Single-mode and ultralow loss bent waveguides

A novel ultralow loss bent waveguide for single-mode operation was designed based on the Euler spiral. The proposed 180° bend (U-bend) was composed of two identical 90° Euler bends with gradient width from 1.6μm to 0.45μm and a 0.45μm wide, 2μm long strip waveguide. The proposed U-bend had ultralow losses (<−0.0005dB) and very low mode excitation ratios (MERs) of high-order modes (<−35dB) over a broad wavelength-band by lumerical simulation software. The effective radius R_eff of the designed U-bend was as small as 42.547μm and the transmission ratio of fundamental-mode was 0.99988 that almost equal to 1. After simulation and computation, only the fundamental-mode could be retained and propagated in the designed U-bend waveguide with ultralow loss.