80 Gbps PAM-4 Data Transmission With 940 nm VCSELs Grown on a 330 μm Ge Substrate

940 nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) on $330~\mu $ m thick Ge bulk substrates were fabricated and characterized, presenting a novel approach to VCSEL manufacturing. The wafer surfaces demonstrated high smoothness and flatness, with a peak-to-valley wafer distortion of $50.3~\mu $ m, a root mean square roughness (Rq) of 1.34 nm, and an average wafer bow-warp of $3.77~\mu $ m. The Fabry-Pérot dip precisely aligned with the target wavelength, while stopband center mapping exhibited excellent uniformity across the wafer, with a 1.937 nm (0.206%) standard deviation. At 300 K, the Ge-based VCSEL with a $6~\mu $ m oxide aperture achieved an optical peak power of 5.5 mW and a maximum modulation bandwidth of 19.8 GHz, with a roll-over current surpassing 16 mA. Furthermore, the device demonstrated successful data transmission at 53.125 Gbps and 80 Gbps using PAM-4 modulation, achieving transmitter and dispersion eye closure quaternary (TDECQ) penalties of 1.36 dB and 4.70 dB, respectively. These results underscore the potential of thin Ge substrates in advancing VCSEL technology for high-speed optical communication applications.