1.55-μm Long-Wavelength VCSEL-Based Optical Interconnects for Short-Reach Networks

We demonstrate the generation and transmission of a 28-Gb/s intensity modulated optical signal over single mode fiber (SMF) links up to 1, 2, 5, and 10 km employing a 18-GHz 3-dB bandwidth monolithic vertical-cavity surface-emitting laser (VCSEL) based on buried tunnel junction technology with regrown n-doped InP material. Inexpensive technologies such as nonreturn-to-zero on-off keying modulation format, direct detection as well as digital signal processing-based receivers make the transmission feasible for chromatic dispersion (CD) uncompensated SMF links up to 10 km at the 7% hard-decision (HD) forward error correction (FEC) limit. Three different equalizers for the receiver side are investigated in this paper in terms of performance optimization, i.e., the maximum likelihood sequence estimation equalizer (MLSE), the feed-forward equalizer (FFE) and a hybrid scheme that comprises of an FFE and an MLSE equalizer (FFE/MLSE) in cascaded form. Our experimental results indicate that MLSE scheme outperforms the other two counterparts in all transmission scenarios. Performance optimization in terms of MLSE complexity is also presented. Less complex implementations like FFE equalizer can provide reliable transmission below the 7% HD-FEC limit for links up to 5 km. The FFE/MLSE scheme provides similar performance as MLSE for links up to 5 km, while its performance deteriorates for 10 and 15 km links as signal distortion due to CD becomes too severe to recover the signal. A timing recovery unit is employed before the equalizers in order to compensate for phase offsets and improve the signal quality. Our proposed scheme proves a promising candidate to enable VCSEL-based short reach optical interconnects in data centers and metro-access area.