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Abstract
The total capacity of optical submarine cable systems as a global communication infrastructure must be continuously enlarged. Multi-core fibers (MCFs) have been studied as methods to maximize the total cable capacity under electrical power and cable space limitations. In particular, standard cladding MCFs, which are expected to have high productivity and mechanical reliability, are attractive for early deployment in submarine cable systems. In this paper, we demonstrate high-capacity trans-Pacific class transmission using standard cladding uncoupled 4-core fibers, achieving a transmission capacity of 55.94 Tbit/s over 12,040 km. In addition, based on the results of this and our previous coupled MCF transmission experiments, we summarize the characteristics of coupled and uncoupled MCFs applied to optical submarine cable systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Optical submarine cable systems are one of the key infrastructures that support high-capacity and high-speed global communication. In 2016, the trans-Pacific optical submarine cable system FASTER [1,2] was launched, which consists of six fiber pairs with a cable capacity of 60 Tbit/s/cable over 11,629 km; the Dunant Submarine Cable System [3] trans-Atlantic optical submarine cable system, which consists of 12 fiber pairs with a cable capacity of 250 Tbit/s/cable over 6,400 km, followed in 2021. In addition, several optical submarine cable systems crossing the Pacific and Atlantic Oceans are scheduled to be developed [4,5]. However, the total cable capacity must be continuously and dramatically enlarged to meet the predicted future global traffic demands. Since optical submarine cable systems require a cable structure that can withstand water pressure at depths of several thousand meters, the space for containing the fibers in the cable is greatly restricted compared to terrestrial optical transmission systems. In addition, the number of repeaters is restricted by the power supply limitation from cable landing stations to optical amplifiers, which is an inherent problem in submarine cable systems. Recently, the expansion of the transmission bandwidth to the C + L band and the increase in the number of fiber pairs (FPs) such as high fiber count cables [6,7] have been studied as methods to maximize the total cable capacity under these limitations. In particular, it has been reported that single-band (C-band only) transmission systems with a larger number of FPs could effectively increase the cable capacity [8]. This finding has triggered work on submarine cables with more FPs, and the full qualification of submarine repeaters and optical cables containing up to 24 FPs has been completed [9]. To further increase FPs in optical submarine cable systems, space-division multiplexing (SDM) technologies such as multi-core fibers (MCFs) and multi-mode fibers (MMFs) could be promising solutions [10]. In particular, 125-µm standard cladding SDM fibers are attractive for early deployment in submarine cable systems since they are expected to have high productivity and high mechanical reliability similar to existing single-mode fibers (SMFs) with the same cladding diameter [11].
Figure 1 shows the relationship between the transmission capacity and distance reported in standard cladding SDM fiber transmission experiments over 1,000 km [12–20]. To date, 34.56-Tbit/s transmission over 5,500 km [16] and 50.47-Tbit/s transmission over 9,150 km (our previous work) [20] have been reported as high-capacity long-haul transmission experiments using standard cladding coupled MCFs. However, to the best of our knowledge, transoceanic-class high-capacity transmission using uncoupled MCFs with standard cladding diameters has not been reported thus far. Unlike coupled MCFs, uncoupled MCFs have a high compatibility with existing SMF transmission systems because conventional transmitters and receivers can be applied via fan-in and fan-out (FIFO) devices.
Fig. 1. Relationship between the transmission capacity and distance reported in standard cladding SDM fiber transmission experiments over 1,000 km.
In this paper, we confirm the possibility of high-capacity, trans-Pacific class long-haul transmission using standard cladding uncoupled 4-core fibers. Moreover, we compare the performance with a transmission experiment of coupled 4-core fibers reported previously [20]. This is an extension of our previous work [18] from 16-WDM signals to full C-band WDM signals. First, to clarify the tolerance for the nonlinear effects in the uncoupled 4-core fiber, the Q2-factors as a function of the launch power were measured using the center channel of 16-WDM 24-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) signals at 6,020 km transmission. Then, we evaluated the Q2-factors as a function of the transmission distance using the typical channels of the full C-band WDM signals to clarify the reachability of the 190-WDM 24-Gbaud DP-QPSK signals in the uncoupled 4-core fibers. In addition, we measured the Q2-factors of all SDM and all WDM channels after 9,150-km and 12,040-km uncoupled 4-core fiber transmission. From the obtained experimental results, a transmission capacity of 63.17 Tbit/s over 9,150 km and 55.94 Tbit/s over 12,040 km was achieved, which is equivalent or better than those of the coupled 4-core fibers [20]. Finally, based on the results of this experiment and a previous coupled MCF transmission experiment [20], we summarized the characteristics of coupled and uncoupled MCF transmission systems for the following items, assuming that each MCF will be applied to an optical submarine cable system.
