A 56-Gb/s PAM-4 Transmitter Using Silicon Photonic Microring Modulator in 40nm CMOS

This work presents a 56-Gb/s pulse amplitude modulation (PAM)-4 transmitter (TX) using a silicon photonic microring modulator (MRM) in 40nm CMOS technology. The TX adopts a quarter-rate architecture with a thermometer-encoded topology for independent single-eye-height adjustment. Key building blocks of the TX include a pulsed tri-state inverter-based 4:1 multiplexer (MUX), a level shifter with active resistor bias, a voltage-mode high-swing driver with swing control, and an output pad-network with passive equalization. A ring oscillator-based wide bandwidth phase-locked loop (PLL) is used in this work as the multiphase clock generator to save power while maintaining acceptable phase accuracy. At 56-Gb/s operation, the TX consumes 71.5mW including the clock path, achieving a 1.28pJ/bit energy efficiency with up to ${3.1V}_{\mathrm{pp}}$ differential output swing and larger than 7.2dB extinction ratio.