A polarization independent optical waveguide structure suited for operation in the third communication window has been developed and optimized towards minimized dependence on deviations in the processing parameters and very low processing complexity. The tolerance analysis and optimization have been based on the thin film parameters of the widely applied silicon oxynitride technology. The silicon oxynitride layers have typically a material birefringence (nTMnTE) between 1-2 x 10-3 and can be deposited within a uniformity and reproducibility of 1% in thickness (d), 5x10-4 in refractive index (n) and 100 nm in channel width (w). The optimized waveguide structure meets the criterion of a channel birefringence (Δneff,TM-TE) within 5x10-5 taking the processing tolerance into account. Moreover, it was found
that the channel birefringence is thickness independent (within the 10-5 criterion) over a range of up to 200 nm
(δΔneff,TM-TE ,/ δd = 0). Furthermore, the optimized waveguide is fulfilling the remaining demands of the application
aimed at, such as monomodality, low fiber to chip coupling loss (< 0.5 dB/facet) and low loss bends with a radius down to 600 nm. This waveguiding structure has been applied for the realization of a passband flattened add-drop multiplexing device (or interleaver) with 0.4 nm free spectral range and 0.03 nm TE-TM shift. Based on this shift, a polarization dependence of 3 x 10-5 was calculated for the optical waveguides.