First Real-Time 221.9 Pb/S∙Km Transmission Capability Demonstration Using Commercial 138-Gbaud 400 Gb/S Backbone OTN System over Field-Deployed Seven-Core Fiber Cable with Multiple Fusion Splicing

Abstract

The core-division-multiplexed (CDM) transmission technique utilizing uncoupled multi-core fiber (MCF) is considered a promising candidate for next-generation long-haul optical transport networks (OTNs) due to its high-capacity potential. For the field implementation of MCF, it is of great significance to explore its long-haul transmission capability using high-speed OTN transceivers over deployed MCF cable. In this paper, we investigate the real-time long-haul transmission capability of a deployed seven-core MCF cable using commercial 138-Gbaud 400 Gb/s backbone OTN transceivers with a dual-polarization quadrature phase shift keying (DP-QPSK) modulation format. Thanks to the highly noise-tolerant DP-QPSK modulation format enabled by the high baud rate, a real-time 256 Tb/s transmission over a 990.64 km (14 × 70.76 km) deployed seven-core fiber cable with more than 600 fusion splices is field demonstrated for the first time, which achieves a real-time capacity–distance product of 221.9 Pb/s∙km. Specifically, the long-haul CDM transmission is simulated by cascading the fiber cores of two segments of 70.76 km seven-core fibers. And dynamic gain equalizers (DGEs) are utilized to mitigate the impacts of stimulated Raman scattering (SRS) and the uneven gain spectra of amplifiers in broadband transmissions by equalizing the power of signals with different wavelengths. This field trial demonstrates the feasibility of applying uncoupled MCF in long-haul OTN transmission systems and will contribute to its field implementation in terrestrial fiber cable systems.

1. Introduction

The relentless surge in global data consumption, propelled by bandwidth-intensive applications such as 5G/6G networks, artificial intelligence (AI)-driven services, and ultra-high-definition video streaming, has led to an explosive demand for the capacity enhancement of optical transmission systems. Although multi-band wavelength-division multiplexing techniques and advanced modulation formats have historically extended the capacity of widely deployed single-mode fibers (SMFs), their scalability is constrained by the available low-attenuation wavelength windows and nonlinear Shannon limit [1,2]. As a result, it is necessary to deploy new fiber cables to meet the capacity requirements of SMF-based optical transmission networks, which is very costly and will occupy massive spatial resources, especially for long-haul backbone networks. Recently, space-division-multiplexed (SDM) transmission techniques utilizing few-mode fibers (FMFs), multi-mode fibers (MMFs), or multi-core fibers (MCFs) have attracted extensive research interests and emerged as a transformative paradigm that can offer a multiplicative increase in transmission capacity by leveraging parallel spatial channels such as fiber modes or fiber cores within a single fiber cladding [3,4]. For the mode-division-multiplexed (MDM) transmission approach based on FMFs or MMFs, the multiplexed fiber modes are transmitted over a single fiber core, which can significantly enhance the transmission capacity while keeping the size of the fiber cladding consistent with that of standard SMF [5]. Thanks to the enormous capacity potential of FMF and MMF, numerous breakthroughs in MDM fibers and optical transmission systems have been achieved [6,7]. For example, a record 1.53 Pb/s C-band transmission has been demonstrated, which is enabled by a customized 55-mode fiber with low differential mode delay (DMD) and advanced coherent 110 × 110 multiple-input multiple-output digital signal processing (MIMO-DSP) [8]. And a 273.6 Tb/s MDM transmission over 1001 km 15-mode fiber has also been achieved using mode multiplexers with low mode-dependent loss (MDL) and insertion loss (IL), as well as a 15-mode fiber optimized for a low-DMD transmission regime, which realizes a record capacity–distance product of 273.9 Pb/s·km for multi-mode transmission [9]. In addition, a field trial over a deployed 15-mode fiber cable has also been reported, which promotes the implementation of the MDM transmission technique [10]. However, due to the inevitable modal crosstalk induced by mode multiplexers/demultiplexers and fiber transmission, complex MIMO-DSP has to be utilized at the receiver to demultiplex different mode channels for achieving ultra-high-speed long-haul transmission [11]. This makes it costly and requires an upgrade of existing transceivers for field implementation. While for the core-division-multiplexed (CDM) technical route utilizing MCFs, multiplexed spatial signals are transmitted over different fiber cores, which makes it easy to suppress the inter-core crosstalk by designing the structure of MCFs and enables kilometer-scale transmission [12,13,14]. Furthermore, MCF-based CDM transmission systems can achieve ultra-high-capacity transmission by expanding the number of fiber cores in a single MCF, effectively addressing the capacity shortage issues in short-reach optical interconnect scenarios [15,16]. Further, the MCF can be directly compatible with existing optical transceivers through fan-in/fan-out (FIFO) devices, making it easy to field implement in optical transmission networks [17].

