Spectral Efficiency and Energy Efficiency of Pulse-Amplitude Modulation Using 1.3 μm Wafer-Fusion VCSELs for Optical Interconnects

We compare the modulation efficiency of on–off keying (OOK) and four-level pulse-amplitude modulation (PAM4) using 1.3 μm wafer-fusion vertical-cavity surface-emitting lasers (VCSELs) for single-channel short-reach optical interconnects. O-band wafer-fusion VCSELs combine high optical gain from InP-based active regions with high-reflectivity, low-absorption, and high-thermal-conductive GaAs-based distributed Bragg reflectors. VCSELs offer a low-cost and low-power-consuming light source for optical interconnects in data centers and fiber-based local-area networks, presenting an attractive alternative to common edge-emitting lasers. Our investigation focuses on the highest error-free bit rates and energy efficiency of OOK and PAM4 signaling. We achieved an error-free bit rate of 38 Gbit/s with PAM4 for devices having a small-signal bandwidth of 9.9 GHz, presenting a spectral efficiency of 3.8 bit/s/Hz. These PAM4 values offer an improved error-free bit rate of 26% and consume 32% less energy-per-bit when co...