The ever-increasing demand for higher data rates in both short-and long-distance communication networks necessitates multilevel modulation schemes [1][2][3][4][5]. The 850 nm vertical-cavity surface-emitting laser (VCSEL)-based fourlevel pulse-amplitude modulation (PAM-4) has been adopted as the standard solution for 100 G/200 G Ethernet [ 6 ]. Compared with the two-level pulse-amplitude modulation (non-return to zero) signal, the PAM-4 signal has half the Nyquist frequency at the same data rate, its interlevel separation that is three times smaller, and its eye-width reduction is larger. Moreover, the timing asymmetry among the lower, middle, and upper eyes is problematic. Consequently, at the transmitter side, these issues require more stringent control of the driving-signal characteristics, component performance, and module-assembly process.AlGaAs-based VCSELs have enabled short-distance data links. Normally, the top-layer material of AlGaAs-based VCSELs is GaAs because the VCSEL structure exhibits low contact and lateral resistances. As the refractive-index step of the air-GaAs interface is the highest among numerous VCSEL interfaces, the optical thickness of the GaAs top layer significantly affects the VCSEL characteristics [ 7 ]. Any deviation from the phase-matched thickness changes the threshold gain, threshold current, and slope efficiency. Moreover, the deviation changes the relaxation-oscillation frequency by changing the photon lifetime.