2025 Theses Doctoral

Co-optimization of Silicon Photonic Link Architectures and Devices

Parsons, Robert

The explosive demand for AI/ML workloads in datacenters and high-performance computing (HPC) centers necessitates low-energy, high-bandwidth integrated silicon photonic interconnects. Specialized design is required to meet these critical design criteria and stringent targets. We present a holistic design methodology, where both the link architecture and devices are co-optimized.

Starting from a link model, we can determine the pain points of a link architecture, such as high insertion loss of a particular component, or negative impacts of dispersion. From there, we design devices to ensure we can meet the power budget and required aggregate bandwidth. Equally important is to design the components and link architecture to enable low energy consumption operation. Geometric variations during fabrication and changes in the environmental temperature can shift the optical properties of these devices, necessitating the use of thermal tuning to stabilize their performance. Thermal tuning can impose a high power consumption burden, so designing components that are robust to fabrication variations is imperative to achieve low energy consumption links. Comprehensive wafer-level statistics from our wafer prober inform the fabrication robustness and tuning efficiencies of these devices, such as whispering gallery mode resonators and Mach Zehnder interferometers (MZIs).

Further, we introduce thermal undercut to our devices, where the substrate is removed and the devices are suspended in air, thereby dramatically increasing their thermal tuning efficiency. We demonstrate, for the first time, wafer-scale analyses comparing devices both with and without undercut and the influence of undercut trench geometry on tuning efficiency.

To address increasing demands for larger link bandwidths, we demonstrate the integration of mode division multiplexing (MDM) into dense wavelength division multiplexing (DWDM) systems. Multiplexing encoded light onto many wavelengths and orthogonal modes of multimode waveguides and fibers allows us to achieve massively parallel interconnects to meet ultra-high bandwidth density requirements. Finally, we develop process design kits (PDKs) in next-generation low-loss active foundry processes for both telecom and quantum applications.