A Low-Power High-BW PAM4 VCSEL Driver With Three-Tap FFE in 12-nm CMOS FinFET Process

This work presents a low-power high-bandwidth (BW) PAM4 VCSEL transmitter. We introduce a new cascode voltage-mode (VM) driver to provide a larger output modulation current while using smaller devices and lower power consumption compared to the conventional VM driver. This improves the power efficiency and relaxes the self-loading concern in the conventional designs. The driver also includes an asymmetric three-tap feedforward equalizer (FFE) for MSB and LSB to effectively compensate for the BW and nonlinearity response of the VCSEL. Further, the driver is implemented using multiple segments to optimize the output impedance and linearity. The proposed transmitter is implemented in a 12-nm CMOS FinFET process and is packaged to facilitate both electrical and optical measurements. Based on the electrical measurement, the transmitter supports up to 50-Gb/s nonreturn to zero (NRZ) and up to 64-Gb/s PAM4 data transmission. The optical measurement is done using a commercial 850-nm anode-driven VCSEL. Based on the optical measurement, the proposed design supports up to 40-Gb/s NRZ and 50-Gb/s PAM4 data transmission with an optical modulation amplitude (OMA) of better than 1.4 dBm and power efficiency of 0.82 (excluding VCSEL power) and 0.97 pJ/b using three supplies of 1, 2, and $-$ 1.5 V providing currents of 11, 15, and 5 mA, respectively.