C2PO: coherent co-packaged optics using offset-QAM-16 for beyond PAM-4 optical I/O

Co-packaged optics (CPO) has emerged as an ultimate solution for achieving the ultra-high bandwidths, shoreline densities, and energy efficiencies required by future GPUs and network switches for AI. Microring modulators (MRMs) are well-suited for transmitters due to their compact size, high energy efficiency, and natural compatibility for dense wavelength-division multiplexing (DWDM). However, extending beyond the recently demonstrated 200 Gb/s will require more advanced modulation formats such as higher-order coherent (e.g., QAM-16). In this work, we show how MRMs can be efficiently used to implement phase-constant amplitude modulators and form the building blocks of a transmitter for offset-QAM-16, which has been shown to simplify carrier-phase recovery relative to QAM with no offset. We simulate and evaluate the performance of our proposed MRM-based coherent CPO (C2PO) transmitters using a foundry-provided commercial silicon photonics process, demonstrating an input-normalized electric field amplitude contrast of 0.64 per dimension. Through complete link-level bit error rate modeling, we show that our design achieves 400 Gb/s using offset-QAM-16 at a total optical laser power of 11.87 dBm, which is comparable to that required by conventional QAM-16 Mach-Zehnder Modulator (MZM)-based links. We further conduct a thermal simulation to assess the proposed transmitter's thermal stability at the MRM input optical power required to meet a target BER at the desired data rates. Finally, as a proof of concept, we demonstrate 25 Gb/s MRM-based offset-QAM-4 modulation with a chip fabricated in the GlobalFoundries 45 nm monolithic silicon photonics process, providing a 10-100x reduction in transmitter size compared to MZM-based transmitters.