Compact and Fabrication-Tolerant Waveguide Bends Based on Quadratic Reflectors

We propose and experimentally demonstrate a broadband, polarization-diverse compact bending design for low-index-contrast waveguides, where light is re-directed via total internal reflection (TIR) on an air-trench quadratic (elliptical or parabolic) reflector surface. Compared to prior work based on flat TIR mirrors, the quadratic reflector design contributes to minimized mode leakage and reduced optical losses, enabling high-density, scalable photonic architectures at the chip and board levels. Moreover, we develop a self-aligned fabrication process where the reflector and the waveguide segments are defined in a single lithography step, thereby circumventing the alignment sensitivity issue common to traditional air trench structures. Our simulations predict bending losses down to <; 0.14 dB per 90° and 180° bend at 850 nm wavelength, and we experimentally measure broadband losses of ~0.3 dB per 90° and 180° bend for both TE and TM polarizations in structures fabricated using standard UV lithography.