Thanks to the inherent advantages of uncoupled MCFs such as low inter-core crosstalk and high transmission stability, long-haul transmission and maintenance techniques based on MCFs have been extensively investigated over a decade, and multiple impressive breakthroughs have been achieved. For example, many types of high-performance FIFO devices enabled by precise fiber bundle alignment techniques or femtosecond laser direct writing approaches have been experimentally demonstrated, which allow for the seamless integration of a wide variety of MCFs into existing optical transmission networks [18,19,20]. And regarding the connection approaches, an ultra-low-loss multi-core fusion splicing technique utilizing an advanced precise azimuthal alignment algorithm and a three-electrode arc-discharging splicer has been reported, which paves the way for the field application of MCF in long-haul transmission scenarios [21]. In addition, integrated four-core erbium-doped fiber amplifiers (EDFAs) with core pumping have also been designed and fabricated, based on which ultra-long-haul transmission over four-core MCF has been experimentally verified [22]. The maturity of CDM transmission and maintenance techniques based on uncoupled MCFs has promoted the exploration of their practical applications. High-speed real-time transmissions utilizing commercial 400 Gb/s and 800 Gb/s optical transport network (OTN) transceivers over more than 300 km seven-core fiber have been verified in the laboratory, which effectively verifies the compatibility between MCF-based transmission systems and next-generation commercial OTN transceivers [17,23]. And various types of experimental MCF cables have also been field deployed, based on which multiple MCF-based high-performance transmission systems including the integrated communication and sensing system, the advanced MIMO distributed acoustic sensing (DAS) system, and the classical/continuous variable quantum key distribution (CVQKD) co-existence transmission system have been investigated and demonstrated in the field [24,25,26,27,28]. These field trials effectively demonstrated the feasibility of implementing MCFs in existing networks and deeply explored the application potential of MCFs with various emerging services. However, to the best of our knowledge, a real-time long-haul field trial over an MCF cable utilizing commercial 400 Gb/s backbone OTN transceivers has not been reported so far. Exploring the high-speed long-haul transmission capability of MCF is of great significance in promoting its field implementation in long-haul transmission scenarios such as terrestrial fiber cable systems.

In this paper, we conduct field investigations of the real-time long-haul transmission capability of MCF using commercial 138 Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) 400 Gb/s 80-wavelength backbone OTN systems over a deployed seven-core MCF cable. The long-haul CDM transmission is simulated by cascading the fiber cores of two segments of MCFs with a span length of 70.76 km. And dynamic gain equalizers (DGEs) are utilized to mitigate the impact of stimulated Raman scattering (SRS) in long-haul C + L-band transmission by equalizing the power of signals with different wavelengths. Thanks to the highly noise-tolerant DP-QPSK modulation format enabled by the high baud rate, a real-time 256 Tb/s transmission over a 990.64 km (14 × 70.76 km) seven-core fiber cable with more than 600 fusion splices is demonstrated for the first time, which achieves a real-time capacity-distance product of 221.9 Pb/s∙km with an optical signal-to-noise ratio (OSNR) margin of over 2.7 dB.

2. The Field-Deployed Seven-Core MCF Cable

In our field trial, the transmission experiment is conducted over a deployed MCF cable, which is located in Jinan, China, and has a length of 17.69 km. The MCF cable connects the Jinxiu Chuan and Xiying data centers with the routing shown in Figure 1a. The entire MCF cable consists of 11 segments of short cables, and the length of each segment of short cable is depicted in Figure 1c. The number of short optical cables and the length of each short cable are designed based on the available resources such as utility poles and manholes during cable deployment. The MCF cable is deployed through a combination of three methods including direct burying, overhead installation, and laying in pipelines. And the 11 segments of short MCF cables are connected through fusion splicing during the deployment process. The fusion splicing process is automatically performed by a fusion splicer. During this process, the splicer automatically aligns all the fiber cores in the MCF by continuously adjusting the position of the MCFs based on an automatic alignment algorithm. And it takes no more than 5 min to achieve simultaneous alignment of the seven fiber cores. The MCF cable contains eight seven-core fibers, and the cross-section of each seven-core fiber is shown in Figure 1b. The core-to-core distance and cladding diameter of the seven-core fiber are 42 μm and 150 μm, respectively. The relatively large core-to-core distance is primarily designed to suppress the inter-core crosstalk and achieve high-capacity transmission. And although the cladding diameter is larger than 125 μm, the diameter of the coating is still 245 μm, which is the same as that of standard SMF and enables compatibility with standard cabling process. Low-refractive-index fluorine-doped trenches are added to each fiber core to reduce the inter-core crosstalk and bending loss. And a tiny high-refractive-index marker core is also processed in the MCF, which can avoid misconnections between different fiber cores and help improve the precision of core alignment during the splicing process.

