Noise Improvement in High-speed VCSELs for PAM-4 Transmission with Low TDECQ Penalties

Noise control in oxide-confined 850-nm vertical-cavity surface-emitting lasers (VCSELs) has become increasingly critical for short-reach optical interconnects, particularly under multilevel modulation formats such as PAM-4, where reduced noise margins heighten sensitivity to modal disturbances. Under PAM-4 modulation, performance is increasingly constrained by modal noise and intensity fluctuations caused by mode beating between spatially degenerate modes with equal energy. In this work, we present an aperture shaping strategy to suppress beating noise at low frequencies by replacing the conventional central symmetric oxide aperture with a deliberately off-center asymmetric design. The introduced off-center asymmetry aperture effectively lifts modal degeneracy and differentiates the energies of splitting modes, eliminating mode beating within the critical relative intensity noise (RIN) frequency range of PAM-4 systems. Experimental characterization demonstrates consistent transmitter dispersion eye closure quaternary (TDECQ) penalties below 1.2dB across a broad temperature range and up to 100m standard OM4 fiber reach. The proposed design offers a scalable, fabrication-compatible solution for enabling low-noise, high-speed VCSELs in 50 Gbps PAM-4 optical links.