Introduction
Silicon nitride (SiN) has emerged as a complementary platform to the well-established silicon-on-insulator (SOI) for photonic integrated circuits. While SOI provides strong optical confinement for compact devices, the high index contrast also increases sensitivity to waveguide sidewall roughness and optical losses. SiN's moderate index contrast results in lower propagation losses, over an order of magnitude less than SOI. Additionally, SiN's transparency window extends down to ~400 nm wavelengths, enabling visible light applications, and its negligible two-photon absorption makes it promising for nonlinear photonics.
A key challenge for SiN photonics has been the design of efficient grating couplers to interface with optical fibers. The lower index contrast compared to SOI limits the grating strength, typically restricting SiN grating coupler efficiencies to around 50%. This tutorial covers a new high-efficiency design that combines subwavelength metamaterial engineering with an apodized-imaging approach to achieve -1.2 dB peak coupling efficiency.
Platform Description
The proposed grating coupler structure is shown in Figures 1a and 1b. It consists of a 400 nm thick SiN waveguide on a 4.5 μm buried oxide layer. A 50 nm oxide buffer layer covers the SiN waveguide, with a 220 nm amorphous silicon (α-Si) layer deposited on top. The grating is formed by fully etching through the α-Si layer.
Hybrid α-Si on SiN Grating Design
The grating period (Λ) and duty cycle were first optimized for maximum directionality using 3D FDTD simulations of uniform gratings. Figure 2 shows the directionality as a function of period and duty cycle, indicating a maximum 93% directionality can be achieved with Λ = 700 nm and 28% duty cycle.
To optimize coupling into a fiber mode, both amplitude and phase apodization were employed using subwavelength metamaterial engineering and grating chirp/curvature respectively. This produces an apodized-imaging effect that focuses the diffracted beam down to the fiber mode field diameter.
The amplitude apodization uses subwavelength metamaterial grating segments with varying fill factors to control the local grating strength. Phase apodization is achieved by chirping the grating pitch to focus the beam longitudinally, and curving the grating grooves to focus transversely.
Key design parameters include the number of apodized periods, total number of periods, range of grating strengths and diffraction angles in the apodized section, and grating groove curvature. All parameters except curvature were optimized using a genetic algorithm on 2D cross-sectional simulations. The curvature was derived by equating the optical path lengths from the input to the fiber tip for each grating groove's transverse coordinate.
The optimized design has a 25 x 17.6 μm2 footprint. Spectral analysis in Figure 3 shows the anticipated peak coupling efficiency is -1.2 dB with a 1-dB bandwidth of 30 nm centered at 1310 nm.
Conclusion
This article covered a high-efficiency grating coupler design for the SiN photonics platform using a hybrid α-Si overlay. By combining subwavelength metamaterial apodization with an apodized-imaging approach, the design demonstrates -1.2 dB peak coupling efficiency with 30 nm 1-dB bandwidth, enabling over 80% fiber coupling for SiN photonic integrated circuits using a single full etch step.
Reference
[1] W. Fraser, D. Benedikovic, R. Korcek, M. Milanizadeh, D.-X. Xu, J. H. Schmid, P. Cheben, and W. N. Ye, "High-Efficiency Single-Etch Grating Coupler on a Hybrid α-Si/SiN Photonic Platform," Silicon Micro/NanoPhotonics Group, Carleton University, National Research Council, Ottawa, Canada; Dept. Multimedia and Information-Communication Technologies, University of Zilina, Zilina, Slovakia, 2024, pp. 1-6, doi: 979-8-3503-9404-7/24/$31.00 ©2024 IEEE.
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