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Abstract
We have designed, developed, and deployed the world's first ultra-dense space division-multiplexing multicore fiber link in a conduit of a metro network. In a 10-mm-diameter fiber optic-cable, 288 4-core multicore fibers are arranged in 24 200-µm spiderweb collapsible ribbons. The multicore fibers are fusion-spliced to 576 fanout devices which provide conventional single-core interfaces at patch panels at both ends of the link.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Physical-space constraints and congestion of the pathways due to optical cables deployed over time present a growing challenge to the expansion of the global infrastructure of interconnected data centers. Maintaining the integrity of the fiber optic infrastructure while upgrading its speed with greener and less eco-disruptive solutions without raising costs, requires pathways with significantly higher optical density. In recent years, smaller-diameter cables have offered some gains, but the ultimate solution must reduce the total number of physical cables added to the infrastructure. Multicore fiber (MCF), especially combined with wavelength division multiplexing (WDM), is a leapfrog solution that removes the risks and barriers mentioned above [1–4]. Industry collaborations integrating MCF, fanout and splicing technologies, are beginning to address commercial deployment of space division multiplexing (SDM) links for data center needs.
Here we describe the design, development, and deployment of an 800-m-long, 1152-core, multicore fiber link, as shown schematically in Fig. 1. We have moved from characterizing links in labs and testbeds trials [5,6,7,8], to the installation and characterization of an SDM link in a conduit of a metro network. We integrated diverse space-saving techniques, including (1) 4-core MCF development, (2) reduction of coating thickness to 200 µm, and (3) SpiderWeb Ribbon (SWR) technology. This resulted in a 1152-core cable of 10 mm outer diameter (OD) with bend radius of 19 cm. This differs from conventional 1152-fiber cable with 26.6 mm OD and 28 cm bend radius [9]. Advances in automated multicore fiber splicing and fanout fabrication allowed for efficient installation of a fiber optic link in which 1102 channels had loss below 3 dB with 1.2 dB and 1.5 dB average loss at 1550 and 1310 nm, respectively. Losses are measured from patch panel to patch panel (PP) with an optical loss test set (OLTS). The measured link loss includes two fanout devices, two MCF-MCF splices, four ribbon splices to inside plant (ISP) cables and MCF propagation loss. The choice of the 4-core fiber for this link was dictated by practical considerations, including the trade-off between crosstalk limit and the benefits of keeping standard outer glass fiber diameter of 125 µm [10].
2. MCF cable fabrication and splicing
The cross section of a 200-µm-coated, 4-core multicore fiber (200-4CF) with 125 µm cladding developed for short-reach transmission in O- and C-band datacom applications is shown in Fig. 2 [11]. The fiber has four homogeneous cores with step-index profiles. The step-index profile has a simpler refractive index structure and is more easily manufactured than a trench-assisted profile. Crosstalk in the C-band is relatively large because of the absence of an index trench. However, it can be used in data-center networks with O-band short-reach transmission and C-band limited-length transmission of around 50 km without degrading performance [12]. Table 1 lists the structural and optical characteristics of the fiber. The 200-4CF is compliant with ITU-T G.657.A1 recommendations. Average, minimum, and maximum core pitches are 40.0 µm, 39.8 µm and 40.2 µm, respectively. Such small fluctuations in the core pitch allow for excellent connectivity of the MCF to the fanouts and other devices. Inter-core crosstalk is smaller than -50 dB/km at 1310 nm and -36 dB/km at 1550 nm. As illustrated in Fig. 3, 288 200-4CF fibers were arranged in 24 space-saving 12-channel SWRs. The fabricated cable was deployed by utilizing the technique of blowing into an underground outdoor 15-mm OD/13-mm ID conduit connecting two buildings. There were no other fibers in the same conduit. The 24-hour average temperature at the installation site during cable installation was 10°C. At both ends of the deployed 800-m cable, the MCFs were spliced one-by-one to 288 fanout devices using a Fujikura FSM-100P fusion splicer. A custom splicing mode, based on the IPA method [13] and tailored to the 200-4CF fiber, enabled efficient automated alignment, splicing, and loss estimation. The automated splicing, which matches the MCF cores by detecting and aligning the marker position, takes about 3 minutes per MCF splice. The single-core pigtails of the fanouts were ribbon-spliced to 25-m ISP cables, which, in turn, were ribbon-spliced to the patch panels.
