A 56-Gb/s PAM-4 VCSEL Transmitter With Piecewise Compensation Scheme in 40-nm CMOS

This article presents a four-level pulse amplitude modulation (PAM-4) quarter-rate transmitter for vertical-cavity surface-emitting lasers (VCSELs). A piecewise compensation scheme is implemented to address VCSELs’ non-idealities, including nonlinearities in bandwidth and electrical-to-optical (E/O) gain versus bias current, and asymmetric responses to rising/falling transitions. The proposed transmitter adopts a unary code-based architecture with variable-transconductance cells and adjustable equalization circuits to independently control the amplitudes and widths of the three optical PAM-4 sub-eyes. Hence, the gain and bandwidth nonlinearities can be settled. Moreover, a falling edge pre-emphasis (PE) circuit is implemented to relieve the asymmetric rising/falling responses. Measurements at 56-Gb/s PAM-4 with a commercial VCSEL show that the proposed piecewise compensation scheme enhances the average optical PAM-4 sub-eye amplitude/width, ratio-of-level mismatch (RLM), and horizontal skew by 14%/12%, 38%, and 63%, respectively. Fabricated in 40-nm CMOS, the transmitter delivers an energy efficiency of 2.05 pJ/b at 56-Gb/s PAM-4.