Maumita Chakrabarti

We report on the design, modeling, and measurements of high-speed, silicon-contacted, waveguide-coupled Germanium-on-Silicon (Ge/Si) avalanche photodiodes (APDs) fabricated on 300 mm Silicon On insulator (SOI) wafers. The devices achieve a peak 3-dB bandwidth (BW) exceeding 100 GHz and maintain 70 GHz BW at a responsivity of 2 A/W in both the O- and C-bands, at a relatively low reverse bias of 7 V. These results demonstrate the strong potential of Ge/Si APDs to be used for next-generation short-reach optical interconnects targeting 400 Gbps per lane operation.

With the rapid development of artificial intelligence (AI) and machine learning (ML) applications, wafer-level optical I/O is becoming increasingly attractive for massive and efficient data interconnects in future wafer-scale multi-processor-units (multi-XPU) compute clusters with unparalleled data bandwidth, energy efficiency, and low latency. In this paper, we present a 300 mm sized wafer reticle-stitched low-pressure chemical vapor deposition (LPCVD) silicon nitride (SiN) waveguide technology and demonstrate a multi-reticle-stitched ~56 cm long waveguides across 20 reticles with propagation loss of 0.13~0.15 dB/cm at 1310 nm wavelength, and <0.001~0.002 dB SiN waveguide stitch loss, which is because of <5 nm high-precision reticle lithography offset. These advantageous features of low-loss reticle-stitched SiN waveguides have the potential to significantly enhance future optically interconnected wafer-scale multi-chip compute systems.

We present a novel compact asymmetric bent directional coupler polarization beam splitter (PBS) fabricated on a silicon-on-insulator (SOI) platform using third-order polynomial interconnected circular (TOPIC) bends. The TOPIC bend design provides continuous curvature and curvature derivatives throughout the structure, which minimizes mode transition losses at connection interfaces. This approach allows for the implementation of tight bend radii to enhance extinction ratio performance without compromising insertion loss, thereby resolving the conventional trade-off limitation in bent-coupler PBS designs. The device performance was characterized through cascaded measurements involving up to 40 PBS units, with single-device loss parameters extracted via linear regression analysis. Comprehensive wafer-level testing across 61 dies confirmed excellent design reproducibility and manufacturing tolerance. The optimized PBS achieves insertion losses as low as 0.017$\pm$0.007 dB for TE and 0.019$\pm$0.018 dB for TM polarizations, accompanied by extinction ratios of 25.5$\pm$0.5 dB and 29.6$\pm$6.1 dB, respectively, at 1295 nm wavelength. These results establish new performance benchmarks for silicon photonic PBS devices and demonstrate significant potential for advanced polarization-handling applications in integrated photonic systems.

In this paper, we present a collective photonic integrated circuits (PIC) die-to-wafer dielectric bonding process, enabling silicon nitride (SiN) waveguide-based die-to-wafer evanescent coupling with insertion losses (IL) of 0.16 ±0.06 dB at optical O-band 1310 nm wavelength. This SiN evanescent coupling is of great significance and offers integration flexibility to effectively reduce optical coupling loss and enable higher-density optical connectivity compared to conventional edge coupling optical fiber arrays. The collective die-to-wafer bonding process enhances manufacturing efficiency by enabling high throughput, reducing handling steps, and lowering overall production costs. Besides, the evanescent coupling misalignment tolerance is up to 2 μm, which can mitigate alignment sensitivity and packaging difficulties. This advancement supports high-volume, low-latency, lower-energy-consumption optical I/O, paving the way for optically interconnected 300 mm wafer-scale multi-XPU compute systems for high-performance AI/ML computing.

We present an RF equivalent circuit model for GeSi quantum confined stark effect (QCSE) electro-absorption modulator (EAM) integrated in a 300 mm Si photonics platform. The model incorporates two key components: the first captures the contributions from interconnects and bondpads, while the second represents the electrical and optical characteristics of the modulator. The model is validated by fitting it to measured S₁₁ parameters across various EAM lengths under different bias conditions. The derived circuit parameters are subsequently employed to simulate the electro-optic S₂₁ response, successfully estimating the EAM 3dB bandwidth in a good agreement with the measured small-signal modulation response. Additionally, deeper insights about the physical interpretation of the S₁₁ data are obtained by converting the S₁₁ to the impedance (Z) domain. Our model facilitates the analysis of EAM performance under high-speed operation, identifying the limiting factors for further speed enhancement.