Fig. 2. (a) Cross section of the 200-4CF fiber, (b) Core refractive index profile. See Table 1 for dimensions.
Table 1. Characteristics of fabricated fiber.
3. Fanout fabrication and characterization
The fabricated fanouts are based on the vanishing-core approach [14] which allows for independent matching of the mode field diameter and the core pitch. Near field profiling was used to optimize the mode size and shape for best coupling to 200-4CF. The best fabricated fanouts had insertion loss below 0.1 dB. Average fanout insertion loss was 0.3 dB at 1550 nm. Measurements of the distributions of insertion loss, return loss, and crosstalk at 1550 nm for 612 fabricated fanouts, which includes 36 backup devices, are shown in Fig. 4. The crosstalk was measured with a broadband amplified spontaneous emission light source averaged over 2-nm band centered at 1550 nm. Each fanout was packaged in a compact, Telcordia-grade package [15] enabling conventional splice tray installation. Typical spectral performance of a fanout across both the O- and C-bands is shown in Fig. 5. The average crosstalk across the C-band is below -47 dB for neighboring cores used for counter-propagating light, and below -60 dB between cores across the diagonal used for co-propagating signals.
Fig. 4. Insertion loss, return loss, and crosstalk distribution at 1550 nm of 2448 channels of 612 fabricated fanouts fusion spliced to the 200-4CF fiber. The horizontal axis ranges cover all the measured fanouts.
Fig. 5. Typical insertion loss of a fabricated fanout fusion spliced to the 200-4CF fiber. Left and right plots represent O- and C-band performance, respectively. The frequency and magnitude of loss variations of this device are typical for both O- and C-bands.
4. Characterization of deployed MCF link
All 1152 channels of the deployed MCF link were tested with an OLTS with performance described above and analyzed with a Luna 6415 optical frequency domain reflectometer (OFDR) from one of the PPs. The OFDR device provides a high-resolution profile of the 1550 nm backscattered light along an optical fiber for the first 100 m of the link, which includes all essential parts of the installation. The amplitude of the backscattered light along the first 50 m of one of the MCF link channels is shown in Fig. 6 with the PP on the left and the MCF cable on the right. Moving from left-to-right, all 5 important junctions of the deployed link can be seen in this figure: (1) reflection peak at ∼1 m corresponding to the PP connector, (2) splice of the PP pigtail to the single-core fiber of the ISP, (3) ISP-fanout pigtail splice, (4) internal fanout-MCF splice, and (5) MCF-MCF splice. Backscattering in the MCF and in the PP pigtail are greater than in the standard ISP cable fiber. The distribution of the insertion loss of the first 90 m of the 1152 channels is shown in Fig. 7. The measured average insertion loss is 0.56 dB. There are 20 channels with loss of greater than 1.5 dB (half of the 3 dB link budget). The high loss in these 20 channels is not related to the MCF deployment, but to the yield of conventional single-core ribbon-to-ribbon splicing and PP connectors.
Fig. 6. Backscattered amplitude of the first 50 meters of one of the 1152 channels in the MCF cable. The remainder of the measured 100-meter trace does not have significant backscattered events.
Fig. 7. Insertion loss distribution of the 1152 channels measured from the PP to a distance of 90 m. The horizontal axis range covers all the measured fanouts.
5. Conclusion
The world’s first ultra-dense, multicore fiber link was deployed in a real-world metro network. The deployed 1152 cores, 800-m-long SDM link has average loss of 1.2 dB and 1.5 dB at 1550 and 1310 nm, respectively, within a 10-mm diameter cable. This was made possible by the co-development of 4-core 200-µm-SWR, low-loss MCF fanouts, and automated MCF-MCF splicing.
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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