A 1.49 pJ/b 4-Channel 256-Gb/s MRM-Based Coherent Co-Packaged Optics with Linear Carrier Phase Recovery

AI interconnect scaling requires compact and low power, low latency co-packaged optics (CPO). We demonstrate a 4-channel 256 Gb/s laser-forwarded coherent CPO optical link using 4-Offset-QAM modulation at 64 Gb/s per channel in 45nm monolithic silicon photonics process for this application. We use micro-ring modulators (MRM) with on-chip analog-based carrier phase recovery (CPR). We also introduced a negative Miller capacitor TIA bandwidth extension method for coherent detections showing ∼5 dB SNR improvement without any power overhead. Measured transmit eye diagrams show 7.9 dB ER and 5 dB IL, with 0.67 pJ/b energy-efficiency. Receiver sensitivity is measured at -12.3 dBm with raw (pre-FEC) BER of <1e-10 at 0.82 pJ/b energy-efficiency.