Data centers around the globe are hosting many applications and services that are increasingly integrated into everyday lives. As data center traffic continues to grow, there is a great demand for high-speed, scalable, and power efficient intra-data center links. With scalability on multiple dimensions, higher spectral efficiency, and improved sensitivity compared with its intensity-modulation and direct detection (IM-DD) counterpart, coherent technology is a promising solution for intra-data center links (< 2km). Conventional coherent links based on digital signal processing (DSP) require high sampling rate analog-to-digital converters (ADC), significantly degrading energy efficiency. An analog coherent scheme performs carrier recovery and phase detection functions using an electronic integrated circuit (EIC) to implement an optical phase locked loop (OPLL) that provides carrier frequency and phase tracking, thereby, eliminating the need for ADCs and improving the energy efficiency.

The focus of this work is to develop high speed O-band QPSK transmitter and coherent receivers to be integrated with energy efficient EICs. O-band QPSK coherent receivers consisting of custom EICs and coherent optical front-end photonic integrated circuit (PIC) fabricated in two different silicon photonic (SiP) platforms are demonstrated at 100~Gb/s per polarization. An EAM-based QPSK transmitter with custom EIC driver is also presented with up to 64~Gb/s operation below SD-FEC BER limit. In addition, QPSK coherent link performance with the EAM-based transmitter and one of the custom EIC-PIC receivers is characterized, showing operation up to 64~Gb/s below SD-FEC BER limit. Discussions on polarization recovery and pros and cons of different photonic technology for data center application are also provided.