Table 1. Measured main characteristics of the 7-core fiber after cabling.

Cladding Diameter (μm)	Coating Diameter (μm)	Core-to-Core Distance (μm)	Mode Field Diameter (μm)	Dispersion Coefficient (ps/nm/km)	Attenuation Coefficient (dB/km)	Inter-Core Crosstalk (dB/10 km)	Bending Loss with R = 30 mm (dB/100 Turns)
150	245	42	9.0	≤22	≤0.22	≤−50	<0.1

shows the main characteristics of the seven-core fiber after the cabling process, measured at 1550 nm. The bending loss after cabling is measured by stripping off the cable jacket and coiling the bare fiber with a bending radius of 30 mm. The reason for testing this parameter is mainly that in optical stations (e.g., transmitter stations, receiver stations), it is necessary to strip off the cable jacket of the MCF cable and connect the MCFs to FIFO devices. When the exposed MCFs after jacket removal are excessively long, they are typically coiled in a fiber management tray with a bending radius of 30 mm. It can be found that the measured characteristics are close to those of standard SMF. And the inter-core crosstalk is remarkably low and will have little impact on the performance of transmission systems, which is mainly due to the relatively large core-to-core distance and the low-refractive-index fluorine-doped trenches. It should be noted that the cabling process has no significant impact on the characteristics of the seven-core fiber. The characteristics of the seven-core fiber before and after cabling all meet the results shown in Table 1.

We cascade every four of the eight seven-core fibers in the MCF cable to form two segments of seven-core fiber links as depicted in Figure 1c. Each segment of the seven-core fiber link has a length of 70.76 km (4 × 17.69 km) and 43 multi-core fusion splices. The two segments of 70.76 km seven-core fiber links contain a total of 86 multi-core fusion splices, which is equivalent to 602 single-core fusion splices. We measure the splicing loss of each single-core fusion splice using an optical time-domain reflectometer (OTDR) and a FIFO device at 1550 nm. And the results are shown in Figure 2. It can be observed that the splicing losses of the 602 single-core fusion splices are all less than 0.5 dB, and the splicing losses of over 83.5% of single-core splices are no more than 0.25 dB. However, it should also be acknowledged that the splicing loss of MCF is larger than that of SMF, which is mainly caused by the slight misalignments between the fiber cores of the two cross-sections during the alignment process and the uneven heating of the seven-fiber cores during the arc discharge process. In our field trial, as shown in Figure 1c, a pair of FIFO devices are connected to each segment of the 70.76 km seven-core fiber link to construct two 70.76 km CDM links. We measured the span loss and the total inter-core crosstalk of the two CDM links and the results are shown in

Table 2. Span loss and the total inter-core crosstalk of the first 70.76 km CDM link (Unit: dB).

	Core #1	Core #2	Core #3	Core #4	Core #5	Core #6	Core #7
Span loss	23.5	23.8	19.7	23.8	22.9	23.5	25.1
Total crosstalk	−49.2	−43.7	−42.9	−50.2	−45.6	−46.5	−48.9

and

Table 3. Span loss and the total inter-core crosstalk of the second 70.76 km CDM link (Unit: dB).

	Core #1	Core #2	Core #3	Core #4	Core #5	Core #6	Core #7
Span loss	22.8	25.3	19.5	22.7	24.7	23	23.8
Total crosstalk	−45.7	−48.7	−43.5	−47.3	−42.3	−46	−47.2

, respectively. The total inter-core crosstalk is defined as the sum of crosstalk from the other six fiber cores to the tested core. We can see that the span loss of each fiber core is no more than 25.3 dB. And the total inter-core crosstalk is lower than −42 dB for each fiber core of the two 70.76 km CDM links.

