Abstract
A detailed analysis of fundamental tradeoffs between ring radius and
coupling gap size is presented to draw realistic borders of the possible
design space for microring resonators (MRRs). The coupling coefficient for
the ring-waveguide structure is estimated based on an integration of the
nonuniform gap between the ring and the waveguide. Combined with the
supermode analysis of two coupled waveguides, this approach is further
expanded into a closed-form equation that describes the coupling strength.
This equation permits to evaluate how the distance separating a waveguide
from a ring resonator, and the ring radius, affect coupling. The effect of
ring radius on the bending loss of the ring is furthermore modeled based
on the measurements for silicon MRRs with different radii. These compact
models for coupling and loss are subsequently used to derive the main
optical properties of MRRs, such as 3-dB optical bandwidth, extinction
ratio of resonance, and insertion loss, hence identifying the design
space. Our results indicate that the design space for add-drop filters in
a wavelength division multiplexed link is currently limited to 5–10
$\mu$
m in radius and gap sizes ranging from 120 to 210 nm. The
good agreement between the results from the proposed compact model for
coupling and the numerical FDTD and experimental measurements indicate the
application of our approach in realizing fast and efficient design space
exploration of MRRs in silicon photonic interconnects.
© 2018 IEEE
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