A 50-Gb/s 1.35-pJ/b PAM-4 VCSEL Transmitter With Three-Tap Asymmetric FFE and Current-Resue Technique in 40-nm CMOS

This work presents an 850-nm vertical-cavity surface-emitting laser (VCSEL)-based optical transmitter (TX) capable of reaching speeds of up to 50 Gb/s in the four-level pulse amplitude modulation (PAM-4) mode. The signal equalization path incorporates a binary-to-thermal (B2T) coder and the three-tap asymmetric feedforward equalization (FFE). The proposed B2T-based signal path realizes an adjustable eye-height optimization function in the PAM-4 mode. The adopted asymmetric FFE enhances the output driver’s bandwidth by shifting the summation node to the equalizer output and also addresses the VCSEL’s nonlinear light-current-voltage (L-I–V) response. Meanwhile, the proposed stacked output stage consisting of the low- pass current reuse pre-driver and high- pass push – pull driver enhances the efficiency by reusing the modulation current and provides independent and flexible equalization capabilities for the PAM-4 signal. Fabricated with a 40-nm CMOS process and occupying an active area of 0.24 mm2, the TX achieves a 50-Gb/s optical data rate with 1.35-pJ/b energy efficiency with a 3.3-V biased VCSEL. When butt-coupled with a 2-m multimode fiber (MMF), the compensated PAM-4 waveform achieves an extinction ratio (ER) of 2.4 dB and an optical modulation amplitude (OMA) of 3.4 dBm at the 50-Gb/s PAM-4 mode.