- • Optical characteristics of the fibers
- • Ease of construction of the systems, including skew compensation between cores
- • Transmission performance
- • Mode-dependent loss (MDL) and spatial mode dispersion (SMD) in coupled MCFs
- • MIMO-digital signal processing for the receivers
- • Spatial efficiency in the cables
2. Experimental setup for uncoupled 4-core fiber transmission
Figure 2 shows the experimental setup for a trans-Pacific class 190-WDM uncoupled 4-core fiber transmission. In the transmitter, the continuous wave (CW) lights generated from eight external cavity lasers were combined with a frequency spacing of 50 GHz for even and odd channels. The even and odd channels were independently modulated using a 4-channel arbitrary waveform generator (AWG) and two IQ modulators (IQMs). The IQMs were driven by 24-Gbaud Nyquist-shaped electrical two-level signals for QPSK, which were generated by the AWG operated at 120 GSample/s for the I and Q components. Pseudo-random bit sequences (PRBS) with lengths of 215-1 were upsampled to two samples/symbol. The delay between two carriers was set to be approximately 10,000 symbols. Following C-band optical amplification, the signals were combined with a 25-GHz spacing and polarization-multiplexed with a delay of 87 ns. Then, we obtained 16 channels of 25-GHz-spaced 24-Gbaud DP-QPSK Nyquist-shaped WDM signals in the C-band.
In addition, we constructed a third rail to load 190 WDM channels in the C-band to maintain an optical signal-to-noise ratio (OSNR) and nonlinear effects in fiber propagation. These 25-GHz spaced 190 tones were generated by a two-cascaded carrier-suppressed modulation of 95 50-GHz-spaced lasers, ranging from 1527.51 nm to 1565.20 nm. These tones were modulated and polarization-multiplexed in the same manner as the measured channel. After three rails were combined and power-equalized with a C-band wavelength selective switch (WSS), we obtained 190-channel WDM Nyquist-shaped DP-QPSK signals with a rate of 96 Gbit/s, including the FEC overhead. The inset in Fig. 2 shows the measured optical spectrum of the WDM signals. Note that when we measured the BER at every channel, the 16 consecutive channels on the C-band loading rails were disabled, and the measured channel and the 15 dummy channels were tuned to the corresponding frequencies in turn.
The generated WDM signal was split into 4 paths, with a relative delay of 200 ns between subsequent paths for decorrelation, and fed into a recirculating loop system consisting of four spans of 60.2-km uncoupled 4-core fibers [21], C-band EDFAs and 2 × 2 optical switches (SWs). Here, conventional single-mode EDFAs were used to evaluate the transmission potential of uncoupled MCFs in trans-Pacific transmission under the same conditions as coupled MCFs [20]. The WDM signals after 4-span transmission were gain-equalized using two 2-channel C-band WSSs. The polarization switches synchronized with the recirculating loop system rotate the polarization state of the signals by 90 degrees per loop to reduce the polarization-dependent loss (PDL). In addition, since signal lights repeatedly pass through the same component in a recirculating loop system, the difference in transmission characteristics between cores tends to be enhanced. Therefore, the signal input cores to the fiber were sequentially switched for each loop in uncoupled MCF transmission. For example, the output signals of core #1 in the first loop were input to core #2 in the second loop, as shown in Fig. 2.
