A 50-Gb/s PAM-4 Silicon-Photonic Transmitter Incorporating Lumped-Segment MZM, Distributed CMOS Driver, and Integrated CDR

This article presents a 50-Gb/s optical transmitter (TX), consisting of a 40-nm distributed CMOS driver and a 180-nm silicon-photonic modulator. A lumped-segment Mach-Zehnder modulator (LS-MZM) is developed for high bandwidth (BW) four-level pulse amplitude (PAM-4) modulation. A multi-segment driver with limiting outputs is co-designed, which is distributed into each LS-MZM segment. By grouping these LS-MSM segments in a thermometer code, high-linearity modulation is realized without the need of power-hungry high-swing linear drivers. To improve the optical PAM-4 signal integrity, in-segment multiplexing along with clock phase interpolation is adopted to synchronize the electrical and optical signals across all segments. The hybrid coupling between the driver and modulator is devised to boost the BW of the high-speed data path, while a half-rate clock and data recovery (CDR) circuit is integrated to remove the accumulated jitter. Measurements show that the TX exhibits an extinction ratio (ER) of up to 9.8 dB and a 0.99 ratio of the level mismatch. A figure-of-merit (FoM) of 1.39 pJ/bit/dB corresponds to a 682-mW power, which can be further reduced by 40%, at the cost of a degraded ER of 4 dB. The PAM-4 CDR helps to achieve < 10^{-12} BER and >0.1- \text{U}I_{\mathrm {pp}} jitter tolerance (JTOL) from 10 to 100 MHz.