A 56-Gb/s PAM-4 Optical Transceiver with Nonlinear FFE for VCSEL Driver in 40nm CMOS

This paper presents a 56Gb/s PAM-4 optical transceiver fabricated in 40 nm CMOS. Differing from the optical transmitters with microring [1] or Mach-Zehnder [2] modulators that require high voltage swing to drive, the VCSEL can be directly driven by a small current, making a power-efficient solution for short reach application $(\lt 100 \mathrm{m})$ through the multimode fiber. The optical TX adopts a DAC-based structure with a 2-tap nonlinear PAM-4 FFE implemented in the DSP domain to compensate for the VCSEL's asymmetric responses. For the optical receiver, it accommodates an external PD+TIA with the electrical RX, increasing the flexibility for system applications. The VCSEL transmitter demonstrates a 56 Gb/s compensated PAM-4 waveform with -0.9 dBm outer OMA while consuming only 97 mW of power (1.73 pJ/b). The RX achieves $10^{-12}$ BER with 0.08 Ul margin under 10 dB interconnection loss between the PD+TIA module and the RX chip while dissipating 278 mW of power (4.96 pJ/b).