The four cores arranged in a square lattice of the uncoupled MCF [21] had almost the same refractive index profile as an ultralow-loss pure-silica-core single-mode fiber used for long-haul transmission. Table 1 shows the optical characteristics at 1550 nm and the cross-sectional image of the fabricated uncoupled MCF. The core-averaged transmission loss, effective area, and core pitch for the uncoupled MCF at 1550 nm were approximately 0.156 dB/km, 87 µm2 and 43.0 µm, respectively. The cut-off wavelength at 7.7 km was approximately 1450 nm. The insertion losses for each fiber-bundled type fan-in (FI) or fan-out (FO) device at 1550 nm ranged from 0.3 to 0.5 dB. In addition, the losses at one splice point were less than 0.4 dB for the uncoupled MCF. Note that one of the four MCF spans has no fusion splice point, and the remaining three spans have one fusion splice point each. Therefore, in this experiment, the averaged total span loss was 10.1 dB. The core-to-core crosstalk for the uncoupled MCF with FO devices was suppressed to less than −57.3 dB/span due to the trench-assisted refractive index profile.
Table 1. Optical Characteristics of Uncoupled MCF
In the coupled MCF experiment [20], the skew between the four cores, which occurs in all devices in a recirculating loop system such as FIFO devices, optical amplifiers, WSSs, and optical switches, needs to be compensated for each span because the skew contributes to an increase in the required number of MIMO taps in MIMO signal processing at the receiver side, as well as SMD [22]. The averaged total span loss was 11.7 dB, including variable optical delay lines (VODLs) with a typical insertion loss of 1.0 dB to compensate for the inter-core skew. Therefore, in uncoupled MCF, the span losses can be reduced compared to coupled MCF because the SMD does not need to be compensated for across all cores by optically suppressing core-to-core crosstalk.
In the receiver, the transmitted WDM signals were detected by four individual digital coherent receivers based on heterodyne detection with a free-running local oscillator (LO) after channel selection with optical bandpass filters (OBPFs). The frequency offset between the LO and the measured signal was adjusted to 20 GHz. The received electrical signals were digitized at 80 GSample/s using four real-time oscilloscopes. For offline processing, the stored samples were processed as follows: The samples were downconverted to the base band. After rectangular Nyquist shaping, the samples were processed by four individual adaptive 2 × 2 MIMO equalizers with 250 taps. The MIMO tap coefficients were updated based on a decision-directed least-mean square (DD-LMS) algorithm [23]. In the LMS, data-aided equalization with training signals was used for initial convergence. After that, it was switched to blind equalization (i.e., decision-directed mode). After the symbols were decoded, the Q2-factors were calculated using only the data after the switch to decision-directed mode. In the uncoupled MCF, compensation for the SMD and core-to-core crosstalk is not required, unlike in the coupled MCF [20], and thus, trans-Pacific transmission was demonstrated using only the conventional 2 × 2 MIMOs with a relatively small number of taps.
3. Results and discussion
3.1 Launch power characteristics
We clarified the tolerance for the nonlinear effects in the uncoupled MCF to determine the optimal launch power. Figure 3 shows the Q2-factors as a function of the power per channel at 6,020-km transmission. The launch power was defined as the input power to the FI devices. Here, we used the center channel of 16-WDM 25 GHz-spaced 24-Gbaud DP-QPSK signals, from 1548.62 nm to 1551.62 nm, to simplify the gain equalization of the transmitted WDM signals using WSSs in the recirculating loop system. The highest average Q2-factor among the four cores was −5 dBm/ch. Therefore, the signal power launched into each core of the uncoupled 4-core fiber was adjusted to −5 dBm/ch in the following trans-Pacific transmission experiment using 190-WDM DP-QPSK signals. From this result, it is clear that the nonlinear tolerance of the uncoupled 4-core fiber is lower than that of the standard cladding coupled 4-core fibers with an optimal launch power of −2 dBm/ch [20]. Note that the launch power in the coupled 4-core fibers was evaluated after 6,020-km transmission using a recirculating loop system consisting of four MCF spans (average span loss: 11.7 dB, average effective area: 113 µm2), as well as that for uncoupled 4-core fibers.
