Minimizing Roughness Loss for Ultra-Compactly Bent High Index-Contrast Waveguides

Roughness-loss in rectangular high index-contrast waveguides may be calculated semi-analytically using a modified coupled-mode theory. The method is verified by experiments and gives rules for the design of straight low-loss optical waveguides. For ultra-compactly bent waveguides, roughness losses may be minimized by choosing whispering-gallery guidance of light along an optimized outer waveguide contour. Our findings are supported by transmission experiments with meander-shaped waveguides having 2, 4, 6 and 40 bends.