Relaxed dimensional tolerance whispering gallery microbends

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 ±100 nm lead to loss variations of only ±0.01 dB in the whispering gallery regime, contrasting favorably with ±0.10 dB loss variations for the single-mode regime for a 20 µm radius and for 1.5 µm 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° and polarization conversion of down to -25 dB/180 are predicted.