3. Experimental Setup of the Long-Haul CDM Transmission System

In this section, we introduce the experimental setup of our real-time long-haul CDM field trial using 400 Gb/s backbone OTN transceivers over a deployed MCF cable, which is shown in Figure 3a. At the transmitter station, a pair of 400 Gb/s C-band and L-band optical transponder units (OTUs) are utilized to transmit high-speed modulated signals in C-band and L-band, respectively. The OTUs have a modulation rate up to 138-Gbaud, which enables the adoption of a highly noise-tolerant DP-QPSK modulation format and thus is suitable for long-haul transmission. And advanced private forward error correction (FEC) coding with the 38-Gbaud overhead is utilized to further improve the long-haul transmission performance of the OTUs, which achieves a soft-decision (SD)-FEC limit up to 4.5 × 10⁻². The central frequencies of the C-band and L-band OTUs are tunable with a channel spacing of 150 GHz and can achieve 80-wavelength WDM transmission with 40 C-band and 40 L-band OTUs over the C + L band. To simulate fully loaded 80-wavelength WDM transmission with insufficient OTUs, dummy light (DL) generated by C-band and L-band amplifier spontaneous emission (ASE) noise sources is utilized to fill the remaining 78 wavelength channels of the C + L band. The utilization of DL can effectively simulate various transmission effects in broadband WDM systems including inter-wavelength crosstalk, cross-phase modulation (XPM), and stimulated Raman scattering (SRS). In addition, filling the remaining wavelength channels using DL can also maintain the stability of the broadband transmission system under the impact of SRS, which is crucial for practical broadband WDM transmission systems where the add/drop operations of wavelength channels occur frequently. The modulated 400 Gb/s signals and DL are multiplexed through a C + L-band-integrated wavelength selection switch (WSS) and then amplified by a pair of C-band and L-band EDFAs. The C-band and L-band amplified signals are then combined by a wavelength multiplexer (WMUX) and further coupled to a fiber core of the 7-core fiber for CDM transmission through a FIFO device. The long-haul optical transmission link is constructed by the two above-mentioned 70.76 km CDM links, whose detailed structure is shown in Figure 1c. The first and the second CDM links are distinguished by A and B, respectively. In our experiment, to investigate the long-haul transmission capability of the system with limited 7-core fiber, we cascade the fiber core channels of the two 70.76 km CDM links one by one in the same direction [17]. As depicted in Figure 3a, each fiber core channel is actually a span of 70.76 km 7-core fiber and a pair of FIFO devices. The cascading sequence of each fiber core channel is also illustrated in Figure 3a and a maximum of 14 spans are cascaded. Commercial optical amplifiers (OAs) are employed after each transmission span to compensate for the span losses, which are also achieved by a pair of C-band and L-band EDFAs as well as a pair of WMUXs and wavelength demultiplexers (WDEMUXs) as shown in Figure 3c. The WMUX and WDEMUX use thin-film filters (TFFs) to achieve the combination and separation of optical signals of the two wavelength bands. And each of the WMUXs and WDEMUXs has an IL of about 1 dB. The C-band and L-band EDFAs adopt a two-stage amplification structure, which can achieve an amplifier gain exceeding 30 dB and effectively compensate for the attenuation induced by the WMUX/WDEMUX and fiber transmission. And a gain flattening filter (GFF) is applied in each EDFA to flatten the gain spectrum of the amplifier. To compensate for the effects of SRS and the residual unevenness of the gain spectra of the EDFAs in the broadband WDM transmission system, two dynamic gain equalizers (DGEs) instead of conventional OAs are employed after the 4th and 9th transmission spans, respectively, as depicted in Figure 3d. The gain equalization is achieved through a C + L-band-integrated WSS in the DGE, which is based on high-resolution liquid crystal on silicon (LCoS) technology and enables the dynamic adjustment of optical power for each wavelength channel. And two pairs of C-band and L-band EDFAs are also utilized to compensate for the attenuation induced by the WSS, WMUX/WDEMUX, and fiber transmission. At the receiver station, the WSS is first utilized to separate the received C-band and L-band signals, which are then amplified by C-band and L-band EDFAs, respectively. And another C + L-band-integrated WSS is followed to demultiplex the modulated signals of the two bands. The demultiplexed C-band and L-band modulated signals are then sent to C-band and L-band OTUs, respectively, for coherently conducting detection and real-time bit error rate (BER) calculation. It should be noted that in our field trial, each vacant core is also filled with C + L-band ASE noise to simulate 7-core fully loaded CDM transmission. This is implemented by employing the same transmission configuration as the transmitter station shown in Figure 3a (excluding the C-band and L-band OTUs) for each vacant fiber core channel and transmitting C + L-band ASE noise to each vacant fiber core channel through FIFO devices. And the OTN equipment boards are all installed in the Jinxiu Chuan data center. Figure 3b shows the photo of the testing site in the Jinxiu Chuan data center.

