Ultra-low-loss 180° adiabatic tapered waveguide bend based on higher-order Bézier curve

In this paper, we propose a compact 180° adiabatic tapered waveguide bend based on a higher-order Bézier curve. By co-optimizing the inner and outer boundaries of the Bézier profile, the design achieves simultaneous high-efficiency adiabatic transmission and 180° optical path reversal between narrow and broadened waveguides. Fabricated on a 400 nm silicon nitride platform, the proposed bend features a small bending radius of 20 μm, achieves a waveguide width variation from 0.8 μm to 2.5 μm, and exhibits an insertion loss below 0.01 dB across the 1290-1330 nm wavelength range. This compact, broadband, and fabrication-tolerant 180° bend provides a vital building block for high-density, low-loss photonic integrated systems.