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

AI interconnect scaling requires compact, 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 GF monolithic silicon photonics process for this application. We use micro-ring modulators (MRM) with on-chip analog-based carrier phase recovery (CPR). We further introduce a negative Miller capacitor TIA bandwidth extension technique for coherent detections, improving SNR by ~5 dB without any power overhead. Measured transmitter eye diagrams show 7.9 dB ER and 5 dB IL, with 0.67 pJ/b energy-efficiency. The receiver achieves -12.3 dBm sensitivity at pre-FEC BER <1e-10 with 0.82 pJ/b.