Multimode optical interconnects based on VCSEL and MMF for more than 100-Gb/s/lane and 100m transmission

Vertical cavity surface emitting laser (VCSEL) and multimode fiber (MMF) based short reach optical solutions are widely deployed as the optical engines in the intra-datacenter communications since its significant advantages of low cost, easy design and test, high yield and low power consumption. With the increasing demand for highspeed interconnects, more than 100-Gb/s per lane and more than 100m transmission distance are highly required in order to meet future 4×100 Gb/s and 800G/1T module deployment in the high performance computing systems including datacenters and supercomputers. In this paper, we propose the comprehensive optical and electrical equalization approaches for the bandwidth limited devices, with the typical VCSEL 3-dB bandwidth around 20 GHz, to deal with the challenges including the inherent modal and chromatic dispersion, modulation nonlinearity and various noises in order that multimode interconnect could reach 100 Gb/s/lane over the distance more than 100m. To mitigate the various noises due to the multimode nature of the interconnect solutions, we propose the spatial phase manipulation scheme to dynamically control the coupling coefficients between the VCSEL and MMF links. We further investigate the physical limitations of the VCSEL during multilevel modulation, and propose the level dependent equalization approaches for the PAM4 modulation. Finally, more advanced nonlinear equalization, Volterra equalization, is investigated and a simplified version named by threshold based pruned retraining Volterra equalization (TRVE) is experimentally demonstrated for the VCSEL enabled 100 Gb/s PAM-4 over 100m MMF. Up to 94.2% and 88.0% complexity reduction has been achieved for B2B and 100m.