3.2 Transmission performance as a function of the transmission distance
The Q2-factors as a function of the transmission distance of the typical four channels, which are the two channels near the center and two channels at both edges in the full C-band, were measured to clarify the reachability of the 190-WDM DP-QPSK signals in the uncoupled MCF. Figure 4 shows the relationship between the transmission distance and the core-averaged Q2-factors at wavelengths of 1533, 1550, 1555, and 1563 nm when using the 190-WDM signals. In this experimental setup, the Q2-factors of the two channels at both edges degraded by approximately 1-2 dB at each transmission distance compared to the center channels due to the insufficient gain of the optical amplifiers in the recirculating loop system, as in the coupled 4-core fiber transmission [20]. In addition, Q2-factors of the two channels at both edges exceeding the FEC threshold of 25.5% [24], the largest overhead among the assumed FECs, were obtained after 12,040-km transmission. Therefore, the transmission distance was set in two patterns: 9,150 km for comparison with coupled 4-core fibers and 12,040 km for longer distance transmission in the following high-capacity transmission experiment using 190-WDM DP-QPSK signals.
3.3 Transmission performance of 190-WDM DP-QPSK signals over an uncoupled 4-core fiber
We evaluated the transmission performance of 190-WDM DP-QPSK signals after 9,150-km and 12,040 km uncoupled 4-core fiber transmission. Figures 5 (a)-(d) show the optical spectra at the output of each core after 9,150-km transmission ((a) core #1, (b) core #2, (c) core #3, and (d) core #4). The flattened WDM channels were maintained across a full C-band after transmission due to gain equalization using WSSs in the recirculating loop system. In addition, no significant difference was observed in the optical spectra between the four cores, as shown in Fig. 5.
Fig. 5. Optical spectra at (a) core#1, (b) core#2, (c) core#3 and (d) core#4 outputs after 9,150-km transmission (0.02 nm res.).
Figure 6 shows the Q2-factors of 748 (4 core × 187 WDM) SDM/WDM channels calculated from the measured BERs after 9,150-km transmission. Note that three WDM channels at both edges in the C-band were removed because of insufficient characteristics. In this experiment, we assumed a rate-adaptive FEC [25] (multirate FEC [26]), which is a promising technology used to maximize system capacity with the OSNR and the nonlinear effect variation between SDM/WDM channels. Three different soft-decision (SD)-FECs based on low-density parity-check (LDPC) codes of a 12.75% overhead (OH) and 6.5 dB FEC limit [27], 20% OH and 5.7 dB FEC limit [28], and 25.5% OH and 4.95 dB FEC limit [24] were employed, and one was selected for each SDM/WDM channel according to the measured BERs. In the results shown in Fig. 6, the Q2-factors of 666, 57, and 25 SDM/WDM channels exceeded the thresholds of 12.75%, 20%, and 25.5% OH FEC, respectively. The Q2-factors at shorter wavelengths were lower than those at longer wavelengths in the C-band due to the insufficient gain of the optical amplifiers used in the recirculating loop system. The worst Q2-factor was 5.01 dB at 1528.09 nm for core #4, which was higher than the FEC limit of 4.95 dB for the 25.5% OH FEC. The maximum difference in the Q2-factors between the 4 cores in each WDM channel was 0.63 dB at 1528.09 nm. From the obtained experimental results, a transmission capacity of 63.17 Tbit/s (15.79 Tbit/s/core) was achieved using the 187-WDM 24-Gbaud DP-QPSK signals over a full C-band after a 9,150-km transmission by assuming the rate-adaptive FEC. This transmission capacity is approximately 3 Tbit/s/core larger than the 9,150 km transmission experiment using coupled 4-core fibers [20]. The lower capacity in the coupled MCF transmission experiment is due to the limitation of the number of WDM channels to 152 for two reasons: the increase in span losses due to the insertion losses of the VODLs to compensate for the skew between the cores, and the insufficient gain of the optical amplifiers in the recirculating loop system at both edge channels in the C-band.
