Estimation of Propagation Losses for Narrow Strip and Rib Waveguides

The dependence of propagation losses on waveguide width and polarization is measured for strip and rib waveguides made of tantalum pentoxide. For strip waveguides, the propagation losses are found to increase rapidly for widths <;3 μ m. The losses were significantly smaller for rib than for strip waveguides, as expected. A method is developed for estimating the dependence of propagation losses on waveguide width. The method is based on approximating sidewall imperfections with an area with complex refractive index in a 2D model and showed a good agreement with the measured dependence on waveguide width. The method is also used to predict that propagation losses will decrease rapidly for rib heights less than 20 nm for TM-polarization.