## 3 Classification of Optical Waveguide Sensors
### Microring Structured Optical Waveguide Sensor
#### 3.1.4 Polymer-Based Microring Optical Waveguide Sensor  
In recent years, integrated optical waveguide sensors have exhibited many advantages, such as being label-free, highly sensitive, and capable of real-time detection, in addition to having excellent potential in biochemical sensing and monitoring [67]. Traditional sensing is based on the evanescent wave principle, and the light-matter interaction in the cladding part is relatively weak. Moreover, the slit waveguide structure increases the interaction between the light and low refractive index, which improves the sensing sensitivity. In addition, research on slit waveguides has primarily focused on SOI materials. Polymer-based optical waveguides have the advantages of speed, high efficiency, and low cost. [PERSON] et al. proposed a polymer-based slit waveguide microring refractive index sensor [68]. First, a silicon master mold was prepared, and a flexible soft mold was prepared using the fluorinated polymer perfluoropolyether (PFFE) on the silicon master mold. A polymer-based slot waveguide was fabricated using a UV soft imprinting process. The polymer slit waveguide microring structure is shown in Figure 12, and the width and height of the waveguide and the width of the slit waveguide were approximately 510, 830, and 234 nm, respectively. The working wavelength was the near-infrared band at 890 nm, and the sensitivity was 130 nm/RIU. Ring optical resonators in single-mode waveguides can improve the sensitivity of devices owing to their low modal dispersion, low propagation loss, and small size. To further realize low-cost and large-scale production, multimode waveguide resonators with linewidths of several micrometers must be designed for refractive index sensing. However, it is difficult to use the resonator as a sensor because of the multiple peaks in the output transmission spectrum owing to modal dispersion. The S-bend resonator can eliminate the multi-peak problem using mode-recognition technology. [PERSON] et al. proposed a polymer S-bend optical resonant cavity structure composed of multimode waveguides  
Figure 12: Polymer-based slit waveguide microring. for refractive index sensing [69]. Optical resonator sensors detect changes in the external effective refractive index by measuring changes in the resonance wavelength peak caused by changes such as antigen-antibody reactions, temperature changes, and protein binding in the sensing region. Figure 13 shows a schematic of an S-shaped curved resonator, where N is the number of curved waveguide layers and \(R\) is the radius of the curved waveguide. The performance of the S-bend resonator is related to the waveguide slope owing to the thermal reflow of the low-viscosity SU8 polymer. It was found that reducing the sidewall angle can increase the range and intensity of the evanescent field, resulting in an enhanced resonator sensitivity owing to the change in the external refractive index. This study provides a possibility for polymer-based millimeter-wave optical waveguide refractive index sensors. Finally, the performance of the microring-based optical waveguide refractive index sensor was compared with that of previously reported sensors, and a summary is shown in Table 2. In addition, the performance of the microring optical waveguide refractive index biochemical sensor with different materials and structures was compared.