Long-distance transmission achieved in a 4-core optical fibre

Researchers from Japan’s National Institute of Information and Communications Technology (NICT) Network System Research Institute say they have achieved the first S-, C- and L-band transmission over long-haul distances in a 4-core optical fibre with standard outer diameter (0.125 mm).

The results of the experiment were accepted as a post-deadline presentation at the International Conference on Optical Fiber Communications (OFC 2021) — one of the largest international conferences related to optical fibre communication.

Over the past decade, research has been carried out worldwide to increase the data rates in optical transmission systems using space-division multiplexing (SDM) as a means to meet the exponentially increasing demand for optical data transmission. More recently, interest in fibres with the same 125 µm cladding diameter as standard singlemode fibres has grown due to their compatibility with conventional cabling infrastructure and concerns over the mechanical reliability of larger fibres. Particularly with multi-core fibres (MCFs), reducing the cladding diameter limits the number of spatial channels, leading to increasing interest in combining such fibres with wider transmission bandwidths in order to meet the expected growth in transmission capacity expected in SDM fibres.

Until now, NICT has built various transmission systems that make use of wavelength division multiplexing (WDM) across the C and L bands together with state-of-the-art modulation technology to explore high data-rate transmission in a range of new optical fibres. Recently, NICT and research groups around the world have begun to explore S-band transmission, leading to several new records for transmission capacity in optical fibres, but transmission distance has been limited to only a few tens of kilometres.

Now NICT has built a long-distance transmission system around a 4-core optical fibre with a standard cladding diameter to exploit wider transmission bandwidth of >120 nm across S, C and L bands. The system exploits WDM and a combination of optical amplification technology to enable long-haul transmission of 552 WDM channels from 1487.8 to 1608.33 nm. Long-distance transmission, not previously demonstrated with S-band signals, was enabled by constructing a recirculating transmission loop experimental set-up that combined two kinds of rare-earth doped fibre amplifiers with Raman amplification distributed along the transmission fibre itself.

Part of the transmission system (Raman amplification section).

The system was used to measure achievable transmission throughput with each channel modulated with PDM-16QAM modulation at distances up to 3001 km, where a data rate of 319 Tbps was achieved. This result may be compared to achievements in other SDM fibres and transmission regimes by calculating the product of transmission capacity and distance, often used as a comparison metric. The data-rate x distance product becomes 957 Pbps x km, which is over 2.7 times larger than previous demonstrations in SDM fibres with standard outer diameter.

The 4-core MCF with standard cladding diameter is attractive for early adoption of SDM fibres in high-throughput, long-distance links, since it is compatible with conventional cable infrastructure and expected to have mechanical reliability comparable to singlemode fibres. Beyond 5G, an explosive increase from new data services is expected and it is therefore crucial to demonstrate how new fibres can meet this demand. Hence, it is hoped that this result will help the realisation of new communication systems that can support new bandwidth-hungry services.

NICT will continue to develop wide-band, long-distance transmission systems and explore how to further increase transmission capacity of low-core-count multi-core fibres and other novel SDM fibres. Furthermore, the researchers will work to extend the transmission range to trans-oceanic distances.

Top image credit: ©stock.adobe.com/au/Sashkin

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