Coherent Communication Over Multi Mode Fibers for Intra-Datacenter Ultra-High Speed Links

Multimode fibers (MMFs) links using vertical cavity surface-emitting lasers (VCSELs) are still the solution of choice for today’s data center shorter distances thanks to their low cost and robustness. Currently, the market is moving towards 400G-capable systems using multiple lanes at 50 Gbps per lane, based on intensity modulation (IM) and direct detection (DD). A technology switch will probably be required to overcome the 100 Gbps per lane limit that will likely be the bottleneck of future IM-DD MMF-based links. In this manuscript we propose an intra-data center system based on coherent detection and MMF, analyzing experimentally and analytically the performance of polarization multiplexed coherent communication. We show that the advantages enabled by the coherent technology can be fully exploited in an MMF-based link when central launch is ensured. In particular, we study in detail the effect of lateral offsets introduced by the connectors at the interface between two MMF facets, in terms of net optical loss and signal-to-noise ratio (SNR) degradation. A statistical analysis is performed analytically exploiting a large database of OM3 and OM4 fibers modal delays and results are presented for various combinations of fiber length, type and number of MMF-to-MMF connections along the link. We show quantitative results on the maximum acceptable lateral offsets in the MMF connectors, highlighting that in practical conditions average offsets of up to 3
$\mu$
m can be tolerable.