Circular Waveguide Grating-Via-Grating for Interlayer Coupling

A circular waveguide grating-via-grating (GVG) coupler is designed to achieve vertical interlayer optical coupling. Interlayer coupling efficiency is simulated to be 41% (single layer efficiency is 64%) for Si
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N
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circular waveguide gratings for TE polarization at 1.55-μm wavelength, which is comparable with that of rectangular SOI gratings incorporating adjacentlayer reflectors. The grating ridges of circular gratings are relatively wide (e.g. >465 nm) and adjacent-layer reflectors are not required, both of which simplify fabrication. In-plane circular distributed Bragg reflectors are incorporated to enhance the waveguide-grating interaction. The resonance-enhanced circular gratings are relatively compact compared with the conventional Si
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N
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-based rectangular gratings, e.g., the inner grating is 20 μm in diameter. Furthermore, this GVG coupling structure is less sensitive to misalignment, works well for a wide range of wavelengths (e.g. ±100 nm), and can operate with larger interlayer separations (e.g. ~5 μm) compared with conventional rectangular gratings.