Design of suspended SU-8 optical waveguides for ultrasmall bending

Small suspended SU-8 optical waveguides with a high refractive index contrast are designed and optimized. The single-mode condition and the bending characteristics of the present waveguide are numerically calculated by using a full-vectorial finite-difference method. Pure bending losses and transition losses of bending waveguides with different bending radii and different core widths are studied. The simulation results show that a wider core is helpful to reduce the pure bending loss. For example, when the core width is chosen as 2 µm, a very small bending radius (∼5 µm) can be achieved. However the transition loss is high. When a small total bending loss (≪0.1dB/90° bend) is required, the minimal bending radius is about 7 µm when an optimal offset is introduced. A crossing structure with two arms to support the suspended waveguides is designed and optimized by using a two-dimensional finite-difference time-domain method. The structure parameters (e.g., the expanding width, the taper length and the length of the insert straight section) of the crossing structure are optimized to minimize the excess loss. The optimal crossing structures exhibits a low excess loss (≪0.1 dB) and performs well in a wide wavelength range (1500 nm∼1600 nm).