission of 40-channel WDM 128-Gbit/s PDM-QPSK signals with 50-GHz spacing has been demonstrated. Compared with long-haul single-core fiber transmission experiments which have already been reported [7,8], there would be sufficient room for increasing the capacity by making the maximum use of the 5-THz bandwidth of the full C-band.
In this paper, we report MC-EDFA-repeatered SDM transmission over 6,370 km with a capacity of 110.9 Tbit/s. Using 45.5-km spans of seven-core fibers and seven-core EDFAs with a 5-THz bandwidth in the full C-band, 264-channel quasi-Nyquist-WDM PDM-QPSK signals are transmitted over 6,370 km. The capacity distance product is achieved to be 706 Pbit/s × km, which is more than double of the previous record of 300 Pbit/s × km [9]. We confirm the feasibility of 100-Tbit/s-class trans-oceanic fiber transmission.
2. Experimental setup
Figure 1 shows the experimental setup. The transmitter was composed of a three-rail configuration as reported in [8]. The two rails were used for eight even and eight odd channels. The even and odd channels were generated by multiplexing eight external cavity lasers (ECLs), and they were then independently modulated by two LiNbO3 optical IQ modulators (IQMs). The third rail was used for generating 264 WDM channels in order to maintain not only an optical signal-to-noise ratio (OSNR) but also nonlinear effects. A total of 132 lasers were combined onto a 37.5-GHz frequency grid. Modulating them by an IQM, we obtained 264 CW tones with 18.75-GHz spacing. The IQM was operated so that the lower-side-band components were suppressed [10]. All 264 channels were then passed through an IQM driven in the same manner as the modulators in the even and odd channels. In the experiment, we disabled 16 consecutive channels on the loading rail, and the 16 channels from the even and odd channel rails were tuned to the corresponding frequencies.
Fig. 1 Experimental setup. IQM: optical IQ modulator, AWG: arbitrary waveform generator, PME: polarization-multiplexing emulator, SW: optical switch, GFF: gain-flattening filter, ECL: external-cavity laser, OBPF: optical band-pass filter, and BPD: balanced photo-diode.
Three IQMs used for the QPSK modulation were driven by 18-GBaud electrical signals with rectangular-shaped spectra, which were generated by an arbitrary waveform generator with a 24-Gsample/s digital-to-analog convertor (DAC) after digital signal processing as follows: Samples of pseudo-random bit sequences (PRBSs) of a length of 215 – 1 were up-sampled twice, and they were then discrete-Fourier-transformed (DFT) and sent to the Nyquist filter with a roll-off factor of less than 0.01. After inverse DFT, a digitally-processed signal was sent to the DAC.
After the modulated WDM signals from three rails were combined, they were fed into the polarization multiplexing emulator (PME). Consequently, we obtained 264-channel 18.75-GHz-spaced quasi-Nyquist-WDM PDM-QPSK signals, resulting in a spectral efficiency of 3.2 bit/s/Hz assuming a 20% overhead for soft-decision forward-error-correction (SD-FEC) [11].
For SDM transmission, the 264-channel WDM signals were launched into a specially configured seven-fold recirculating loop consisting of a span of 45.5-km seven-core fiber, a seven-core EDFA, external gain-flattening filters (GFFs) based on wavelength-selective switches, and optical switches. The seven loops were synchronously operated as reported in [5]. The seven re-circulating loops shared a common load switch that launched identical copies of the WDM signals into each of the loops through a 1 × 8 power splitter with different delays between cores for the signal deccorelation. After amplification by the MC-EDFA with external GFFs, the signals were launched into the cores of the 45.5-km MCF. The output signals from one core were sent to the re-circulating loop input of the next core, in a cyclic fashion. The core-to-core configuration can average out variations in span loss, dispersion, and other component imperfections [5]. A polarization scrambler was inserted at the input of the third core. The speed of the polarization scrambling was about 142 deg/ms. The polarization rotation was estimated to be more than 100 degrees for one lap time of the loop consisting of seven cores of the MCF. By scrambling the polarization of the WDM signals, the temporal fluctuation of the spectral profile was suppressed.
