22.6 A 25Gb/s 4.4V-swing AC-coupled Si-photonic microring transmitter with 2-tap asymmetric FFE and dynamic thermal tuning in 65nm CMOS

Silicon photonic microring modulators (MRMs) offer a promising approach for realizing energy-efficient wavelength-division multiplexing (WDM) optical interconnects. For data-rates greater than 10Gb/s, depletion-mode MRMs are generally preferred over their injection-mode counterparts due to their shorter carrier lifetimes and resulting higher bandwidths. Unfortunately, these depletion-mode MRMs typically exhibit low PN junction tunability, thereby requiring higher modulation voltages in order to provide >6dB extinction ratios (ER). Furthermore, negative DC-biasing of the MRMs is necessary to maintain reverse-biased depletion-mode operation. In this work, a 5×25Gb/s hybrid-integrated MRM WDM transmitter is demonstrated that incorporates the following key advances: 1) an AC-coupled differential output driver that applies a 4.4Vpp-diff output-swing on the MRM while providing a tunable on-chip negative DC-bias; 2) a 2-tap non-linear digital FFE that compensates for optical-dynamics-induced bandwidth limitations; 3) a dynamic thermal tuning loop that stabilizes the MRM by minimizing thermally-induced wavelength fluctuations.