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Sumitomo designs 8-core fibre for compact optical cables

Sumitomo Electric Industries has developed an optical fibre with eight cores inside a standard 125μm-diameter cladding, which has allowed the company to manufacture fibre-optic cable based on the new fibre. The resulting 3mm cable has the highest core density ever reported, the company claims.

The results were presented at the postdeadline paper (Th5C.6) at OFC 2015 on 26 March 2015 in Los Angeles, California.

Sumitomo’s new fibre is aimed at short-reach optical interconnects used to connect servers and switches inside data centres and parallel processing in supercomputers. Multi-core fibre (MCF) is also being intensively researched and developed to help realise ultra-high-capacity transmission systems, but transmission experiments so far have only achieved the shorter distances.

Most previously reported MCF developments used cladding diameter that is thicker than the standard 125μm cladding used in optical fibres because this makes it possible to increase the number of cores while keeping sufficient distance between them to suppress core-to-core crosstalk, says Sumitomo. However, such fibres are not compatible with standard connectors, processes and tools. Sumitomo’s new MCF fibre has mechanical reliability and dimensions equivalent to that of standard optical fibres and therefore is more amenable to manufacture into fibre-optic cables.

“Owing to the standard 125-μm cladding diameter, we were able to fabricate the cable by using our manufacturing facilities for current optical fibre cable products without any modifications, and able to use standard ferrules for connectorisation,” the company wrote in the paper.

The eight cores were arranged in a simple circle with a 31-μm pitch between nearest neighbours. Sumitomo used a trench-assisted design to confine the light, achieving good suppression in the O-band at 1.3μm but accepting higher leakage loss in longer wavelength bands. Optimising the fibre around the 1.3-μm wavelength band provides compatibility with devices made in silicon photonics technology, the company points out.

Sumitomo says the core count of eight is highly desirable because it enables full-duplex (2x) 100-Gb/s (4 × 25 Gb/s) signals to be transmitted over one MCF using commercially available transmitters and receivers designed to support 25-Gb/s data rates.

Sumitomo also evaluated the transmission characteristics in 1.3-μm band of the 1.1-km-long MCF cable. Using a 100 Gigabit Ethernet (100GBASE-LR4) transceiver, the experiment achieved error-free transmission of 800 Gb/s (8 cores × 4 wavelengths × 25 Gb/s) signals, which is eight times higher than standard singlemode fibre – as one would expect.

The 12-fibre cable manufactured from the developed MCFs contained 96 cores within an outer diameter of 3mm. The results indicate that this new cable could support transmission capacity of at least 9.6Tb/s (12 fibres × 8 cores × 4 wavelengths x 25Gb/s) – and more as transmitter technologies improve.

The research was supported in part by the National Institute of Information and Communications Technology (NICT), Japan under ‘R&D of Innovative Optical Fibre and Communication Technology’.

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