The transmitted signals were incident on a digital coherent receiver after they were amplified and passed through an optical band-pass filter (OBPF) with a bandwidth of 1 nm. Electrical signals from the receiver were stored in sets of 2M samples by using a four-channel digital oscilloscope operating at 50 Gsample/s. The stored data were processed offline by DSP as follows: The received four-channel signals were down-sampled to two sample/symbol. After Nyquist filtering and dispersion compensation in the frequency domain, polarization demultiplexing and signal equalization were performed by half-symbol-spaced finite impulse response (FIR) filters with 40 taps in a butterfly configuration, which were adapted by the decision-directed least-mean square (DD-LMS) algorithm [12]. After the symbols were decoded, bit errors were counted.
Figure 2(a) shows a schematic configuration of the fabricated 45.5-km MCF with fan-in (FI) / fan-out (FO) devices. A cross section of the MCF is shown in Fig. 2(b). The core number is also indicated. The cladding diameter and the core pitch of the MCF were designed to be 196 μm and 56 μm, respectively. We adopted the trench index core profile in order to obtain a large effective area (Aeff) while maintaining low crosstalk. The measured Aeffs of all cores at a wavelength of 1550 nm were 117.5 ~125.2 μm2, and the dispersion values of all cores were constant to be 20.6 ps/nm/km at a wavelength of 1550 nm. The measured crosstalk for all pairs of cores including the 45.5-km MCF and FIFO devices was suppressed to be less than −51 dB. Figure 3 shows the measured loss of each core of the MCF as a function of the wavelength. The losses of all cores at a wavelength of 1550 nm were 0.196 ~0.200 dB/km. The total span loss between the FI/FO ports including the 45.5-km MCF and two fusion-splice points ranged from 9.9 to 10.5 dB at a wavelength of 1550 nm.
Fig. 2 (a) Schematic configuration of the fabricated MCF with FI/FO devices. (b) Cross section of the MCF.
The fabricated MC-EDFA had a two-stage configuration, as shown in Fig. 4. Fiber-bundle-type FI/FO devices were used to extract independently the optical signals from the seven-core EDF into seven single-core fibers [13]. At the first stage, each core of the MC-EDFA was forward-pumped by a 980-nm laser diode (LD) through an FI device. The second stage was pumped with a bi-directional configuration with two 980-nm LDs by using FI/FO devices. The core pitch of the MC-EDF was 45 μm. In order to flatten the gain over the full C-band, GFFs based on dielectric multilayer films were inserted between the first and the second stages.
Figure 5 shows the measured gain and noise figure (NF) as a function of the wavelength with 22-channel WDM signals. The input power per channel before the MC-EDFA was −8.5 dBm/ch, and the power of each pump LD was 26.5 dBm. We obtained a gain of larger than 13 dB and an NF of lower than 5.5 dB in the 5-THz bandwidth.
The fiber-bundle-type FI/FO devices were fabricated by bundling thin-cladding fibers with mode field diameters (MFD) of 7.0 μm at a wavelength of 1.55 µm. Since the MFC can match that of an MC-EDF of 7.3 µm, the insertion loss was improved by 0.4 dB per connection, resulting in a low NF of less than 5.5 dB in the two-stage MC-EDFA.
3. Measured back-to-back performance and design of the MCF launched power
Here, we discuss the OSNR and the launched power into the MCF required for long-haul transmission over 6,000 km.
First, we evaluated the back-to-back performance of the transmitter and the receiver. Figure 6 shows the sample spectra of the PDM-QPSK signals with and without adjacent WDM channels. These were measured by a 50-GSampling/s digital oscilloscope with a 16-GHz bandwidth. We observed the nearly-ideal rectangular-shaped spectrum with a bandwidth of 18 GHz. With adjacent WDM channels, we found a clear frequency gap of less than 0.75 GHz. The crosstalk from the adjacent WDM channels can be suppressed to be over 10 dB.
