High density vertical optical interconnects for passive assembly

Area of Science:

• Photonics and Optical Engineering
• Integrated Optics
• Semiconductor Device Technology

Background:

• Co-packaging of optics and electronics is key for next-generation data center switches exceeding 50 Tbps.
• Scaling bandwidth, cost, and energy efficiency in co-packaged designs depends on optical transceiver (TxRx) density, not just transistor scaling.
• Vertical stacking of optical TxRx chips is necessary due to the larger footprint of optical components.

Purpose of the Study:

• To propose and simulate a novel chip-to-chip optical coupler for dense, vertical integration in co-packaged optics.
• To enable efficient, high-bandwidth optical connections between silicon nitride and silicon on insulator waveguides.

Main Methods:

• Proposed a novel coupler scheme using overlapping, inverse double tapers for vertical coupling.
• Utilized Lumerical's 3D Finite Difference Time Domain (FDTD) solver for detailed optical simulations.
• Investigated performance metrics including insertion loss, alignment tolerances, bandwidth, and angular tolerances.

Main Results:

• Achieved insertion losses below -0.13 dB with a 1 µm inter-chip spacing.
• Demonstrated significant alignment tolerances: ~2.6 µm vertically and ±2.8 µm laterally.
• Obtained a 1 dB bandwidth exceeding 300 nm and angular tolerances of ±2.3 degrees (twist) and 0.4 degrees (tilt).

Conclusions:

• The proposed coupler is a viable enabling technology for high-performance, high-density co-packaged optical designs.
• Demonstrated CMOS-compatible, out-of-plane optical connections suitable for scaling optical transceivers (TxRx).
• The coupler's wide tolerances and low loss support practical implementation in future switch architectures.