Design and application of compact and highly tolerant polarization-independent waveguides

In this paper, the design, fabrication, and application of a highly tolerant polarization-independent optical-waveguide structure suited for operation in the third communication window is presented. The waveguide structure has been optimized toward minimized sensitivity to technological tolerances and low fabrication complexity. The tolerance analysis has been based on the typical processing tolerances of the widely applied silicon-oxynitride technology, being +/- $3 x 10^{-4}$ in refractive index, +/- 1% in thickness, and +/- 0.1 micron in channel width. The optimized waveguide design fulfills the criterion of a channel birefringence within $5 x 10^{-5}$, including processing tolerance. It also enables a fiber-to-chip coupling loss below 1 dB/facet and is suited for the realization of low-loss bends with a radius down to 600 micron. Based on this waveguide design, a passband-flattened optical wavelength filter with 50-GHz free spectral range has been realized and tested. The measured TE–TM shift of 0.03 nm confirms the polarization dependence of the optical waveguides being as low as $3 x 10^{-5}$.