Figure 7 shows the PDLs of each SDM/WDM channel after 9,150-km transmission. Each PDL is the difference in the singular values between two polarizations averaged over the 24-GHz signal bandwidth. The singular values were obtained by singular value decomposition of a MIMO matrix [29]. The PDLs were suppressed to less than 3 dB in most SDM/WDM channels at the full C-band. Since the MDLs between the eight tributaries (4-spatial tributaries × 2 polarizations) were approximately 6-9 dB across the C-band in the coupled 4CF transmission [20], the PDLs of each SDM/WDM channel in uncoupled 4CF transmission were sufficiently small in comparison. Figure 8 shows the MIMO impulse responses at (a) 1534.545 nm, (b) 1550.016 nm, and (c) 1564.781 nm after 9,150-km transmission. Note that the wavelength dependency was negligible in these MIMO impulse responses. In coupled MCF transmission [20], approximately 250 or more MIMO taps were required across the full C-band due to the SMD, which increases with the square root of the transmission distance [30]. On the other hand, since the core-to-core crosstalk and SMD between cores do not need to be compensated for in uncoupled MCFs, a few dozen MIMO taps are sufficient even after 9,150 km transmission, as shown in Fig. 8.
Fig. 8. MIMO impulse responses at (a) 1534.545 nm, (b) 1550.016 nm, and (c) 1564.781 nm after 9,150-km transmission.
Figure 9 shows the Q2-factors of 684 (4 core × 171 WDM) SDM/WDM channels calculated from the measured BERs after 12,040-km transmission. Note that 19 WDM channels at both edges in the C-band were removed because of insufficient characteristics. In the results shown in Fig. 9, the Q2-factors of 334, 208, and 142 SDM/WDM channels exceeded the thresholds of 12.75%, 20%, and 25.5% OH FEC, respectively. The worst Q2-factor was 4.98 dB at 1532.58 nm for core #3, which was higher than the FEC limit of 4.95 dB for the 25.5% OH FEC. The maximum difference in the Q2-factors between the 4 cores in each WDM channel was 1.4 dB at 1545.02 nm. From the obtained experimental results, a transmission capacity of 55.94 Tbit/s (13.99 Tbit/s/core) was achieved using the 171-WDM 24-Gbaud DP-QPSK signals over a full C-band after a 12,040-km transmission.
4. Characteristics of standard cladding uncoupled and coupled MCFs for optical submarine cable systems
In this section, based on the results of this experiment and a previous coupled MCF transmission experiment [20], we summarize the characteristics of coupled and uncoupled MCFs, as shown in Table 2, assuming that each MCF will be applied to an optical submarine cable system.
Table 2. Characteristics of Uncoupled and Coupled Multicore Fibers
First, the core-averaged transmission losses of the uncoupled [21] and coupled [31] 4-core fibers with standard cladding diameters are almost the same because both fibers have pure silica cores. The insertion losses for the fiber-bundled fan-out devices for uncoupled MCF and lens-coupled fan-out devices for coupled MCF at 1550 nm ranged from 0.3-0.5 dB and 0.3-0.6 dB, respectively. Here, since the core pitch of the coupled MCFs is as small as 20 µm and it is difficult to manufacture fiber-bundled FIFO devices, we selected lens-coupled FIFO devices. In addition, the losses at one splice point were less than 0.4 dB for the uncoupled MCF and 0.1 dB for the coupled MCF. This is partly because the core pitch of the coupled MCFs is smaller than that of the uncoupled MCFs, and the cores are located near the center of the fibers, which has a higher tolerance to rotational misalignment. Furthermore, as described in Section 2, since the coupled MCF needs to reduce the skew between cores, a mechanism to adjust the optical path length, such as VODLs, is required if the transmission line is composed of FIFO devices and conventional single-mode amplifiers, as in this experiment. Considering only the transmission losses of fibers, splicing loss, and insertion losses of the FIFOs, the total losses in each fiber are almost the same. However, the span loss in the coupled MCF is increased by the insertion loss of the skew adjustment mechanism such as VODLs. Therefore, in the case of a coupled MCF, it is desirable to construct a FIFO-less all-multicore transmission line that does not require skew adjustment between cores by using multicore amplifiers [32] with the same number of cores as the transmission fiber in the repeaters instead of FIFO devices and single-mode amplifiers. Furthermore, the realization of FIFO-less multicore repeaters is expected to further reduce the span losses for the insertion losses of FIFOs and improve the spatial efficiency of the repeaters in both uncoupled and coupled MCF systems.
