A 64-GBaud 64-QAM Optical Coherent Transmitter with Monolithically Integrated Driver and I/Q Modulator in 45-nm SOI CMOS

This paper presents an optical coherent transmitter consisting of an analog output stage and two Mach-Zehnder Modulators (MZM). For monolithic integration of electrical and optical parts, a Cherry-Hooper-based 3-bit high speed R-2R ladder DAC and a push-pull output driver is realized to achieve high speed pulse amplitude modulation (PAM) signal generation, amplification and electrical-optical connection. The designed MOSFET variable resistors in DAC also provide pre-distortion to compensate nonlinearity of later stages. Simulations results show the combination of DAC, and the output driver achieves over 59 GHz 3-dB bandwidth with differential inputs, permitting 64GBaud PAM-8 signal processing. Implemented in a 45 nm silicon photonic (Si-Ph) CMOS SOI process, the optical transmitter reaches $384 \mathrm{~Gb} / \mathbf{s}$ channel speed with $64-Q A M$ modulation, at the electrical power cost of about 600 mW.