4 × 25 Gb/s PAM4 optical transmitter for micro‐ring modulator with thermal control

Abstract The explosive growth of bandwidth demands of data center and super‐computing systems spur the appliance and development of optoelectronic‐based solutions such as micro‐ring modulator (MRM) transmitters, and advanced modulation schemes such as four‐level pulse‐amplitude modulation (PAM4). In this paper, a 4 × 25 Gb/s PAM4 MRM transmitter fabricated in 0.13 μm SiGe BiCMOS technology is presented. It consists of four‐channel MRM driver and thermal control loop. Capacitor and resistor degeneration techniques and inductor peaking techniques are adopted to expand channel bandwidth. Meanwhile, the predistortion technique is utilized to compensate for MRM nonlinearity, and auto gain control (AGC) circuits to enable the MRM driver to realize a fixed differential output swing of 3.5 Vpp. Furthermore, a thermal control method based on analog to digital converter, digital to analog converter, and logic control circuits is proposed to compensate for the temperature sensitivity of MRM. The measurement results show that the 25 Gb/s nonreturn‐to‐zero optical eye achieves an extinction ratio of 5.63 dB, and the PAM4 eye can be observed up to 40 Gb/s.