Second, we compared the transmission performance of each fiber. Since the optimal fiber input power was −5 dBm/ch for the uncoupled MCF and −2 dBm/ch for the coupled MCF, as shown in Table 2, the coupled MCF had a better nonlinear performance. However, as described in Section 3, the transmission capacity of the uncoupled 4-core fiber slightly exceeded that of the coupled 4-core fiber after 9,150-km transmission due to the larger span losses of the coupled MCF and insufficient gain of the amplifiers in the recirculating loop system. In the uncoupled MCF, PDLs of less than 5.5 dB (and less than 3 dB for most SDM/WDM channels) were observed in each channel after 9,150-km transmission. On the other hand, larger MDLs of 6-9 dB were observed in the coupled MCF transmission because the MDL occurs between all cores and polarizations. Since the PDLs and MDLs contribute to the degradation of transmission performance [33–35], it is necessary to reduce them as much as possible.
Next, we compared the MIMO receivers in each system. In the uncoupled MCF system, compensation for crosstalk between cores is not required, so the 2 × 2 MIMO signal processing used in conventional receivers can be applied to each core. On the other hand, a coupled multicore fiber system requires a larger MIMO size corresponding to the square of the number of cores to compensate for crosstalk between cores. Although real-time 8 × 8 MIMO long-haul transmission using coupled 4-core fibers has recently been reported [36], further research and development of MIMO signal processing is essential for practical applications. In addition, the coupled MCF also needs to compensate for the SMD and inter-core skew in the fiber, which requires a larger number of MIMO taps than the uncoupled MCF, increasing the burden and complexity of the receivers.
The above experimental results were obtained under the condition that uncoupled MCFs and coupled MCFs are wound onto the fiber bobbin. In practical optical submarine cable systems, the core-to-core crosstalk in uncoupled MCFs and SMD in coupled MCFs will be reduced because the fiber bending and twisting are expected to be less than in the experimental environment. Therefore, the uncoupled MCFs are unlikely to degrade the transmission characteristics compared to the experimental results. For coupled MCFs, the number of MIMO taps (MIMO complexity) at the receivers is expected to be reduced, while the nonlinear tolerance is degraded [37] due to the reduced SMD.
From the above, we believe that uncoupled MCFs are suitable for the early deployment of MCFs in optical submarine cable systems. However, it is difficult to further increase the number of cores in the standard cladding uncoupled MCFs because a core pitch of approximately 40 µm and a trench structure are necessary to suppress the crosstalk between cores [21]. On the other hand, 7-core fibers [15] and 12-core fibers [38] with a standard cladding diameter have already been reported for coupled MCFs, which are expected to improve the spatial efficiency in the limited space of a cable. Although large MIMO and FIFO-less multicore repeaters must be developed, coupled MCFs can be an attractive solution to further increase transmission capacity in the future.
5. Conclusion
We successfully demonstrated the high-capacity and trans-Pacific class transmission of standard cladding ultralow-loss uncoupled 4-core fibers. To clarify the tolerance for the nonlinear effects in uncoupled 4-core fibers, the Q2-factors as a function of the launch power were measured using the 16-WDM 24-Gbaud DP-QPSK signals at 6,020 km transmission. In addition, we evaluated the Q2-factors as a function of the transmission distance using the typical channels of the full C-band WDM signals to clarify the reachability of the 190-WDM 24-Gbaud DP-QPSK signals. Moreover, we measured the transmission performance of all SDM/WDM channels after 9,150-km and 12,040-km uncoupled 4-core fiber transmission. From the obtained experimental results, a transmission capacity of 63.17 Tbit/s over 9,150 km and 55.94 Tbit/s over 12,040 km was achieved. Based on the results of this experiment and a previous coupled MCF transmission experiment, we summarized the characteristics of uncoupled and coupled MCFs, assuming that each MCF will be applied to an optical submarine cable system. Considering the transmission performance after long-haul transmission and the applicability of conventional MIMO signal processing, uncoupled MCFs are more suitable for early deployment in optical submarine cable systems.
Funding
Ministry of Internal Affairs and Communications (JPMI00316).
Acknowledgments
We would like to thank the staff of Furukawa Electric Co. for their assistance in this study. Some of the research results were achieved via the Ministry of Internal Affairs and Communications (MIC)/Research and Development of Innovative Optical Network Technology for a Novel Social Infrastructure (JPMI00316) (Technological Theme II: OCEANS), Japan.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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