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

Area of Science:

• Photonics and Optical Engineering
• Integrated Optics
• High-Speed Communication Systems

Background:

• Co-packaged optics (CPO) are crucial for future AI hardware, demanding high bandwidth, density, and energy efficiency.
• Microring modulators (MRMs) offer compact size and energy efficiency for dense wavelength-division multiplexing (DWDM) transmitters.
• Achieving >200 Gb/s requires advanced modulation formats like higher-order quadrature amplitude modulation (QAM).

Purpose of the Study:

• To demonstrate the feasibility of using MRMs for higher-order modulation formats.
• To develop MRM-based coherent CPO (C2PO) transmitters for 400 Gb/s data rates.
• To evaluate the performance and thermal stability of MRM-based transmitters.

Main Methods:

• Simulated MRM-based phase-constant amplitude modulators for offset-QAM-16.
• Evaluated transmitter performance using foundry-provided silicon photonics process.
• Conducted link-level bit error rate (BER) modeling and thermal simulations.
• Fabricated a proof-of-concept chip demonstrating 25 Gb/s MRM-based offset-QAM-4.

Main Results:

• Achieved 400 Gb/s using offset-QAM-16 with 11.87 dBm optical power, comparable to MZM-based links.
• Demonstrated an input-normalized electric field amplitude contrast of 0.64 per dimension.
• Validated thermal stability for meeting target BER at desired data rates.
• Proof-of-concept chip achieved 25 Gb/s offset-QAM-4, showing 10-100x size reduction.

Conclusions:

• MRM-based C2PO transmitters are a viable solution for future ultra-high bandwidth applications.
• The proposed design enables efficient implementation of advanced modulation formats like offset-QAM-16.
• MRM technology offers significant size and potential power advantages over MZM-based solutions.