A 50-GBaud QPSK Optical Receiver With a Phase/Frequency Detector for Energy-Efficient Intra-Data Center Interconnects

This paper describes the energy-efficient realization of a QPSK optical receiver (CoRX) for short-reach intra-datacenter interconnects based on analog coherent detection. The CoRX comprises inphase and quadrature channels for each polarization and a high-speed phase-frequency detector (PFD) that provides feedback to stabilize an optical local oscillator (LO) and maintain coherence with the received optical signal. Each receive (RX) channel consists of a transimpedance amplifier (TIA) based on a Cherry-Hooper emitter follower (CHEF). The electronic RX is implemented in a 130-nm SiGe HBT technology ( $f_{T} = 300$ GHz), consumes 534 mW of DC power for a total electrical RX energy efficiency of 5.34 pJ/bit, and occupies 2.8 $mm^{2}$ . Electrical characterization of the CoRX on an FR-4 PCB assembly demonstrates operation up to 60 GBaud with a bit error rate (BER) of less than 10 ⁻¹² . A co-packaged optical/electrical CoRX assembly with a silicon photonic receiver is characterized using a commercial-off-the-shelf quadrature phase-shift keying (QPSK) transmitter for constellations up to 50 GBaud (100 Gbps) at BER below KP4-FEC ( $2.2\times 10^{-4}$ ).