Universal Fibre for Coherent Data Centre Interconnect Applications

Multimode fibers have consistently represented a significant portion of deployed fibres in data centre applications, driven mostly by cost and power advantages. While fibre infrastructure is typically pre-deployed, transceiver upgrades are desirable, and have been made simpler with recent adoption of topologies based on optical circuit switching. This upgrade path benefit can be further enhanced by using versatile fibres supporting multiple transceiver technologies. In this work, universal fibre (UF), a novel multimode fibre type compatible with both multimode and single-mode transmission is experimentally assessed. Using coherent single-mode transmission technology and a training sequence (TS)-based digital signal processing (DSP) chain, C-band transmission over UF for data centre interconnect (DCI) applications is investigated with single-channel and dense wavelength division multiplexing transmission. The results are compared against a state-of-the-art single-mode fibre (SMF), and show small received signal-to-noise ratio penalties ($\lesssim$1 dB), allowing for a combined rate of 40.8 Tb/s for 75 channels on a 50-GHz grid. The impact of splicing on performance is studied, and no significant performance degradation due to multipath interference is observed for the standard precision of splicers and connectors. This paper also presents an in-depth analysis of equalization requirements, including TS length and channel estimation rate. The effectiveness of single-mode nonlinear compensation techniques is also tested, namely single-mode digital backpropagation. In all cases the UF is shown to achieve a performance consistent with that of SMFs. This study demonstrates the potential of UF to meet current DCI demands with coherent single-mode transmission, offering a scalable upgrade path from systems employing vertical-cavity surface-emitting lasers to single-mode transmission and to potential future compatibility with space-division multiplexing.