Abstract
A new class of highly compact photonic microbend exploiting whispering
gallery propagation is analyzed. Critical design dimensions are relaxed by
using multimode waveguiding. A small outer sidewall radius enables tight arbitrary
angle of rotation for the guided light, and the inner radius is reduced beyond
the caustic radius to minimize the impact of the microbend dimensions on losses.
Whispering gallery operation is compared with single-mode microbend operation
using a combination of full-vectorial wave-equation models and 2-D and 3-D
finite-difference time domain models. The dependence of loss on fabricated
dimension is shown to be reduced by an order of magnitude. Waveguide width
variations of ${\pm} 100$ nm lead to loss variations of only ${\pm} 0.01$ dB in the whispering gallery
regime, contrasting favorably with ${\pm}
0.10$ dB loss variations for the single-mode regime for a
20 $\mu$m
radius and for 1.5 $\mu$m input waveguide widths. Smaller radius microbends show comparable
trends albeit with higher losses and increased sensitivity to width variations.
The sensitivity of microbend loss to small changes in narrow waveguide width
is attributed to the excitation of a restricted mode group. Moving the inner
sidewall beyond the caustic radius allows relaxed tolerance light propagation
in the whispering gallery regime. Losses down to 0.2 dB/180$^{\circ}$ and polarization conversion
of down to ${-}25$ dB/180$^{\circ}$ are predicted.
© 2011 IEEE
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