High Density Vertical Optical Interconnects for Passive Assembly

TL;DR

This paper introduces a novel vertical optical coupling scheme for dense, high-performance co-packaged optics, enabling scalable data center interconnects with low loss and high alignment tolerance.

Contribution

It proposes an innovative overlapping inverse double taper design for vertical coupling between silicon nitride and silicon waveguides, demonstrating promising simulation results.

Findings

Insertion losses below -0.13 dB at 1 μm spacing

Alignment tolerances of approximately ±2.7 μm

Bandwidth exceeding 300 nm

Abstract

The co-packaging of optics and electronics provides a potential path forward to achieving beyond 50 Tbps top of rack switch packages. In a co-packaged design, the scaling of bandwidth, cost, and energy is governed by the number of optical transceivers (TxRx) per package as opposed to transistor shrink. Due to the large footprint of optical components relative to their electronic counterparts, the vertical stacking of optical TxRx chips in a co-packaged optics design will become a necessity. As a result, development of efficient, dense, and wide alignment tolerance chip-to-chip optical couplers will be an enabling technology for continued TxRx scaling. In this paper, we propose a novel scheme to vertically couple into standard 220 nm silicon on insulator waveguides from 220 nm silicon nitride on glass waveguides using overlapping, inverse double tapers. Simulation results using Lumerical's 3D Finite Difference Time Domain solver solver are presented, demonstrating insertion losses below -0.13 dB for an inter-chip spacing of 1 $\mu$m, 1 dB vertical and lateral alignment tolerances of approximately $\pm$ 2.7 $\mu$m, a greater than 300 nm 1 dB bandwidth, and 1 dB twist and tilt tolerance of approximately 2.3 degrees and 0.4 degrees, respectively. These results demonstrate the potential of our coupler for use in co-packaged designs requiring high performance, high density, CMOS compatible out of plane optical connections.