4. Experimental Results of the Long-Haul Transmission

In this section, we introduce the experimental results of our long-haul field trial utilizing a commercial 400 Gb/s backbone OTN system. We first measure the BER values after transmission through each span to evaluate the long-haul transmission capability of the high-speed WDM-CDM transmission system utilizing the setup shown in Figure 3a. As mentioned above, each vacant core is also filled with C + L-band ASE noise to simulate seven-core fully loaded CDM transmission during this measurement process. The total input power of the C-band and L-band signals is approximately 24 dBm and 23 dBm, respectively. The input power of the C-band signal is slightly higher than that of the L-band signal, which is mainly to balance the influence of the SRS effect. The measured results at 1550.92 nm and 1590.83 nm are shown in Figure 4. We can find that the C-band and L-band signals exhibit similar BER performance. And after transmission through 14 spans, the BERs of the test signals are still significantly lower than the SD-FEC limit, which demonstrates that the CDM transmission system using 400 Gb/s backbone OTN transceivers has the capability to transmit over 990.64 km. The BER values under different OSNRs after 990.64 km transmission are further measured and the results at 1550.92 nm and 1590.83 nm are shown in Figure 5a and Figure 5b, respectively. The results under back-to-back (B2B) configuration are also depicted for reference. We can find that the BER–OSNR characteristics of C-band and L-band signals are similar. And under the SD-FEC limit, the corresponding OSNR thresholds are no more than 16.6 dB for both C-band and L-band signals under 990.64 km CDM transmission. Compared to the results under the B2B configuration, the degradation of the OSNR threshold does not exceed 1.1 dB. These results demonstrate that the CDM system utilizing 400 Gb/s backbone OTN transceivers has good noise tolerance and can compensate for the impacts brought by fiber transmission well.

Lastly, we adjust the central wavelength of the OTUs at both transmitter and receiver stations to investigate the BER values at the 80 wavelength channels. The total input power of C-band and L-band signals is also set to 24 dBm and 23 dBm, respectively, by adjusting the EDFAs and C + L-integrated WSSs. The experimental results are shown in Figure 6. It is obvious that after 990.64 km WDM–CDM transmission, the BERs of all 80 wavelength channels are still significantly lower than the SD-FEC limit. And the OSNRs of the 80 wavelength channels are all higher than 19.3 dB, which proves that an OSNR threshold exceeding 2.7 dB is reserved for each wavelength channel. The optical spectra of the transmitted 400 Gb/s 80-wavelength signals at the transmitter, after the first DGE and after the second DGE, are shown in Figure 7a–c, respectively. And the optical spectra of the received signals before the first DGE, before the second DGE, and at the receiver are also depicted in Figure 7d–f, respectively. We can find that the signals at shorter wavelengths are pre-equalized by the WSSs at the transmitter and DGEs, which is mainly to mitigate the effects induced by SRS and the uneven gain spectra of the EDFAs. And the signals at longer wavelengths in the L-band are also pre-emphasized by the WSSs. This is mainly due to the gain of L-band EDFAs for long-wavelength signals being relatively low. Pre-emphasizing the signals at longer wavelengths can compensate for the influence of the EDFAs and thus ensure a relatively flat optical spectrum at the receiver. These results demonstrate the feasibility of real-time 448 Tb/s (400 Gb/s × 80 × 7) transmission over a 990.64 km field-deployed MCF cable. To the best of our knowledge, this is the first real-time long-haul 80-wavelength seven-core WDM–CDM field trial using commercial DP-QPSK 400Gb/s backbone OTN transceivers, and a real-time capacity–distance product of 221.9 Pb/s∙km for MCF cable is field demonstrated.

5. Discussion

In our field trial, we investigate the long-haul transmission capability of a deployed seven-core fiber cable utilizing commercial 138-Gbaud DP-QPSK 400 Gb/s backbone OTN transceivers. And thanks to the highly noise-tolerant DP-QPSK modulation format enabled by the high modulation rate, a transmission distance of up to 990.64 km is achieved with an over 2.7 dB OSNR margin. Although the long-haul transmission is simulated by cascading the fiber core of two 70.76 km CDM links, it effectively simulated the transmission effects in the CDM system including inter-core crosstalk by filling all the fiber core channels. To further verify the superiority of our experimental system, we compared our work with recent high-speed transmission experiments based on MCFs, and the results are presented in

Table 4. Comparisons with recent MCF-based high-speed transmission experiments.

