Fig. 1. Basic principles and simulation results. (a) Cross-section diagram of thin-film lithium niobate waveguide; (b) schematic diagram of TM mode bending model (w is the waveguide width,θ=60°,h_slab=200 nm,h_total=400 nm); (c) effective refractive index variation curve of each mode with the variation of waveguide width at φ=0° and 90°; (d) corresponding TM₀ mode hybrid point widths for different waveguide transmission directions φ (red region indicates the range of waveguide widths without mode hybridization); (e) mode field distribution of the second mode in the waveguide cross-section when φ=0° and w=1.6 μm (near the hybrid point of TM₀ and TE₁ modes); (f) mode field distribution of the fourth mode in the waveguide cross-section when φ=90° and w=2.2 μm (near the hybrid point of TM₀ and TE₂ modes). The bending radius is 200 μm in all cases

Fig. 2. Simulation results. (a) Simulated bending transmission efficiencies for the TM₀ mode at various waveguide widths; transmission efficiencies of TM₀ mode to TE₀, TM₀, TE₁, and TE₂ at a width of (b) 1.5 μm, (c) 2.1 μm, and (d) 2.3 μm

Fig. 3. Electric field intensity distribution for TM₀ mode input under waveguide widths of (a) 1.5 μm, (b) 1.8 μm, (c) 2.1 μm, and (d) 2.3 μm (insets: corresponding output cross-section field distribution)

Fig. 4. Experimental results of 40 cascaded bending waveguides with different widths. (a) Microscopic image; (b) coupling losses between fiber and grating under different grating interface widths; (c) normalized transmission losses