The measured back-to-back bit-error rates (BERs) of the quasi-Nyquist-WDM PDM-QPSK signals are plotted by dots in Fig. 7. Open circles indicate the results without adjacent channels, and the dashed curve indicates the theoretical results. The OSNR was defined with a noise bandwidth of 0.1 nm. Thanks to the well-confined Nyquist-pulse-shaped spectrum, no additional penalty was incurred in the WDM case, although the penalty from the dashed line was observed to be about 2 ~3 dB.
Fig. 7 Measured back-to-back performance of the PDM-QPSK signals. Open circles: the single-channel case, Dots: the WDM case. Inset: recovered constellation at the OSNR of 16.2 dB.
Based on the measured back-to-back performance, we design the OSNR and the launched power into the MCF transmission line required for long-haul transmission over 6,000 km. Using an SD-FEC with a 20% overhead, the error-free operation can be achieved as long as the BER is less than 2.4 × 10−2 corresponding to a Q factor of 5.9 dB [11]. Supposing a margin of about 1.5 dB for the Q factor, we set the target Q factor at 7.3 dB which corresponds to the BER of 1.0 × 10−2. Using our PDM-QPSK transmitter and receiver, the OSNR would be required to be over 10 dB for a BER of less than 1.0 × 10−2, as shown in Fig. 7. Assuming a 1- ~2-dB penalty after transmission, the required OSNR after transmission is estimated to be over 11 ~12 dB. Figure 8 shows the calculated launched power required for an OSNR of 11 ~12 dB as a function of the transmission distance. The launched powers required for 6,000-km transmission are calculated to be −7.3 and −8.3 dBm/ch required for an OSNR of 12 dB and 11 dB, respectively. In our experiment, the launched power per channel was set at −7 dBm/ch corresponding to a total power of all 264 WDM channels of 17 dBm.
Fig. 8 Calculated launched power required for an OSNR of 11 dB and 12 dB as a function of transmission distance.
4. Results of transmission experiments
Figure 9 shows the measured Q factor and OSNR of the channel at a frequency of 193.175 THz as a function of the transmission distance with a fiber launched power of −7 dBm/ch as mentioned in Section 3. The Q factors were calculated from the measured BERs. The degradation of the Q factor almost corresponds to the decrease in the OSNR, although the penalty was estimated to be less than 2 dB by referring to the back-to-back BER performance. This result suggests that the nonlinear effect could be suppressed in the transmission experiment.
Fig. 9 Measured Q factor and OSNR as a function of transmission distance for a channel of 193.175 THz.
Figure 10 shows the optical spectra before and after transmission over 6,370 km. We measured the Q factors for all 264 WDM channels for seven cores after the 6,370-km transmission. The total number of measured channels was 1,848. Figure 11 shows the measured Q factors for all channels. The Q factors were confirmed to be over 6.2 dB, which exceeds the limit of the SD-FEC with a 20% overhead, namely 5.9 dB [11].
Fig. 11 Measured Q factors for all 264 WDM channels for each of the seven cores of the MCF. Inset: typical recovered constellation of PM-QPSK signals.
5. Conclusion
We demonstrated 110.9-Tbit/s MCF-based SDM transmission over 6,370 km. Using a seven-core fiber with a seven-core EDFA, 110.9 Tbit/s (7 × 264 channels × 60 Gbit/s) quasi-Nyquist-WDM PDM-QPSK signals were transmitted with 18.75-GHz spacing in a 5-THz bandwidth of the full C-band. We confirmed the feasibility of 100-Tbit/s-class trans-oceanic fiber transmission.
Acknowledgment
The authors would like to thank E. L. T. de Gabory, M. Arikawa, and T. Ito from NEC Corporation for supporting the experiment. Some of the research results have been achieved by “Research on Innovative Optical Fiber Technology” and “R&D of Innovative Optical Communication Infrastructure,” Commissioned Research of NICT, Japan.
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