Types of MCF	Achieved Net Bit Rate	Transmission Distance (km)	Capacity–Distance Product (Pb/s∙km)	Type of the Experiment	Form of the Experiment	References
4-core fiber	45.7 Tb/s	12,053	550.82	Laboratory testing	Offline	[22]
4-core fiber	63.58 Tb/s	1800	114.44	Field trial	Offline	[25]
4-core fiber	319 Tb/s	3001	957.32	Laboratory testing	Offline	[13]
7-core fiber	179.2 Tb/s	350	62.72	Laboratory testing	Real-time	[17]
7-core fiber	616 Tb/s	360	221.76	Laboratory testing	Real-time	[23]
7-core fiber	187.49 Tb/s	264	49.5	Field trial	Offline	[28]
19-core fiber	3.61 Pb/s	1	3.61	Laboratory testing	Offline	[15]
38-core 3-mode fiber	22.9 Pb/s	13	297.7	Laboratory testing	Offline	[16]
7-core fiber	224 Tb/s	990.64	221.9	Field trial	Real-time	Our work

. It can be observed that the achieved capacity–distance product in our work has reached a state-of-the-art level among the real-time CDM transmission experiments using commercial high-speed OTN transceivers [10,16]. This is primarily attributed to the high-performance 138 Gbaud 400 Gb/s OTN transceivers with a highly noise-tolerant DP-QPSK modulation format and the utilization of DGEs for equalizing the differences between different wavelength channels in broadband transmission. And it should be emphasized that, in contrast to previous laboratory-based demonstrations using MCFs and offline DSP, our work is implemented using commercial 400 Gb/s OTN transceivers over a field-deployed MCF cable with more than 600 fusion splices, which is more aligned with real-world long-haul transmission applications and will inevitably affect system performance compared to ideal lab conditions. Our work indicates that the CDM transmission technique utilizing uncoupled MCF has the potential to be implemented in high-speed 400 Gb/s backbone OTN systems. In our future work, we will further investigate the long-haul transmission capability of seven-core fiber cable with next-generation high-speed 800 Gb/s OTN transceivers.

Moreover, we find that the splicing loss of seven-core fiber in our experiment is larger than the state-of-the-art value [21]. One primary reason is that our fusion splicing process is performed over a deployed MCF cable in the field. And unlike ideal lab conditions, inevitable vibrations in the field (such as passing vehicles) will affect the precise alignment of fiber cores during the splicing process. Compared to the average fusion splicing loss of deployed four-core MCF cable in recent work [25], the average fusion splicing loss of our seven-core MCF cable is only slightly larger. This indicates that the fusion splicing technique used in our experiment is relatively advanced. While it should also be acknowledged that, in our field trial, the splicing loss of seven-core fiber is significantly larger than that of standard SMF, this increases the span loss of the seven-core fiber cable and affects the transmission performance of the CDM system. By further reducing the splicing losses of the MCF, the transmission performance can be significantly improved, and a longer transmission distance can be achieved. Therefore, in our future work, we will continuously improve our fusion splicing technique for MCFs, using more precise and stable automatic alignment algorithms and developing multi-electrode arc-discharge systems, and further promote the field implementation of MCF cables in long-haul transmission systems.

6. Conclusions

In conclusion, we investigate the real-time long-haul transmission capability of field-deployed seven-core MCF cable using commercial 138-Gbaud DP-QPSK 400 Gb/s backbone OTN transceivers manufactured by Fiberhome Telecommunication Technologies Co., Ltd. In Wuhan, China. Thanks to the highly noise-tolerant DP-QPSK modulation format enabled by the high baud rate, a real-time 256 Tb/s transmission over a 990.64 km deployed seven-core fiber cable with more than 600 fusion splices is demonstrated, which achieves a real-time capacity–distance product of 221.9 Pb/s∙km with an over 2.7 dB OSNR margin. The long-haul CDM transmission is simulated by cascading the fiber cores of two 70.76 km CDM links. And DGEs are utilized to mitigate the impacts of SRS and the uneven gain spectra of EDFAs in broadband transmission. To the best of our knowledge, our work is the first real-time long-haul field trial using commercial DP-QPSK 400 Gb/s backbone OTN transceivers over deployed MCF cable, and we believe it will contribute to the field implementation of uncoupled MCF in long-haul terrestrial fiber cable systems.