A 56Gb/s PAM-4 VCSEL driver circuit

We present a driver circuit that can modulate a VCSEL with a 56Gb/s (28Gbaud/s) 4-level pulse-amplitude-modulated (PAM-4) signal. High power efficiency is achieved using a common-source driver topology that only draws the modulation current corresponding to the PAM-4 symbol being transmitted. A Gray encoder based on MOS Current-Mode Logic (MCML) gates is included on chip, as well as programmable PAM-4 bias and modulation levels.