Ultra-compact and efficient photonic waveguide bends with different configurations designed by topology optimization

Transporting light signals over the corners and sharp bends imposes high optical loss and distortion on the mode profiles. Usually, bends with larger radii are used in circuits to minimize the loss over transmission, resulting in a severe limitation in integration density. In this paper, we propose novel topology-optimized optimized L-bend and U-bend structures designed for a 220 nm silicon-on-insulator (SOI) platform. Optimized L-bends with footprints of 2.5 µm × 2.5 µm, 1.5 µm × 1.5 µm, and 1 µm × 1 µm show maximum insertion losses of only 0.07 dB, 0.26 dB, and 0.78 dB, respectively. For optimized U-bends with footprints of 3 µm × 3.6 µm, 2.5 µm × 2.5 µm, and 1.5 µm × 1.5 µm, the maximum insertion losses are 0.07 dB, 0.21 dB, and 3.16 dB. These optimized bends reduce the maximum insertion loss by over 50% compared to un-optimized arc-type bends across a broad wavelength range of 1450–1650 nm. Experimental verification of a meander line with 16 optimized U-bends (3 µm × 3.6 µm) demonstrates an averaged insertion loss of 1.23 dB in the wavelength range of 1520–1580 nm, agreeing with simulated results and indicating a high potential of loss reduction with optimized bends.