Thermal Reflow Effect in Multi-Mode Waveguide of S-Bend Resonator With Mode Discrimination

Polymer-based S-bend optical resonator consisting of multi-mode waveguide has been used as refractive index sensing devices due to their cost-effectiveness and ease of fabrication. However, during fabrication, the thermal reflow that occurs during the baking process and overdevelopment induces an inclination in the sidewalls of the waveguide. The construction of the angled sidewalls can influence the propagation mode properties of the waveguide, such as the evanescent field and loss. Hence, in this study, we designed and analyzed S-bend resonators to investigate the influence of the sidewall slope on the optical modes of the waveguide and the sensing performance of the resonator with respect to the variation of slope angles. The simulation results revealed that, as the sidewall slope deviated from the right angle (90°–65°), the sensitivity of the device increased by a factor of 1.8 due to the field leakage, whereas, the extinction ratio and Q-factor decreased by 54% and 37%, respectively. The results indicated that the sidewall slope induces a trade-off between the resolution and the sensitivity of the S-bend resonator. The findings of this study can be used as a reference for further research on MMW-based optical sensors using polymer materials.